X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2FbasicTypes%2FDataCon.lhs;fp=compiler%2FbasicTypes%2FDataCon.lhs;h=d8fdfb55aeaea09ecbc125d014a99b681cafed1a;hp=df8af8e221045eadb411b98a3733ff9eb4b92549;hb=9ffadf219cbc4f8ec57264786df936a3cee88aec;hpb=24a5fdb5fe20290cbb9b58b2901e8d2fd651d3f3

diff --git a/compiler/basicTypes/DataCon.lhs b/compiler/basicTypes/DataCon.lhs
index df8af8e..d8fdfb5 100644
--- a/compiler/basicTypes/DataCon.lhs
+++ b/compiler/basicTypes/DataCon.lhs
@@ -318,7 +318,6 @@ data DataCon
 	dcOrigArgTys :: [Type],		-- Original argument types
 					-- (before unboxing and flattening of strict fields)
 	dcOrigResTy :: Type,		-- Original result type, as seen by the user
-		-- INVARIANT: mentions only dcUnivTyVars
 		-- NB: for a data instance, the original user result type may 
 		-- differ from the DataCon's representation TyCon.  Example
 		--	data instance T [a] where MkT :: a -> T [a]
@@ -636,8 +635,10 @@ dataConFieldLabels = dcFields
 
 -- | Extract the type for any given labelled field of the 'DataCon'
 dataConFieldType :: DataCon -> FieldLabel -> Type
-dataConFieldType con label = expectJust "unexpected label" $
-    lookup label (dcFields con `zip` dcOrigArgTys con)
+dataConFieldType con label
+  = case lookup label (dcFields con `zip` dcOrigArgTys con) of
+      Just ty -> ty
+      Nothing -> pprPanic "dataConFieldType" (ppr con <+> ppr label)
 
 -- | The strictness markings decided on by the compiler.  Does not include those for
 -- existential dictionaries.  The list is in one-to-one correspondence with the arity of the 'DataCon'
@@ -726,7 +727,7 @@ dataConUserType :: DataCon -> Type
 --
 -- rather than:
 --
--- > T :: forall a c. forall b. (c=[a]) => a -> b -> T c
+-- > T :: forall a c. forall b. (c~[a]) => a -> b -> T c
 --
 -- NB: If the constructor is part of a data instance, the result type
 -- mentions the family tycon, not the internal one.