I'm trying to write a validating XML library. I'm struggling with implementnig the choice alternative. Once piece of the code is Where st is the state representing the subelemnts which may still be attached to the current element. (Basically some tree containing Seq, Or, one or more, 0 or more, 0 or 1) Each xml tag is represented by it's own type. el is the element which should be added. (1): match of types, I'd like to return a C(onsumed) (2): no match, I'd like to return a F(ailure) class Consume st el r | st el -> r -- result is on of C,R,F -- element instance Consume ANY el C instance Consume (Elem el) el C -- (1) instance Consume (Elem el') el (F ExpectedButGot el' el) --(2) Of course this results in: || Functional dependencies conflict between instance declarations: Is there a nice way to implement this type level equality function? Of course I could give an instance for each possible comparison instance EQ TAG_A TAG_A HTrue instance EQ TAG_A TAG_DIV HFalse instance EQ TAG_A TAG_HTML HFalse which would work find but would also be a mess if a xml dtd has many elements. (100 elements means 100 * 100 comparisons = 10.000 *shrug*) I fear this slowing down compilation time a way too much. So my current idea is to assign numbers to all tags and do some type level number comparison. This will work resulting in something like this: data Zero data Succ a class Eq a b c | a b -> c instances: Eq (Succ a) Zero HFalse Eq Zero Zero HFalse Eq Zero (Succ a) HFalse Eq (Eq a b c) => (Succ b) (Succ a) c I want to know wether you know about a more elegant solution. Sincerly Marc Weber