Hello, This post is (hopefully) literate Haskell. I recently noticed that there are two ways to specify instances in a common situation. Suppose I have something like this:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, NoMonomorphismRestriction, OverlappingInstances #-}
data A = A data B = B data C = C
newtype Repr a = Repr { unRepr :: State MyState a }
class SomeClass a b where
If I want to make instances of SomeClass for Repr, A and Repr, B, I have two choices:
instance SomeClass Repr A where instance SomeClass Repr B where
or I can introduce a new class and make an instance with a context,
class RClass c where
instance RClass A where instance RClass B where -- no C instance for RClass
instance RClass x => SomeClass Repr x
is there any reason to prefer one form over the other? Of course the first requires more instance declarations, but they're auto-generated so that doesn't bother me. Thanks, John L.