How to write extension descriptions

For pros & cons, please include how difficult this feature is to implement, how much it is used, whether it is a convenience or the only way to get a particular feature, whether any formal description exists, etc. See also HaskellExtensions.

Use this template:

= Extension Name =
[[PageOutline]]

See ExtensionDescriptionHowto for information on how to write these extension descriptions.  Please add any new extensions to the list of HaskellExtensions.

== Brief Explanation ==

== References ==
 * [http://www.haskell.org a link] to some reference.

== Pros ==
 * Pro
 * Pro

== Cons ==
 * Con
 * Con