answer20.rb

def evaluate(expr)
  case expr
  in ["value", x]
    x
  in ["+", x, y]
    evaluate(x) + evaluate(y)
  in ["-", x, y]
    evaluate(x) - evaluate(y)
  in ["*", x, y]
    evaluate(x) * evaluate(y)
  in ["/", x, y]
    evaluate(x) / evaluate(y)
  end
end

def answer20(expr)
  evaluate(expr)
end

p answer20(["+", ["value", 1], ["*", ["value", 2], ["value", 3]]])