add displayln\*

from syntax-objects/Summer2021#2

cc @Lazerbeak12345

displaylns/displaylns-test.rkt

@@ -0,0 +1,11 @@
+#lang racket/base
+(module+ test
+  (require rackunit racket/port syntax-parse-example/displaylns/displaylns)
+
+  (check-equal?
+    (with-output-to-string
+      (lambda ()
+        (displayln* 1 2 3 4 5 '(this is a list of datums 1 2 3 "hi"))))
+    "1 2 3 4 5 (this is a list of datums 1 2 3 hi)\n")
+
+)

displaylns/displaylns.rkt

@@ -0,0 +1,6 @@
+#lang racket
+(provide displayln*)
+(require syntax/parse/define)
+
+(define-syntax-parse-rule (displayln* items:expr ...)
+  (displayln (string-join (map ~a (list items ...)) " ")))

displaylns/displaylns.scrbl

@@ -0,0 +1,46 @@
+#lang syntax-parse-example
+@require[
+  (for-label racket/base syntax/parse syntax/parse/define syntax-parse-example/displaylns/displaylns)]
+
+@(define displaylns-eval
+   (make-base-eval '(require syntax-parse-example/displaylns/displaylns) '(define a-port (current-output-port))))
+
+@title{@tt{displaylns}}
+
+@; =============================================================================
+
+@defmodule[syntax-parse-example/displaylns/displaylns]{}
+@stxbee2021["Lazerbeak12345" 2]
+
+@defform[(displayln* expr ...)]{
+  This macro is intended to make debugging easier by allowing a programmer to
+  print a batch of values all in one go much like Python's
+  @hyperlink["https://docs.python.org/3/library/functions.html#print" "print"].
+
+  To change the output port use @racket[parameterize].
+
+  @examples[#:eval displaylns-eval
+    (displayln* 1 2 3 4 5 '(this is a list of datums 1 2 3 "hi"))
+    (parameterize ([current-output-port a-port])
+      (displayln* 1 2 '(a b)))
+  ]
+
+  With @racket[define-syntax-parse-rule], this macro is a one-liner:
+
+  @racketfile{displaylns.rkt}
+
+  A function could express the same behavior.
+  Furthermore, using a function instead would help reduce code size ---
+  assuming the compiler does not inline every call to @racket[displayln*].
+  That said, a macro has two advantages for this debugging tool:
+  @itemize[
+  @item{
+    You can easily to hide all the debug expressions by changing the macro
+    body to @racket[(void)].
+  }
+  @item{
+    You might extend the macro to compute source locations from its arguments.
+  }
+  ]
+
+}

index.scrbl

@@ -16,6 +16,7 @@
 
 @; =============================================================================
 
+@include-example{displaylns}
 @include-example{first-class-or}
 @include-example{optional-assert}
 @include-example{make-variable}