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Trouble with doubly-linked, circular list using lazy evaluation
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Date: 2006-11-17 (15:01)
From: Damien Doligez <damien.doligez@i...>
Subject: Re: [Caml-list] Trouble with doubly-linked, circular list using lazy evaluation

On 2006-10-25, at 03:46, Denis Bueno wrote:

> I get a "Bus error" when running a unit test on a doubly-linked
> circular list (of length 2). A self-contained test case is included
> [1]. I have 8 fields in a record which includes two left & right
> pointers (not ref types).
> I am running a PowerMac G5 2.5GHz (uname -a: Darwin ford.local 8.8.0
> Darwin Kernel Version 8.8.0: Fri Sep  8 17:18:57 PDT 2006;
> root:xnu-792.12.6.obj~1/RELEASE_PPC Power Macintosh powerpc) and
> ocaml 3.09.3.
> Interestingly, if I remove a field (such as the mark field), the test
> case succeeds.

That was an interesting bug.  Thanks for reporting it.

It has nothing to do with lazy values: it's an interference between
"let rec" of values and an optimization in the implementation of
the { ... with ... } construct.

To work around the bug, you can remove all instances of

   let rec ... = { ... with ... }

(i.e. the "with" construct as the toplevel expression of a letrec-bound
variable) by expanding the "with" construct by hand.

I am fixing it in the 3.09 branch (3.09.4+dev1).

-- Damien