X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcRnTypes.lhs;h=9a08e9a6535d65661922adc6ecf5a5e1523ccd82;hb=382fce427106197b5e7dec81165af20329ac6cc2;hp=42f4ff43a67532515ccdfb43087ab5241788c16f;hpb=a3a15a646977ab98f9150bb2b926d960796077e4;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index 42f4ff4..9a08e9a 100644
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -598,15 +598,20 @@ data Inst
   | ImplicInst {	-- An implication constraint
 			-- forall tvs. (reft, given) => wanted
 	tci_name   :: Name,
-	tci_tyvars :: [TcTyVar],    -- Includes coercion variables
+	tci_tyvars :: [TcTyVar],    -- Quantified type variables
+				    -- Includes coercion variables
 				    --   mentioned in tci_reft
 	tci_reft   :: Refinement,
 	tci_given  :: [Inst],	    -- Only Dicts
 				    --   (no Methods, LitInsts, ImplicInsts)
 	tci_wanted :: [Inst],	    -- Only Dicts and ImplicInsts
 				    --   (no Methods or LitInsts)
+
 	tci_loc    :: InstLoc
     }
+	-- NB: the tci_given are not necessarily rigid,
+	--     although they will be if the tci_reft is non-trivial
+	-- NB: the tci_reft is already applied to tci_given and tci_wanted
 
   | Method {
 	tci_id :: TcId,		-- The Id for the Inst
@@ -702,10 +707,20 @@ emptyLIE          = emptyBag
 unitLIE inst 	  = unitBag inst
 mkLIE insts	  = listToBag insts
 plusLIE lie1 lie2 = lie1 `unionBags` lie2
-consLIE inst lie  = inst `consBag` lie
 plusLIEs lies	  = unionManyBags lies
 lieToList	  = bagToList
 listToLIE	  = listToBag
+
+consLIE inst lie  = lie `snocBag` inst
+-- Putting the new Inst at the *end* of the bag is a half-hearted attempt
+-- to ensure that we tend to report the *leftmost* type-constraint error
+-- E.g. 	f :: [a]
+--		f = [1,2,3]
+-- we'd like to complain about the '1', not the '3'.
+--
+-- "Half-hearted" because the rest of the type checker makes no great
+-- claims for retaining order in the constraint set.  Still, this 
+-- seems to improve matters slightly.  Exampes: mdofail001, tcfail015
 \end{code}