Re: [Caml-list] [Camlp4][trivial ?] Anonymous bind (for monads)

Nicolas Pouillard
2007-08-09T18:30:17Z

Excerpts from Gabriel Kerneis's message of Tue Jul 31 18:55:05 +0200 2007:
> Hello,
>
> I've got a little problem with camlp4. The following works perfectly
> well :
> #####################################################################
> ~/ocaml/camlp4% cat pa_bind.ml
> module Id = struct
> let name = "pa_bind"
> let version = "$Id:$"
> end
>
> module Example (Syntax : Camlp4.Sig.Camlp4Syntax) = struct
> include Syntax
>
> EXTEND Gram
> expr: LEVEL "top"
> [[ t=expr; "foo"; f=expr ->
> <:expr<
> Lwt.bind $t$ (fun () -> $f$)
> >>]];
> END
> end
>
> module M = Camlp4.Register.OCamlSyntaxExtension(Id)(Example)
>
> ~/ocaml/camlp4% ocamlc -c -pp camlp4of -I +camlp4 pa_bind.ml
>
> ~/ocaml/camlp4% cat test_bind.ml
> let r = (Lwt_unix.sleep 5) foo (print_endline "42")
>
> ~/ocaml/camlp4% camlp4of -parser pa_bind.cmo test_bind.ml
> let r = Lwt.bind (Lwt_unix.sleep 5) (fun () -> print_endline "42")
>
> #####################################################################
>
> But if I change "foo" to ">>" (which is the usual way to write an
> anonymous bind for monads), it doesn't work anymore. I first thought the
> reason was that ">>" is also the symbol for closing a quotation, but
> trying "++" instead proved useless as well.
>
> I'm certainly not a camlp4 guru so... what am I missing?

When you use "foo", you create a new keyword. You can therefor choose its
priority. However the parsing of ">>" is predefined since OCaml supports
user-defined operators. In your example using t=expr is too greedy since it
contains ">>" in the infixop0 rule.

However if you use SELF instead of expr it works fine. Indeed SELF handles
the left recursion and don't blindly parse an expr but remember that ">>" is
waited.

...
EXTEND Gram
expr: LEVEL "top"
[[ t = SELF; ">>"; f = SELF ->
<:expr<
Lwt.bind $t$ (fun () -> $f$)
>>]];
END
...

Best regards,

--
Nicolas Pouillard aka Ertai