let rec x = lazy x

let rec x = lazy x

[Alessandro Baretta]

Strange as it may seem, it seems to work! With -rectypes enabled of course.

Obviously, forcing x does not produce anything sensible, but it does produce 
something. Am I justified in believing that the result is a long chain of 
lazy-blocks, the last one containing a Stack_overflow exception?

Alex


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Re: [Caml-list] let rec x = lazy x

[Mackenzie Straight]

On 1/11/06, Alessandro Baretta <a.baretta@barettadeit.com> wrote:
> Strange as it may seem, it seems to work! With -rectypes enabled of course.
>
> Obviously, forcing x does not produce anything sensible, but it does produce
> something. Am I justified in believing that the result is a long chain of
> lazy-blocks, the last one containing a Stack_overflow exception?

http://caml.inria.fr/pub/docs/manual-ocaml/libref/Lazy.html

You don't want to do this.

Re: [Caml-list] let rec x = lazy x

[Alessandro Baretta]

Mackenzie Straight wrote:
> On 1/11/06, Alessandro Baretta <a.baretta@barettadeit.com> wrote:
> 
>>Strange as it may seem, it seems to work! With -rectypes enabled of course.
> 
> http://caml.inria.fr/pub/docs/manual-ocaml/libref/Lazy.html
> 
> You don't want to do this.

I did it specifically because it is forbidden ;)

Alex

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Re: [Caml-list] let rec x = lazy x

[Mackenzie Straight]

Apologies for the duplicate, I forgot to send this to the list.

On 1/11/06, Alessandro Baretta <a.baretta@barettadeit.com> wrote:
> Mackenzie Straight wrote:
> > On 1/11/06, Alessandro Baretta <a.baretta@barettadeit.com> wrote:
> >
> >>Strange as it may seem, it seems to work! With -rectypes enabled of course.
> >
> > http://caml.inria.fr/pub/docs/manual-ocaml/libref/Lazy.html
> >
> > You don't want to do this.
>
> I did it specifically because it is forbidden ;)

Oh well in that case... a lazy value is a block with 1 field. Before
the computation is forced, it's tagged Lazy_tag and contains a pointer
to the closure to execute. Once forced, it becomes a forwarding
pointer (Forward_tag) to the forced value. Thus, you would get a
forwarding pointer which points to itself. The GC may then replace
references to the forwarding pointer with its value.

So, what you've got with let rec x = lazy x is a forwarding pointer
that points to itself.