What I always do is to write it like this:
equal pair@Pair{} = foo pair == bar pair
The {} syntax ensures that it doesn't matter how complex the Pair constructor
is.
Sjoerd
On Jul 18, 2013, at 1:52 PM, harry <volderm...@hotmail.com> wrote:
> All of the proposed solutions seem to rely on pattern matching in the
> constructor, which isn't always feasible. Here's a slightly better example:
>
> data Pair a = (Num a, Eq a) => Pair {x::a,y::a}
>
> equal :: Pair a -> Bool
> equal pair = (foo pair) == (bar pair)
>
> foo pair = (x pair) * (y pair)
>
> bar pair = (y pair) + (x pair)
>
> Now imagine that foo and bar are part of a library which I don't control, or
> that for some other reason I can't just change foo and bar to take only the
> components of Pair (e.g. it's a complex type with numerous components and
> subcomponents).
>
>
>
> --
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>
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