Hi All, I'm trying to write a simplified dimensional library where where quantities in meters, seconds and meters/second can all co-exist adjusting their respective units when multiplied and/or divided. Also, meter+meter is allowed, but meter+second should cause the type checker to complain. This is a bit like a *much* simplified version of the Units library. However, I am having trouble understanding why my instance declaration below appears to be under specified. Here's the code: ------------ module Dimensional where import qualified Prelude -- A Group allows you to add and subtract (but not multiply or divide) class Group a where (+) :: a -> a -> a (-) :: a -> a -> a x - y = x + negate y negate :: a -> a negate x = fromInteger 0 - x fromInteger :: Prelude.Integer -> a class Unit a where (*) :: (Unit a, Unit b, Unit c) => a -> b -> c (/) :: (Unit a, Unit b, Unit c) => a -> b -> c toDouble :: a -> Prelude.Double instance Unit Prelude.Double where (*) x y = (Prelude.*) x (toDouble y) -- <----- Error here (/) x y = (Prelude./) x (toDouble y) toDouble x = x ------------ GHC complains as follows: Could not deduce (c ~ Prelude.Double) from the context (Unit Prelude.Double, Unit b, Unit c) bound by the type signature for * :: (Unit Prelude.Double, Unit b, Unit c) => Prelude.Double -> b -> c at /code/haskell/dimensional.hs:25:5-43 `c' is a rigid type variable bound by the type signature for * :: (Unit Prelude.Double, Unit b, Unit c) => Prelude.Double -> b -> c at /code/haskell/dimensional.hs:25:5 In the first argument of `(Prelude.*)', namely `x' In the expression: (Prelude.*) x (toDouble y) In an equation for `*': * x y = (Prelude.*) x (toDouble y) Any pointers would be much appreciated! Thanks, Dimitri