Concerning (1) yes, this is a recent and substantial improvement in GHC (HEAD only).
Concerning (3), this is a plain bug. I've just fixed it in the HEAD. I very much doubt I'll fix it in 6.6, because type inference in the HEAD now works rather differently. Thanks for the example; I've added it to the test suite! Simon | -----Original Message----- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim | Apple | Sent: 09 January 2007 01:52 | To: haskell-cafe@haskell.org | Subject: Re: [Haskell-cafe] Monad Set via GADT | | On 1/3/07, Roberto Zunino <[EMAIL PROTECTED]> wrote: | > 1) Why the first version did not typececk? | > 2) Why the second one does? | > 3) If I replace (Teq a w) with (Teq w a), as in | > SM :: Ord w => Teq w a -> Set.Set w -> SetM a | > then union above does not typecheck! Why? I guess the type variable | > unification deriving from matching Teq is not symmetric as I expect it | > to be... | | These are very interesting questions that I forgot about until | reminded by Haskell Weekly News. Thanks, HWN! | | 1) Class constraints can't be used on pattern matching. They ARE | restrictive on construction, however. This is arguably bug in the | Haskell standard. It is fixed in GHC HEAD for datatypes declared in | the GADT way, so as not to break H98 code: | | http://article.gmane.org/gmane.comp.lang.haskell.cvs.all/29458/match=gadt+class+context | | 2) The second one works because Class constraints can be used when | pattern matching existentials. | | 3) I imagine this might have something to do with the coercions that | System FC uses. With one ordering, a coercion might occur that in | another one is unnecessary. This coercion might allow the use of Ord w | by using it before the coercion from S.Set a to S.Set w. | | #3 is just a guess. | | Jim | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
