Quite right. I've fixed the documentation (in CVS). Simon | -----Original Message----- | From: haskell-bounces@haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Wolfgang | Jeltsch | Sent: 17 October 2005 14:04 | To: Haskell ML | Subject: [Haskell] multiple occurence of the same type variable in instancehead | | Hello, | | the following is not Haskell 98: | | class C a | | instance C (a,a) | | Well, GHC allows this with the -fglasgow-exts flag. Surprisingly, I cannot | find a section in the GHC User's Guide which states that -fglasgow-exts can | be used to allow this kind of instance declarations. Well, in the section on | undecidable instances (7.4.4.3), one can read: | | Note that instance heads may contain repeated type variables. For example, | this is OK: | | instance Stateful (ST s) (MutVar s) where ... | | But one has to note that in this expample, the different occurences of s | aren't in the same type. Furthermore, multiple occurences of the same type | variable cannot result in undecidability (as far as I understand). The | above-mentioned statement talks about the things that are allowed *without* | -fallow-undecidable-instances, and premises that multiple occurences of a | type variable are okay. | | So am I missing something? | | Best wishes, | Wolfgang | _______________________________________________ | Haskell mailing list | Haskell@haskell.org | http://www.haskell.org/mailman/listinfo/haskell