Simple calculator make in one ruby function

calculator.rb

def calculator(expression, *args)
  {
    add:      -> (a,b) { a + b },
    subtract: -> (a,b) { a - b },
    divide:   -> (a,b) { a / b },
    multiply: -> (a,b) { a * b},
    power:    -> (a,b) { a ** b }
  }[expression].call(*args)
end