I would not say that it's well-specified, no. What we do know is this: GHC may loop if you use -fallow-undecidable-instances -- but if it terminates, the program is well typed and should not "go wrong" at runtime. S | -----Original Message----- | From: haskell-prime-bounces@haskell.org [mailto:haskell-prime-bounces@haskell.org] On Behalf Of | Ashley Yakeley | Sent: 21 February 2006 20:13 | To: haskell-prime@haskell.org | Subject: Re: the MPTC Dilemma (please solve) | | Simon Peyton-Jones wrote: | | > Of course -fallow-undecidable-instances still lifts all restrictions, | > and then all bets are off. | | Is the behaviour of GHC with -fallow-undecidable-instances (and | -fcontext-stack) well-understood and specifiable? It is a very useful | option, as you can join (meet?) classes like this: | | class P a | class Q a | | class (P a,Q a) => PQ a | instance (P a,Q a) => PQ a | | -- | Ashley Yakeley | | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://haskell.org/mailman/listinfo/haskell-prime