Re: [Caml-list] Implementing include "file" statement in menhir

["Yann Régis-Gianas" <yrg@irif.fr>]

Hello Richard,

if I had to do that, I would use the incremental mode of Menhir:
indeed, this mode allows you to manage complex lexing without
interfering with the parser.

In that mode, Menhir produces a function that executes parsing
step-by-step and between each step, you can easily update the state of
the lexer. Besides, that technique solves the circular dependency
problem since the interaction between the lexer and the parser is to
be implemented in a third module, which only depends on the lexer and
the parser.

More precisely, I think that you could intercept the reduction of the
production related to your include directive and enrich your lexer
buffer with the content of the included file on-the-fly.

Cheers,

On Tue, Jan 21, 2020 at 7:48 AM Richard W.M. Jones <rich@annexia.org> wrote:
>
> [Resend, apologies if you get this twice, but I sent it
> earlier and that seems to have disappeared.]
>
> I'm writing a parser which needs to have a C-like include directive.
> There's an old thread on this describing a rather complicated way to
> do this for ocamllex:
> https://groups.google.com/forum/#!topic/fa.caml/_v_k4WTQV_Q
>
> I thought I'd have a go at writing an include statement in menhir, and
> I did come up with something which works but it's quite a large hack.
> What I did is documented below, but I wonder if someone can think of a
> simpler way to do this?  Also two related questions:
>
> How do you pass extra parameters to menhir's generated parser
> functions?
>
> Is there a nice way to export values into menhir's generated
> parser.mli file?
>
> ----
>
> The concept behind my include statement uses the following grammar:
>
>   %token INCLUDE
>   %token <string> STRING
>   %start file
>   %%
>   file: list(stmt) ;
>
>   stmt:
>       | INCLUDE STRING
>       {
>         let filename = $2 in
>         let fp = open_in filename in
>         let lexbuf = Lexing.from_channel fp in
>         lexbuf.lex_curr_p <- { lexbuf.lex_curr_p with pos_fname = filename };
>         (* Recursively call Parser.file: *)
>         file Lexer.read lexbuf;
>         close_in fp;
>       }
>       | ... other statements ...
>       ;
>
>
> Unfortunately as written the above code cannot work because it
> introduces a circular dependency between the Parser and the Lexer
> modules (normally the Lexer module depends on the Parser, and so the
> Parser cannot use any functions from the Lexer module).
>
> To break the cycle we have to add:
>
>   %{
>   let lexer_read = ref None
>   %}
>
> and replace Lexer.read with:
>
>   let reader =
>      match !lexer_read with None -> assert false | Some r -> r in
>   file reader lexbuf;
>
> Then to initialize lexer_read, we have to export it by doing this
> hack:
>
>   menhir parser.mly
>   echo 'val lexer_read : (Lexing.lexbuf -> token) option ref' >> parser.mli
>
> and we can set it from the main program.
>
> Rich.