underscore.md

-# Treatment of names beginning with underscore
+# Caseless Underscore
 
 
-It is a convention that the compiler will not warn about unused variables that begin with an underscore. An underscore also counts as a lower case letter meaning that you cannot start type, class, or data constructors with it.
+It is a convention that the compiler will not warn about unused variables that begin with an underscore. In Haskell 98, an underscore also counts as a lower case letter meaning that you cannot start type, class, or data constructors with it.
 
-## proposal
+## Proposal
 
 
 Make underscore 'caseless' meaning when deciding whether a name is a constructor or not, look through underscores. so 
@@ -21,7 +21,27 @@ we can either treat identifiers containing only underscores as lowercase names o
 
 The leading underscore convention is not only useful as a way to suppress warnings, but as a form of documentation that we expect a name to be unused so being able to use it in these cases would be helpful.
 
-## implementations
+## Change to the Report
+
+
+Change the productions for *varid* and *conid* in section 2.4 to
+
+```wiki
+varid -> ({ _ } small {small | large | digit | _ | ' })<reservedid>
+conid -> { _ } large {small | large | digit | _ | ' }
+```
+
+
+In the above, there are no underscore-only identifiers, but single underscore is a reservedid.
+
+
+For full backward compatibility, we would need to allow underscore-only varids, changing the first production to:
+
+```wiki
+varid -> ({ _ } ( _ | small {small | large | digit | _ | ' }))<reservedid>
+```
+
+## Implementations
 
 
 Caseless underscore is implemented in nhc98 by default, H'98 behaviour requires the -underscore option.