Perhaps you want
data UnitValue u n =
(Unit u, Num n) => UnitValue {uUnit :: u, uValue :: n}
On 13/06/05, Adde
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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