22 Jan
2010
22 Jan
'10
6:37 a.m.
On Fri, Jan 22, 2010 at 12:24:37PM +0100, Emil Axelsson wrote:
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 [...] 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?
I'm afraid you voided the warranty when you used UndecidableInstances. You really do have a circularity between A and B here, so it's not surprising that you get a loop. By changing the context, you demanded more instances, undecidable ones in fact.