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Code Block
require 'rparsec'
include RParsec
class Calculator
  include Parsers
  include Functors
  def parser
    ops = OperatorTable.new.
      infixl(char(?+) >> Plus, 20).
      infixl(char(?-) >> Minus, 20).
      infixl(char(?*) >> Mul, 40).
      infixl(char(?/) >> Div, 40).
      prefix(char(?-) >> Neg, 60)
    expr = nil
    term = integer.map(&To_i) | char('(') >> lazy{expr} << char (')')
    delim = whitespace.many_
    expr = delim >> Expressions.build(term, ops, delim)
  end
end

Calculator.new.parser.parse '1+2*(3-1)' # => 5

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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
    whitespace.many_
  end
  def ignore parser
    parser << delim
  end
  def lparen
    ignore(char('('))
  end
  def rparen
    ignore(char(')'))
  end
  def parser
    expr = nil
    lazy_expr = lazy{expr}
    term = number.map(&To_f) | 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|
      op.call(e1, e2)
    end
    expr = delim >> (term | binary)
  end
  def test1
    assert_equal(4, parser.parse('- + 1 (* 2 2.0) (1)'))
  end
end

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