I would like to make a two-parameter type class where some choices of the first type uniquely determine the second type, and other choices of the first type promise to work with any choice of the second type. This code seems a bit suspicious, but is accepted thanks to UndecidableInstances.
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-}
class C a b | a -> b where update :: b -> a -> a
instance C (Maybe a) a where update x m = fmap (const x) m
instance C Int b where update _ n = n
The application is a program transformation problem, where I'm using distinct types for Statements, Expressions, etc, but want a single environment that contains the bindings for variables of each different type for implementing rewriting rules. I'm trying to make a type class for binding things in the environment. Some of the terms, such as Statement, have a type parameter for semantic analysis information, so the Environment type has the type parameter as well. Then the instance a statement variable needs to require that the info type of the statement matches that of the environment, but the operation for binding an expression should be polymorphic in the info type of the environment. I usually have enough type information around that type checking succeeds without the fundep, so it's not currently a big problem. Brandon