[m-dev.] question about polymorphic higher order terms

[Julien Fischer]

On Sun, 24 May 2009, Ian MacLarty wrote:

> Hi,
>
> Suppose I have a higher order function that converts a value of type T
> to a string.  It's type is func(T) = string.  Now suppose I want to
> pass this function to a predicate and apply it to values of different
> types.  Something like:
>
> :- pred p((func(T) = string)::in, io::di, io::uo) is det.
>
> p(F, !IO) :-
>    io.write_string(F(1), !IO),
>    io.write_string(F("one"), !IO).
>
> won't work, because the caller binds T, so F is monomorphic.
>
> So my question: Is there a way to pass a polymorphic higher order term
> to a predicate so that it is still polymorphic inside that predicate?
> I realise that one could define a typeclass with the function as a
> method, but that would require the function to have an extra dummy
> argument.  Does anyone know of a more elegant solution, or could
> someone point me to any previous discussions or literature on the
> topic?

What you want here is called rank-2 polymorphism, which Mercury doesn't
(currently) support.  Search for literature on rank-2, rank-N, or
higher-ranked polymoprhism or types -- there's a vast amount of it.

Haskell (ghc at least) supports this.

Julien.
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