[Haskell-cafe] Re: Generic types

[Cale Gibbard]

On 14/06/05, Adde <adde at trialcode.com> 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).

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