\begin{code}
module TcInteract (
solveInteract, AtomicInert,
- InertSet, emptyInert, extendInertSet, extractUnsolved, solveOne,
+ InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
listToWorkList
) where
import TypeRep
import Id
+import VarEnv
import Var
import TcType
import TcRnTypes
import TcErrors
import TcSMonad
-import qualified Bag as Bag
+import Bag
import qualified Data.Map as Map
import Maybes
import DynFlags
\end{code}
-Note [InsertSet invariants]
+Note [InertSet invariants]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
An InertSet is a bag of canonical constraints, with the following invariants:
Note that we must switch wanted inert items to given when going under an
implication constraint (when in top-level inference mode).
+Note [InertSet FlattenSkolemEqClass]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The inert_fsks field of the inert set contains an "inverse map" of all the
+flatten skolem equalities in the inert set. For instance, if inert_cts looks
+like this:
+
+ fsk1 ~ fsk2
+ fsk3 ~ fsk2
+ fsk4 ~ fsk5
+
+Then, the inert_fsks fields holds the following map:
+ fsk2 |-> { fsk1, fsk3 }
+ fsk5 |-> { fsk4 }
+Along with the necessary coercions to convert fsk1 and fsk3 back to fsk2
+and fsk4 back to fsk5. Hence, the invariants of the inert_fsks field are:
+
+ (a) All TcTyVars in the domain and range of inert_fsks are flatten skolems
+ (b) All TcTyVars in the domain of inert_fsk occur naked as rhs in some
+ equalities of inert_cts
+ (c) For every mapping fsk1 |-> { (fsk2,co), ... } it must be:
+ co : fsk2 ~ fsk1
+
+The role of the inert_fsks is to make it easy to maintain the equivalence
+class of each flatten skolem, which is much needed to correctly do spontaneous
+solving. See Note [Loopy Spontaneous Solving]
\begin{code}
-- See Note [InertSet invariants]
data InertSet
= IS { inert_cts :: Bag.Bag CanonicalCt
, inert_fsks :: Map.Map TcTyVar [(TcTyVar,Coercion)] }
--- inert_fsks contains the *FlattenSkolem* equivalence classes.
--- inert_fsks extra invariants:
--- (a) all TcTyVars in the domain and range of inert_fsks are flatten skolems
--- (b) for every mapping tv1 |-> (tv2,co), co : tv2 ~ tv1
+ -- See Note [InertSet FlattenSkolemEqClass]
--- newtype InertSet = IS (Bag.Bag CanonicalCt)
instance Outputable InertSet where
ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_cts is))
, vcat (map (\(v,rest) -> ppr v <+> text "|->" <+> hsep (map (ppr.fst) rest))
)
]
-
-
emptyInert :: InertSet
emptyInert = IS { inert_cts = Bag.emptyBag, inert_fsks = Map.empty }
-
-extendInertSet :: InertSet -> AtomicInert -> InertSet
--- Simply extend the bag of constraints rebuilding an inert set
-extendInertSet (IS { inert_cts = cts
- , inert_fsks = fsks }) item
- = IS { inert_cts = cts `Bag.snocBag` item
- , inert_fsks = fsks }
-
-
updInertSet :: InertSet -> AtomicInert -> InertSet
-- Introduces an element in the inert set for the first time
updInertSet (IS { inert_cts = cts, inert_fsks = fsks })
FlatSkol {} <- tcTyVarDetails tv2
= let cts' = cts `Bag.snocBag` item
fsks' = Map.insertWith (++) tv2 [(tv1, mkCoVarCoercion cv)] fsks
+ -- See Note [InertSet FlattenSkolemEqClass]
in IS { inert_cts = cts', inert_fsks = fsks' }
updInertSet (IS { inert_cts = cts
, inert_fsks = fsks }) item
= let cts' = cts `Bag.snocBag` item
in IS { inert_cts = cts', inert_fsks = fsks }
-
foldlInertSetM :: (Monad m) => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
foldlInertSetM k z (IS { inert_cts = cts })
= Bag.foldlBagM k z cts
getFskEqClass :: InertSet -> TcTyVar -> [(TcTyVar,Coercion)]
--- Precondition: tv is a FlatSkol
+-- Precondition: tv is a FlatSkol. See Note [InertSet FlattenSkolemEqClass]
getFskEqClass (IS { inert_cts = cts, inert_fsks = fsks }) tv
= case lkpTyEqCanByLhs of
Nothing -> fromMaybe [] (Map.lookup tv fsks)
, sr_inerts = inerts
, sr_stop = Stop }
}
-{--
+
+{-- This is all old code, but does not quite work now. The problem is that due to
+ Note [Loopy Spontaneous Solving] we may have unflattened a type, to be able to
+ perform a sneaky unification. This unflattening means that we may have to recanonicalize
+ a given (solved) equality, this is why the result of trySpontaneousSolve is now a list
+ of constraints (instead of an atomic solved constraint). We would have to react all of
+ them once again with the worklist but that is very tiresome. Instead we throw them back
+ in the worklist.
+
| isWantedCt workItem
-- Original was wanted we have now made him given so
-- we have to ineract him with the inerts again because
-- Note, 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
+ = return Nothing
| Just tv2 <- tcGetTyVar_maybe xi
= do { tch1 <- isTouchableMetaTyVar tv1
; tch2 <- isTouchableMetaTyVar tv2
it and keep it as wanted. In inference mode we'll end up quantifying over
(alpha ~ Maybe (E alpha))
Hence, 'solveWithIdentity' performs a small occurs check before
-actually solving. But this occurs check *must look through* flatten
-skolems.
+actually solving. But this occurs check *must look through* flatten skolems.
+
+However, it may be the case that the flatten skolem in hand is equal to some other
+flatten skolem whith *does not* mention our unification variable. Here's a typical example:
+
+Original wanteds:
+ g: F alpha ~ F beta
+ w: alpha ~ F alpha
+After canonicalization:
+ g: F beta ~ f1
+ g: F alpha ~ f1
+ w: alpha ~ f2
+ g: F alpha ~ f2
+After some reactions:
+ g: f1 ~ f2
+ g: F beta ~ f1
+ w: alpha ~ f2
+ g: F alpha ~ f2
+At this point, we will try to spontaneously solve (alpha ~ f2) which remains as yet unsolved.
+We will look inside f2, which immediately mentions (F alpha), so it's not good to unify! However
+by looking at the equivalence class of the flatten skolems, we can see that it is fine to
+unify (alpha ~ f1) which solves our goals!
+
+A similar problem happens because of other spontaneous solving. Suppose we have the
+following wanteds, arriving in this exact order:
+ (first) w: beta ~ alpha
+ (second) w: alpha ~ fsk
+ (third) g: F beta ~ fsk
+Then, we first spontaneously solve the first constraint, making (beta := alpha), and having
+(beta ~ alpha) as given. *Then* we encounter the second wanted (alpha ~ fsk). "fsk" does not
+obviously mention alpha, so naively we can also spontaneously solve (alpha := fsk). But
+that is wrong since fsk mentions beta, which has already secretly been unified to alpha!
+
+To avoid this problem, the same occurs check must unveil rewritings that can happen because
+of spontaneously having solved other constraints.
+
Note [Avoid double unifications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
variable *on the left* of the equality. Here is what happens if not:
Original wanted: (a ~ alpha), (alpha ~ Int)
We spontaneously solve the first wanted, without changing the order!
- given : a ~ alpha [having unifice alpha := a]
+ given : a ~ alpha [having unified alpha := a]
Now the second wanted comes along, but he cannot rewrite the given, so we simply continue.
At the end we spontaneously solve that guy, *reunifying* [alpha := Int]
-We avoid this problem by orienting the given so that the unification variable is on the left.
-[Note that alternatively we could attempt to enforce this at canonicalization]
+We avoid this problem by orienting the given so that the unification
+variable is on the left. [Note that alternatively we could attempt to
+enforce this at canonicalization]
-Avoiding double unifications is yet another reason to disallow touchable unification variables
-as RHS of type family equations: F xis ~ alpha. Consider having already spontaneously solved
-a wanted (alpha ~ [b]) by setting alpha := [b]. So the inert set looks like:
- given : alpha ~ [b]
-And now a new wanted (F tau ~ alpha) comes along. Since it does not react with anything
-we will be left with a constraint (F tau ~ alpha) that must cause a unification of
-(alpha := F tau) at some point (either in spontaneous solving, or at the end). But alpha
-is *already* unified so we must not do anything to it. By disallowing naked touchables in
-the RHS of constraints (in favor of introduced flatten skolems) we do not have to worry at
-all about unifying or spontaneously solving (F xis ~ alpha) by unification.
+See also Note [No touchables as FunEq RHS] in TcSMonad; avoiding
+double unifications is the main reason we disallow touchable
+unification variables as RHS of type family equations: F xis ~ alpha.
\begin{code}
----------------
-> TcS (Maybe SWorkList)
-- Solve with the identity coercion
-- Precondition: kind(xi) is a sub-kind of kind(tv)
--- See [New Wanted Superclass Work] to see why we do this for *given* as well
+-- Precondition: CtFlavor is Wanted or Derived
+-- See [New Wanted Superclass Work] to see why solveWithIdentity
+-- must work for Derived as well as Wanted
solveWithIdentity inerts cv gw tv xi
- | not (isGiven gw)
- = do { m <- passOccursCheck inerts tv xi
- ; case m of
- Nothing -> return Nothing
- Just (xi_unflat,coi) -- coi : xi_unflat ~ xi
- -> do { traceTcS "Sneaky unification:" $
+ = do { tybnds <- getTcSTyBindsMap
+ ; case occurCheck tybnds inerts tv xi of
+ Nothing -> return Nothing
+ Just (xi_unflat,coi) -> solve_with xi_unflat coi }
+ where
+ solve_with xi_unflat coi -- coi : xi_unflat ~ xi
+ = do { traceTcS "Sneaky unification:" $
vcat [text "Coercion variable: " <+> ppr gw,
text "Coercion: " <+> pprEq (mkTyVarTy tv) xi,
text "Left Kind is : " <+> ppr (typeKind (mkTyVarTy tv)),
text "Right Kind is : " <+> ppr (typeKind xi)
- ]
- ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat
- ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat
- ; let flav = mkGivenFlavor gw UnkSkol
- ; (cts, co) <- case coi of
- ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat
- ; return (can_eqs, co) }
- IdCo co -> return $
- (singleCCan (CTyEqCan { cc_id = cv_given
- , cc_flavor = mkGivenFlavor gw UnkSkol
- , cc_tyvar = tv, cc_rhs = xi }
- -- xi, *not* xi_unflat!
- ), co)
- ; case gw of
- Wanted {} -> setWantedCoBind cv co
- Derived {} -> setDerivedCoBind cv co
- _ -> pprPanic "Can't spontaneously solve *given*" empty
-
- -- See Note [Avoid double unifications]
-
- -- The reason that we create a new given variable (cv_given) instead of reusing cv
- -- is because we do not want to end up with coercion unification variables in the givens.
- ; return (Just cts) }
- }
- | otherwise
- = return Nothing
-
-
-passOccursCheck :: InertSet -> TcTyVar -> TcType -> TcS (Maybe (TcType,CoercionI))
--- passOccursCheck inerts tv ty
--- Traverse the type and make sure that 'tv' does not appear under
--- some flatten skolem. If it appears under some flatten skolem
--- look in that flatten skolem equivalence class to see if you can
--- find a different flatten skolem to use, which does not mention the
--- variable.
--- Postcondition: Just (ty',coi) <- passOccursCheck tv ty
+ ]
+ ; setWantedTyBind tv xi_unflat -- Set tv := xi_unflat
+ ; cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat
+ ; let flav = mkGivenFlavor gw UnkSkol
+ ; (cts, co) <- case coi of
+ ACo co -> do { can_eqs <- canEq flav cv_given (mkTyVarTy tv) xi_unflat
+ ; return (can_eqs, co) }
+ IdCo co -> return $
+ (singleCCan (CTyEqCan { cc_id = cv_given
+ , cc_flavor = mkGivenFlavor gw UnkSkol
+ , cc_tyvar = tv, cc_rhs = xi }
+ -- xi, *not* xi_unflat because
+ -- xi_unflat may require flattening!
+ ), co)
+ ; case gw of
+ Wanted {} -> setWantedCoBind cv co
+ Derived {} -> setDerivedCoBind cv co
+ _ -> pprPanic "Can't spontaneously solve *given*" empty
+ -- See Note [Avoid double unifications]
+ ; return (Just cts) }
+
+occurCheck :: VarEnv (TcTyVar, TcType) -> InertSet
+ -> TcTyVar -> TcType -> Maybe (TcType,CoercionI)
+-- Traverse @ty@ to make sure that @tv@ does not appear under some flatten skolem.
+-- If it appears under some flatten skolem look in that flatten skolem equivalence class
+-- (see Note [InertSet FlattenSkolemEqClass], [Loopy Spontaneous Solving]) to see if you
+-- can find a different flatten skolem to use, that is, one that does not mention @tv@.
+--
+-- Postcondition: Just (ty', coi) = occurCheck binds inerts tv ty
-- coi :: ty' ~ ty
--- NB: I believe there is no need to do the tcView thing here
-passOccursCheck is tv (TyConApp tc tys)
- = do { tys_mbs <- mapM (passOccursCheck is tv) tys
- ; case allMaybes tys_mbs of
- Nothing -> return Nothing
- Just tys_cois ->
- let (tys',cois') = unzip tys_cois
- in return $
- Just (TyConApp tc tys', mkTyConAppCoI tc cois')
- }
-passOccursCheck is tv (PredTy sty)
- = do { sty_mb <- passOccursCheckPred tv sty
- ; case sty_mb of
- Nothing -> return Nothing
- Just (sty',coi) -> return (Just (PredTy sty', coi))
- }
- where passOccursCheckPred tv (ClassP cn tys)
- = do { tys_mbs <- mapM (passOccursCheck is tv) tys
- ; case allMaybes tys_mbs of
- Nothing -> return Nothing
- Just tys_cois ->
- let (tys', cois') = unzip tys_cois
- in return $
- Just (ClassP cn tys', mkClassPPredCoI cn cois')
- }
- passOccursCheckPred tv (IParam nm ty)
- = do { mty <- passOccursCheck is tv ty
- ; case mty of
- Nothing -> return Nothing
- Just (ty',co')
- -> return (Just (IParam nm ty',
- mkIParamPredCoI nm co'))
- }
- passOccursCheckPred tv (EqPred ty1 ty2)
- = do { mty1 <- passOccursCheck is tv ty1
- ; mty2 <- passOccursCheck is tv ty2
- ; case (mty1,mty2) of
- (Just (ty1',coi1), Just (ty2',coi2))
- -> return $
- Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2)
- _ -> return Nothing
- }
-
-passOccursCheck is tv (FunTy arg res)
- = do { arg_mb <- passOccursCheck is tv arg
- ; res_mb <- passOccursCheck is tv res
- ; case (arg_mb,res_mb) of
- (Just (arg',coiarg), Just (res',coires))
- -> return $
- Just (FunTy arg' res', mkFunTyCoI coiarg coires)
- _ -> return Nothing
- }
-
-passOccursCheck is tv (AppTy fun arg)
- = do { fun_mb <- passOccursCheck is tv fun
- ; arg_mb <- passOccursCheck is tv arg
- ; case (fun_mb,arg_mb) of
- (Just (fun',coifun), Just (arg',coiarg))
- -> return $
- Just (AppTy fun' arg', mkAppTyCoI coifun coiarg)
- _ -> return Nothing
- }
-
-passOccursCheck is tv (ForAllTy tv1 ty1)
- = do { ty1_mb <- passOccursCheck is tv ty1
- ; case ty1_mb of
- Nothing -> return Nothing
- Just (ty1',coi)
- -> return $
- Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi)
- }
-
-passOccursCheck _is tv (TyVarTy tv')
- | tv == tv'
- = return Nothing
-
-passOccursCheck is tv (TyVarTy fsk)
- | FlatSkol ty <- tcTyVarDetails fsk
- = do { zty <- zonkFlattenedType ty -- Must zonk as it contains unif. vars
- ; occ <- passOccursCheck is tv zty
- ; case occ of
- Nothing -> go_down_eq_class $ getFskEqClass is fsk
- Just (zty',ico) -> return $ Just (zty',ico)
- }
- where go_down_eq_class [] = return Nothing
- go_down_eq_class ((fsk1,co1):rest)
- = do { occ1 <- passOccursCheck is tv (TyVarTy fsk1)
- ; case occ1 of
- Nothing -> go_down_eq_class rest
- Just (ty1,co1i')
- -> return $ Just (ty1, mkTransCoI co1i' (ACo co1)) }
-passOccursCheck _is _tv ty
- = return (Just (ty,IdCo ty))
-
-{--
-Problematic situation:
-~~~~~~~~~~~~~~~~~~~~~~
- Suppose we have a flatten skolem f1 := F f6
- Suppose we are chasing for 'alpha', and:
- f6 := G alpha with eq.class f7,f8
-
- Then we will return F f7 potentially.
---}
-
-
+-- NB: The returned type ty' may not be flat!
+occurCheck ty_binds inerts the_tv the_ty
+ = ok emptyVarSet the_ty
+ where
+ -- If (fsk `elem` bad) then tv occurs in any rendering
+ -- of the type under the expansion of fsk
+ ok bad this_ty@(TyConApp tc tys)
+ | Just tys_cois <- allMaybes (map (ok bad) tys)
+ , (tys',cois') <- unzip tys_cois
+ = Just (TyConApp tc tys', mkTyConAppCoI tc cois')
+ | isSynTyCon tc, Just ty_expanded <- tcView this_ty
+ = ok bad ty_expanded -- See Note [Type synonyms and the occur check] in TcUnify
+ ok bad (PredTy sty)
+ | Just (sty',coi) <- ok_pred bad sty
+ = Just (PredTy sty', coi)
+ ok bad (FunTy arg res)
+ | Just (arg', coiarg) <- ok bad arg, Just (res', coires) <- ok bad res
+ = Just (FunTy arg' res', mkFunTyCoI coiarg coires)
+ ok bad (AppTy fun arg)
+ | Just (fun', coifun) <- ok bad fun, Just (arg', coiarg) <- ok bad arg
+ = Just (AppTy fun' arg', mkAppTyCoI coifun coiarg)
+ ok bad (ForAllTy tv1 ty1)
+ -- WARNING: What if it is a (t1 ~ t2) => t3? It's not handled properly at the moment.
+ | Just (ty1', coi) <- ok bad ty1
+ = Just (ForAllTy tv1 ty1', mkForAllTyCoI tv1 coi)
+
+ -- Variable cases
+ ok bad this_ty@(TyVarTy tv)
+ | tv == the_tv = Nothing -- Occurs check error
+ | not (isTcTyVar tv) = Just (this_ty, IdCo this_ty) -- Bound var
+ | FlatSkol zty <- tcTyVarDetails tv = ok_fsk bad tv zty
+ | Just (_,ty) <- lookupVarEnv ty_binds tv = ok bad ty
+ | otherwise = Just (this_ty, IdCo this_ty)
+
+ -- Check if there exists a ty bind already, as a result of sneaky unification.
+ -- Fall through
+ ok _bad _ty = Nothing
+
+ -----------
+ ok_pred bad (ClassP cn tys)
+ | Just tys_cois <- allMaybes $ map (ok bad) tys
+ = let (tys', cois') = unzip tys_cois
+ in Just (ClassP cn tys', mkClassPPredCoI cn cois')
+ ok_pred bad (IParam nm ty)
+ | Just (ty',co') <- ok bad ty
+ = Just (IParam nm ty', mkIParamPredCoI nm co')
+ ok_pred bad (EqPred ty1 ty2)
+ | Just (ty1',coi1) <- ok bad ty1, Just (ty2',coi2) <- ok bad ty2
+ = Just (EqPred ty1' ty2', mkEqPredCoI coi1 coi2)
+ ok_pred _ _ = Nothing
+
+ -----------
+ ok_fsk bad fsk zty
+ | fsk `elemVarSet` bad
+ -- We are already trying to find a rendering of fsk,
+ -- and to do that it seems we need a rendering, so fail
+ = Nothing
+ | otherwise
+ = firstJusts (ok new_bad zty : map (go_under_fsk new_bad) fsk_equivs)
+ where
+ fsk_equivs = getFskEqClass inerts fsk
+ new_bad = bad `extendVarSetList` (fsk : map fst fsk_equivs)
+
+ -----------
+ go_under_fsk bad_tvs (fsk,co)
+ | FlatSkol zty <- tcTyVarDetails fsk
+ = case ok bad_tvs zty of
+ Nothing -> Nothing
+ Just (ty,coi') -> Just (ty, mkTransCoI coi' (ACo co))
+ | otherwise = pprPanic "go_down_equiv" (ppr fsk)
\end{code}
| fl2 `canRewrite` fl1 && tv2 `elemVarSet` tyVarsOfType xi1
= do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1)
; mkIRContinue workItem DropInert rewritten_eq }
--- Finally, if workitem is a flatten equivalence class constraint and the
--- inert is a wanted constraints, even when the workitem cannot rewrite the
--- inert, drop the inert out because you may have to reconsider solving him
--- using the equivalence class you created.
+
+-- Finally, if workitem is a Flatten Equivalence Class constraint and the
+-- inert is a wanted constraint, even when the workitem cannot rewrite the
+-- inert, drop the inert out because you may have to reconsider solving the
+-- inert *using* the equivalence class you created. See note [Loopy Spontaneous Solving]
+-- and [InertSet FlattenSkolemEqClass]
| not $ isGiven fl1, -- The inert is wanted or derived
isMetaTyVar tv1, -- and has a unification variable lhs
= mkIRContinue workItem DropInert (singletonWorkList inert)
--- Fall-through case for all other cases
+-- Fall-through case for all other situations
doInteractWithInert _ workItem = noInteraction workItem
---------------------------------------------
-combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc
--- Precondition: At least one of them should be wanted
-combineCtLoc (Wanted loc) _ = loc
-combineCtLoc _ (Wanted loc) = loc
-combineCtLoc _ _ = panic "Expected one of wanted constraints (BUG)"
-
-
+-------------------------
-- Equational Rewriting
rewriteDict :: (CoVar, TcTyVar, Xi) -> (DictId, CtFlavor, Class, [Xi]) -> TcS CanonicalCt
rewriteDict (cv,tv,xi) (dv,gw,cl,xis)
mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
; mkCanonical gw cv2' }
--- ->
--- if isWanted gw then
--- do { cv2' <- newWantedCoVar xi1 xi2
--- ; setWantedCoBind cv2 $
--- co1 `mkTransCoercion` mkCoVarCoercion cv2'
--- ; return cv2' }
--- else newGivOrDerCoVar xi1 xi2 $
--- mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
--- ; mkCanonical gw cv2' }
solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
Now suppose that we have:
given: C a b
wanted: C a beta
- By interacting the given we will get that (F a ~ b) which is not
+ By interacting the given we will get given (F a ~ b) which is not
enough by itself to make us discharge (C a beta). However, we
- may create a new given equality from the super-class that we promise
- to solve: (F a ~ beta). Now we may interact this with the rest of
- constraint to finally get:
- given : beta ~ b
-
+ may create a new derived equality from the super-class of the
+ wanted constraint (C a beta), namely derived (F a ~ beta).
+ Now we may interact this with given (F a ~ b) to get:
+ derived : beta ~ b
But 'beta' is a touchable unification variable, and hence OK to
- unify it with 'b', replacing the given evidence with the identity.
+ unify it with 'b', replacing the derived evidence with the identity.
- This requires trySpontaneousSolve to solve given equalities that
- have a touchable in their RHS, *in addition* to solving wanted
- equalities.
+ This requires trySpontaneousSolve to solve *derived*
+ equalities that have a touchable in their RHS, *in addition*
+ to solving wanted equalities.
Here is another example where this is useful.
}
}
\end{code}
-
-
projects / ghc-hetmet.git / blobdiff