Versions Compared


  • This line was added.
  • This line was removed.
  • Formatting was changed.
Comment: Migrated to Confluence 4.0

Jparsec is now ported to Ruby, with a way lighter syntax than Java.

Calculator demo

The following is the ruby version of calculator:

Code Block
require 'rparsec'
include RParsec
class Calculator
  include Parsers
  include Functors
  def parser
    ops =
      infixl(char(?+) >> Plus, 20).
      infixl(char(?-) >> Minus, 20).
      infixl(char(?*) >> Mul, 40).
      infixl(char(?/) >> Div, 40).
      prefix(char(?-) >> Neg, 60)
    expr = nil
    term = | char('(') >> lazy{expr} << char (')')
    delim = whitespace.many_
    expr = delim >>, ops, delim)
end '1+2*(3-1)' # => 5

As simple as that!

S-expression demo

The above example utilizes the pre-built Expressions class to help building expression parser. Another example is for a simpler s-expression parser (in lisp syntax, "- (+ 1 (* 2 2)) (1)" sort of thing). As s-expression is way simpler to parse than expressions with infix operators, we will build the parser without using Expressions class:

Code Block
include RParsec
class SExpressionTestCase < RUNIT::TestCase
  include Parsers
  include Functors
  def delim
  def ignore parser
    parser << delim
  def lparen
  def rparen
  def parser
    expr = nil
    lazy_expr = lazy{expr}
    term = | lparen >> lazy_expr << rparen
    binop = char('+') >> Plus | char('-') >> Minus | char('*') >> Mul | char('/') >> Div
    binop = ignore binop
    term = ignore term
    binary = sequence(binop, lazy_expr, lazy_expr) do |op, e1, e2|, e2)
    expr = delim >> (term | binary)
  def test1
    assert_equal(4, parser.parse('- + 1 (* 2 2.0) (1)'))

The code is pretty self-explanatory. The only thing to note is that we had to manually skip whitespaces using the "ignore" function. (Whitespace skipping can be more elegant if we do a 2-phase parsing, where lexer takes care of them before grammar rule kicks in. Such use is demonstrated in detail in the test code.)

More examples

Take a look at the SQL Parser for a more sophisticated parser, where more traditional lex/parse approach is used.

Installing rparsec

Code Block
gem install rparsec

rparsec can be downloaded at

Rdoc is available online.

Children Display

Created by benyu benyu
On Sun Oct 08 14:53:56 CDT 2006
Using TimTam