[Haskell-cafe] nice simple problem for someone struggling....

[David Menendez]

On Dec 21, 2007 11:50 AM, Nicholls, Mark <Nicholls.Mark at mtvne.com> wrote:

> Now I have....
>
> module Main where
>
> data SquareType numberType = Num numberType => SquareConstructor
> numberType


This is a valid declaration, but I don't think it does what you want it to.
The constraint on numberType applies only to the data constructor.

That is, given an unknown value of type SquareType a for some a, we do not
have enough information to infer Num a.

For your code, you want something like:

instance (Num a) => ShapeInterface (SquareType a) where
    area (SquareConstructor side) = side * side

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>
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