A good first approximation already exists: Searching by function type in languages (such as Haskell[1]) that have sufficiently expressive type systems. There might be integration with e.g. the Emacs mode for Haskell* , and if not, it would certainly be feasible.

  [1]: Hoogle: http://www.haskell.org/hoogle/

* I haven't used Haskell for a while, though, so I don't remember if the Emacs mode currently supports searching by type.