> So doesn't my proposal give you precisely what you want, and no more?
Because my interest is as much in documentation as in constraints on
how the program executes (I admit your method achieves the latter).
If I write, say,
data Eq a => Assoc a b
then I'd like the signatures to ensure that the first argument of Assoc
is a member of class Eq wherever it appears. Your technique guarantees
this is Assoc is used in a constructor, but not if used in a destructor.
> But when you take a constructor *apart*, the invariant must hold
> by construction: you couldn't have built the thing you are taking
> apart unless invariant held. So enforcing the invariant again is
> redundant; and in addition it pollutes the type of selectors.
Your `redundant pollution' is exactly the effect I want to achieve!
Clearly, tastes differ. Hope this clarifies, -- P
