Example module that replaces the last space in a tagged X-expression with the HTML nbsp entity

example-nbsp-replacer.rkt

#lang racket

(require txexpr)

(define (string-reverse str)
  (list->string (reverse (string->list str))))

(define (replace-first-space str)
  (string-replace str " " " " #:all? #f))

(define (replace-last-space str)
  (string-reverse (string-replace (string-reverse str) " " ";psbn&" #:all? #f)))

(define (nbsp xpr [found? #f])
  (cond
    [found? (values xpr found?)]
    [(and (string? xpr) (string-contains? xpr " "))
     (values (replace-last-space xpr) #t)]
    [(txexpr? xpr)
     (define-values (result-elements found-within?)
       (for/fold ([result-xprs null]
                  [found-yet? #f])
                 ([this-expr (in-list (reverse (get-elements xpr)))])
         (define-values (this-result found-this-time?) (nbsp this-expr found-yet?))
         (values (cons this-result result-xprs) found-this-time?)))
     (values (txexpr (get-tag xpr) (get-attrs xpr) result-elements) found-within?)]
    [else (values xpr found?)]))

(define (last-nbsp xpr)
  (let-values ([(result _) (nbsp xpr)]) result))

(module+ test
  (require rackunit)
  (check-equal? (last-nbsp '(p "This is my only number"))
                '(p "This is my only number"))

  ;; What if we break up the x-expression?
  (check-equal? (last-nbsp '(p "This is my " (em "only") " number"))
                '(p "This is my " (em "only") " number"))

  ;; Hide the space in the middle tag 
  (check-equal? (last-nbsp '(p "This is my " (em "only ") "number"))
                '(p "This is my " (em "only ") "number"))

  ;; Hide the space in between two tags
  (check-equal? (last-nbsp '(p "This is my " (em "only") " " (strong "number")))
                '(p "This is my " (em "only") " " (strong "number")))

  ;; Doesn't mess with attributes
  (check-equal? (last-nbsp '(p "This is my " (em "only") " " (strong [[class "c1 c2"]] "number")))
                '(p "This is my " (em "only") " " (strong ((class "c1 c2")) "number"))))

;; Doesn't properly handle multiple spaces
;; such as:
;;   '(p "This is my only  number")
;; OR:
;;   '(p "This is my " (em "only ") " number")