TcSMonad.lhs 58.1 KB
Newer Older
1
\begin{code}
2
{-# OPTIONS -Wwarn -fno-warn-tabs #-}
Ian Lynagh's avatar
Ian Lynagh committed
3
4
5
6
7
8
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

9
10
11
-- Type definitions for the constraint solver
module TcSMonad ( 

12
       -- Canonical constraints, definition is now in TcRnTypes
13

14
15
16
17
    WorkList(..), isEmptyWorkList, emptyWorkList,
    workListFromEq, workListFromNonEq, workListFromCt, 
    extendWorkListEq, extendWorkListNonEq, extendWorkListCt, 
    appendWorkListCt, appendWorkListEqs, unionWorkList,
18

19
20
21
22
    getTcSWorkList, updWorkListTcS, updWorkListTcS_return, keepWanted,

    Ct(..), Xi, tyVarsOfCt, tyVarsOfCts, tyVarsOfCDicts, 
    emitFrozenError,
23

dimitris's avatar
dimitris committed
24
25
26
    isWanted, isGivenOrSolved, isDerived,
    isGivenOrSolvedCt, isGivenCt_maybe, 
    isWantedCt, isDerivedCt, pprFlavorArising,
27

28
29
    isFlexiTcsTv,

30
    canRewrite, canSolve,
dimitris's avatar
dimitris committed
31
32
    combineCtLoc, mkSolvedFlavor, mkGivenFlavor,
    mkWantedFlavor,
33
    getWantedLoc,
34

35
    TcS, runTcS, failTcS, panicTcS, traceTcS, -- Basic functionality 
36
37
38
39
    traceFireTcS, bumpStepCountTcS, doWithInert,
    tryTcS, nestImplicTcS, recoverTcS,
    wrapErrTcS, wrapWarnTcS,

40
41
    SimplContext(..), isInteractive, simplEqsOnly, performDefaulting,

42
       -- Creation of evidence variables
43
44
45
46
    newEvVar, forceNewEvVar, delCachedEvVar, updateFlatCache, flushFlatCache,
    newGivenEqVar,
    newEqVar, newKindConstraint,
    EvVarCreated (..), isNewEvVar, FlatEqOrigin ( .. ), origin_matches,
47
48

       -- Setting evidence variables 
batterseapower's avatar
batterseapower committed
49
50
51
52
    setEqBind,
    setIPBind,
    setDictBind,
    setEvBind,
53
54
55

    setWantedTyBind,

56
    getInstEnvs, getFamInstEnvs,                -- Getting the environments
57
    getTopEnv, getGblEnv, getTcEvBinds, getUntouchables,
58
59
    getTcEvBindsMap, getTcSContext, getTcSTyBinds, getTcSTyBindsMap,
    getTcSEvVarCacheMap, getTcSEvVarFlatCache, setTcSEvVarCacheMap, pprEvVarCache,
60
61

    newFlattenSkolemTy,                         -- Flatten skolems 
62

63
64
65
66
67
68
69
70
71
        -- Inerts 
    InertSet(..), 
    getInertEqs, rewriteFromInertEqs, liftInertEqsTy,
    emptyInert, getTcSInerts, updInertSet, extractUnsolved,
    extractUnsolvedTcS, modifyInertTcS,
    updInertSetTcS, partitionCCanMap, partitionEqMap,
    getRelevantCts, extractRelevantInerts,
    CCanMap (..), CtTypeMap, pprCtTypeMap, mkPredKeyForTypeMap, partitionCtTypeMap,

72
73

    instDFunTypes,                              -- Instantiation
74
    instDFunConstraints,          
75
    newFlexiTcSTy, instFlexiTcS,
76

dreixel's avatar
dreixel committed
77
    compatKind, compatKindTcS, isSubKindTcS, unifyKindTcS,
78

79
    TcsUntouchables,
80
    isTouchableMetaTyVar,
81
    isTouchableMetaTyVar_InRange, 
82
83
84
85
86
87

    getDefaultInfo, getDynFlags,

    matchClass, matchFam, MatchInstResult (..), 
    checkWellStagedDFun, 
    warnTcS,
88
    pprEq                                    -- Smaller utils, re-exported from TcM
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
                                             -- TODO (DV): these are only really used in the 
                                             -- instance matcher in TcSimplify. I am wondering
                                             -- if the whole instance matcher simply belongs
                                             -- here 
) where 

#include "HsVersions.h"

import HscTypes
import BasicTypes 

import Inst
import InstEnv 
import FamInst 
import FamInstEnv

import qualified TcRnMonad as TcM
import qualified TcMType as TcM
import qualified TcEnv as TcM 
108
       ( checkWellStaged, topIdLvl, tcGetDefaultTys )
dreixel's avatar
dreixel committed
109
import {-# SOURCE #-} qualified TcUnify as TcM ( unifyKindEq, mkKindErrorCtxt )
110
import Kind
111
112
import TcType
import DynFlags
113
import Type
114
115
116
117

import Coercion
import Class
import TyCon
118
119
import TypeRep 

120
121
import Name
import Var
122
import VarEnv
123
124
125
126
import Outputable
import Bag
import MonadUtils
import VarSet
127
128

import Pair ( pSnd )
129
import FastString
Ian Lynagh's avatar
Ian Lynagh committed
130
import Util
131
132
133

import HsBinds               -- for TcEvBinds stuff 
import Id 
Ian Lynagh's avatar
Ian Lynagh committed
134
import TcRnTypes
135

136
137
138
139
140
141
import Unique 
import UniqFM
import Maybes ( orElse )

import Control.Monad( when )
import StaticFlags( opt_PprStyle_Debug )
Ian Lynagh's avatar
Ian Lynagh committed
142
import Data.IORef
143

144
import TrieMap
145

146
\end{code}
147
148

\begin{code}
149
150
compatKind :: Kind -> Kind -> Bool
compatKind k1 k2 = k1 `isSubKind` k2 || k2 `isSubKind` k1 
151

dreixel's avatar
dreixel committed
152
153
154
155
156
157
158
159
160
161
compatKindTcS :: Kind -> Kind -> TcS Bool
-- Because kind unification happens during constraint solving, we have
-- to make sure that two kinds are zonked before we compare them.
compatKindTcS k1 k2 = wrapTcS (TcM.compatKindTcM k1 k2)

isSubKindTcS :: Kind -> Kind -> TcS Bool
isSubKindTcS k1 k2 = wrapTcS (TcM.isSubKindTcM k1 k2)

unifyKindTcS :: Type -> Type     -- Context
             -> Kind -> Kind     -- Corresponding kinds
162
             -> TcS Bool
dreixel's avatar
dreixel committed
163
unifyKindTcS ty1 ty2 ki1 ki2
164
165
166
  = wrapTcS $ TcM.addErrCtxtM ctxt $ do
      (_errs, mb_r) <- TcM.tryTc (TcM.unifyKindEq ki1 ki2)
      return (maybe False (const True) mb_r)
dreixel's avatar
dreixel committed
167
168
  where ctxt = TcM.mkKindErrorCtxt ty1 ki1 ty2 ki2

169
170
\end{code}

171
172
173
174
175
176
177
178
%************************************************************************
%*									*
%*                            Worklists                                *
%*  Canonical and non-canonical constraints that the simplifier has to  *
%*  work on. Including their simplification depths.                     *
%*                                                                      *
%*									*
%************************************************************************
179

180
181
Note [WorkList]
~~~~~~~~~~~~~~~
182

183
184
185
A WorkList contains canonical and non-canonical items (of all flavors). 
Notice that each Ct now has a simplification depth. We may 
consider using this depth for prioritization as well in the future. 
186

187
188
189
190
191
As a simple form of priority queue, our worklist separates out
equalities (wl_eqs) from the rest of the canonical constraints, 
so that it's easier to deal with them first, but the separation 
is not strictly necessary. Notice that non-canonical constraints 
are also parts of the worklist. 
192

193
194
195
196
197
198
Note [NonCanonical Semantics]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Note that canonical constraints involve a CNonCanonical constructor. In the worklist
we use this constructor for constraints that have not yet been canonicalized such as 
   [Int] ~ [a] 
In other words, all constraints start life as NonCanonicals. 
199

200
201
On the other hand, in the Inert Set (see below) the presence of a NonCanonical somewhere
means that we have a ``frozen error''. 
202

203
204
205
NonCanonical constraints never interact directly with other constraints -- but they can
be rewritten by equalities (for instance if a non canonical exists in the inert, we'd 
better rewrite it as much as possible before reporting it as an error to the user)
206

207
\begin{code}
208

209
210
-- See Note [WorkList]
data WorkList = WorkList { wl_eqs  :: [Ct], wl_rest :: [Ct] }
211

batterseapower's avatar
batterseapower committed
212

213
214
215
216
unionWorkList :: WorkList -> WorkList -> WorkList
unionWorkList new_wl orig_wl = 
   WorkList { wl_eqs = wl_eqs new_wl ++ wl_eqs orig_wl
            , wl_rest = wl_rest new_wl ++ wl_rest orig_wl }
217

218
219
220
extendWorkListEq :: Ct -> WorkList -> WorkList
-- Extension by equality
extendWorkListEq ct wl = wl { wl_eqs = ct : wl_eqs wl }
221

222
223
224
extendWorkListNonEq :: Ct -> WorkList -> WorkList
-- Extension by non equality
extendWorkListNonEq ct wl = wl { wl_rest = ct : wl_rest wl }
225

226
227
228
229
230
extendWorkListCt :: Ct -> WorkList -> WorkList
-- Agnostic
extendWorkListCt ct wl
 | isLCoVar (cc_id ct) = extendWorkListEq ct wl
 | otherwise = extendWorkListNonEq ct wl
231

232
233
234
appendWorkListCt :: [Ct] -> WorkList -> WorkList
-- Agnostic
appendWorkListCt cts wl = foldr extendWorkListCt wl cts
235

236
237
238
appendWorkListEqs :: [Ct] -> WorkList -> WorkList
-- Append a list of equalities
appendWorkListEqs cts wl = foldr extendWorkListEq wl cts
239
240

isEmptyWorkList :: WorkList -> Bool
241
isEmptyWorkList wl = null (wl_eqs wl) &&  null (wl_rest wl)
242
243

emptyWorkList :: WorkList
244
emptyWorkList = WorkList { wl_eqs  = [], wl_rest = [] }
245

246
247
workListFromEq :: Ct -> WorkList
workListFromEq ct = WorkList { wl_eqs = [ct], wl_rest = [] }
248

249
250
workListFromNonEq :: Ct -> WorkList
workListFromNonEq ct = WorkList { wl_eqs = [], wl_rest = [ct] }
251

252
253
254
255
workListFromCt :: Ct -> WorkList
-- Agnostic 
workListFromCt ct | isLCoVar (cc_id ct) = workListFromEq ct 
                  | otherwise           = workListFromNonEq ct
256

257
258
259
260
261
-- Pretty printing 
instance Outputable WorkList where 
  ppr wl = vcat [ text "WorkList (eqs)   = " <+> ppr (wl_eqs wl)
                , text "WorkList (rest)  = " <+> ppr (wl_rest wl)
                ]
262

263
264
265
266
267
keepWanted :: Cts -> Cts
keepWanted = filterBag isWantedCt
    -- DV: there used to be a note here that read: 
    -- ``Important: use fold*r*Bag to preserve the order of the evidence variables'' 
    -- DV: Is this still relevant? 
268

269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
\end{code}

%************************************************************************
%*									*
%*                            Inert sets                                *
%*                                                                      *
%*									*
%************************************************************************


Note [InertSet invariants]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
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]
289

290
291
292
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
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
  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

  4 The entire set of equalities is acyclic.

  5 Wanted dictionaries are inert with the top-level axiom set 

  6 Equalities of the form tv1 ~ tv2 always have a touchable variable
    on the left (if possible).

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

  8 No Given constraint mentions a touchable unification variable, but 
    Given/Solved may do so. 

  9 Given constraints will also have their superclasses in the inert set, 
    but Given/Solved will not. 
 
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}

data CCanMap a = CCanMap { cts_given   :: UniqFM Cts
                                          -- Invariant: all Given
                         , cts_derived :: UniqFM Cts 
                                          -- Invariant: all Derived
                         , cts_wanted  :: UniqFM Cts } 
                                          -- Invariant: all Wanted

cCanMapToBag :: CCanMap a -> Cts 
cCanMapToBag cmap = foldUFM unionBags rest_wder (cts_given cmap)
  where rest_wder = foldUFM unionBags rest_der  (cts_wanted cmap) 
        rest_der  = foldUFM unionBags emptyCts  (cts_derived cmap)

emptyCCanMap :: CCanMap a 
emptyCCanMap = CCanMap { cts_given = emptyUFM, cts_derived = emptyUFM, cts_wanted = emptyUFM } 

updCCanMap:: Uniquable a => (a,Ct) -> CCanMap a -> CCanMap a 
updCCanMap (a,ct) cmap 
  = case cc_flavor ct of 
      Wanted {}  -> cmap { cts_wanted  = insert_into (cts_wanted cmap)  } 
      Given {}   -> cmap { cts_given   = insert_into (cts_given cmap)   }
      Derived {} -> cmap { cts_derived = insert_into (cts_derived cmap) }
  where 
    insert_into m = addToUFM_C unionBags m a (singleCt ct)

getRelevantCts :: Uniquable a => a -> CCanMap a -> (Cts, CCanMap a) 
-- Gets the relevant constraints and returns the rest of the CCanMap
getRelevantCts a cmap 
    = let relevant = lookup (cts_wanted cmap) `unionBags`
                     lookup (cts_given cmap)  `unionBags`
                     lookup (cts_derived cmap) 
          residual_map = cmap { cts_wanted  = delFromUFM (cts_wanted cmap) a
                              , cts_given   = delFromUFM (cts_given cmap) a
                              , cts_derived = delFromUFM (cts_derived cmap) a }
      in (relevant, residual_map) 
  where
    lookup map = lookupUFM map a `orElse` emptyCts


getCtTypeMapRelevants :: PredType -> TypeMap Ct -> (Cts, TypeMap Ct)
getCtTypeMapRelevants key_pty tmap
  = partitionCtTypeMap (\ct -> mkPredKeyForTypeMap ct `eqType` key_pty) tmap


partitionCCanMap :: (Ct -> Bool) -> CCanMap a -> (Cts,CCanMap a) 
-- All constraints that /match/ the predicate go in the bag, the rest remain in the map
partitionCCanMap pred cmap
  = let (ws_map,ws) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_wanted cmap) 
        (ds_map,ds) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_derived cmap)
        (gs_map,gs) = foldUFM_Directly aux (emptyUFM,emptyCts) (cts_given cmap) 
    in (ws `andCts` ds `andCts` gs, cmap { cts_wanted  = ws_map
                                         , cts_given   = gs_map
                                         , cts_derived = ds_map }) 
  where aux k this_cts (mp,acc_cts) = (new_mp, new_acc_cts)
                                    where new_mp      = addToUFM mp k cts_keep
                                          new_acc_cts = acc_cts `andCts` cts_out
                                          (cts_out, cts_keep) = partitionBag pred this_cts

partitionEqMap :: (Ct -> Bool) -> TyVarEnv (Ct,Coercion) -> ([Ct], TyVarEnv (Ct,Coercion))
partitionEqMap pred isubst 
  = let eqs_out = foldVarEnv extend_if_pred [] isubst
        eqs_in  = filterVarEnv_Directly (\_ (ct,_) -> not (pred ct)) isubst
    in (eqs_out, eqs_in)
  where extend_if_pred (ct,_) cts = if pred ct then ct : cts else cts


extractUnsolvedCMap :: CCanMap a -> (Cts, CCanMap a)
-- Gets the wanted or derived constraints and returns a residual
-- CCanMap with only givens.
extractUnsolvedCMap cmap =
  let wntd = foldUFM unionBags emptyCts (cts_wanted cmap)
      derd = foldUFM unionBags emptyCts (cts_derived cmap)
  in (wntd `unionBags` derd, 
      cmap { cts_wanted = emptyUFM, cts_derived = emptyUFM })

-- See Note [InertSet invariants]
data InertSet 
  = IS { inert_eqs     :: TyVarEnv (Ct,Coercion) 
         -- Must all be CTyEqCans! If an entry exists of the form: 
         --   a |-> ct,co
         -- Then ct = CTyEqCan { cc_tyvar = a, cc_rhs = xi } 
         -- And  co : a ~ xi
       , inert_eq_tvs  :: InScopeSet -- Invariant: superset of inert_eqs tvs

       , inert_dicts        :: CCanMap Class -- Dictionaries only, index is the class
       , inert_ips          :: CCanMap (IPName Name)      -- Implicit parameters 
         -- NB: We do not want to use TypeMaps here because functional dependencies
         -- will only match on the class but not the type. Similarly IPs match on the
         -- name but not on the whole datatype

       , inert_funeqs       :: CtTypeMap -- Map from family heads to CFunEqCan constraints

       , inert_irreds       :: Cts  -- Irreducible predicates
       , inert_frozen       :: Cts  -- All non-canonicals are kept here (as frozen errors)
       }


type CtTypeMap = TypeMap Ct

pprCtTypeMap :: TypeMap Ct -> SDoc 
pprCtTypeMap ctmap = ppr (foldTM (:) ctmap [])

ctTypeMapCts :: TypeMap Ct -> Cts
ctTypeMapCts ctmap = foldTM (\ct cts -> extendCts cts ct) ctmap emptyCts

mkPredKeyForTypeMap :: Ct -> PredType
-- Create a key from a constraint to use in the inert CtTypeMap.
-- The only interesting case is for family applications, where the 
-- key is not the whole PredType of cc_id, but rather the family 
-- equality left hand side (head)
mkPredKeyForTypeMap (CFunEqCan { cc_fun = fn, cc_tyargs = xis }) 
  = mkTyConApp fn xis
mkPredKeyForTypeMap ct 
  = evVarPred (cc_id ct)

partitionCtTypeMap :: (Ct -> Bool)
                   -> TypeMap Ct -> (Cts, TypeMap Ct)
-- Kick out the ones that match the predicate and keep the rest in the typemap
partitionCtTypeMap f ctmap
  = foldTM upd_acc ctmap (emptyBag,ctmap)
  where upd_acc ct (cts,acc_map)
         | f ct      = (extendCts cts ct, alterTM ct_key (\_ -> Nothing) acc_map)
         | otherwise = (cts,acc_map)
         where ct_key = mkPredKeyForTypeMap ct


instance Outputable InertSet where
  ppr is = vcat [ vcat (map ppr (varEnvElts (inert_eqs is)))
                , vcat (map ppr (Bag.bagToList $ inert_irreds is)) 
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_dicts is)))
                , vcat (map ppr (Bag.bagToList $ cCanMapToBag (inert_ips is))) 
                , vcat (map ppr (Bag.bagToList $ ctTypeMapCts (inert_funeqs is)))
                , text "Frozen errors =" <+> -- Clearly print frozen errors
                    braces (vcat (map ppr (Bag.bagToList $ inert_frozen is)))
                , text "Warning: Not displaying cached (solved) constraints"
                ]
                       
emptyInert :: InertSet
emptyInert = IS { inert_eqs     = emptyVarEnv
                , inert_eq_tvs  = emptyInScopeSet
                , inert_frozen  = emptyCts
                , inert_irreds  = emptyCts
                , inert_dicts   = emptyCCanMap
                , inert_ips     = emptyCCanMap
                , inert_funeqs  = emptyTM
                }


type AtomicInert = Ct 

updInertSet :: InertSet -> AtomicInert -> InertSet 
-- Add a new inert element to the inert set. 
updInertSet is item 
  | isCTyEqCan item                     
  = let upd_err a b = pprPanic "updInertSet" $ 
                      vcat [text "Multiple inert equalities:", ppr a, ppr b]
        eqs'     = extendVarEnv_C upd_err (inert_eqs is)
                                          (cc_tyvar item)
                                          (item, mkEqVarLCo (cc_id item))
        inscope' = extendInScopeSetSet (inert_eq_tvs is) (tyVarsOfCt item)
    in is { inert_eqs = eqs', inert_eq_tvs = inscope' }

{-
       -- /Solved/ non-equalities go to the solved map
  | Just GivenSolved <- isGiven_maybe (cc_flavor item)
  = let pty = mkPredKeyForTypeMap item
        solved_orig = inert_solved is
    in is { inert_solved = alterTM pty (\_ -> Just item) solved_orig }
-}

  | Just x  <- isCIPCan_Maybe item      -- IP 
  = is { inert_ips   = updCCanMap (x,item) (inert_ips is) }  
  | isCIrredEvCan item                  -- Presently-irreducible evidence
  = is { inert_irreds = inert_irreds is `Bag.snocBag` item }


  | Just cls <- isCDictCan_Maybe item   -- Dictionary 
  = is { inert_dicts = updCCanMap (cls,item) (inert_dicts is) }

  | Just _tc <- isCFunEqCan_Maybe item  -- Function equality
  = let pty = mkPredKeyForTypeMap item
        upd_funeqs Nothing = Just item
        upd_funeqs (Just _alredy_there) = panic "updInertSet: item already there!"
    in is { inert_funeqs = alterTM pty upd_funeqs (inert_funeqs is) }
     
  | otherwise 
  = is { inert_frozen = inert_frozen is `Bag.snocBag` item }

updInertSetTcS :: AtomicInert -> TcS ()
-- Add a new item in the inerts of the monad
updInertSetTcS item
  = do { traceTcS "updInertSetTcs {" $ 
         text "Trying to insert new inert item:" <+> ppr item

       ; modifyInertTcS (\is -> ((), updInertSet is item)) 
                        
       ; traceTcS "updInertSetTcs }" $ empty }


modifyInertTcS :: (InertSet -> (a,InertSet)) -> TcS a 
-- Modify the inert set with the supplied function
modifyInertTcS upd 
  = do { is_var <- getTcSInertsRef
       ; curr_inert <- wrapTcS (TcM.readTcRef is_var)
       ; let (a, new_inert) = upd curr_inert
       ; wrapTcS (TcM.writeTcRef is_var new_inert)
       ; return a }

extractUnsolvedTcS :: TcS (Cts,Cts) 
-- Extracts frozen errors and remaining unsolved and sets the 
-- inert set to be the remaining! 
extractUnsolvedTcS = 
  modifyInertTcS extractUnsolved 

extractUnsolved :: InertSet -> ((Cts,Cts), InertSet)
-- Postcondition
-- -------------
-- When: 
--   ((frozen,cts),is_solved) <- extractUnsolved inert
-- Then: 
-- -----------------------------------------------------------------------------
--  cts       |  The unsolved (Derived or Wanted only) residual 
--            |  canonical constraints, that is, no CNonCanonicals.
-- -----------|-----------------------------------------------------------------
--  frozen    | The CNonCanonicals of the original inert (frozen errors), 
--            | of all flavors
-- -----------|-----------------------------------------------------------------
--  is_solved | Whatever remains from the inert after removing the previous two. 
-- -----------------------------------------------------------------------------
extractUnsolved is@(IS {inert_eqs = eqs, inert_irreds = irreds}) 
  = let is_solved  = is { inert_eqs    = solved_eqs
                        , inert_eq_tvs = inert_eq_tvs is
                        , inert_dicts  = solved_dicts
                        , inert_ips    = solved_ips
                        , inert_irreds = solved_irreds
                        , inert_frozen = emptyCts
                        , inert_funeqs = solved_funeqs
                        }
    in ((inert_frozen is, unsolved), is_solved)

  where solved_eqs = filterVarEnv_Directly (\_ (ct,_) -> isGivenOrSolvedCt ct) eqs
        unsolved_eqs = foldVarEnv (\(ct,_co) cts -> cts `extendCts` ct) emptyCts $
                       eqs `minusVarEnv` solved_eqs

        (unsolved_irreds, solved_irreds) = Bag.partitionBag (not.isGivenOrSolvedCt) irreds
        (unsolved_ips, solved_ips)       = extractUnsolvedCMap (inert_ips is) 
        (unsolved_dicts, solved_dicts)   = extractUnsolvedCMap (inert_dicts is) 

        (unsolved_funeqs, solved_funeqs) = extractUnsolvedCtTypeMap (inert_funeqs is)

        unsolved = unsolved_eqs `unionBags` unsolved_irreds `unionBags`
                   unsolved_ips `unionBags` unsolved_dicts `unionBags` unsolved_funeqs

extractUnsolvedCtTypeMap :: TypeMap Ct -> (Cts,TypeMap Ct)
extractUnsolvedCtTypeMap
  = partitionCtTypeMap (not . isGivenOrSolved . cc_flavor)


extractRelevantInerts :: Ct -> TcS Cts
-- Returns the constraints from the inert set that are 'relevant' to react with 
-- this constraint. The monad is left with the 'thinner' inerts. 
-- NB: This function contains logic specific to the constraint solver, maybe move there?
extractRelevantInerts wi 
  = modifyInertTcS (extract_inert_relevants wi)
  where extract_inert_relevants (CDictCan {cc_class = cl}) is = 
            let (cts,dict_map) = getRelevantCts cl (inert_dicts is) 
            in (cts, is { inert_dicts = dict_map })
        extract_inert_relevants (CFunEqCan {cc_fun = tc, cc_tyargs = xis}) is = 
            let (cts,feqs_map)  = getCtTypeMapRelevants (mkTyConApp tc xis) (inert_funeqs is)
            in (cts, is { inert_funeqs = feqs_map })
        extract_inert_relevants (CIPCan { cc_ip_nm = nm } ) is = 
            let (cts, ips_map) = getRelevantCts nm (inert_ips is) 
            in (cts, is { inert_ips = ips_map })
        extract_inert_relevants (CIrredEvCan { }) is = 
            let cts = inert_irreds is 
            in (cts, is { inert_irreds = emptyCts })
        extract_inert_relevants _ is = (emptyCts,is)
601
602
\end{code}

603
604


605

606
607
608
609
610
611
612
613
%************************************************************************
%*									*
                    CtFlavor
         The "flavor" of a canonical constraint
%*									*
%************************************************************************

\begin{code}
614
getWantedLoc :: Ct -> WantedLoc
615
616
617
618
619
620
getWantedLoc ct 
  = ASSERT (isWanted (cc_flavor ct))
    case cc_flavor ct of 
      Wanted wl -> wl 
      _         -> pprPanic "Can't get WantedLoc of non-wanted constraint!" empty

621
isWantedCt :: Ct -> Bool
622
isWantedCt ct = isWanted (cc_flavor ct)
623
isDerivedCt :: Ct -> Bool
624
isDerivedCt ct = isDerived (cc_flavor ct)
625

626
isGivenCt_maybe :: Ct -> Maybe GivenKind
dimitris's avatar
dimitris committed
627
628
isGivenCt_maybe ct = isGiven_maybe (cc_flavor ct)

629
isGivenOrSolvedCt :: Ct -> Bool
dimitris's avatar
dimitris committed
630
631
632
isGivenOrSolvedCt ct = isGivenOrSolved (cc_flavor ct)


633
634
635
canSolve :: CtFlavor -> CtFlavor -> Bool 
-- canSolve ctid1 ctid2 
-- The constraint ctid1 can be used to solve ctid2 
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
636
637
638
639
640
-- "to solve" means a reaction where the active parts of the two constraints match.
--  active(F xis ~ xi) = F xis 
--  active(tv ~ xi)    = tv 
--  active(D xis)      = D xis 
--  active(IP nm ty)   = nm 
641
642
--
-- NB:  either (a `canSolve` b) or (b `canSolve` a) must hold
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
643
-----------------------------------------
644
canSolve (Given {})   _            = True 
645
canSolve (Wanted {})  (Derived {}) = True
646
canSolve (Wanted {})  (Wanted {})  = True
647
648
canSolve (Derived {}) (Derived {}) = True  -- Important: derived can't solve wanted/given
canSolve _ _ = False  	       	     	   -- (There is no *evidence* for a derived.)
649

650
651
canRewrite :: CtFlavor -> CtFlavor -> Bool 
-- canRewrite ctid1 ctid2 
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
652
-- The *equality_constraint* ctid1 can be used to rewrite inside ctid2 
653
canRewrite = canSolve 
dimitris@microsoft.com's avatar
dimitris@microsoft.com committed
654

655
656
combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc
-- Precondition: At least one of them should be wanted 
dimitris's avatar
dimitris committed
657
658
659
660
combineCtLoc (Wanted loc) _    = loc
combineCtLoc _ (Wanted loc)    = loc
combineCtLoc (Derived loc ) _  = loc
combineCtLoc _ (Derived loc )  = loc
661
combineCtLoc _ _ = panic "combineCtLoc: both given"
662

dimitris's avatar
dimitris committed
663
664
665
666
667
mkSolvedFlavor :: CtFlavor -> SkolemInfo -> CtFlavor
-- To be called when we actually solve a wanted/derived (perhaps leaving residual goals)
mkSolvedFlavor (Wanted  loc) sk  = Given (setCtLocOrigin loc sk) GivenSolved
mkSolvedFlavor (Derived loc) sk  = Given (setCtLocOrigin loc sk) GivenSolved
mkSolvedFlavor fl@(Given {}) _sk = pprPanic "Solving a given constraint!" $ ppr fl
668

dimitris's avatar
dimitris committed
669
670
671
672
mkGivenFlavor :: CtFlavor -> SkolemInfo -> CtFlavor
mkGivenFlavor (Wanted  loc) sk  = Given (setCtLocOrigin loc sk) GivenOrig
mkGivenFlavor (Derived loc) sk  = Given (setCtLocOrigin loc sk) GivenOrig
mkGivenFlavor fl@(Given {}) _sk = pprPanic "Solving a given constraint!" $ ppr fl
673
674

mkWantedFlavor :: CtFlavor -> CtFlavor
675
676
mkWantedFlavor (Wanted  loc) = Wanted loc
mkWantedFlavor (Derived loc) = Wanted loc
dimitris's avatar
dimitris committed
677
mkWantedFlavor fl@(Given {}) = pprPanic "mkWantedFlavor" (ppr fl)
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
\end{code}

%************************************************************************
%*									*
%*		The TcS solver monad                                    *
%*									*
%************************************************************************

Note [The TcS monad]
~~~~~~~~~~~~~~~~~~~~
The TcS monad is a weak form of the main Tc monad

All you can do is
    * fail
    * allocate new variables
    * fill in evidence variables

Filling in a dictionary evidence variable means to create a binding
for it, so TcS carries a mutable location where the binding can be
added.  This is initialised from the innermost implication constraint.

\begin{code}
data TcSEnv
  = TcSEnv { 
702
703
704
705
      tcs_ev_binds    :: EvBindsVar,
      tcs_evvar_cache :: IORef EvVarCache,
          -- Evidence bindings and a cache from predicate types to the created evidence 
          -- variables. The scope of the cache will be the same as the scope of tcs_ev_binds
706

707
      tcs_ty_binds :: IORef (TyVarEnv (TcTyVar, TcType)),
708
709
          -- Global type bindings

710
      tcs_context :: SimplContext,
711
                     
712
713
      tcs_untch :: TcsUntouchables,

dimitris's avatar
dimitris committed
714
715
716
      tcs_ic_depth   :: Int,       -- Implication nesting depth
      tcs_count      :: IORef Int, -- Global step count

717
718
      tcs_inerts   :: IORef InertSet, -- Current inert set
      tcs_worklist :: IORef WorkList  -- Current worklist
dimitris's avatar
dimitris committed
719
720


721
722
723
    -- TcSEnv invariant: the tcs_evvar_cache is a superset of tcs_inerts, tcs_worklist, tcs_ev_binds which must 
    --                   all be disjoint with each other.
    }
dimitris's avatar
dimitris committed
724

725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
data EvVarCache
  = EvVarCache { evc_cache     :: TypeMap (EvVar,CtFlavor)    
                     -- Map from PredTys to Evidence variables
                     -- used to avoid creating new goals
               , evc_flat_cache :: TypeMap (Coercion,(Xi,CtFlavor,FlatEqOrigin))
                     -- Map from family-free heads (F xi) to family-free types.
                     -- Useful during flattening to share flatten skolem generation
                     -- The boolean flag:
                     --   True  <-> This equation was generated originally during flattening
                     --   False <-> This equation was generated by having solved a goal
               }

data FlatEqOrigin = WhileFlattening  -- Was it generated during flattening?
                  | WhenSolved       -- Was it generated when a family equation was solved?
                  | Any

origin_matches :: FlatEqOrigin -> FlatEqOrigin -> Bool
origin_matches Any _                           = True
origin_matches WhenSolved WhenSolved           = True
origin_matches WhileFlattening WhileFlattening = True
origin_matches _ _ = False
dimitris's avatar
dimitris committed
746
747


748
749
750
751
type TcsUntouchables = (Untouchables,TcTyVarSet)
-- Like the TcM Untouchables, 
-- but records extra TcsTv variables generated during simplification
-- See Note [Extra TcsTv untouchables] in TcSimplify
752
753
754
\end{code}

\begin{code}
755
data SimplContext
756
757
758
759
  = SimplInfer SDoc	   -- Inferring type of a let-bound thing
  | SimplRuleLhs RuleName  -- Inferring type of a RULE lhs
  | SimplInteractive	   -- Inferring type at GHCi prompt
  | SimplCheck SDoc	   -- Checking a type signature or RULE rhs
760
761

instance Outputable SimplContext where
762
763
764
  ppr (SimplInfer d)   = ptext (sLit "SimplInfer") <+> d
  ppr (SimplCheck d)   = ptext (sLit "SimplCheck") <+> d
  ppr (SimplRuleLhs n) = ptext (sLit "SimplRuleLhs") <+> doubleQuotes (ftext n)
765
766
767
768
769
770
771
772
773
774
  ppr SimplInteractive = ptext (sLit "SimplInteractive")

isInteractive :: SimplContext -> Bool
isInteractive SimplInteractive = True
isInteractive _                = False

simplEqsOnly :: SimplContext -> Bool
-- Simplify equalities only, not dictionaries
-- This is used for the LHS of rules; ee
-- Note [Simplifying RULE lhs constraints] in TcSimplify
775
776
simplEqsOnly (SimplRuleLhs {}) = True
simplEqsOnly _                 = False
777
778

performDefaulting :: SimplContext -> Bool
779
780
781
782
performDefaulting (SimplInfer {})   = False
performDefaulting (SimplRuleLhs {}) = False
performDefaulting SimplInteractive  = True
performDefaulting (SimplCheck {})   = True
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817

---------------
newtype TcS a = TcS { unTcS :: TcSEnv -> TcM a } 

instance Functor TcS where
  fmap f m = TcS $ fmap f . unTcS m

instance Monad TcS where 
  return x  = TcS (\_ -> return x) 
  fail err  = TcS (\_ -> fail err) 
  m >>= k   = TcS (\ebs -> unTcS m ebs >>= \r -> unTcS (k r) ebs)

-- Basic functionality 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
wrapTcS :: TcM a -> TcS a 
-- Do not export wrapTcS, because it promotes an arbitrary TcM to TcS,
-- and TcS is supposed to have limited functionality
wrapTcS = TcS . const -- a TcM action will not use the TcEvBinds

wrapErrTcS :: TcM a -> TcS a 
-- The thing wrapped should just fail
-- There's no static check; it's up to the user
-- Having a variant for each error message is too painful
wrapErrTcS = wrapTcS

wrapWarnTcS :: TcM a -> TcS a 
-- The thing wrapped should just add a warning, or no-op
-- There's no static check; it's up to the user
wrapWarnTcS = wrapTcS

failTcS, panicTcS :: SDoc -> TcS a
failTcS      = wrapTcS . TcM.failWith
panicTcS doc = pprPanic "TcCanonical" doc

traceTcS :: String -> SDoc -> TcS ()
818
traceTcS herald doc = wrapTcS (TcM.traceTc herald doc)
819

820
821
822
823
824
bumpStepCountTcS :: TcS ()
bumpStepCountTcS = TcS $ \env -> do { let ref = tcs_count env
                                    ; n <- TcM.readTcRef ref
                                    ; TcM.writeTcRef ref (n+1) }

825
traceFireTcS :: SubGoalDepth -> SDoc -> TcS ()
826
827
828
829
830
831
832
833
834
-- Dump a rule-firing trace
traceFireTcS depth doc 
  = TcS $ \env -> 
    TcM.ifDOptM Opt_D_dump_cs_trace $ 
    do { n <- TcM.readTcRef (tcs_count env)
       ; let msg = int n 
                <> text (replicate (tcs_ic_depth env) '>')
                <> brackets (int depth) <+> doc
       ; TcM.dumpTcRn msg }
835
836

runTcS :: SimplContext
837
       -> Untouchables 	       -- Untouchables
838
839
       -> InertSet             -- Initial inert set
       -> WorkList             -- Initial work list
840
       -> TcS a		       -- What to run
841
       -> TcM (a, Bag EvBind)
842
runTcS context untouch is wl tcs 
843
  = do { ty_binds_var <- TcM.newTcRef emptyVarEnv
844
845
       ; ev_cache_var <- TcM.newTcRef $ 
                         EvVarCache { evc_cache = emptyTM, evc_flat_cache = emptyTM }
846
       ; ev_binds_var@(EvBindsVar evb_ref _) <- TcM.newTcEvBinds
847
       ; step_count <- TcM.newTcRef 0
848
849
850
851

       ; inert_var <- TcM.newTcRef is 
       ; wl_var <- TcM.newTcRef wl

852
       ; let env = TcSEnv { tcs_ev_binds = ev_binds_var
853
                          , tcs_evvar_cache = ev_cache_var
854
                          , tcs_ty_binds = ty_binds_var
855
                          , tcs_context  = context
856
                          , tcs_untch    = (untouch, emptyVarSet) -- No Tcs untouchables yet
857
858
			  , tcs_count    = step_count
			  , tcs_ic_depth = 0
859
860
                          , tcs_inerts   = inert_var
                          , tcs_worklist = wl_var }
861
862

	     -- Run the computation
863
       ; res <- unTcS tcs env
864
865
	     -- Perform the type unifications required
       ; ty_binds <- TcM.readTcRef ty_binds_var
866
       ; mapM_ do_unification (varEnvElts ty_binds)
867

Ian Lynagh's avatar
Ian Lynagh committed
868
869
870
871
872
873
       ; when debugIsOn $ do {
             count <- TcM.readTcRef step_count
           ; when (opt_PprStyle_Debug && count > 0) $
             TcM.debugDumpTcRn (ptext (sLit "Constraint solver steps =") 
                                <+> int count <+> ppr context)
         }
874
             -- And return
875
       ; ev_binds      <- TcM.readTcRef evb_ref
876
       ; return (res, evBindMapBinds ev_binds) }
877
878
  where
    do_unification (tv,ty) = TcM.writeMetaTyVar tv ty
879

880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899

doWithInert :: InertSet -> TcS a -> TcS a 
doWithInert inert (TcS action)
  = TcS $ \env -> do { new_inert_var <- TcM.newTcRef inert
                     ; orig_cache_var <- TcM.readTcRef (tcs_evvar_cache env)
                     ; new_cache_var <- TcM.newTcRef orig_cache_var
                     ; action (env { tcs_inerts = new_inert_var 
                                   , tcs_evvar_cache = new_cache_var }) }


nestImplicTcS :: EvBindsVar -> TcsUntouchables -> TcS a -> TcS a 
nestImplicTcS ref (inner_range, inner_tcs) (TcS thing_inside) 
  = TcS $ \ TcSEnv { tcs_ty_binds = ty_binds
                   , tcs_evvar_cache = orig_evvar_cache_var
                   , tcs_untch = (_outer_range, outer_tcs)
                   , tcs_count = count
                   , tcs_ic_depth = idepth
                   , tcs_context = ctxt
                   , tcs_inerts = inert_var
                   , tcs_worklist = wl_var } -> 
dimitris's avatar
dimitris committed
900
    do { let inner_untch = (inner_range, outer_tcs `unionVarSet` inner_tcs)
901
902
903
904
       		   -- The inner_range should be narrower than the outer one
		   -- (thus increasing the set of untouchables) but 
		   -- the inner Tcs-untouchables must be unioned with the
		   -- outer ones!
dimitris's avatar
dimitris committed
905

906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
         -- Inherit the inerts from the outer scope
       ; orig_inerts <- TcM.readTcRef inert_var
       ; new_inert_var <- TcM.newTcRef orig_inerts
                          
         -- Inherit EvVar cache
       ; orig_evvar_cache <- TcM.readTcRef orig_evvar_cache_var
       ; evvar_cache <- TcM.newTcRef orig_evvar_cache
 
       ; let nest_env = TcSEnv { tcs_ev_binds    = ref
                               , tcs_evvar_cache = evvar_cache
                               , tcs_ty_binds    = ty_binds
                               , tcs_untch       = inner_untch
                               , tcs_count       = count
                               , tcs_ic_depth    = idepth+1
                               , tcs_context     = ctxtUnderImplic ctxt 
                               , tcs_inerts      = new_inert_var
                               , tcs_worklist    = wl_var 
                               -- NB: worklist is going to be empty anyway, 
                               -- so reuse the same ref cell
                               }
       ; thing_inside nest_env } 
927

928
929
930
931
932
recoverTcS :: TcS a -> TcS a -> TcS a
recoverTcS (TcS recovery_code) (TcS thing_inside)
  = TcS $ \ env ->
    TcM.recoverM (recovery_code env) (thing_inside env)

933
934
ctxtUnderImplic :: SimplContext -> SimplContext
-- See Note [Simplifying RULE lhs constraints] in TcSimplify
935
936
937
ctxtUnderImplic (SimplRuleLhs n) = SimplCheck (ptext (sLit "lhs of rule") 
                                               <+> doubleQuotes (ftext n))
ctxtUnderImplic ctxt              = ctxt
938

939
tryTcS :: TcS a -> TcS a
940
941
942
-- Like runTcS, but from within the TcS monad 
-- Completely afresh inerts and worklist, be careful! 
-- Moreover, we will simply throw away all the evidence generated. 
dimitris's avatar
dimitris committed
943
tryTcS tcs
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
  = TcS (\env -> 
             do { wl_var <- TcM.newTcRef emptyWorkList
                ; is_var <- TcM.newTcRef emptyInert

                ; ty_binds_var <- TcM.newTcRef emptyVarEnv
                ; ev_binds_var <- TcM.newTcEvBinds

                ; ev_binds_cache_var <- TcM.newTcRef (EvVarCache emptyTM emptyTM)
                    -- Empty cache: Don't inherit cache from above, see 
                    -- Note [tryTcS for defaulting] in TcSimplify

                ; let env1 = env { tcs_ev_binds = ev_binds_var
                                 , tcs_evvar_cache = ev_binds_cache_var
                                 , tcs_ty_binds = ty_binds_var
                                 , tcs_inerts   = is_var
                                 , tcs_worklist = wl_var } 
                ; unTcS tcs env1 })

-- Getters and setters of TcEnv fields
963
964
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
-- Getter of inerts and worklist
getTcSInertsRef :: TcS (IORef InertSet)
getTcSInertsRef = TcS (return . tcs_inerts)

getTcSWorkListRef :: TcS (IORef WorkList) 
getTcSWorkListRef = TcS (return . tcs_worklist) 

getTcSInerts :: TcS InertSet 
getTcSInerts = getTcSInertsRef >>= wrapTcS . (TcM.readTcRef) 

getTcSWorkList :: TcS WorkList
getTcSWorkList = getTcSWorkListRef >>= wrapTcS . (TcM.readTcRef) 

updWorkListTcS :: (WorkList -> WorkList) -> TcS () 
updWorkListTcS f 
  = updWorkListTcS_return (\w -> ((),f w))

updWorkListTcS_return :: (WorkList -> (a,WorkList)) -> TcS a
updWorkListTcS_return f
  = do { wl_var <- getTcSWorkListRef
       ; wl_curr <- wrapTcS (TcM.readTcRef wl_var)
       ; let (res,new_work) = f wl_curr
       ; wrapTcS (TcM.writeTcRef wl_var new_work)
       ; return res }

emitFrozenError :: CtFlavor -> EvVar -> SubGoalDepth -> TcS ()
-- Emits a non-canonical constraint that will stand for a frozen error in the inerts. 
emitFrozenError fl ev depth 
  = do { traceTcS "Emit frozen error" (ppr ev <+> dcolon <+> ppr (evVarPred ev))
       ; inert_ref <- getTcSInertsRef 
       ; inerts <- wrapTcS (TcM.readTcRef inert_ref)
       ; let ct = CNonCanonical { cc_id = ev
                                , cc_flavor = fl
                                , cc_depth = depth } 
             inerts_new = inerts { inert_frozen = extendCts (inert_frozen inerts) ct } 
       ; wrapTcS (TcM.writeTcRef inert_ref inerts_new) }

1002
1003
1004
1005
1006
1007
1008
1009
1010
getDynFlags :: TcS DynFlags
getDynFlags = wrapTcS TcM.getDOpts

getTcSContext :: TcS SimplContext
getTcSContext = TcS (return . tcs_context)

getTcEvBinds :: TcS EvBindsVar
getTcEvBinds = TcS (return . tcs_ev_binds) 

1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
getTcSEvVarCache :: TcS (IORef EvVarCache)
getTcSEvVarCache = TcS (return . tcs_evvar_cache)

flushFlatCache :: TcS ()
flushFlatCache
  = do { cache_var <- getTcSEvVarCache
       ; the_cache <- wrapTcS $ TcM.readTcRef cache_var
       ; wrapTcS $ TcM.writeTcRef cache_var (the_cache { evc_flat_cache = emptyTM }) }


getTcSEvVarCacheMap :: TcS (TypeMap (EvVar,CtFlavor))
getTcSEvVarCacheMap = do { cache_var <- getTcSEvVarCache 
                         ; the_cache <- wrapTcS $ TcM.readTcRef cache_var 
                         ; return (evc_cache the_cache) }

getTcSEvVarFlatCache :: TcS (TypeMap (Coercion,(Type,CtFlavor,FlatEqOrigin)))
getTcSEvVarFlatCache = do { cache_var <- getTcSEvVarCache 
                          ; the_cache <- wrapTcS $ TcM.readTcRef cache_var 
                          ; return (evc_flat_cache the_cache) }

setTcSEvVarCacheMap :: TypeMap (EvVar,CtFlavor) -> TcS () 
setTcSEvVarCacheMap cache = do { cache_var <- getTcSEvVarCache 
                               ; orig_cache <- wrapTcS $ TcM.readTcRef cache_var
                               ; let new_cache = orig_cache { evc_cache = cache } 
                               ; wrapTcS $ TcM.writeTcRef cache_var new_cache }

1037
getUntouchables :: TcS TcsUntouchables
1038
1039
getUntouchables = TcS (return . tcs_untch)

1040
getTcSTyBinds :: TcS (IORef (TyVarEnv (TcTyVar, TcType)))
1041
1042
getTcSTyBinds = TcS (return . tcs_ty_binds)

1043
getTcSTyBindsMap :: TcS (TyVarEnv (TcTyVar, TcType))
1044
getTcSTyBindsMap = getTcSTyBinds >>= wrapTcS . (TcM.readTcRef) 
1045

1046
1047
1048

getTcEvBindsMap :: TcS EvBindMap
getTcEvBindsMap
1049
1050
1051
  = do { EvBindsVar ev_ref _ <- getTcEvBinds 
       ; wrapTcS $ TcM.readTcRef ev_ref }

1052

batterseapower's avatar
batterseapower committed
1053
1054
setEqBind :: EqVar -> LCoercion -> TcS () 
setEqBind eqv co = setEvBind eqv (EvCoercionBox co)
1055
1056
1057

setWantedTyBind :: TcTyVar -> TcType -> TcS () 
-- Add a type binding
1058
-- We never do this twice!
1059
1060
1061
1062
setWantedTyBind tv ty 
  = do { ref <- getTcSTyBinds
       ; wrapTcS $ 
         do { ty_binds <- TcM.readTcRef ref
Ian Lynagh's avatar
Ian Lynagh committed
1063
1064
1065
1066
1067
            ; when debugIsOn $
                  TcM.checkErr (not (tv `elemVarEnv` ty_binds)) $
                  vcat [ text "TERRIBLE ERROR: double set of meta type variable"
                       , ppr tv <+> text ":=" <+> ppr ty
                       , text "Old value =" <+> ppr (lookupVarEnv_NF ty_binds tv)]
1068
            ; TcM.writeTcRef ref (extendVarEnv ty_binds tv (tv,ty)) } }
1069
1070
1071
1072
1073
1074
1075
1076
1077

setIPBind :: EvVar -> EvTerm -> TcS () 
setIPBind = setEvBind 

setDictBind :: EvVar -> EvTerm -> TcS () 
setDictBind = setEvBind 

setEvBind :: EvVar -> EvTerm -> TcS () 
-- Internal
batterseapower's avatar
batterseapower committed
1078
setEvBind ev t
1079
  = do { tc_evbinds <- getTcEvBinds
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
       ; wrapTcS $ TcM.addTcEvBind tc_evbinds ev t

#ifdef DEBUG
       ; binds <- getTcEvBindsMap
       ; let cycle = any (reaches binds) (evterm_evs t)
       ; when cycle (fail_if_co_loop binds)
#endif
       }

#ifdef DEBUG
  where fail_if_co_loop binds
          = pprTrace "setEvBind" (vcat [ text "Cycle in evidence binds, evvar =" <+> ppr ev
                                       , ppr (evBindMapBinds binds) ]) $
            when (isLCoVar ev) (pprPanic "setEvBind" (text "BUG: Coercion loop!"))

        reaches :: EvBindMap -> Var -> Bool 
        -- Does this evvar reach ev? 
        reaches ebm ev0 = go ev0
          where go ev0
                  | ev0 == ev = True
                  | Just (EvBind _ evtrm) <- lookupEvBind ebm ev0
                  = any go (evterm_evs evtrm)
                  | otherwise = False

        evterm_evs (EvId v) = [v]
        evterm_evs (EvCoercionBox lco) = varSetElems $ coVarsOfCo lco
        evterm_evs (EvDFunApp _ _ evs) = evs
        evterm_evs (EvTupleSel v _)    = [v]
        evterm_evs (EvSuperClass v _)  = [v]
        evterm_evs (EvCast v co)       = v : varSetElems (coVarsOfCo co)
        evterm_evs (EvTupleMk evs)     = evs
#endif


1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153

warnTcS :: CtLoc orig -> Bool -> SDoc -> TcS ()
warnTcS loc warn_if doc 
  | warn_if   = wrapTcS $ TcM.setCtLoc loc $ TcM.addWarnTc doc
  | otherwise = return ()

getDefaultInfo ::  TcS (SimplContext, [Type], (Bool, Bool))
getDefaultInfo 
  = do { ctxt <- getTcSContext
       ; (tys, flags) <- wrapTcS (TcM.tcGetDefaultTys (isInteractive ctxt))
       ; return (ctxt, tys, flags) }

-- Just get some environments needed for instance looking up and matching
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

getInstEnvs :: TcS (InstEnv, InstEnv) 
getInstEnvs = wrapTcS $ Inst.tcGetInstEnvs 

getFamInstEnvs :: TcS (FamInstEnv, FamInstEnv) 
getFamInstEnvs = wrapTcS $ FamInst.tcGetFamInstEnvs

getTopEnv :: TcS HscEnv 
getTopEnv = wrapTcS $ TcM.getTopEnv 

getGblEnv :: TcS TcGblEnv 
getGblEnv = wrapTcS $ TcM.getGblEnv 

-- Various smaller utilities [TODO, maybe will be absorbed in the instance matcher]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

checkWellStagedDFun :: PredType -> DFunId -> WantedLoc -> TcS () 
checkWellStagedDFun pred dfun_id loc 
  = wrapTcS $ TcM.setCtLoc loc $ 
    do { use_stage <- TcM.getStage
       ; TcM.checkWellStaged pp_thing bind_lvl (thLevel use_stage) }
  where
    pp_thing = ptext (sLit "instance for") <+> quotes (ppr pred)
    bind_lvl = TcM.topIdLvl dfun_id

pprEq :: TcType -> TcType -> SDoc
batterseapower's avatar
batterseapower committed
1154
pprEq ty1 ty2 = pprType $ mkEqPred (ty1,ty2)
1155
1156

isTouchableMetaTyVar :: TcTyVar -> TcS Bool
1157
isTouchableMetaTyVar tv 
1158
1159
1160
  = do { untch <- getUntouchables
       ; return $ isTouchableMetaTyVar_InRange untch tv } 

1161
1162
isTouchableMetaTyVar_InRange :: TcsUntouchables -> TcTyVar -> Bool 
isTouchableMetaTyVar_InRange (untch,untch_tcs) tv 
1163
  = case tcTyVarDetails tv of 
1164
1165
      MetaTv TcsTv _ -> not (tv `elemVarSet` untch_tcs)
                        -- See Note [Touchable meta type variables] 
1166
1167
1168
1169
      MetaTv {}      -> inTouchableRange untch tv 
      _              -> False 


1170
1171
\end{code}

1172

1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
Note [Touchable meta type variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Meta type variables allocated *by the constraint solver itself* are always
touchable.  Example: 
   instance C a b => D [a] where...
if we use this instance declaration we "make up" a fresh meta type
variable for 'b', which we must later guess.  (Perhaps C has a
functional dependency.)  But since we aren't in the constraint *generator*
we can't allocate a Unique in the touchable range for this implication
constraint.  Instead, we mark it as a "TcsTv", which makes it always-touchable.
1183
1184


1185
\begin{code}
1186
1187
1188
1189
1190
-- Flatten skolems
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

newFlattenSkolemTy :: TcType -> TcS TcType
newFlattenSkolemTy ty = mkTyVarTy <$> newFlattenSkolemTyVar ty
1191
1192
1193

newFlattenSkolemTyVar :: TcType -> TcS TcTyVar
newFlattenSkolemTyVar ty
1194
  = do { tv <- wrapTcS $ do { uniq <- TcM.newUnique
1195
                            ; let name = TcM.mkTcTyVarName uniq (fsLit "f")
1196
1197
1198
1199
                            ; return $ mkTcTyVar name (typeKind ty) (FlatSkol ty) } 
       ; traceTcS "New Flatten Skolem Born" $ 
           (ppr tv <+> text "[:= " <+> ppr ty <+> text "]")
       ; return tv }
1200
1201
1202
1203
1204

-- Instantiations 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

instDFunTypes :: [Either TyVar TcType] -> TcS [TcType] 
1205
1206
1207
1208
instDFunTypes mb_inst_tys 
  = mapM inst_tv mb_inst_tys
  where
    inst_tv :: Either TyVar TcType -> TcS Type
1209
    inst_tv (Left tv)  = mkTyVarTy <$> instFlexiTcS tv
1210
    inst_tv (Right ty) = return ty 
1211

1212
1213
1214
instDFunConstraints :: TcThetaType -> CtFlavor -> TcS [EvVarCreated] 
instDFunConstraints preds fl
  = mapM (newEvVar fl) preds
1215

1216
instFlexiTcS :: TyVar -> TcS TcTyVar 
1217
1218
1219
-- Like TcM.instMetaTyVar but the variable that is created is always
-- touchable; we are supposed to guess its instantiation. 
-- See Note [Touchable meta type variables] 
1220
instFlexiTcS tv = instFlexiTcSHelper (tyVarName tv) (tyVarKind tv) 
1221

1222
1223
1224
1225
1226
newFlexiTcSTy :: Kind -> TcS TcType  
newFlexiTcSTy knd 
  = wrapTcS $
    do { uniq <- TcM.newUnique 
       ; ref  <- TcM.newMutVar  Flexi 
1227
       ; let name = TcM.mkTcTyVarName uniq (fsLit "uf")
1228
1229
       ; return $ mkTyVarTy (mkTcTyVar name knd (MetaTv TcsTv ref)) }

1230
1231
1232
1233
1234
1235
isFlexiTcsTv :: TyVar -> Bool
isFlexiTcsTv tv
  | not (isTcTyVar tv)                  = False
  | MetaTv TcsTv _ <- tcTyVarDetails tv = True
  | otherwise                           = False

1236
newKindConstraint :: TcTyVar -> Kind -> CtFlavor -> TcS EvVarCreated
1237
-- Create new wanted CoVar that constrains the type to have the specified kind. 
1238
newKindConstraint tv knd fl
1239
  = do { tv_k <- instFlexiTcSHelper (tyVarName tv) knd 
1240
       ; let ty_k = mkTyVarTy tv_k
1241
       ; eqv <- newEqVar fl (mkTyVarTy tv) ty_k
batterseapower's avatar
batterseapower committed
1242
       ; return eqv }
1243

1244
1245
instFlexiTcSHelper :: Name -> Kind -> TcS TcTyVar
instFlexiTcSHelper tvname tvkind
1246
1247
1248
1249
1250
1251
  = wrapTcS $ 
    do { uniq <- TcM.newUnique 
       ; ref  <- TcM.newMutVar  Flexi 
       ; let name = setNameUnique tvname uniq 
             kind = tvkind 
       ; return (mkTcTyVar name kind (MetaTv TcsTv ref)) }
1252
1253
1254
1255

-- Superclasses and recursive dictionaries 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
data EvVarCreated 
  = EvVarCreated { evc_is_new    :: Bool    -- True iff the variable was just created
                 , evc_the_evvar :: EvVar } -- The actual evidence variable could be cached or new

isNewEvVar :: EvVarCreated -> Bool
isNewEvVar = evc_is_new

newEvVar :: CtFlavor -> TcPredType -> TcS EvVarCreated
-- Post: If Given then evc_is_new is True
-- Hence it is safe to do a setEvBind right after a newEvVar with a Given flavor
-- NB: newEvVar may temporarily break the TcSEnv invariant but it is expected in 
--     the call sites for this invariant to be quickly restored.
newEvVar fl pty
  | isGivenOrSolved fl    -- Create new variable and update the cache
  = do { new <- forceNewEvVar fl pty
       ; return (EvVarCreated True new) }

  | otherwise             -- Otherwise lookup first
  = do { eref <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; case lookupTM pty (evc_cache ecache) of
           Just (cached_evvar, cached_flavor)
             | cached_flavor `canSolve` fl -- NB: 
                                           -- We want to use the cache /only/ if he can solve
                                           -- the workitem. If cached_flavor is Derived
                                           -- but we have a real Wanted, we want to create
                                           -- new evidence, otherwise we are in danger to
                                           -- have unsolved goals in the end. 
                                           -- (Remember: Derived's are just unification hints
                                           --            but they don't come with guarantees
                                           --            that they can be solved and we don't 
                                           --            quantify over them.
             -> do { traceTcS "newEvVar"  $  text "already cached, doing nothing"
                   ; return (EvVarCreated False cached_evvar) }
           _   -- Not cached or cached with worse flavor
             -> do { new <- force_new_ev_var eref ecache fl pty
                   ; return (EvVarCreated True new) } }

forceNewEvVar :: CtFlavor -> TcPredType -> TcS EvVar
-- Create a new EvVar, regardless of whether or not the
-- cache already contains one like it, and update the cache
forceNewEvVar fl pty 
  = do { eref   <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; force_new_ev_var eref ecache fl pty }

force_new_ev_var :: IORef EvVarCache -> EvVarCache -> CtFlavor -> TcPredType -> TcS EvVar
-- Create a new EvVar, and update the cache with it
force_new_ev_var eref ecache fl pty
  = wrapTcS $
    do { TcM.traceTc "newEvVar" $ text "updating cache"

       ; new_evvar <-TcM.newEvVar pty
            -- This is THE PLACE where we finally call TcM.newEvVar

       ; let new_cache = updateCache ecache (new_evvar,fl,pty)
       ; TcM.writeTcRef eref new_cache 
       ; return new_evvar }

updateCache :: EvVarCache -> (EvVar,CtFlavor,Type) -> EvVarCache
updateCache ecache (ev,fl,pty)
  | IPPred {} <- classifier
  = ecache
  | otherwise
  = ecache { evc_cache = ecache' }
  where classifier = classifyPredType pty
        ecache'    = alterTM pty (\_ -> Just (ev,fl)) $
                     evc_cache ecache

delCachedEvVar :: EvVar -> TcS ()
delCachedEvVar ev
  = do { eref   <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; wrapTcS $ TcM.writeTcRef eref (delFromCache ecache ev) }

delFromCache :: EvVarCache -> EvVar -> EvVarCache 
delFromCache (EvVarCache { evc_cache      = ecache
                         , evc_flat_cache = flat_cache }) ev
  = EvVarCache { evc_cache = ecache', evc_flat_cache = flat_cache }
  where ecache' = alterTM pty x_del ecache
        x_del Nothing = Nothing
        x_del r@(Just (ev0,_))
           | ev0 == ev = Nothing
           | otherwise = r
        pty = evVarPred ev



updateFlatCache :: EvVar -> CtFlavor 
                -> TyCon -> [Xi] -> TcType 
                -> FlatEqOrigin
                -> TcS () 
updateFlatCache ev fl fn xis rhs_ty feq_origin
  = do { eref <- getTcSEvVarCache
       ; ecache <- wrapTcS (TcM.readTcRef eref)
       ; let flat_cache     = evc_flat_cache ecache
             new_flat_cache = alterTM fun_ty x_flat_cache flat_cache
             new_evc = ecache { evc_flat_cache = new_flat_cache }
       ; wrapTcS $ TcM.writeTcRef eref new_evc }
  where x_flat_cache _ = Just (mkEqVarLCo ev,(rhs_ty,fl,feq_origin))
        fun_ty = mkTyConApp fn xis


pprEvVarCache :: TypeMap (Coercion,a) -> SDoc
pprEvVarCache tm = ppr (foldTM mk_pair tm [])
 where mk_pair (co,_) cos = (co, liftedCoercionKind co) : cos


newGivenEqVar :: CtFlavor -> TcType -> TcType -> Coercion -> TcS EvVar
-- Pre: fl is Given
newGivenEqVar fl ty1 ty2 co 
  = do { ecv <- newEqVar fl ty1 ty2
       ; let v = evc_the_evvar ecv -- Will be a new EvVar by post of newEvVar
       ; setEvBind v (EvCoercionBox co)
       ; return v }

newEqVar :: CtFlavor -> TcType -> TcType -> TcS EvVarCreated
newEqVar fl ty1 ty2 
  = newEvVar fl (mkEqPred (ty1,ty2))
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394


\end{code} 


\begin{code} 
-- Matching and looking up classes and family instances
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

data MatchInstResult mi
  = MatchInstNo         -- No matching instance 
  | MatchInstSingle mi  -- Single matching instance
  | MatchInstMany       -- Multiple matching instances


matchClass :: Class -> [Type] -> TcS (MatchInstResult (DFunId, [Either TyVar TcType])) 
-- Look up a class constraint in the instance environment
matchClass clas tys
  = do	{ let pred = mkClassPred clas tys 
        ; instEnvs <- getInstEnvs
1395
        ; case lookupInstEnv instEnvs clas tys of {
1396
            ([], unifs, _)               -- Nothing matches  
1397
1398
1399
1400
1401
                -> do { traceTcS "matchClass not matching"
                                 (vcat [ text "dict" <+> ppr pred, 
                                         text "unifs" <+> ppr unifs ]) 
                      ; return MatchInstNo  
                      } ;  
1402
	    ([(ispec, inst_tys)], [], _) -- A single match 
1403
1404
1405
1406
		-> do	{ let dfun_id = is_dfun ispec
			; traceTcS "matchClass success"
				   (vcat [text "dict" <+> ppr pred, 
				          text "witness" <+> ppr dfun_id
1407
                                           <+> ppr (idType dfun_id) ])
1408
				  -- Record that this dfun is needed
1409
                        ; return $ MatchInstSingle (dfun_id, inst_tys)
1410
                        } ;
1411
     	    (matches, unifs, _)          -- More than one matches 
1412
1413
1414
1415
1416
1417
1418
1419
		-> do	{ traceTcS "matchClass multiple matches, deferring choice"
			           (vcat [text "dict" <+> ppr pred,
				   	  text "matches" <+> ppr matches,
				   	  text "unifs" <+> ppr unifs])
                        ; return MatchInstMany 
		        }
	}
        }
1420

1421
matchFam :: TyCon -> [Type] -> TcS (Maybe (TyCon, [Type]))
1422
matchFam tycon args = wrapTcS $ tcLookupFamInst tycon args
1423
\end{code}
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518


-- Rewriting with respect to the inert equalities 
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}

getInertEqs :: TcS (TyVarEnv (Ct,Coercion), InScopeSet)
getInertEqs = do { inert <- getTcSInerts
                 ; return (inert_eqs inert, inert_eq_tvs inert) }

rewriteFromInertEqs :: (TyVarEnv (Ct,Coercion), InScopeSet)
                    -- Precondition: Ct are CTyEqCans only!
                    -> CtFlavor 
                    -> EvVar 
                    -> TcS (EvVar,Bool)
-- Boolean flag returned: True <-> no rewriting happened
rewriteFromInertEqs (subst,inscope) fl v 
  = do { let co = liftInertEqsTy (subst,inscope) fl (evVarPred v)
       ; if isReflCo co then return (v,True)
         else do { traceTcS "rewriteFromInertEqs" $
                   text "Original item =" <+> ppr v <+> dcolon <+> ppr (evVarPred v)
                 ; v' <- forceNewEvVar fl (pSnd (liftedCoercionKind co))
                 ; case fl of 
                     Wanted {}  -> setEvBind v (EvCast v' (mkSymCo co)) 
                     Given {}   -> setEvBind v' (EvCast v co) 
                     Derived {} -> return ()
                 ; traceTcS "rewriteFromInertEqs" $
                   text "Rewritten item =" <+> ppr v' <+> dcolon <+> ppr (evVarPred v')
                 ; return (v',False) } }


-- See Note [LiftInertEqs]
liftInertEqsTy :: (TyVarEnv (Ct,Coercion),InScopeSet)
                 -> CtFlavor
                 -> PredType -> Coercion
liftInertEqsTy (subst,inscope) fl pty
  = ty_cts_subst subst inscope fl pty


ty_cts_subst :: TyVarEnv (Ct,Coercion)
             -> InScopeSet -> CtFlavor -> Type -> Coercion
ty_cts_subst subst inscope fl ty 
  = go ty 
  where 
        go ty = go' ty

        go' (TyVarTy tv)      = tyvar_cts_subst tv `orElse` Refl (TyVarTy tv)
        go' (AppTy ty1 ty2)   = mkAppCo (go ty1) (go ty2) 
        go' (TyConApp tc tys) = mkTyConAppCo tc (map go tys)  

        go' (ForAllTy v ty)   = mkForAllCo v' $! co
                             where 
                               (subst',inscope',v') = upd_tyvar_bndr subst inscope v
                               co = ty_cts_subst subst' inscope' fl ty 

        go' (FunTy ty1 ty2)   = mkFunCo (go ty1) (go ty2)


        tyvar_cts_subst tv  
          | Just (ct,co) <- lookupVarEnv subst tv, cc_flavor ct `canRewrite` fl  
          = Just co -- Warn: use cached, not cc_id directly, because of alpha-renamings!
          | otherwise = Nothing 

        upd_tyvar_bndr subst inscope v 
          = (new_subst, (inscope `extendInScopeSet` new_v), new_v)
          where new_subst 
                    | no_change = delVarEnv subst v
                        -- Otherwise we have to extend the environment with /something/. 
                        -- But we do not want to monadically create a new EvVar. So, we
                        -- create an 'unused_ct' but we cache reflexivity as the 
                        -- associated coercion. 
                    | otherwise = extendVarEnv subst v (unused_ct, Refl (TyVarTy new_v))

                no_change = new_v == v 
                new_v     = uniqAway inscope v 

                unused_ct = CTyEqCan { cc_id     = unused_evvar
                                     , cc_flavor = fl -- canRewrite is reflexive.
                                     , cc_tyvar  = v 
                                     , cc_rhs    = mkTyVarTy new_v 
                                     , cc_depth  = unused_depth }
                unused_depth = panic "ty_cts_subst: This depth should not be accessed!"
                unused_evvar = panic "ty_cts_subst: This var is just an alpha-renaming!"
\end{code}

Note [LiftInertEqsPred]
~~~~~~~~~~~~~~~~~~~~~~~ 
The function liftInertEqPred behaves almost like liftCoSubst (in
Coercion), but accepts a map TyVarEnv (Ct,Coercion) instead of a
LiftCoSubst. This data structure is more convenient to use since we
must apply the inert substitution /only/ if the inert equality 
`canRewrite` the work item. There's admittedly some duplication of 
functionality but it would be more tedious to cache and maintain 
different flavors of LiftCoSubst structures in the inerts.