It's a feature! You have * B is a superclass of A * Eq is a superclass of B So every A dictionary has a B dictionary inside it, and every B dictionary has an Eq dictionary inside it. Now, your instance declaration instance (A a, Eq a) => B a says "if you give me an A dictionary and an Eq dictionary, I'll make you a B dictionary". Now, 'test' needs a (B Int) dictionary. To get one, we need an (A Int) dictionary and an (Eq Int) dictionary. But when solving these sub-problems, GHC assumes that you have in hand a solution to the original problem, this case (B Int) Why? Read the SYB3 paper. OK so now you see the problem: we can solve the (A Int) and (Eq Int) sub-problems by selection from the (B Int) dictionary. Still, I confess that I have not fully grokked the relationship between the SYB3-style recursion stuff and the question of superclasses. So I will think about your example some more, thank you. Meanwhile, it's clear that you are on thin ice. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Emil Axelsson | Sent: 22 January 2010 11:25 | To: Haskell Cafe | Subject: [Haskell-cafe] Non-termination due to context | | Hello all! | | Consider the following program: | | > {-# LANGUAGE FlexibleInstances, OverlappingInstances, UndecidableInstances #-} | > | > class B a => A a | > | > instance A Int | > | > class Eq a => B a | > | > instance (A a, Eq a) => B a | > | > eq :: B a => a -> a -> Bool | > eq = (==) | > | > test = 1 `eq` (2::Int) | | (This is a condensed version of a much larger program that I've been | debugging.) | | It compiles just fine, but `test` doesn't terminate (GHCi 6.10.4). If I | change the context `B a` to `Eq a` for the function `eq`, it terminates. | | Although I don't know all the details of the class system, it seems | unintuitive that I can make a program non-terminating just by changing | the context of a function (regardless of UndecidableInstances etc.). | | Is this a bug or a feature? | | / Emil | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe