diff --git a/lib/opal/compiler.rb b/lib/opal/compiler.rb
index <HASH>..<HASH> 100644
--- a/lib/opal/compiler.rb
+++ b/lib/opal/compiler.rb
@@ -108,7 +108,7 @@ module Opal
     # method simply appends the filename and curent line number onto
     # the message and issues a warning.
     def warning(msg, line = nil)
-      warn "WARNING: #{msg} :#{file}:#{line}"
+      warn "WARNING: #{msg} -- #{file}:#{line}"
     end
 
     # Instances of `Scope` can use this to determine the current
Make filename in compiler warnings look better