[Hat] Qualified variable in pattern: T.mkRoot

[Malcolm Wallace]

Thomas Davie <tatd2 at kent.ac.uk> writes:

> > Just foo = undefined
> 
> The haskell syntax states:
> 
> decl -> gendecl
>       | (funlhs | pat0) rhs
> 
> where, gendecl covers type definitions, funlhs covers functions (and  
> must start with a lower case character), and pat0 covers 0 arity  
> patterns.

The notation pat_0 does not mean zero-arity, but zero-fixity.  Thus,
since any pat_i can be a pat_i+1, and

   pat_10 -> apat
           | gcon apat_1 ... apat_k

the given declaration is syntactically valid, even though it is
certainly unusual.

hat-trans is generating bad code for this, because the pattern binding
is at the toplevel.  If the pattern binding were in a let or where
clause, the generated code is different.  When not at the toplevel,
the parent, p, of the binding is always a variable passed in.
But at the toplevel, there is no static parent, so hat-trans has
wrongly substituted T.mkRoot (which explicitly represents no parent),
in both the generated pattern and the generated expression:

    gx px T.mkRoot = T.constUse px T.mkRoot sx	-- incorrect
vs.
    gx px p = T.constUse px p sx		-- correct

I think the attached patch for hat-trans should fix the problem (but
would like someone to confirm this is not going to break anything else,
before I will commit it to CVS).

Regards,
    Malcolm
-------------- next part --------------
Index: TraceTrans.hs
===================================================================
RCS file: /home/cvs/root/hat/src/hattrans/TraceTrans.hs,v
retrieving revision 1.73
diff -u -r1.73 src/hattrans/TraceTrans.hs
--- src/hattrans/TraceTrans.hs	27 Sep 2004 11:15:51 -0000	1.73
+++ src/hattrans/TraceTrans.hs	29 Jun 2005 12:34:34 -0000
@@ -701,13 +701,14 @@
   (Fun [pat'] (Unguarded exp') decls',altConsts) = 
      tFun traced False parent failContinuation (Fun [pat] rhs decls)
   useSR = ExpVar pos (nameSR firstId)
+  useParent = ExpVar pos nameParent
 
   useDef :: (Pos,TraceId) -> Decl TokenId
   useDef (pos,id) =
     DeclFun pos (nameTransLetVar id)
-      [Fun [useSR,parent]
+      [Fun [useSR,useParent]
         (Unguarded (ExpApplication pos
-          [combConstUse pos traced,useSR,parent,ExpVar pos (nameShare id)]))
+          [combConstUse pos traced,useSR,useParent,ExpVar pos (nameShare id)]))
         noDecls]
 
   projDef :: (Pos,TraceId) -> Decl TokenId