parslet_demo.rb

#!/usr/bin/env ruby

require 'parslet'

class MyParser < Parslet::Parser
  rule(:sentence) { (greeting >> space >> name >> exclamation.maybe).as(:result) }
  rule(:greeting) { (str('Hello') | str('Goodbye')).as(:greeting) }
  rule(:name) { match(/[A-Za-z]/).repeat(1).as(:name) }
  rule(:space) { match(/\s/).repeat(1) }
  rule(:exclamation) { str('!') }
  root(:sentence)
end

class Transformer < Parslet::Transform
  rule(name: simple(:n), greeting: simple(:g)) do
    { name: n.to_s, presence: (g == 'Hello' ? true : false) }
  end
end

class Person
  def initialize(name, here)
    @name = name
    @here = here
  end

  attr_accessor :name, :here

  def present?
    here
  end
end


raw = MyParser.new.parse("Hello Laura!")
transformed = Transformer.new.apply(raw)[:result]

person = Person.new(transformed[:name], transformed[:presence])

if person.present?
  puts person.name + ' is here'
else
  puts person.name + ' is not here'
end