Associated Parameter Restriction using Functional Dependency

Question

The function f below, for a given type \'a\', takes a parameter of type \'c\'. For different types \'a\', \'c\' is restricted in different ways. Concretely, when \'a\' is any In

Answer 1

A possibly better way, is to use constraint kinds and type families (GHC extensions, requires GHC 7.4, I think). This allows you to specify the constraint as part of the class instance.

{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances, UndecidableInstances #-}

import GHC.Exts (Constraint)

class Foo a where
   type ParamConstraint a b :: Constraint
   f :: ParamConstraint a b => b -> a

instance Integral i => Foo i where
   type ParamConstraint i b = Real b
   f = fIntegral

_{EDIT: Upon further experimentation, there are some subtleties that mean that this doesn't work as expected, specifically, type ParamConstraint i b = Real b is too general. I don't know a solution (or if one exists) right now.}