[Redirecting to GHC users] The <<loop>> is a bug. Thank you! I'll Trac it. Simon | -----Original Message----- | From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] | On Behalf Of Sittampalam, Ganesh | Sent: Monday, January 19, 2009 2:05 PM | To: haskell@haskell.org | Subject: RE: [Haskell] type inference & instance extensions | | Doug McIlroy wrote: | > A fragment of an attempt to make pairs serve as complex numbers, | using | > ghc/hugs extensions: | > | > instance Num a => Num (a,a) where | > (x,y) * (u,v) = (x*u-y*v, x*v+y*u) | > | > Unfortunately, type inference isn't strong enough to cope with | > | > (1,1)*(1,1) | > | > Why shouldn't it be strengthened to do so? | | The problem is that type classes are an "open" system. Although it's | obvious that your instance is the only one in this code that can be | used to type-check (1,1), that doesn't preclude new code adding an | instance that could make it behave differently. | | I had hoped that the code below (GHC 6.10+) would work, but it just | sends GHC into a loop when you actually try to typecheck (1,1). I don't | know if that's a bug in GHC or a misunderstanding on my part of how the | typechecking should work. | | {-# LANGUAGE FlexibleInstances, TypeFamilies #-} | | instance (a~b, Num a) => Num (a, b) where | fromInteger k = (fromInteger k, fromInteger 0) | (x,y) * (u,v) = (x*u-y*v, x*v+y*u) | | PS: Sorry, I didn't mean to say "typecheck (1,1)". I meant to say | "evaluate (1,1)*(1,1)" - I had a suspicion that some kind of infinite | polymorphic recursion through the Num instance is happening and | accidentally turned that thought into a completely different statement. | | | Ganesh | | ======================================================================= | ======= | Please access the attached hyperlink for an important electronic | communications disclaimer: | | http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html | ======================================================================= | ======= | | _______________________________________________ | Haskell mailing list | Haskell@haskell.org | http://www.haskell.org/mailman/listinfo/haskell