Hoogle is a Haskell API search engine, which allows you to search many standard Haskell libraries by either function name, or by approximate type signature.
Example searches:
$query=map$ $#search$
$query=(a -> b) -> [a] -> [b]$ $#search$
$query=Ord a => [a] -> [a]$ $#search$
$query=Data.Map.insert$ $#search$
Enter your own search at the top of the page.
The Hoogle manual contains more details, including further details on search queries, how to install Hoogle as a command line application and how to integrate Hoogle with Firefox/Emacs/Vim etc.
I am very interested in any feedback you may have. Please email me, or add an entry to my bug tracker.
#export parseErrorParse error: $&errMessage$
For information on what queries should look like, see the user manual.