* module/ice-9/atomic.scm (atomic-box-update!): New variable.
---
Changes since v1. Use single-argument proc to avoid potential perfomance
problems cause of call to apply.

 module/ice-9/atomic.scm | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/module/ice-9/atomic.scm b/module/ice-9/atomic.scm
index 2a8af901d..6bfa2e8ee 100644
--- a/module/ice-9/atomic.scm
+++ b/module/ice-9/atomic.scm
@@ -25,7 +25,8 @@
             atomic-box-ref
             atomic-box-set!
             atomic-box-swap!
-            atomic-box-compare-and-swap!))
+            atomic-box-compare-and-swap!
+            atomic-box-update!))
 
 (eval-when (expand load eval)
   (load-extension (string-append "libguile-" (effective-version))
@@ -36,3 +37,14 @@
   (add-interesting-primitive! 'atomic-box-set!)
   (add-interesting-primitive! 'atomic-box-swap!)
   (add-interesting-primitive! 'atomic-box-compare-and-swap!))
+
+(define (atomic-box-update! box proc)
+  "Atomically updates value of BOX to (PROC BOX-VALUE), returns new
+value.  PROC may be called multiple times, and thus PROC should be
+free of side effects."
+  (let loop ()
+    (let* ((old-value (atomic-box-ref box))
+           (new-value (proc old-value)))
+      (if (eq? old-value (atomic-box-compare-and-swap! box old-value new-value))
+          new-value
+          (loop)))))
-- 
2.40.1