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# Class Method Types
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## Brief Explanation
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In [ Haskell 98, s4.3.1](http://www.haskell.org/onlinereport/decls.html#class-decls), the signatures of methods in a class may contain constraints, but these constraints must not mention the argument of the type class. The following is illegal:
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```wiki
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class Foo a where
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op :: Num a => a -> a -> a
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The motivation was perhaps that without such constraints, class dictionaries could be represented as records with [PolymorphicComponents](polymorphic-components).
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However this restriction is not implemented by Hugs, following a suggestion of Mark Jones in
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[ Typing Haskell in Haskell](http://www.cse.ogi.edu/~mpj/thih/), and can be turned off in GHC with `-fglasgow-exts`.
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## interaction with existentials
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If this were allowed than existential types of the form
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```wiki
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data SomeFoo = exists a . Foo a => SomeFoo a
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```
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would have to carry around a dictionary for Num as well as Foo in dictionary passing implementation of type classes if it wished to use methods of this form.
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Typecase based implementations of classes such as jhcs do not have a problem with classes or existentials of this form since all classes are determined from the single type parameter no matter how many classes are actually needed.
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## Examples
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Can someone submit some real world examples of where this would be useful?
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## Cons
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- There are alternative type-preserving translations, so this doesn't really matter much anymore: In the absence of this restriction, a type-preserving translation of a type system including either [AssociatedTypes](associated-types) or [FunctionalDependencies](functional-dependencies) becomes much more difficult (see Section 4.3.4 of [ Associated Types with Class](http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html), POPL'05, and [ClassMethodTypes/TypedTrans](class-method-types/typed-trans)). Even if we adopt neither of these two type system features for Haskell', we will impact future directions of Haskell.
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## Tickets
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<table><tr><th>[\#17](https://gitlab.haskell.org//haskell/prime/issues/17)</th>
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<td>relax restriction on signatures of class methods</td></tr></table> |
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\ No newline at end of file |
