Why can't one put type signatures in instance declarations in Haskell?

Question

I like to put type signatures for all top-level definitions in my code. However, type signatures in instance declarations don't seem to be allowed, and if I put one I get a "Misplaced type signature" error from GHC. Why is this so? Why can't GHC check if the type signature is the same as what it was expecting, and reject (or warn) if it isn't?

Answer 1

You can create the functions separately, outside the instance body, if you really want the type declarations.

class Class a where
    f1 :: a -> a

instance Class Foo where
    f1 = foo_f1

--monomorphic version of f1 for Foo:
foo_f1 :: Foo -> Foo
foo_f1 = ...

Answer 2

You can add type signatures for instances using [the new] -XInstanceSigs, which is especially useful for bringing type variables in scope. You can find more information in the official docs.

Answer 3

Most of the other answers here are pretty old... there's now a language extension:

stick the following at the top of your file:

{-# Language InstanceSigs #-}