Re: [Caml-list] camlp4 scope issue

["Nicolas Pouillard" <nicolas.pouillard@inria.fr>]

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,

-- 
Nicolas Pouillard