[Haskell-beginners] Instance declaration needs more info?

[Bob Ippolito]

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 at 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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