[Haskell-cafe] Generic types

[Cale Gibbard]

Perhaps you want
data UnitValue u n = 
     (Unit u, Num n) => UnitValue {uUnit :: u, uValue :: n}

On 13/06/05, Adde <adde at trialcode.com> wrote:
> I'm writing a small unit library to excercise my barely existing
> Haskell skills.
> However I can't figure out how to make Haskell accept any type
> of a specific class.
> 
> This is what I've got so far:
> <code>
> class Unit u where
>     shortName :: u -> String
> 
> data Meter = Meter
> 
> instance Unit Meter where
>     shortName u = "m"
> 
> data UnitValue = UnitValue  {uUnit :: Unit u => u, uValue :: Num n => n}
> </code>
> 
> Meter is supposed to be a type in the Unit class.
> UnitValue is supposed to represent unit values regardless
> of the Unit type.
> 
> Meter appears to be a subclass of Unit, so far so good:
> 
> *Units> shortName Meter
> "m"
> 
> But I can't get UnitValue to accept Meter as a Unit:
> 
> *Units> UnitValue Meter 10
> 
> <interactive>:1:10:
>     Couldn't match the rigid variable `u' against `Meter'
>       `u' is bound by the polymorphic type `forall u. (Unit u) => u'
>                         at <interactive>:1:0-17
>       Expected type: u
>       Inferred type: Meter
>     In the first argument of `UnitValue', namely `Meter'
>     In the definition of `it': it = UnitValue Meter 10
> 
> I strongly suspect that I've got the declaration of UnitValue wrong
> but I've spent two hours searching the web and trying to get it to
> work without any success.
> 
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