condd4: The Design Space of Conditionals with Embedded Defines

condd4.rkt

#lang racket
(require (for-syntax racket/base
                     syntax/parse)
         racket/list
         racket/match)

;; Related post: http://jeapostrophe.github.io/2013-11-12-condd-post.html

(define (f-condd4 l)
  (condd4
   [(empty? l) empty]
   #:def (match-define (cons fst rst) l)
   [(zero? (modulo fst 3))
    (cons 3 (f-condd4 rst))]
   [(zero? (modulo fst 4))
    (cons (+ fst 4) rst)]
   [else
    (list fst)]))

(define-syntax (condd4 stx)
  (syntax-parse stx
    #:literals (else)
    [(_)
     #'(error 'condd4 "Fell through without else clause")]
    [(_ [else . d])
     #'(let () . d)]
    [(_ #:def def . tail)
     #'(let ()
         def
         (condd4 . tail))]
    [(_ [test . b] . more)
     #'(if test
         (let () . b)
         (condd4 . more))]))


(f-condd4 (range 10))