mutable and polymorphism
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Date:   (:) 
From:  Radu Grigore <radugrigore@g...> 
Subject:  Re: [Camllist] mutable and polymorphism 
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' 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) 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.) regards, radu