Refactor, improve, and document the deriving mechanism

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

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 007a717..62a7151 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -1680,7 +1680,11 @@ reduceContext env wanteds
 
        ; let givens                    = red_givens env
              (given_eqs0,given_dicts0) = partitionGivenEqInsts  givens
-             (wanted_eqs,wanted_dicts) = partitionWantedEqInsts wanteds
+             (wanted_eqs0,wanted_dicts) = partitionWantedEqInsts wanteds
+
+       ; wanted_ancestor_eqs <- (mapM wantedAncestorEqualities wanted_dicts >>= \ls -> return (concat ls))
+       ; traceTc (text "test wanted SCs" <+> ppr wanted_ancestor_eqs)
+       ; let wanted_eqs = wanted_ancestor_eqs ++ wanted_eqs0
 
        ; -- 1. Normalise the *given* *equality* constraints
          (given_eqs,eliminate_skolems) <- normaliseGivens given_eqs0
@@ -2476,6 +2480,31 @@ addSCs is_loop avails dict
     is_given sc_dict = case findAvail avails sc_dict of
                          Just (Given _) -> True        -- Given is cheaper than superclass selection
                          other          -> False       
+
+
+wantedAncestorEqualities :: Inst -> TcM [Inst]
+wantedAncestorEqualities dict 
+  | isClassDict dict
+  = mapM mkWantedEqInst $ filter isEqPred $ bagToList $ wantedAncestorEqualities' (dictPred dict) emptyBag
+  | otherwise
+  = return []
+
+wantedAncestorEqualities' :: PredType -> Bag PredType -> Bag PredType
+wantedAncestorEqualities' pred bag
+  = ASSERT( isClassPred pred )
+    let (clas, tys)             = getClassPredTys pred 
+        (tyvars, sc_theta, _, _) = classBigSig clas
+        sc_theta'               = substTheta (zipTopTvSubst tyvars tys) sc_theta
+        add_sc bag sc_pred
+          | elemBag sc_pred bag  = bag
+         | not (isEqPred sc_pred)
+            && not (isClassPred sc_pred)
+                                = bag
+         | isEqPred sc_pred     = consBag sc_pred bag
+          | otherwise           = let bag' = consBag sc_pred bag
+                                  in wantedAncestorEqualities' sc_pred bag'
+    in foldl add_sc bag sc_theta'
+
 \end{code}
 
 %************************************************************************
@@ -2740,7 +2769,6 @@ tcSimplifyDeriv :: InstOrigin
                -> TcM ThetaType        -- Needed
 -- Given  instance (wanted) => C inst_ty 
 -- Simplify 'wanted' as much as possible
--- The inst_ty is needed only for the termination check
 
 tcSimplifyDeriv orig tyvars theta
   = do { (tvs, _, tenv) <- tcInstTyVars tyvars
@@ -2750,8 +2778,9 @@ tcSimplifyDeriv orig tyvars theta
        ; wanteds <- newDictBndrsO orig (substTheta tenv theta)
        ; (irreds, _) <- tryHardCheckLoop doc wanteds
 
-       ; let (tv_dicts, others) = partition isTyVarDict irreds
+       ; let (tv_dicts, others) = partition ok irreds
        ; addNoInstanceErrs others
+       -- See Note [Exotic derived instance contexts] in TcMType
 
        ; let rev_env = zipTopTvSubst tvs (mkTyVarTys tyvars)
              simpl_theta = substTheta rev_env (map dictPred tv_dicts)
@@ -2761,49 +2790,10 @@ tcSimplifyDeriv orig tyvars theta
        ; return simpl_theta }
   where
     doc = ptext SLIT("deriving classes for a data type")
-\end{code}
-
-Note [Exotic derived instance contexts]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider
-       data T a b c = MkT (Foo a b c) deriving( Eq )
-       instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
-
-Notice that this instance (just) satisfies the Paterson termination 
-conditions.  Then we *could* derive an instance decl like this:
-
-       instance (C Int a, Eq b, Eq c) => Eq (T a b c) 
-
-even though there is no instance for (C Int a), because there just
-*might* be an instance for, say, (C Int Bool) at a site where we
-need the equality instance for T's.  
 
-However, this seems pretty exotic, and it's quite tricky to allow
-this, and yet give sensible error messages in the (much more common)
-case where we really want that instance decl for C.
-
-So for now we simply require that the derived instance context
-should have only type-variable constraints.
-
-Here is another example:
-       data Fix f = In (f (Fix f)) deriving( Eq )
-Here, if we are prepared to allow -fallow-undecidable-instances we
-could derive the instance
-       instance Eq (f (Fix f)) => Eq (Fix f)
-but this is so delicate that I don't think it should happen inside
-'deriving'. If you want this, write it yourself!
-
-NB: if you want to lift this condition, make sure you still meet the
-termination conditions!  If not, the deriving mechanism generates
-larger and larger constraints.  Example:
-  data Succ a = S a
-  data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
-
-Note the lack of a Show instance for Succ.  First we'll generate
-  instance (Show (Succ a), Show a) => Show (Seq a)
-and then
-  instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
-and so on.  Instead we want to complain of no instance for (Show (Succ a)).
+    ok dict | isDict dict = validDerivPred (dictPred dict)
+           | otherwise   = False
+\end{code}
 
 
 @tcSimplifyDefault@ just checks class-type constraints, essentially;