From 10cb7e12c1cfb4759ffeff3bc2f8e54900019e6a Mon Sep 17 00:00:00 2001
From: Doug Coleman <doug.coleman@gmail.com>
Date: Mon, 4 Apr 2016 11:45:21 -0700
Subject: [PATCH] assocs.extras: Add a word set-once-at that throws if the key
 is already set.

---
 extra/assocs/extras/extras.factor | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/extra/assocs/extras/extras.factor b/extra/assocs/extras/extras.factor
index cf63ee8472..80e2521d4f 100644
--- a/extra/assocs/extras/extras.factor
+++ b/extra/assocs/extras/extras.factor
@@ -44,3 +44,11 @@ GENERIC: delete-value-at ( value assoc -- )
 
 M: assoc delete-value-at
     [ value-at* ] keep swap [ delete-at ] [ 2drop ] if ;
+
+ERROR: key-exists value key assoc ;
+: set-once-at ( value key assoc -- )
+    2dup ?at [
+        key-exists
+    ] [
+        drop set-at
+    ] if ;
\ No newline at end of file
-- 
2.34.1