Minimal lambda calculus interpreter

lambda.rb

# This can be used as a starting point to build a Scheme or other
# dynamic language interpreter.
module Lambda

  def evaluate(expression, environment)
    if expression.is_a? Symbol
      environment[expression]
    elsif expression[0] == "\\"
      [expression, environment]
    else
      apply evaluate(expression[0], environment), evaluate(expression[1], environment)
    end
  end

  def apply(closure, argument)
    function, environment = *closure
    expression = function[2]
    extended_environment = { function[1] => argument }.merge environment
    evaluate expression, extended_environment
  end
end

include Lambda

function = ["\\", :identity, :identity]
application = [function, :answer]

environment = {
  :answer => 42
}

puts evaluate(application, environment)