On 14/06/05, Adde <[EMAIL PROTECTED]> wrote: > While we're at it, I tried adding a class Length in between Unit and Meter, > like > this: > > class Unit u where > shortName :: u -> String > > class (Unit u) => Length u > > instance Length Meter where > shortName u = "m" > > >From the information I can find on the web subclassing should include all > of the methods in the superclasses but I get this error: > > `shortName' is not a (visible) method of class `Length' >
It's saying that shortName isn't a method of the class Length, which is right, it's a method of the class Unit, so you should put it in the Unit instance rather than in the Length instance. The declarations in an instance of a class C may contain bindings only for the class methods of C. The class methods of C are exactly those whose type signatures are given in the class declaration of C (which may not overlap with other class declarations or top level bindings). > _______________________________________________ > 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
