Difference between revisions of "Type signatures as good style"

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(mention rank n types)
(type inference not always decidable)
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Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>.
 
Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>.
   
''I remember that for some type extensions the automatic inference fails. Examples?''
+
Even more, for some type extensions the automatic inference fails,
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e.g. the higher-order types used by <hask>Control.Monad.ST.runST</hask>
Higher-order types, e.g., the type of <hask>Control.Monad.ST.runST</hask>:
 
 
<haskell>
 
<haskell>
 
runST :: (forall s . ST s a) -> a
 
runST :: (forall s . ST s a) -> a

Revision as of 10:40, 25 December 2008

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.

Even more, for some type extensions the automatic inference fails, e.g. the higher-order types used by 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.