Gather constraints in program order

[ghc-hetmet.git] / compiler / typecheck / TcRnTypes.lhs

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index d01710c..b624a14 100644 (file)
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -30,7 +30,7 @@ module TcRnTypes(
        -- Insts
        Inst(..), InstOrigin(..), InstLoc(..), 
        pprInstLoc, pprInstArising, instLocSpan, instLocOrigin,
-       LIE, emptyLIE, unitLIE, plusLIE, consLIE, 
+       LIE, emptyLIE, unitLIE, plusLIE, consLIE, instLoc, instSpan,
        plusLIEs, mkLIE, isEmptyLIE, lieToList, listToLIE,
 
        -- Misc other types
@@ -702,10 +702,20 @@ 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}
 
 
@@ -726,6 +736,12 @@ functions that deal with it.
 -------------------------------------------
 data InstLoc = InstLoc InstOrigin SrcSpan ErrCtxt
 
+instLoc :: Inst -> InstLoc
+instLoc inst = tci_loc inst
+
+instSpan :: Inst -> SrcSpan
+instSpan wanted = instLocSpan (instLoc wanted)
+
 instLocSpan :: InstLoc -> SrcSpan
 instLocSpan (InstLoc _ s _) = s