parslet_parser_instance.rb

require "parslet"

class Parser < Parslet::Parser
  def initialize
    super
    rule(:integer) { an_integer }
    root(:integer)
  end

  def an_integer
    match["0-9"].repeat(1).as(:integer)
  end
end

class Transform < Parslet::Transform
  def initialize
    super
    rule integer: simple(:x) do |context|
      to_integer context[:x]
    end
  end

  def to_integer(x)
    x.to_i
  end
end

p Transform.new.apply integer: "1"
p Parser.new.parse "1"