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[Caml-list] mutability analysis too strict?
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Date: 2001-12-10 (08:13)
From: Francois Pottier <francois.pottier@i...>
Subject: Re: [Caml-list] mutability analysis too strict?

On Sun, Dec 09, 2001 at 05:43:41PM +0200, Ohad Rodeh wrote:
>       let h = Hashtbl.create 10;;
>       h : ('_a, '_b) Hashtbl.t
> The objects and keys in the table are infered to be mutable. However,
> in my case, they are immutable and I have to coerce them using Obj.magic
> from '_b to 'b.

You are slightly wrong here: the analysis infers the table itself (not the
keys or objects in it) to be mutable, which it indeed is. If the table was
given a polymorphic type, you would be able to store objects of a certain
type and to retrieve them at another type (by taking different instances of
'b), which would be unsound.

Furthermore, I'm surprised to hear that using Obj.magic helps; indeed, any
application of Obj.magic is itself deemed `dangerous' by O'Caml, leading
to the following behavior:

  # let h = Hashtbl.create 10;;  
  val h : ('_a, '_b) Hashtbl.t = <abstr>
  # let h = (Obj.magic h : ('a, 'b) Hashtbl.t);;
  val h : ('_a, '_b) Hashtbl.t = <abstr>

That is, Obj.magic doesn't help at all in this case.

Perhaps you could tell us what you are trying to achieve? Any polymorphic,
mutable structure is unsound and rightly rejected. A monomorphic, mutable
structure that contains polymorphic data is sound, but cannot be expressed
in ML's type system where universal quantification must be prenex.

François Pottier
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