Difference between revisions of "Type signatures as good style"

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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]]

Latest revision as of 15:18, 6 February 2021

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.