All of the proposed solutions seem to rely on pattern matching in the constructor, which isn't always feasible. Here's a slightly better example: data Pair a = (Num a, Eq a) => Pair {x::a,y::a} equal :: Pair a -> Bool equal pair = (foo pair) == (bar pair) foo pair = (x pair) * (y pair) bar pair = (y pair) + (x pair) Now imagine that foo and bar are part of a library which I don't control, or that for some other reason I can't just change foo and bar to take only the components of Pair (e.g. it's a complex type with numerous components and subcomponents). -- View this message in context: http://haskell.1045720.n5.nabble.com/How-to-fix-DatatypeContexts-tp5733103p5... Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.