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Type signatures as good style

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1 Question

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

2 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
a -> b -> a
, but the purpose of
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
String -> String
rather than
a -> a

Even more, for some type extensions the automatic inference fails,

e.g. the higher-order types used by
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.

3 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.