[luchs]

Yes, it's a special search engine: https://www.haskell.org/hoogle/

You don't search for words in the documentation etc., but enter the type of the function you want. This often works pretty well.

[TJSomething]

FPComplete's Hoogle is a bit more complete: https://www.fpcomplete.com/hoogle

[michaelochurch]

It's good overall but sometimes the ranking is weird. For example, if you search for "ByteString", what you'd expect is only the #3 hit.

For beginners who are likely to be searching for core libraries, the other Hoogle might be more useful.