scheme version of clojure's ->

->.scm

;; scheme version of clojure's ->

;; ->
;; macro
;; Usage: (-> x)
;;        (-> x form)
;;        (-> x form & more)
;; Threads the expr through the forms. Inserts x as the
;; second item in the first form, making a list of it if it is not a
;; list already. If there are more forms, inserts the first form as the
;; second item in second form, etc.

(define-syntax ->
  (lambda (stx)
    (define (combine x form)
      (syntax-case form ()
        [(proc args ...)
         #`(proc #,x args ...)]
        [id
         (identifier? #'id)
         #`(id #,x)]
        [else
         (syntax-violation '-> "Invalid form in ->" form)]))
    (syntax-case stx ()
      [(-> x) #'x]
      [(-> x form forms ...)
       (let ((form-list (syntax->list #'(form forms ...))))
         (fold-left combine #'x form-list))])))