utils: Require arity for mapn. HEAD main

With an empty list, mapn cannot know the number of values proc would
return. mapn therefore needs to know the arity of proc. To provide for
existing callers of mapn, we add a map2 function variant. Finally, we
add a test case testing mapn on an empty list.

1 files changed, 12 insertions, 2 deletions

diff --git a/tests/utils.scm b/tests/utils.scm
index 50c3396..d786d02 100644
--- a/tests/utils.scm
+++ b/tests/utils.scm
@@ -1,5 +1,5 @@
 ;;; ccwl --- Concise Common Workflow Language
-;;; Copyright © 2021, 2022, 2023 Arun Isaac <arunisaac@systemreboot.net>
+;;; Copyright © 2021, 2022, 2023, 2025 Arun Isaac <arunisaac@systemreboot.net>
 ;;;
 ;;; This file is part of ccwl.
 ;;;
@@ -194,7 +194,17 @@
   (let ((squares cubes (mapn (lambda (n)
                                (values (expt n 2)
                                        (expt n 3)))
-                             (iota 5))))
+                             (iota 5)
+                             2)))
+    (list squares cubes)))
+
+(test-equal "mapn on an empty list"
+  '(() ())
+  (let ((squares cubes (mapn (lambda (n)
+                               (values (expt n 2)
+                                       (expt n 3)))
+                             '()
+                             2)))
     (list squares cubes)))
 
 (test-equal "foldn"