simple decode in racket

#lang racket

(define cyphertext
"")

(define code
  #hash(("a" . "_")
        ("b" . "_")
        ("c" . "_")
        ("d" . "_")
        ("e" . "R")
        ("f" . "_")
        ("g" . "_")
        ("h" . "_")
        ("i" . "_")
        ("j" . "_")
        ("k" . "_")
        ("l" . "E")
        ("m" . "_")
        ("n" . "T")
        ("o" . "_")
        ("p" . "_")
        ("q" . "_")
        ("r" . "_")
        ("s" . "_")
        ("t" . "_")
        ("u" . "N")
        ("v" . "_")
        ("w" . "_")
        ("x" . "_")
        ("y" . "_")
        ("z" . "_")))

(define (replacer a b) (string-replace b a (dict-ref code a)))

(define text (foldl replacer cyphertext (dict-keys code)))

(display text)
(println "")

; max is 26 for a..z
(define (char-plus-int c n max)
  (integer->char (+ (modulo (+ (- (char->integer c) 97) n) max) 97)))

(define (shifter text n)
  (let ((letters (string->list text)))
    (list->string (map (lambda (c) (char-plus-int c n 26)) letters))
    ))

;(map (lambda (n) (println n) (display (shifter cyphertext n)) (println "")) (range 0 27))