same type from different type arguments.
-Note [Kinding]
-~~~~~~~~~~~~~~
-The canonicalizer assumes that it's provided with well-kinded equalities
-as wanted or given, that is LHS kind and the RHS kind agree, modulo subkinding.
-
-Both canonicalization and interaction solving must preserve this invariant.
-DV: TODO TODO: Check!
-
Note [Canonical ordering for equality constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Implemented as (<+=) below:
reOrient (VarCls tv1) (FunCls {}) = not (isMetaTyVar tv1)
reOrient (VarCls {}) (OtherCls {}) = False
+reOrient (VarCls tv1) (VarCls tv2) = False
+{-
-- Variables-variables are oriented according to their kind
--- so that the invariant of CTyEqCan has the best chance of
+-- so that the following property has the best chance of
-- holding: tv ~ xi
-- * If tv is a MetaTyVar, then typeKind xi <: typeKind tv
-- a skolem, then typeKind xi = typeKind tv
-reOrient (VarCls tv1) (VarCls tv2)
+
| k1 `eqKind` k2 = False
| otherwise = k1 `isSubKind` k2
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
+-}
------------------
canEqLeaf :: CtFlavor -> CoVar
-> TypeClassifier -> TcType -> TcS CanonicalCts
-- First argument is not OtherCls
canEqLeafOriented fl cv cls1@(FunCls fn tys) s2
- | not (kindAppResult (tyConKind fn) tys `eqKind` typeKind s2 )
+ | let k1 = kindAppResult (tyConKind fn) tys,
+ let k2 = typeKind s2,
+ isGiven fl && not (k1 `eqKind` k2) -- Establish the kind invariant for CFunEqCan
= do { kindErrorTcS fl (unClassify cls1) s2
; return emptyCCan }
| otherwise
-- Otherwise, we have a variable on the left, so we flatten the RHS
-- and then do an occurs check.
canEqLeafOriented fl cv (VarCls tv) s2
- | not (k1 `eqKind` k2 || (isMetaTyVar tv && k2 `isSubKind` k1))
- -- Establish the kind invariant for CTyEqCan
+ | isGiven fl && not (k1 `eqKind` k2) -- Establish the kind invariant for CTyEqCan
= do { kindErrorTcS fl (mkTyVarTy tv) s2
; return emptyCCan }
-- * Nothing if we were not able to solve it
-- * Just wi' if we solved it, wi' (now a "given") should be put in the work list.
-- See Note [Touchables and givens]
--- Note, just passing the inerts through for the skolem equivalence classes
+-- NB: just passing the inerts through for the skolem equivalence classes
trySpontaneousSolve :: WorkItem -> InertSet -> TcS (Maybe SWorkList)
trySpontaneousSolve (CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi }) inerts
| isGiven gw
; case (tch1, tch2) of
(True, True) -> trySpontaneousEqTwoWay inerts cv gw tv1 tv2
(True, False) -> trySpontaneousEqOneWay inerts cv gw tv1 xi
- (False, True) | tyVarKind tv1 `isSubKind` tyVarKind tv2
- -> trySpontaneousEqOneWay inerts cv gw tv2 (mkTyVarTy tv1)
+ (False, True) -> trySpontaneousEqOneWay inerts cv gw tv2 (mkTyVarTy tv1)
_ -> return Nothing }
| otherwise
= do { tch1 <- isTouchableMetaTyVar tv1
trySpontaneousEqOneWay :: InertSet -> CoVar -> CtFlavor -> TcTyVar -> Xi
-> TcS (Maybe SWorkList)
-- tv is a MetaTyVar, not untouchable
--- Precondition: kind(xi) is a sub-kind of kind(tv)
trySpontaneousEqOneWay inerts cv gw tv xi
- | not (isSigTyVar tv) || isTyVarTy xi
+ | not (isSigTyVar tv) || isTyVarTy xi,
+ typeKind xi `isSubKind` tyVarKind tv
= solveWithIdentity inerts cv gw tv xi
| otherwise
= return Nothing
-- Both tyvars are *touchable* MetaTyvars
-- By the CTyEqCan invariant, k2 `isSubKind` k1
trySpontaneousEqTwoWay inerts cv gw tv1 tv2
- | k1 `eqKind` k2
+ | k1 `isSubKind` k2
, nicer_to_update_tv2 = solveWithIdentity inerts cv gw tv2 (mkTyVarTy tv1)
- | otherwise = ASSERT( k2 `isSubKind` k1 )
- solveWithIdentity inerts cv gw tv1 (mkTyVarTy tv2)
+ | k2 `isSubKind` k1
+ = solveWithIdentity inerts cv gw tv1 (mkTyVarTy tv2)
+ | otherwise = return Nothing
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}
+
+Note [Spontaneous solving and kind compatibility]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Note that our canonical constraints insist that only *given* equalities (tv ~ xi)
+or (F xis ~ rhs) require the LHS and the RHS to have exactly the same kinds.
+
+ - We have to require this because:
+ Given equalities can be freely used to rewrite inside
+ other types or constraints.
+ - We do not have to do the same for wanteds because:
+ First, wanted equations (tv ~ xi) where tv is a touchable unification variable
+ may have kinds that do not agree (the kind of xi must be a sub kind of the kind of tv).
+ Second, any potential kind mismatch will result in the constraint not being soluble,
+ which will be reported anyway. This is the reason that @trySpontaneousOneWay@ and
+ @trySpontaneousTwoWay@ will perform a kind compatibility check, and only then will
+ they proceed to @solveWithIdentity@.
+
+Caveat:
+ - Givens from higher-rank, such as:
+ type family T b :: * -> * -> *
+ type instance T Bool = (->)
+
+ f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
+ flop = f (...) True
+ Whereas we would be able to apply the type instance, we would not be able to
+ use the given (T Bool ~ (->)) in the body of 'flop'
+
Note [Loopy spontaneous solving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the original wanted:
, cc_tyargs = args1, cc_rhs = xi1 })
workItem@(CFunEqCan { cc_id = cv2, cc_flavor = fl2, cc_fun = tc2
, cc_tyargs = args2, cc_rhs = xi2 })
- | fl1 `canRewrite` fl2 && lhss_match
+ | fl1 `canSolve` fl2 && lhss_match
= do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
; mkIRStop KeepInert cans }
- | fl2 `canRewrite` fl1 && lhss_match
+ | fl2 `canSolve` fl1 && lhss_match
= do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
; mkIRContinue workItem DropInert cans }
where
inert@(CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
workItem@(CTyEqCan { cc_id = cv2, cc_flavor = fl2, cc_tyvar = tv2, cc_rhs = xi2 })
-- Check for matching LHS
- | fl1 `canRewrite` fl2 && tv1 == tv2
+ | fl1 `canSolve` fl2 && tv1 == tv2
= do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
; mkIRStop KeepInert cans }
- | fl2 `canRewrite` fl1 && tv1 == tv2
+ | fl2 `canSolve` fl1 && tv1 == tv2
= do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
; mkIRContinue workItem DropInert cans }
-- Used to ineratct two equalities of the following form:
-- First Equality: co1: (XXX ~ xi1)
-- Second Equality: cv2: (XXX ~ xi2)
--- Where the cv1 `canRewrite` cv2 equality
+-- Where the cv1 `canSolve` cv2 equality
-- We have an option of creating new work (xi1 ~ xi2) OR (xi2 ~ xi1). This
-- depends on whether the left or the right equality comes from the inert set.
-- We must:
| isDerived ifl && isDerived wfl
= noInteraction workItem
- | ifl `canRewrite` wfl
+ | ifl `canSolve` wfl
= do { unless (isGiven wfl) $ setEvBind wid (EvId iid)
-- Overwrite the binding, if one exists
-- For Givens, which are lambda-bound, nothing to overwrite,
; dischargeWorkItem }
- | otherwise -- wfl `canRewrite` ifl
+ | otherwise -- wfl `canSolve` ifl
= do { unless (isGiven ifl) $ setEvBind iid (EvId wid)
; mkIRContinue workItem DropInert emptyCCan }
mkWantedConstraints, deCanonicaliseWanted,
makeGivens, makeSolved,
- CtFlavor (..), isWanted, isGiven, isDerived, canRewrite,
+ CtFlavor (..), isWanted, isGiven, isDerived, canRewrite, canSolve,
combineCtLoc, mkGivenFlavor,
TcS, runTcS, failTcS, panicTcS, traceTcS, traceTcS0, -- Basic functionality
| CTyEqCan { -- tv ~ xi (recall xi means function free)
-- Invariant:
-- * tv not in tvs(xi) (occurs check)
- -- * If tv is a MetaTyVar, then typeKind xi <: typeKind tv
- -- a skolem, then typeKind xi = typeKind tv
+ -- * If constraint is given then typeKind xi == typeKind tv
+ -- See Note [Spontaneous solving and kind compatibility]
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_tyvar :: TcTyVar,
-- Invariant: * isSynFamilyTyCon cc_fun
-- * cc_rhs is not a touchable unification variable
-- See Note [No touchables as FunEq RHS]
- -- * typeKind (TyConApp cc_fun cc_tyargs) == typeKind cc_rhs
+ -- * If constraint is given then
+ -- typeKind (TyConApp cc_fun cc_tyargs) == typeKind cc_rhs
cc_id :: EvVar,
cc_flavor :: CtFlavor,
cc_fun :: TyCon, -- A type function
isDerived (Derived {}) = True
isDerived _ = False
+canSolve :: CtFlavor -> CtFlavor -> Bool
+-- canSolve ctid1 ctid2
+-- The constraint ctid1 can be used to solve ctid2
+canSolve (Given {}) _ = True
+canSolve (Derived {}) (Wanted {}) = True
+canSolve (Derived {}) (Derived {}) = True
+canSolve (Wanted {}) (Wanted {}) = True
+canSolve _ _ = False
+
canRewrite :: CtFlavor -> CtFlavor -> Bool
-- canRewrite ctid1 ctid2
--- The constraint ctid1 can be used to rewrite ctid2
+-- The *equality* constraint ctid1 can be used to rewrite inside ctid2
canRewrite (Given {}) _ = True
canRewrite (Derived {}) (Wanted {}) = True
canRewrite (Derived {}) (Derived {}) = True
-canRewrite (Wanted {}) (Wanted {}) = True
+ -- Never use a wanted to rewrite anything!
+canRewrite (Wanted {}) (Wanted {}) = False
canRewrite _ _ = False
combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc
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