Another expression tree evaluator

Expression Tree

# Another version of the code at https://gist.github.com/2934374

Number   = lambda { |env, num| num }
Variable = lambda { |env, var| env[var] }
Add      = lambda { |env, a, b| evaluate(env, a) + evaluate(env, b) }
Multiply = lambda { |env, a, b| evaluate(env, a) * evaluate(env, b) }

def evaluate(env, exp)
  op, *args = exp
  op.(env, *args)
end

ExpressionTree = [Add, [Variable, :a], [Multiply, [Number, 2], [Variable, :b]]]
Env = { a: 3, b: 4, c: 5 }

puts evaluate(Env, ExpressionTree)