add while / break

from syntax-objects/Summer2021#20
cc @countvajhula

Author: bennn

index.scrbl

@@ -20,6 +20,7 @@
 @include-example{optional-assert}
 @include-example{cross-macro-communication}
 @include-example{let-star}
+@include-example{while-break}
 @include-example{def}
 @include-example{conditional-require}
 @include-example{multi-check-true}

while-break/while-break-test.rkt

@@ -0,0 +1,30 @@
+#lang racket/base
+(module+ test
+  (require rackunit racket/port syntax-parse-example/while-break/while-break)
+
+  (test-begin
+    (define x 5)
+
+    (test-case "ex1"
+      (check-equal?
+        (with-output-to-string
+          (lambda ()
+            (while (> x 0)
+              (displayln x)
+              (set! x (sub1 x)))))
+        "5\n4\n3\n2\n1\n"))
+
+    (set! x 5)
+
+    (test-case "ex2"
+      (check-equal?
+        (with-output-to-string
+          (lambda ()
+            (while #t
+              (displayln x)
+              (set! x (sub1 x))
+              (unless (> x 0)
+                (break)))))
+        "5\n4\n3\n2\n1\n")))
+
+)

while-break/while-break.rkt

@@ -0,0 +1,17 @@
+#lang racket/base
+(provide while break)
+(require syntax/parse/define racket/stxparam (for-syntax racket/base))
+
+(define-syntax-parameter break
+  (lambda (stx)
+    (raise-syntax-error (syntax-e stx) "can only be used inside `while`")))
+
+(define-syntax-parse-rule (while condition body ...)
+  (call/ec
+   (λ (return)
+     (syntax-parameterize ([break (make-rename-transformer #'return)])
+       (let loop ()
+         (when condition
+           (begin body ...
+                  (loop))))))))
+

while-break/while-break.scrbl

@@ -0,0 +1,46 @@
+#lang syntax-parse-example
+@require[
+  (for-label racket/base syntax/parse syntax-parse-example/while-break/while-break)]
+
+@(define while-break-eval
+   (make-base-eval '(require syntax-parse-example/while-break/while-break)))
+
+@title{Basic while loop equipped with break}
+@stxbee2021["countvajhula" 20]
+
+@; =============================================================================
+
+@defmodule[syntax-parse-example/while-break/while-break]{}
+
+@defform[(while test body ...+)]{
+
+  @examples[#:eval while-break-eval
+    (define x 5)
+
+    (while (> x 0)
+      (displayln x)
+      (set! x (sub1 x)))
+
+    (set! x 5)
+
+    (while #t
+      (displayln x)
+      (set! x (sub1 x))
+      (unless (> x 0)
+        (break)))
+  ]
+
+
+  @; TODO add description here
+}
+
+@defidform[break]{
+  May only appear within an @racket[while] form.
+}
+
+The macro uses an escape continuation to provide the semantics of @racket[break], and
+leverages it using a syntax parameter so that the continuation is accessible
+in the lexical scope of the @racket[while] body, and also so that @racket[break] is a syntax
+error outside the @racket[while] loop.
+
+@racketfile{while-break.rkt}