Hidden features of Haskell
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). 

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

This follows after the following excellent questions:

See also:

  - [Hidden features of C][1]
  - [Hidden features of C#][2]
  - [Hidden features of C++][3]
  - [Hidden features of Python][10]
  - [Hidden features of Java][4]
  - [Hidden features of JavaScript][5]
  - [Hidden features of Ruby][6]
  - [Hidden features of PHP][7]
  - [Hidden features of Perl][8]
  - [Hidden features of VB.Net][9]


  [1]: http://stackoverflow.com/questions/132241/hidden-features-of-c
  [2]: http://stackoverflow.com/questions/9033/hidden-features-of-c
  [3]: http://stackoverflow.com/questions/75538/hidden-features-of-c
  [4]: http://stackoverflow.com/questions/15496/hidden-features-of-java
  [5]: http://stackoverflow.com/questions/61088/hidden-features-of-javascript
  [6]: http://stackoverflow.com/questions/63998/hidden-features-of-ruby
  [7]: http://stackoverflow.com/questions/61401/hidden-features-of-php
  [8]: http://stackoverflow.com/questions/161872/hidden-features-of-perl
  [9]: http://stackoverflow.com/questions/102084/hidden-features-of-vbnet 
  [10]: http://stackoverflow.com/questions/101268/hidden-features-of-python

Hidden features of Haskell
===

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). 

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

This follows after the following excellent questions:

See also:

  - [Hidden features of C][1]
  - [Hidden features of C#][2]
  - [Hidden features of C++][3]
  - [Hidden features of Python][10]
  - [Hidden features of Java][4]
  - [Hidden features of JavaScript][5]
  - [Hidden features of Ruby][6]
  - [Hidden features of PHP][7]
  - [Hidden features of Perl][8]
  - [Hidden features of VB.Net][9]


  [1]: http://stackoverflow.com/questions/132241/hidden-features-of-c
  [2]: http://stackoverflow.com/questions/9033/hidden-features-of-c
  [3]: http://stackoverflow.com/questions/75538/hidden-features-of-c
  [4]: http://stackoverflow.com/questions/15496/hidden-features-of-java
  [5]: http://stackoverflow.com/questions/61088/hidden-features-of-javascript
  [6]: http://stackoverflow.com/questions/63998/hidden-features-of-ruby
  [7]: http://stackoverflow.com/questions/61401/hidden-features-of-php
  [8]: http://stackoverflow.com/questions/161872/hidden-features-of-perl
  [9]: http://stackoverflow.com/questions/102084/hidden-features-of-vbnet 
  [10]: http://stackoverflow.com/questions/101268/hidden-features-of-python
