GADTs and fundeps don't seem to interact in the way that I (perhaps naively) expect. I expected that for each case, the type variables would be instantiated according to the type of the constructors, and then the fundep would be used to figure out the result type. This does not seem to work, or am I doing something stupid? /Bjorn {-# OPTIONS_GHC -fglasgow-exts #-} data Succ n data Zero class Plus x y z | x y -> z instance Plus Zero x x instance Plus x y z => Plus (Succ x) y (Succ z) infixr 5 ::: data List :: * -> * -> * where Nil :: List a Zero (:::) :: a -> List a n -> List a (Succ n) {- GHC 6.4 says: Couldn't match the rigid variable `y' against `Succ z' `y' is bound by the type signature for `append' Expected type: List a y Inferred type: List a (Succ z) In the expression: x ::: (append xs ys) In the definition of `append': append (x ::: xs) ys = x ::: (append xs ys) -} append :: Plus x y z => List a x -> List a y -> List a z append Nil ys = ys append (x ::: xs) ys = x ::: append xs ys