[Haskell-beginners] Instance declaration needs more info?

[Dimitri DeFigueiredo]

Got it. I was mistaken in what the final "Unit c =>" meant. Thanks Bob!

Em 21/05/14 03:28, Bob Ippolito escreveu:
> 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 <mailto: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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