TcInteract.lhs 86.8 KB
Newer Older
1 2
\begin{code}
module TcInteract ( 
3 4 5
     solveInteract, solveInteractGiven, solveInteractWanted,
     AtomicInert, tyVarsOfInert, 
     InertSet, emptyInert, updInertSet, extractUnsolved, solveOne,
6 7 8 9
  ) where  

#include "HsVersions.h"

10

11 12 13 14 15 16 17 18 19
import BasicTypes 
import TcCanonical
import VarSet
import Type

import Id 
import Var

import TcType
20
import HsBinds
21

22
import Inst( tyVarsOfEvVar )
23 24
import Class
import TyCon
25 26 27 28 29 30 31
import Name

import FunDeps

import Coercion
import Outputable

32
import TcRnTypes
33
import TcErrors
34
import TcSMonad
35
import Bag
36
import qualified Data.Map as Map
37

38 39
import Control.Monad( when )

40 41 42 43
import FastString ( sLit ) 
import DynFlags
\end{code}

44
Note [InertSet invariants]
45 46 47 48 49 50 51 52 53 54 55 56 57 58
~~~~~~~~~~~~~~~~~~~~~~~~~~~
An InertSet is a bag of canonical constraints, with the following invariants:

  1 No two constraints react with each other. 
    
    A tricky case is when there exists a given (solved) dictionary 
    constraint and a wanted identical constraint in the inert set, but do 
    not react because reaction would create loopy dictionary evidence for 
    the wanted. See note [Recursive dictionaries]

  2 Given equalities form an idempotent substitution [none of the
    given LHS's occur in any of the given RHS's or reactant parts]

  3 Wanted equalities also form an idempotent substitution
59

60 61 62 63 64 65
  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).
66 67

  7 No wanted constraints tv1 ~ tv2 with tv1 touchable. Such constraints
68 69
    will be marked as solved right before being pushed into the inert set. 
    See note [Touchables and givens].
70 71 72

  8 No Given constraint mentions a touchable unification variable,
    except if the
73 74 75 76 77 78 79 80 81 82 83 84 85 86
 
Note that 6 and 7 are /not/ enforced by canonicalization but rather by 
insertion in the inert list, ie by TcInteract. 

During the process of solving, the inert set will contain some
previously given constraints, some wanted constraints, and some given
constraints which have arisen from solving wanted constraints. For
now we do not distinguish between given and solved constraints.

Note that we must switch wanted inert items to given when going under an
implication constraint (when in top-level inference mode).

\begin{code}

87 88 89 90
data CCanMap a = CCanMap { cts_given   :: Map.Map a CanonicalCts
                                          -- Invariant: all Given
                         , cts_derived :: Map.Map a CanonicalCts 
                                          -- Invariant: all Derived
91 92
                         , cts_wanted  :: Map.Map a CanonicalCts } 
                                          -- Invariant: all Wanted
93

94
cCanMapToBag :: Ord a => CCanMap a -> CanonicalCts 
95 96 97
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)
98 99

emptyCCanMap :: CCanMap a 
100 101
emptyCCanMap = CCanMap { cts_given = Map.empty
                       , cts_derived = Map.empty, cts_wanted = Map.empty } 
102 103 104 105 106 107

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) } 
108 109 110 111
      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) }
112 113 114 115 116
  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 
117 118 119
    = let relevant = unionManyBags [ Map.findWithDefault emptyCCan a (cts_wanted cmap)
                                   , Map.findWithDefault emptyCCan a (cts_given cmap)
                                   , Map.findWithDefault emptyCCan a (cts_derived cmap) ]
120
          residual_map = cmap { cts_wanted = Map.delete a (cts_wanted cmap) 
121 122
                              , cts_given = Map.delete a (cts_given cmap) 
                              , cts_derived = Map.delete a (cts_derived cmap) }
123 124
      in (relevant, residual_map) 

125 126 127 128 129 130 131 132 133
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 })

134

135
-- See Note [InertSet invariants]
136
data InertSet 
137
  = IS { inert_eqs          :: CanonicalCts               -- Equalities only (CTyEqCan)
138
       , inert_dicts        :: CCanMap Class              -- Dictionaries only
139
       , inert_ips          :: CCanMap (IPName Name)      -- Implicit parameters 
140 141
       , inert_frozen       :: CanonicalCts
       , inert_funeqs       :: CCanMap TyCon              -- Type family equalities only
142 143
               -- This representation allows us to quickly get to the relevant 
               -- inert constraints when interacting a work item with the inert set.
144
       }
145

146 147 148 149
tyVarsOfInert :: InertSet -> TcTyVarSet 
tyVarsOfInert (IS { inert_eqs    = eqs
                  , inert_dicts  = dictmap
                  , inert_ips    = ipmap
150 151 152 153 154
                  , inert_frozen = frozen
                  , inert_funeqs = funeqmap }) = tyVarsOfCanonicals cts
  where
    cts = eqs `andCCan` frozen `andCCan` cCanMapToBag dictmap
              `andCCan` cCanMapToBag ipmap `andCCan` cCanMapToBag funeqmap
155

156
instance Outputable InertSet where
157
  ppr is = vcat [ vcat (map ppr (Bag.bagToList $ inert_eqs is))
158
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts is)))
159 160
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips is))) 
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_funeqs is)))
161
                , vcat (map ppr (Bag.bagToList $ inert_frozen is))
162 163
                ]
                       
164
emptyInert :: InertSet
165
emptyInert = IS { inert_eqs    = Bag.emptyBag
166
                , inert_frozen = Bag.emptyBag
167 168
                , inert_dicts  = emptyCCanMap
                , inert_ips    = emptyCCanMap
169
                , inert_funeqs = emptyCCanMap }
170 171

updInertSet :: InertSet -> AtomicInert -> InertSet 
172 173 174 175 176 177 178 179 180 181
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) }
182
  | otherwise 
183
  = is { inert_frozen = inert_frozen is `Bag.snocBag` item }
184

185
extractUnsolved :: InertSet -> (InertSet, CanonicalCts)
186
-- Postcondition: the returned canonical cts are either Derived, or Wanted.
187
extractUnsolved is@(IS {inert_eqs = eqs}) 
188 189 190
  = let is_solved  = is { inert_eqs    = solved_eqs
                        , inert_dicts  = solved_dicts
                        , inert_ips    = solved_ips
191 192
                        , inert_frozen = emptyCCan
                        , inert_funeqs = solved_funeqs }
193
    in (is_solved, unsolved)
194

195
  where (unsolved_eqs, solved_eqs)       = Bag.partitionBag (not.isGivenCt) eqs
196 197 198
        (unsolved_ips, solved_ips)       = extractUnsolvedCMap (inert_ips is) 
        (unsolved_dicts, solved_dicts)   = extractUnsolvedCMap (inert_dicts is) 
        (unsolved_funeqs, solved_funeqs) = extractUnsolvedCMap (inert_funeqs is) 
199

200
        unsolved = unsolved_eqs `unionBags` inert_frozen is `unionBags`
201
                   unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs
202 203 204 205 206 207 208 209 210 211 212 213 214 215
\end{code}

%*********************************************************************
%*                                                                   * 
*                      Main Interaction Solver                       *
*                                                                    *
**********************************************************************

Note [Basic plan] 
~~~~~~~~~~~~~~~~~
1. Canonicalise (unary)
2. Pairwise interaction (binary)
    * Take one from work list 
    * Try all pair-wise interactions with each constraint in inert
216 217 218 219
   
   As an optimisation, we prioritize the equalities both in the 
   worklist and in the inerts. 

220 221 222 223 224 225 226 227
3. Try to solve spontaneously for equalities involving touchables 
4. Top-level interaction (binary wrt top-level)
   Superclass decomposition belongs in (4), see note [Superclasses]

\begin{code}
type AtomicInert = CanonicalCt     -- constraint pulled from InertSet
type WorkItem    = CanonicalCt     -- constraint pulled from WorkList

228
------------------------
229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
data StopOrContinue 
  = Stop			-- Work item is consumed
  | ContinueWith WorkItem	-- Not consumed

instance Outputable StopOrContinue where
  ppr Stop             = ptext (sLit "Stop")
  ppr (ContinueWith w) = ptext (sLit "ContinueWith") <+> ppr w

-- Results after interacting a WorkItem as far as possible with an InertSet
data StageResult
  = SR { sr_inerts     :: InertSet
           -- The new InertSet to use (REPLACES the old InertSet)
       , sr_new_work   :: WorkList
           -- Any new work items generated (should be ADDED to the old WorkList)
           -- Invariant: 
           --    sr_stop = Just workitem => workitem is *not* in sr_inerts and
           --                               workitem is inert wrt to sr_inerts
       , sr_stop       :: StopOrContinue
       }

instance Outputable StageResult where
  ppr (SR { sr_inerts = inerts, sr_new_work = work, sr_stop = stop })
    = ptext (sLit "SR") <+> 
      braces (sep [ ptext (sLit "inerts =") <+> ppr inerts <> comma
             	  , ptext (sLit "new work =") <+> ppr work <> comma
             	  , ptext (sLit "stop =") <+> ppr stop])

256 257 258
type SubGoalDepth = Int	  -- Starts at zero; used to limit infinite
     		    	  -- recursion of sub-goals
type SimplifierStage = SubGoalDepth -> WorkItem -> InertSet -> TcS StageResult 
259 260

-- Combine a sequence of simplifier 'stages' to create a pipeline 
261 262 263
runSolverPipeline :: SubGoalDepth
                  -> [(String, SimplifierStage)]
		  -> InertSet -> WorkItem 
264 265
                  -> TcS (InertSet, WorkList)
-- Precondition: non-empty list of stages 
266
runSolverPipeline depth pipeline inerts workItem
267 268 269 270 271
  = do { traceTcS "Start solver pipeline" $ 
            vcat [ ptext (sLit "work item =") <+> ppr workItem
                 , ptext (sLit "inerts    =") <+> ppr inerts]

       ; let itr_in = SR { sr_inerts = inerts
272 273
                         , sr_new_work = emptyWorkList
                         , sr_stop = ContinueWith workItem }
274 275 276 277
       ; itr_out <- run_pipeline pipeline itr_in
       ; let new_inert 
              = case sr_stop itr_out of 
       	          Stop              -> sr_inerts itr_out
278
                  ContinueWith item -> sr_inerts itr_out `updInertSet` item
279 280 281 282 283 284 285 286 287 288 289
       ; return (new_inert, sr_new_work itr_out) }
  where 
    run_pipeline :: [(String, SimplifierStage)]
                 -> StageResult -> TcS StageResult
    run_pipeline [] itr                         = return itr
    run_pipeline _  itr@(SR { sr_stop = Stop }) = return itr

    run_pipeline ((name,stage):stages) 
                 (SR { sr_new_work = accum_work
                     , sr_inerts   = inerts
                     , sr_stop     = ContinueWith work_item })
290
      = do { itr <- stage depth work_item inerts 
291
           ; traceTcS ("Stage result (" ++ name ++ ")") (ppr itr)
292
           ; let itr' = itr { sr_new_work = accum_work `unionWorkList` sr_new_work itr }
293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323
           ; run_pipeline stages itr' }
\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}
-- Main interaction solver: we fully solve the worklist 'in one go', 
-- returning an extended inert set.
--
-- See Note [Touchables and givens].
324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342
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
343 344
solveInteract inert ws 
  = do { dyn_flags <- getDynFlags
345 346 347 348 349 350 351
       ; sctx <- getTcSContext

       ; traceTcS "solveInteract, before clever canonicalization:" $
         vcat [ text "ws = " <+>  ppr (mapBag (\(EvVarX ev ct)
                                                   -> (ct,evVarPred ev)) ws)
              , text "inert = " <+> ppr inert ]

352 353 354 355
       ; can_ws <- mkCanonicalFEVs ws

       ; (flag, inert_ret)
           <- foldrWorkListM (tryPreSolveAndInteract sctx dyn_flags) (True,inert) can_ws
356 357 358 359 360 361 362 363 364

       ; 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
365
                       -> CanonicalCt
366
                       -> (Bool, InertSet)
367 368
                       -> TcS (Bool, InertSet)
-- Returns: True if it was able to discharge this constraint AND all previous ones
369
tryPreSolveAndInteract sctx dyn_flags ct (all_previous_discharged, inert)
370 371
  = do { let inert_cts = get_inert_cts (evVarPred ev_var)

372 373 374 375 376
       ; this_one_discharged <- 
           if isCFrozenErr ct then 
               return False
           else
               dischargeFromCCans inert_cts ev_var fl
377 378 379

       ; if this_one_discharged
         then return (all_previous_discharged, inert)
380

381
         else do
382
       { inert_ret <- solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) ct inert
383 384 385
       ; return (False, inert_ret) } }

  where
386 387 388
    ev_var = cc_id ct
    fl = cc_flavor ct 

389 390 391 392 393 394 395 396 397 398
    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)

399
dischargeFromCCans :: CanonicalCts -> EvVar -> CtFlavor -> TcS Bool
400 401 402 403
-- See if this (pre-canonicalised) work-item is identical to a 
-- one already in the inert set. Reasons:
--    a) Avoid creating superclass constraints for millions of incoming (Num a) constraints
--    b) Termination for improve_eqs in TcSimplify.simpl_loop
404
dischargeFromCCans cans ev fl
405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422
  = Bag.foldrBag discharge_ct (return False) cans
  where 
    the_pred = evVarPred ev

    discharge_ct :: CanonicalCt -> TcS Bool -> TcS Bool
    discharge_ct ct _rest
      | evVarPred (cc_id ct) `tcEqPred` the_pred
      , cc_flavor ct `canSolve` fl
      = do { when (isWanted fl) $ set_ev_bind ev (cc_id ct) 
           	 -- Deriveds need no evidence
    	         -- For Givens, we already have evidence, and we don't need it twice 
           ; return True }
      where 
         set_ev_bind x y
            | EqPred {} <- evVarPred y = setEvBind x (EvCoercion (mkCoVarCoercion y))
            | otherwise                = setEvBind x (EvId y)

    discharge_ct _ct rest = rest
423 424 425 426
\end{code}

Note [Avoiding the superclass explosion] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
427 428 429 430 431 432 433 434 435 436 437 438
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.
439 440

\begin{code}
441 442
solveOne :: WorkItem -> InertSet -> TcS InertSet 
solveOne workItem inerts 
443
  = do { dyn_flags <- getDynFlags
444
       ; solveOneWithDepth (ctxtStkDepth dyn_flags,0,[]) workItem inerts
445 446 447 448
       }

-----------------
solveInteractWithDepth :: (Int, Int, [WorkItem])
449 450
                       -> WorkList -> InertSet -> TcS InertSet
solveInteractWithDepth ctxt@(max_depth,n,stack) ws inert
451 452 453 454 455 456 457 458
  | isEmptyWorkList ws
  = return inert

  | n > max_depth 
  = solverDepthErrorTcS n stack

  | otherwise 
  = do { traceTcS "solveInteractWithDepth" $ 
459
              vcat [ text "Current depth =" <+> ppr n
460 461
                   , text "Max depth =" <+> ppr max_depth
                   , text "ws =" <+> ppr ws ]
462

463 464 465

       ; foldrWorkListM (solveOneWithDepth ctxt) inert ws }
              -- use foldr to preserve the order
466 467 468 469 470

------------------
-- 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])
471 472
                  -> WorkItem -> InertSet -> TcS InertSet
solveOneWithDepth (max_depth, depth, stack) work inert
473 474
  = do { traceFireTcS depth (text "Solving {" <+> ppr work)
       ; (new_inert, new_work) <- runSolverPipeline depth thePipeline inert work
475 476 477
         
	 -- Recursively solve the new work generated 
         -- from workItem, with a greater depth
478
       ; res_inert <- solveInteractWithDepth (max_depth, depth+1, work:stack) new_work new_inert 
479

480 481
       ; traceFireTcS depth (text "Done }" <+> ppr work) 

482 483 484
       ; return res_inert }

thePipeline :: [(String,SimplifierStage)]
485 486 487 488
thePipeline = [ ("interact with inert eqs", interactWithInertEqsStage)
              , ("interact with inerts",    interactWithInertsStage)
              , ("spontaneous solve",       spontaneousSolveStage)
              , ("top-level reactions",     topReactionsStage) ]
489 490 491 492 493 494 495 496
\end{code}

*********************************************************************************
*                                                                               * 
                       The spontaneous-solve Stage
*                                                                               *
*********************************************************************************

497 498 499 500 501 502
Note [Efficient Orientation] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are two cases where we have to be careful about 
orienting equalities to get better efficiency. 

503
Case 1: In Rewriting Equalities (function rewriteEqLHS) 
504

505 506 507 508 509 510 511 512 513 514
    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. 
515

516 517 518 519 520
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. 
521

522 523
\begin{code}
spontaneousSolveStage :: SimplifierStage 
524
spontaneousSolveStage depth workItem inerts 
525 526
  = do { mSolve <- trySpontaneousSolve workItem

527
       ; case mSolve of 
528
           SPCantSolve -> -- No spontaneous solution for him, keep going
529 530
               return $ SR { sr_new_work   = emptyWorkList
                           , sr_inerts     = inerts
531 532
                           , sr_stop       = ContinueWith workItem }

533
           SPSolved workItem'
534 535 536 537 538 539
               | 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 
540 541 542
               -> do { bumpStepCountTcS
	       	     ; traceFireTcS depth (ptext (sLit "Spontaneous (w/d)") <+> ppr workItem)
                     ; (new_inert, new_work) <- runSolverPipeline depth
543 544 545
                             [ ("recursive interact with inert eqs", interactWithInertEqsStage)
                             , ("recursive interact with inerts", interactWithInertsStage)
                             ] inerts workItem'
546 547 548
                     ; return $ SR { sr_new_work = new_work 
                                   , sr_inerts   = new_inert -- will include workItem' 
                                   , sr_stop     = Stop }
549
                     }
550 551 552
               | otherwise 
                   -> -- Original was given; he must then be inert all right, and
                      -- workList' are all givens from flattening
553 554 555 556 557
                      do { bumpStepCountTcS
	       	         ; traceFireTcS depth (ptext (sLit "Spontaneous (g)") <+> ppr workItem)
                         ; return $ SR { sr_new_work = emptyWorkList
                                       , sr_inerts   = inerts `updInertSet` workItem' 
                                       , sr_stop     = Stop } }
558 559 560 561
           SPError -> -- Return with no new work
               return $ SR { sr_new_work = emptyWorkList
                           , sr_inerts   = inerts
                           , sr_stop     = Stop }
562
       }
563

564 565 566 567 568 569
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


570
-- @trySpontaneousSolve wi@ solves equalities where one side is a
571
-- touchable unification variable.
572
--     	    See Note [Touchables and givens] 
573 574
trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
trySpontaneousSolve workItem@(CTyEqCan { cc_id = cv, cc_flavor = gw, cc_tyvar = tv1, cc_rhs = xi })
575
  | isGiven gw
576
  = return SPCantSolve
577 578 579 580
  | Just tv2 <- tcGetTyVar_maybe xi
  = do { tch1 <- isTouchableMetaTyVar tv1
       ; tch2 <- isTouchableMetaTyVar tv2
       ; case (tch1, tch2) of
581 582 583 584
           (True,  True)  -> trySpontaneousEqTwoWay cv gw tv1 tv2
           (True,  False) -> trySpontaneousEqOneWay cv gw tv1 xi
           (False, True)  -> trySpontaneousEqOneWay cv gw tv2 (mkTyVarTy tv1)
	   _ -> return SPCantSolve }
585 586
  | otherwise
  = do { tch1 <- isTouchableMetaTyVar tv1
587
       ; if tch1 then trySpontaneousEqOneWay cv gw tv1 xi
588 589
                 else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" 
                                    (ppr workItem) 
590
                         ; return SPCantSolve }
591
       }
592 593 594 595

  -- No need for 
  --      trySpontaneousSolve (CFunEqCan ...) = ...
  -- See Note [No touchables as FunEq RHS] in TcSMonad
596
trySpontaneousSolve _ = return SPCantSolve
597 598

----------------
599
trySpontaneousEqOneWay :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
600
-- tv is a MetaTyVar, not untouchable
601
trySpontaneousEqOneWay cv gw tv xi	
602
  | not (isSigTyVar tv) || isTyVarTy xi 
603 604
  = do { let kxi = typeKind xi -- NB: 'xi' is fully rewritten according to the inerts 
                               -- so we have its more specific kind in our hands
605
       ; if kxi `isSubKind` tyVarKind tv then
606
             solveWithIdentity cv gw tv xi
607 608 609
         else return SPCantSolve
{-
         else if tyVarKind tv `isSubKind` kxi then
610 611 612 613 614 615
             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 }
616
-}
617
       }
618
  | otherwise -- Still can't solve, sig tyvar and non-variable rhs
619
  = return SPCantSolve
620 621

----------------
622
trySpontaneousEqTwoWay :: CoVar -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
623
-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
624
trySpontaneousEqTwoWay cv gw tv1 tv2
625
  | k1 `isSubKind` k2
626
  , nicer_to_update_tv2 = solveWithIdentity cv gw tv2 (mkTyVarTy tv1)
627
  | k2 `isSubKind` k1 
628
  = solveWithIdentity cv gw tv1 (mkTyVarTy tv2)
629
  | otherwise -- None is a subkind of the other, but they are both touchable! 
630 631 632
  = return SPCantSolve
    -- do { addErrorTcS KindError gw (mkTyVarTy tv1) (mkTyVarTy tv2)
    --   ; return SPError }
633 634 635 636 637 638
  where
    k1 = tyVarKind tv1
    k2 = tyVarKind tv2
    nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}

639 640 641 642 643 644
Note [Kind errors] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the wanted problem: 
      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' 
645
simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and 
646
get quantified over in inference mode. That's bad because we do know at this point that the 
647
constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.
648 649

The same applies in canonicalization code in case of kind errors in the givens. 
650

651
However, when we canonicalize givens we only check for compatibility (@compatKind@). 
652
If there were a kind error in the givens, this means some form of inconsistency or dead code.
653

654 655 656 657 658
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. 
659

660 661
Note [Spontaneous solving and kind compatibility] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
662 663 664
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.)
665

666 667 668 669 670 671 672 673 674 675 676 677 678 679
  - 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:
680 681 682 683 684 685 686 687 688
  - 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' 

689 690 691 692 693 694 695

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! 
696
      given : a ~ alpha      [having unified alpha := a] 
697 698 699
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] 

700
We avoid this problem by orienting the resulting given so that the unification
701 702
variable is on the left.  [Note that alternatively we could attempt to
enforce this at canonicalization]
703

704 705 706
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.
707 708 709

\begin{code}
----------------
710 711

solveWithIdentity :: CoVar -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
712 713
-- Solve with the identity coercion 
-- Precondition: kind(xi) is a sub-kind of kind(tv)
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
714 715 716
-- Precondition: CtFlavor is Wanted or Derived
-- See [New Wanted Superclass Work] to see why solveWithIdentity 
--     must work for Derived as well as Wanted
717
-- Returns: workItem where 
718
--        workItem = the new Given constraint
719 720 721
solveWithIdentity cv wd tv xi 
  = do { traceTcS "Sneaky unification:" $ 
                       vcat [text "Coercion variable:  " <+> ppr wd, 
722 723 724
                             text "Coercion:           " <+> pprEq (mkTyVarTy tv) xi,
                             text "Left  Kind is     : " <+> ppr (typeKind (mkTyVarTy tv)),
                             text "Right Kind is     : " <+> ppr (typeKind xi)
725
                  ]
726

727 728
       ; setWantedTyBind tv xi
       ; cv_given <- newGivenCoVar (mkTyVarTy tv) xi xi
729

730
       ; when (isWanted wd) (setCoBind cv xi)
731
           -- We don't want to do this for Derived, that's why we use 'when (isWanted wd)'
732 733 734

       ; return $ SPSolved (CTyEqCan { cc_id = cv_given
                                     , cc_flavor = mkGivenFlavor wd UnkSkol
735
                                     , cc_tyvar  = tv, cc_rhs = xi }) }
736 737 738 739 740 741 742 743 744
\end{code}


*********************************************************************************
*                                                                               * 
                       The interact-with-inert Stage
*                                                                               *
*********************************************************************************

745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769
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

770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785
\begin{code}
-- Interaction result of  WorkItem <~> AtomicInert
data InteractResult
   = IR { ir_stop         :: StopOrContinue
            -- Stop
            --   => Reagent (work item) consumed.
            -- ContinueWith new_reagent
            --   => Reagent transformed but keep gathering interactions. 
            --      The transformed item remains inert with respect 
            --      to any previously encountered inerts.

        , ir_inert_action :: InertAction
            -- Whether the inert item should remain in the InertSet.

        , ir_new_work     :: WorkList
            -- new work items to add to the WorkList
786 787

        , ir_fire :: Maybe String    -- Tells whether a rule fired, and if so what
788 789 790
        }

-- What to do with the inert reactant.
791
data InertAction = KeepInert | DropInert 
792

793 794 795 796
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 }
797

798 799
mkIRStopK :: String -> WorkList -> TcS InteractResult
mkIRStopK rule newWork
800 801
  = return $ IR { ir_stop = Stop, ir_inert_action = KeepInert
                , ir_new_work = newWork, ir_fire = Just rule }
802

803 804 805 806 807
mkIRStopD :: String -> WorkList -> TcS InteractResult
mkIRStopD rule newWork
  = return $ IR { ir_stop = Stop, ir_inert_action = DropInert
                , ir_new_work = newWork, ir_fire = Just rule }

808
noInteraction :: Monad m => WorkItem -> m InteractResult
809 810 811
noInteraction wi
  = return $ IR { ir_stop = ContinueWith wi, ir_inert_action = KeepInert
                , ir_new_work = emptyWorkList, ir_fire = Nothing }
812

dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
813
data WhichComesFromInert = LeftComesFromInert | RightComesFromInert 
814
     -- See Note [Efficient Orientation] 
815

816

817
---------------------------------------------------
818 819 820
-- 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
821 822 823
-- interact the WorkItem with the entire equalities of the InertSet

interactWithInertEqsStage :: SimplifierStage 
824
interactWithInertEqsStage depth workItem inert
825
  = Bag.foldrBagM (interactNext depth) initITR (inert_eqs inert)
826
                        -- use foldr to preserve the order          
827 828 829 830
  where
    initITR = SR { sr_inerts   = inert { inert_eqs = emptyCCan }
                 , sr_new_work = emptyWorkList
                 , sr_stop     = ContinueWith workItem }
831

832 833 834 835
---------------------------------------------------
-- 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 
836 837
-- "Other" constraints it contains!

838
interactWithInertsStage :: SimplifierStage
839
interactWithInertsStage depth workItem inert
840 841 842 843
  = let (relevant, inert_residual) = getISRelevant workItem inert 
        initITR = SR { sr_inerts   = inert_residual
                     , sr_new_work = emptyWorkList
                     , sr_stop     = ContinueWith workItem } 
844 845
    in Bag.foldrBagM (interactNext depth) initITR relevant 
                        -- use foldr to preserve the order
846
  where 
847
    getISRelevant :: CanonicalCt -> InertSet -> (CanonicalCts, InertSet) 
848 849 850 851 852 853
    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)
854 855 856 857 858 859 860 861 862 863 864 865 866
        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` 
867 868 869
                    cCanMapToBag (inert_dicts is) `unionBags` cCanMapToBag (inert_funeqs is)
        in (cts, is { inert_dicts  = emptyCCanMap
                    , inert_ips    = emptyCCanMap
870
                    , inert_funeqs = emptyCCanMap })
871

872 873
interactNext :: SubGoalDepth -> AtomicInert -> StageResult -> TcS StageResult 
interactNext depth inert it
874 875 876 877 878 879 880 881 882 883 884
  | 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
885
      	                  , ppUnless (isEmptyWorkList new_work) $
886 887 888 889 890 891 892 893 894 895 896 897 898
                            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
899 900

       ; return $ SR { sr_inerts   = inerts_new
901
                     , sr_new_work = sr_new_work it `unionWorkList` new_work
902
                     , sr_stop     = stop } }
903 904
  | otherwise 
  = return $ it { sr_inerts = (sr_inerts it) `updInertSet` inert }
905 906

-- Do a single interaction of two constraints.
907
interactWithInert :: AtomicInert -> WorkItem -> TcS InteractResult
908 909 910
interactWithInert inert workItem 
  = do { ctxt <- getTcSContext
       ; let is_allowed  = allowedInteraction (simplEqsOnly ctxt) inert workItem 
911

912 913
       ; if is_allowed then 
              doInteractWithInert inert workItem 
914
          else 
915 916
              noInteraction workItem 
       }
917 918 919 920 921 922 923 924

allowedInteraction :: Bool -> AtomicInert -> WorkItem -> Bool 
-- Allowed interactions 
allowedInteraction eqs_only (CDictCan {}) (CDictCan {}) = not eqs_only
allowedInteraction eqs_only (CIPCan {})   (CIPCan {})   = not eqs_only
allowedInteraction _ _ _ = True 

--------------------------------------------
925
doInteractWithInert :: CanonicalCt -> CanonicalCt -> TcS InteractResult
926 927
-- Identical class constraints.

928
doInteractWithInert
929 930
  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 })
931
  | cls1 == cls2 && (and $ zipWith tcEqType tys1 tys2)
932
  = solveOneFromTheOther "Cls/Cls" (EvId d1,fl1) workItem 
933 934 935

  | cls1 == cls2 && (not (isGiven fl1 && isGiven fl2))
  = 	 -- See Note [When improvement happens]
936 937 938
    do { let pty1 = ClassP cls1 tys1
             pty2 = ClassP cls2 tys2
             inert_pred_loc     = (pty1, pprFlavorArising fl1)
939 940 941 942 943 944 945 946 947 948 949 950
             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
951 952 953 954 955 956 957 958 959 960 961 962
	     	case fl2 of
	          Given   {} -> pprPanic "Unexpected given" (ppr inertItem $$ ppr workItem)
                  Derived {} -> mkIRStopK "Cls/Cls fundep (solved)" fd_work
		  Wanted  {} 
		    | isDerived fl1 
                   -> do { setDictBind d2 (EvCast d1 dict_co)
			 ; let inert_w = inertItem { cc_flavor = fl2 }
			   -- A bit naughty: we take the inert Derived, 
			   -- turn it into a Wanted, use it to solve the work-item
			   -- and put it back into the work-list
			   -- Maybe rather than starting again, we could *replace* the
			   -- inert item, but its safe and simple to restart
963 964
                         ; mkIRStopD "Cls/Cls fundep (solved)" $ 
                           workListFromNonEq inert_w `unionWorkList` fd_work }
965 966 967
		    | otherwise 
                    -> do { setDictBind d2 (EvCast d1 dict_co)
                          ; mkIRStopK "Cls/Cls fundep (solved)" fd_work }
968

969 970
             | otherwise
             -> -- We could not quite solve him, but we still rewrite him
971 972 973 974 975 976
	        -- 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.
977 978 979 980 981 982 983 984 985 986 987 988
		-- However it is not necessarily inert wrt previous inert-set items.
                --      class C a b c d |  a -> b, b c -> d
		--      Inert: c1: C b Q R S, c2: C P Q a b
		--      Work: C P alpha R beta
		--      Does not react with c1; reacts with c2, with alpha:=Q
		--      NOW it reacts with c1!
		-- So we must stop, and put the rewritten constraint back in the work list
                do { d2' <- newDictVar cls1 rewritten_tys2
                   ; case fl2 of
                       Given {}   -> pprPanic "Unexpected given" (ppr inertItem $$ ppr workItem)
                       Wanted {}  -> setDictBind d2 (EvCast d2' dict_co)
                       Derived {} -> return ()
989
                   ; let workItem' = workItem { cc_id = d2', cc_tyargs = rewritten_tys2 }
990 991
                   ; mkIRStopK "Cls/Cls fundep (partial)" $ 
                     workListFromNonEq workItem' `unionWorkList` fd_work } 
992

993 994
             where
               dict_co = mkTyConCoercion (classTyCon cls1) cos2
995
  }
996 997 998

-- Class constraint and given equality: use the equality to rewrite
-- the class constraint. 
999
doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi }) 
1000 1001 1002
                    (CDictCan { cc_id = dv, cc_flavor = wfl, cc_class = cl, cc_tyargs = xis }) 
  | ifl `canRewrite` wfl 
  , tv `elemVarSet` tyVarsOfTypes xis
1003
  = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,wfl,cl,xis)
1004 1005
            -- Continue with rewritten Dictionary because we can only be in the 
            -- interactWithEqsStage, so the dictionary is inert. 
1006
       ; mkIRContinue "Eq/Cls" rewritten_dict KeepInert emptyWorkList }
1007
    
1008
doInteractWithInert (CDictCan { cc_id = dv, cc_flavor = ifl, cc_class = cl, cc_tyargs = xis }) 
1009 1010 1011
           workItem@(CTyEqCan { cc_id = cv, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi })
  | wfl `canRewrite` ifl
  , tv `elemVarSet` tyVarsOfTypes xis
1012
  = do { rewritten_dict <- rewriteDict (cv,tv,xi) (dv,ifl,cl,xis)
1013
       ; mkIRContinue "Cls/Eq" workItem DropInert (workListFromNonEq rewritten_dict) }
1014 1015 1016

-- Class constraint and given equality: use the equality to rewrite
-- the class constraint.
1017
doInteractWithInert (CTyEqCan { cc_id = cv, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi }) 
1018 1019 1020 1021
                    (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) 
1022
       ; mkIRContinue "Eq/IP" rewritten_ip KeepInert emptyWorkList } 
1023

1024
doInteractWithInert (CIPCan { cc_id = ipid, cc_flavor = ifl, cc_ip_nm = nm, cc_ip_ty = ty }) 
1025 1026 1027 1028
           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) 
1029
       ; mkIRContinue "IP/Eq" workItem DropInert (workListFromNonEq rewritten_ip) }
1030 1031 1032 1033 1034 1035

-- 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.
1036
doInteractWithInert (CIPCan { cc_id = id1, cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 }) 
1037
           workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
1038 1039 1040 1041
  | 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
1042 1043 1044
	-- 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
1045

1046
  | nm1 == nm2 && ty1 `tcEqType` ty2 
1047
  = solveOneFromTheOther "IP/IP" (EvId id1,ifl) workItem 
1048

1049
  | nm1 == nm2
1050
  =  	-- See Note [When improvement happens]
1051
    do { co_var <- newCoVar ty2 ty1 -- See Note [Efficient Orientation]
1052
       ; let flav = Wanted (combineCtLoc ifl wfl) 
1053
       ; cans <- mkCanonical flav co_var 
1054
       ; mkIRContinue "IP/IP fundep" workItem KeepInert cans }
1055 1056

-- Never rewrite a given with a wanted equality, and a type function
1057 1058 1059
-- 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.
1060

1061
-- Inert: equality, work item: function equality
1062
doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = ifl, cc_tyvar = tv, cc_rhs = xi1 }) 
1063 1064 1065
                    (CFunEqCan { cc_id = cv2, cc_flavor = wfl, cc_fun = tc
                               , cc_tyargs = args, cc_rhs = xi2 })
  | ifl `canRewrite` wfl 
1066
  , tv `elemVarSet` tyVarsOfTypes (xi2:args) -- Rewrite RHS as well
1067
  = do { rewritten_funeq <- rewriteFunEq (cv1,tv,xi1) (cv2,wfl,tc,args,xi2) 
1068
       ; mkIRStopK "Eq/FunEq" (workListFromEq rewritten_funeq) } 
1069
         -- Must Stop here, because we may no longer be inert after the rewritting.
1070 1071

-- Inert: function equality, work item: equality
1072
doInteractWithInert (CFunEqCan {cc_id = cv1, cc_flavor = ifl, cc_fun = tc
1073 1074 1075
                              , cc_tyargs = args, cc_rhs = xi1 }) 
           workItem@(CTyEqCan { cc_id = cv2, cc_flavor = wfl, cc_tyvar = tv, cc_rhs = xi2 })
  | wfl `canRewrite` ifl
1076
  , tv `elemVarSet` tyVarsOfTypes (xi1:args) -- Rewrite RHS as well
1077
  = do { rewritten_funeq <- rewriteFunEq (cv2,tv,xi2) (cv1,ifl,tc,args,xi1) 
1078
       ; mkIRContinue "FunEq/Eq" workItem DropInert (workListFromEq rewritten_funeq) } 
1079 1080 1081 1082 1083 1084 1085 1086
         -- 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.
1087

1088
doInteractWithInert (CFunEqCan { cc_id = cv1, cc_flavor = fl1, cc_fun = tc1
1089 1090 1091
                               , cc_tyargs = args1, cc_rhs = xi1 }) 
           workItem@(CFunEqCan { cc_id = cv2, cc_flavor = fl2, cc_fun = tc2
                               , cc_tyargs = args2, cc_rhs = xi2 })
1092
  | fl1 `canSolve` fl2 && lhss_match
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
1093
  = do { cans <- rewriteEqLHS LeftComesFromInert  (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2) 
1094
       ; mkIRStopK "FunEq/FunEq" cans } 
1095
  | fl2 `canSolve` fl1 && lhss_match
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
1096
  = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1) 
1097
       ; mkIRContinue "FunEq/FunEq" workItem DropInert cans }
1098 1099 1100
  where
    lhss_match = tc1 == tc2 && and (zipWith tcEqType args1 args2) 

1101
doInteractWithInert (CTyEqCan { cc_id = cv1, cc_flavor = fl1, cc_tyvar = tv1, cc_rhs = xi1 }) 
1102 1103
           workItem@(CTyEqCan { cc_id = cv2, cc_flavor = fl2, cc_tyvar = tv2, cc_rhs = xi2 })
-- Check for matching LHS 
1104
  | fl1 `canSolve` fl2 && tv1 == tv2 
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
1105
  = do { cans <- rewriteEqLHS LeftComesFromInert (mkCoVarCoercion cv1,xi1) (cv2,fl2,xi2) 
1106
       ; mkIRStopK "Eq/Eq lhs" cans } 
1107

1108
  | fl2 `canSolve` fl1 && tv1 == tv2 
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
1109
  = do { cans <- rewriteEqLHS RightComesFromInert (mkCoVarCoercion cv2,xi2) (cv1,fl1,xi1) 
1110 1111
       ; mkIRContinue "Eq/Eq lhs" workItem DropInert cans }

1112 1113 1114
-- Check for rewriting RHS 
  | fl1 `canRewrite` fl2 && tv1 `elemVarSet` tyVarsOfType xi2 
  = do { rewritten_eq <- rewriteEqRHS (cv1,tv1,xi1) (cv2,fl2,tv2,xi2) 
1115
       ; mkIRStopK "Eq/Eq rhs" rewritten_eq }
1116

1117 1118
  | fl2 `canRewrite` fl1 && tv2 `elemVarSet` tyVarsOfType xi1
  = do { rewritten_eq <- rewriteEqRHS (cv2,tv2,xi2) (cv1,fl1,tv1,xi1) 
1119
       ; mkIRContinue "Eq/Eq rhs" workItem DropInert rewritten_eq } 
1120

1121 1122 1123 1124
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)
1125
       ; mkIRStopK "Frozen/Eq" rewritten_frozen }
1126 1127 1128 1129 <