Ugh!  Thank you for clarifying this, as it was driving me insane in 
trying to figure out what the revised construct was *supposed to do* 
rather than what it *did*.

Not sure if this is a proper place to ask, but would it be possible to 
document this feature?

Frankly, in presence of the << value ...; >> construct, I don't see the 
benefit behind << let ... ; >> inside the << do { ... } >> clause other 
than causing confusion.  Though I realize that people might have a 
different view of this...

Regards,

Serge

-- 
Serge Aleynikov
Routing R&D, IDT Telecom
Tel: +1 (973) 438-3436
Fax: +1 (973) 438-1464


Nicolas Pouillard wrote:
> On 10/25/06, Serge Aleynikov <serge@hq.idt.net> wrote:
>> Perhaps I am misunderstanding the meaning of ";" in the revised syntax,
>> however, the 6.2 chapter
>> (http://caml.inria.fr/pub/docs/manual-camlp4/manual007.html) says that:
>>
>> do { e1; e2; e3; e4 }
>>
>> is an iterative sequence of expressions, whereas "let ... in" is
>> reserved for local constructs.
>>
>> If so, wouldn't the scope of y in
>>
>> let y = 1 in do { a; b; c };
>>
>> be different from:
>>
>> let y = 1 in a; b; c;
>>
>> Or else how to we indicate in the *revised syntax* the boundary of the
>> "let ... in" scope?
> 
> It's not a bug it's a feature :)
> 
> But a not documented one.
> 
> Inside a << do { ... } >> you can use << let var = expr1; expr2 >>
> like << let var = expr1 in expr2 >>.
> 
> The main goal is to facilitate imperative coding inside a << do {} >>:
> 
> do {
>  let x = 42;
>  do_that_on x;
>  let y = x + 2;
>  play_with y;
> }
> 
> That's nice but undocumented :(
> 
> Without such a syntax the regular one will make you nest do { ... } 
> notations.
> 
> do {
>  foo 1;
>  let x = 43 in do {
>     bar x;
>  };
>  (* x should be out of the scope *)
> }
> 
> Alas << let ... in >> and << let ... ; >> have the same semantics
> inside a << do { ... } >> what I regret because << let ... in >> is
> not local anymore.
> 
> In plain OCaml it's different since << ; >> is a binary operator so
> you must see << let a = () in a; a >> like << let a = () in (a; a) >>.
> 
> Hope this helps...
> 
> Best regards,
>