parens language (in scheme)

parens-lang.scm

;;; ref. http://blog.livedoor.jp/dankogai/archives/51524639.html

(define S (lambda (x) (lambda (y) (lambda (z) ((x z) (y z))))))
(define K (lambda (x) (lambda (y) x)))
(define U (lambda (x) ((x S) K)))

(define (%parens base compose)
  (lambda (code)
    (let r ((c code))
      (unless (list? c) (error "proper list required, but got" c))
      (fold (lambda (a b) (compose b (r a))) base c))))

(define parens        (%parens U (cut <> <>)))
(define parens->lisp  (%parens 'U list))
(define parens->jsstr (%parens "U" (cut string-append <> "(" <> ")")))