Re: [Haskell-beginners] Instance declaration needs more info?

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 mailto: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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