[Haskell-cafe] Re: Generic types

[Cale Gibbard]

On 14/06/05, Adde <adde at trialcode.com> wrote:
> Cale Gibbard <cgibbard <at> gmail.com> writes:
> 
> >
> > Perhaps you want
> > data UnitValue u n =
> >      (Unit u, Num n) => UnitValue {uUnit :: u, uValue :: n}
> >
> 
> I tried adding UnitValue as an instance of class Show, but I can't figure out
> how to tell the compiler that u is a Unit and m is a Num (shouldn't it be able
> to figure that out from the above declaration?):
> 
> instance Show (UnitValue u m) where
>    show (UnitValue u m) = show m ++ (shortName u)
> 
You indicate the class restrictions just before the name of the class
and type, with the usual implication arrow:

instance (Num m, Unit u) => Show (UnitValue u m) where
  show (UnitValue u m) = show m ++ (shortName u)

> This is the error I'm getting:
> 
>     Could not deduce (Show m) from the context (Show (UnitValue u m))
>       arising from use of `show' at /Users/adde/Projects/Haskell/units.hs:18:26-29
>     Probable fix: add (Show m) to the class or instance method `show'
>     In the first argument of `(++)', namely `show m'
>     In the definition of `show': show (UnitValue u m) = (show m) ++ (shortName u)
>     In the definition for method `show'
> 
>     Could not deduce (Unit u) from the context (Show (UnitValue u m))
>       arising from use of `shortName' at
> /Users/adde/Projects/Haskell/units.hs:18:37-45
>     Probable fix: add (Unit u) to the class or instance method `show'
>     In the second argument of `(++)', namely `(shortName u)'
>     In the definition of `show': show (UnitValue u m) = (show m) ++ (shortName u)
>     In the definition for method `show'
> 
When you get errors like this, there are actually two probable fixes:
the one mentioned, to add the class restraints to the method (if
you're allowed to, in this case you can't/don't want to), or to add
the class restraints to the instance, which is what you want to do.

 - Cale