I'm working with data structures that contain class methods from the HList library (classes with functional dependencies). What I want to be able to do is operate on high-level parts of the structure while ignoring the type constraints of the lower-level components. For example, say I have two data types and a class method:

data Box x = Box x data Carton x = Carton x class Func x y | x -> y where func :: x -> y

I want to define a function 'isBoxed' that identifies whether a function is in a Box or in a Carton that I can use like `isBoxed (Carton func)`. Naively:

class IsBoxed f b | f -> b where isBoxed :: f -> b instance IsBoxed (Box x) HTrue where isBoxed _ = hTrue instance IsBoxed (Carton x) HFalse where isBoxed _ = hFalse

(Where hTrue is a constructorish thing for the empty datatype HTrue.) But `isBoxed (Box func)` fails in GHC with "Ambiguous type variables `x', `y' in the constraint: `Func x y' arising from use of `func'...". I realise that it's solvable if I have a finite number of instances for Func, but what I actually want to box is functions with recursive constraints like HList's hLookupByLabel.* I tried to borrow some ideas (like TypeCast and empty data types) from the examples that come along with HList (like keyword-arguments.hs) to no avail. Can anybody give me a hand? Thanks, Jared Warren <warren@cs.queensu.ca> * I could satisfy the type constraints with the infinite HList Record: {Label hZero -> undefined, Label (hSucc hZero) -> undefined, ...} ...but that obviously runs into an infinite type.