I've had to use some fold functions recently and I've come to the conclusion that Haskell should automatically generate fold functions for data definitions. To clarify what I mean, for the data definition: data MyData = This Int Char | That String Int Int there should be a matching function definition: foldMyData f g (This a b) = f a b foldMyData f g (That a b c) = g a b c This definition is as natural as the constructors "This" and "That". Consider the tuple definition and its fold: data (,) a b = (a, b) foldTuple f (x, y) = f x y and the definition of Either and its fold: data Either a b = Left a | Right b foldEither f g (Left x) = f a foldEither f g (Right x) = g x In logic these constructors define the introduction rules ((,), Left and Right) and the folds define the elimination rules (exactly for Either, indirectly for tuples) for conjunction and disjunction. Further, while pattern-matching is very convenient for accessing the constituents of the data type, patterns are not first-class objects in Haskell. Fold functions, on the othe hand, are. They can be passed around and used in higher-order functions to extract constituents in a points-free manner. I know the short-term answer is "use TH" to derive folds if I want them, but I think such an important concept should probably be part of the language. Tim Newsham http://www.thenewsh.com/~newsham/