Re: seeking an OO or functional parser generator for Java

[Johan Ovlinger <address@hidden>]

Lauri Alanko wrote:

>It has been done. It is not pretty.
>
>http://citeseer.nj.nec.com/dijkstra01lazy.html
>http://www.cs.uu.nl/~atze/Programming/JPC/readme.html
>  
>
the last link:

> The basic idea of writing a parser in Java using the parsercombinator 
> package is to write a parser using the errorcorrecting 
> parsercombinators <http://www.cs.uu.nl/groups/ST/Software/UU_Parsing/> 
> written in Haskell, checking them for correctness and then handcompile 
> the resulting parser to Java.

I was hoping for perhaps a slightly more java-esq solution.  Something 
like (completely untested)

class Parser {
  Object parse(TokenIterator ti) { throw new ParseFailed(); }
}
class TokenParser extends Parser {
  String str;
  TokenParser(String s) { str = s; }
  Object parse(TokenIterator ti) {
    Token t = ti.next();
    if (!t.toString().equals(str))
      ti.unread();
      throw new ParseFailed();
    }
    return t;
  }
}
class OrParser extends Parser {
  Parser first,second;
  OrParser(Parser f, Parser s) { first = f; second =s; }
  Object parse(TokenIterator ti) {
    try { return first.parse(ti) }
    catch (ParseFailed e) { return second.parse(); }
  }
}

...
// To parse the grammar "foo" | "bar"
OrParser op =  new OrParser(new TokenParser("foo"), new TokenParser("bar"));
// now will only parse "bar"
op.first = new Parser();
...

Appart from casting from Object, it's not entirely horrible. A more 
advanced solution would build up lookahead sets and guards.