update sugar/list

list.rkt

@@ -1,4 +1,5 @@
 #lang racket/base
+(require (for-syntax racket/base))
 (require racket/list racket/set)
 (require "define.rkt" "len.rkt" "coerce.rkt")
 
@@ -59,3 +60,13 @@
                                   "item isn’t"
                                   "items aren’t") " unique:") duplicate-keys))
       result))
+
+
+;; for use inside quasiquote
+;; instead of ,(when ...) use ,@(when/splice ...)
+;; to avoid voids
+(provide when/splice)
+(define-syntax (when/splice stx)
+  (syntax-case stx ()
+    [(_ test body)
+      #'(if test (list body) '())]))

macro.rkt

@@ -1,32 +0,0 @@
-#lang racket/base
-(require (for-syntax racket/base))
-(require racket/syntax)
-
-(provide define->macro)
-
-
-(define-syntax define->macro  
-  (syntax-rules ()
-    [(_ (proc-name arg ... . rest-arg) proc-expr ...) (define->macro proc-name (λ(arg ... . rest-arg) proc-expr ...))]
-    [(_ proc-name proc) (define-syntax proc-name 
-                          (syntax-id-rules ()
-                            [(proc-name expr (... ...)) (proc expr (... ...))]
-                            [proc-name proc]))]))
-
-
-;(define foo (λ(x) (add1 x)))
-;(define->macro foo (λ(x) (add1 x)))
-
-;(foo 4)
-;(map foo '(2 4 6))
-
-;(define (bar x) (add1 x))
-;(define->macro (bar x y) (+ x y))
-
-;(bar 4 1)
-;(map bar '(2 4 6) '(2 4 6))
-
-;(define-syntax zam (add1 x))
-
-;(zam 4)
-;(map zam '(2 4 6))

misc.rkt

@@ -21,11 +21,3 @@
     [else (format "~a bytes" bytecount)]))
 
 
-;; for use inside quasiquote
-;; instead of ,(when ...) use ,@(when/splice ...)
-;; to avoid voids
-(provide when/splice)
-(define-syntax (when/splice stx)
-  (syntax-case stx ()
-    [(_ test body)
-      #'(if test (list body) '())]))

scribblings/list.scrbl

@@ -73,4 +73,12 @@ Same as @racket[members-unique?], but if the members are not unique, raises a de
 (members-unique?/error '(a b c d e f a b))
 ]
 
+@defform[(when/splice test expr)]
+A special version of @racket[when] that you can use inside @racket[quasiquote] to suppress @racket[void] values when @racket[_test] is @racket[#f]. As the name suggests, it works in conjunction with the @litchar["@"] splicing operator.
 
+@examples[#:eval my-eval
+`(,(when (even? 2) "hooray"))
+`(,(when (even? 3) "hooray"))
+`(,@(when/splice (even? 2) "hooray"))
+`(,@(when/splice (even? 3) "hooray"))
+]