a Python3 style string-template for Scheme

string-template.scm

(define *stpl-SRE* '(or (=> dollar "$$")
                        (: "${" (=> name (+ (~ #\}))) "}")))
(define (make-string-template str . opts)
  (define ll '())
  (define lv '())
  (define template
    (irregex-replace/all 
     ;;"(\\$\\{([^$])+\\})"
     *stpl-SRE* str
     (lambda (m) 
       (if (irregex-match-substring m 'dollar)
           "$"
           (let* ((key (symbol->keyword (string->symbol 
                                         (irregex-match-substring m 'name))))
                  (v (kw-arg-ref opts key)))
             (and v (set! lv (cons (cons key v) lv))) ; default value
             (set! ll (cons key ll))
             "~a")))))
  (lambda args
    (let ((vals (map (lambda (x) (or (kw-arg-ref args x) (assoc-ref lv x) "NONE")) ll)))
    (format #f "~?" template (reverse vals)))))

(define tpl (make-string-template "${name1}, I'm ${name2}"))
(tpl #:name1 "madam" #:name2 "adam")
==> "madam, I'm adam"

and for default value

(define tpl (make-string-template "${name1}, I'm ${name2}" #:name1 "Dassy"))
(tpl #:name2 "adam")
==> "Dassy, I'm adam"