mbpew.clj

(defn eval-expr [expr env]
  (cond (symbol? expr) (env expr)

        (and (seq? expr) (= 'lambda (first expr)))
        (let [[_ [a] body] expr]
          (fn [arg]
            (eval-expr body (fn [x] (if (= a x)
                              arg
                              (env x))))))

        (seq? expr)
        (let [rtor (eval-expr (first expr) env)
              rnd  (eval-expr (second expr) env)]
          (rtor rnd))

        :else
        (throw (ex-info "uncool" {:expr expr}))))

;; test code

(eval-expr '((lambda (n) n) hello) {'hello "hi!"})

(eval-expr '(((lambda (x)
                (lambda (y)
                  x))
              foo)
             bar)
           '{foo 1, bar 2})