--[[
Copyright (c) 2016-2019 Calvin Rose and contributors
Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
]]

-- Make global variables local.
local setmetatable = setmetatable
local getmetatable = getmetatable
local type = type
local assert = assert
local pairs = pairs
local ipairs = ipairs
local tostring = tostring
local unpack = unpack or table.unpack

--
-- Main Types and support functions
--

local function deref(self) return self[1] end

local SYMBOL_MT = { 'SYMBOL', __tostring = deref }
local EXPR_MT = { 'EXPR', __tostring = deref }
local VARARG = setmetatable({ '...' }, { 'VARARG', __tostring = deref })
local LIST_MT = { 'LIST',
    __tostring = function (self)
        local strs = {}
        for _, s in ipairs(self) do
            table.insert(strs, tostring(s))
        end
        return '(' .. table.concat(strs, ' ', 1, #self) .. ')'
    end
}
local SEQUENCE_MT = { 'SEQUENCE' }

-- Load code with an environment in all recent Lua versions
local function loadCode(code, environment, filename)
    environment = environment or _ENV or _G
    if setfenv and loadstring then
        local f = assert(loadstring(code, filename))
        setfenv(f, environment)
        return f
    else
        return assert(load(code, filename, "t", environment))
    end
end

-- Create a new list
local function list(...)
    return setmetatable({...}, LIST_MT)
end

-- Create a new symbol
local function sym(str, scope, meta)
    local s = {str, scope = scope}
    if meta then
        for k, v in pairs(meta) do
            if type(k) == 'string' then s[k] = v end
        end
    end
    return setmetatable(s, SYMBOL_MT)
end

-- Create a new sequence
local function sequence(...)
   return setmetatable({...}, SEQUENCE_MT)
end

-- Create a new expr
-- etype should be one of
--   "literal", -- literals like numbers, strings, nil, true, false
--   "expression", -- Complex strings of Lua code, may have side effects, etc, but is an expression
--   "statement", -- Same as expression, but is also a valid statement (function calls).
--   "vargs", -- varargs symbol
--   "sym", -- symbol reference
local function expr(strcode, etype)
    return setmetatable({ strcode, type = etype }, EXPR_MT)
end

local function varg()
    return VARARG
end

local function isVarg(x)
    return x == VARARG and x
end

-- Checks if an object is a List. Returns the object if is a List.
local function isList(x)
    return type(x) == 'table' and getmetatable(x) == LIST_MT and x
end

-- Checks if an object is a symbol. Returns the object if it is a symbol.
local function isSym(x)
    return type(x) == 'table' and getmetatable(x) == SYMBOL_MT and x
end

-- Checks if an object any kind of table, EXCEPT list or symbol
local function isTable(x)
    return type(x) == 'table' and
        x ~= VARARG and
        getmetatable(x) ~= LIST_MT and getmetatable(x) ~= SYMBOL_MT and x
end

-- Checks if an object is a sequence (created with a [] literal)
local function isSequence(x)
   return type(x) == 'table' and getmetatable(x) == SEQUENCE_MT and x
end

--
-- Parser
--

-- Convert a stream of chunks to a stream of bytes.
-- Also returns a second function to clear the buffer in the byte stream
local function granulate(getchunk)
    local c = ''
    local index = 1
    local done = false
    return function (parserState)
        if done then return nil end
        if index <= #c then
            local b = c:byte(index)
            index = index + 1
            return b
        else
            c = getchunk(parserState)
            if not c or c == '' then
                done = true
                return nil
            end
            index = 2
            return c:byte(1)
        end
    end, function ()
        c = ''
    end
end

-- Convert a string into a stream of bytes
local function stringStream(str)
    local index = 1
    return function()
        local r = str:byte(index)
        index = index + 1
        return r
    end
end

-- Table of delimiter bytes - (, ), [, ], {, }
-- Opener keys have closer as the value, and closers keys
-- have true as their value.
local delims = {
    [40] = 41,        -- (
    [41] = true,      -- )
    [91] = 93,        -- [
    [93] = true,      -- ]
    [123] = 125,      -- {
    [125] = true      -- }
}

local function iswhitespace(b)
    return b == 32 or (b >= 9 and b <= 13)
end

local function issymbolchar(b)
    return b > 32 and
        not delims[b] and
        b ~= 127 and -- "<BS>"
        b ~= 34 and -- "\""
        b ~= 39 and -- "'"
        b ~= 126 and -- "~"
        b ~= 59 and -- ";"
        b ~= 44 and -- ","
        b ~= 64 and -- "@"
        b ~= 96 -- "`"
end

local prefixes = { -- prefix chars substituted while reading
    [96] = 'quote', -- `
    [44] = 'unquote', -- ,
    [39] = 'quote', -- '
    [35] = 'hashfn' -- #
}

-- Parse one value given a function that
-- returns sequential bytes. Will throw an error as soon
-- as possible without getting more bytes on bad input. Returns
-- if a value was read, and then the value read. Will return nil
-- when input stream is finished.
local function parser(getbyte, filename)

    -- Stack of unfinished values
    local stack = {}

    -- Provide one character buffer and keep
    -- track of current line and byte index
    local line = 1
    local byteindex = 0
    local lastb
    local function ungetb(ub)
        if ub == 10 then line = line - 1 end
        byteindex = byteindex - 1
        lastb = ub
    end
    local function getb()
        local r
        if lastb then
            r, lastb = lastb, nil
        else
            r = getbyte({ stackSize = #stack })
        end
        byteindex = byteindex + 1
        if r == 10 then line = line + 1 end
        return r
    end
    local function parseError(msg)
        return error(msg .. ' in ' .. (filename or 'unknown') .. ':' .. line, 0)
    end

    -- Parse stream
    return function()

        -- Dispatch when we complete a value
        local done, retval
        local whitespaceSinceDispatch = true
        local function dispatch(v)
            if #stack == 0 then
                retval = v
                done = true
            elseif stack[#stack].prefix then
                local stacktop = stack[#stack]
                stack[#stack] = nil
                return dispatch(list(sym(stacktop.prefix), v))
            else
                table.insert(stack[#stack], v)
            end
            whitespaceSinceDispatch = false
        end

        -- Throw nice error when we expect more characters
        -- but reach end of stream.
        local function badend()
            local accum = {}
            for _, item in ipairs(stack) do
                accum[#accum + 1] = item.closer
            end
            parseError(('expected closing delimiter%s %s'):format(
                #stack == 1 and "" or "s",
                string.char(unpack(accum))))
        end

        -- The main parse loop
        repeat
            local b

            -- Skip whitespace
            repeat
                b = getb()
                if b and iswhitespace(b) then
                    whitespaceSinceDispatch = true
                end
            until not b or not iswhitespace(b)
            if not b then
                if #stack > 0 then badend() end
                return nil
            end

            if b == 59 then -- ; Comment
                repeat
                    b = getb()
                until not b or b == 10 -- newline
            elseif type(delims[b]) == 'number' then -- Opening delimiter
                if not whitespaceSinceDispatch then
                    parseError('expected whitespace before opening delimiter '
                                   .. string.char(b))
                end
                table.insert(stack, setmetatable({
                    closer = delims[b],
                    line = line,
                    filename = filename,
                    bytestart = byteindex
                }, LIST_MT))
            elseif delims[b] then -- Closing delimiter
                if #stack == 0 then parseError('unexpected closing delimiter '
                                                   .. string.char(b)) end
                local last = stack[#stack]
                local val
                if last.closer ~= b then
                    parseError('unexpected delimiter ' .. string.char(b) ..
                               ', expected ' .. string.char(last.closer))
                end
                last.byteend = byteindex -- Set closing byte index
                if b == 41 then -- ; )
                    val = last
                elseif b == 93 then -- ; ]
                    val = sequence()
                    for i = 1, #last do
                        val[i] = last[i]
                    end
                else -- ; }
                    if #last % 2 ~= 0 then
                        parseError('expected even number of values in table literal')
                    end
                    val = {}
                    for i = 1, #last, 2 do
                        if tostring(last[i]) == ":" and isSym(last[i + 1]) then
                            last[i] = tostring(last[i + 1])
                        end
                        val[last[i]] = last[i + 1]
                    end
                end
                stack[#stack] = nil
                dispatch(val)
            elseif b == 34 then -- Quoted string
                local state = "base"
                local chars = {34}
                stack[#stack + 1] = {closer = 34}
                repeat
                    b = getb()
                    chars[#chars + 1] = b
                    if state == "base" then
                        if b == 92 then
                            state = "backslash"
                        elseif b == 34 then
                            state = "done"
                        end
                    else
                        -- state == "backslash"
                        state = "base"
                    end
                until not b or (state == "done")
                if not b then badend() end
                stack[#stack] = nil
                local raw = string.char(unpack(chars))
                local formatted = raw:gsub("[\1-\31]", function (c) return '\\' .. c:byte() end)
                local loadFn = loadCode(('return %s'):format(formatted), nil, filename)
                dispatch(loadFn())
            elseif prefixes[b] then
                -- expand prefix byte into wrapping form eg. '`a' into '(quote a)'
                table.insert(stack, {
                    prefix = prefixes[b]
                })
                local nextb = getb()
                if iswhitespace(nextb) then
                    if b == 35 then
                        stack[#stack] = nil
                        dispatch(sym('#'))
                    else
                        parseError('invalid whitespace after quoting prefix')
                    end
                end
                ungetb(nextb)
            elseif issymbolchar(b) or b == string.byte("~") then -- Try symbol
                local chars = {}
                local bytestart = byteindex
                repeat
                    chars[#chars + 1] = b
                    b = getb()
                until not b or not issymbolchar(b)
                if b then ungetb(b) end
                local rawstr = string.char(unpack(chars))
                if rawstr == 'true' then dispatch(true)
                elseif rawstr == 'false' then dispatch(false)
                elseif rawstr == '...' then dispatch(VARARG)
                elseif rawstr:match('^:.+$') then -- keyword style strings
                    dispatch(rawstr:sub(2))
                elseif rawstr:match("^~") and rawstr ~= "~=" then
                    -- for backwards-compatibility, special-case allowance of ~=
                    -- but all other uses of ~ are disallowed
                    parseError("illegal character: ~")
                else
                    local forceNumber = rawstr:match('^%d')
                    local numberWithStrippedUnderscores = rawstr:gsub("_", "")
                    local x
                    if forceNumber then
                        x = tonumber(numberWithStrippedUnderscores) or
                            parseError('could not read token "' .. rawstr .. '"')
                    else
                        x = tonumber(numberWithStrippedUnderscores) or
                            (rawstr:match("%.[0-9]") and
                                 parseError("can't start multisym segment " ..
                                                "with digit: ".. rawstr)) or
                            ((rawstr:match(":%.") or
                                  rawstr:match("%.:") or
                                  rawstr:match("::") or
                                  (rawstr:match("%.%.") and rawstr ~= "..")) and
                                    parseError("malformed multisym: " .. rawstr)) or
                            (rawstr:match(":.+:") and
                                 parseError("method call must be last component " ..
                                                "of multisym: " .. rawstr)) or
                            sym(rawstr, nil, { line = line,
                                               filename = filename,
                                               bytestart = bytestart,
                                               byteend = byteindex, })
                    end
                    dispatch(x)
                end
            else
                parseError("illegal character: " .. string.char(b))
            end
        until done
        return true, retval
    end, function ()
        stack = {}
    end
end

--
-- Compilation
--

-- Top level compilation bindings.
local rootChunk
local rootScope
local rootOptions

-- Create a new Scope, optionally under a parent scope. Scopes are compile time constructs
-- that are responsible for keeping track of local variables, name mangling, and macros.
-- They are accessible to user code via the '*compiler' special form (may change). They
-- use metatables to implement nesting via inheritance.
local function makeScope(parent)
    return {
        unmanglings = setmetatable({}, {
            __index = parent and parent.unmanglings
        }),
        manglings = setmetatable({}, {
            __index = parent and parent.manglings
        }),
        specials = setmetatable({}, {
            __index = parent and parent.specials
        }),
        symmeta = setmetatable({}, {
            __index = parent and parent.symmeta
        }),
        includes = setmetatable({}, {
            __index = parent and parent.includes
        }),
        autogensyms = {},
        parent = parent,
        vararg = parent and parent.vararg,
        depth = parent and ((parent.depth or 0) + 1) or 0,
        hashfn = parent and parent.hashfn
    }
end

-- Assert a condition and raise a compile error with line numbers. The ast arg
-- should be unmodified so that its first element is the form being called.
local function assertCompile(condition, msg, ast)
    -- if we use regular `assert' we can't provide the `level' argument of zero
    if not condition then
        error(string.format("Compile error in '%s' %s:%s: %s",
                            isSym(ast[1]) and ast[1][1] or ast[1] or '()',
                            ast.filename or "unknown", ast.line or '?', msg), 0)
    end
    return condition
end

local GLOBAL_SCOPE = makeScope()
GLOBAL_SCOPE.vararg = true
local SPECIALS = GLOBAL_SCOPE.specials
local COMPILER_SCOPE = makeScope(GLOBAL_SCOPE)

local luaKeywords = {
    'and', 'break', 'do', 'else', 'elseif', 'end', 'false', 'for', 'function',
    'if', 'in', 'local', 'nil', 'not', 'or', 'repeat', 'return', 'then', 'true',
    'until', 'while'
}
for i, v in ipairs(luaKeywords) do
    luaKeywords[v] = i
end

local function isValidLuaIdentifier(str)
    return (str:match('^[%a_][%w_]*$') and not luaKeywords[str])
end

-- Allow printing a string to Lua, also keep as 1 line.
local serializeSubst = {
    ['\a'] = '\\a',
    ['\b'] = '\\b',
    ['\f'] = '\\f',
    ['\n'] = 'n',
    ['\t'] = '\\t',
    ['\v'] = '\\v'
}
local function serializeString(str)
    local s = ("%q"):format(str)
    s = s:gsub('.', serializeSubst):gsub("[\128-\255]", function(c)
        return "\\" .. c:byte()
    end)
    return s
end

-- A multi symbol is a symbol that is actually composed of
-- two or more symbols using the dot syntax. The main differences
-- from normal symbols is that they cannot be declared local, and
-- they may have side effects on invocation (metatables)
local function isMultiSym(str)
    if isSym(str) then
        return isMultiSym(tostring(str))
    end
    if type(str) ~= 'string' then return end
    local parts = {}
    for part in str:gmatch('[^%.%:]+[%.%:]?') do
        local lastChar = part:sub(-1)
        if lastChar == ":" then
            parts.multiSymMethodCall = true
        end
        if lastChar == ":" or lastChar == "." then
            parts[#parts + 1] = part:sub(1, -2)
        else
            parts[#parts + 1] = part
        end
    end
    return #parts > 0 and
        (str:match('%.') or str:match(':')) and
        (not str:match('%.%.')) and
        str:byte() ~= string.byte '.' and
        str:byte(-1) ~= string.byte '.' and
        parts
end

local function isQuoted(symbol) return symbol.quoted end

-- Mangler for global symbols. Does not protect against collisions,
-- but makes them unlikely. This is the mangling that is exposed to
-- to the world.
local function globalMangling(str)
    if isValidLuaIdentifier(str) then
        return str
    end
    -- Use underscore as escape character
    return '__fnl_global__' .. str:gsub('[^%w]', function (c)
        return ('_%02x'):format(c:byte())
    end)
end

-- Reverse a global mangling. Takes a Lua identifier and
-- returns the fennel symbol string that created it.
local function globalUnmangling(identifier)
    local rest = identifier:match('^__fnl_global__(.*)$')
    if rest then
        return rest:gsub('_[%da-f][%da-f]', function (code)
            return string.char(tonumber(code:sub(2), 16))
        end)
    else
        return identifier
    end
end

-- Creates a symbol from a string by mangling it.
-- ensures that the generated symbol is unique
-- if the input string is unique in the scope.
local function localMangling(str, scope, ast)
    if scope.manglings[str] then
        return scope.manglings[str]
    end
    local append = 0
    local mangling = str
    assertCompile(not isMultiSym(str), 'did not expect multi symbol ' .. str, ast)

    -- Mapping mangling to a valid Lua identifier
    if luaKeywords[mangling] or mangling:match('^%d') then
        mangling = '_' .. mangling
    end
    mangling = mangling:gsub('-', '_')
    mangling = mangling:gsub('[^%w_]', function (c)
        return ('_%02x'):format(c:byte())
    end)

    local raw = mangling
    while scope.unmanglings[mangling] do
        mangling = raw .. append
        append = append + 1
    end
    scope.unmanglings[mangling] = str
    scope.manglings[str] = mangling
    return mangling
end

-- Combine parts of a symbol
local function combineParts(parts, scope)
    local ret = scope.manglings[parts[1]] or globalMangling(parts[1])
    for i = 2, #parts do
        if isValidLuaIdentifier(parts[i]) then
            if parts.multiSymMethodCall and i == #parts then
                ret = ret .. ':' .. parts[i]
            else
                ret = ret .. '.' .. parts[i]
            end
        else
            ret = ret .. '[' .. serializeString(parts[i]) .. ']'
        end
    end
    return ret
end

-- Generates a unique symbol in the scope.
local function gensym(scope, base)
    local mangling
    local append = 0
    repeat
        mangling = (base or '') .. '_' .. append .. '_'
        append = append + 1
    until not scope.unmanglings[mangling]
    scope.unmanglings[mangling] = true
    return mangling
end

-- Generates a unique symbol in the scope based on the base name. Calling
-- repeatedly with the same base and same scope will return existing symbol
-- rather than generating new one.
local function autogensym(base, scope)
    if scope.autogensyms[base] then return scope.autogensyms[base] end
    local mangling = gensym(scope, base)
    scope.autogensyms[base] = mangling
    return mangling
end

-- Check if a binding is valid
local function checkBindingValid(symbol, scope, ast)
    -- Check if symbol will be over shadowed by special
    local name = symbol[1]
    assertCompile(not scope.specials[name],
    ("symbol %s may be overshadowed by a special form or macro"):format(name), ast)
    assertCompile(not isQuoted(symbol), 'macro tried to bind ' .. name ..
                      ' without gensym; try ' .. name .. '# instead', ast)

end

-- Declare a local symbol
local function declareLocal(symbol, meta, scope, ast)
    checkBindingValid(symbol, scope, ast)
    local name = symbol[1]
    assertCompile(not isMultiSym(name), "did not expect mutltisym", ast)
    local mangling = localMangling(name, scope, ast)
    scope.symmeta[name] = meta
    return mangling
end

-- If there's a provided list of allowed globals, don't let references
-- thru that aren't on the list. This list is set at the compiler
-- entry points of compile and compileStream.
local allowedGlobals

local function globalAllowed(name)
    if not allowedGlobals then return true end
    for _, g in ipairs(allowedGlobals) do
        if g == name then return true end
    end
end

-- Convert symbol to Lua code. Will only work for local symbols
-- if they have already been declared via declareLocal
local function symbolToExpression(symbol, scope, isReference)
    local name = symbol[1]
    local multiSymParts = isMultiSym(name)
    if scope.hashfn then
       if name == '$' then name = '$1' end
       if multiSymParts then
          if multiSymParts[1] == "$" then
             multiSymParts[1] = "$1"
             name = table.concat(multiSymParts, ".")
          end
       end
    end
    local parts = multiSymParts or {name}
    local etype = (#parts > 1) and "expression" or "sym"
    local isLocal = scope.manglings[parts[1]]
    if isLocal and scope.symmeta[parts[1]] then scope.symmeta[parts[1]].used = true end
    -- if it's a reference and not a symbol which introduces a new binding
    -- then we need to check for allowed globals
    assertCompile(not isReference or isLocal or globalAllowed(parts[1]),
                  'unknown global in strict mode: ' .. parts[1], symbol)
    return expr(combineParts(parts, scope), etype)
end


-- Emit Lua code
local function emit(chunk, out, ast)
    if type(out) == 'table' then
        table.insert(chunk, out)
    else
        table.insert(chunk, {leaf = out, ast = ast})
    end
end

-- Do some peephole optimization.
local function peephole(chunk)
    if chunk.leaf then return chunk end
    -- Optimize do ... end in some cases.
    if #chunk == 3 and
        chunk[1].leaf == 'do' and
        not chunk[2].leaf and
        chunk[3].leaf == 'end' then
        return peephole(chunk[2])
    end
    -- Recurse
    for i, v in ipairs(chunk) do
        chunk[i] = peephole(v)
    end
    return chunk
end

-- correlate line numbers in input with line numbers in output
local function flattenChunkCorrelated(mainChunk)
    local function flatten(chunk, out, lastLine, file)
        if chunk.leaf then
            out[lastLine] = (out[lastLine] or "") .. " " .. chunk.leaf
        else
            for _, subchunk in ipairs(chunk) do
                -- Ignore empty chunks
                if subchunk.leaf or #subchunk > 0 then
                    -- don't increase line unless it's from the same file
                    if subchunk.ast and file == subchunk.ast.file then
                        lastLine = math.max(lastLine, subchunk.ast.line or 0)
                    end
                    lastLine = flatten(subchunk, out, lastLine, file)
                end
            end
        end
        return lastLine
    end
    local out = {}
    local last = flatten(mainChunk, out, 1, mainChunk.file)
    for i = 1, last do
        if out[i] == nil then out[i] = "" end
    end
    return table.concat(out, "\n")
end

-- Flatten a tree of indented Lua source code lines.
-- Tab is what is used to indent a block.
local function flattenChunk(sm, chunk, tab, depth)
    if type(tab) == 'boolean' then tab = tab and '  ' or '' end
    if chunk.leaf then
        local code = chunk.leaf
        local info = chunk.ast
        -- Just do line info for now to save memory
        if sm then sm[#sm + 1] = info and info.line or -1 end
        return code
    else
        local parts = {}
        for i = 1, #chunk do
            -- Ignore empty chunks
            if chunk[i].leaf or #(chunk[i]) > 0 then
                local sub = flattenChunk(sm, chunk[i], tab, depth + 1)
                if depth > 0 then sub = tab .. sub:gsub('\n', '\n' .. tab) end
                table.insert(parts, sub)
            end
        end
        return table.concat(parts, '\n')
    end
end

-- Some global state for all fennel sourcemaps. For the time being,
-- this seems the easiest way to store the source maps.
-- Sourcemaps are stored with source being mapped as the key, prepended
-- with '@' if it is a filename (like debug.getinfo returns for source).
-- The value is an array of mappings for each line.
local fennelSourcemap = {}
-- TODO: loading, unloading, and saving sourcemaps?

local function makeShortSrc(source)
    source = source:gsub('\n', ' ')
    if #source <= 49 then
        return '[fennel "' .. source .. '"]'
    else
        return '[fennel "' .. source:sub(1, 46) .. '..."]'
    end
end

-- Return Lua source and source map table
local function flatten(chunk, options)
    chunk = peephole(chunk)
    if(options.correlate) then
        return flattenChunkCorrelated(chunk), {}
    else
        local sm = {}
        local ret = flattenChunk(sm, chunk, options.indent, 0)
        if sm then
            local key, short_src
            if options.filename then
                short_src = options.filename
                key = '@' .. short_src
            else
                key = ret
                short_src = makeShortSrc(options.source or ret)
            end
            sm.short_src = short_src
            sm.key = key
            fennelSourcemap[key] = sm
        end
        return ret, sm
    end
end

-- module-wide state for metadata
-- create metadata table with weakly-referenced keys
local function makeMetadata()
    return setmetatable({}, {
        __mode = 'k',
        __index = {
            get = function(self, tgt, key)
                if self[tgt] then return self[tgt][key] end
            end,
            set = function(self, tgt, key, value)
                self[tgt] = self[tgt] or {}
                self[tgt][key] = value
                return tgt
            end,
            setall = function(self, tgt, ...)
                local kvLen, kvs = select('#', ...), {...}
                if kvLen % 2 ~= 0 then
                    error('metadata:setall() expected even number of k/v pairs')
                end
                self[tgt] = self[tgt] or {}
                for i = 1, kvLen, 2 do self[tgt][kvs[i]] = kvs[i + 1] end
                return tgt
            end,
        }})
end

local metadata = makeMetadata()
local doc = function(tgt, name)
    if(not tgt) then return name .. " not found" end
    local docstring = (metadata:get(tgt, 'fnl/docstring') or
                           '#<undocumented>'):gsub('\n$', ''):gsub('\n', '\n  ')
    if type(tgt) == "function" then
        local arglist = table.concat(metadata:get(tgt, 'fnl/arglist') or
                                         {'#<unknown-arguments>'}, ' ')
        return string.format("(%s%s%s)\n  %s", name, #arglist > 0 and ' ' or '',
                             arglist, docstring)
    else
        return string.format("%s\n  %s", name, docstring)
    end
end

local function docSpecial(name, arglist, docstring)
    metadata[SPECIALS[name]] =
        { ['fnl/docstring'] = docstring, ['fnl/arglist'] = arglist }
end

-- Convert expressions to Lua string
local function exprs1(exprs)
    local t = {}
    for _, e in ipairs(exprs) do
        t[#t + 1] = e[1]
    end
    return table.concat(t, ', ')
end

-- Compile side effects for a chunk
local function keepSideEffects(exprs, chunk, start, ast)
    start = start or 1
    for j = start, #exprs do
        local se = exprs[j]
        -- Avoid the rogue 'nil' expression (nil is usually a literal,
        -- but becomes an expression if a special form
        -- returns 'nil'.)
        if se.type == 'expression' and se[1] ~= 'nil' then
            emit(chunk, ('do local _ = %s end'):format(tostring(se)), ast)
        elseif se.type == 'statement' then
            local code = tostring(se)
            emit(chunk, code:byte() == 40 and ("do end " .. code) or code , ast)
        end
    end
end

-- Does some common handling of returns and register
-- targets for special forms. Also ensures a list expression
-- has an acceptable number of expressions if opts contains the
-- "nval" option.
local function handleCompileOpts(exprs, parent, opts, ast)
    if opts.nval then
        local n = opts.nval
        if n ~= #exprs then
            local len = #exprs
            if len > n then
                -- Drop extra
                keepSideEffects(exprs, parent, n + 1, ast)
                for i = n + 1, len do
                    exprs[i] = nil
                end
            else
                -- Pad with nils
                for i = #exprs + 1, n do
                    exprs[i] = expr('nil', 'literal')
                end
            end
        end
    end
    if opts.tail then
        emit(parent, ('return %s'):format(exprs1(exprs)), ast)
    end
    if opts.target then
        emit(parent, ('%s = %s'):format(opts.target, exprs1(exprs)), ast)
    end
    if opts.tail or opts.target then
        -- Prevent statements and expression from being used twice if they
        -- have side-effects. Since if the target or tail options are set,
        -- the expressions are already emitted, we should not return them. This
        -- is fine, as when these options are set, the caller doesn't need the result
        -- anyways.
        exprs = {}
    end
    return exprs
end

-- Compile an AST expression in the scope into parent, a tree
-- of lines that is eventually compiled into Lua code. Also
-- returns some information about the evaluation of the compiled expression,
-- which can be used by the calling function. Macros
-- are resolved here, as well as special forms in that order.
-- the 'ast' param is the root AST to compile
-- the 'scope' param is the scope in which we are compiling
-- the 'parent' param is the table of lines that we are compiling into.
-- add lines to parent by appending strings. Add indented blocks by appending
-- tables of more lines.
-- the 'opts' param contains info about where the form is being compiled.
-- Options include:
--   'target' - mangled name of symbol(s) being compiled to.
--      Could be one variable, 'a', or a list, like 'a, b, _0_'.
--   'tail' - boolean indicating tail position if set. If set, form will generate a return
--   instruction.
--   'nval' - The number of values to compile to if it is known to be a fixed value.
local function compile1(ast, scope, parent, opts)
    opts = opts or {}
    local exprs = {}

    -- Compile the form
    if isList(ast) then
        -- Function call or special form
        local len = #ast
        assertCompile(len > 0, "expected a function to call", ast)
        -- Test for special form
        local first = ast[1]
        if isSym(first) then -- Resolve symbol
            first = first[1]
        end
        local multiSymParts = isMultiSym(first)
        local special = scope.specials[first]
        if special and isSym(ast[1]) then
            -- Special form
            exprs = special(ast, scope, parent, opts) or expr('nil', 'literal')
            -- Be very accepting of strings or expression
            -- as well as lists or expressions
            if type(exprs) == 'string' then exprs = expr(exprs, 'expression') end
            if getmetatable(exprs) == EXPR_MT then exprs = {exprs} end
            -- Unless the special form explicitly handles the target, tail, and
            -- nval properties, (indicated via the 'returned' flag), handle
            -- these options.
            if not exprs.returned then
                exprs = handleCompileOpts(exprs, parent, opts, ast)
            elseif opts.tail or opts.target then
                exprs = {}
            end
            exprs.returned = true
            return exprs
        elseif multiSymParts and multiSymParts.multiSymMethodCall then
            local tableWithMethod = table.concat({
                    unpack(multiSymParts, 1, #multiSymParts - 1)
                                                 }, '.')
            local methodToCall = multiSymParts[#multiSymParts]
            local newAST = list(sym(':', scope), sym(tableWithMethod, scope), methodToCall)
            for i = 2, len do
                newAST[#newAST + 1] = ast[i]
            end
            local compiled = compile1(newAST, scope, parent, opts)
            exprs = compiled
        else
            -- Function call
            local fargs = {}
            local fcallee = compile1(ast[1], scope, parent, {
                nval = 1
            })[1]
            assertCompile(fcallee.type ~= 'literal',
                          'cannot call literal value', ast)
            fcallee = tostring(fcallee)
            for i = 2, len do
                local subexprs = compile1(ast[i], scope, parent, {
                    nval = i ~= len and 1 or nil
                })
                fargs[#fargs + 1] = subexprs[1] or expr('nil', 'literal')
                if i == len then
                    -- Add sub expressions to function args
                    for j = 2, #subexprs do
                        fargs[#fargs + 1] = subexprs[j]
                    end
                else
                    -- Emit sub expression only for side effects
                    keepSideEffects(subexprs, parent, 2, ast[i])
                end
            end
            local call = ('%s(%s)'):format(tostring(fcallee), exprs1(fargs))
            exprs = handleCompileOpts({expr(call, 'statement')}, parent, opts, ast)
        end
    elseif isVarg(ast) then
        assertCompile(scope.vararg, "unexpected vararg", ast)
        exprs = handleCompileOpts({expr('...', 'varg')}, parent, opts, ast)
    elseif isSym(ast) then
        local e
        local multiSymParts = isMultiSym(ast)
        assertCompile(not (multiSymParts and multiSymParts.multiSymMethodCall),
                      "multisym method calls may only be in call position", ast)
        -- Handle nil as special symbol - it resolves to the nil literal rather than
        -- being unmangled. Alternatively, we could remove it from the lua keywords table.
        if ast[1] == 'nil' then
            e = expr('nil', 'literal')
        else
            e = symbolToExpression(ast, scope, true)
        end
        exprs = handleCompileOpts({e}, parent, opts, ast)
    elseif type(ast) == 'nil' or type(ast) == 'boolean' then
        exprs = handleCompileOpts({expr(tostring(ast), 'literal')}, parent, opts)
    elseif type(ast) == 'number' then
        local n = ('%.17g'):format(ast)
        exprs = handleCompileOpts({expr(n, 'literal')}, parent, opts)
    elseif type(ast) == 'string' then
        local s = serializeString(ast)
        exprs = handleCompileOpts({expr(s, 'literal')}, parent, opts)
    elseif type(ast) == 'table' then
        local buffer = {}
        for i = 1, #ast do -- Write numeric keyed values.
            local nval = i ~= #ast and 1
            buffer[#buffer + 1] = exprs1(compile1(ast[i], scope, parent, {nval = nval}))
        end
        local keys = {}
        for k, _ in pairs(ast) do -- Write other keys.
            if type(k) ~= 'number' or math.floor(k) ~= k or k < 1 or k > #ast then
                local kstr
                if type(k) == 'string' and isValidLuaIdentifier(k) then
                    kstr = k
                else
                    kstr = '[' .. tostring(compile1(k, scope, parent, {nval = 1})[1]) .. ']'
                end
                table.insert(keys, { kstr, k })
            end
        end
        table.sort(keys, function (a, b) return a[1] < b[1] end)
        for _, k in ipairs(keys) do
            local v = ast[k[2]]
            buffer[#buffer + 1] = ('%s = %s'):format(
                k[1], tostring(compile1(v, scope, parent, {nval = 1})[1]))
        end
        local tbl = '{' .. table.concat(buffer, ', ') ..'}'
        exprs = handleCompileOpts({expr(tbl, 'expression')}, parent, opts, ast)
    else
        assertCompile(false, 'could not compile value of type ' .. type(ast), ast)
    end
    exprs.returned = true
    return exprs
end

-- SPECIALS --

-- For statements and expressions, put the value in a local to avoid
-- double-evaluating it.
local function once(val, ast, scope, parent)
    if val.type == 'statement' or val.type == 'expression' then
        local s = gensym(scope)
        emit(parent, ('local %s = %s'):format(s, tostring(val)), ast)
        return expr(s, 'sym')
    else
        return val
    end
end

-- Implements destructuring for forms like let, bindings, etc.
-- Takes a number of options to control behavior.
-- var: Whether or not to mark symbols as mutable
-- declaration: begin each assignment with 'local' in output
-- nomulti: disallow multisyms in the destructuring. Used for (local) and (global).
-- noundef: Don't set undefined bindings. (set)
-- forceglobal: Don't allow local bindings
local function destructure(to, from, ast, scope, parent, opts)
    opts = opts or {}
    local isvar = opts.isvar
    local declaration = opts.declaration
    local nomulti = opts.nomulti
    local noundef = opts.noundef
    local forceglobal = opts.forceglobal
    local forceset = opts.forceset
    local setter = declaration and "local %s = %s" or "%s = %s"

    -- Get Lua source for symbol, and check for errors
    local function getname(symbol, up1)
        local raw = symbol[1]
        assertCompile(not (nomulti and isMultiSym(raw)),
            'did not expect multisym', up1)
        if declaration then
            return declareLocal(symbol, {var = isvar}, scope, symbol)
        else
            local parts = isMultiSym(raw) or {raw}
            local meta = scope.symmeta[parts[1]]
            if #parts == 1 and not forceset then
                assertCompile(not(forceglobal and meta),
                    'expected global, found var', up1)
                assertCompile(meta or not noundef,
                    'expected local var ' .. parts[1], up1)
                assertCompile(not (meta and not meta.var),
                    'expected local var', up1)
            end
            return symbolToExpression(symbol, scope)[1]
        end
    end

    -- Compile the outer most form. We can generate better Lua in this case.
    local function compileTopTarget(lvalues)
        -- Calculate initial rvalue
        local inits = {}
        for _, x in ipairs(lvalues) do
            table.insert(inits, scope.manglings[x] and x or 'nil')
        end
        local init = table.concat(inits, ', ')
        local lvalue = table.concat(lvalues, ', ')

        local plen = #parent
        local ret = compile1(from, scope, parent, {target = lvalue})
        if declaration then
            if #parent == plen + 1 and parent[#parent].leaf then
                -- A single leaf emitted means an simple assignment a = x was emitted
                parent[#parent].leaf = 'local ' .. parent[#parent].leaf
            else
                table.insert(parent, plen + 1, { leaf = 'local ' .. lvalue ..
                                                     ' = ' .. init, ast = ast})
            end
        end
        return ret
    end

    -- Recursive auxiliary function
    local function destructure1(left, rightexprs, up1, top)
        if isSym(left) and left[1] ~= "nil" then
            checkBindingValid(left, scope, left)
            local lname = getname(left, up1)
            if top then
                compileTopTarget({lname})
            else
                emit(parent, setter:format(lname, exprs1(rightexprs)), left)
            end
        elseif isTable(left) then -- table destructuring
            if top then rightexprs = compile1(from, scope, parent) end
            local s = gensym(scope)
            emit(parent, ("local %s = %s"):format(s, exprs1(rightexprs)), left)
            for k, v in pairs(left) do
                if isSym(left[k]) and left[k][1] == "&" then
                    assertCompile(type(k) == "number" and not left[k+2],
                        "expected rest argument in final position", left)
                    local subexpr = expr(('{(table.unpack or unpack)(%s, %s)}'):format(s, k),
                        'expression')
                    destructure1(left[k+1], {subexpr}, left)
                    return
                else
                    if isSym(k) and tostring(k) == ":" and isSym(v) then k = tostring(v) end
                    if type(k) ~= "number" then k = serializeString(k) end
                    local subexpr = expr(('%s[%s]'):format(s, k), 'expression')
                    destructure1(v, {subexpr}, left)
                end
            end
        elseif isList(left) then -- values destructuring
            local leftNames, tables = {}, {}
            for i, name in ipairs(left) do
                local symname
                if isSym(name) then -- binding directly to a name
                    symname = getname(name, up1)
                else -- further destructuring of tables inside values
                    symname = gensym(scope)
                    tables[i] = {name, expr(symname, 'sym')}
                end
                table.insert(leftNames, symname)
            end
            if top then
                compileTopTarget(leftNames)
            else
                local lvalue = table.concat(leftNames, ', ')
                emit(parent, setter:format(lvalue, exprs1(rightexprs)), left)
            end
            for _, pair in pairs(tables) do -- recurse if left-side tables found
                destructure1(pair[1], {pair[2]}, left)
            end
        else
            assertCompile(false, 'unable to bind ' .. tostring(left), up1)
        end
        if top then return {returned = true} end
    end

    return destructure1(to, nil, ast, true)
end

-- Unlike most expressions and specials, 'values' resolves with multiple
-- values, one for each argument, allowing multiple return values. The last
-- expression can return multiple arguments as well, allowing for more than the number
-- of expected arguments.
local function values(ast, scope, parent)
    local len = #ast
    local exprs = {}
    for i = 2, len do
        local subexprs = compile1(ast[i], scope, parent, {
            nval = (i ~= len) and 1
        })
        exprs[#exprs + 1] = subexprs[1]
        if i == len then
            for j = 2, #subexprs do
                exprs[#exprs + 1] = subexprs[j]
            end
        end
    end
    return exprs
end

-- Compile a list of forms for side effects
local function compileDo(ast, scope, parent, start)
    start = start or 2
    local len = #ast
    local subScope = makeScope(scope)
    for i = start, len do
        compile1(ast[i], subScope, parent, {
            nval = 0
        })
    end
end

-- Implements a do statement, starting at the 'start' element. By default, start is 2.
local function doImpl(ast, scope, parent, opts, start, chunk, subScope)
    start = start or 2
    subScope = subScope or makeScope(scope)
    chunk = chunk or {}
    local len = #ast
    local outerTarget = opts.target
    local outerTail = opts.tail
    local retexprs = {returned = true}

    -- See if we need special handling to get the return values
    -- of the do block
    if not outerTarget and opts.nval ~= 0 and not outerTail then
        if opts.nval then
            -- Generate a local target
            local syms = {}
            for i = 1, opts.nval do
                local s = gensym(scope)
                syms[i] = s
                retexprs[i] = expr(s, 'sym')
            end
            outerTarget = table.concat(syms, ', ')
            emit(parent, ('local %s'):format(outerTarget), ast)
            emit(parent, 'do', ast)
        else
            -- We will use an IIFE for the do
            local fname = gensym(scope)
            local fargs = scope.vararg and '...' or ''
            emit(parent, ('local function %s(%s)'):format(fname, fargs), ast)
            retexprs = expr(fname .. '(' .. fargs .. ')', 'statement')
            outerTail = true
            outerTarget = nil
        end
    else
        emit(parent, 'do', ast)
    end
    -- Compile the body
    if start > len then
        -- In the unlikely case we do a do with no arguments.
        compile1(nil, subScope, chunk, {
            tail = outerTail,
            target = outerTarget
        })
        -- There will be no side effects
    else
        for i = start, len do
            local subopts = {
                nval = i ~= len and 0 or opts.nval,
                tail = i == len and outerTail or nil,
                target = i == len and outerTarget or nil
            }
            local subexprs = compile1(ast[i], subScope, chunk, subopts)
            if i ~= len then
                keepSideEffects(subexprs, parent, nil, ast[i])
            end
        end
    end
    emit(parent, chunk, ast)
    emit(parent, 'end', ast)
    return retexprs
end

SPECIALS['do'] = doImpl
docSpecial('do', {'...'}, 'Evaluate multiple forms; return last value.')

SPECIALS['values'] = values
docSpecial('values', {'...'},
           'Return multiple values from a function.  Must be in tail position.')

-- The fn special declares a function. Syntax is similar to other lisps;
-- (fn optional-name [arg ...] (body))
-- Further decoration such as docstrings, meta info, and multibody functions a possibility.
SPECIALS['fn'] = function(ast, scope, parent)
    local fScope = makeScope(scope)
    local fChunk = {}
    local index = 2
    local fnName = isSym(ast[index])
    local isLocalFn
    local docstring
    fScope.vararg = false
    if fnName and fnName[1] ~= 'nil' then
        isLocalFn = not isMultiSym(fnName[1])
        if isLocalFn then
            fnName = declareLocal(fnName, {}, scope, ast)
        else
            fnName = symbolToExpression(fnName, scope)[1]
        end
        index = index + 1
    else
        isLocalFn = true
        fnName = gensym(scope)
    end
    local argList = assertCompile(isTable(ast[index]),
                                  'expected vector arg list [a b ...]', ast)
    local argNameList = {}
    for i = 1, #argList do
        if isVarg(argList[i]) then
            assertCompile(i == #argList, "expected vararg in last parameter position", ast)
            argNameList[i] = '...'
            fScope.vararg = true
        elseif(isSym(argList[i]) and argList[i][1] ~= "nil"
               and not isMultiSym(argList[i][1])) then
            argNameList[i] = declareLocal(argList[i], {}, fScope, ast)
        elseif isTable(argList[i]) then
            local raw = sym(gensym(scope))
            argNameList[i] = declareLocal(raw, {}, fScope, ast)
            destructure(argList[i], raw, ast, fScope, fChunk,
                        { declaration = true, nomulti = true })
        else
            assertCompile(false, 'expected symbol for function parameter', ast)
        end
    end
    if type(ast[index + 1]) == 'string' and index + 1 < #ast then
        index = index + 1
        docstring = ast[index]
    end
    for i = index + 1, #ast do
        compile1(ast[i], fScope, fChunk, {
            tail = i == #ast,
            nval = i ~= #ast and 0 or nil,
        })
    end
    if isLocalFn then
        emit(parent, ('local function %s(%s)')
                 :format(fnName, table.concat(argNameList, ', ')), ast)
    else
        emit(parent, ('%s = function(%s)')
                 :format(fnName, table.concat(argNameList, ', ')), ast)
    end

    emit(parent, fChunk, ast)
    emit(parent, 'end', ast)

    if rootOptions.useMetadata then
        local args = {}
        for i, v in ipairs(argList) do
            -- TODO: show destructured args properly instead of replacing
            args[i] =  isTable(v) and '"#<table>"' or string.format('"%s"', tostring(v))
        end

        local metaFields = {
            '"fnl/arglist"', '{' .. table.concat(args, ', ') .. '}',
        }
        if docstring then
            table.insert(metaFields, '"fnl/docstring"')
            table.insert(metaFields, '"' .. docstring:gsub('%s+$', '')
                             :gsub('\\', '\\\\'):gsub('\n', '\\n')
                             :gsub('"', '\\"') .. '"')
        end
        local metaStr = ('require("%s").metadata'):format(rootOptions.moduleName or "fennel")
        emit(parent, string.format('%s:setall(%s, %s)', metaStr,
                                   fnName, table.concat(metaFields, ', ')))
    end

    return expr(fnName, 'sym')
end
docSpecial('fn', {'name?', 'args', 'docstring?', '...'},
           'Function syntax. May optionally include a name and docstring.'
               ..'\nIf a name is provided, the function will be bound in the current scope.'
               ..'\nWhen called with the wrong number of args, excess args will be discarded'
               ..'\nand lacking args will be nil; use lambda for arity-checked functions.')

-- (lua "print('hello!')") -> prints hello, evaluates to nil
-- (lua "print 'hello!'" "10") -> prints hello, evaluates to the number 10
-- (lua nil "{1,2,3}") -> Evaluates to a table literal
SPECIALS['lua'] = function(ast, _, parent)
    assertCompile(#ast == 2 or #ast == 3,
        "expected 2 or 3 arguments in 'lua' special form", ast)
    if ast[2] ~= nil then
        table.insert(parent, {leaf = tostring(ast[2]), ast = ast})
    end
    if #ast == 3 then
        return tostring(ast[3])
    end
end

SPECIALS['doc'] = function(ast, scope, parent)
    assert(rootOptions.useMetadata, "can't look up doc with metadata disabled.")
    assertCompile(#ast == 2, "expected one argument", ast)

    local target = deref(ast[2])
    local special = scope.specials[target]
    if special then
        return ("print([[%s]])"):format(doc(special, target))
    else
        local value = tostring(compile1(ast[2], scope, parent, {nval = 1})[1])
        -- need to require here since the metadata is stored in the module
        -- and we need to make sure we look it up in the same module it was
        -- declared from.
        return ("print(require('%s').doc(%s, '%s'))")
            :format(rootOptions.moduleName or "fennel", value, tostring(ast[2]))
    end
end
docSpecial('doc', {'x'},
           'Print the docstring and arglist for a function, macro, or special form.')

-- Table lookup
SPECIALS['.'] = function(ast, scope, parent)
    local len = #ast
    assertCompile(len > 1, "expected table argument", ast)
    local lhs = compile1(ast[2], scope, parent, {nval = 1})
    if len == 2 then
        return tostring(lhs[1])
    else
        local indices = {}
        for i = 3, len do
            local index = ast[i]
            if type(index) == 'string' and isValidLuaIdentifier(index) then
                table.insert(indices, '.' .. index)
            else
                index = compile1(index, scope, parent, {nval = 1})[1]
                table.insert(indices, '[' .. tostring(index) .. ']')
            end
        end
        -- extra parens are needed for table literals
        if isTable(ast[2]) then
            return '(' .. tostring(lhs[1]) .. ')' .. table.concat(indices)
        else
            return tostring(lhs[1]) .. table.concat(indices)
        end
    end
end
docSpecial('.', {'tbl', 'key1', '...'},
           'Look up key1 in tbl table. If more args are provided, do a nested lookup.')

SPECIALS['global'] = function(ast, scope, parent)
    assertCompile(#ast == 3, "expected name and value", ast)
    -- globals tracking doesn't currently work with multi-values/destructuring
    if allowedGlobals and isSym(ast[2]) then
        for _,global in ipairs(isList(ast[2]) and ast[2] or {ast[2]}) do
            table.insert(allowedGlobals, deref(global))
        end
    end
    destructure(ast[2], ast[3], ast, scope, parent, {
        nomulti = true,
        forceglobal = true
    })
end
docSpecial('global', {'name', 'val'}, 'Set name as a global with val.')

SPECIALS['set'] = function(ast, scope, parent)
    assertCompile(#ast == 3, "expected name and value", ast)
    destructure(ast[2], ast[3], ast, scope, parent, {
        noundef = true
    })
end
docSpecial('set', {'name', 'val'},
           'Set a local variable to a new value. Only works on locals using var.')

SPECIALS['set-forcibly!'] = function(ast, scope, parent)
    assertCompile(#ast == 3, "expected name and value", ast)
    destructure(ast[2], ast[3], ast, scope, parent, {
        forceset = true
    })
end

SPECIALS['local'] = function(ast, scope, parent)
    assertCompile(#ast == 3, "expected name and value", ast)
    destructure(ast[2], ast[3], ast, scope, parent, {
        declaration = true,
        nomulti = true
    })
end
docSpecial('local', {'name', 'val'},
           'Introduce new top-level immutable local.')

SPECIALS['var'] = function(ast, scope, parent)
    assertCompile(#ast == 3, "expected name and value", ast)
    destructure(ast[2], ast[3], ast, scope, parent, {
        declaration = true,
        nomulti = true,
        isvar = true
    })
end
docSpecial('var', {'name', 'val'},
           'Introduce new mutable local.')

SPECIALS['let'] = function(ast, scope, parent, opts)
    local bindings = ast[2]
    assertCompile(isList(bindings) or isTable(bindings),
                  'expected table for destructuring', ast)
    assertCompile(#bindings % 2 == 0,
                  'expected even number of name/value bindings', ast)
    assertCompile(#ast >= 3, 'missing body expression', ast)
    local subScope = makeScope(scope)
    local subChunk = {}
    for i = 1, #bindings, 2 do
        destructure(bindings[i], bindings[i + 1], ast, subScope, subChunk, {
            declaration = true,
            nomulti = true
        })
    end
    return doImpl(ast, scope, parent, opts, 3, subChunk, subScope)
end
docSpecial('let', {'[name1 val1 ... nameN valN]', '...'},
           'Introduces a new scope in which a given set of local bindings are used.')

-- For setting items in a table
SPECIALS['tset'] = function(ast, scope, parent)
    assertCompile(#ast > 3, ('tset form needs table, key, and value'), ast)
    local root = compile1(ast[2], scope, parent, {nval = 1})[1]
    local keys = {}
    for i = 3, #ast - 1 do
        local key = compile1(ast[i], scope, parent, {nval = 1})[1]
        keys[#keys + 1] = tostring(key)
    end
    local value = compile1(ast[#ast], scope, parent, {nval = 1})[1]
    local rootstr = tostring(root)
    -- Prefix 'do end ' so parens are not ambiguous (grouping or function call?)
    local fmtstr = (rootstr:match('^{')) and 'do end (%s)[%s] = %s' or '%s[%s] = %s'
    emit(parent, fmtstr:format(tostring(root),
                               table.concat(keys, ']['),
                               tostring(value)), ast)
end
docSpecial('tset', {'tbl', 'key1', '...', 'keyN', 'val'},
           'Set the value of a table field. Can take additional keys to set'
        .. 'nested values,\nbut all parents must contain an existing table.')

-- The if special form behaves like the cond form in
-- many languages
SPECIALS['if'] = function(ast, scope, parent, opts)
    local doScope = makeScope(scope)
    local branches = {}
    local elseBranch = nil

    -- Calculate some external stuff. Optimizes for tail calls and what not
    local wrapper, innerTail, innerTarget, targetExprs
    if opts.tail or opts.target or opts.nval then
        if opts.nval and opts.nval ~= 0 and not opts.target then
            -- We need to create a target
            targetExprs = {}
            local accum = {}
            for i = 1, opts.nval do
                local s = gensym(scope)
                accum[i] = s
                targetExprs[i] = expr(s, 'sym')
            end
            wrapper = 'target'
            innerTail = opts.tail
            innerTarget = table.concat(accum, ', ')
        else
            wrapper = 'none'
            innerTail = opts.tail
            innerTarget = opts.target
        end
    else
        wrapper = 'iife'
        innerTail = true
        innerTarget = nil
    end

    -- Compile bodies and conditions
    local bodyOpts = {
        tail = innerTail,
        target = innerTarget,
        nval = opts.nval
    }
    local function compileBody(i)
        local chunk = {}
        local cscope = makeScope(doScope)
        keepSideEffects(compile1(ast[i], cscope, chunk, bodyOpts),
        chunk, nil, ast[i])
        return {
            chunk = chunk,
            scope = cscope
        }
    end
    for i = 2, #ast - 1, 2 do
        local condchunk = {}
        local res = compile1(ast[i], doScope, condchunk, {nval = 1})
        local cond = res[1]
        local branch = compileBody(i + 1)
        branch.cond = cond
        branch.condchunk = condchunk
        branch.nested = i ~= 2 and next(condchunk, nil) == nil
        table.insert(branches, branch)
    end
    local hasElse = #ast > 3 and #ast % 2 == 0
    if hasElse then elseBranch = compileBody(#ast) end

    -- Emit code
    local s = gensym(scope)
    local buffer = {}
    local lastBuffer = buffer
    for i = 1, #branches do
        local branch = branches[i]
        local fstr = not branch.nested and 'if %s then' or 'elseif %s then'
        local cond = tostring(branch.cond)
        local condLine = (cond == "true" and branch.nested and i == #branches)
            and "else"
            or fstr:format(cond)
        if branch.nested then
            emit(lastBuffer, branch.condchunk, ast)
        else
            for _, v in ipairs(branch.condchunk) do emit(lastBuffer, v, ast) end
        end
        emit(lastBuffer, condLine, ast)
        emit(lastBuffer, branch.chunk, ast)
        if i == #branches then
            if hasElse then
                emit(lastBuffer, 'else', ast)
                emit(lastBuffer, elseBranch.chunk, ast)
            end
            emit(lastBuffer, 'end', ast)
        elseif not branches[i + 1].nested then
            emit(lastBuffer, 'else', ast)
            local nextBuffer = {}
            emit(lastBuffer, nextBuffer, ast)
            emit(lastBuffer, 'end', ast)
            lastBuffer = nextBuffer
        end
    end

    if wrapper == 'iife' then
        local iifeargs = scope.vararg and '...' or ''
        emit(parent, ('local function %s(%s)'):format(tostring(s), iifeargs), ast)
        emit(parent, buffer, ast)
        emit(parent, 'end', ast)
        return expr(('%s(%s)'):format(tostring(s), iifeargs), 'statement')
    elseif wrapper == 'none' then
        -- Splice result right into code
        for i = 1, #buffer do
            emit(parent, buffer[i], ast)
        end
        return {returned = true}
    else -- wrapper == 'target'
        emit(parent, ('local %s'):format(innerTarget), ast)
        for i = 1, #buffer do
            emit(parent, buffer[i], ast)
        end
        return targetExprs
    end
end
docSpecial('if', {'cond1', 'body1', '...', 'condN', 'bodyN'},
           'Conditional form.\n' ..
               'Takes any number of condition/body pairs and evaluates the first body where'
               .. '\nthe condition evaluates to truthy. Similar to cond in other lisps.')

-- (each [k v (pairs t)] body...) => []
SPECIALS['each'] = function(ast, scope, parent)
    local binding = assertCompile(isTable(ast[2]), 'expected binding table', ast)
    local iter = table.remove(binding, #binding) -- last item is iterator call
    local bindVars = {}
    local destructures = {}
    for _, v in ipairs(binding) do
        assertCompile(isSym(v) or isTable(v),
                      'expected iterator symbol or table', ast)
        if(isSym(v)) then
            table.insert(bindVars, declareLocal(v, {}, scope, ast))
        else
            local raw = sym(gensym(scope))
            destructures[raw] = v
            table.insert(bindVars, declareLocal(raw, {}, scope, ast))
        end
    end
    emit(parent, ('for %s in %s do'):format(
             table.concat(bindVars, ', '),
             tostring(compile1(iter, scope, parent, {nval = 1})[1])), ast)
    local chunk = {}
    for raw, args in pairs(destructures) do
        destructure(args, raw, ast, scope, chunk,
                    { declaration = true, nomulti = true })
    end
    compileDo(ast, scope, chunk, 3)
    emit(parent, chunk, ast)
    emit(parent, 'end', ast)
end
docSpecial('each', {'[key value (iterator)]', '...'},
           'Runs the body once for each set of values provided by the given iterator.'
           ..'\nMost commonly used with ipairs for sequential tables or pairs for'
               ..' undefined\norder, but can be used with any iterator.')

-- (while condition body...) => []
SPECIALS['while'] = function(ast, scope, parent)
    local len1 = #parent
    local condition = compile1(ast[2], scope, parent, {nval = 1})[1]
    local len2 = #parent
    local subChunk = {}
    if len1 ~= len2 then
        -- Compound condition
        emit(parent, 'while true do', ast)
        -- Move new compilation to subchunk
        for i = len1 + 1, len2 do
            subChunk[#subChunk + 1] = parent[i]
            parent[i] = nil
        end
        emit(parent, ('if %s then break end'):format(condition[1]), ast)
    else
        -- Simple condition
        emit(parent, 'while ' .. tostring(condition) .. ' do', ast)
    end
    compileDo(ast, makeScope(scope), subChunk, 3)
    emit(parent, subChunk, ast)
    emit(parent, 'end', ast)
end
docSpecial('while', {'condition', '...'},
           'The classic while loop. Evaluates body until a condition is non-truthy.')

SPECIALS['for'] = function(ast, scope, parent)
    local ranges = assertCompile(isTable(ast[2]), 'expected binding table', ast)
    local bindingSym = assertCompile(isSym(table.remove(ast[2], 1)),
                                     'expected iterator symbol', ast)
    local rangeArgs = {}
    for i = 1, math.min(#ranges, 3) do
        rangeArgs[i] = tostring(compile1(ranges[i], scope, parent, {nval = 1})[1])
    end
    emit(parent, ('for %s = %s do'):format(
             declareLocal(bindingSym, {}, scope, ast),
             table.concat(rangeArgs, ', ')), ast)
    local chunk = {}
    compileDo(ast, scope, chunk, 3)
    emit(parent, chunk, ast)
    emit(parent, 'end', ast)
end
docSpecial('for', {'[index start stop step?]', '...'}, 'Numeric loop construct.' ..
               '\nEvaluates body once for each value between start and stop (inclusive).')

SPECIALS[':'] = function(ast, scope, parent)
    assertCompile(#ast >= 3, 'expected at least 3 arguments', ast)
    -- Compile object
    local objectexpr = compile1(ast[2], scope, parent, {nval = 1})[1]
    -- Compile method selector
    local methodstring
    local methodident = false
    if type(ast[3]) == 'string' and isValidLuaIdentifier(ast[3]) then
        methodident = true
        methodstring = ast[3]
    else
        methodstring = tostring(compile1(ast[3], scope, parent, {nval = 1})[1])
        objectexpr = once(objectexpr, ast[2], scope, parent)
    end
    -- Compile arguments
    local args = {}
    for i = 4, #ast do
        local subexprs = compile1(ast[i], scope, parent, {
            nval = i ~= #ast and 1 or nil
        })
        for j = 1, #subexprs do
            args[#args + 1] = tostring(subexprs[j])
        end
    end
    local fstring
    if not methodident then
        -- Make object first argument
        table.insert(args, 1, tostring(objectexpr))
        fstring = objectexpr.type == 'sym'
            and '%s[%s](%s)'
            or '(%s)[%s](%s)'
    elseif(objectexpr.type == 'literal' or objectexpr.type == 'expression') then
        fstring = '(%s):%s(%s)'
    else
        fstring = '%s:%s(%s)'
    end
    return expr(fstring:format(
        tostring(objectexpr),
        methodstring,
        table.concat(args, ', ')), 'statement')
end
docSpecial(':', {'tbl', 'method-name', '...'},
           'Call the named method on tbl with the provided args.'..
           '\nMethod name doesn\'t have to be known at compile-time; if it is, use'
               ..'\n(tbl:method-name ...) instead.')

SPECIALS['comment'] = function(ast, _, parent)
    local els = {}
    for i = 2, #ast do
        els[#els + 1] = tostring(ast[i]):gsub('\n', ' ')
    end
    emit(parent, '              -- ' .. table.concat(els, ' '), ast)
end
docSpecial('comment', {'...'}, 'Comment which will be emitted in Lua output.')

SPECIALS['hashfn'] = function(ast, scope, parent)
    assertCompile(#ast == 2, "expected one argument", ast)
    local fScope = makeScope(scope)
    local fChunk = {}
    local name = gensym(scope)
    local symbol = sym(name)
    declareLocal(symbol, {}, scope, ast)
    fScope.vararg = false
    fScope.hashfn = true
    local args = {}
    for i = 1, 9 do args[i] = declareLocal(sym('$' .. i), {}, fScope, ast) end
    -- Compile body
    compile1(ast[2], fScope, fChunk, {tail = true})
    local maxUsed = 0
    for i = 1, 9 do if fScope.symmeta['$' .. i].used then maxUsed = i end end
    local argStr = table.concat(args, ', ', 1, maxUsed)
    emit(parent, ('local function %s(%s)'):format(name, argStr), ast)
    emit(parent, fChunk, ast)
    emit(parent, 'end', ast)
    return expr(name, 'sym')
end
docSpecial('hashfn', {'...'}, 'Function literal shorthand; args are $1, $2, etc.')

local function defineArithmeticSpecial(name, zeroArity, unaryPrefix)
    local paddedOp = ' ' .. name .. ' '
    SPECIALS[name] = function(ast, scope, parent)
        local len = #ast
        if len == 1 then
            assertCompile(zeroArity ~= nil, 'Expected more than 0 arguments', ast)
            return expr(zeroArity, 'literal')
        else
            local operands = {}
            for i = 2, len do
                local subexprs = compile1(ast[i], scope, parent, {
                    nval = (i == 1 and 1 or nil)
                })
                for j = 1, #subexprs do
                    operands[#operands + 1] = tostring(subexprs[j])
                end
            end
            if #operands == 1 then
                if unaryPrefix then
                    return '(' .. unaryPrefix .. paddedOp .. operands[1] .. ')'
                else
                    return operands[1]
                end
            else
                return '(' .. table.concat(operands, paddedOp) .. ')'
            end
        end
    end
    docSpecial(name, {'a', 'b', '...'},
               'Arithmetic operator; works the same as Lua but accepts more arguments.')
end

defineArithmeticSpecial('+', '0')
defineArithmeticSpecial('..', "''")
defineArithmeticSpecial('^')
defineArithmeticSpecial('-', nil, '')
defineArithmeticSpecial('*', '1')
defineArithmeticSpecial('%')
defineArithmeticSpecial('/', nil, '1')
defineArithmeticSpecial('//', nil, '1')
defineArithmeticSpecial('or', 'false')
defineArithmeticSpecial('and', 'true')

docSpecial('and', {'a', 'b', '...'},
           'Boolean operator; works the same as Lua but accepts more arguments.')
docSpecial('or', {'a', 'b', '...'},
           'Boolean operator; works the same as Lua but accepts more arguments.')
docSpecial('..', {'a', 'b', '...'},
           'String concatenation operator; works the same as Lua but accepts more arguments.')

local function defineComparatorSpecial(name, realop, chainOp)
    local op = realop or name
    SPECIALS[name] = function(ast, scope, parent)
        local len = #ast
        assertCompile(len > 2, 'expected at least two arguments', ast)
        local lhs = compile1(ast[2], scope, parent, {nval = 1})[1]
        local lastval = compile1(ast[3], scope, parent, {nval = 1})[1]
        -- avoid double-eval by introducing locals for possible side-effects
        if len > 3 then lastval = once(lastval, ast[3], scope, parent) end
        local out = ('(%s %s %s)'):
            format(tostring(lhs), op, tostring(lastval))
        if len > 3 then
            for i = 4, len do -- variadic comparison
                local nextval = once(compile1(ast[i], scope, parent, {nval = 1})[1],
                                     ast[i], scope, parent)
                out = (out .. " %s (%s %s %s)"):
                    format(chainOp or 'and', tostring(lastval), op, tostring(nextval))
                lastval = nextval
            end
            out = '(' .. out .. ')'
        end
        return out
    end
    docSpecial(name, {'a', 'b', '...'},
               'Comparison operator; works the same as Lua but accepts more arguments.')
end

defineComparatorSpecial('>')
defineComparatorSpecial('<')
defineComparatorSpecial('>=')
defineComparatorSpecial('<=')
defineComparatorSpecial('=', '==')
defineComparatorSpecial('not=', '~=', 'or')
SPECIALS["~="] = SPECIALS["not="] -- backwards-compatibility alias

local function defineUnarySpecial(op, realop)
    SPECIALS[op] = function(ast, scope, parent)
        assertCompile(#ast == 2, 'expected one argument', ast)
        local tail = compile1(ast[2], scope, parent, {nval = 1})
        return (realop or op) .. tostring(tail[1])
    end
end

defineUnarySpecial('not', 'not ')
docSpecial('not', {'x'}, 'Boolean operator; works the same as Lua.')

defineUnarySpecial('length', '#')
docSpecial('length', {'x'}, 'Returns the length of a table or string.')
SPECIALS['#'] = SPECIALS['length']

-- Save current macro scope
local macroCurrentScope = GLOBAL_SCOPE

-- Covert a macro function to a special form
local function macroToSpecial(mac)
    local special = function(ast, scope, parent, opts)
        local oldScope = macroCurrentScope
        macroCurrentScope = scope
        local ok, transformed = pcall(mac, unpack(ast, 2))
        macroCurrentScope = oldScope
        assertCompile(ok, transformed, ast)
        local result = compile1(transformed, scope, parent, opts)
        return result
    end
    if metadata[mac] then
        -- copy metadata from original function to special form function
        metadata[mac], metadata[special] = nil, metadata[mac]
    end
    return special
end

local requireSpecial
local function compile(ast, options)
    options = options or {}
    local oldGlobals = allowedGlobals
    local oldChunk = rootChunk
    local oldScope = rootScope
    local oldOptions = rootOptions
    allowedGlobals = options.allowedGlobals
    if options.indent == nil then options.indent = '  ' end
    local chunk = {}
    local scope = options.scope or makeScope(GLOBAL_SCOPE)
    rootChunk = chunk
    rootScope = scope
    rootOptions = options
    if options.requireAsInclude then scope.specials.require = requireSpecial end
    local exprs = compile1(ast, scope, chunk, {tail = true})
    keepSideEffects(exprs, chunk, nil, ast)
    allowedGlobals = oldGlobals
    rootChunk = oldChunk
    rootScope = oldScope
    rootOptions = oldOptions
    return flatten(chunk, options)
end

-- map a function across all pairs in a table
local function quoteTmap(f, t)
    local res = {}
    for k,v in pairs(t) do
        local nk, nv = f(k, v)
        if nk then
            res[nk] = nv
        end
    end
    return res
end

-- make a transformer for key / value table pairs, preserving all numeric keys
local function entryTransform(fk,fv)
    return function(k, v)
        if type(k) == 'number' then
            return k,fv(v)
        else
            return fk(k),fv(v)
        end
    end
end

-- consume everything return nothing
local function no() end

local function mixedConcat(t, joiner)
    local ret = ""
    local s = ""
    local seen = {}
    for k,v in ipairs(t) do
        table.insert(seen, k)
        ret = ret .. s .. v
        s = joiner
    end
    for k,v in pairs(t) do
        if not(seen[k]) then
            ret = ret .. s .. '[' .. k .. ']' .. '=' .. v
            s = joiner
        end
    end
    return ret
end

-- expand a quoted form into a data literal, evaluating unquote
local function doQuote (form, scope, parent, runtime)
    local q = function (x) return doQuote(x, scope, parent, runtime) end
    -- vararg
    if isVarg(form) then
        assertCompile(not runtime, "quoted ... may only be used at compile time", form)
        return "_VARARG"
    -- symbol
    elseif isSym(form) then
        assertCompile(not runtime, "symbols may only be used at compile time", form)
        if deref(form):find("#$") then -- autogensym
            return ("sym('%s')"):format(autogensym(deref(form), scope))
        else -- prevent non-gensymmed symbols from being bound as an identifier
            return ("sym('%s', nil, {quoted=true})"):format(deref(form))
        end
    -- unquote
    elseif isList(form) and isSym(form[1]) and (deref(form[1]) == 'unquote') then
        local payload = form[2]
        local res = unpack(compile1(payload, scope, parent))
        return res[1]
    -- list
    elseif isList(form) then
        assertCompile(not runtime, "lists may only be used at compile time", form)
        local mapped = quoteTmap(entryTransform(no, q), form)
        return 'list(' .. mixedConcat(mapped, ", ") .. ')'
    -- table
    elseif type(form) == 'table' then
        local mapped = quoteTmap(entryTransform(q, q), form)
        return '{' .. mixedConcat(mapped, ", ") .. '}'
    -- string
    elseif type(form) == 'string' then
        return serializeString(form)
    else
        return tostring(form)
    end
end

SPECIALS['quote'] = function(ast, scope, parent)
    assertCompile(#ast == 2, "quote only takes a single form")
    local runtime, thisScope = true, scope
    while thisScope do
        thisScope = thisScope.parent
        if thisScope == COMPILER_SCOPE then runtime = false end
    end
    return doQuote(ast[2], scope, parent, runtime)
end
docSpecial('quote', {'x'}, 'Quasiquote the following form. Only works in macro/compiler scope.')

local function compileStream(strm, options)
    options = options or {}
    local oldGlobals = allowedGlobals
    local oldChunk = rootChunk
    local oldScope = rootScope
    local oldOptions = rootOptions
    allowedGlobals = options.allowedGlobals
    if options.indent == nil then options.indent = '  ' end
    local scope = options.scope or makeScope(GLOBAL_SCOPE)
    if options.requireAsInclude then scope.specials.require = requireSpecial end
    local vals = {}
    for ok, val in parser(strm, options.filename) do
        if not ok then break end
        vals[#vals + 1] = val
    end
    local chunk = {}
    rootChunk = chunk
    rootScope = scope
    rootOptions = options
    for i = 1, #vals do
        local exprs = compile1(vals[i], scope, chunk, {
            tail = i == #vals,
        })
        keepSideEffects(exprs, chunk, nil, vals[i])
    end
    allowedGlobals = oldGlobals
    rootChunk = oldChunk
    rootScope = oldScope
    rootOptions = oldOptions
    return flatten(chunk, options)
end

local function compileString(str, options)
    local strm = stringStream(str)
    return compileStream(strm, options)
end

---
--- Evaluation
---

-- Convert a fennel environment table to a Lua environment table.
-- This means automatically unmangling globals when getting a value,
-- and mangling values when setting a value. This means the original
-- env will see its values updated as expected, regardless of mangling rules.
local function wrapEnv(env)
    return setmetatable({}, {
        __index = function(_, key)
            if type(key) == 'string' then
                key = globalUnmangling(key)
            end
            return env[key]
        end,
        __newindex = function(_, key, value)
            if type(key) == 'string' then
                key = globalMangling(key)
            end
            env[key] = value
        end,
        -- checking the __pairs metamethod won't work automatically in Lua 5.1
        -- sadly, but it's important for 5.2+ and can be done manually in 5.1
        __pairs = function()
            local pt = {}
            for key, value in pairs(env) do
                if type(key) == 'string' then
                    pt[globalUnmangling(key)] = value
                else
                    pt[key] = value
                end
            end
            return next, pt, nil
        end,
    })
end

-- A custom traceback function for Fennel that looks similar to
-- the Lua's debug.traceback.
-- Use with xpcall to produce fennel specific stacktraces.
local function traceback(msg, start)
    local level = start or 2 -- Can be used to skip some frames
    local lines = {}
    if msg then
        local stripped = msg:gsub('^[^:]*:%d+:%s+', 'runtime error: ')
        table.insert(lines, stripped)
    end
    table.insert(lines, 'stack traceback:')
    while true do
        local info = debug.getinfo(level, "Sln")
        if not info then break end
        local line
        if info.what == "C" then
            if info.name then
                line = ('  [C]: in function \'%s\''):format(info.name)
            else
                line = '  [C]: in ?'
            end
        else
            local remap = fennelSourcemap[info.source]
            if remap and remap[info.currentline] then
                -- And some global info
                info.short_src = remap.short_src
                local mapping = remap[info.currentline]
                -- Overwrite info with values from the mapping (mapping is now
                -- just integer, but may eventually be a table)
                info.currentline = mapping
            end
            if info.what == 'Lua' then
                local n = info.name and ("'" .. info.name .. "'") or '?'
                line = ('  %s:%d: in function %s'):format(info.short_src, info.currentline, n)
            elseif info.short_src == '(tail call)' then
                line = '  (tail call)'
            else
                line = ('  %s:%d: in main chunk'):format(info.short_src, info.currentline)
            end
        end
        table.insert(lines, line)
        level = level + 1
    end
    return table.concat(lines, '\n')
end

local function currentGlobalNames(env)
    local names = {}
    for k in pairs(env or _G) do
       k = globalUnmangling(k)
       table.insert(names, k)
    end
    return names
end

local function eval(str, options, ...)
    options = options or {}
    -- eval and dofile are considered "live" entry points, so we can assume
    -- that the globals available at compile time are a reasonable allowed list
    -- UNLESS there's a metatable on env, in which case we can't assume that
    -- pairs will return all the effective globals; for instance openresty
    -- sets up _G in such a way that all the globals are available thru
    -- the __index meta method, but as far as pairs is concerned it's empty.
    if options.allowedGlobals == nil and not getmetatable(options.env) then
        options.allowedGlobals = currentGlobalNames(options.env)
    end
    local env = options.env and wrapEnv(options.env)
    local luaSource = compileString(str, options)
    local loader = loadCode(luaSource, env,
        options.filename and ('@' .. options.filename) or str)
    return loader(...)
end

local function dofileFennel(filename, options, ...)
    options = options or {}
    if options.allowedGlobals == nil then
        options.allowedGlobals = currentGlobalNames(options.env)
    end
    local f = assert(io.open(filename, "rb"))
    local source = f:read("*all"):gsub("^#![^\n]*\n", "")
    f:close()
    options.filename = options.filename or filename
    return eval(source, options, ...)
end

-- Implements a configurable repl
local function repl(options)

    local opts = options or {}
    -- This would get set for us when calling eval, but we want to seed it
    -- with a value that is persistent so it doesn't get reset on each eval.
    if opts.allowedGlobals == nil then
        opts.allowedGlobals = currentGlobalNames(opts.env)
    end

    opts.useMetadata = options.useMetadata ~= false
    opts.moduleName = options.moduleName
    local oldRootOptions = rootOptions

    local env = opts.env and wrapEnv(opts.env) or setmetatable({}, {
        __index = _ENV or _G
    })

    local function defaultReadChunk(parserState)
        io.write(parserState.stackSize > 0 and '.. ' or '>> ')
        io.flush()
        local input = io.read()
        return input and input .. '\n'
    end

    local function defaultOnValues(xs)
        io.write(table.concat(xs, '\t'))
        io.write('\n')
    end

    local function defaultOnError(errtype, err, luaSource)
        if (errtype == 'Lua Compile') then
            io.write('Bad code generated - likely a bug with the compiler:\n')
            io.write('--- Generated Lua Start ---\n')
            io.write(luaSource .. '\n')
            io.write('--- Generated Lua End ---\n')
        end
        if (errtype == 'Runtime') then
            io.write(traceback(err, 4))
            io.write('\n')
        else
            io.write(('%s error: %s\n'):format(errtype, tostring(err)))
        end
    end

    local envdbg = (opts.env or _G)["debug"]
    -- if the environment doesn't support debug.getlocal you can't save locals
    local saveLocals = opts.saveLocals ~= false and envdbg and envdbg.getlocal
    local saveSource = table.
       concat({"local ___i___ = 1",
               "while true do",
               " local name, value = debug.getlocal(1, ___i___)",
               " if(name and name ~= \"___i___\") then",
               " ___replLocals___[name] = value",
               " ___i___ = ___i___ + 1",
               " else break end end"}, "\n")

    -- we do some source munging in order to save off locals from each chunk
    -- and reintroduce them to the beginning of the next chunk, allowing
    -- locals to work in the repl the way you'd expect them to.
       local spliceSaveLocals = function(luaSource)
        env.___replLocals___ = env.___replLocals___ or {}
        local splicedSource = {}
        for line in luaSource:gmatch("([^\n]+)\n?") do
            table.insert(splicedSource, line)
        end
        -- reintroduce locals from the previous time around
        local bind = "local %s = ___replLocals___['%s']"
        for name in pairs(env.___replLocals___) do
            table.insert(splicedSource, 1, bind:format(name, name))
        end
        -- save off new locals at the end - if safe to do so (i.e. last line is a return)
        if (string.match(splicedSource[#splicedSource], "^ *return .*$")) then
            if (#splicedSource > 1) then
                table.insert(splicedSource, #splicedSource, saveSource)
            end
        end
        return table.concat(splicedSource, "\n")
    end

    -- Read options
    local readChunk = opts.readChunk or defaultReadChunk
    local onValues = opts.onValues or defaultOnValues
    local onError = opts.onError or defaultOnError
    local pp = opts.pp or tostring

    -- Make parser
    local bytestream, clearstream = granulate(readChunk)
    local chars = {}
    local read, reset = parser(function (parserState)
        local c = bytestream(parserState)
        chars[#chars + 1] = c
        return c
    end)

    local scope = makeScope(GLOBAL_SCOPE)

    local replCompleter = function(text)
        local matches = {}
        local inputFragment = text:gsub(".*[%s)(]+", "")

        -- adds partial key matches in tbl to the match list
        local function addPartials(input, tbl, prefix)
          for k in pairs(tbl) do
            if #matches >= 40 then break -- cap completions at 40 to avoid overwhelming
            elseif input == k:sub(0, #input) then
              table.insert(matches, prefix .. k)
            end
          end
        end
        -- adds matches to the match list, descending into table fields
        local function addMatches(input, tbl, prefix)
          prefix = prefix and prefix .. "." or ""
          if not string.find(input, "%.") then -- no (more) dots, so add matches
            return addPartials(input, tbl, prefix)
          end
          local head, tail = string.match(input, "^([^.]+)%.(.*)")
          -- check for table access field.child, and if field is a table, recur
          if type(tbl[head]) == "table" then
            return addMatches(tail, tbl[head], prefix .. head)
          end
        end

        addMatches(inputFragment, env._ENV or env._G or {})
        addMatches(inputFragment, env.___replLocals___ or {})
        addMatches(inputFragment, SPECIALS or {})
        addMatches(inputFragment, scope.specials or {})
        return matches
    end
    if opts.registerCompleter then opts.registerCompleter(replCompleter) end

    -- REPL loop
    while true do
        chars = {}
        local ok, parseok, x = pcall(read)
        local srcstring = string.char(unpack(chars))
        if not ok then
            onError('Parse', parseok)
            clearstream()
            reset()
        else
            rootOptions = opts
            if not parseok then break end -- eof
            local compileOk, luaSource = pcall(compile, x, {
                correlate = opts.correlate,
                source = srcstring,
                scope = scope,
                useMetadata = opts.useMetadata,
                moduleName = opts.moduleName,
            })
            if not compileOk then
                clearstream()
                onError('Compile', luaSource) -- luaSource is error message in this case
            else
                if saveLocals then
                    luaSource = spliceSaveLocals(luaSource)
                end
                local luacompileok, loader = pcall(loadCode, luaSource, env)
                if not luacompileok then
                    clearstream()
                    onError('Lua Compile', loader, luaSource)
                else
                    local loadok, ret = xpcall(function () return {loader()} end,
                        function (runtimeErr)
                            onError('Runtime', runtimeErr)
                        end)
                    if loadok then
                        env._ = ret[1]
                        env.__ = ret
                        for i = 1, #ret do ret[i] = pp(ret[i]) end
                        onValues(ret)
                    end
                end
            end
            rootOptions = oldRootOptions
        end
    end
end

local macroLoaded = {}

local pathTable = {"./?.fnl", "./?/init.fnl"}
local osPath = os and os.getenv and os.getenv("FENNEL_PATH")
if osPath then
    table.insert(pathTable, osPath)
end

local module = {
    parser = parser,
    granulate = granulate,
    stringStream = stringStream,
    compile = compile,
    compileString = compileString,
    compileStream = compileStream,
    compile1 = compile1,
    mangle = globalMangling,
    unmangle = globalUnmangling,
    list = list,
    sym = sym,
    varg = varg,
    scope = makeScope,
    gensym = gensym,
    eval = eval,
    repl = repl,
    dofile = dofileFennel,
    macroLoaded = macroLoaded,
    path = table.concat(pathTable, ";"),
    traceback = traceback,
    version = "0.4.0-dev",
}

local function searchModule(modulename, pathstring)
    modulename = modulename:gsub("%.", "/")
    for path in string.gmatch((pathstring or module.path)..";", "([^;]*);") do
        local filename = path:gsub("%?", modulename)
        local file = io.open(filename, "rb")
        if(file) then
            file:close()
            return filename
        end
    end
end

module.makeSearcher = function(options)
    return function(modulename)
      -- this will propagate options from the repl but not from eval, because
      -- eval unsets rootOptions after compiling but before running the actual
      -- calls to require.
      local opts = {}
      for k,v in pairs(rootOptions or {}) do opts[k] = v end
      for k,v in pairs(options or {}) do opts[k] = v end
      local filename = searchModule(modulename)
      if filename then
         return function(modname)
            return dofileFennel(filename, opts, modname)
         end
      end
   end
end

-- Add metadata and docstrings to fennel module
module.metadata = metadata
module.doc = doc

-- This will allow regular `require` to work with Fennel:
-- table.insert(package.loaders, fennel.searcher)
module.searcher = module.makeSearcher()
module.make_searcher = module.makeSearcher -- oops backwards compatibility

local function makeCompilerEnv(ast, scope, parent)
    return setmetatable({
        -- State of compiler if needed
        _SCOPE = scope,
        _CHUNK = parent,
        _AST = ast,
        _IS_COMPILER = true,
        _SPECIALS = SPECIALS,
        _VARARG = VARARG,
        -- Expose the module in the compiler
        fennel = module,
        -- Useful for macros and meta programming. All of Fennel can be accessed
        -- via fennel.myfun, for example (fennel.eval "(print 1)").
        list = list,
        sym = sym,
        unpack = unpack,
        gensym = function() return sym(gensym(macroCurrentScope)) end,
        ["list?"] = isList,
        ["multi-sym?"] = isMultiSym,
        ["sym?"] = isSym,
        ["table?"] = isTable,
        ["sequence?"] = isSequence,
        ["varg?"] = isVarg,
        ["get-scope"] = function() return macroCurrentScope end,
        ["in-scope?"] = function(symbol)
            return macroCurrentScope.manglings[tostring(symbol)]
        end
    }, { __index = _ENV or _G })
end

local function macroGlobals(env, globals)
    local allowed = {}
    for k in pairs(env) do
        local g = globalUnmangling(k)
        table.insert(allowed, g)
    end
    if globals then
        for _, k in pairs(globals) do
            table.insert(allowed, k)
        end
    end
    return allowed
end

local function addMacros(macros, ast, scope)
    assertCompile(isTable(macros), 'expected macros to be table', ast)
    for k, v in pairs(macros) do
        scope.specials[k] = macroToSpecial(v)
    end
end

local function loadMacros(modname, ast, scope, parent)
    local filename = assertCompile(searchModule(modname),
                                   modname .. " not found.", ast)
    local env = makeCompilerEnv(ast, scope, parent)
    local globals = macroGlobals(env, currentGlobalNames())
    return dofileFennel(filename, { env = env, allowedGlobals = globals,
                                    useMetadata = rootOptions.useMetadata,
                                    scope = COMPILER_SCOPE })
end

SPECIALS['require-macros'] = function(ast, scope, parent)
    assertCompile(#ast == 2, "Expected one module name argument", ast)
    local modname = ast[2]
    if not macroLoaded[modname] then
        macroLoaded[modname] = loadMacros(modname, ast, scope, parent)
    end
    addMacros(macroLoaded[modname], ast, scope, parent)
end
docSpecial('require-macros', {'macro-module-name'},
           'Load given module and use its contents as macro definitions in current scope.'
               ..'\nMacro module should return a table of macro functions with string keys.')

SPECIALS['include'] = function(ast, scope, parent, opts)
    assertCompile(#ast == 2, 'expected one argument', ast)

    -- Compile mod argument
    local modexpr = compile1(ast[2], scope, parent, {nval = 1})[1]
    if modexpr.type ~= 'literal' or modexpr[1]:byte() ~= 34 then
        if opts.fallback then
            return opts.fallback(modexpr)
        else
            assertCompile(false, 'module name must resolve to a string literal', ast)
        end
    end
    local code = 'return ' .. modexpr[1]
    local mod = loadCode(code)()

    -- Check cache
    local includeExpr = scope.includes[mod]
    if includeExpr then
        return includeExpr
    end

    -- Find path to source
    local path = searchModule(mod)
    local isFennel = true
    if not path then
        isFennel = false
        path = searchModule(mod, package.path)
        if not path then
            if opts.fallback then
                return opts.fallback(modexpr)
            else
                assertCompile(false, 'could not find module ' .. mod, ast)
            end
        end
    end

    -- Read source
    local f = io.open(path)
    local s = f:read('*all')
    f:close()

    -- splice in source and memoize it
    -- so we can include it again without duplication
    local target = gensym(scope)
    local ret = expr(target, 'sym')
    if isFennel then
        local p = parser(stringStream(s), path)
        local forms = list(sym('do'))
        for _, val in p do table.insert(forms, val) end
        local subscope = makeScope(rootScope.parent)
        if rootOptions.requireAsInclude then
            subscope.specials.require = requireSpecial
        end
        local subopts = {
            nval = 1,
            target = target
        }
        emit(rootChunk, 'local ' .. target, ast)
        compile1(forms, subscope, rootChunk, subopts)
    else
        emit(rootChunk, 'local ' .. target .. ' = (function() ' .. s .. ' end)()', ast)
    end

    -- Put in cache and return
    rootScope.includes[mod] = ret
    return ret
end
docSpecial('include', {'module-name-literal'},
           'Like require, but load the target module during compilation and embed it in the\n'
        .. 'Lua output. The module must be a string literal and resolvable at compile time.')

local function requireFallback(e)
    local code = ('require(%s)'):format(tostring(e))
    return expr(code, 'statement')
end

requireSpecial = function (ast, scope, parent, opts)
    opts.fallback = requireFallback
    return SPECIALS['include'](ast, scope, parent, opts)
end

local function evalCompiler(ast, scope, parent)
    local luaSource = compile(ast, { scope = makeScope(COMPILER_SCOPE),
                                     useMetadata = rootOptions.useMetadata })
    local loader = loadCode(luaSource, wrapEnv(makeCompilerEnv(ast, scope, parent)))
    return loader()
end

SPECIALS['macros'] = function(ast, scope, parent)
    assertCompile(#ast == 2, "Expected one table argument", ast)
    local macros = evalCompiler(ast[2], scope, parent)
    addMacros(macros, ast, scope, parent)
end
docSpecial('macros', {'{:macro-name-1 (fn [...] ...) ... :macro-name-N macro-body-N}'},
           'Define all functions in the given table as macros local to the current scope.')

SPECIALS['eval-compiler'] = function(ast, scope, parent)
    local oldFirst = ast[1]
    ast[1] = sym('do')
    local val = evalCompiler(ast, scope, parent)
    ast[1] = oldFirst
    return val
end
docSpecial('eval-compiler', {'...'}, 'Evaluate the body at compile-time.'
               .. ' Use the macro system instead if possible.')

-- Load standard macros
local stdmacros = [===[
{"->" (fn [val ...]
        "Thread-first macro.
Take the first value and splice it into the second form as its first argument.
The value of the second form is spliced into the first arg of the third, etc."
        (var x val)
        (each [_ e (ipairs [...])]
          (let [elt (if (list? e) e (list e))]
            (table.insert elt 2 x)
            (set x elt)))
        x)
 "->>" (fn [val ...]
         "Thread-last macro.
Same as ->, except splices the value into the last position of each form
rather than the first."
         (var x val)
         (each [_ e (pairs [...])]
           (let [elt (if (list? e) e (list e))]
             (table.insert elt x)
             (set x elt)))
         x)
 "-?>" (fn [val ...]
         "Nil-safe thread-first macro.
Same as -> except will short-circuit with nil when it encounters a nil value."
         (if (= 0 (select "#" ...))
             val
             (let [els [...]
                   e (table.remove els 1)
                   el (if (list? e) e (list e))
                   tmp (gensym)]
               (table.insert el 2 tmp)
               `(let [,tmp ,val]
                  (if ,tmp
                      (-?> ,el ,(unpack els))
                      ,tmp)))))
 "-?>>" (fn [val ...]
         "Nil-safe thread-last macro.
Same as ->> except will short-circuit with nil when it encounters a nil value."
          (if (= 0 (select "#" ...))
              val
              (let [els [...]
                    e (table.remove els 1)
                    el (if (list? e) e (list e))
                    tmp (gensym)]
                (table.insert el tmp)
                `(let [,tmp ,val]
                   (if ,tmp
                       (-?>> ,el ,(unpack els))
                       ,tmp)))))
 :doto (fn [val ...]
         "Evaluates val and splices it into the first argument of subsequent forms."
         (let [name (gensym)
               form `(let [,name ,val])]
           (each [_ elt (pairs [...])]
             (table.insert elt 2 name)
             (table.insert form elt))
           (table.insert form name)
           form))
 :when (fn [condition body1 ...]
         "Evaluate body for side-effects only when condition is truthy."
         (assert body1 "expected body")
         `(if ,condition
              (do ,body1 ,...)))
 :partial (fn [f ...]
            "Returns a function with all arguments partially applied to f."
            (let [body (list f ...)]
              (table.insert body _VARARG)
              `(fn [,_VARARG] ,body)))
 :lambda (fn [...]
           "Function literal with arity checking.
Will throw an exception if a declared argument is passed in as nil, unless
that argument name begins with ?."
           (let [args [...]
                 has-internal-name? (sym? (. args 1))
                 arglist (if has-internal-name? (. args 2) (. args 1))
                 docstring-position (if has-internal-name? 3 2)
                 has-docstring? (and (> (# args) docstring-position)
                                     (= :string (type (. args docstring-position))))
                 arity-check-position (- 4 (if has-internal-name? 0 1) (if has-docstring? 0 1))]
             (fn check! [a]
               (if (table? a)
                   (each [_ a (pairs a)]
                     (check! a))
                   (and (not (: (tostring a) :match "^?"))
                        (not= (tostring a) "&")
                        (not= (tostring a) "..."))
                   (table.insert args arity-check-position
                                 `(assert (not= nil ,a)
                                          (: "Missing argument %s on %s:%s"
                                             :format ,(tostring a)
                                             ,(or a.filename "unknown")
                                             ,(or a.line "?"))))))
             (assert (> (length args) 1) "missing body expression")
             (each [_ a (ipairs arglist)]
               (check! a))
             `(fn ,(unpack args))))
 :macro (fn macro [name ...]
          "Define a single macro."
          (assert (sym? name) "expected symbol for macro name")
          (local args [...])
          `(macros { ,(tostring name) (fn ,name ,(unpack args))}))
 :match
(fn match [val ...]
  "Perform pattern matching on val. See reference for details."
  ;; this function takes the AST of values and a single pattern and returns a
  ;; condition to determine if it matches as well as a list of bindings to
  ;; introduce for the duration of the body if it does match.
  (fn match-pattern [vals pattern unifications]
    ;; we have to assume we're matching against multiple values here until we
    ;; know we're either in a multi-valued clause (in which case we know the #
    ;; of vals) or we're not, in which case we only care about the first one.
    (let [[val] vals]
      (if (or (and (sym? pattern) ; unification with outer locals (or nil)
                   (not= :_ (tostring pattern)) ; never unify _
                   (or (in-scope? pattern)
                       (= :nil (tostring pattern))))
              (and (multi-sym? pattern)
                   (in-scope? (. (multi-sym? pattern) 1))))
          (values `(= ,val ,pattern) [])
          ;; unify a local we've seen already
          (and (sym? pattern)
               (. unifications (tostring pattern)))
          (values `(= ,(. unifications (tostring pattern)) ,val) [])
          ;; bind a fresh local
          (sym? pattern)
          (let [wildcard? (= (tostring pattern) "_")]
            (if (not wildcard?) (tset unifications (tostring pattern) val))
            (values (if (or wildcard? (: (tostring pattern) :find "^?"))
                        true `(not= ,(sym :nil) ,val))
                    [pattern val]))
          ;; guard clause
          (and (list? pattern) (sym? (. pattern 2)) (= :? (tostring (. pattern 2))))
          (let [(pcondition bindings) (match-pattern vals (. pattern 1)
                                                     unifications)
                condition `(and ,pcondition)]
            (for [i 3 (# pattern)] ; splice in guard clauses
              (table.insert condition (. pattern i)))
            (values `(let ,bindings ,condition) bindings))

          ;; multi-valued patterns (represented as lists)
          (list? pattern)
          (let [condition `(and)
                bindings []]
            (each [i pat (ipairs pattern)]
              (let [(subcondition subbindings) (match-pattern [(. vals i)] pat
                                                              unifications)]
                (table.insert condition subcondition)
                (each [_ b (ipairs subbindings)]
                  (table.insert bindings b))))
            (values condition bindings))
          ;; table patterns)
          (= (type pattern) :table)
          (let [condition `(and (= (type ,val) :table))
                bindings []]
            (each [k pat (pairs pattern)]
              (if (and (sym? pat) (= "&" (tostring pat)))
                  (do (assert (not (. pattern (+ k 2)))
                              "expected rest argument in final position")
                      (table.insert bindings (. pattern (+ k 1)))
                      (table.insert bindings [`(select ,k ((or unpack table.unpack)
                                                           ,val))]))
                  (and (= :number (type k))
                       (= "&" (tostring (. pattern (- k 1)))))
                  nil ; don't process the pattern right after &; already got it
                  (let [subval `(. ,val ,k)
                        (subcondition subbindings) (match-pattern [subval] pat
                                                                  unifications)]
                    (table.insert condition subcondition)
                    (each [_ b (ipairs subbindings)]
                      (table.insert bindings b)))))
            (values condition bindings))
          ;; literal value
          (values `(= ,val ,pattern) []))))
  (fn match-condition [vals clauses]
    (let [out `(if)]
      (for [i 1 (length clauses) 2]
        (let [pattern (. clauses i)
              body (. clauses (+ i 1))
              (condition bindings) (match-pattern vals pattern {})]
          (table.insert out condition)
          (table.insert out `(let ,bindings ,body))))
      out))
  ;; how many multi-valued clauses are there? return a list of that many gensyms
  (fn val-syms [clauses]
    (let [syms (list (gensym))]
      (for [i 1 (length clauses) 2]
        (if (list? (. clauses i))
            (each [valnum (ipairs (. clauses i))]
              (if (not (. syms valnum))
                  (tset syms valnum (gensym))))))
      syms))
  ;; wrap it in a way that prevents double-evaluation of the matched value
  (let [clauses [...]
        vals (val-syms clauses)]
    (if (not= 0 (% (length clauses) 2)) ; treat odd final clause as default
        (table.insert clauses (length clauses) (sym :_)))
    ;; protect against multiple evaluation of the value, bind against as
    ;; many values as we ever match against in the clauses.
    (list (sym :let) [vals val]
          (match-condition vals clauses))))
 }
]===]
do
    -- docstrings rely on having a place to "put" metadata; we use the module
    -- system for that. but if you try to require the module while it's being
    -- loaded, you get a stack overflow. so we fake out the module for the
    -- purposes of boostrapping the built-in macros here.
    local moduleName = "__fennel-bootstrap__"
    package.preload[moduleName] = function() return module end
    local env = makeCompilerEnv(nil, COMPILER_SCOPE, {})
    for name, fn in pairs(eval(stdmacros, {
        env = env,
        scope = makeScope(COMPILER_SCOPE),
        -- assume the code to load globals doesn't have any mistaken globals,
        -- otherwise this can be problematic when loading fennel in contexts
        -- where _G is an empty table with an __index metamethod. (openresty)
        allowedGlobals = false,
        useMetadata = true,
        filename = "built-ins",
        moduleName = moduleName,
    })) do
        SPECIALS[name] = macroToSpecial(fn, name)
    end
    package.preload[moduleName] = nil
end
SPECIALS['λ'] = SPECIALS['lambda']

return module
