\begin{code}
module TcInteract (
- solveInteract, AtomicInert,
- InertSet, emptyInert, updInertSet, extractUnsolved, solveOne
+ solveInteract, solveInteractGiven, solveInteractWanted,
+ AtomicInert, tyVarsOfInert,
+ InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
) where
#include "HsVersions.h"
import TcCanonical
import VarSet
import Type
-import TypeRep
import Id
-import VarEnv
import Var
import TcType
-import HsBinds
+import HsBinds
-import InstEnv
-import Class
-import TyCon
+import Inst( tyVarsOfEvVar )
+import Class
+import TyCon
import Name
import FunDeps
-import Control.Monad ( when )
-
import Coercion
import Outputable
-import TcRnTypes
+import TcRnTypes
import TcErrors
-import TcSMonad
+import TcSMonad
import Bag
-import qualified Data.Map as Map
-import Maybes
+import qualified Data.Map as Map
+
+import Control.Monad( when )
-import Control.Monad( zipWithM, unless )
import FastString ( sLit )
import DynFlags
\end{code}
Note [InertSet invariants]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
An InertSet is a bag of canonical constraints, with the following invariants:
1 No two constraints react with each other.
given LHS's occur in any of the given RHS's or reactant parts]
3 Wanted equalities also form an idempotent substitution
+
4 The entire set of equalities is acyclic.
5 Wanted dictionaries are inert with the top-level axiom set
6 Equalities of the form tv1 ~ tv2 always have a touchable variable
on the left (if possible).
- 7 No wanted constraints tv1 ~ tv2 with tv1 touchable. Such constraints
+
+ 7 No wanted constraints tv1 ~ tv2 with tv1 touchable. Such constraints
will be marked as solved right before being pushed into the inert set.
See note [Touchables and givens].
+
+ 8 No Given constraint mentions a touchable unification variable,
+ except if the
Note that 6 and 7 are /not/ enforced by canonicalization but rather by
insertion in the inert list, ie by TcInteract.
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}
+data CCanMap a = CCanMap { cts_given :: Map.Map a CanonicalCts
+ -- Invariant: all Given
+ , cts_derived :: Map.Map a CanonicalCts
+ -- Invariant: all Derived
+ , cts_wanted :: Map.Map a CanonicalCts }
+ -- Invariant: all Wanted
+
+cCanMapToBag :: Ord a => CCanMap a -> CanonicalCts
+cCanMapToBag cmap = Map.fold unionBags rest_wder (cts_given cmap)
+ where rest_wder = Map.fold unionBags rest_der (cts_wanted cmap)
+ rest_der = Map.fold unionBags emptyCCan (cts_derived cmap)
+
+emptyCCanMap :: CCanMap a
+emptyCCanMap = CCanMap { cts_given = Map.empty
+ , cts_derived = Map.empty, cts_wanted = Map.empty }
+
+updCCanMap:: Ord a => (a,CanonicalCt) -> CCanMap a -> CCanMap a
+updCCanMap (a,ct) cmap
+ = case cc_flavor ct of
+ Wanted {}
+ -> cmap { cts_wanted = Map.insertWith unionBags a this_ct (cts_wanted cmap) }
+ Given {}
+ -> cmap { cts_given = Map.insertWith unionBags a this_ct (cts_given cmap) }
+ Derived {}
+ -> cmap { cts_derived = Map.insertWith unionBags a this_ct (cts_derived cmap) }
+ where this_ct = singleCCan ct
+
+getRelevantCts :: Ord a => a -> CCanMap a -> (CanonicalCts, CCanMap a)
+-- Gets the relevant constraints and returns the rest of the CCanMap
+getRelevantCts a cmap
+ = let relevant = unionManyBags [ Map.findWithDefault emptyCCan a (cts_wanted cmap)
+ , Map.findWithDefault emptyCCan a (cts_given cmap)
+ , Map.findWithDefault emptyCCan a (cts_derived cmap) ]
+ residual_map = cmap { cts_wanted = Map.delete a (cts_wanted cmap)
+ , cts_given = Map.delete a (cts_given cmap)
+ , cts_derived = Map.delete a (cts_derived cmap) }
+ in (relevant, residual_map)
+
+extractUnsolvedCMap :: Ord a => CCanMap a -> (CanonicalCts, CCanMap a)
+-- Gets the wanted or derived constraints and returns a residual
+-- CCanMap with only givens.
+extractUnsolvedCMap cmap =
+ let wntd = Map.fold unionBags emptyCCan (cts_wanted cmap)
+ derd = Map.fold unionBags emptyCCan (cts_derived cmap)
+ in (wntd `unionBags` derd,
+ cmap { cts_wanted = Map.empty, cts_derived = Map.empty })
+
+
-- See Note [InertSet invariants]
data InertSet
- = IS { inert_eqs :: Bag.Bag CanonicalCt -- Equalities only **CTyEqCan**
- , inert_cts :: Bag.Bag CanonicalCt -- Other constraints
- , inert_fds :: FDImprovements -- List of pairwise improvements that have kicked in already
- -- and reside either in the worklist or in the inerts
- , inert_fsks :: Map.Map TcTyVar [(TcTyVar,Coercion)] }
- -- See Note [InertSet FlattenSkolemEqClass]
+ = IS { inert_eqs :: CanonicalCts -- Equalities only (CTyEqCan)
+ , inert_dicts :: CCanMap Class -- Dictionaries only
+ , inert_ips :: CCanMap (IPName Name) -- Implicit parameters
+ , inert_frozen :: CanonicalCts
+ , inert_funeqs :: CCanMap TyCon -- Type family equalities only
+ -- This representation allows us to quickly get to the relevant
+ -- inert constraints when interacting a work item with the inert set.
+ }
-type FDImprovement = (PredType,PredType)
-type FDImprovements = [(PredType,PredType)]
+tyVarsOfInert :: InertSet -> TcTyVarSet
+tyVarsOfInert (IS { inert_eqs = eqs
+ , inert_dicts = dictmap
+ , inert_ips = ipmap
+ , inert_frozen = frozen
+ , inert_funeqs = funeqmap }) = tyVarsOfCanonicals cts
+ where
+ cts = eqs `andCCan` frozen `andCCan` cCanMapToBag dictmap
+ `andCCan` cCanMapToBag ipmap `andCCan` cCanMapToBag funeqmap
instance Outputable InertSet where
ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_eqs is))
- , vcat (map ppr (Bag.bagToList $ inert_cts is))
- , vcat (map (\(v,rest) -> ppr v <+> text "|->" <+> hsep (map (ppr.fst) rest))
- (Map.toList $ inert_fsks is)
- )
+ , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts is)))
+ , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips is)))
+ , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_funeqs is)))
+ , vcat (map ppr (Bag.bagToList $ inert_frozen is))
]
emptyInert :: InertSet
-emptyInert = IS { inert_eqs = Bag.emptyBag
- , inert_cts = Bag.emptyBag, inert_fsks = Map.empty, inert_fds = [] }
+emptyInert = IS { inert_eqs = Bag.emptyBag
+ , inert_frozen = Bag.emptyBag
+ , inert_dicts = emptyCCanMap
+ , inert_ips = emptyCCanMap
+ , inert_funeqs = emptyCCanMap }
updInertSet :: InertSet -> AtomicInert -> InertSet
--- Introduces an element in the inert set for the first time
-updInertSet (IS { inert_eqs = eqs, inert_cts = cts, inert_fsks = fsks, inert_fds = fdis })
- item@(CTyEqCan { cc_id = cv
- , cc_tyvar = tv1
- , cc_rhs = xi })
- | Just tv2 <- tcGetTyVar_maybe xi,
- FlatSkol {} <- tcTyVarDetails tv1,
- FlatSkol {} <- tcTyVarDetails tv2
- = let eqs' = eqs `Bag.snocBag` item
- fsks' = Map.insertWith (++) tv2 [(tv1, mkCoVarCoercion cv)] fsks
- -- See Note [InertSet FlattenSkolemEqClass]
- in IS { inert_eqs = eqs', inert_cts = cts, inert_fsks = fsks', inert_fds = fdis }
-updInertSet (IS { inert_eqs = eqs, inert_cts = cts
- , inert_fsks = fsks, inert_fds = fdis }) item
- | isTyEqCCan item
- = let eqs' = eqs `Bag.snocBag` item
- in IS { inert_eqs = eqs', inert_cts = cts, inert_fsks = fsks, inert_fds = fdis }
+updInertSet is item
+ | isCTyEqCan item -- Other equality
+ = let eqs' = inert_eqs is `Bag.snocBag` item
+ in is { inert_eqs = eqs' }
+ | Just cls <- isCDictCan_Maybe item -- Dictionary
+ = is { inert_dicts = updCCanMap (cls,item) (inert_dicts is) }
+ | Just x <- isCIPCan_Maybe item -- IP
+ = is { inert_ips = updCCanMap (x,item) (inert_ips is) }
+ | Just tc <- isCFunEqCan_Maybe item -- Function equality
+ = is { inert_funeqs = updCCanMap (tc,item) (inert_funeqs is) }
| otherwise
- = let cts' = cts `Bag.snocBag` item
- in IS { inert_eqs = eqs, inert_cts = cts', inert_fsks = fsks, inert_fds = fdis }
-
-updInertSetFDImprs :: InertSet -> Maybe FDImprovement -> InertSet
-updInertSetFDImprs is (Just fdi) = is { inert_fds = fdi : inert_fds is }
-updInertSetFDImprs is Nothing = is
-
-foldISEqCtsM :: Monad m => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
--- Fold over the equalities of the inerts
-foldISEqCtsM k z IS { inert_eqs = eqs }
- = Bag.foldlBagM k z eqs
-
-foldISOtherCtsM :: Monad m => (a -> AtomicInert -> m a) -> a -> InertSet -> m a
--- Fold over other constraints in the inerts
-foldISOtherCtsM k z IS { inert_cts = cts }
- = Bag.foldlBagM k z cts
+ = is { inert_frozen = inert_frozen is `Bag.snocBag` item }
extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
-extractUnsolved is@(IS {inert_eqs = eqs, inert_cts = cts, inert_fds = fdis })
- = let is_init = is { inert_eqs = emptyCCan
- , inert_cts = solved_cts, inert_fsks = Map.empty, inert_fds = fdis }
- is_final = Bag.foldlBag updInertSet is_init solved_eqs -- Add equalities carefully
- in (is_final, unsolved)
- where (unsolved_cts, solved_cts) = Bag.partitionBag isWantedCt cts
- (unsolved_eqs, solved_eqs) = Bag.partitionBag isWantedCt eqs
- unsolved = unsolved_cts `unionBags` unsolved_eqs
-
-
-getFskEqClass :: InertSet -> TcTyVar -> [(TcTyVar,Coercion)]
--- 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)
- Just ceq ->
- case tcGetTyVar_maybe (cc_rhs ceq) of
- Just tv_rhs | FlatSkol {} <- tcTyVarDetails tv_rhs
- -> let ceq_co = mkSymCoercion $ mkCoVarCoercion (cc_id ceq)
- mk_co (v,c) = (v, mkTransCoercion c ceq_co)
- in (tv_rhs, ceq_co): map mk_co (fromMaybe [] $ Map.lookup tv fsks)
- _ -> []
- where lkpTyEqCanByLhs = Bag.foldlBag lkp Nothing cts
- lkp :: Maybe CanonicalCt -> CanonicalCt -> Maybe CanonicalCt
- lkp Nothing ct@(CTyEqCan {cc_tyvar = tv'}) | tv' == tv = Just ct
- lkp other _ct = other
-
-haveBeenImproved :: FDImprovements -> PredType -> PredType -> Bool
-haveBeenImproved [] _ _ = False
-haveBeenImproved ((pty1,pty2):fdimprs) pty1' pty2'
- | tcEqPred pty1 pty1' && tcEqPred pty2 pty2'
- = True
- | tcEqPred pty1 pty2' && tcEqPred pty2 pty1'
- = True
- | otherwise
- = haveBeenImproved fdimprs pty1' pty2'
-
-getFDImprovements :: InertSet -> FDImprovements
--- Return a list of the improvements that have kicked in so far
-getFDImprovements = inert_fds
-
-
-isWantedCt :: CanonicalCt -> Bool
-isWantedCt ct = isWanted (cc_flavor ct)
-
-{- TODO: Later ...
-data Inert = IS { class_inerts :: FiniteMap Class Atomics
- ip_inerts :: FiniteMap Class Atomics
- tyfun_inerts :: FiniteMap TyCon Atomics
- tyvar_inerts :: FiniteMap TyVar Atomics
- }
-
-Later should we also separate out givens and wanteds?
--}
-
+-- Postcondition: the returned canonical cts are either Derived, or Wanted.
+extractUnsolved is@(IS {inert_eqs = eqs})
+ = let is_solved = is { inert_eqs = solved_eqs
+ , inert_dicts = solved_dicts
+ , inert_ips = solved_ips
+ , inert_frozen = emptyCCan
+ , inert_funeqs = solved_funeqs }
+ in (is_solved, unsolved)
+
+ where (unsolved_eqs, solved_eqs) = Bag.partitionBag (not.isGivenCt) eqs
+ (unsolved_ips, solved_ips) = extractUnsolvedCMap (inert_ips is)
+ (unsolved_dicts, solved_dicts) = extractUnsolvedCMap (inert_dicts is)
+ (unsolved_funeqs, solved_funeqs) = extractUnsolvedCMap (inert_funeqs is)
+
+ unsolved = unsolved_eqs `unionBags` inert_frozen is `unionBags`
+ unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs
\end{code}
-Note [Touchables and givens]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Touchable variables will never show up in givens which are inputs to
-the solver. However, touchables may show up in givens generated by the flattener.
-For example,
-
- axioms:
- G Int ~ Char
- F Char ~ Int
-
- wanted:
- F (G alpha) ~w Int
-
-canonicalises to
-
- G alpha ~g b
- F b ~w Int
-
-which can be put in the inert set. Suppose we also have a wanted
-
- alpha ~w Int
-
-We cannot rewrite the given G alpha ~g b using the wanted alpha ~w
-Int. Instead, after reacting alpha ~w Int with the whole inert set,
-we observe that we can solve it by unifying alpha with Int, so we mark
-it as solved and put it back in the *work list*. [We also immediately unify
-alpha := Int, without telling anyone, see trySpontaneousSolve function, to
-avoid doing this in the end.]
-
-Later, because it is solved (given, in effect), we can use it to rewrite
-G alpha ~g b to G Int ~g b, which gets put back in the work list. Eventually,
-we will dispatch the remaining wanted constraints using the top-level axioms.
-
-Finally, note that after reacting a wanted equality with the entire inert set
-we may end up with something like
-
- b ~w alpha
-
-which we should flip around to generate the solved constraint alpha ~s b.
-
%*********************************************************************
%* *
* Main Interaction Solver *
-- A mixture of Given, Wanted, and Derived constraints.
-- We split between equalities and the rest to process equalities first.
type WorkList = CanonicalCts
-type SWorkList = WorkList -- A worklist of solved
unionWorkLists :: WorkList -> WorkList -> WorkList
unionWorkLists = andCCan
, ptext (sLit "new work =") <+> ppr work <> comma
, ptext (sLit "stop =") <+> ppr stop])
-type SimplifierStage = WorkItem -> InertSet -> TcS StageResult
+type SubGoalDepth = Int -- Starts at zero; used to limit infinite
+ -- recursion of sub-goals
+type SimplifierStage = SubGoalDepth -> WorkItem -> InertSet -> TcS StageResult
-- Combine a sequence of simplifier 'stages' to create a pipeline
-runSolverPipeline :: [(String, SimplifierStage)]
- -> InertSet -> WorkItem
+runSolverPipeline :: SubGoalDepth
+ -> [(String, SimplifierStage)]
+ -> InertSet -> WorkItem
-> TcS (InertSet, WorkList)
-- Precondition: non-empty list of stages
-runSolverPipeline pipeline inerts workItem
+runSolverPipeline depth pipeline inerts workItem
= do { traceTcS "Start solver pipeline" $
vcat [ ptext (sLit "work item =") <+> ppr workItem
, ptext (sLit "inerts =") <+> ppr inerts]
; let itr_in = SR { sr_inerts = inerts
- , sr_new_work = emptyWorkList
- , sr_stop = ContinueWith workItem }
+ , sr_new_work = emptyWorkList
+ , sr_stop = ContinueWith workItem }
; itr_out <- run_pipeline pipeline itr_in
; let new_inert
= case sr_stop itr_out of
(SR { sr_new_work = accum_work
, sr_inerts = inerts
, sr_stop = ContinueWith work_item })
- = do { itr <- stage work_item inerts
+ = do { itr <- stage depth work_item inerts
; traceTcS ("Stage result (" ++ name ++ ")") (ppr itr)
; let itr' = itr { sr_new_work = accum_work `unionWorkLists` sr_new_work itr }
; run_pipeline stages itr' }
-- returning an extended inert set.
--
-- See Note [Touchables and givens].
-solveInteract :: InertSet -> CanonicalCts -> TcS InertSet
+solveInteractGiven :: InertSet -> GivenLoc -> [EvVar] -> TcS InertSet
+solveInteractGiven inert gloc evs
+ = do { (_, inert_ret) <- solveInteract inert $ listToBag $
+ map mk_given evs
+ ; return inert_ret }
+ where
+ flav = Given gloc
+ mk_given ev = mkEvVarX ev flav
+
+solveInteractWanted :: InertSet -> [WantedEvVar] -> TcS InertSet
+solveInteractWanted inert wvs
+ = do { (_,inert_ret) <- solveInteract inert $ listToBag $
+ map wantedToFlavored wvs
+ ; return inert_ret }
+
+solveInteract :: InertSet -> Bag FlavoredEvVar -> TcS (Bool, InertSet)
+-- Post: (True, inert_set) means we managed to discharge all constraints
+-- without actually doing any interactions!
+-- (False, inert_set) means some interactions occurred
solveInteract inert ws
= do { dyn_flags <- getDynFlags
- ; solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[]) inert ws
- }
-solveOne :: InertSet -> WorkItem -> TcS InertSet
-solveOne inerts workItem
+ ; sctx <- getTcSContext
+
+ ; traceTcS "solveInteract, before clever canonicalization:" $
+ vcat [ text "ws = " <+> ppr (mapBag (\(EvVarX ev ct)
+ -> (ct,evVarPred ev)) ws)
+ , text "inert = " <+> ppr inert ]
+
+ ; (flag, inert_ret) <- foldrBagM (tryPreSolveAndInteract sctx dyn_flags) (True,inert) ws
+ -- use foldr to preserve the order
+
+ ; traceTcS "solveInteract, after clever canonicalization (and interaction):" $
+ vcat [ text "No interaction happened = " <+> ppr flag
+ , text "inert_ret = " <+> ppr inert_ret ]
+
+ ; return (flag, inert_ret) }
+
+
+tryPreSolveAndInteract :: SimplContext
+ -> DynFlags
+ -> FlavoredEvVar
+ -> (Bool, InertSet)
+ -> TcS (Bool, InertSet)
+-- Returns: True if it was able to discharge this constraint AND all previous ones
+tryPreSolveAndInteract sctx dyn_flags flavev@(EvVarX ev_var fl) (all_previous_discharged, inert)
+ = do { let inert_cts = get_inert_cts (evVarPred ev_var)
+
+ ; this_one_discharged <- dischargeFromCCans inert_cts flavev
+
+ ; if this_one_discharged
+ then return (all_previous_discharged, inert)
+
+ else do
+ { extra_cts <- mkCanonical fl ev_var
+ ; inert_ret <- solveInteractWithDepth (ctxtStkDepth dyn_flags,0,[]) extra_cts inert
+ ; return (False, inert_ret) } }
+
+ where
+ get_inert_cts (ClassP clas _)
+ | simplEqsOnly sctx = emptyCCan
+ | otherwise = fst (getRelevantCts clas (inert_dicts inert))
+ get_inert_cts (IParam {})
+ = emptyCCan -- We must not do the same thing for IParams, because (contrary
+ -- to dictionaries), work items /must/ override inert items.
+ -- See Note [Overriding implicit parameters] in TcInteract.
+ get_inert_cts (EqPred {})
+ = inert_eqs inert `unionBags` cCanMapToBag (inert_funeqs inert)
+
+dischargeFromCCans :: CanonicalCts -> FlavoredEvVar -> TcS Bool
+dischargeFromCCans cans (EvVarX ev fl)
+ = Bag.foldlBagM discharge_ct False cans
+ where discharge_ct :: Bool -> CanonicalCt -> TcS Bool
+ discharge_ct True _ct = return True
+ discharge_ct False ct
+ | evVarPred (cc_id ct) `tcEqPred` evVarPred ev
+ , cc_flavor ct `canSolve` fl
+ = do { when (isWanted fl) $ set_ev_bind ev (cc_id ct)
+ ; return True }
+ where set_ev_bind x y
+ | EqPred {} <- evVarPred y
+ = setEvBind x (EvCoercion (mkCoVarCoercion y))
+ | otherwise = setEvBind x (EvId y)
+ discharge_ct False _ct = return False
+\end{code}
+
+Note [Avoiding the superclass explosion]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This note now is not as significant as it used to be because we no
+longer add the superclasses of Wanted as Derived, except only if they
+have equality superclasses or superclasses with functional
+dependencies. The fear was that hundreds of identical wanteds would
+give rise each to the same superclass or equality Derived's which
+would lead to a blo-up in the number of interactions.
+
+Instead, what we do with tryPreSolveAndCanon, is when we encounter a
+new constraint, we very quickly see if it can be immediately
+discharged by a class constraint in our inert set or the previous
+canonicals. If so, we add nothing to the returned canonical
+constraints.
+
+\begin{code}
+solveOne :: WorkItem -> InertSet -> TcS InertSet
+solveOne workItem inerts
= do { dyn_flags <- getDynFlags
- ; solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) inerts workItem
+ ; solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) workItem inerts
}
-----------------
solveInteractWithDepth :: (Int, Int, [WorkItem])
- -> InertSet -> WorkList -> TcS InertSet
-solveInteractWithDepth ctxt@(max_depth,n,stack) inert ws
+ -> WorkList -> InertSet -> TcS InertSet
+solveInteractWithDepth ctxt@(max_depth,n,stack) ws inert
| isEmptyWorkList ws
= return inert
| otherwise
= do { traceTcS "solveInteractWithDepth" $
vcat [ text "Current depth =" <+> ppr n
- , text "Max depth =" <+> ppr max_depth ]
+ , text "Max depth =" <+> ppr max_depth
+ , text "ws =" <+> ppr ws ]
-- Solve equalities first
- ; let (eqs, non_eqs) = Bag.partitionBag isTyEqCCan ws
- ; is_from_eqs <- Bag.foldlBagM (solveOneWithDepth ctxt) inert eqs
- ; Bag.foldlBagM (solveOneWithDepth ctxt) is_from_eqs non_eqs }
+ ; let (eqs, non_eqs) = Bag.partitionBag isCTyEqCan ws
+ ; is_from_eqs <- Bag.foldrBagM (solveOneWithDepth ctxt) inert eqs
+ ; Bag.foldrBagM (solveOneWithDepth ctxt) is_from_eqs non_eqs }
+ -- use foldr to preserve the order
------------------
-- Fully interact the given work item with an inert set, and return a
-- new inert set which has assimilated the new information.
solveOneWithDepth :: (Int, Int, [WorkItem])
- -> InertSet -> WorkItem -> TcS InertSet
-solveOneWithDepth (max_depth, n, stack) inert work
- = do { traceTcS0 (indent ++ "Solving {") (ppr work)
- ; (new_inert, new_work) <- runSolverPipeline thePipeline inert work
+ -> WorkItem -> InertSet -> TcS InertSet
+solveOneWithDepth (max_depth, depth, stack) work inert
+ = do { traceFireTcS depth (text "Solving {" <+> ppr work)
+ ; (new_inert, new_work) <- runSolverPipeline depth thePipeline inert work
- ; traceTcS0 (indent ++ "Subgoals:") (ppr new_work)
-
-- Recursively solve the new work generated
-- from workItem, with a greater depth
- ; res_inert <- solveInteractWithDepth (max_depth, n+1, work:stack)
- new_inert new_work
+ ; res_inert <- solveInteractWithDepth (max_depth, depth+1, work:stack) new_work new_inert
+
+ ; traceFireTcS depth (text "Done }" <+> ppr work)
- ; traceTcS0 (indent ++ "Done }") (ppr work)
; return res_inert }
- where
- indent = replicate (2*n) ' '
thePipeline :: [(String,SimplifierStage)]
thePipeline = [ ("interact with inert eqs", interactWithInertEqsStage)
There are two cases where we have to be careful about
orienting equalities to get better efficiency.
-Case 1: In Spontaneous Solving
-
- The OrientFlag is used to preserve the original orientation of a
- spontaneously solved equality (insofar the canonical constraints
- invariants allow it). This way we hope to be more efficient since
- when reaching the spontaneous solve stage, a particular
- constraint has already been inert-ified wrt to the preexisting
- inerts.
-
- Example:
- Inerts: [w1] : D alpha
- [w2] : C beta
- [w3] : F alpha ~ Int
- [w4] : H beta ~ Int
- Untouchables = [beta]
- Then a wanted (beta ~ alpha) comes along.
-
- 1) While interacting with the inerts it is going to kick w2,w4
- out of the inerts
- 2) Then, it will spontaneoulsy be solved by (alpha := beta)
- 3) Now (and here is the tricky part), to add him back as
- solved (alpha ~ beta) is no good because, in the next
- iteration, it will kick out w1,w3 as well so we will end up
- with *all* the inert equalities back in the worklist!
-
- So, we instead solve (alpha := beta), but we preserve the
- original orientation, so that we get a given (beta ~ alpha),
- which will result in no more inerts getting kicked out of the
- inert set in the next iteration.
-
-Case 2: In Rewriting Equalities (function rewriteEqLHS)
+Case 1: In Rewriting Equalities (function rewriteEqLHS)
When rewriting two equalities with the same LHS:
(a) (tv ~ xi1)
This choice is implemented using the WhichComesFromInert flag.
-Case 3: Functional Dependencies and IP improvement work
- TODO. Optimisation not yet implemented there.
+Case 2: Functional Dependencies
+ Again, we should prefer, if possible, the inert variables on the RHS
+
+Case 3: IP improvement work
+ We must always rewrite so that the inert type is on the right.
\begin{code}
spontaneousSolveStage :: SimplifierStage
-spontaneousSolveStage workItem inerts
- = do { mSolve <- trySpontaneousSolve workItem inerts
+spontaneousSolveStage depth workItem inerts
+ = do { mSolve <- trySpontaneousSolve workItem
+
; case mSolve of
- Nothing -> -- no spontaneous solution for him, keep going
+ SPCantSolve -> -- No spontaneous solution for him, keep going
return $ SR { sr_new_work = emptyWorkList
, sr_inerts = inerts
, sr_stop = ContinueWith workItem }
- Just workList' -> -- He has been solved; workList' are all givens
- return $ SR { sr_new_work = workList'
- , sr_inerts = inerts
+ SPSolved workItem'
+ | not (isGivenCt workItem)
+ -- Original was wanted or derived but we have now made him
+ -- given so we have to interact him with the inerts due to
+ -- its status change. This in turn may produce more work.
+ -- We do this *right now* (rather than just putting workItem'
+ -- back into the work-list) because we've solved
+ -> do { bumpStepCountTcS
+ ; traceFireTcS depth (ptext (sLit "Spontaneous (w/d)") <+> ppr workItem)
+ ; (new_inert, new_work) <- runSolverPipeline depth
+ [ ("recursive interact with inert eqs", interactWithInertEqsStage)
+ , ("recursive interact with inerts", interactWithInertsStage)
+ ] inerts workItem'
+ ; return $ SR { sr_new_work = new_work
+ , sr_inerts = new_inert -- will include workItem'
+ , sr_stop = Stop }
+ }
+ | otherwise
+ -> -- Original was given; he must then be inert all right, and
+ -- workList' are all givens from flattening
+ do { bumpStepCountTcS
+ ; traceFireTcS depth (ptext (sLit "Spontaneous (g)") <+> ppr workItem)
+ ; return $ SR { sr_new_work = emptyWorkList
+ , sr_inerts = inerts `updInertSet` workItem'
+ , sr_stop = Stop } }
+ SPError -> -- Return with no new work
+ return $ SR { sr_new_work = emptyWorkList
+ , sr_inerts = inerts
, sr_stop = Stop }
}
+data SPSolveResult = SPCantSolve | SPSolved WorkItem | SPError
+-- SPCantSolve means that we can't do the unification because e.g. the variable is untouchable
+-- SPSolved workItem' gives us a new *given* to go on
+-- SPError means that it's completely impossible to solve this equality, eg due to a kind error
-data OrientFlag = OrientThisWay
- | OrientOtherWay -- See Note [Efficient Orientation]
-- @trySpontaneousSolve wi@ solves equalities where one side is a
--- touchable unification variable. Returns:
--- * 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.
+-- touchable unification variable.
-- See Note [Touchables and givens]
--- NB: just passing the inerts through for the skolem equivalence classes
-trySpontaneousSolve :: WorkItem -> InertSet -> TcS (Maybe SWorkList)
-trySpontaneousSolve workItem@(CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi }) inerts
+trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
+trySpontaneousSolve workItem@(CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi })
| isGiven gw
- = return Nothing
+ = return SPCantSolve
| Just tv2 <- tcGetTyVar_maybe xi
= do { tch1 <- isTouchableMetaTyVar tv1
; tch2 <- isTouchableMetaTyVar tv2
; case (tch1, tch2) of
- (True, True) -> trySpontaneousEqTwoWay inerts cv gw tv1 tv2
- (True, False) -> trySpontaneousEqOneWay OrientThisWay inerts cv gw tv1 xi
- (False, True) -> trySpontaneousEqOneWay OrientOtherWay inerts cv gw tv2 (mkTyVarTy tv1)
- _ -> return Nothing }
+ (True, True) -> trySpontaneousEqTwoWay cv gw tv1 tv2
+ (True, False) -> trySpontaneousEqOneWay cv gw tv1 xi
+ (False, True) -> trySpontaneousEqOneWay cv gw tv2 (mkTyVarTy tv1)
+ _ -> return SPCantSolve }
| otherwise
= do { tch1 <- isTouchableMetaTyVar tv1
- ; if tch1 then trySpontaneousEqOneWay OrientThisWay inerts cv gw tv1 xi
- else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" (ppr workItem)
- ; return Nothing }
+ ; if tch1 then trySpontaneousEqOneWay cv gw tv1 xi
+ else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:"
+ (ppr workItem)
+ ; return SPCantSolve }
}
-- No need for
-- trySpontaneousSolve (CFunEqCan ...) = ...
-- See Note [No touchables as FunEq RHS] in TcSMonad
-trySpontaneousSolve _ _ = return Nothing
+trySpontaneousSolve _ = return SPCantSolve
----------------
-trySpontaneousEqOneWay :: OrientFlag
- -> InertSet -> CoVar -> CtFlavor -> TcTyVar -> Xi
- -> TcS (Maybe SWorkList)
--- NB: The OrientFlag is there to help us recover the orientation of the original
--- constraint which is important for enforcing the canonical constraints invariants.
--- Also, tv is a MetaTyVar, not untouchable
-trySpontaneousEqOneWay or_flag inerts cv gw tv xi
+trySpontaneousEqOneWay :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
+-- tv is a MetaTyVar, not untouchable
+trySpontaneousEqOneWay cv gw tv xi
| not (isSigTyVar tv) || isTyVarTy xi
- = do { kxi <- zonkTcTypeTcS xi >>= return . typeKind -- Must look through the TcTyBinds
- -- hence kxi and not typeKind xi
- -- See Note [Kind Errors]
+ = do { let kxi = typeKind xi -- NB: 'xi' is fully rewritten according to the inerts
+ -- so we have its more specific kind in our hands
; if kxi `isSubKind` tyVarKind tv then
- solveWithIdentity or_flag inerts cv gw tv xi
- else if tyVarKind tv `isSubKind` kxi then
- return Nothing -- kinds are compatible but we can't solveWithIdentity this way
- -- This case covers the a_touchable :: * ~ b_untouchable :: ??
- -- which has to be deferred or floated out for someone else to solve
- -- it in a scope where 'b' is no longer untouchable.
- else kindErrorTcS gw (mkTyVarTy tv) xi -- See Note [Kind errors]
+ solveWithIdentity cv gw tv xi
+ else return SPCantSolve
+{-
+ else if tyVarKind tv `isSubKind` kxi then
+ return SPCantSolve -- kinds are compatible but we can't solveWithIdentity this way
+ -- This case covers the a_touchable :: * ~ b_untouchable :: ??
+ -- which has to be deferred or floated out for someone else to solve
+ -- it in a scope where 'b' is no longer untouchable.
+ else do { addErrorTcS KindError gw (mkTyVarTy tv) xi -- See Note [Kind errors]
+ ; return SPError }
+-}
}
| otherwise -- Still can't solve, sig tyvar and non-variable rhs
- = return Nothing
+ = return SPCantSolve
----------------
-trySpontaneousEqTwoWay :: InertSet -> CoVar -> CtFlavor -> TcTyVar -> TcTyVar
- -> TcS (Maybe SWorkList)
+trySpontaneousEqTwoWay :: CoVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
-trySpontaneousEqTwoWay inerts cv gw tv1 tv2
+trySpontaneousEqTwoWay cv gw tv1 tv2
| k1 `isSubKind` k2
- , nicer_to_update_tv2 = solveWithIdentity OrientOtherWay inerts cv gw tv2 (mkTyVarTy tv1)
+ , nicer_to_update_tv2 = solveWithIdentity cv gw tv2 (mkTyVarTy tv1)
| k2 `isSubKind` k1
- = solveWithIdentity OrientThisWay inerts cv gw tv1 (mkTyVarTy tv2)
+ = solveWithIdentity cv gw tv1 (mkTyVarTy tv2)
| otherwise -- None is a subkind of the other, but they are both touchable!
- = kindErrorTcS gw (mkTyVarTy tv1) (mkTyVarTy tv2) -- See Note [Kind errors]
+ = return SPCantSolve
+ -- do { addErrorTcS KindError gw (mkTyVarTy tv1) (mkTyVarTy tv2)
+ -- ; return SPError }
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
alpha ~ (# Int, Int #)
where alpha :: ?? and (# Int, Int #) :: (#). We can't spontaneously solve this constraint,
but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay'
-simply returns @Nothing@ then that wanted constraint is going to propagate all the way and
+simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and
get quantified over in inference mode. That's bad because we do know at this point that the
-constraint is insoluble. Instead, we call 'kindErrorTcS' here, which immediately fails.
+constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.
The same applies in canonicalization code in case of kind errors in the givens.
However, when we canonicalize givens we only check for compatibility (@compatKind@).
-If there were a kind error in the givens, this means some form of inconsistency or dead code.
-
-When we spontaneously solve wanteds we may have to look through the bindings, hence we
-call zonkTcTypeTcS above. The reason is that maybe xi is @alpha@ where alpha :: ? and
-a previous spontaneous solving has set (alpha := f) with (f :: *). The reason that xi is
-still alpha and not f is becasue the solved constraint may be oriented as (f ~ alpha) instead
-of (alpha ~ f). Then we should be using @xi@s "real" kind, which is * and not ?, when we try
-to detect whether spontaneous solving is possible.
+If there were a kind error in the givens, this means some form of inconsistency or dead code.
+You may think that when we spontaneously solve wanteds we may have to look through the
+bindings to determine the right kind of the RHS type. E.g one may be worried that xi is
+@alpha@ where alpha :: ? and a previous spontaneous solving has set (alpha := f) with (f :: *).
+But we orient our constraints so that spontaneously solved ones can rewrite all other constraint
+so this situation can't happen.
Note [Spontaneous solving and kind compatibility]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note that our canonical constraints insist that *all* equalities (tv ~
+xi) or (F xis ~ rhs) require the LHS and the RHS to have *compatible*
+the same kinds. ("compatible" means one is a subKind of the other.)
-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:
+ - It can't be *equal* kinds, because
+ b) wanted constraints don't necessarily have identical kinds
+ eg alpha::? ~ Int
+ b) a solved wanted constraint becomes a given
+
+ - SPJ thinks that *given* constraints (tv ~ tau) always have that
+ tau has a sub-kind of tv; and when solving wanted constraints
+ in trySpontaneousEqTwoWay we re-orient to achieve this.
+
+ - Note that the kind invariant is maintained by rewriting.
+ Eg wanted1 rewrites wanted2; if both were compatible kinds before,
+ wanted2 will be afterwards. Similarly givens.
+
+Caveat:
- Givens from higher-rank, such as:
type family T b :: * -> * -> *
type instance T Bool = (->)
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]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Example 1: (The problem of loopy spontaneous solving)
-****************************************************************************
-Consider the original wanted:
- wanted : Maybe (E alpha) ~ alpha
-where E is a type family, such that E (T x) = x. After canonicalization,
-as a result of flattening, we will get:
- given : E alpha ~ fsk
- wanted : alpha ~ Maybe fsk
-where (fsk := E alpha, on the side). Now, if we spontaneously *solve*
-(alpha := Maybe fsk) we are in trouble! Instead, we should refrain from solving
-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.
-
-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:
-
-Example 2: (The need of keeping track of flatten skolem equivalence classes)
-****************************************************************************
-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!
-
-Example 3: (The need of looking through TyBinds for already spontaneously solved variables)
-*******************************************************************************************
-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.
-
-Example 4: (Orientation of (tv ~ xi) equalities)
-************************************************
-We orient equalities (tv ~ xi) so that flatten skolems appear on the left, if possible. Here
-is an example of why this is needed:
-
- [Wanted] w1: alpha ~ fsk
- [Given] g1: F alpha ~ fsk
- [Given] g2: b ~ fsk
- Flatten skolem equivalence class = []
-
-Assume that g2 is *not* oriented properly, as shown above. Then we would like to spontaneously
-solve w1 but we can't set alpha := fsk, since fsk hides the type F alpha. However, by using
-the equation g2 it would be possible to solve w1 by setting alpha := b. In other words, it is
-not enough to look at a flatten skolem equivalence class to try to find alternatives to unify
-with. We may have to go to other variables.
-
-By orienting the equalities so that flatten skolems are in the LHS we are eliminating them
-as much as possible from the RHS of other wanted equalities, and hence it suffices to look
-in their flatten skolem equivalence classes.
-
-This situation appears in the IndTypesPerf test case, inside indexed-types/.
Note [Avoid double unifications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
+We avoid this problem by orienting the resulting given so that the unification
variable is on the left. [Note that alternatively we could attempt to
enforce this at canonicalization]
\begin{code}
----------------
-solveWithIdentity :: OrientFlag
- -> InertSet
- -> CoVar -> CtFlavor -> TcTyVar -> Xi
- -> TcS (Maybe SWorkList)
+
+solveWithIdentity :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
-- Solve with the identity coercion
-- Precondition: kind(xi) is a sub-kind of kind(tv)
-- Precondition: CtFlavor is Wanted or Derived
-- See [New Wanted Superclass Work] to see why solveWithIdentity
-- must work for Derived as well as Wanted
-solveWithIdentity or_flag inerts cv gw tv xi
- = 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,
+-- Returns: workItem where
+-- workItem = the new Given constraint
+solveWithIdentity cv wd tv xi
+ = do { traceTcS "Sneaky unification:" $
+ vcat [text "Coercion variable: " <+> ppr wd,
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 { cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi_unflat xi_unflat
- ; setWantedTyBind tv xi_unflat
- ; can_eqs <- case or_flag of
- OrientThisWay -> canEq flav cv_given (mkTyVarTy tv) xi_unflat
- OrientOtherWay -> canEq flav cv_given xi_unflat (mkTyVarTy tv)
- ; return (can_eqs, co) }
- IdCo co -> do { cv_given <- newGivOrDerCoVar (mkTyVarTy tv) xi xi
- ; setWantedTyBind tv xi
- ; can_eqs <- case or_flag of
- OrientThisWay -> canEq flav cv_given (mkTyVarTy tv) xi
- OrientOtherWay -> canEq flav cv_given xi (mkTyVarTy tv)
- ; return (can_eqs, 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: 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)
+
+ ; setWantedTyBind tv xi
+ ; cv_given <- newGivenCoVar (mkTyVarTy tv) xi xi
+
+ ; when (isWanted wd) (setWantedCoBind cv xi)
+ -- We don't want to do this for Derived, that's why we use 'when (isWanted wd)'
+
+ ; return $ SPSolved (CTyEqCan { cc_id = cv_given
+ , cc_flavor = mkGivenFlavor wd UnkSkol
+ , cc_tyvar = tv, cc_rhs = xi }) }
\end{code}
+
+
*********************************************************************************
* *
The interact-with-inert Stage
, ir_new_work :: WorkList
-- new work items to add to the WorkList
- , ir_improvement :: Maybe FDImprovement -- In case improvement kicked in
+ , ir_fire :: Maybe String -- Tells whether a rule fired, and if so what
}
-- What to do with the inert reactant.
-data InertAction = KeepInert | DropInert
- deriving Eq
-
-mkIRContinue :: Monad m => WorkItem -> InertAction -> WorkList -> m InteractResult
-mkIRContinue wi keep newWork = return $ IR (ContinueWith wi) keep newWork Nothing
-
-mkIRStop :: Monad m => InertAction -> WorkList -> m InteractResult
-mkIRStop keep newWork = return $ IR Stop keep newWork Nothing
+data InertAction = KeepInert | DropInert
-mkIRStop_RecordImprovement :: Monad m => InertAction -> WorkList -> FDImprovement -> m InteractResult
-mkIRStop_RecordImprovement keep newWork fdimpr = return $ IR Stop keep newWork (Just fdimpr)
+mkIRContinue :: String -> WorkItem -> InertAction -> WorkList -> TcS InteractResult
+mkIRContinue rule wi keep newWork
+ = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = keep
+ , ir_new_work = newWork, ir_fire = Just rule }
-
-dischargeWorkItem :: Monad m => m InteractResult
-dischargeWorkItem = mkIRStop KeepInert emptyWorkList
+mkIRStop :: String -> WorkList -> TcS InteractResult
+mkIRStop rule newWork
+ = return $ IR { ir_stop = Stop, ir_inert_action = KeepInert
+ , ir_new_work = newWork, ir_fire = Just rule }
noInteraction :: Monad m => WorkItem -> m InteractResult
-noInteraction workItem = mkIRContinue workItem KeepInert emptyWorkList
+noInteraction wi
+ = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = KeepInert
+ , ir_new_work = emptyWorkList, ir_fire = Nothing }
data WhichComesFromInert = LeftComesFromInert | RightComesFromInert
- -- See Note [Efficient Orientation, Case 2]
+ -- See Note [Efficient Orientation]
---------------------------------------------------
--- Interact a single WorkItem with the equalities of an inert set as far as possible, i.e. until we
--- get a Stop result from an individual reaction (i.e. when the WorkItem is consumed), or until we've
+-- Interact a single WorkItem with the equalities of an inert set as
+-- far as possible, i.e. until we get a Stop result from an individual
+-- reaction (i.e. when the WorkItem is consumed), or until we've
-- interact the WorkItem with the entire equalities of the InertSet
interactWithInertEqsStage :: SimplifierStage
-interactWithInertEqsStage workItem inert
- = foldISEqCtsM interactNext initITR inert
- where initITR = SR { sr_inerts = IS { inert_eqs = emptyCCan -- We will fold over the equalities
- , inert_fsks = Map.empty -- which will generate those two again
- , inert_cts = inert_cts inert
- , inert_fds = inert_fds inert
- }
- , sr_new_work = emptyWorkList
- , sr_stop = ContinueWith workItem }
-
+interactWithInertEqsStage depth workItem inert
+ = Bag.foldrBagM (interactNext depth) initITR (inert_eqs inert)
+ -- use foldr to preserve the order
+ where
+ initITR = SR { sr_inerts = inert { inert_eqs = emptyCCan }
+ , sr_new_work = emptyWorkList
+ , sr_stop = ContinueWith workItem }
---------------------------------------------------
-- Interact a single WorkItem with *non-equality* constraints in the inert set.
-- Precondition: equality interactions must have already happened, hence we have
-- to pick up some information from the incoming inert, before folding over the
--- "Other" constraints it contains!
+-- "Other" constraints it contains!
+
interactWithInertsStage :: SimplifierStage
-interactWithInertsStage workItem inert
- = foldISOtherCtsM interactNext initITR inert
+interactWithInertsStage depth workItem inert
+ = let (relevant, inert_residual) = getISRelevant workItem inert
+ initITR = SR { sr_inerts = inert_residual
+ , sr_new_work = emptyWorkList
+ , sr_stop = ContinueWith workItem }
+ in Bag.foldrBagM (interactNext depth) initITR relevant
+ -- use foldr to preserve the order
where
- initITR = SR { -- Pick up: (1) equations, (2) FD improvements, (3) FlatSkol equiv. classes
- sr_inerts = IS { inert_eqs = inert_eqs inert
- , inert_cts = emptyCCan
- , inert_fds = inert_fds inert
- , inert_fsks = inert_fsks inert }
- , sr_new_work = emptyWorkList
- , sr_stop = ContinueWith workItem }
-
-interactNext :: StageResult -> AtomicInert -> TcS StageResult
-interactNext it inert
- | ContinueWith workItem <- sr_stop it
- = do { let inerts = sr_inerts it
- fdimprs_old = getFDImprovements inerts
-
- ; ir <- interactWithInert fdimprs_old inert workItem
-
- -- New inerts depend on whether we KeepInert or not and must
- -- be updated with FD improvement information from the interaction result (ir)
- ; let inerts_new = updInertSetFDImprs upd_inert (ir_improvement ir)
- upd_inert = if ir_inert_action ir == KeepInert
- then inerts `updInertSet` inert else inerts
+ getISRelevant :: CanonicalCt -> InertSet -> (CanonicalCts, InertSet)
+ getISRelevant (CFrozenErr {}) is = (emptyCCan, is)
+ -- Nothing s relevant; we have alread interacted
+ -- it with the equalities in the inert set
+
+ getISRelevant (CDictCan { cc_class = cls } ) is
+ = let (relevant, residual_map) = getRelevantCts cls (inert_dicts is)
+ in (relevant, is { inert_dicts = residual_map })
+ getISRelevant (CFunEqCan { cc_fun = tc } ) is
+ = let (relevant, residual_map) = getRelevantCts tc (inert_funeqs is)
+ in (relevant, is { inert_funeqs = residual_map })
+ getISRelevant (CIPCan { cc_ip_nm = nm }) is
+ = let (relevant, residual_map) = getRelevantCts nm (inert_ips is)
+ in (relevant, is { inert_ips = residual_map })
+ -- An equality, finally, may kick everything except equalities out
+ -- because we have already interacted the equalities in interactWithInertEqsStage
+ getISRelevant _eq_ct is -- Equality, everything is relevant for this one
+ -- TODO: if we were caching variables, we'd know that only
+ -- some are relevant. Experiment with this for now.
+ = let cts = cCanMapToBag (inert_ips is) `unionBags`
+ cCanMapToBag (inert_dicts is) `unionBags` cCanMapToBag (inert_funeqs is)
+ in (cts, is { inert_dicts = emptyCCanMap
+ , inert_ips = emptyCCanMap
+ , inert_funeqs = emptyCCanMap })
+
+interactNext :: SubGoalDepth -> AtomicInert -> StageResult -> TcS StageResult
+interactNext depth inert it
+ | ContinueWith work_item <- sr_stop it
+ = do { let inerts = sr_inerts it
+
+ ; IR { ir_new_work = new_work, ir_inert_action = inert_action
+ , ir_fire = fire_info, ir_stop = stop }
+ <- interactWithInert inert work_item
+
+ ; let mk_msg rule
+ = text rule <+> keep_doc
+ <+> vcat [ ptext (sLit "Inert =") <+> ppr inert
+ , ptext (sLit "Work =") <+> ppr work_item
+ , ppUnless (isEmptyBag new_work) $
+ ptext (sLit "New =") <+> ppr new_work ]
+ keep_doc = case inert_action of
+ KeepInert -> ptext (sLit "[keep]")
+ DropInert -> ptext (sLit "[drop]")
+ ; case fire_info of
+ Just rule -> do { bumpStepCountTcS
+ ; traceFireTcS depth (mk_msg rule) }
+ Nothing -> return ()
+
+ -- New inerts depend on whether we KeepInert or not
+ ; let inerts_new = case inert_action of
+ KeepInert -> inerts `updInertSet` inert
+ DropInert -> inerts
; return $ SR { sr_inerts = inerts_new
- , sr_new_work = sr_new_work it `unionWorkLists` ir_new_work ir
- , sr_stop = ir_stop ir } }
+ , sr_new_work = sr_new_work it `unionWorkLists` new_work
+ , sr_stop = stop } }
| otherwise
= return $ it { sr_inerts = (sr_inerts it) `updInertSet` inert }
-- Do a single interaction of two constraints.
-interactWithInert :: FDImprovements -> AtomicInert -> WorkItem -> TcS InteractResult
-interactWithInert fdimprs inert workitem
+interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
+interactWithInert inert workitem
= do { ctxt <- getTcSContext
; let is_allowed = allowedInteraction (simplEqsOnly ctxt) inert workitem
- inert_ev = cc_id inert
- work_ev = cc_id workitem
-
- -- Never interact a wanted and a derived where the derived's evidence
- -- mentions the wanted evidence in an unguarded way.
- -- See Note [Superclasses and recursive dictionaries]
- -- and Note [New Wanted Superclass Work]
- -- We don't have to do this for givens, as we fully know the evidence for them.
- ; rec_ev_ok <-
- case (cc_flavor inert, cc_flavor workitem) of
- (Wanted loc, Derived {}) -> isGoodRecEv work_ev (WantedEvVar inert_ev loc)
- (Derived {}, Wanted loc) -> isGoodRecEv inert_ev (WantedEvVar work_ev loc)
- _ -> return True
-
- ; if is_allowed && rec_ev_ok then
- doInteractWithInert fdimprs inert workitem
+
+ ; if is_allowed then
+ doInteractWithInert inert workitem
else
noInteraction workitem
}
allowedInteraction _ _ _ = True
--------------------------------------------
-doInteractWithInert :: FDImprovements -> CanonicalCt -> CanonicalCt -> TcS InteractResult
+doInteractWithInert :: CanonicalCt -> CanonicalCt -> TcS InteractResult
-- Identical class constraints.
-doInteractWithInert fdimprs
+doInteractWithInert
(CDictCan { cc_id = d1, cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 })
workItem@(CDictCan { cc_id = d2, cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })
| cls1 == cls2 && (and $ zipWith tcEqType tys1 tys2)
| cls1 == cls2 && (not (isGiven fl1 && isGiven fl2))
= -- See Note [When improvement happens]
- do { let pty1 = ClassP cls1 tys1
- pty2 = ClassP cls2 tys2
- work_item_pred_loc = (pty2, ppr d2)
- inert_pred_loc = (pty1, ppr d1)
- loc = combineCtLoc fl1 fl2
- eqn_pred_locs = improveFromAnother work_item_pred_loc inert_pred_loc
-
- ; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
- ; fd_work <- canWanteds wevvars
- -- See Note [Generating extra equalities]
- ; traceTcS "Checking if improvements existed." (ppr fdimprs)
- ; if isEmptyWorkList fd_work || haveBeenImproved fdimprs pty1 pty2 then
- -- Must keep going
- mkIRContinue workItem KeepInert fd_work
- else do { traceTcS "Recording improvement and throwing item back in worklist." (ppr (pty1,pty2))
- ; mkIRStop_RecordImprovement KeepInert
- (fd_work `unionWorkLists` workListFromCCan workItem) (pty1,pty2)
- }
- -- See Note [FunDep Reactions]
- }
+ do { let pty1 = ClassP cls1 tys1
+ pty2 = ClassP cls2 tys2
+ inert_pred_loc = (pty1, pprFlavorArising fl1)
+ work_item_pred_loc = (pty2, pprFlavorArising fl2)
+ fd_eqns = improveFromAnother
+ inert_pred_loc -- the template
+ work_item_pred_loc -- the one we aim to rewrite
+ -- See Note [Efficient Orientation]
+
+ ; m <- rewriteWithFunDeps fd_eqns tys2 fl2
+ ; case m of
+ Nothing -> noInteraction workItem
+ Just (rewritten_tys2, cos2, fd_work)
+
+ | tcEqTypes tys1 rewritten_tys2
+ -> -- Solve him on the spot in this case
+ do { let dict_co = mkTyConCoercion (classTyCon cls1) cos2
+ ; when (isWanted fl2) $ setDictBind d2 (EvCast d1 dict_co)
+ ; mkIRStop "Cls/Cls fundep (solved)" fd_work }
+
+ | isWanted fl2
+ -> -- We could not quite solve him, but we stil rewrite him
+ -- Example: class C a b c | a -> b
+ -- Given: C Int Bool x, Wanted: C Int beta y
+ -- Then rewrite the wanted to C Int Bool y
+ -- but note that is still not identical to the given
+ -- The important thing is that the rewritten constraint is
+ -- inert wrt the given.
+ -- In fact, it is inert wrt all the previous inerts too, so
+ -- we can keep on going rather than sending it back to the work list
+ do { let dict_co = mkTyConCoercion (classTyCon cls1) cos2
+ ; d2' <- newDictVar cls1 rewritten_tys2
+ ; setDictBind d2 (EvCast d2' dict_co)
+ ; let workItem' = workItem { cc_id = d2', cc_tyargs = rewritten_tys2 }
+ ; mkIRContinue "Cls/Cls fundep (partial)" workItem' KeepInert fd_work }
+
+ | otherwise
+ -> ASSERT (isDerived fl2) -- Derived constraints have no evidence,
+ -- so just produce the rewritten constraint
+ let workItem' = workItem { cc_tyargs = rewritten_tys2 }
+ in mkIRContinue "Cls/Cls fundep" workItem' KeepInert fd_work
+ }
-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
-doInteractWithInert _fdimprs
- (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
+doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
(CDictCan { cc_id = dv, cc_flavor = wfl, cc_class = cl, cc_tyargs = xis })
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfTypes xis
- = if isDerivedSC wfl then
- mkIRStop KeepInert $ emptyWorkList -- See Note [Adding Derived Superclasses]
- else do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
+ = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
-- Continue with rewritten Dictionary because we can only be in the
-- interactWithEqsStage, so the dictionary is inert.
- ; mkIRContinue rewritten_dict KeepInert emptyWorkList }
+ ; mkIRContinue "Eq/Cls" rewritten_dict KeepInert emptyWorkList }
-doInteractWithInert _fdimprs
- (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis })
+doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis })
workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
| wfl `canRewrite` ifl
, tv `elemVarSet` tyVarsOfTypes xis
- = if isDerivedSC ifl then
- mkIRContinue workItem DropInert emptyWorkList -- No need to do any rewriting,
- -- see Note [Adding Derived Superclasses]
- else do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
- ; mkIRContinue workItem DropInert (workListFromCCan rewritten_dict) }
+ = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
+ ; mkIRContinue "Cls/Eq" workItem DropInert (workListFromCCan rewritten_dict) }
-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
-doInteractWithInert _fdimprs
- (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
+doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi })
(CIPCan { cc_id = ipid, cc_flavor = wfl, cc_ip_nm = nm, cc_ip_ty = ty })
| ifl `canRewrite` wfl
, tv `elemVarSet` tyVarsOfType ty
= do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,wfl,nm,ty)
- ; mkIRContinue rewritten_ip KeepInert emptyWorkList }
+ ; mkIRContinue "Eq/IP" rewritten_ip KeepInert emptyWorkList }
-doInteractWithInert _fdimprs
- (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty })
+doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty })
workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
| wfl `canRewrite` ifl
, tv `elemVarSet` tyVarsOfType ty
= do { rewritten_ip <- rewriteIP (cv,tv,xi) (ipid,ifl,nm,ty)
- ; mkIRContinue workItem DropInert (workListFromCCan rewritten_ip) }
+ ; mkIRContinue "IP/Eq" workItem DropInert (workListFromCCan rewritten_ip) }
-- Two implicit parameter constraints. If the names are the same,
-- but their types are not, we generate a wanted type equality
-- that equates the type (this is "improvement").
-- However, we don't actually need the coercion evidence,
-- so we just generate a fresh coercion variable that isn't used anywhere.
-doInteractWithInert _fdimprs
- (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 })
+doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 })
workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
| nm1 == nm2 && isGiven wfl && isGiven ifl
= -- See Note [Overriding implicit parameters]
-- Dump the inert item, override totally with the new one
-- Do not require type equality
- mkIRContinue workItem DropInert emptyWorkList
+ -- For example, given let ?x::Int = 3 in let ?x::Bool = True in ...
+ -- we must *override* the outer one with the inner one
+ mkIRContinue "IP/IP override" workItem DropInert emptyWorkList
| nm1 == nm2 && ty1 `tcEqType` ty2
= solveOneFromTheOther (id1,ifl) workItem
| nm1 == nm2
= -- See Note [When improvement happens]
- do { co_var <- newWantedCoVar ty1 ty2
+ do { co_var <- newWantedCoVar ty2 ty1 -- See Note [Efficient Orientation]
; let flav = Wanted (combineCtLoc ifl wfl)
; cans <- mkCanonical flav co_var
- ; mkIRContinue workItem KeepInert cans }
+ ; mkIRContinue "IP/IP fundep" workItem KeepInert cans }
+-- Never rewrite a given with a wanted equality, and a type function
+-- equality can never rewrite an equality. We rewrite LHS *and* RHS
+-- of function equalities so that our inert set exposes everything that
+-- we know about equalities.
-- Inert: equality, work item: function equality
-
--- Never rewrite a given with a wanted equality, and a type function
--- equality can never rewrite an equality. Note also that if we have
--- F x1 ~ x2 and a ~ x3, and a occurs in x2, we don't rewrite it. We
--- can wait until F x1 ~ x2 matches another F x1 ~ x4, and only then
--- we will ``expose'' x2 and x4 to rewriting.
-
--- Otherwise, we can try rewriting the type function equality with the equality.
-doInteractWithInert _fdimprs
- (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 })
+doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 })
(CFunEqCan { cc_id = cv2, cc_flavor = wfl, cc_fun = tc
, cc_tyargs = args, cc_rhs = xi2 })
| ifl `canRewrite` wfl
- , tv `elemVarSet` tyVarsOfTypes args
+ , tv `elemVarSet` tyVarsOfTypes (xi2:args) -- Rewrite RHS as well
= do { rewritten_funeq <- rewriteFunEq (cv1,tv,xi1) (cv2,wfl,tc,args,xi2)
- ; mkIRStop KeepInert (workListFromCCan rewritten_funeq) }
- -- must Stop here, because we may no longer be inert after the rewritting.
+ ; mkIRStop "Eq/FunEq" (workListFromCCan rewritten_funeq) }
+ -- Must Stop here, because we may no longer be inert after the rewritting.
-- Inert: function equality, work item: equality
-doInteractWithInert _fdimprs
- (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
+doInteractWithInert (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
, cc_tyargs = args, cc_rhs = xi1 })
workItem@(CTyEqCan { cc_id = cv2, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi2 })
| wfl `canRewrite` ifl
- , tv `elemVarSet` tyVarsOfTypes args
+ , tv `elemVarSet` tyVarsOfTypes (xi1:args) -- Rewrite RHS as well
= do { rewritten_funeq <- rewriteFunEq (cv2,tv,xi2) (cv1,ifl,tc,args,xi1)
- ; mkIRContinue workItem DropInert (workListFromCCan rewritten_funeq) }
-
-doInteractWithInert _fdimprs
- (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
+ ; mkIRContinue "FunEq/Eq" workItem DropInert (workListFromCCan rewritten_funeq) }
+ -- One may think that we could (KeepTransformedInert rewritten_funeq)
+ -- but that is wrong, because it may end up not being inert with respect
+ -- to future inerts. Example:
+ -- Original inert = { F xis ~ [a], b ~ Maybe Int }
+ -- Work item comes along = a ~ [b]
+ -- If we keep { F xis ~ [b] } in the inert set we will end up with:
+ -- { F xis ~ [b], b ~ Maybe Int, a ~ [Maybe Int] }
+ -- At the end, which is *not* inert. So we should unfortunately DropInert here.
+
+doInteractWithInert (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
, cc_tyargs = args1, cc_rhs = xi1 })
workItem@(CFunEqCan { cc_id = cv2, cc_flavor = fl2, cc_fun = tc2
, cc_tyargs = args2, cc_rhs = xi2 })
| fl1 `canSolve` fl2 && lhss_match
= do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
- ; mkIRStop KeepInert cans }
+ ; mkIRStop "FunEq/FunEq" cans }
| fl2 `canSolve` fl1 && lhss_match
= do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
- ; mkIRContinue workItem DropInert cans }
+ ; mkIRContinue "FunEq/FunEq" workItem DropInert cans }
where
lhss_match = tc1 == tc2 && and (zipWith tcEqType args1 args2)
-doInteractWithInert _fdimprs
- inert@(CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
+doInteractWithInert (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 `canSolve` fl2 && tv1 == tv2
= do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2)
- ; mkIRStop KeepInert cans }
+ ; mkIRStop "Eq/Eq lhs" cans }
| fl2 `canSolve` fl1 && tv1 == tv2
= do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1)
- ; mkIRContinue workItem DropInert cans }
+ ; mkIRContinue "Eq/Eq lhs" workItem DropInert cans }
-- Check for rewriting RHS
| fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfType xi2
= do { rewritten_eq <- rewriteEqRHS (cv1,tv1,xi1) (cv2,fl2,tv2,xi2)
- ; mkIRStop KeepInert rewritten_eq }
+ ; mkIRStop "Eq/Eq rhs" rewritten_eq }
+
| 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 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]
+ ; mkIRContinue "Eq/Eq rhs" workItem DropInert rewritten_eq }
- | not $ isGiven fl1, -- The inert is wanted or derived
- isMetaTyVar tv1, -- and has a unification variable lhs
- FlatSkol {} <- tcTyVarDetails tv2, -- And workitem is a flatten skolem equality
- Just tv2' <- tcGetTyVar_maybe xi2, FlatSkol {} <- tcTyVarDetails tv2'
- = mkIRContinue workItem DropInert (workListFromCCan inert)
+doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
+ (CFrozenErr { cc_id = cv2, cc_flavor = fl2 })
+ | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfEvVar cv2
+ = do { rewritten_frozen <- rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
+ ; mkIRStop "Frozen/Eq" rewritten_frozen }
+doInteractWithInert (CFrozenErr { cc_id = cv2, cc_flavor = fl2 })
+ workItem@(CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 })
+ | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfEvVar cv2
+ = do { rewritten_frozen <- rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
+ ; mkIRContinue "Frozen/Eq" workItem DropInert rewritten_frozen }
-- Fall-through case for all other situations
-doInteractWithInert _fdimprs _ workItem = noInteraction workItem
+doInteractWithInert _ workItem = noInteraction workItem
-------------------------
-- Equational Rewriting
; dv' <- newDictVar cl args
; case gw of
Wanted {} -> setDictBind dv (EvCast dv' (mkSymCoercion dict_co))
- _given_or_derived -> setDictBind dv' (EvCast dv dict_co)
+ Given {} -> setDictBind dv' (EvCast dv dict_co)
+ Derived {} -> return () -- Derived dicts we don't set any evidence
+
; return (CDictCan { cc_id = dv'
, cc_flavor = gw
, cc_class = cl
; ipid' <- newIPVar nm ty'
; case gw of
Wanted {} -> setIPBind ipid (EvCast ipid' (mkSymCoercion ip_co))
- _given_or_derived -> setIPBind ipid' (EvCast ipid ip_co)
+ Given {} -> setIPBind ipid' (EvCast ipid ip_co)
+ Derived {} -> return () -- Derived ips: we don't set any evidence
+
; return (CIPCan { cc_id = ipid'
, cc_flavor = gw
, cc_ip_nm = nm
, cc_ip_ty = ty' }) }
rewriteFunEq :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TyCon, [Xi], Xi) -> TcS CanonicalCt
-rewriteFunEq (cv1,tv,xi1) (cv2,gw, tc,args,xi2)
+rewriteFunEq (cv1,tv,xi1) (cv2,gw, tc,args,xi2) -- cv2 :: F args ~ xi2
= do { let arg_cos = substTysWith [tv] [mkCoVarCoercion cv1] args
args' = substTysWith [tv] [xi1] args
- fun_co = mkTyConCoercion tc arg_cos
+ fun_co = mkTyConCoercion tc arg_cos -- fun_co :: F args ~ F args'
+
+ xi2' = substTyWith [tv] [xi1] xi2
+ xi2_co = substTyWith [tv] [mkCoVarCoercion cv1] xi2 -- xi2_co :: xi2 ~ xi2'
; cv2' <- case gw of
- Wanted {} -> do { cv2' <- newWantedCoVar (mkTyConApp tc args') xi2
+ Wanted {} -> do { cv2' <- newWantedCoVar (mkTyConApp tc args') xi2'
; setWantedCoBind cv2 $
- mkTransCoercion fun_co (mkCoVarCoercion cv2')
+ fun_co `mkTransCoercion`
+ mkCoVarCoercion cv2' `mkTransCoercion` mkSymCoercion xi2_co
; return cv2' }
- _giv_or_der -> newGivOrDerCoVar (mkTyConApp tc args') xi2 $
- mkTransCoercion (mkSymCoercion fun_co) (mkCoVarCoercion cv2)
+ Given {} -> newGivenCoVar (mkTyConApp tc args') xi2' $
+ mkSymCoercion fun_co `mkTransCoercion`
+ mkCoVarCoercion cv2 `mkTransCoercion` xi2_co
+ Derived {} -> newDerivedId (EqPred (mkTyConApp tc args') xi2')
+
; return (CFunEqCan { cc_id = cv2'
, cc_flavor = gw
, cc_tyargs = args'
, cc_fun = tc
- , cc_rhs = xi2 }) }
+ , cc_rhs = xi2' }) }
rewriteEqRHS :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor,TcTyVar,Xi) -> TcS WorkList
, tv2 == tv2' -- In this case xi2[xi1/tv1] = tv2, so we have tv2~tv2
= do { when (isWanted gw) (setWantedCoBind cv2 (mkSymCoercion co2'))
; return emptyCCan }
- | otherwise
- = do { cv2' <-
- case gw of
- Wanted {}
- -> do { cv2' <- newWantedCoVar (mkTyVarTy tv2) xi2'
- ; setWantedCoBind cv2 $
+ | otherwise
+ = do { cv2' <-
+ case gw of
+ Wanted {}
+ -> do { cv2' <- newWantedCoVar (mkTyVarTy tv2) xi2'
+ ; setWantedCoBind cv2 $
mkCoVarCoercion cv2' `mkTransCoercion` mkSymCoercion co2'
- ; return cv2' }
- _giv_or_der
- -> newGivOrDerCoVar (mkTyVarTy tv2) xi2' $
+ ; return cv2' }
+ Given {}
+ -> newGivenCoVar (mkTyVarTy tv2) xi2' $
mkCoVarCoercion cv2 `mkTransCoercion` co2'
+ Derived {}
+ -> newDerivedId (EqPred (mkTyVarTy tv2) xi2')
- ; xi2'' <- canOccursCheck gw tv2 xi2' -- we know xi2' is *not* tv2
- ; canEq gw cv2' (mkTyVarTy tv2) xi2''
+ ; canEq gw cv2' (mkTyVarTy tv2) xi2'
}
where
xi2' = substTyWith [tv1] [xi1] xi2
co1 `mkTransCoercion` mkCoVarCoercion cv2'
; return cv2' }
(False,LeftComesFromInert) ->
- newGivOrDerCoVar xi2 xi1 $
- mkSymCoercion (mkCoVarCoercion cv2) `mkTransCoercion` co1
+ if isGiven gw then
+ newGivenCoVar xi2 xi1 $
+ mkSymCoercion (mkCoVarCoercion cv2) `mkTransCoercion` co1
+ else newDerivedId (EqPred xi2 xi1)
(False,RightComesFromInert) ->
- newGivOrDerCoVar xi1 xi2 $
- mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
- ; mkCanonical gw cv2'
- }
+ if isGiven gw then
+ newGivenCoVar xi1 xi2 $
+ mkSymCoercion co1 `mkTransCoercion` mkCoVarCoercion cv2
+ else newDerivedId (EqPred xi1 xi2)
+ ; mkCanonical gw cv2' }
-solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
--- First argument inert, second argument workitem. They both represent
+rewriteFrozen :: (CoVar,TcTyVar,Xi) -> (CoVar,CtFlavor) -> TcS WorkList
+rewriteFrozen (cv1, tv1, xi1) (cv2, fl2)
+ = do { cv2' <-
+ case fl2 of
+ Wanted {} -> do { cv2' <- newWantedCoVar ty2a' ty2b'
+ -- ty2a[xi1/tv1] ~ ty2b[xi1/tv1]
+ ; setWantedCoBind cv2 $
+ co2a' `mkTransCoercion`
+ mkCoVarCoercion cv2' `mkTransCoercion`
+ mkSymCoercion co2b'
+ ; return cv2' }
+
+ Given {} -> newGivenCoVar ty2a' ty2b' $
+ mkSymCoercion co2a' `mkTransCoercion`
+ mkCoVarCoercion cv2 `mkTransCoercion`
+ co2b'
+
+ Derived {} -> newDerivedId (EqPred ty2a' ty2b')
+ ; return (singleCCan $ CFrozenErr { cc_id = cv2', cc_flavor = fl2 }) }
+ where
+ (ty2a, ty2b) = coVarKind cv2 -- cv2 : ty2a ~ ty2b
+ ty2a' = substTyWith [tv1] [xi1] ty2a
+ ty2b' = substTyWith [tv1] [xi1] ty2b
+
+ co2a' = substTyWith [tv1] [mkCoVarCoercion cv1] ty2a -- ty2a ~ ty2a[xi1/tv1]
+ co2b' = substTyWith [tv1] [mkCoVarCoercion cv1] ty2b -- ty2b ~ ty2b[xi1/tv1]
+
+solveOneFromTheOther :: (EvVar, CtFlavor) -> CanonicalCt -> TcS InteractResult
+-- First argument inert, second argument work-item. They both represent
-- wanted/given/derived evidence for the *same* predicate so we try here to
-- discharge one directly from the other.
--
-- Precondition: value evidence only (implicit parameters, classes)
-- not coercion
-solveOneFromTheOther (iid,ifl) workItem
- -- Both derived needs a special case. You might think that we do not need
- -- two evidence terms for the same claim. But, since the evidence is partial,
- -- either evidence may do in some cases; see TcSMonad.isGoodRecEv.
- -- See also Example 3 in Note [Superclasses and recursive dictionaries]
- | isDerived ifl && isDerived wfl
- = noInteraction workItem
+solveOneFromTheOther (iid,ifl) workItem
+ | isDerived wfl
+ = mkIRStop "Solved (derived)" emptyWorkList
| ifl `canSolve` wfl
- = do { unless (isGiven wfl) $ setEvBind wid (EvId iid)
+ = do { when (isWanted wfl) $ setEvBind wid (EvId iid)
-- Overwrite the binding, if one exists
-- For Givens, which are lambda-bound, nothing to overwrite,
- ; dischargeWorkItem }
+ ; mkIRStop "Solved" emptyWorkList }
- | otherwise -- wfl `canSolve` ifl
- = do { unless (isGiven ifl) $ setEvBind iid (EvId wid)
- ; mkIRContinue workItem DropInert emptyWorkList }
+ | wfl `canSolve` ifl
+ = do { when (isWanted ifl) $ setEvBind iid (EvId wid)
+ ; mkIRContinue "Solved inert" workItem DropInert emptyWorkList }
+ | otherwise -- The inert item is Derived, we can just throw it away,
+ = mkIRContinue "Discard derived inert" workItem DropInert emptyWorkList
+
where
wfl = cc_flavor workItem
wid = cc_id workItem
When we simplify a wanted constraint, if we first see a matching
instance, we may produce new wanted work. To (1) avoid doing this work
twice in the future and (2) to handle recursive dictionaries we may ``cache''
-this item as solved (in effect, given) into our inert set and with that add
-its superclass constraints (as given) in our worklist.
+this item as given into our inert set WITHOUT adding its superclass constraints,
+otherwise we'd be in danger of creating a loop [In fact this was the exact reason
+for doing the isGoodRecEv check in an older version of the type checker].
But now we have added partially solved constraints to the worklist which may
interact with other wanteds. Consider the example:
instance Eq a => Foo [a] a --- fooDFun
and wanted (Foo [t] t). We are first going to see that the instance matches
-and create an inert set that includes the solved (Foo [t] t) and its
-superclasses.
+and create an inert set that includes the solved (Foo [t] t) but not its superclasses:
d1 :_g Foo [t] t d1 := EvDFunApp fooDFun d3
- d2 :_g Eq t d2 := EvSuperClass d1 0
Our work list is going to contain a new *wanted* goal
d3 :_w Eq t
-It is wrong to react the wanted (Eq t) with the given (Eq t) because that would
-construct loopy evidence. Hence the check isGoodRecEv in doInteractWithInert.
-OK, so we have ruled out bad behaviour, but how do we ge recursive dictionaries,
-at all? Consider
+Ok, so how do we get recursive dictionaries, at all:
Example 2:
\begin{code}
-- If a work item has any form of interaction with top-level we get this
data TopInteractResult
- = NoTopInt -- No top-level interaction
+ = NoTopInt -- No top-level interaction
+ -- Equivalent to (SomeTopInt emptyWorkList (ContinueWith work_item))
| SomeTopInt
{ tir_new_work :: WorkList -- Sub-goals or new work (could be given,
-- for superclasses)
-- arising from top-level instances.
topReactionsStage :: SimplifierStage
-topReactionsStage workItem inerts
+topReactionsStage depth workItem inerts
= do { tir <- tryTopReact workItem
; case tir of
NoTopInt ->
, sr_new_work = emptyWorkList
, sr_stop = ContinueWith workItem }
SomeTopInt tir_new_work tir_new_inert ->
- return $ SR { sr_inerts = inerts
- , sr_new_work = tir_new_work
- , sr_stop = tir_new_inert
- }
+ do { bumpStepCountTcS
+ ; traceFireTcS depth (ptext (sLit "Top react")
+ <+> vcat [ ptext (sLit "Work =") <+> ppr workItem
+ , ptext (sLit "New =") <+> ppr tir_new_work ])
+ ; return $ SR { sr_inerts = inerts
+ , sr_new_work = tir_new_work
+ , sr_stop = tir_new_inert
+ } }
}
tryTopReact :: WorkItem -> TcS TopInteractResult
else return NoTopInt
}
-allowedTopReaction :: Bool -> WorkItem -> Bool
+allowedTopReaction :: Bool -> WorkItem -> Bool
allowedTopReaction eqs_only (CDictCan {}) = not eqs_only
-allowedTopReaction _ _ = True
-
+allowedTopReaction _ _ = True
doTopReact :: WorkItem -> TcS TopInteractResult
--- The work item does not react with the inert set,
--- so try interaction with top-level instances
+-- The work item does not react with the inert set, so try interaction with top-level instances
+-- NB: The place to add superclasses in *not* in doTopReact stage. Instead superclasses are
+-- added in the worklist as part of the canonicalisation process.
+-- See Note [Adding superclasses] in TcCanonical.
--- Given dictionary; just add superclasses
+-- Given dictionary
-- See Note [Given constraint that matches an instance declaration]
-doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Given loc
+doTopReact (CDictCan { cc_flavor = Given {} })
+ = return NoTopInt -- NB: Superclasses already added since it's canonical
+
+-- Derived dictionary: just look for functional dependencies
+doTopReact workItem@(CDictCan { cc_flavor = fl@(Derived loc)
, cc_class = cls, cc_tyargs = xis })
- = do { sc_work <- newGivenSCWork dv loc cls xis
- ; return $ SomeTopInt sc_work (ContinueWith workItem) }
-
--- Derived dictionary
--- Do not add any further derived superclasses; their
--- full transitive closure has already been added.
--- But do look for functional dependencies
-doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Derived loc _
+ = do { instEnvs <- getInstEnvs
+ ; let fd_eqns = improveFromInstEnv instEnvs
+ (ClassP cls xis, pprArisingAt loc)
+ ; m <- rewriteWithFunDeps fd_eqns xis fl
+ ; case m of
+ Nothing -> return NoTopInt
+ Just (xis',_,fd_work) ->
+ let workItem' = workItem { cc_tyargs = xis' }
+ -- Deriveds are not supposed to have identity (cc_id is unused!)
+ in return $ SomeTopInt { tir_new_work = fd_work
+ , tir_new_inert = ContinueWith workItem' } }
+
+-- Wanted dictionary
+doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = fl@(Wanted loc)
, cc_class = cls, cc_tyargs = xis })
- = do { fd_work <- findClassFunDeps dv cls xis loc
- ; if isEmptyWorkList fd_work then
- return NoTopInt
- else return $ SomeTopInt { tir_new_work = fd_work
- , tir_new_inert = ContinueWith workItem } }
-
-doTopReact workItem@(CDictCan { cc_id = dv, cc_flavor = Wanted loc
- , cc_class = cls, cc_tyargs = xis })
= do { -- See Note [MATCHING-SYNONYMS]
; lkp_inst_res <- matchClassInst cls xis loc
- ; case lkp_inst_res of
- NoInstance ->
- do { traceTcS "doTopReact/ no class instance for" (ppr dv)
- ; fd_work <- findClassFunDeps dv cls xis loc
- ; if isEmptyWorkList fd_work then
- do { sc_work <- newDerivedSCWork dv loc cls xis
- -- See Note [Adding Derived Superclasses]
- -- NB: workItem is inert, but it isn't solved
- -- keep it as inert, although it's not solved
- -- because we have now reacted all its
- -- top-level fundep-induced equalities!
- ; return $ SomeTopInt
- { tir_new_work = fd_work `unionWorkLists` sc_work
- , tir_new_inert = ContinueWith workItem } }
-
- else -- More fundep work produced, don't do any superclass stuff,
- -- just thow him back in the worklist, which will prioritize
- -- the solution of fd equalities
- return $ SomeTopInt
- { tir_new_work = fd_work `unionWorkLists`
- workListFromCCan workItem
- , tir_new_inert = Stop } }
-
- GenInst wtvs ev_term -> -- Solved
+ ; case lkp_inst_res of
+ NoInstance ->
+ do { traceTcS "doTopReact/ no class instance for" (ppr dv)
+
+ ; instEnvs <- getInstEnvs
+ ; let fd_eqns = improveFromInstEnv instEnvs
+ (ClassP cls xis, pprArisingAt loc)
+ ; m <- rewriteWithFunDeps fd_eqns xis fl
+ ; case m of
+ Nothing -> return NoTopInt
+ Just (xis',cos,fd_work) ->
+ do { let dict_co = mkTyConCoercion (classTyCon cls) cos
+ ; dv'<- newDictVar cls xis'
+ ; setDictBind dv (EvCast dv' dict_co)
+ ; let workItem' = CDictCan { cc_id = dv', cc_flavor = fl,
+ cc_class = cls, cc_tyargs = xis' }
+ ; return $
+ SomeTopInt { tir_new_work = singleCCan workItem' `andCCan` fd_work
+ , tir_new_inert = Stop } } }
+
+ GenInst wtvs ev_term -- Solved
-- No need to do fundeps stuff here; the instance
-- matches already so we won't get any more info
-- from functional dependencies
- do { traceTcS "doTopReact/ found class instance for" (ppr dv)
- ; setDictBind dv ev_term
- ; inst_work <- canWanteds wtvs
- ; if null wtvs
+ | null wtvs
+ -> do { traceTcS "doTopReact/ found nullary class instance for" (ppr dv)
+ ; setDictBind dv ev_term
-- Solved in one step and no new wanted work produced.
-- i.e we directly matched a top-level instance
- -- No point in caching this in 'inert', nor in adding superclasses
- then return $ SomeTopInt { tir_new_work = emptyWorkList
- , tir_new_inert = Stop }
-
- -- Solved and new wanted work produced, you may cache the
- -- (tentatively solved) dictionary as Derived and its superclasses
- else do { let solved = makeSolvedByInst workItem
- ; sc_work <- newDerivedSCWork dv loc cls xis
- -- See Note [Adding Derived Superclasses]
- ; return $ SomeTopInt
- { tir_new_work = inst_work `unionWorkLists` sc_work
- , tir_new_inert = ContinueWith solved } }
- } }
+ -- No point in caching this in 'inert'; hence Stop
+ ; return $ SomeTopInt { tir_new_work = emptyWorkList
+ , tir_new_inert = Stop } }
+
+ | otherwise
+ -> do { traceTcS "doTopReact/ found nullary class instance for" (ppr dv)
+ ; setDictBind dv ev_term
+ -- Solved and new wanted work produced, you may cache the
+ -- (tentatively solved) dictionary as Given! (used to be: Derived)
+ ; let solved = workItem { cc_flavor = given_fl }
+ given_fl = Given (setCtLocOrigin loc UnkSkol)
+ ; inst_work <- canWanteds wtvs
+ ; return $ SomeTopInt { tir_new_work = inst_work
+ , tir_new_inert = ContinueWith solved } }
+ }
-- Type functions
doTopReact (CFunEqCan { cc_id = cv, cc_flavor = fl
coe `mkTransCoercion`
mkCoVarCoercion cv'
; return cv' }
- _ -> newGivOrDerCoVar xi rhs_ty $
- mkSymCoercion (mkCoVarCoercion cv) `mkTransCoercion` coe
-
+ Given {} -> newGivenCoVar xi rhs_ty $
+ mkSymCoercion (mkCoVarCoercion cv) `mkTransCoercion` coe
+ Derived {} -> newDerivedId (EqPred xi rhs_ty)
; can_cts <- mkCanonical fl cv'
; return $ SomeTopInt can_cts Stop }
_
-- Any other work item does not react with any top-level equations
doTopReact _workItem = return NoTopInt
-
-----------------------
-findClassFunDeps :: EvVar -> Class -> [Xi] -> WantedLoc -> TcS WorkList
--- Look for a fundep reaction beween the wanted item
--- and a top-level instance declaration
-findClassFunDeps dv cls xis loc
- = do { instEnvs <- getInstEnvs
- ; let eqn_pred_locs = improveFromInstEnv (classInstances instEnvs)
- (ClassP cls xis, ppr dv)
- ; wevvars <- mkWantedFunDepEqns loc eqn_pred_locs
- -- NB: fundeps generate some wanted equalities, but
- -- we don't use their evidence for anything
- ; canWanteds wevvars }
\end{code}
-Note [Adding Derived Superclasses]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Generally speaking, we want to be able to add derived superclasses of
-unsolved wanteds, and wanteds that have been partially being solved
-via an instance. This is important to be able to simplify the inferred
-constraints more (and to allow for recursive dictionaries, less
-importantly). Example:
-
-Inferred wanted constraint is (Eq a, Ord a), but we'd only like to
-quantify over Ord a, hence we would like to be able to add the
-superclass of Ord a as Derived and use it to solve the wanted Eq a.
-
-Hence we will add Derived superclasses in the following two cases:
- (1) When we meet an unsolved wanted in top-level reactions
- (2) When we partially solve a wanted in top-level reactions using an instance decl.
-
-At that point, we have two options:
- (1) Add transitively add *ALL* of the superclasses of the Derived
- (2) Add only the immediate ones, but whenever we meet a Derived in
- the future, add its own superclasses as Derived.
-
-Option (2) is terrible, because deriveds may be rewritten or kicked
-out of the inert set, which will result in slightly rewritten
-superclasses being reintroduced in the worklist and the inert set. Eg:
-
- class C a => B a
- instance Foo a => B [a]
-
-Original constraints:
-[Wanted] d : B [a]
-[Given] co : a ~ Int
-
-We apply the instance to the wanted and put it and its superclasses as
-as Deriveds in the inerts:
-
-[Derived] d : B [a]
-[Derived] (sel d) : C [a]
-
-The work is now:
-[Given] co : a ~ Int
-[Wanted] d' : Foo a
-
-Now, suppose that we interact the Derived with the Given equality, and
-kick him out of the inert, the next time around a superclass C [Int]
-will be produced -- but we already *have* C [a] in the inerts which
-will anyway get rewritten to C [Int].
-
-So we choose (1), and *never* introduce any more superclass work from
-Deriveds. This enables yet another optimisation: If we ever meet an
-equality that can rewrite a Derived, if that Derived is a superclass
-derived (like C [a] above), i.e. not a partially solved one (like B
-[a]) above, we may simply completely *discard* that Derived. The
-reason is because somewhere in the inert lies the original wanted, or
-partially solved constraint that gave rise to that superclass, and
-that constraint *will* be kicked out, and *will* result in the
-rewritten superclass to be added in the inerts later on, anyway.
-
-
Note [FunDep and implicit parameter reactions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note [New Wanted Superclass Work]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Even in the case of wanted constraints, we add all of its superclasses as
-new given work. There are several reasons for this:
- a) to minimise error messages;
- eg suppose we have wanted (Eq a, Ord a)
- then we report only (Ord a) unsoluble
-
- b) to make the smallest number of constraints when *inferring* a type
- (same Eq/Ord example)
+Even in the case of wanted constraints, we may add some superclasses
+as new given work. The reason is:
- c) for recursive dictionaries we *must* add the superclasses
- so that we can use them when solving a sub-problem
-
- d) To allow FD-like improvement for type families. Assume that
+ To allow FD-like improvement for type families. Assume that
we have a class
class C a b | a -> b
and we have to solve the implication constraint:
equalities that have a touchable in their RHS, *in addition*
to solving wanted equalities.
-Here is another example where this is useful.
+We also need to somehow use the superclasses to quantify over a minimal,
+constraint see note [Minimize by Superclasses] in TcSimplify.
+
+
+Finally, here is another example where this is useful.
Example 1:
----------
that participate in recursive dictionary bindings.
\begin{code}
-
-newGivenSCWork :: EvVar -> GivenLoc -> Class -> [Xi] -> TcS WorkList
-newGivenSCWork ev loc cls xis
- | NoScSkol <- ctLocOrigin loc -- Very important!
- = return emptyWorkList
- | otherwise
- = newImmSCWorkFromFlavored ev (Given loc) cls xis >>= return
-
-newDerivedSCWork :: EvVar -> WantedLoc -> Class -> [Xi] -> TcS WorkList
-newDerivedSCWork ev loc cls xis
- = do { ims <- newImmSCWorkFromFlavored ev flavor cls xis
- ; rec_sc_work ims }
- where
- rec_sc_work :: CanonicalCts -> TcS CanonicalCts
- rec_sc_work cts
- = do { bg <- mapBagM (\c -> do { ims <- imm_sc_work c
- ; recs_ims <- rec_sc_work ims
- ; return $ consBag c recs_ims }) cts
- ; return $ concatBag bg }
- imm_sc_work (CDictCan { cc_id = dv, cc_flavor = fl, cc_class = cls, cc_tyargs = xis })
- = newImmSCWorkFromFlavored dv fl cls xis
- imm_sc_work _ct = return emptyCCan
-
- flavor = Derived loc DerSC
-
-newImmSCWorkFromFlavored :: EvVar -> CtFlavor -> Class -> [Xi] -> TcS WorkList
--- Returns immediate superclasses
-newImmSCWorkFromFlavored ev flavor cls xis
- = do { let (tyvars, sc_theta, _, _) = classBigSig cls
- sc_theta1 = substTheta (zipTopTvSubst tyvars xis) sc_theta
- ; sc_vars <- zipWithM inst_one sc_theta1 [0..]
- ; mkCanonicals flavor sc_vars }
- where
- inst_one pred n = newGivOrDerEvVar pred (EvSuperClass ev n)
-
-
data LookupInstResult
= NoInstance
| GenInst [WantedEvVar] EvTerm
; tys <- instDFunTypes mb_inst_tys
; let (theta, _) = tcSplitPhiTy (applyTys (idType dfun_id) tys)
; if null theta then
- return (GenInst [] (EvDFunApp dfun_id tys []))
+ return (GenInst [] (EvDFunApp dfun_id tys []))
else do
{ ev_vars <- instDFunConstraints theta
- ; let wevs = [WantedEvVar w loc | w <- ev_vars]
+ ; let wevs = [EvVarX w loc | w <- ev_vars]
; return $ GenInst wevs (EvDFunApp dfun_id tys ev_vars) }
}
}
projects / ghc-hetmet.git / blobdiff