Comment by labria

  (define eval-expr
    (lambda (expr env)
      (pmatch expr
        [`,x (guard (symbol? x))
          (env x)]
        [`(lambda (,x) ,body)
          (lambda (arg)
            (eval-expr body (lambda (y)
                              (if (eq? x y)
                                  arg
                                  (env y)))))]
        [`(,rator ,rand)
         ((eval-expr rator env)
          (eval-expr rand env))])))

There's an amazing talk about it: https://www.youtube.com/watch?v=OyfBQmvr2Hc