[mercury-users] Polymorphism / Type classes

[Michael Day]

> You have written the type of your 'with_useful' method as:
>
> 	all [U] pred with_useful(pred(U, U), F, F) <= useful(U)
>
> whereas what I think you are trying to say is something like
>
> 	pred with_useful(all [U] pred(U, U) <= useful(U), F, F)

Right, that's exactly what I wrote, in fact. Though of course it didn't
work...

> First, you can re-arrange your code so that the type that you call
> `do_stuff' with is bound at the point at which you call with_useful. Not
> always easy to do...

I managed to do it this time; still seems like this polymorphism would be
nice to have sometimes though.

> The other way is to use a trick with type classes. This trick takes advantage
> of the fact that the implementation of type classes effectively uses
> second order polymorphism to encode the type class dictionaries (although
> this is hidden in a layer of C code because our compiler back end can't
> handle second order polymorphism at the moment).

Yeah that's what I was doing at first, but it seemed like unnecessary
indirection. Oh well.

> Enough babble from me... you just managed to ask me a question about
> exactly the material that I'm currently writing in my thesis...

Nice to make an on-topic post for once :)

Michael

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