Scheme Interpreter Essensentials

type0.rb

# run on irb as follows.
# $ irb -r './type0.rb'
# irb> e [:define, :f, [:lambda, [:v], ["+", :v, 1]]]
# irb> e [:f 1]
# => 2
# ...

module Type0
  Environment = Struct.new(:parent, :hash)
  Lambda = Struct.new(:env, :params, :exp)

  extend self

  def resolve(env, var_name)
    if env.parent
      env.hash[var_name] || resolve(env.parent, var_name)
    else
      env.hash[var_name]
    end
  end

  def apply(lambda, *args)
    eval(Environment.new(lambda.env, lambda.params.zip(args).to_h), lambda.exp)
  end

  def call_ruby_method(method_name, receiver, *args)
    receiver.send(method_name, *args)
  end

  def eval(env, exp)
    case exp
    when Numeric, String, TrueClass, FalseClass
      exp
    when Symbol
      resolve(env, exp)
    when Array
      case exp[0]
      when :lambda
        Lambda.new(env, exp[1], exp[2])
      when :define
        env.hash[exp[1]] = eval(env, exp[2])
      when :if
        eval(env, exp[1]) ? eval(env, exp[2]) : eval(env, exp[3])
      when String
        call_ruby_method(*exp.map{|e| eval(env, e)})
      else
        apply(*exp.map{|e| eval(env, e)})
      end
    end
  end
end

def e(exp)
  $env ||= Type0::Environment.new(nil, {})
  Type0.eval($env, exp)
end