The issue is that this definition can't be generalized to lists of  
arbitrary size.
The code
let list_cycle l =
  let rec loop = l @ loop in
  loop
will not be accepted.

I don't know the exact rule, but I guess that on the right-hand side  
of a
let rec defining a ground value named foo you can only write a term  
which
evaluates to a finite ground term on the currently defined variables +  
foo.
That is to say something that evaluates to a finite tree of  
constructors with
constants or defined variables as leaves.
Maybe someone more knowledgeable could state the exact rule.

- marc

P.S. : the code using Obj is far from a solution as it modifies the  
existing structure
of the list to add cycling and thus, breaks persistency.

Le 23 oct. 2009 à 17:35, Damien Guichard a écrit :

>
> let list_cycle2 a b =
>   let rec loop = a::b::loop
>   in loop
>
>
> - damien
>
> En réponse au message
> de : Lukasz Stafiniak
> du : 2009-10-23 01:10:37
> À : caml-list
> CC :
> Sujet : Re: [Caml-list] forbidden construct as right hand side of  
> "let rec"
>
> While we are at it, what is the best way to convert a "straight" list
> into a cyclic list?
>
> i.e. convert
>
> let l = a::b::[]
>
> into
>
> let rec l = a::b::l
>
> (for arbitrary length lists). (The answer I recall from the archives
> was using Obj.magic to mutate the [] in the original list).
>
> On Fri, Oct 23, 2009 at 12:34 AM, Stéphane Glondu  <steph@glondu.net  
> > wrote:
> > Mathias Kende a écrit :
> > >       let rec g = f g
> >
> > What about:
> >
> >  let rec g x = f g x
>
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