X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FdeSugar%2FDesugar.lhs;h=073e87390f3c009fd98e4f9de4716bf96fffbbd3;hb=5723262f616ac02ddf637f6ff480a599c737ea0d;hp=d154e045ab8860634d5bb63378ea54597c35b26e;hpb=92267aa26adb1ab5a6d8004a80fdf6aa06ea4e44;p=ghc-hetmet.git

diff --git a/compiler/deSugar/Desugar.lhs b/compiler/deSugar/Desugar.lhs
index d154e04..073e873 100644
--- a/compiler/deSugar/Desugar.lhs
+++ b/compiler/deSugar/Desugar.lhs
@@ -34,7 +34,6 @@ import CoreMonad	( endPass, CoreToDo(..) )
 import ErrUtils
 import Outputable
 import SrcLoc
-import FastString
 import Coverage
 import Util
 import MonadUtils
@@ -338,16 +337,17 @@ dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
   = putSrcSpanDs loc $ 
     do	{ let bndrs' = [var | RuleBndr (L _ var) <- vars]
 
-	; lhs' <- unsetOptM Opt_EnableRewriteRules $
-	          dsLExpr lhs	-- Note [Desugaring RULE left hand sides]
+        ; lhs' <- unsetOptM Opt_EnableRewriteRules $
+                  unsetOptM Opt_WarnIdentities $
+                  dsLExpr lhs   -- Note [Desugaring RULE left hand sides]
 
 	; rhs' <- dsLExpr rhs
 
 	-- Substitute the dict bindings eagerly,
 	-- and take the body apart into a (f args) form
-	; case decomposeRuleLhs lhs' of {
-		Nothing -> do { warnDs msg; return Nothing } ;
-		Just (fn_id, args) -> do
+	; case decomposeRuleLhs bndrs' lhs' of {
+		Left msg -> do { warnDs msg; return Nothing } ;
+		Right (final_bndrs, fn_id, args) -> do
 	
 	{ let is_local = isLocalId fn_id
 		-- NB: isLocalId is False of implicit Ids.  This is good becuase
@@ -356,12 +356,9 @@ dsRule (L loc (HsRule name act vars lhs _tv_lhs rhs _fv_rhs))
 	      fn_name   = idName fn_id
 	      final_rhs = simpleOptExpr rhs'	-- De-crap it
 	      rule      = mkRule False {- Not auto -} is_local 
-                                 name act fn_name bndrs' args final_rhs
+                                 name act fn_name final_bndrs args final_rhs
 	; return (Just rule)
 	} } }
-  where
-    msg = hang (ptext (sLit "RULE left-hand side too complicated to desugar; ignored"))
-	     2 (ppr lhs)
 \end{code}
 
 Note [Desugaring RULE left hand sides]
@@ -374,4 +371,6 @@ switching off EnableRewriteRules.  See DsExpr.dsExplicitList.
 
 That keeps the desugaring of list comprehensions simple too.
 
-
+Nor do we want to warn of conversion identities on the LHS;
+the rule is precisly to optimise them:
+  {-# RULES "fromRational/id" fromRational = id :: Rational -> Rational #-}