Re: [Caml-list] What's wrong with my parser ?

["Ryan Tarpine" <rtarpine@hotmail.com>]

>From: Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr>
>To: Nicolas FRANCOIS (AKA El Bofo) <nicolas.francois@free.fr>
>CC: caml-list@inria.fr
>Subject: Re: [Caml-list] What's wrong with my parser ?
>Date: Sat, 13 Jul 2002 13:27:34 +0200
>
>OK, this is my new polynom parser, it seems to work OK, thanks to your
>explanations.
>
>   let rec parse s =
><snip>
>     let parse_coeff = parser
>       | [< n = R.parse >] -> n
>       | [< >] -> R.one
>     in
>     let parse_monome = parser
>       | [< n = parse_coeff; e = parse_x >] ->
>	  monome n e
>       | [< e = parse_x >] ->
>	  monome R.one e
><snip>
>     in
>       match s with parser
>	| [< m = parse_monome >] ->
>	    match parse_suite s with
>	      | p -> m ++ p
>
><snip>
># print (P.parse (Stream.of_string ""));;
>[1: 1]- : unit = ()
>

Parsing the empty string calls parse_monome, which calls parse_coeff, which 
returns R.one.  My take at this is that you need 2 versions of parse_x: one 
that requires an X to be there, and one that can parse an X but doesn't need 
it.

If there is no leading coefficient (the second option to parse_monome), the 
X *must* be there.  If the coeffecient is there (the first option), then the 
X doesn't have to be.  The empty string, which has no coefficient, should 
fail parse_monome because it doesn't have an X either.  If passed the string 
"X" or "2", though, parse_monome should succeed.

Ryan Tarpine, rtarpine@hotmail.com
"To err is human, to compute divine.  Trust your computer but not its 
programmer."
  - Morris Kingston

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