\begin{code}
module TcInteract (
solveInteractWanted,
solveInteractGiven,
solveInteractCts,
) where
#include "HsVersions.h"
import BasicTypes ()
import TcCanonical
import VarSet
import Type
import Unify
import Id
import Var
import VarEnv ( )
import TcType
import Class
import TyCon
import Name
import IParam
import FunDeps
import TcEvidence
import Outputable
import TcRnTypes
import TcErrors
import TcSMonad
import Maybes( orElse )
import Bag
import Control.Monad ( foldM )
import TrieMap
import VarEnv
import qualified Data.Traversable as Traversable
import Control.Monad( when )
import Pair ( pSnd )
import UniqFM
import FastString ( sLit )
import DynFlags
\end{code}
**********************************************************************
* *
* Main Interaction Solver *
* *
**********************************************************************
Note [Basic Simplifier Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1. Pick an element from the WorkList if there exists one with depth
less thanour context-stack depth.
2. Run it down the 'stage' pipeline. Stages are:
- canonicalization
- inert reactions
- spontaneous reactions
- top-level intreactions
Each stage returns a StopOrContinue and may have sideffected
the inerts or worklist.
The threading of the stages is as follows:
- If (Stop) is returned by a stage then we start again from Step 1.
- If (ContinueWith ct) is returned by a stage, we feed 'ct' on to
the next stage in the pipeline.
4. If the element has survived (i.e. ContinueWith x) the last stage
then we add him in the inerts and jump back to Step 1.
If in Step 1 no such element exists, we have exceeded our context-stack
depth and will simply fail.
\begin{code}
solveInteractCts :: [Ct] -> TcS ()
solveInteractCts cts
= do { evvar_cache <- getTcSEvVarCacheMap
; (cts_thinner, new_evvar_cache) <- add_cts_in_cache evvar_cache cts
; traceTcS "solveInteractCts" (vcat [ text "cts_original =" <+> ppr cts,
text "cts_thinner =" <+> ppr cts_thinner
])
; setTcSEvVarCacheMap new_evvar_cache
; updWorkListTcS (appendWorkListCt cts_thinner) >> solveInteract }
where
add_cts_in_cache evvar_cache cts
= do { ctxt <- getTcSContext
; foldM (solve_or_cache (simplEqsOnly ctxt)) ([],evvar_cache) cts }
solve_or_cache :: Bool
-> ([Ct],TypeMap (EvVar,CtFlavor))
-> Ct
-> TcS ([Ct],TypeMap (EvVar,CtFlavor))
solve_or_cache eqs_only (acc_cts,acc_cache) ct
| dont_cache eqs_only (classifyPredType pred_ty)
= return (ct:acc_cts,acc_cache)
| Just (ev',fl') <- lookupTM pred_ty acc_cache
, fl' `canSolve` fl
, isWanted fl
= do { _ <- setEvBind ev (EvId ev') fl
; return (acc_cts,acc_cache) }
| otherwise
= return (ct:acc_cts, alterTM pred_ty (\_ -> Just (ev,fl)) acc_cache)
where fl = cc_flavor ct
ev = cc_id ct
pred_ty = ctPred ct
dont_cache :: Bool -> PredTree -> Bool
dont_cache _ (IPPred {}) = True
dont_cache _ (EqPred ty1 ty2)
| Just tc1 <- tyConAppTyCon_maybe ty1
, Just tc2 <- tyConAppTyCon_maybe ty2
, tc1 /= tc2
= isDecomposableTyCon tc1 && isDecomposableTyCon tc2
| otherwise = False
dont_cache eqs_only _ = eqs_only
solveInteractGiven :: GivenLoc -> [EvVar] -> TcS ()
solveInteractGiven gloc evs
= solveInteractCts (map mk_noncan evs)
where mk_noncan ev = CNonCanonical { cc_id = ev
, cc_flavor = Given gloc GivenOrig
, cc_depth = 0 }
solveInteractWanted :: [WantedEvVar] -> TcS ()
solveInteractWanted wevs
= solveInteractCts (map mk_noncan wevs)
where mk_noncan (EvVarX v w)
= CNonCanonical { cc_id = v, cc_flavor = Wanted w, cc_depth = 0 }
solveInteract :: TcS ()
solveInteract
=
do { dyn_flags <- getDynFlags
; let max_depth = ctxtStkDepth dyn_flags
solve_loop
=
do { sel <- selectNextWorkItem max_depth
; case sel of
NoWorkRemaining
-> return ()
MaxDepthExceeded ct
-> solverDepthErrorTcS (cc_depth ct) [ct]
NextWorkItem ct
-> runSolverPipeline thePipeline ct >> solve_loop }
; solve_loop }
type WorkItem = Ct
type SimplifierStage = WorkItem -> TcS StopOrContinue
continueWith :: WorkItem -> TcS StopOrContinue
continueWith work_item = return (ContinueWith work_item)
data SelectWorkItem
= NoWorkRemaining
| MaxDepthExceeded Ct
| NextWorkItem Ct
selectNextWorkItem :: SubGoalDepth
-> TcS SelectWorkItem
selectNextWorkItem max_depth
= updWorkListTcS_return pick_next
where
pick_next :: WorkList -> (SelectWorkItem, WorkList)
pick_next wl = case selectWorkItem wl of
(Nothing,_)
-> (NoWorkRemaining,wl)
(Just ct, new_wl)
| cc_depth ct > max_depth
-> (MaxDepthExceeded ct,new_wl)
(Just ct, new_wl)
-> (NextWorkItem ct, new_wl)
runSolverPipeline :: [(String,SimplifierStage)]
-> WorkItem
-> TcS ()
runSolverPipeline pipeline workItem
= do { initial_is <- getTcSInerts
; traceTcS "Start solver pipeline {" $
vcat [ ptext (sLit "work item = ") <+> ppr workItem
, ptext (sLit "inerts = ") <+> ppr initial_is]
; final_res <- run_pipeline pipeline (ContinueWith workItem)
; final_is <- getTcSInerts
; case final_res of
Stop -> do { traceTcS "End solver pipeline (discharged) }"
(ptext (sLit "inerts = ") <+> ppr final_is)
; return () }
ContinueWith ct -> do { traceTcS "End solver pipeline (not discharged) }" $
vcat [ ptext (sLit "final_item = ") <+> ppr ct
, ptext (sLit "inerts = ") <+> ppr final_is]
; updInertSetTcS ct }
}
where run_pipeline :: [(String,SimplifierStage)] -> StopOrContinue -> TcS StopOrContinue
run_pipeline [] res = return res
run_pipeline _ Stop = return Stop
run_pipeline ((stg_name,stg):stgs) (ContinueWith ct)
= do { traceTcS ("runStage " ++ stg_name ++ " {")
(text "workitem = " <+> ppr ct)
; res <- stg ct
; traceTcS ("end stage " ++ stg_name ++ " }") empty
; run_pipeline stgs res
}
\end{code}
Example 1:
Inert: {c ~ d, F a ~ t, b ~ Int, a ~ ty} (all given)
Reagent: a ~ [b] (given)
React with (c~d) ==> IR (ContinueWith (a~[b])) True []
React with (F a ~ t) ==> IR (ContinueWith (a~[b])) False [F [b] ~ t]
React with (b ~ Int) ==> IR (ContinueWith (a~[Int]) True []
Example 2:
Inert: {c ~w d, F a ~g t, b ~w Int, a ~w ty}
Reagent: a ~w [b]
React with (c ~w d) ==> IR (ContinueWith (a~[b])) True []
React with (F a ~g t) ==> IR (ContinueWith (a~[b])) True [] (can't rewrite given with wanted!)
etc.
Example 3:
Inert: {a ~ Int, F Int ~ b} (given)
Reagent: F a ~ b (wanted)
React with (a ~ Int) ==> IR (ContinueWith (F Int ~ b)) True []
React with (F Int ~ b) ==> IR Stop True [] -- after substituting we re-canonicalize and get nothing
\begin{code}
thePipeline :: [(String,SimplifierStage)]
thePipeline = [ ("canonicalization", canonicalizationStage)
, ("spontaneous solve", spontaneousSolveStage)
, ("interact with inerts", interactWithInertsStage)
, ("top-level reactions", topReactionsStage) ]
\end{code}
\begin{code}
canonicalizationStage :: SimplifierStage
canonicalizationStage = TcCanonical.canonicalize
\end{code}
*********************************************************************************
* *
The spontaneous-solve Stage
* *
*********************************************************************************
Note [Efficient Orientation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There are two cases where we have to be careful about
orienting equalities to get better efficiency.
Case 1: In Rewriting Equalities (function rewriteEqLHS)
When rewriting two equalities with the same LHS:
(a) (tv ~ xi1)
(b) (tv ~ xi2)
We have a choice of producing work (xi1 ~ xi2) (up-to the
canonicalization invariants) However, to prevent the inert items
from getting kicked out of the inerts first, we prefer to
canonicalize (xi1 ~ xi2) if (b) comes from the inert set, or (xi2
~ xi1) if (a) comes from the inert set.
This choice is implemented using the WhichComesFromInert flag.
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
= do { mSolve <- trySpontaneousSolve workItem
; spont_solve mSolve }
where spont_solve SPCantSolve
| isCTyEqCan workItem
= do { kickOutRewritableInerts workItem
; return Stop }
| otherwise
= continueWith workItem
spont_solve (SPSolved workItem')
= do { bumpStepCountTcS
; traceFireTcS (cc_depth workItem) $
ptext (sLit "Spontaneous")
<+> parens (ppr (cc_flavor workItem)) <+> ppr workItem
; kickOutRewritableInerts workItem'
; return Stop }
kickOutRewritableInerts :: Ct -> TcS ()
kickOutRewritableInerts ct
=
do { (wl,ieqs) <-
modifyInertTcS (kick_out_rewritable ct)
; let ct_coercion = getCtCoercion ct
; new_ieqs <-
rewriteInertEqsFromInertEq (cc_tyvar ct,ct_coercion, cc_flavor ct) ieqs
; modifyInertTcS (\is -> ((), is { inert_eqs = new_ieqs }))
; updInertSetTcS ct
; traceTcS "Kick out" (ppr ct $$ ppr wl)
; updWorkListTcS (unionWorkList wl) }
rewriteInertEqsFromInertEq :: (TcTyVar, TcCoercion, CtFlavor)
-> TyVarEnv (Ct, TcCoercion)
-> TcS (TyVarEnv (Ct,TcCoercion))
rewriteInertEqsFromInertEq (subst_tv, subst_co, subst_fl) ieqs
= do { mieqs <- Traversable.mapM do_one ieqs
; traceTcS "Original inert equalities:" (ppr ieqs)
; let flatten_justs elem venv
| Just (act,aco) <- elem = extendVarEnv venv (cc_tyvar act) (act,aco)
| otherwise = venv
final_ieqs = foldVarEnv flatten_justs emptyVarEnv mieqs
; traceTcS "Remaining inert equalities:" (ppr final_ieqs)
; return final_ieqs }
where do_one (ct,inert_co)
| subst_fl `canRewrite` fl && (subst_tv `elemVarSet` tyVarsOfCt ct)
= if fl `canRewrite` subst_fl then
do { (ct',inert_co') <- rewrite_on_the_spot (ct,inert_co)
; return $ Just (ct',inert_co') }
else
do { updWorkListTcS (extendWorkListEq ct)
; return Nothing }
| otherwise
= return $ Just (ct,inert_co)
where
rewrite_on_the_spot (ct,_inert_co)
= do { let rhs' = pSnd (tcCoercionKind co)
; delCachedEvVar ev fl
; evc <- newEqVar fl (mkTyVarTy tv) rhs'
; let ev' = evc_the_evvar evc
; let evco' = mkTcCoVarCo ev'
; fl' <- if isNewEvVar evc then
do { case fl of
Wanted {}
-> setEqBind ev (evco' `mkTcTransCo` mkTcSymCo co) fl
Given {}
-> setEqBind ev' (mkTcCoVarCo ev `mkTcTransCo` co) fl
Derived {}
-> return fl }
else
if isWanted fl then
setEqBind ev (evco' `mkTcTransCo` mkTcSymCo co) fl
else return fl
; let ct' = ct { cc_id = ev', cc_flavor = fl', cc_rhs = rhs' }
; return (ct',evco') }
ev = cc_id ct
fl = cc_flavor ct
tv = cc_tyvar ct
rhs = cc_rhs ct
co = liftTcCoSubstWith [subst_tv] [subst_co] rhs
kick_out_rewritable :: Ct -> InertSet -> ((WorkList,TyVarEnv (Ct,TcCoercion)), InertSet)
kick_out_rewritable ct (IS { inert_eqs = eqmap
, inert_eq_tvs = inscope
, inert_dicts = dictmap
, inert_ips = ipmap
, inert_funeqs = funeqmap
, inert_irreds = irreds
, inert_frozen = frozen
} )
= ((kicked_out, eqmap), remaining)
where
kicked_out = WorkList { wl_eqs = []
, wl_funeqs = bagToList feqs_out
, wl_rest = bagToList (fro_out `andCts` dicts_out
`andCts` ips_out `andCts` irs_out) }
remaining = IS { inert_eqs = emptyVarEnv
, inert_eq_tvs = inscope
, inert_dicts = dicts_in
, inert_ips = ips_in
, inert_funeqs = feqs_in
, inert_irreds = irs_in
, inert_frozen = fro_in
}
fl = cc_flavor ct
tv = cc_tyvar ct
(ips_out, ips_in) = partitionCCanMap rewritable ipmap
(feqs_out, feqs_in) = partitionCtTypeMap rewritable funeqmap
(dicts_out, dicts_in) = partitionCCanMap rewritable dictmap
(irs_out, irs_in) = partitionBag rewritable irreds
(fro_out, fro_in) = partitionBag rewritable frozen
rewritable ct = (fl `canRewrite` cc_flavor ct) &&
(tv `elemVarSet` tyVarsOfCt ct)
\end{code}
Note [Delicate equality kick-out]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Delicate:
When kicking out rewritable constraints, it would be safe to simply
kick out all rewritable equalities, but instead we only kick out those
that, when rewritten, may result in occur-check errors. We rewrite the
rest on the spot. Example:
WorkItem = [S] a ~ b
Inerts = { [W] b ~ [a] }
Now at this point the work item cannot be further rewritten by the
inert (due to the weaker inert flavor), so we are examining if we can
instead rewrite the inert from the workitem. But if we rewrite it on
the spot we have to recanonicalize because of the danger of occurs
errors. On the other hand if the inert flavor was just as powerful or
more powerful than the workitem flavor, the work-item could not have
reached this stage (because it would have already been rewritten by
the inert).
The coclusion is: we kick out the 'dangerous' equalities that may
require recanonicalization (occurs checks) and the rest we rewrite
unconditionally without further checks, on-the-spot with function
rewriteInertEqsFromInertEq.
\begin{code}
data SPSolveResult = SPCantSolve
| SPSolved WorkItem
trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
trySpontaneousSolve workItem@(CTyEqCan { cc_id = eqv, cc_flavor = gw
, cc_tyvar = tv1, cc_rhs = xi, cc_depth = d })
| isGivenOrSolved gw
= return SPCantSolve
| Just tv2 <- tcGetTyVar_maybe xi
= do { tch1 <- isTouchableMetaTyVar tv1
; tch2 <- isTouchableMetaTyVar tv2
; case (tch1, tch2) of
(True, True) -> trySpontaneousEqTwoWay d eqv gw tv1 tv2
(True, False) -> trySpontaneousEqOneWay d eqv gw tv1 xi
(False, True) -> trySpontaneousEqOneWay d eqv gw tv2 (mkTyVarTy tv1)
_ -> return SPCantSolve }
| otherwise
= do { tch1 <- isTouchableMetaTyVar tv1
; if tch1 then trySpontaneousEqOneWay d eqv gw tv1 xi
else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" $
ppr workItem
; return SPCantSolve }
}
trySpontaneousSolve _ = return SPCantSolve
trySpontaneousEqOneWay :: SubGoalDepth
-> EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
trySpontaneousEqOneWay d eqv gw tv xi
| not (isSigTyVar tv) || isTyVarTy xi
= do { let kxi = typeKind xi
; is_sub_kind <- kxi `isSubKindTcS` tyVarKind tv
; if is_sub_kind then
solveWithIdentity d eqv gw tv xi
else return SPCantSolve
}
| otherwise
= return SPCantSolve
trySpontaneousEqTwoWay :: SubGoalDepth
-> EqVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
trySpontaneousEqTwoWay d eqv gw tv1 tv2
= do { k1_sub_k2 <- k1 `isSubKindTcS` k2
; if k1_sub_k2 && nicer_to_update_tv2
then solveWithIdentity d eqv gw tv2 (mkTyVarTy tv1)
else do
{ k2_sub_k1 <- k2 `isSubKindTcS` k1
; MASSERT( k2_sub_k1 )
; solveWithIdentity d eqv gw tv1 (mkTyVarTy tv2) } }
where
k1 = tyVarKind tv1
k2 = tyVarKind tv2
nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}
Note [Kind errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the wanted problem:
alpha ~ (# Int, Int #)
where alpha :: ArgKind 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 @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 '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.
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.)
- 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 = (->)
f :: forall a. ((T a ~ (->)) => ...) -> a -> ...
flop = f (...) True
Whereas we would be able to apply the type instance, we would not be able to
use the given (T Bool ~ (->)) in the body of 'flop'
Note [Avoid double unifications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The spontaneous solver has to return a given which mentions the unified unification
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 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 resulting given so that the unification
variable is on the left. [Note that alternatively we could attempt to
enforce this at canonicalization]
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}
solveWithIdentity :: SubGoalDepth
-> EqVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
solveWithIdentity d eqv wd tv xi
= do { traceTcS "Sneaky unification:" $
vcat [text "Coercion variable: " <+> ppr eqv <+> 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
; let refl_xi = mkTcReflCo xi
; let solved_fl = mkSolvedFlavor wd UnkSkol (EvCoercion refl_xi)
; (_,eqv_given) <- newGivenEqVar solved_fl (mkTyVarTy tv) xi refl_xi
; when (isWanted wd) $ do { _ <- setEqBind eqv refl_xi wd; return () }
; return $ SPSolved (CTyEqCan { cc_id = eqv_given
, cc_flavor = solved_fl
, cc_tyvar = tv, cc_rhs = xi, cc_depth = d }) }
\end{code}
*********************************************************************************
* *
The interact-with-inert Stage
* *
*********************************************************************************
Note [The Solver Invariant]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
We always add Givens first. So you might think that the solver has
the invariant
If the work-item is Given,
then the inert item must Given
But this isn't quite true. Suppose we have,
c1: [W] beta ~ [alpha], c2 : [W] blah, c3 :[W] alpha ~ Int
After processing the first two, we get
c1: [G] beta ~ [alpha], c2 : [W] blah
Now, c3 does not interact with the the given c1, so when we spontaneously
solve c3, we must re-react it with the inert set. So we can attempt a
reaction between inert c2 [W] and work-item c3 [G].
It *is* true that [Solver Invariant]
If the work-item is Given,
AND there is a reaction
then the inert item must Given
or, equivalently,
If the work-item is Given,
and the inert item is Wanted/Derived
then there is no reaction
\begin{code}
data InteractResult
= IRWorkItemConsumed { ir_fire :: String }
| IRInertConsumed { ir_fire :: String }
| IRKeepGoing { ir_fire :: String }
irWorkItemConsumed :: String -> TcS InteractResult
irWorkItemConsumed str = return (IRWorkItemConsumed str)
irInertConsumed :: String -> TcS InteractResult
irInertConsumed str = return (IRInertConsumed str)
irKeepGoing :: String -> TcS InteractResult
irKeepGoing str = return (IRKeepGoing str)
interactWithInertsStage :: WorkItem -> TcS StopOrContinue
interactWithInertsStage wi
= do { ctxt <- getTcSContext
; if simplEqsOnly ctxt then
return (ContinueWith wi)
else
extractRelevantInerts wi >>=
foldlBagM interact_next (ContinueWith wi) }
where interact_next Stop atomic_inert
= updInertSetTcS atomic_inert >> return Stop
interact_next (ContinueWith wi) atomic_inert
= do { ir <- doInteractWithInert atomic_inert wi
; let mk_msg rule keep_doc
= text rule <+> keep_doc
<+> vcat [ ptext (sLit "Inert =") <+> ppr atomic_inert
, ptext (sLit "Work =") <+> ppr wi ]
; case ir of
IRWorkItemConsumed { ir_fire = rule }
-> do { bumpStepCountTcS
; traceFireTcS (cc_depth wi)
(mk_msg rule (text "WorkItemConsumed"))
; updInertSetTcS atomic_inert
; return Stop }
IRInertConsumed { ir_fire = rule }
-> do { bumpStepCountTcS
; traceFireTcS (cc_depth atomic_inert)
(mk_msg rule (text "InertItemConsumed"))
; return (ContinueWith wi) }
IRKeepGoing {}
-> do { updInertSetTcS atomic_inert
; return (ContinueWith wi) }
}
data WhichComesFromInert = LeftComesFromInert | RightComesFromInert
doInteractWithInert :: Ct -> Ct -> TcS InteractResult
doInteractWithInert
inertItem@(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
= do { let pty1 = mkClassPred cls1 tys1
pty2 = mkClassPred cls2 tys2
inert_pred_loc = (pty1, pprFlavorArising fl1)
work_item_pred_loc = (pty2, pprFlavorArising fl2)
; traceTcS "doInteractWithInert" (vcat [ text "inertItem = " <+> ppr inertItem
, text "workItem = " <+> ppr workItem ])
; any_fundeps
<- if isGivenOrSolved fl1 && isGivenOrSolved fl2 then return Nothing
else let fd_eqns = improveFromAnother inert_pred_loc work_item_pred_loc
wloc = get_workitem_wloc fl2
in rewriteWithFunDeps fd_eqns tys2 wloc
; case any_fundeps of
Nothing
| eqTypes tys1 tys2 -> solveOneFromTheOther "Cls/Cls" (EvId d1,fl1) workItem
| otherwise -> irKeepGoing "NOP"
Just (_rewritten_tys2,_cos2,fd_work)
-> do { emitFDWorkAsDerived fd_work (cc_depth workItem)
; irKeepGoing "Cls/Cls (new fundeps)" }
}
where get_workitem_wloc (Wanted wl) = wl
get_workitem_wloc (Derived wl) = wl
get_workitem_wloc (Given {}) = panic "Unexpected given!"
doInteractWithInert (CIrredEvCan { cc_id = id1, cc_flavor = ifl, cc_ty = ty1 })
workItem@(CIrredEvCan { cc_ty = ty2 })
| ty1 `eqType` ty2
= solveOneFromTheOther "Irred/Irred" (EvId id1,ifl) workItem
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 && isGivenOrSolved wfl && isGivenOrSolved ifl
=
irInertConsumed "IP/IP (override inert)"
| nm1 == nm2 && ty1 `eqType` ty2
= solveOneFromTheOther "IP/IP" (EvId id1,ifl) workItem
| nm1 == nm2
=
do { let flav = Wanted (combineCtLoc ifl wfl)
; eqv <- newEqVar flav ty2 ty1
; when (isNewEvVar eqv) $
(let ct = CNonCanonical { cc_id = evc_the_evvar eqv
, cc_flavor = flav
, cc_depth = cc_depth workItem }
in updWorkListTcS (extendWorkListEq ct))
; case wfl of
Given {} -> pprPanic "Unexpected given IP" (ppr workItem)
Derived {} -> pprPanic "Unexpected derived IP" (ppr workItem)
Wanted {} ->
do { _ <- setEvBind (cc_id workItem)
(mkEvCast id1 (mkTcSymCo (mkTcTyConAppCo (ipTyCon nm1) [mkTcCoVarCo (evc_the_evvar eqv)]))) wfl
; irWorkItemConsumed "IP/IP (solved by rewriting)" } }
doInteractWithInert (CFunEqCan { cc_id = eqv1, cc_flavor = fl1, cc_fun = tc1
, cc_tyargs = args1, cc_rhs = xi1, cc_depth = d1 })
(CFunEqCan { cc_id = eqv2, cc_flavor = fl2, cc_fun = tc2
, cc_tyargs = args2, cc_rhs = xi2, cc_depth = d2 })
| lhss_match
, Just (GivenSolved {}) <- isGiven_maybe fl1
, isGivenOrSolved fl2
= irInertConsumed "FunEq/FunEq"
| lhss_match
, Just (GivenSolved {}) <- isGiven_maybe fl2
, isGivenOrSolved fl1
= irWorkItemConsumed "FunEq/FunEq"
| fl1 `canSolve` fl2 && lhss_match
= do { rewriteEqLHS LeftComesFromInert (eqv1,xi1) (eqv2,d2,fl2,xi2)
; irWorkItemConsumed "FunEq/FunEq" }
| fl2 `canSolve` fl1 && lhss_match
= do { rewriteEqLHS RightComesFromInert (eqv2,xi2) (eqv1,d1,fl1,xi1)
; irInertConsumed "FunEq/FunEq"}
where
lhss_match = tc1 == tc2 && eqTypes args1 args2
doInteractWithInert _ _ = irKeepGoing "NOP"
rewriteEqLHS :: WhichComesFromInert -> (EqVar,Xi) -> (EqVar,SubGoalDepth,CtFlavor,Xi) -> TcS ()
rewriteEqLHS LeftComesFromInert (eqv1,xi1) (eqv2,d,gw,xi2)
= do { delCachedEvVar eqv2 gw
; evc <- newEqVar gw xi2 xi1
; let eqv2' = evc_the_evvar evc
; gw' <- case gw of
Wanted {}
-> setEqBind eqv2
(mkTcCoVarCo eqv1 `mkTcTransCo` mkTcSymCo (mkTcCoVarCo eqv2')) gw
Given {}
-> setEqBind eqv2'
(mkTcSymCo (mkTcCoVarCo eqv2) `mkTcTransCo` mkTcCoVarCo eqv1) gw
Derived {}
-> return gw
; when (isNewEvVar evc) $
updWorkListTcS (extendWorkListEq (CNonCanonical { cc_id = eqv2'
, cc_flavor = gw'
, cc_depth = d } ) ) }
rewriteEqLHS RightComesFromInert (eqv1,xi1) (eqv2,d,gw,xi2)
= do { delCachedEvVar eqv2 gw
; evc <- newEqVar gw xi1 xi2
; let eqv2' = evc_the_evvar evc
; gw' <- case gw of
Wanted {}
-> setEqBind eqv2
(mkTcCoVarCo eqv1 `mkTcTransCo` mkTcCoVarCo eqv2') gw
Given {}
-> setEqBind eqv2'
(mkTcSymCo (mkTcCoVarCo eqv1) `mkTcTransCo` mkTcCoVarCo eqv2) gw
Derived {}
-> return gw
; when (isNewEvVar evc) $
updWorkListTcS (extendWorkListEq (CNonCanonical { cc_id = eqv2'
, cc_flavor = gw'
, cc_depth = d } ) ) }
solveOneFromTheOther :: String
-> (EvTerm, CtFlavor)
-> Ct
-> TcS InteractResult
solveOneFromTheOther info (ev_term,ifl) workItem
| isDerived wfl
= irWorkItemConsumed ("Solved[DW] " ++ info)
| isDerived ifl
= irInertConsumed ("Solved[DI] " ++ info)
| Just (GivenSolved {}) <- isGiven_maybe ifl, isGivenOrSolved wfl
= irInertConsumed ("Solved[SI] " ++ info)
| otherwise
= ASSERT( ifl `canSolve` wfl )
do { when (isWanted wfl) $ do { _ <- setEvBind wid ev_term wfl; return () }
; irWorkItemConsumed ("Solved " ++ info) }
where
wfl = cc_flavor workItem
wid = cc_id workItem
\end{code}
Note [Superclasses and recursive dictionaries]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Overlaps with Note [SUPERCLASS-LOOP 1]
Note [SUPERCLASS-LOOP 2]
Note [Recursive instances and superclases]
ToDo: check overlap and delete redundant stuff
Right before adding a given into the inert set, we must
produce some more work, that will bring the superclasses
of the given into scope. The superclass constraints go into
our worklist.
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 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:
Example 1:
class Eq b => Foo a b --- 0-th selector
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) but not its superclasses:
d1 :_g Foo [t] t d1 := EvDFunApp fooDFun d3
Our work list is going to contain a new *wanted* goal
d3 :_w Eq t
Ok, so how do we get recursive dictionaries, at all:
Example 2:
data D r = ZeroD | SuccD (r (D r));
instance (Eq (r (D r))) => Eq (D r) where
ZeroD == ZeroD = True
(SuccD a) == (SuccD b) = a == b
_ == _ = False;
equalDC :: D [] -> D [] -> Bool;
equalDC = (==);
We need to prove (Eq (D [])). Here's how we go:
d1 :_w Eq (D [])
by instance decl, holds if
d2 :_w Eq [D []]
where d1 = dfEqD d2
*BUT* we have an inert set which gives us (no superclasses):
d1 :_g Eq (D [])
By the instance declaration of Eq we can show the 'd2' goal if
d3 :_w Eq (D [])
where d2 = dfEqList d3
d1 = dfEqD d2
Now, however this wanted can interact with our inert d1 to set:
d3 := d1
and solve the goal. Why was this interaction OK? Because, if we chase the
evidence of d1 ~~> dfEqD d2 ~~-> dfEqList d3, so by setting d3 := d1 we
are really setting
d3 := dfEqD2 (dfEqList d3)
which is FINE because the use of d3 is protected by the instance function
applications.
So, our strategy is to try to put solved wanted dictionaries into the
inert set along with their superclasses (when this is meaningful,
i.e. when new wanted goals are generated) but solve a wanted dictionary
from a given only in the case where the evidence variable of the
wanted is mentioned in the evidence of the given (recursively through
the evidence binds) in a protected way: more instance function applications
than superclass selectors.
Here are some more examples from GHC's previous type checker
Example 3:
This code arises in the context of "Scrap Your Boilerplate with Class"
class Sat a
class Data ctx a
instance Sat (ctx Char) => Data ctx Char -- dfunData1
instance (Sat (ctx [a]), Data ctx a) => Data ctx [a] -- dfunData2
class Data Maybe a => Foo a
instance Foo t => Sat (Maybe t) -- dfunSat
instance Data Maybe a => Foo a -- dfunFoo1
instance Foo a => Foo [a] -- dfunFoo2
instance Foo [Char] -- dfunFoo3
Consider generating the superclasses of the instance declaration
instance Foo a => Foo [a]
So our problem is this
d0 :_g Foo t
d1 :_w Data Maybe [t]
We may add the given in the inert set, along with its superclasses
[assuming we don't fail because there is a matching instance, see
tryTopReact, given case ]
Inert:
d0 :_g Foo t
WorkList
d01 :_g Data Maybe t -- d2 := EvDictSuperClass d0 0
d1 :_w Data Maybe [t]
Then d2 can readily enter the inert, and we also do solving of the wanted
Inert:
d0 :_g Foo t
d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
WorkList
d2 :_w Sat (Maybe [t])
d3 :_w Data Maybe t
d01 :_g Data Maybe t
Now, we may simplify d2 more:
Inert:
d0 :_g Foo t
d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
d1 :_g Data Maybe [t]
d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
WorkList:
d3 :_w Data Maybe t
d4 :_w Foo [t]
d01 :_g Data Maybe t
Now, we can just solve d3.
Inert
d0 :_g Foo t
d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
WorkList
d4 :_w Foo [t]
d01 :_g Data Maybe t
And now we can simplify d4 again, but since it has superclasses we *add* them to the worklist:
Inert
d0 :_g Foo t
d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
d4 :_g Foo [t] d4 := dfunFoo2 d5
WorkList:
d5 :_w Foo t
d6 :_g Data Maybe [t] d6 := EvDictSuperClass d4 0
d01 :_g Data Maybe t
Now, d5 can be solved! (and its superclass enter scope)
Inert
d0 :_g Foo t
d1 :_s Data Maybe [t] d1 := dfunData2 d2 d3
d2 :_g Sat (Maybe [t]) d2 := dfunSat d4
d4 :_g Foo [t] d4 := dfunFoo2 d5
d5 :_g Foo t d5 := dfunFoo1 d7
WorkList:
d7 :_w Data Maybe t
d6 :_g Data Maybe [t]
d8 :_g Data Maybe t d8 := EvDictSuperClass d5 0
d01 :_g Data Maybe t
Now, two problems:
[1] Suppose we pick d8 and we react him with d01. Which of the two givens should
we keep? Well, we *MUST NOT* drop d01 because d8 contains recursive evidence
that must not be used (look at case interactInert where both inert and workitem
are givens). So we have several options:
- Drop the workitem always (this will drop d8)
This feels very unsafe -- what if the work item was the "good" one
that should be used later to solve another wanted?
- Don't drop anyone: the inert set may contain multiple givens!
[This is currently implemented]
The "don't drop anyone" seems the most safe thing to do, so now we come to problem 2:
[2] We have added both d6 and d01 in the inert set, and we are interacting our wanted
d7. Now the [isRecDictEv] function in the ineration solver
[case inert-given workitem-wanted] will prevent us from interacting d7 := d8
precisely because chasing the evidence of d8 leads us to an unguarded use of d7.
So, no interaction happens there. Then we meet d01 and there is no recursion
problem there [isRectDictEv] gives us the OK to interact and we do solve d7 := d01!
Note [SUPERCLASS-LOOP 1]
~~~~~~~~~~~~~~~~~~~~~~~~
We have to be very, very careful when generating superclasses, lest we
accidentally build a loop. Here's an example:
class S a
class S a => C a where { opc :: a -> a }
class S b => D b where { opd :: b -> b }
instance C Int where
opc = opd
instance D Int where
opd = opc
From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
Simplifying, we may well get:
$dfCInt = :C ds1 (opd dd)
dd = $dfDInt
ds1 = $p1 dd
Notice that we spot that we can extract ds1 from dd.
Alas! Alack! We can do the same for (instance D Int):
$dfDInt = :D ds2 (opc dc)
dc = $dfCInt
ds2 = $p1 dc
And now we've defined the superclass in terms of itself.
Two more nasty cases are in
tcrun021
tcrun033
Solution:
- Satisfy the superclass context *all by itself*
(tcSimplifySuperClasses)
- And do so completely; i.e. no left-over constraints
to mix with the constraints arising from method declarations
Note [SUPERCLASS-LOOP 2]
~~~~~~~~~~~~~~~~~~~~~~~~
We need to be careful when adding "the constaint we are trying to prove".
Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where
class Ord a => C a where
instance Ord [a] => C [a] where ...
Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
superclasses of C [a] to avails. But we must not overwrite the binding
for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
build a loop!
Here's another variant, immortalised in tcrun020
class Monad m => C1 m
class C1 m => C2 m x
instance C2 Maybe Bool
For the instance decl we need to build (C1 Maybe), and it's no good if
we run around and add (C2 Maybe Bool) and its superclasses to the avails
before we search for C1 Maybe.
Here's another example
class Eq b => Foo a b
instance Eq a => Foo [a] a
If we are reducing
(Foo [t] t)
we'll first deduce that it holds (via the instance decl). We must not
then overwrite the Eq t constraint with a superclass selection!
At first I had a gross hack, whereby I simply did not add superclass constraints
in addWanted, though I did for addGiven and addIrred. This was sub-optimal,
becuase it lost legitimate superclass sharing, and it still didn't do the job:
I found a very obscure program (now tcrun021) in which improvement meant the
simplifier got two bites a the cherry... so something seemed to be an Stop
first time, but reducible next time.
Now we implement the Right Solution, which is to check for loops directly
when adding superclasses. It's a bit like the occurs check in unification.
Note [Recursive instances and superclases]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this code, which arises in the context of "Scrap Your
Boilerplate with Class".
class Sat a
class Data ctx a
instance Sat (ctx Char) => Data ctx Char
instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]
class Data Maybe a => Foo a
instance Foo t => Sat (Maybe t)
instance Data Maybe a => Foo a
instance Foo a => Foo [a]
instance Foo [Char]
In the instance for Foo [a], when generating evidence for the superclasses
(ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]).
Using the instance for Data, we therefore need
(Sat (Maybe [a], Data Maybe a)
But we are given (Foo a), and hence its superclass (Data Maybe a).
So that leaves (Sat (Maybe [a])). Using the instance for Sat means
we need (Foo [a]). And that is the very dictionary we are bulding
an instance for! So we must put that in the "givens". So in this
case we have
Given: Foo a, Foo [a]
Wanted: Data Maybe [a]
BUT we must *not not not* put the *superclasses* of (Foo [a]) in
the givens, which is what 'addGiven' would normally do. Why? Because
(Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted
by selecting a superclass from Foo [a], which simply makes a loop.
On the other hand we *must* put the superclasses of (Foo a) in
the givens, as you can see from the derivation described above.
Conclusion: in the very special case of tcSimplifySuperClasses
we have one 'given' (namely the "this" dictionary) whose superclasses
must not be added to 'givens' by addGiven.
There is a complication though. Suppose there are equalities
instance (Eq a, a~b) => Num (a,b)
Then we normalise the 'givens' wrt the equalities, so the original
given "this" dictionary is cast to one of a different type. So it's a
bit trickier than before to identify the "special" dictionary whose
superclasses must not be added. See test
indexed-types/should_run/EqInInstance
We need a persistent property of the dictionary to record this
special-ness. Current I'm using the InstLocOrigin (a bit of a hack,
but cool), which is maintained by dictionary normalisation.
Specifically, the InstLocOrigin is
NoScOrigin
then the no-superclass thing kicks in. WATCH OUT if you fiddle
with InstLocOrigin!
Note [MATCHING-SYNONYMS]
~~~~~~~~~~~~~~~~~~~~~~~~
When trying to match a dictionary (D tau) to a top-level instance, or a
type family equation (F taus_1 ~ tau_2) to a top-level family instance,
we do *not* need to expand type synonyms because the matcher will do that for us.
Note [RHS-FAMILY-SYNONYMS]
~~~~~~~~~~~~~~~~~~~~~~~~~~
The RHS of a family instance is represented as yet another constructor which is
like a type synonym for the real RHS the programmer declared. Eg:
type instance F (a,a) = [a]
Becomes:
:R32 a = [a] -- internal type synonym introduced
F (a,a) ~ :R32 a -- instance
When we react a family instance with a type family equation in the work list
we keep the synonym-using RHS without expansion.
*********************************************************************************
* *
The top-reaction Stage
* *
*********************************************************************************
\begin{code}
topReactionsStage :: SimplifierStage
topReactionsStage workItem
= tryTopReact workItem
tryTopReact :: WorkItem -> TcS StopOrContinue
tryTopReact wi
= do { inerts <- getTcSInerts
; ctxt <- getTcSContext
; if simplEqsOnly ctxt then return (ContinueWith wi)
else
do { tir <- doTopReact inerts wi
; case tir of
NoTopInt
-> return (ContinueWith wi)
SomeTopInt rule what_next
-> do { bumpStepCountTcS
; traceFireTcS (cc_depth wi) $
ptext (sLit "Top react:") <+> text rule
; return what_next }
} }
data TopInteractResult
= NoTopInt
| SomeTopInt { tir_rule :: String, tir_new_item :: StopOrContinue }
doTopReact :: InertSet -> WorkItem -> TcS TopInteractResult
doTopReact _inerts (CDictCan { cc_flavor = Given {} })
= return NoTopInt
doTopReact _inerts workItem@(CDictCan { cc_flavor = Derived loc
, cc_class = cls, cc_tyargs = xis })
= do { instEnvs <- getInstEnvs
; let fd_eqns = improveFromInstEnv instEnvs
(mkClassPred cls xis, pprArisingAt loc)
; m <- rewriteWithFunDeps fd_eqns xis loc
; case m of
Nothing -> return NoTopInt
Just (xis',_,fd_work) ->
let workItem' = workItem { cc_tyargs = xis' }
in do { emitFDWorkAsDerived fd_work (cc_depth workItem)
; return $
SomeTopInt { tir_rule = "Derived Cls fundeps"
, tir_new_item = ContinueWith workItem' } }
}
doTopReact inerts workItem@(CDictCan { cc_flavor = fl@(Wanted loc)
, cc_class = cls, cc_tyargs = xis })
= do { traceTcS "doTopReact" (ppr workItem)
; instEnvs <- getInstEnvs
; let fd_eqns = improveFromInstEnv instEnvs
(mkClassPred cls xis, pprArisingAt loc)
; any_fundeps <- rewriteWithFunDeps fd_eqns xis loc
; case any_fundeps of
Nothing ->
do { lkup_inst_res <- matchClassInst inerts cls xis loc
; case lkup_inst_res of
GenInst wtvs ev_term
-> doSolveFromInstance wtvs ev_term workItem
NoInstance
-> return NoTopInt
}
Just (_xis',_cos,fd_work) ->
do { emitFDWorkAsDerived fd_work (cc_depth workItem)
; return SomeTopInt { tir_rule = "Dict/Top (fundeps)"
, tir_new_item = ContinueWith workItem } } }
where doSolveFromInstance :: [WantedEvVar]
-> EvTerm
-> Ct
-> TcS TopInteractResult
doSolveFromInstance wtvs ev_term workItem
| null wtvs
= do { traceTcS "doTopReact/found nullary instance for" (ppr (cc_id workItem))
; _ <- setEvBind (cc_id workItem) ev_term fl
; return $
SomeTopInt { tir_rule = "Dict/Top (solved, no new work)"
, tir_new_item = Stop } }
| otherwise
= do { traceTcS "doTopReact/found non-nullary instance for" $
ppr (cc_id workItem)
; _ <- setEvBind (cc_id workItem) ev_term fl
; let ct_from_wev (EvVarX v fl)
= CNonCanonical { cc_id = v, cc_flavor = Wanted fl
, cc_depth = cc_depth workItem + 1 }
wtvs_cts = map ct_from_wev wtvs
; updWorkListTcS (appendWorkListCt wtvs_cts)
; return $
SomeTopInt { tir_rule = "Dict/Top (solved, more work)"
, tir_new_item = Stop }
}
doTopReact _inerts (CFunEqCan { cc_flavor = fl })
| Just (GivenSolved {}) <- isGiven_maybe fl
= return NoTopInt
doTopReact _inerts workItem@(CFunEqCan { cc_id = eqv, cc_flavor = fl
, cc_fun = tc, cc_tyargs = args, cc_rhs = xi })
= ASSERT (isSynFamilyTyCon tc)
do { match_res <- matchFam tc args
; case match_res of
Nothing -> return NoTopInt
Just (rep_tc, rep_tys)
-> do { let Just coe_tc = tyConFamilyCoercion_maybe rep_tc
Just rhs_ty = tcView (mkTyConApp rep_tc rep_tys)
coe = mkTcAxInstCo coe_tc rep_tys
; case fl of
Wanted {} -> do { evc <- newEqVar fl rhs_ty xi
; let eqv' = evc_the_evvar evc
; let coercion = coe `mkTcTransCo` mkTcCoVarCo eqv'
; _ <- setEqBind eqv coercion fl
; when (isNewEvVar evc) $
(let ct = CNonCanonical { cc_id = eqv'
, cc_flavor = fl
, cc_depth = cc_depth workItem + 1}
in updWorkListTcS (extendWorkListEq ct))
; let _solved = workItem { cc_flavor = solved_fl }
solved_fl = mkSolvedFlavor fl UnkSkol (EvCoercion coercion)
; updateFlatCache eqv solved_fl tc args xi WhenSolved
; return $
SomeTopInt { tir_rule = "Fun/Top (solved, more work)"
, tir_new_item = Stop } }
Given {} -> do { (fl',eqv') <- newGivenEqVar fl xi rhs_ty $
mkTcSymCo (mkTcCoVarCo eqv) `mkTcTransCo` coe
; let ct = CNonCanonical { cc_id = eqv'
, cc_flavor = fl'
, cc_depth = cc_depth workItem + 1}
; updWorkListTcS (extendWorkListEq ct)
; return $
SomeTopInt { tir_rule = "Fun/Top (given)"
, tir_new_item = ContinueWith workItem } }
Derived {} -> do { evc <- newEvVar fl (mkEqPred (xi, rhs_ty))
; let eqv' = evc_the_evvar evc
; when (isNewEvVar evc) $
(let ct = CNonCanonical { cc_id = eqv'
, cc_flavor = fl
, cc_depth = cc_depth workItem + 1 }
in updWorkListTcS (extendWorkListEq ct))
; return $
SomeTopInt { tir_rule = "Fun/Top (derived)"
, tir_new_item = Stop } }
}
}
doTopReact _inerts _workItem = return NoTopInt
\end{code}
Note [FunDep and implicit parameter reactions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently, our story of interacting two dictionaries (or a dictionary
and top-level instances) for functional dependencies, and implicit
paramters, is that we simply produce new wanted equalities. So for example
class D a b | a -> b where ...
Inert:
d1 :g D Int Bool
WorkItem:
d2 :w D Int alpha
We generate the extra work item
cv :w alpha ~ Bool
where 'cv' is currently unused. However, this new item reacts with d2,
discharging it in favour of a new constraint d2' thus:
d2' :w D Int Bool
d2 := d2' |> D Int cv
Now d2' can be discharged from d1
We could be more aggressive and try to *immediately* solve the dictionary
using those extra equalities. With the same inert set and work item we
might dischard d2 directly:
cv :w alpha ~ Bool
d2 := d1 |> D Int cv
But in general it's a bit painful to figure out the necessary coercion,
so we just take the first approach. Here is a better example. Consider:
class C a b c | a -> b
And:
[Given] d1 : C T Int Char
[Wanted] d2 : C T beta Int
In this case, it's *not even possible* to solve the wanted immediately.
So we should simply output the functional dependency and add this guy
[but NOT its superclasses] back in the worklist. Even worse:
[Given] d1 : C T Int beta
[Wanted] d2: C T beta Int
Then it is solvable, but its very hard to detect this on the spot.
It's exactly the same with implicit parameters, except that the
"aggressive" approach would be much easier to implement.
Note [When improvement happens]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We fire an improvement rule when
* Two constraints match (modulo the fundep)
e.g. C t1 t2, C t1 t3 where C a b | a->b
The two match because the first arg is identical
* At least one is not Given. If they are both given, we don't fire
the reaction because we have no way of constructing evidence for a
new equality nor does it seem right to create a new wanted goal
(because the goal will most likely contain untouchables, which
can't be solved anyway)!
Note that we *do* fire the improvement if one is Given and one is Derived.
The latter can be a superclass of a wanted goal. Example (tcfail138)
class L a b | a -> b
class (G a, L a b) => C a b
instance C a b' => G (Maybe a)
instance C a b => C (Maybe a) a
instance L (Maybe a) a
When solving the superclasses of the (C (Maybe a) a) instance, we get
Given: C a b ... and hance by superclasses, (G a, L a b)
Wanted: G (Maybe a)
Use the instance decl to get
Wanted: C a b'
The (C a b') is inert, so we generate its Derived superclasses (L a b'),
and now we need improvement between that derived superclass an the Given (L a b)
Note [Overriding implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
f :: (?x::a) -> Bool -> a
g v = let ?x::Int = 3
in (f v, let ?x::Bool = True in f v)
This should probably be well typed, with
g :: Bool -> (Int, Bool)
So the inner binding for ?x::Bool *overrides* the outer one.
Hence a work-item Given overrides an inert-item Given.
Note [Given constraint that matches an instance declaration]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What should we do when we discover that one (or more) top-level
instances match a given (or solved) class constraint? We have
two possibilities:
1. Reject the program. The reason is that there may not be a unique
best strategy for the solver. Example, from the OutsideIn(X) paper:
instance P x => Q [x]
instance (x ~ y) => R [x] y
wob :: forall a b. (Q [b], R b a) => a -> Int
g :: forall a. Q [a] => [a] -> Int
g x = wob x
will generate the impliation constraint:
Q [a] => (Q [beta], R beta [a])
If we react (Q [beta]) with its top-level axiom, we end up with a
(P beta), which we have no way of discharging. On the other hand,
if we react R beta [a] with the top-level we get (beta ~ a), which
is solvable and can help us rewrite (Q [beta]) to (Q [a]) which is
now solvable by the given Q [a].
However, this option is restrictive, for instance [Example 3] from
Note [Recursive instances and superclases] will fail to work.
2. Ignore the problem, hoping that the situations where there exist indeed
such multiple strategies are rare: Indeed the cause of the previous
problem is that (R [x] y) yields the new work (x ~ y) which can be
*spontaneously* solved, not using the givens.
We are choosing option 2 below but we might consider having a flag as well.
Note [New Wanted Superclass Work]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Even in the case of wanted constraints, we may add some superclasses
as new given work. The reason is:
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:
C a b => C a beta
Then, FD improvement can help us to produce a new wanted (beta ~ b)
We want to have the same effect with the type family encoding of
functional dependencies. Namely, consider:
class (F a ~ b) => C a b
Now suppose that we have:
given: C a b
wanted: C a beta
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 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 derived evidence with the identity.
This requires trySpontaneousSolve to solve *derived*
equalities that have a touchable in their RHS, *in addition*
to solving wanted equalities.
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:
----------
class (F a ~ b) => C a b
And we are given the wanteds:
w1 : C a b
w2 : C a c
w3 : b ~ c
We surely do *not* want to quantify over (b ~ c), since if someone provides
dictionaries for (C a b) and (C a c), these dictionaries can provide a proof
of (b ~ c), hence no extra evidence is necessary. Here is what will happen:
Step 1: We will get new *given* superclass work,
provisionally to our solving of w1 and w2
g1: F a ~ b, g2 : F a ~ c,
w1 : C a b, w2 : C a c, w3 : b ~ c
The evidence for g1 and g2 is a superclass evidence term:
g1 := sc w1, g2 := sc w2
Step 2: The givens will solve the wanted w3, so that
w3 := sym (sc w1) ; sc w2
Step 3: Now, one may naively assume that then w2 can be solve from w1
after rewriting with the (now solved equality) (b ~ c).
But this rewriting is ruled out by the isGoodRectDict!
Conclusion, we will (correctly) end up with the unsolved goals
(C a b, C a c)
NB: The desugarer needs be more clever to deal with equalities
that participate in recursive dictionary bindings.
\begin{code}
data LookupInstResult
= NoInstance
| GenInst [WantedEvVar] EvTerm
matchClassInst :: InertSet -> Class -> [Type] -> WantedLoc -> TcS LookupInstResult
matchClassInst inerts clas tys loc
= do { let pred = mkClassPred clas tys
; mb_result <- matchClass clas tys
; untch <- getUntouchables
; case mb_result of
MatchInstNo -> return NoInstance
MatchInstMany -> return NoInstance
MatchInstSingle (_,_)
| given_overlap untch ->
do { traceTcS "Delaying instance application" $
vcat [ text "Workitem=" <+> pprType (mkClassPred clas tys)
, text "Relevant given dictionaries=" <+> ppr givens_for_this_clas ]
; return NoInstance
}
MatchInstSingle (dfun_id, mb_inst_tys) ->
do { checkWellStagedDFun pred dfun_id loc
; tys <- instDFunTypes mb_inst_tys
; let (theta, _) = tcSplitPhiTy (applyTys (idType dfun_id) tys)
; if null theta then
return (GenInst [] (EvDFunApp dfun_id tys []))
else do
{ evc_vars <- instDFunConstraints theta (Wanted loc)
; let ev_vars = map evc_the_evvar evc_vars
new_evc_vars = filter isNewEvVar evc_vars
wevs = map (\v -> EvVarX (evc_the_evvar v) loc) new_evc_vars
; return $ GenInst wevs (EvDFunApp dfun_id tys ev_vars) }
}
}
where
givens_for_this_clas :: Cts
givens_for_this_clas
= lookupUFM (cts_given (inert_dicts inerts)) clas `orElse` emptyCts
given_overlap :: TcsUntouchables -> Bool
given_overlap untch = anyBag (matchable untch) givens_for_this_clas
matchable untch (CDictCan { cc_class = clas_g, cc_tyargs = sys, cc_flavor = fl })
| Just GivenOrig <- isGiven_maybe fl
= ASSERT( clas_g == clas )
case tcUnifyTys (\tv -> if isTouchableMetaTyVar_InRange untch tv &&
tv `elemVarSet` tyVarsOfTypes tys
then BindMe else Skolem) tys sys of
Nothing -> False
Just _ -> True
| otherwise = False
matchable _tys ct = pprPanic "Expecting dictionary!" (ppr ct)
\end{code}
Note [Instance and Given overlap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Assume that we have an inert set that looks as follows:
[Given] D [Int]
And an instance declaration:
instance C a => D [a]
A new wanted comes along of the form:
[Wanted] D [alpha]
One possibility is to apply the instance declaration which will leave us
with an unsolvable goal (C alpha). However, later on a new constraint may
arise (for instance due to a functional dependency between two later dictionaries),
that will add the equality (alpha ~ Int), in which case our ([Wanted] D [alpha])
will be transformed to [Wanted] D [Int], which could have been discharged by the given.
The solution is that in matchClassInst and eventually in topReact, we get back with
a matching instance, only when there is no Given in the inerts which is unifiable to
this particular dictionary.
The end effect is that, much as we do for overlapping instances, we delay choosing a
class instance if there is a possibility of another instance OR a given to match our
constraint later on. This fixes bugs #4981 and #5002.
This is arguably not easy to appear in practice due to our aggressive prioritization
of equality solving over other constraints, but it is possible. I've added a test case
in typecheck/should-compile/GivenOverlapping.hs
Moreover notice that our goals here are different than the goals of the top-level
overlapping checks. There we are interested in validating the following principle:
If we inline a function f at a site where the same global instance environment
is available as the instance environment at the definition site of f then we
should get the same behaviour.
But for the Given Overlap check our goal is just related to completeness of
constraint solving.