<p>What are the lesser-known but useful features of the Haskell programming language. (I understand the language itself is lesser-known, but work with me. Even explanations of the simple things in Haskell, like defining the Fibonacci sequence with one line of code, will get upvoted by me.) </p>

<ul>
<li>Try to limit answers to the Haskell core</li>
<li>One feature per answer</li>
<li>Give an example and short description of the feature, not just a link to documentation</li>
<li>Label the feature using bold title as the first line</li>
</ul>

Optional Layout
You can use explicit braces and semicolons instead of whitespace (aka layout) to delimit blocks.
```
```
let {
      x = 40;
      y = 2
     } in
 x + y
```
```
... or equivalently...
```
```
let { x = 40; y = 2 } in x + y
```
```
... instead of ...
```
```
let x = 40
    y = 2
 in x + y
```
```
Because layout is not required, Haskell programs can be straightforwardly produced by other programs.