http://research.microsoft.com/%7Esimonpj/Papers/type-class-design-space will fix manual Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Scott J. | Sent: 25 May 2005 13:29 | To: haskell-cafe | Subject: Re: [Haskell-cafe] Question about classes from a Haskell newbie. | | Hello, | | the web page http://research.microsoft.com/~simonpj/multi.ps.gz mentioned | in | | http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions. html#multi-param-type- | classes | | is not reachable or does not exist anymore. | | Any hints how I can get my hands on this paper are welcome | | Jan | | | ----- Original Message ----- | From: "robert dockins" | To: | Cc: | Sent: Tuesday, May 24, 2005 10:07 PM | Subject: Re: [Haskell-cafe] Question about classes from a Haskell newbie. | | | > | >> However, I didn't understand the syntax of the class declaration. | >> | >> Lemmih wrote: | >> > import Complex | >> > | >> > class ConvertibleToComplex a b | a -> b where | >> > toComplex :: RealFloat b => a -> Complex b | >> | >> I've looked at several sources to try to understand this declaration. I | >> can't find any examples where a class declaration takes two type | >> variables or uses the pipe symbol. One of the sources I used was | >> _Haskell 98 Language and Libraries The Revised Report_. I'm not an | >> expert at BNF notation, but the definitions for topdecl, tycls, and tyvar | >> in the grammar seems to exclude Lemmih's declaration. | >> | >> If someone could point me to some sources that explain this notation, I'd | >> be very grateful. | > | > This is a multi-parameter typeclass (with functional dependencies), which | > is not a part of Haskell 98, so its no surprise that you didn't find it in | > the Report. The Muli-parameter part is pretty easy to understand: the | > type class simply introduces more than one type variable. Functional | > dependencies let you say which types should determine which other types. | > Here are some pages on these ideas: | > | > http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions. html#multi-param-type- | classes | > http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions. html#functional- | dependencies | > http://haskell.org/hawiki/FunDeps | > | > In this particular instance, what the class definition means is that | > 'ConvertaibleToComplex' is a class which relates two types 'a' and 'b' AND | > the type 'a' uniquely determines the type 'b'. | > | > | > Hope that helps, | > Robert Dockins | > | > _______________________________________________ | > 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