hwparser.rb

require "scanner.rb"

class ParseException < Exception
  
end

class Parser

  def initialize input = nil
    @sc = Scanner.new(input) unless input == nil
  end

  def tp tree
    tree.each do |x|
      if (x.class == Array) then
        tp x
      else
        puts "#{x.type} :: #{x.value}"
      end
    end
  end

  def match? type
    token = @sc.next
    @sc.prev
    token.type == type
  end
  
  def take type
    token = @sc.next
    raise ParseException unless token.type == type
    token
  end

  def parse
    b_program
  end

  def b_program
    [take(:PROGRAM), b_ident,take(:SEMICOLON), take(:BEGIN), b_statlist, take(:END)
  end

  def b_statlist
    result = []
    until match?(:END)
      result << [b_statement, take(:SEMICOLON)]
    end
    result
  end

  def b_statement
    [b_ident,
     take(:OPEN),
     b_literal,
     take(:CLOSE)]
  end

  def b_ident
    take(:IDENT)
  end

  def b_literal
    b_string
  end

  def b_string
    take(:STRING)    
  end

end