From: Caleb Clausen <caleb@...>
Date: 2009-08-30T05:55:19+09:00
Subject: [ruby-core:25186] Re: RegOOps: An Object-Oriented Approach to Pattern Matching

Run Paint Run Run wrote:
> Regexps in Ruby can feel like a jagged edge to the otherwise smooth
> curves. For all the syntactical smarts we employ so as to avoid
> Perl-like punctuation, regular expressions take us two steps back,
[snip]
> Imagine a String as an Array of Char objects. The Char class would
> define predicate methods corresponding to Unicode and regexp
> properties. For instance Char#lowercase? or Char#digit?. The
> implementation is trivial because Regexps are used internally.
> 
> Matching the String against a pattern is now a case of calling the
> appropriate predicate methods on the underlying characters. For
> example, match(/\w+/) could be expressed as match(:word). This would
> succeed if at least one character in the string returns true for
> Char#word?. Similarly, match(:hex, :blank, :digit) would:

This all reminds me intensely of Eric Mahurin's Grammar library/DSL. 
Much of what you're talking about is in there. Any ruby-based parser 
system (eg treetop or racc) is likely to have at least some similarity 
to this too.

http://rubyforge.org/projects/grammar/


> I've so far talked in terms of matching against Strings, but the
> generality of this approach suggests that it could be associated with
> any enumerator. (I've overloaded String#chars to return Char objects
> for String). This could allow functional-style matching against data
> structures.

Ah, now you're really reminding me of my own project, Reg (Ruby Extended 
  Grammar). It has pattern-matching constructs for arrays, hashes, 
objects. With backtracking in arrays, even. A whole DSL for matching 
patterns in ruby object trees. The github project is here:

http://github.com/coatl/reg

but, I see you're already following it. :) (There's also a gem, but its 
really old.)

I've gotten some good use out of Reg, for instance, I wrote a ruby 
parser (RedParse) using it.

I find this subject deeply fascinating. I think it would be cool if ruby 
had more pattern matching built in, so that for instance, you could 
specify a pattern in a block or method parameter list or the left side 
of an assignment. Using unary star and parens, there's already some 
capability in this direction, but it's fairly limited as a pattern 
matching language compared to what Reg can do.