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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>.
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''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>
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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

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
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.

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.
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Categories: FAQ | Style