Another expression tree evaluator

clojure.clj

(defn evaluate [env [op & args]]
   (apply op env args))
  
(defn number   [env num] num)
(defn variable [env var] (env var))
(defn add      [env a b] (+ (evaluate env a) (evaluate env b)))
(defn multiply [env a b] (* (evaluate env a) (evaluate env b)))

(def expression-tree [add [variable :a] [multiply [number 2] [variable :b]]])
(def env {:a 3, :b 4, :c 5})

(evaluate env expression-tree)

ExpressionTree.rb

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)