# Difference between revisions of "Type signatures as good style"

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(→Answer: link to type signature) |
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− | == Question == |
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− | Since Haskell type checkers can automatically [[Determining_the_type_of_an_expression|derive types of expressions]] |
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− | why shall I put explicit type signatures in my programs? |
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− | == Answer == |
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− | Using explicit [[type signature]]s is good style and [[GHC]] with option <code>-Wall</code> warns about missing signatures. |
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− | Signatures are a good documentation and not all Haskell program readers have a type inference algorithm built-in. |
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− | There are also some cases where the infered signature is too general for your purposes. |
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− | E.g. the infered (most general) type for <hask>asTypeOf</hask> is <hask>a -> b -> a</hask>, |
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− | but the purpose of <hask>asTypeOf</hask> is to unify the types of both operands. |
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− | The more special signature <hask>a -> a -> a</hask> is what you want and it cannot be infered automatically. |
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− | Another example: |
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− | <haskell> |
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− | emptyString :: ShowS |
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− | emptyString = id |
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− | </haskell> |
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− | Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>. |
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− | 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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− | <haskell> |
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− | runST :: (forall s . ST s a) -> a |
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− | </haskell> |
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− | cannot be inferred in general, because the problem is undecidable. In GHC, they are enabled with the language pragma <code>RankNTypes</code>. |
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− | == How to add a bunch of signatures? == |
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− | Ok, this convinced me. How can I add all the signatures I did not write so far? |
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− | : You can start [[GHC|GHCi]] or [[Hugs]] and use the <code>:browse Modulename</code> directive. This will list all type signatures including the infered ones. |
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− | [[Category:FAQ]] |
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− | [[Category:Style]] |