Scheme expression threading (->>)

chain.scm

; Scheme implementation of 'expression threading'.
; chains the given argument through each expression; such as
; (->> 5
;      (+ 3)
;      (* 2)) === 16
;
; or even better:
;
; (->>
;   "lets find some even chars!"
;   string->list
;   (map char->integer)
;   (filter even?)
;   (map integer->char)
;   list->string) === "lt fnd vn hr"


(load "~~/lib/syntax-case")

(define (curry f . c) (lambda x (apply f (append c x))))

(define-syntax expr-or-func
  (syntax-rules
    ()
    ((_ (f default-arg ...))
     (curry f default-arg ...))
    ((_ f)
     f)))

(define-syntax ->>
  (syntax-rules
    ()
    ((_ s expr ...)
     (let loop
       ((current s)
        (rest (list (expr-or-func expr) ...)))
       (if (null? rest)
         current
         (loop ((car rest) current) (cdr rest)))))))