mutable and polymorphism
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Date:  20100915 (19:10) 
From:  Goswin von Brederlow <goswinvb@w...> 
Subject:  Re: [Camllist] mutable and polymorphism 
Radu Grigore <radugrigore@gmail.com> writes: > On Wed, Sep 15, 2010 at 6:40 PM, Andreas Rossberg <rossberg@mpisws.org> wrote: >> Have a look at this variant of c.ml and its "potential" if it typechecked: >>> Â let f = let xs = ref [] in fun x > xs := x :: !xs in f 1; f 'a' > > Interesting. If I would be a type checker, here's what I'd do. > 1. "ref []" tells me that (xs : '_a), because I know refs can't be polymorphic. > 2. "x :: !xs" tells me that (xs : '_a list) and (x : '_a) > 3. "x > x :: !xs" tells me that (f : '_a > '_a list) > 4. "f 1" tells me that (f : int > int list) > 5. "f 'a'" tells me... OOPS > > Moreover, I'd have similar thoughts about a variant of a.ml (and a.ml > does typecheck!) > let xs = ref [] in let f = fun x > xs:=x::!xs in f 1; f 'a' In your original a.ml you didn't use the reference inside f so f was polymorphic. That is was then further below the scope of a reference didn't matter since the application of f was too. > On the other hand, here's what I'd do for the original c.ml, which > with a few names added is > let f = let x = ref () in fun y > () in f 1; f 'a' > 1. "ref ()" tells me that (x : unit ref) > 2. "y > ()" tells me that (f : 'a > unit) 2. "y > ()" tells me that (f : '_a > unit) Because the compiler is stupid and things with a ref can't polymorphic. It isn't smart enough to see that the type of the ref is independent of the type of the function and there could remain polymorphic. > 3. "f 1" returns () > 4. "f 'a'" return () > > In other words, I would have thought that in your example the problem > is that you tried to use a polymorphic reference. (And this problem > even appears in the FAQ.) The solution is called lifting I think? Anyway, you need to move the polymorphic argument outside the scope of the reference: let f y = let x = ref () in (fun y > ()) y in f 1; f 'a' > regards, > radu MfG Goswin