Type signatures as good style

Question

Since Haskell type checkers can automatically derive types of expressions why shall I put explicit type signatures in my programs?

Answer

Using explicit type signatures is good style and GHC with option -Wall warns about missing signatures. Signatures are a good documentation and not all Haskell program readers have a type inference algorithm built-in. There are also some cases where the infered signature is too general for your purposes. E.g. the infered (most general) type for asTypeOf is a -> b -> a, but the purpose of asTypeOf is to unify the types of both operands. The more special signature a -> a -> a is what you want and it cannot be infered automatically. Another example:

emptyString :: ShowS
emptyString = id

Where ShowS is String -> String rather than a -> a.

I remember that for some type extensions the automatic inference fails. Examples?

Higher-order types, e.g., the type of Control.Monad.ST.runST:

runST :: (forall s . ST s a) -> a

cannot be inferred in general, because the problem is undecidable. In GHC, they are enabled with the language pragma RankNTypes.

How to add a bunch of signatures?

Ok, this convinced me. How can I add all the signatures I did not write so far?

You can start GHCi or Hugs and use the :browse Modulename directive. This will list all type signatures including the infered ones.