Parses a tokenized stream of elements.

element_parser.rb

require 'parslet'
include Parslet

input = %w(a aa b)

class El < Parslet::Atoms::Base
  def initialize(pattern)
    @pattern = pattern
    super()
  end
  def to_s_inner(prec)
    "'#{@pattern}'"
  end
  def try(source, context)
    return succ(source.consume(1)) if source.matches?(@pattern)
    
    # Failures: 
    error_pos = source.pos  
    return context.err_at(
      self, source, 
      "Failed to match #@pattern", error_pos) 
  end
end
def el(*args)
  El.new(*args)
end

parser1 = el('a') >> el('aa') >> el('b')
parser2 = el('aa') >> el('a') >> el('b')

class ArySource
  def initialize(ary)
    @ary = ary
    @pos = 0
  end

  def line_and_column(pos=nil)
    [1, (pos || @pos)+1]
  end

  attr_accessor :pos

  def matches?(pattern)
    @ary[@pos] == pattern
  end
  def consume(n)
    @ary.slice(@pos, n).join(',').tap {
      @pos += n
    }
  end
  
  def eof?
    @pos >= @ary.size
  end
  def chars_left
    @ary.size-@pos.size
  end
end
s = ArySource.new(input)

require 'parslet/convenience'
puts "parser1: "
p parser1.parse_with_debug(s)

s.pos = 0
puts "parser2: "
p parser2.parse_with_debug(s)