Ambiguous defaults

[Thomas L. Bevan]

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I've never been very clear on this.
As I understand it, Andrew's code specifies a Compound class where the type 
the compound determines the type (and consequently class) of the value of 
"decompose c".
In Ferenc's example, the Compound class specifies that the class of the value 
of "decompose c" will be Component.
So, Ferenc's example is ambiguous in so far as a Compound type does not 
determine the Component type, but it does determine the Class and isn't that 
all we need to know to be able to call the 'property' function?

Tom

On Fri, 3 Jan 2003 10:18 am, Andrew J Bromage wrote:
> G'day all.
>
> On Thu, Jan 02, 2003 at 05:49:41PM +0100, Ferenc Wagner wrote:
> > What's the way to express the following: a compound object
> > is generally made up of two components with identical type.
>
> This should work:
>
> \begin{code}
> module Test where
>
> class Component b where
>     property :: b -> Int
>
> class (Component b) => Compound a b | a -> b where
>     decompose :: a -> (b,b)
>     additive :: a -> Int
>     additive x = property l + property r
>         where (l,r) = decompose x
> \end{code}
>
> Cheers,
> Andrew Bromage
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