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David Menendez | 21 Dec 18:04
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Re: nice simple problem for someone struggling....

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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