This is all you need for an interpreter

interp.scm

; From https://www.youtube.com/watch?v=OyfBQmvr2Hc

(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))]))