diff --git a/whalesong/selfhost/js-assembler/cache.rkt b/whalesong/selfhost/js-assembler/cache.rkt
new file mode 100644
index 0000000..3a094fe
--- /dev/null
+++ b/whalesong/selfhost/js-assembler/cache.rkt
@@ -0,0 +1,7 @@
+#lang whalesong
+(define-struct cached-entry (real-path ;; path to a module. 
+                             whalesong-version ;; string
+                             md5   ;; md5 of the original source in real-path
+                             bytes)
+  #:transparent) ;; bytes
+