X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcExpr.lhs;h=93d3fe93caa49942e9c8f44f064828d949db97e1;hp=784d466bde1ea9eca0e34cb5e358c84db3f3aff3;hb=4a343629ebe5be2c5b27e84c031e38abd81122fa;hpb=97a8fe8780307e95829034117efa98d2e27109cd

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 784d466..93d3fe9 100644
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -457,12 +457,22 @@ field isn't part of the existential. For example, this should be ok.
   data T a where { MkT { f1::a, f2::b->b } :: T a }
   f :: T a -> b -> T b
   f t b = t { f1=b }
+
 The criterion we use is this:
 
   The types of the updated fields
   mention only the universally-quantified type variables
   of the data constructor
 
+NB: this is not (quite) the same as being a "naughty" record selector
+(See Note [Naughty record selectors]) in TcTyClsDecls), at least 
+in the case of GADTs. Consider
+   data T a where { MkT :: { f :: a } :: T [a] }
+Then f is not "naughty" because it has a well-typed record selector.
+But we don't allow updates for 'f'.  (One could consider trying to
+allow this, but it makes my head hurt.  Badly.  And no one has asked
+for it.)
+
 In principle one could go further, and allow
   g :: T a -> T a
   g t = t { f2 = \x -> x }
@@ -529,7 +539,7 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
 	      con1_flds = dataConFieldLabels con1
 	      con1_res_ty = mkFamilyTyConApp tycon (mkTyVarTys con1_tvs)
    	      
-	-- STEP 2
+	-- Step 2
 	-- Check that at least one constructor has all the named fields
 	-- i.e. has an empty set of bad fields returned by badFields
 	; checkTc (not (null relevant_cons)) (badFieldsUpd rbinds)