From: simonpj@microsoft.com <unknown>
Date: Mon, 8 Feb 2010 16:37:51 +0000 (+0000)
Subject: Fix Trac #3850
X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=commitdiff_plain;h=dafabe653da4e6cf5aea6b5281c8f77de8d0964a

Fix Trac #3850

This patch simply avoids a needless difference in behaviour from
6.10, and one that happens to affect HList. See Note [Stupid theta].
---

diff --git a/compiler/rename/RnSource.lhs b/compiler/rename/RnSource.lhs
index bfecfd6..6984a4b 100644
--- a/compiler/rename/RnSource.lhs
+++ b/compiler/rename/RnSource.lhs
@@ -726,9 +726,10 @@ rnTyClDecl tydecl@TyData {tcdND = new_or_data, tcdCtxt = context,
 		   else emptyFVs))
         }
   where
-    h98_style = case condecls of
-		     L _ (ConDecl { con_res = ResTyH98 }) : _  -> True
-		     _    		                       -> False
+    h98_style = case condecls of	 -- Note [Stupid theta]
+		     L _ (ConDecl { con_res = ResTyGADT {} }) : _  -> False
+		     _    		                           -> True
+               		     						  
     data_doc = text "In the data type declaration for" <+> quotes (ppr tycon)
 
     rn_derivs Nothing   = return (Nothing, emptyFVs)
@@ -824,6 +825,15 @@ badGadtStupidTheta _
 	  ptext (sLit "(You can put a context on each contructor, though.)")]
 \end{code}
 
+Note [Stupid theta]
+~~~~~~~~~~~~~~~~~~~
+Trac #3850 complains about a regression wrt 6.10 for 
+     data Show a => T a
+There is no reason not to allow the stupid theta if there are no data
+constructors.  It's still stupid, but does no harm, and I don't want
+to cause programs to break unnecessarily (notably HList).  So if there
+are no data constructors we allow h98_style = True
+
 
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