I think he may mean searching by type, not searching for the type of something. Hoogle does this for Haskell and it's a fantastic tool. Let's say I want to find if an element is in a list but I don't know the name of the function to do that. It should have a signature like [a]->a->Bool (take a list of a's and one a, and return true or false). Well, here it is:

http://www.haskell.org/hoogle/?hoogle=%5Ba%5D+-%3E+a+-%3E+Bo...