Re: polymorphic recursion

[Simon Helsen <helsen@informatik.uni-tuebingen.de>]

> > I don't know why Caml allows more general type constraints, but it might
> > be a good idea to follow SML on this matter (and I am interested to know
> > if there are good reasons for not doing this)
> 
> If we use it to get polymorphic recursion, there is a good reason to
> do this.

right, but there is no polymorphic recursion in Ocaml, is there?

> Another problem is the scope of type variables in type
> constraints. What's the meaning of
> 
> let f (x : 'a) (y : 'a) = y;;
> 
> We may need explicit Forall keywords to express type schemes in constraints.

Indeed, this is a problem. Standard ML solves this by defining some
explicit rules for free type variables (section 4.6 of the definition -
p18) But, admitted, it's ugly and difficult. It would probably be better
to forbid free variables in type constraints altogether.

	Simon

----------------------- Simon Helsen ------------------------ 
--       Wilhelm-Schickard-Institut fuer Informatik        --
--           Arbeitsbereich Programmierung (PU)            --
--            Universitaet Tuebingen, Germany              --
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