The trouble is that your specification says that Unit c is the return type (the caller can choose any Unit instance), but this implementation can only evaluate to a Prelude.Double. One way to solve this is to add a fromDouble :: Double -> Unit a and wrap the expression with that in order to satisfy Unit c.

On Wednesday, May 21, 2014, Dimitri DeFigueiredo <defigueiredo@ucdavis.edu> wrote:

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


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