Re: polymorphic recursion

Pierre Weis
 Simon Helsen
 Pierre CREGUT  FT.BD/CNET/DTL/MSV
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Date:  19980922 (16:41) 
From:  Pierre CREGUT  FT.BD/CNET/DTL/MSV <pierre.cregut@c...> 
Subject:  Re: polymorphic recursion 
Quoting Pierre Weis (Pierre.Weis@inria.fr): > > This might be the case for OCaml, but note that SML97 disallows more > > general typeconstraints 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 declaration. 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  pierre.cregut@cnet.francetelecom.fr  +33 2 96 05 16 28 FT.CNET  DTL/MSV  2 avenue Pierre Marzin  22307 Lannion Cedex  France