Magicloud Magiclouds wrote:
> So I think I got what you guys meant, I limited ClassB to only H.
> Then how to archive my requirement, that from and to only return items
> that instanced ClassB?
If you are willing to go beyond Haskell98 (or Haskell2010), you can use a
multi-parameter class. Enable the extension:
{-# LANGUAGE MultiParamTypeClasses #-}
An then, instead of
class (ClassA a) => ClassC a where
from :: (ClassB b) => a -> [b]
to :: (ClassB c) => a -> [c]
you say
class (ClassA a, ClassB b) => ClassC a b c where
from :: c -> [b]
to :: c -> [a]
This means that for each triple of concrete types (a,b,c) that you wish to
be an instance of ClassC, you must provide an instance declaration, e.g.
instance ClassC Test H H where
from = ...whatever...
to = ...whatever...
Now you have the fixed type H in the instance declaration and not a
universally quantified type variable.
Cheers
Ben
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