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
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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners