Re: [Caml-list] Strange physical equality behavior

[William Lovas <wlovas@stwing.upenn.edu>]

On Tue, Nov 11, 2003 at 08:46:56AM -0800, David Brown wrote:
> On Tue, Nov 11, 2003 at 01:48:22AM -0500, Oleg Trott wrote:
> > On Sunday 09 November 2003 08:33 pm, Jacques Garrigue wrote:
> > >  On mutable structures, [e1 == e2] is true if and only if
> > >    physical modification of [e1] also affects [e2].
> > 
> > By the way, either "mutable structures" or "physical modification" need
> > to be clarified, because if (int ref list) is "mutable" then the above
> > is wrong:
> 
> If you take structure to mean a single data type, rather than a more
> complicated data structure, then it is true.

Well, what do you mean by "a single data type", then?  Surely a record is a
single data type, but ...

    type r = { mutable a: int; mutable r: r }

    let rec r1 = { a = 5; r = r2 }
        and r2 = { a = 7; r = r1 }

Surely you wouldn't argue that this is an immutable data structure, either
-- it contains nothing *but* mutable fields!  And yet,

    r1.a <- 6

also "affects" r2, but r1 != r2.  (Admittedly, though, the ambiguity may
lie in the usage of the word "affects".)

*shrug* Maybe it's a bit contrived, but i would err on the side of caution
and say that the documentation should be made clearer.

William

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