Re: polymorphic recursion

Date: Tue Sep 22 1998 - 17:50:23 MET DST

Date: Tue, 22 Sep 1998 17:50:23 +0200
From: "Pierre CREGUT - FT.BD/CNET/DTL/MSV" <>
To: Pierre Weis <>
Subject: Re: polymorphic recursion
In-Reply-To: <>; from Pierre Weis on Tue, Sep 22, 1998 at 05:06:41PM +0200

Quoting Pierre Weis (
> > This might be the case for OCaml, but note that SML97 disallows more
> > general type-constraints than the type apparent in the expression without
> > the constraint (cf. rule (9) and comment in the 97 Definition - p22)
> That's a good point. It's simple to state and understand.
This way of handling type constraints is inherited from Hope.

> 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.

This has already been solved in the SML standard and even if it is not
necessarily easy to understand when formalized, this is quite intuitive :

  First un occurrence of 'a in a value declaration [val valbind] is said to
  be unguarded if the occurrence is not part of a smaller value declaration
  within [valbind]. In this case we say that 'a occurs unguarded in the value

  Then we say that 'a is scoped at a particular occurrence O of [val valbind]
  in a program if (1) 'a occurs unguarded in this value declaration, and (2)
   'a does not occur unguarded in any larger value declaration containing
  the occurrence O.

                        Old Definition of Standard ML p 20

According to this definition, the 'a's denote the same type variable in
your example.

let g (x : 'a) = let f (x:'a) = x in x
let h (x : 'a) = x

is equivalent to

let g (x : 'a) = let f (x:'a) = x in x
let h (x : 'b) = x

but not to

let g (x : 'a) = let f (x:'c) = x in x
let h (x : 'b) = x

The only risk of this solution is that you overconstrain an expression
because one of your type variable got caught in the scope of another unrelated
variable and then you get stuck with your compiler complaining about
a constraint it cannot fulfill. This is a safe risk.

Pierre Cregut - - +33 2 96 05 16 28
FT.CNET - DTL/MSV - 2 avenue Pierre Marzin - 22307 Lannion Cedex - France

This archive was generated by hypermail 2b29 : Sun Jan 02 2000 - 11:58:16 MET