allow `(setcar nil nil)' and `(setcdr nil nil)'

* module/language/elisp/runtime/subrs.scm (setcar, setcdr): Allow
  setting the car or cdr of `nil' to `nil'.

Signed-off-by: Andy Wingo <wingo@pobox.com>

diff --git a/module/language/elisp/runtime/subrs.scm b/module/language/elisp/runtime/subrs.scm
index e01261a..c981b38 100644 (file)
--- a/module/language/elisp/runtime/subrs.scm
+++ b/module/language/elisp/runtime/subrs.scm
@@ -265,12 +265,16 @@
 
 (built-in-func setcar
   (lambda (cell val)
-    (prim set-car! cell val)
+    (if (and (null? cell) (null? val))
+        #nil
+        (prim set-car! cell val))
     val))
 
 (built-in-func setcdr
   (lambda (cell val)
-    (prim set-cdr! cell val)
+    (if (and (null? cell) (null? val))
+        #nil
+        (prim set-cdr! cell val))
     val))
 
 ;;; Accessing symbol bindings for symbols known only at runtime.