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

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 708b8e4..95250f8 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -849,10 +849,11 @@ bindIrredsR loc qtvs co_vars reft givens irreds
                -- The givens can include methods
                -- See Note [Pruning the givens in an implication constraint]
 
-          -- If there are no 'givens', then it's safe to 
+          -- If there are no 'givens' *and* the refinement is empty
+          -- (the refinement is like more givens), then it's safe to 
           -- partition the 'wanteds' by their qtvs, thereby trimming irreds
           -- See Note [Freeness and implications]
-       ; irreds' <- if null givens'
+       ; irreds' <- if null givens' && isEmptyRefinement reft
                     then do
                        { let qtv_set = mkVarSet qtvs
                              (frees, real_irreds) = partition (isFreeWrtTyVars qtv_set) irreds
@@ -862,7 +863,8 @@ bindIrredsR loc qtvs co_vars reft givens irreds
        
        ; let all_tvs = qtvs ++ co_vars -- Abstract over all these
        ; (implics, bind) <- makeImplicationBind loc all_tvs reft givens' irreds'
-                               -- This call does the real work
+                       -- This call does the real work
+                       -- If irreds' is empty, it does something sensible
        ; extendLIEs implics
        ; return bind } 
 
@@ -875,6 +877,8 @@ makeImplicationBind :: InstLoc -> [TcTyVar] -> Refinement
 -- The binding looks like
 --     (ir1, .., irn) = f qtvs givens
 -- where f is (evidence for) the new implication constraint
+--     f :: forall qtvs. {reft} givens => (ir1, .., irn)
+-- qtvs includes coercion variables
 --
 -- This binding must line up the 'rhs' in reduceImplication
 makeImplicationBind loc all_tvs reft
@@ -2256,7 +2260,12 @@ disambiguate extended_defaulting insts
                                        -- use [Integer, Double]
                                do { integer_ty <- tcMetaTy integerTyConName
                                   ; checkWiredInTyCon doubleTyCon
-                                  ; return [integer_ty, doubleTy] }
+                                  ; string_ty <- tcMetaTy stringTyConName
+                                  ; ovl_str <- doptM Opt_OverloadedStrings
+                                  ; if ovl_str         -- Add String if -foverloaded-strings
+                                       then return [integer_ty,doubleTy,string_ty] 
+                                       else return [integer_ty,doubleTy] }
+
        ; traceTc (text "disambigutate" <+> vcat [ppr unaries, ppr bad_tvs, ppr defaultable_groups])
        ; mapM_ (disambigGroup default_tys) defaultable_groups  }
   where
@@ -2278,13 +2287,13 @@ disambiguate extended_defaulting insts
 
    defaultable_classes clss 
        | extended_defaulting = any isInteractiveClass clss
-       | otherwise = all isStandardClass clss && any isNumericClass clss
+       | otherwise = all isStandardClass clss && (any isNumericClass clss || any ((== isStringClassKey) . classKey) clss)
 
        -- In interactive mode, or with -fextended-default-rules,
        -- we default Show a to Show () to avoid graututious errors on "show []"
    isInteractiveClass cls 
        = isNumericClass cls
-       || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey])
+       || (classKey cls `elem` [showClassKey, eqClassKey, ordClassKey, isStringClassKey])
 
 
 disambigGroup :: [Type]                        -- The default types