The following code works. I must admit though that I don't understand the problem. What is the expected result? Note that bmap is actually a type transformer, and 'funny' introduces a constraint via a run-time witness. data A = A deriving Show data B = B deriving Show data Cons x xs = Cons x xs deriving Show data Nil = Nil deriving Show class XTERM u where xtval:: u -> Char instance XTERM A where xtval _ = 'A' instance XTERM B where xtval _ = 'B' data BTERM = forall u. (XTERM u) => BTERM u xtmake 'A' = BTERM A xtmake 'B' = BTERM B instance Show BTERM where show (BTERM u) = show $ xtval u class XSEQ u where xsval:: u -> String isnil:: u -> Bool car:: u -> BTERM cdr:: u -> BSEQ instance XSEQ Nil where xsval _ = [] isnil = const True car = undefined cdr = undefined instance (XTERM v, XSEQ w) => XSEQ (Cons v w) where xsval _ = (xtval (undefined::v)):(xsval (undefined::w)) isnil = const False car _ = BTERM (undefined::v) cdr _ = BSEQ (undefined::w) data BSEQ = forall u. (XSEQ u) => BSEQ u instance Show BSEQ where show (BSEQ u) = show $ xsval u bcons (BTERM u) (BSEQ t) = BSEQ $ Cons u t xsmake [] = BSEQ Nil xsmake (x:xs) = bcons (xtmake x) $ xsmake xs bcar (BSEQ u) = car u bcdr (BSEQ u) = cdr u bmap f (BSEQ u) | isnil u = BSEQ Nil bmap f seq = bcons (f $ bcar seq) $ bmap f (bcdr seq) class FunnyMap u v | u -> v where funnymap:: u -> v instance FunnyMap A B where funnymap _ = (undefined::B) funny (BTERM u) = case xtval u of 'A' -> BTERM $ funnymap (undefined::A) -- 'B' -> BTERM $ funnymap (undefined::B) *Main> bmap funny $ xsmake "AAA" "BBB"