[Haskell-cafe] Generic types

[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.