From 7bdc4a32ff775bca0158533c9e9af250f16cceb7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@lysator.liu.se>
Date: Thu, 23 Feb 2023 01:08:16 +0100
Subject: Extend `for'-macro to allow improper list elements.

---
 module/hnh/util.scm | 13 +++++++++++++
 tests/test/util.scm | 10 ++++++++++
 2 files changed, 23 insertions(+)

diff --git a/module/hnh/util.scm b/module/hnh/util.scm
index bbb1a5ec..ea7c0dd1 100644
--- a/module/hnh/util.scm
+++ b/module/hnh/util.scm
@@ -133,6 +133,19 @@
                         b1 body ...)))])
                  <collection>)))))
 
+    ((for (<var> <vars> ... . <tail>) in <collection> b1 body ...)
+     #'(call/ec
+        (lambda (break)
+          (map ((@ (ice-9 match) match-lambda)
+                [(<var> <vars> ... . <tail>)
+                 (call/ec
+                  (lambda (raw-continue)
+                    (let ((continue
+                           (case-lambda
+                             (() #f)
+                             (args (apply raw-continue args)))))
+                      b1 body ...)))])
+               <collection>))))
     ((for <var> in <collection> b1 body ...)
      (with-syntax ((break (datum->syntax stx 'break))
                    (continue (datum->syntax stx 'continue)))
diff --git a/tests/test/util.scm b/tests/test/util.scm
index d2fc2d81..41dbd7a3 100644
--- a/tests/test/util.scm
+++ b/tests/test/util.scm
@@ -51,6 +51,16 @@
    (for (x c) in (zip (iota 12) (string->list "Hello, World"))
         x))
 
+ (test-equal "for with improper list elements"
+   `(3 7)
+   (for (a . b) in '((1 . 2) (3 . 4))
+        (+ a b)))
+
+ (test-equal "for with longer improper list elements"
+   '(1 2 4)
+   (for (a b . c) in '((1 -1 . 1) (2 -2 . 2) (4 -4 . 4))
+        (* c (+ 1 a b))))
+
  (test-equal "for break"
    'x
    (for x in (iota 10)
-- 
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