Evaluator for Little Schemer chapter 1

atom.rb

class Atom
  attr_reader :symbol

  def initialize(symbol)
    @symbol = symbol
  end

  def ==(other)
    self.symbol == other.symbol
  end
end

evaluator.rb

class Evaluator
  def initialize(program)
    @program = program
  end

  def evaluate(env={})
    function, argument = @program.car, @program.cdr.car

    operation = function.symbol
    case argument
    when Atom
      env[argument.symbol].send(operation)
    when List
      self.class.new(argument).evaluate(env).send(operation)
    else
      raise
    end
  end
end

evaluator_test.rb

require 'minitest/autorun'

require_relative 'atom'
require_relative 'list'
require_relative 'evaluator'

class EvaluatorTest < Minitest::Test
  def test_car
    assert_equal Atom.new(:a), evaluate(
      List.new(Atom.new(:car), Atom.new(:l)),
      l: List.new(Atom.new(:a), Atom.new(:b), Atom.new(:c))
    )
  end

  def test_car_car
    assert_equal Atom.new(:hotdogs), evaluate(
      List.new(Atom.new(:car), List.new(Atom.new(:car), Atom.new(:l))),
      l: List.new(List.new(Atom.new(:hotdogs), Atom.new(:and)))
    )
  end

  def test_cdr
    assert_equal List.new(Atom.new(:b), Atom.new(:c)), evaluate(
      List.new(Atom.new(:cdr), Atom.new(:l)),
      l: List.new(Atom.new(:a), Atom.new(:b), Atom.new(:c))
    )
  end

  private

  def evaluate(program, env={})
    Evaluator.new(program).evaluate(env)
  end
end

list.rb

class List
  def initialize(*array)
    @array = array
  end

  def car
    @array.first
  end

  def cdr
    self.class.new(*@array[1..-1])
  end

  def ==(other)
    self.array == other.array
  end

  protected

  attr_reader :array
end

Great!

The book hasn’t yet told us how to provide a literal (i.e. unevaluated) list argument to car and cdr, so IMO only the *_with_environment_lookup tests make sense.

I guess what I mean is: the book doesn’t say “(car (a b c)) is a” or “(cdr (a b c)) is (b c)”, but the tests do.