[Haskell-beginners] why do classes require the type variable in type signatures?

[Peter Hall]

Consider two instances of your class:

     data Foo = Foo
     data Bar = Bar

     instance AlsoSpecial Foo where
        -- This implementation *must* be constant True or False because
        -- we don't constrain b at all, so it has to work for all b.
        isAlsoSpecial b = True

     instance AlsoSpecial Bar where
        isAlsoSpecial b = False


Now what would you expect this to output?

     isAlsoSpecial "hello"



Peter




On 23 April 2014 09:23, Dimitri DeFigueiredo <defigueiredo at ucdavis.edu>wrote:

> Hello All,
>
> Why does this compile
>
> class Special a where
>     isSpecial :: a -> Bool
>
> whereas, GHC 7.6 complains about this
>
> class AlsoSpecial a where
>     isAlsoSpecial :: b -> Bool
>
>
> This is the error message I get:
>
>     The class method `isAlsoSpecial'
>     mentions none of the type variables of the class AlsoSpecial a
>     When checking the class method:
>       isAlsoSpecial :: forall b. b -> Bool
>     In the class declaration for `AlsoSpecial'
>
> My question is: Why must I use the type variable of the class declaration
> (i.e. *a*) in the type signature for the associated method? Is there a
> fundamental reason for this or is it just a GHC specific limitation? I
> didn't see a need for it when watching this video
>
> http://channel9.msdn.com/posts/MDCC-TechTalk-Classes-
> Jim-but-not-as-we-know-them
>
> that explains the translation that GHC does to turn type classes into core.
>
>
> Thanks!
>
>
> Dimitri
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