Difference between revisions of "Type signatures as good style"
(signatures are good documentation and cannot always be infered automatically) |
(mention rank n types) |
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''I remember that for some type extensions the automatic inference fails. Examples?'' |
''I remember that for some type extensions the automatic inference fails. Examples?'' |
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+ | |||
+ | Higher-order types, e.g., the type of <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? == |
== How to add a bunch of signatures? == |
Revision as of 20:55, 8 July 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
.
I remember that for some type extensions the automatic inference fails. Examples?
Higher-order types, e.g., the type of 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?