Hello,
On 01-07-2009, Andreas Rossberg <rossb...@mpi-sws.org> wrote:
> Mike Lin wrote:
>> OK, now I'm curious :) how does your lexer match balanced parentheses,
>> or in this case comments?
>>
>
> Easily, with a bit of side effects (I think that's roughly how all ML
> compilers do it):
>
> ------------------------------------------------
> let error l s = (* ... *)
> let commentDepth = ref 0
> let start = ref 0
> let loc length = let pos = !start in (pos, pos+length)
>
> rule lex =
> parse eof { EOF }
> (* | ... *)
> | "{-" { start := pos lexbuf;
> lexNestComment lexbuf }
>
> and lexNestComment =
> parse eof { error (loc 2) "unterminated comment" }
> | "(*" { incr commentDepth;
> lexNestComment lexbuf }
> | "*)" { decr commentDepth;
> if !commentDepth > 0
> then lexNestComment lexbuf
> else lex lexbuf }
> | _ { lexNestComment lexbuf }
> ------------------------------------------------
>
> If you also want to treat strings in comments specially (like OCaml),
> then you need to do a bit more work, but it's basically the same idea.
>
May I recommend you to write this in a more simple way:
-------------------------------------------------------------------------
rule lex =
parse eof { () }
| "(*" { start := pos lexbuf; lexNestComment lexbuf; lex lexbuf }
and lexNestComment =
parse eof { error (loc 2) "unterminated comment" }
| "(*" { lexNestComment lexbuf }
| "*)" { () }
| _ { lexNestComment lexbuf }
-------------------------------------------------------------------------
I think it works the same way, except that it uses less global
variables.
Regards,
Sylvain Le Gall
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