(define eval-expr
    (lambda (expr env)
        (pmatch expr
            [,n (guard number? n)
                n]
            [,x (guard symbol? x)
                x]
            [(lambda (,arg) ,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))])))