gistfile1.rb

require 'yaml'
require 'parslet'

module Prolog
  include Parslet

  rule(:collection) { statement.repeat.as(:collection) }
  rule(:statement) { ((headed|headless) >> str('.') >> newline?).as(:statement) }

  rule(:headed) { (structure >> space? >> str(':-') >> space? >> conjunction).as(:headed) }
  rule(:headless) { structure.as(:headless) }

  rule(:conjunction) { (structure >> divider).repeat >> structure | structure }

  rule(:parameter_list) { (term >> divider).repeat >> term | term }
  rule(:term) { atom | variable | structure }
  rule(:variable) { match('[A-Z_]') >> match('[a-z0-9A-Z_]').repeat }
  rule(:atom) { match('[a-z]') >> match('[a-zA-Z0-9_]').repeat }
  rule(:structure) { atom.as(:functor) >> str('(') >> parameter_list.as(:params) >> str(')') }

  # helper rules
  rule(:divider) { space? >> str(',') >> space? }
  rule(:newline?) { (str("\n") | str("\r\n") | str("\r")).maybe }
  rule(:space) { str(' ').repeat(1) }
  rule(:space?) { space.maybe }
end

class Parser
  include Prolog

  def initialize(str)
    puts collection.parse(str).to_yaml
  end
end

contents = File.open(ARGV[0]).read()
Parser.new(contents)