TcInteract.lhs 83.2 KB
Newer Older
1
\begin{code}
Ian Lynagh's avatar
Ian Lynagh committed
2 3 4 5 6 7 8
{-# OPTIONS -fno-warn-tabs #-}
-- 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
module TcInteract ( 
10 11
     solveInteractGiven,  -- Solves [EvVar],GivenLoc
     solveInteractCts,    -- Solves [Cts]
12 13 14 15
  ) where  

#include "HsVersions.h"

16

17
import BasicTypes ()
18 19 20
import TcCanonical
import VarSet
import Type
dimitris's avatar
dimitris committed
21
import Unify
22 23
import FamInstEnv
import Coercion( mkAxInstRHS )
24 25 26 27 28

import Id 
import Var

import TcType
29
import PrelNames (singIClassName)
30

31 32
import Class
import TyCon
33
import Name
34
import IParam
35

dimitris's avatar
dimitris committed
36
import TysWiredIn ( eqTyCon )
37 38
import FunDeps

39
import TcEvidence
40 41
import Outputable

42 43
import TcMType ( zonkTcPredType )

44
import TcRnTypes
45
import TcErrors
46
import TcSMonad
47
import Maybes( orElse )
48
import Bag
49

50 51 52
import Control.Monad ( foldM )
import TrieMap

dimitris's avatar
dimitris committed
53 54
import VarEnv
import qualified Data.Traversable as Traversable
55
import Data.Maybe ( isJust )
dimitris's avatar
dimitris committed
56

57
import Control.Monad( when, unless )
58
import Pair ()
59
import UniqFM
60 61 62
import FastString ( sLit ) 
import DynFlags
\end{code}
63 64
**********************************************************************
*                                                                    * 
65 66 67 68
*                      Main Interaction Solver                       *
*                                                                    *
**********************************************************************

69 70
Note [Basic Simplifier Plan] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
71

72 73
1. Pick an element from the WorkList if there exists one with depth 
   less thanour context-stack depth. 
74

75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91
2. Run it down the 'stage' pipeline. Stages are: 
      - canonicalization
      - inert reactions
      - spontaneous reactions
      - top-level intreactions
   Each stage returns a StopOrContinue and may have sideffected 
   the inerts or worklist.
  
   The threading of the stages is as follows: 
      - If (Stop) is returned by a stage then we start again from Step 1. 
      - If (ContinueWith ct) is returned by a stage, we feed 'ct' on to 
        the next stage in the pipeline. 
4. If the element has survived (i.e. ContinueWith x) the last stage 
   then we add him in the inerts and jump back to Step 1.

If in Step 1 no such element exists, we have exceeded our context-stack 
depth and will simply fail.
92 93
\begin{code}

94 95 96
solveInteractCts :: [Ct] -> TcS (Bag Implication)
-- Returns a bag of residual implications that have arisen while solving
-- this particular worklist.
97
solveInteractCts cts 
dimitris's avatar
dimitris committed
98
  = do { traceTcS "solveInteractCtS" (vcat [ text "cts =" <+> ppr cts ]) 
99 100 101 102 103 104 105 106 107 108
       ; updWorkListTcS (appendWorkListCt cts) >> solveInteract 
       ; impls <- getTcSImplics
       ; updTcSImplics (const emptyBag) -- Nullify residual implications
       ; return impls }

solveInteractGiven :: GivenLoc -> [EvVar] -> TcS (Bag Implication)
-- In principle the givens can kick out some wanteds from the inert
-- resulting in solving some more wanted goals here which could emit
-- implications. That's why I return a bag of implications. Not sure
-- if this can happen in practice though.
109 110
solveInteractGiven gloc evs
  = solveInteractCts (map mk_noncan evs)
dimitris's avatar
dimitris committed
111
  where mk_noncan ev = CNonCanonical { cc_flavor = Given gloc ev
112 113 114 115 116 117 118
                                     , cc_depth = 0 }

-- The main solver loop implements Note [Basic Simplifier Plan]
---------------------------------------------------------------
solveInteract :: TcS ()
-- Returns the final InertSet in TcS, WorkList will be eventually empty.
solveInteract
119 120
  = {-# SCC "solveInteract" #-}
    do { dyn_flags <- getDynFlags
121 122
       ; let max_depth = ctxtStkDepth dyn_flags
             solve_loop
123 124
              = {-# SCC "solve_loop" #-}
                do { sel <- selectNextWorkItem max_depth
125 126 127 128
                   ; case sel of 
                      NoWorkRemaining     -- Done, successfuly (modulo frozen)
                        -> return ()
                      MaxDepthExceeded ct -- Failure, depth exceeded
129
                        -> wrapErrTcS $ solverDepthErrorTcS (cc_depth ct) [ct]
130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149
                      NextWorkItem ct     -- More work, loop around!
                        -> runSolverPipeline thePipeline ct >> solve_loop }
       ; solve_loop }

type WorkItem = Ct
type SimplifierStage = WorkItem -> TcS StopOrContinue

continueWith :: WorkItem -> TcS StopOrContinue
continueWith work_item = return (ContinueWith work_item) 

data SelectWorkItem 
       = NoWorkRemaining      -- No more work left (effectively we're done!)
       | MaxDepthExceeded Ct  -- More work left to do but this constraint has exceeded
                              -- the max subgoal depth and we must stop 
       | NextWorkItem Ct      -- More work left, here's the next item to look at 

selectNextWorkItem :: SubGoalDepth -- Max depth allowed
                   -> TcS SelectWorkItem
selectNextWorkItem max_depth
  = updWorkListTcS_return pick_next
150
  where 
151
    pick_next :: WorkList -> (SelectWorkItem, WorkList)
dimitris's avatar
dimitris committed
152 153 154 155 156 157 158 159
    pick_next wl = case selectWorkItem wl of
                     (Nothing,_) 
                         -> (NoWorkRemaining,wl)           -- No more work
                     (Just ct, new_wl) 
                         | cc_depth ct > max_depth         -- Depth exceeded
                         -> (MaxDepthExceeded ct,new_wl)
                     (Just ct, new_wl) 
                         -> (NextWorkItem ct, new_wl)      -- New workitem and worklist
160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192

runSolverPipeline :: [(String,SimplifierStage)] -- The pipeline 
                  -> WorkItem                   -- The work item 
                  -> TcS () 
-- Run this item down the pipeline, leaving behind new work and inerts
runSolverPipeline pipeline workItem 
  = do { initial_is <- getTcSInerts 
       ; traceTcS "Start solver pipeline {" $ 
                  vcat [ ptext (sLit "work item = ") <+> ppr workItem 
                       , ptext (sLit "inerts    = ") <+> ppr initial_is]

       ; final_res  <- run_pipeline pipeline (ContinueWith workItem)

       ; final_is <- getTcSInerts
       ; case final_res of 
           Stop            -> do { traceTcS "End solver pipeline (discharged) }" 
                                       (ptext (sLit "inerts    = ") <+> ppr final_is)
                                 ; return () }
           ContinueWith ct -> do { traceTcS "End solver pipeline (not discharged) }" $
                                       vcat [ ptext (sLit "final_item = ") <+> ppr ct
                                            , ptext (sLit "inerts     = ") <+> ppr final_is]
                                 ; updInertSetTcS ct }
       }
  where run_pipeline :: [(String,SimplifierStage)] -> StopOrContinue -> TcS StopOrContinue
        run_pipeline [] res = return res 
        run_pipeline _ Stop = return Stop 
        run_pipeline ((stg_name,stg):stgs) (ContinueWith ct)
          = do { traceTcS ("runStage " ++ stg_name ++ " {")
                          (text "workitem   = " <+> ppr ct) 
               ; res <- stg ct 
               ; traceTcS ("end stage " ++ stg_name ++ " }") empty
               ; run_pipeline stgs res 
               }
193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218
\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}
219
thePipeline :: [(String,SimplifierStage)]
220 221
thePipeline = [ ("lookup-in-inerts",        lookupInInertsStage)
              , ("canonicalization",        canonicalizationStage)
222 223 224
              , ("spontaneous solve",       spontaneousSolveStage)
              , ("interact with inerts",    interactWithInertsStage)
              , ("top-level reactions",     topReactionsStage) ]
225 226 227 228
\end{code}


\begin{code}
229

230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248
-- A quick lookup everywhere to see if we know about this constraint
--------------------------------------------------------------------
lookupInInertsStage :: SimplifierStage
lookupInInertsStage ct
  | isWantedCt ct
  = do { is <- getTcSInerts
       ; ctxt <- getTcSContext
       ; case lookupInInerts is (ctPred ct) of
           Just ct_cached 
             | (not $ isDerivedCt ct) && (not $ simplEqsOnly ctxt) 
               -- Don't share if we are simplifying a RULE 
               -- see Note [Simplifying RULE lhs constraints]
             -> setEvBind (ctId ct) (EvId (ctId ct_cached)) >> 
                return Stop
           _ -> continueWith ct }
  | otherwise -- I could do something like that for givens 
              -- as well I suppose but it is not a big deal
  = continueWith ct

dimitris's avatar
dimitris committed
249

250 251 252 253
-- The canonicalization stage, see TcCanonical for details
----------------------------------------------------------
canonicalizationStage :: SimplifierStage
canonicalizationStage = TcCanonical.canonicalize 
254

255 256 257 258 259 260 261 262
\end{code}

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

263 264 265 266 267 268
Note [Efficient Orientation] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

269
Case 1: In Rewriting Equalities (function rewriteEqLHS) 
270

271 272 273 274 275 276 277 278 279
    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.
    
280 281 282 283 284
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. 
285

286 287
\begin{code}
spontaneousSolveStage :: SimplifierStage 
288
spontaneousSolveStage workItem
289
  = do { mSolve <- trySpontaneousSolve workItem
290
       ; spont_solve mSolve } 
291 292 293 294 295 296 297
  where spont_solve SPCantSolve 
          | isCTyEqCan workItem                    -- Unsolved equality
          = do { kickOutRewritableInerts workItem  -- NB: will add workItem in inerts
               ; return Stop }
          | otherwise
          = continueWith workItem
        spont_solve (SPSolved workItem')           -- Post: workItem' must be equality
298 299
          = do { bumpStepCountTcS
               ; traceFireTcS (cc_depth workItem) $
300
                 ptext (sLit "Spontaneous:") <+> ppr workItem
301 302 303 304

                 -- NB: will add the item in the inerts
               ; kickOutRewritableInerts workItem'
               -- .. and Stop
305 306 307 308
               ; return Stop }

kickOutRewritableInerts :: Ct -> TcS () 
-- Pre:  ct is a CTyEqCan 
309 310 311
-- Post: The TcS monad is left with the thinner non-rewritable inerts; but which
--       contains the new constraint.
--       The rewritable end up in the worklist
dimitris's avatar
dimitris committed
312
kickOutRewritableInerts ct
313
  = {-# SCC "kickOutRewritableInerts" #-}
dimitris's avatar
dimitris committed
314 315
    do { traceTcS "kickOutRewritableInerts" $ text "workitem = " <+> ppr ct
       ; (wl,ieqs) <- {-# SCC "kick_out_rewritable" #-}
316
                      modifyInertTcS (kick_out_rewritable ct)
dimitris's avatar
dimitris committed
317 318 319
       ; traceTcS "Kicked out the following constraints" $ ppr wl
       ; is <- getTcSInerts 
       ; traceTcS "Remaining inerts are" $ ppr is
320

dimitris's avatar
dimitris committed
321 322 323 324 325
       -- Step 1: Rewrite as many of the inert_eqs on the spot!
       -- NB: if it is a given constraint just use the cached evidence
       -- to optimize e.g. mkRefl coercions from spontaneously solved cts.
       ; bnds <- getTcEvBindsMap
       ; let ct_coercion = getCtCoercion bnds ct 
326 327

       ; new_ieqs <- {-# SCC "rewriteInertEqsFromInertEq" #-}
dimitris's avatar
dimitris committed
328 329 330 331 332 333 334 335 336 337 338
                     rewriteInertEqsFromInertEq (cc_tyvar ct,
                                                 ct_coercion,cc_flavor ct) ieqs
       ; let upd_eqs is = is { inert_cans = new_ics }
                        where ics     = inert_cans is
                              new_ics = ics { inert_eqs = new_ieqs }
       ; modifyInertTcS (\is -> ((), upd_eqs is)) 
         
       ; is <- getTcSInerts 
       ; traceTcS "Final inerts are" $ ppr is
       
         -- Step 2: Add the new guy in
339
       ; updInertSetTcS ct
340 341 342

       ; traceTcS "Kick out" (ppr ct $$ ppr wl)
       ; updWorkListTcS (unionWorkList wl) }
dimitris's avatar
dimitris committed
343

344
rewriteInertEqsFromInertEq :: (TcTyVar, TcCoercion, CtFlavor) -- A new substitution
dimitris's avatar
dimitris committed
345 346
                           -> TyVarEnv Ct                     -- All the inert equalities
                           -> TcS (TyVarEnv Ct)               -- The new inert equalities
347 348 349
rewriteInertEqsFromInertEq (subst_tv, _subst_co, subst_fl) ieqs
-- The goal: traverse the inert equalities and throw some of them back to the worklist
-- if you have to rewrite and recheck them for occurs check errors. 
350
-- To see which ones we must throw out see Note [Delicate equality kick-out]
351 352 353
 = do { mieqs <- Traversable.mapM do_one ieqs 
      ; traceTcS "Original inert equalities:" (ppr ieqs)
      ; let flatten_justs elem venv
dimitris's avatar
dimitris committed
354
              | Just act <- elem = extendVarEnv venv (cc_tyvar act) act
355 356 357 358
              | otherwise = venv                                     
            final_ieqs = foldVarEnv flatten_justs emptyVarEnv mieqs
      ; traceTcS "Remaining inert equalities:" (ppr final_ieqs)
      ; return final_ieqs }
359

dimitris's avatar
dimitris committed
360
 where do_one ct
361
         | subst_fl `canRewrite` fl && (subst_tv `elemVarSet` tyVarsOfCt ct) 
dimitris's avatar
dimitris committed
362
         = if fl `canRewrite` subst_fl then
363 364
               -- If also the inert can rewrite the subst then there is no danger of 
               -- occurs check errors sor keep it there. No need to rewrite the inert equality
365 366
               -- (as we did in the past) because of point (8) of 
               -- Note [Detailed InertCans Invariants] and 
367 368
             return (Just ct)
             -- used to be: rewrite_on_the_spot ct >>= ( return . Just )
369
           else -- We have to throw inert back to worklist for occurs checks 
370
             updWorkListTcS (extendWorkListEq ct) >> return Nothing
371
         | otherwise -- Just keep it there
372
         = return (Just ct)
373
         where 
dimitris's avatar
dimitris committed
374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389
           fl  = cc_flavor ct

kick_out_rewritable :: Ct 
                    -> InertSet 
                    -> ((WorkList, TyVarEnv Ct),InertSet)
-- Post: returns ALL inert equalities, to be dealt with later
-- 
kick_out_rewritable ct is@(IS { inert_cans = 
                                   IC { inert_eqs    = eqmap
                                      , inert_eq_tvs = inscope
                                      , inert_dicts  = dictmap
                                      , inert_ips    = ipmap
                                      , inert_funeqs = funeqmap
                                      , inert_irreds = irreds }
                              , inert_frozen = frozen })
  = ((kicked_out,eqmap), remaining)
390
  where
391 392
    rest_out = fro_out `andCts` dicts_out 
                   `andCts` ips_out `andCts` irs_out
393
    kicked_out = WorkList { wl_eqs    = []
dimitris's avatar
dimitris committed
394
                          , wl_funeqs = bagToList feqs_out
395
                          , wl_rest   = bagToList rest_out }
396
  
dimitris's avatar
dimitris committed
397 398 399 400 401 402 403 404 405 406 407 408
    remaining = is { inert_cans = IC { inert_eqs = emptyVarEnv
                                     , inert_eq_tvs = inscope 
                                       -- keep the same, safe and cheap
                                     , inert_dicts = dicts_in
                                     , inert_ips = ips_in
                                     , inert_funeqs = feqs_in
                                     , inert_irreds = irs_in }
                   , inert_frozen = fro_in } 
                -- NB: Notice that don't rewrite 
                -- inert_solved, inert_flat_cache and inert_solved_funeqs
                -- optimistically. But when we lookup we have to take the 
                -- subsitution into account
409 410
    fl = cc_flavor ct
    tv = cc_tyvar ct
411 412
                               
    (ips_out,   ips_in)     = partitionCCanMap rewritable ipmap
413

dimitris's avatar
dimitris committed
414
    (feqs_out,  feqs_in)    = partCtFamHeadMap rewritable funeqmap
415
    (dicts_out, dicts_in)   = partitionCCanMap rewritable dictmap
416 417 418

    (irs_out,   irs_in)   = partitionBag rewritable irreds
    (fro_out,   fro_in)   = partitionBag rewritable frozen
dimitris's avatar
dimitris committed
419 420

    rewritable ct = (fl `canRewrite` cc_flavor ct)  &&
421 422 423 424 425 426 427 428 429 430
                    (tv `elemVarSet` tyVarsOfCt ct) 
                    -- NB: tyVarsOfCt will return the type 
                    --     variables /and the kind variables/ that are 
                    --     directly visible in the type. Hence we will
                    --     have exposed all the rewriting we care about
                    --     to make the most precise kinds visible for 
                    --     matching classes etc. No need to kick out 
                    --     constraints that mention type variables whose
                    --     kinds could contain this variable!

431
\end{code}
432

433 434
Note [Delicate equality kick-out]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
dimitris's avatar
dimitris committed
435

436 437 438
Delicate:
When kicking out rewritable constraints, it would be safe to simply
kick out all rewritable equalities, but instead we only kick out those
439
that, when rewritten, may result in occur-check errors. Example:
440

441
          WorkItem =   [G] a ~ b
442 443
          Inerts   = { [W] b ~ [a] }
Now at this point the work item cannot be further rewritten by the
444 445 446 447 448 449
inert (due to the weaker inert flavor). Instead the workitem can 
rewrite the inert leading to potential occur check errors. So we must
kick the inert out. On the other hand, if the inert flavor was as 
powerful or more powerful than the workitem flavor, the work-item could 
not have reached this stage (because it would have already been 
rewritten by the inert).
450 451

The coclusion is: we kick out the 'dangerous' equalities that may
452 453
require recanonicalization (occurs checks) and the rest we keep 
there in the inerts without further checks.
454

455 456
In the past we used to rewrite-on-the-spot those equalities that we keep in,
but this is no longer necessary see Note [Non-idempotent inert substitution].
457 458

\begin{code}
459 460
data SPSolveResult = SPCantSolve
                   | SPSolved WorkItem 
461

462 463 464
-- 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 

465
-- @trySpontaneousSolve wi@ solves equalities where one side is a
466
-- touchable unification variable.
467
--     	    See Note [Touchables and givens] 
468
trySpontaneousSolve :: WorkItem -> TcS SPSolveResult
dimitris's avatar
dimitris committed
469
trySpontaneousSolve workItem@(CTyEqCan { cc_flavor = gw
470
                                       , cc_tyvar = tv1, cc_rhs = xi, cc_depth = d })
dimitris's avatar
dimitris committed
471
  | isGivenOrSolved gw
472
  = return SPCantSolve
473 474 475 476
  | Just tv2 <- tcGetTyVar_maybe xi
  = do { tch1 <- isTouchableMetaTyVar tv1
       ; tch2 <- isTouchableMetaTyVar tv2
       ; case (tch1, tch2) of
dimitris's avatar
dimitris committed
477 478 479
           (True,  True)  -> trySpontaneousEqTwoWay d gw tv1 tv2
           (True,  False) -> trySpontaneousEqOneWay d gw tv1 xi
           (False, True)  -> trySpontaneousEqOneWay d gw tv2 (mkTyVarTy tv1)
480
	   _ -> return SPCantSolve }
481 482
  | otherwise
  = do { tch1 <- isTouchableMetaTyVar tv1
dimitris's avatar
dimitris committed
483
       ; if tch1 then trySpontaneousEqOneWay d gw tv1 xi
484 485
                 else do { traceTcS "Untouchable LHS, can't spontaneously solve workitem:" $
                           ppr workItem 
486
                         ; return SPCantSolve }
487
       }
488 489 490 491

  -- No need for 
  --      trySpontaneousSolve (CFunEqCan ...) = ...
  -- See Note [No touchables as FunEq RHS] in TcSMonad
492
trySpontaneousSolve _ = return SPCantSolve
493 494

----------------
495
trySpontaneousEqOneWay :: SubGoalDepth 
dimitris's avatar
dimitris committed
496
                       -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
497
-- tv is a MetaTyVar, not untouchable
dimitris's avatar
dimitris committed
498
trySpontaneousEqOneWay d gw tv xi
499
  | not (isSigTyVar tv) || isTyVarTy xi
dimitris's avatar
dimitris committed
500
  = solveWithIdentity d gw tv xi
501
  | otherwise -- Still can't solve, sig tyvar and non-variable rhs
502
  = return SPCantSolve
503 504

----------------
505
trySpontaneousEqTwoWay :: SubGoalDepth 
dimitris's avatar
dimitris committed
506
                       -> CtFlavor -> TcTyVar -> TcTyVar -> TcS SPSolveResult
507
-- Both tyvars are *touchable* MetaTyvars so there is only a chance for kind error here
508

dimitris's avatar
dimitris committed
509
trySpontaneousEqTwoWay d gw tv1 tv2
Simon Peyton Jones's avatar
Simon Peyton Jones committed
510
  = do { let k1_sub_k2 = k1 `tcIsSubKind` k2
dreixel's avatar
dreixel committed
511
       ; if k1_sub_k2 && nicer_to_update_tv2
dimitris's avatar
dimitris committed
512 513
         then solveWithIdentity d gw tv2 (mkTyVarTy tv1)
         else solveWithIdentity d gw tv1 (mkTyVarTy tv2) }
514 515 516 517 518 519
  where
    k1 = tyVarKind tv1
    k2 = tyVarKind tv2
    nicer_to_update_tv2 = isSigTyVar tv1 || isSystemName (Var.varName tv2)
\end{code}

520 521 522 523
Note [Kind errors] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the wanted problem: 
      alpha ~ (# Int, Int #) 
524
where alpha :: ArgKind and (# Int, Int #) :: (#). We can't spontaneously solve this constraint, 
525
but we should rather reject the program that give rise to it. If 'trySpontaneousEqTwoWay' 
526
simply returns @CantSolve@ then that wanted constraint is going to propagate all the way and 
527
get quantified over in inference mode. That's bad because we do know at this point that the 
528
constraint is insoluble. Instead, we call 'recKindErrorTcS' here, which will fail later on.
529 530

The same applies in canonicalization code in case of kind errors in the givens. 
531

532
However, when we canonicalize givens we only check for compatibility (@compatKind@). 
533
If there were a kind error in the givens, this means some form of inconsistency or dead code.
534

535 536 537 538 539
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. 
540

541 542
Note [Spontaneous solving and kind compatibility] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
543 544 545
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.)
546

547 548 549 550 551 552 553 554 555 556 557 558 559 560
  - 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:
561 562 563 564 565 566 567 568 569
  - 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' 

570 571 572 573 574 575 576

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! 
577
      given : a ~ alpha      [having unified alpha := a] 
578 579 580
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] 

581
We avoid this problem by orienting the resulting given so that the unification
582 583
variable is on the left.  [Note that alternatively we could attempt to
enforce this at canonicalization]
584

585 586 587
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.
588 589 590

\begin{code}
----------------
591

592
solveWithIdentity :: SubGoalDepth 
dimitris's avatar
dimitris committed
593
                  -> CtFlavor -> TcTyVar -> Xi -> TcS SPSolveResult
594 595
-- Solve with the identity coercion 
-- Precondition: kind(xi) is a sub-kind of kind(tv)
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
596 597 598
-- Precondition: CtFlavor is Wanted or Derived
-- See [New Wanted Superclass Work] to see why solveWithIdentity 
--     must work for Derived as well as Wanted
599
-- Returns: workItem where 
600
--        workItem = the new Given constraint
dimitris's avatar
dimitris committed
601 602 603 604 605 606 607
solveWithIdentity d wd tv xi 
  = do { let tv_ty = mkTyVarTy tv
       ; traceTcS "Sneaky unification:" $ 
                       vcat [text "Constraint:" <+> ppr wd,
                             text "Coercion:" <+> pprEq tv_ty xi,
                             text "Left Kind is:" <+> ppr (typeKind tv_ty),
                             text "Right Kind is:" <+> ppr (typeKind xi) ]
608

609 610 611 612 613 614 615
       ; let xi' = defaultKind xi      
               -- We only instantiate kind unification variables
               -- with simple kinds like *, not OpenKind or ArgKind
               -- cf TcUnify.uUnboundKVar

       ; setWantedTyBind tv xi'
       ; let refl_xi = mkTcReflCo xi'
616

dimitris's avatar
dimitris committed
617 618
       ; when (isWanted wd) $ 
              setEvBind (flav_evar wd) (EvCoercion refl_xi)
619

dimitris's avatar
dimitris committed
620 621 622 623 624 625 626
       ; ev_given <- newGivenEvVar (mkTcEqPred tv_ty xi') 
                                   (EvCoercion refl_xi) >>= (return . mn_thing)
       ; let given_fl = Given (mkGivenLoc (flav_wloc wd) UnkSkol) ev_given
             
       ; return $ 
         SPSolved (CTyEqCan { cc_flavor = given_fl
                            , cc_tyvar  = tv, cc_rhs = xi', cc_depth = d }) }
627 628
\end{code}

629 630 631 632 633 634 635

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

636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660
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

661 662 663
\begin{code}
-- Interaction result of  WorkItem <~> AtomicInert

664 665 666 667
data InteractResult 
    = IRWorkItemConsumed { ir_fire :: String } 
    | IRInertConsumed    { ir_fire :: String } 
    | IRKeepGoing        { ir_fire :: String }
668

669 670
irWorkItemConsumed :: String -> TcS InteractResult
irWorkItemConsumed str = return (IRWorkItemConsumed str) 
671

672 673
irInertConsumed :: String -> TcS InteractResult
irInertConsumed str = return (IRInertConsumed str) 
674

675 676 677 678
irKeepGoing :: String -> TcS InteractResult 
irKeepGoing str = return (IRKeepGoing str) 
-- You can't discard neither workitem or inert, but you must keep 
-- going. It's possible that new work is waiting in the TcS worklist. 
679 680


681 682 683 684
interactWithInertsStage :: WorkItem -> TcS StopOrContinue 
-- Precondition: if the workitem is a CTyEqCan then it will not be able to 
-- react with anything at this stage. 
interactWithInertsStage wi 
685
  = do { ctxt <- getTcSContext
686
       ; if simplEqsOnly ctxt && not (isCFunEqCan wi) then 
687
                    -- Why not just "simplEqsOnly"? See Note [SimplEqsOnly and InteractWithInerts]
688 689
             return (ContinueWith wi)
         else 
dimitris's avatar
dimitris committed
690 691 692 693
           do { traceTcS "interactWithInerts" $ text "workitem = " <+> ppr wi
              ; rels <- extractRelevantInerts wi 
              ; traceTcS "relevant inerts are:" $ ppr rels
              ; foldlBagM interact_next (ContinueWith wi) rels } }
694 695 696 697 698 699

  where interact_next Stop atomic_inert 
          = updInertSetTcS atomic_inert >> return Stop
        interact_next (ContinueWith wi) atomic_inert 
          = do { ir <- doInteractWithInert atomic_inert wi
               ; let mk_msg rule keep_doc 
700 701 702
                       = vcat [ text rule <+> keep_doc
                              , ptext (sLit "InertItem =") <+> ppr atomic_inert
                              , ptext (sLit "WorkItem  =") <+> ppr wi ]
703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718
               ; case ir of 
                   IRWorkItemConsumed { ir_fire = rule } 
                       -> do { bumpStepCountTcS
                             ; traceFireTcS (cc_depth wi) 
                                            (mk_msg rule (text "WorkItemConsumed"))
                             ; updInertSetTcS atomic_inert
                             ; return Stop } 
                   IRInertConsumed { ir_fire = rule }
                       -> do { bumpStepCountTcS
                             ; traceFireTcS (cc_depth atomic_inert) 
                                            (mk_msg rule (text "InertItemConsumed"))
                             ; return (ContinueWith wi) }
                   IRKeepGoing {} -- Should we do a bumpStepCountTcS? No for now.
                       -> do { updInertSetTcS atomic_inert
                             ; return (ContinueWith wi) }
               }
719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748

\end{code}

Note [SimplEqsOnly and InteractWithInerts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It may be possible when we are simplifying a RULE that we have two wanted constraints
of the form: 
  [W] c1 : F Int ~ Bool
  [W] c2 : F Int ~ alpha

When we simplify RULES we only do equality reactions (simplEqsOnly). So the question is:
are we allowed to do type family interactions? We definitely do not want to apply top-level
family and dictionary instances but what should we do with the constraint set above? 

Suppose that c1 gets processed first and enters the inert. Remember that he will enter a 
CtFamHead map with (F Int) as the index. Now c2 comes along, we can't add him to the inert
set since it has exactly the same key, so we'd better react him with the inert c1. In fact 
one might think that we should react him anyway to learn that (alpha ~ Bool). This is why
we allow CFunEqCan's to perform reactions with the inerts. 

If we don't allow this, we will try to add both elements to the inert set and will panic! 
The relevant example that fails when we don't allow such family reactions is:

        indexed_types/should_compile/T2291.hs

NB: In previous versions of TcInteract the extra guard (not (isCFunEqCan wi)) was not there 
but family reactions were actually happening earlier, during canonicalization. So the behaviour 
has not changed -- previously this tricky point was completely lost and worked by accident.

749
\begin{code}
750 751
--------------------------------------------

752 753
doInteractWithInert :: Ct -> Ct -> TcS InteractResult
-- Identical class constraints.
754
doInteractWithInert
dimitris's avatar
dimitris committed
755 756
  inertItem@(CDictCan { cc_flavor = fl1, cc_class = cls1, cc_tyargs = tys1 }) 
   workItem@(CDictCan { cc_flavor = fl2, cc_class = cls2, cc_tyargs = tys2 })
757

758
  | cls1 == cls2  
batterseapower's avatar
batterseapower committed
759 760
  = do { let pty1 = mkClassPred cls1 tys1
             pty2 = mkClassPred cls2 tys2
761
             inert_pred_loc     = (pty1, pprFlavorArising fl1)
762
             work_item_pred_loc = (pty2, pprFlavorArising fl2)
763

764 765 766
       ; traceTcS "doInteractWithInert" (vcat [ text "inertItem = " <+> ppr inertItem
                                              , text "workItem  = " <+> ppr workItem ])

767 768 769 770 771 772
       ; any_fundeps 
           <- if isGivenOrSolved fl1 && isGivenOrSolved fl2 then return Nothing
              -- NB: We don't create fds for given (and even solved), have not seen a useful
              -- situation for these and even if we did we'd have to be very careful to only
              -- create Derived's and not Wanteds. 

773 774 775
              else do { let fd_eqns = improveFromAnother inert_pred_loc work_item_pred_loc
                      ; wloc  <- get_workitem_wloc fl2 
                      ; rewriteWithFunDeps fd_eqns tys2 wloc }
776 777 778 779 780
                      -- See Note [Efficient Orientation], [When improvement happens]

       ; case any_fundeps of
           -- No Functional Dependencies 
           Nothing             
dimitris's avatar
dimitris committed
781
               | eqTypes tys1 tys2 -> solveOneFromTheOther "Cls/Cls" fl1 workItem
782
               | otherwise         -> irKeepGoing "NOP"
783 784

           -- Actual Functional Dependencies
785 786
           Just (_rewritten_tys2,_cos2,fd_work)
              -- Standard thing: create derived fds and keep on going. Importantly we don't
787
               -- throw workitem back in the worklist because this can cause loops. See #5236.
788 789
               -> do { emitFDWorkAsDerived fd_work (cc_depth workItem)
                     ; irKeepGoing "Cls/Cls (new fundeps)" } -- Just keep going without droping the inert 
790
       }
dimitris's avatar
dimitris committed
791 792 793 794 795 796 797 798 799 800 801
  where get_workitem_wloc (Wanted wl _)  = return wl 
        get_workitem_wloc (Derived wl _) = return wl
        get_workitem_wloc _ = pprPanic "Unexpected given workitem!" $
                              vcat [ text "Work item =" <+> ppr workItem
                                   , text "Inert item=" <+> ppr inertItem]

-- Two pieces of irreducible evidence: if their types are *exactly identical* 
-- we can rewrite them. We can never improve using this: 
-- if we want ty1 :: Constraint and have ty2 :: Constraint it clearly does not 
-- mean that (ty1 ~ ty2)
doInteractWithInert (CIrredEvCan { cc_flavor = ifl, cc_ty = ty1 })
802 803
           workItem@(CIrredEvCan { cc_ty = ty2 })
  | ty1 `eqType` ty2
dimitris's avatar
dimitris committed
804
  = solveOneFromTheOther "Irred/Irred" ifl workItem
805

806 807 808 809 810
-- 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.
dimitris's avatar
dimitris committed
811
doInteractWithInert (CIPCan { cc_flavor = ifl, cc_ip_nm = nm1, cc_ip_ty = ty1 }) 
812
           workItem@(CIPCan { cc_flavor = wfl, cc_ip_nm = nm2, cc_ip_ty = ty2 })
dimitris's avatar
dimitris committed
813
  | nm1 == nm2 && isGivenOrSolved wfl && isGivenOrSolved ifl
814 815 816
  = 	-- See Note [Overriding implicit parameters]
        -- Dump the inert item, override totally with the new one
	-- Do not require type equality
817 818
	-- For example, given let ?x::Int = 3 in let ?x::Bool = True in ...
	--              we must *override* the outer one with the inner one
819
    irInertConsumed "IP/IP (override inert)"
820

821
  | nm1 == nm2 && ty1 `eqType` ty2 
dimitris's avatar
dimitris committed
822
  = solveOneFromTheOther "IP/IP" ifl workItem 
823

824
  | nm1 == nm2
825
  =  	-- See Note [When improvement happens]
dimitris's avatar
dimitris committed
826 827 828 829 830 831 832 833 834 835 836 837
    do { mb_eqv <- newWantedEvVar (mkEqPred ty2 ty1)
         -- co :: ty2 ~ ty1, see Note [Efficient orientation]
       ; cv <- case mb_eqv of
                 Fresh eqv  -> 
                   do { updWorkListTcS $ extendWorkListEq $ 
                        CNonCanonical { cc_flavor = Wanted new_wloc eqv
                                      , cc_depth = cc_depth workItem }
                      ; return eqv }
                 Cached eqv -> return eqv
       ; case wfl of
            Wanted  {} ->
              let ip_co = mkTcTyConAppCo (ipTyCon nm1) [mkTcCoVarCo cv]
838
              in do { setEvBind (ctId workItem) $
dimitris's avatar
dimitris committed
839 840 841 842 843 844 845 846 847 848
                      mkEvCast (flav_evar ifl) (mkTcSymCo ip_co)
                    ; irWorkItemConsumed "IP/IP (solved by rewriting)" }
            _ -> pprPanic "Unexpected IP constraint" (ppr workItem) }
  where new_wloc
          | Wanted wl _  <- wfl = wl
          | Derived wl _ <- wfl = wl
          | Wanted wl _  <- ifl = wl
          | Derived wl _ <- ifl = wl
          | otherwise = panic "Solve IP: no WantedLoc!"
    
849

dimitris's avatar
dimitris committed
850 851 852 853
doInteractWithInert ii@(CFunEqCan { cc_flavor = fl1, cc_fun = tc1
                                  , cc_tyargs = args1, cc_rhs = xi1, cc_depth = d1 }) 
                    wi@(CFunEqCan { cc_flavor = fl2, cc_fun = tc2
                                  , cc_tyargs = args2, cc_rhs = xi2, cc_depth = d2 })
854
  | lhss_match  
dimitris's avatar
dimitris committed
855 856
  , isSolved fl1 -- Inert is solved and we can simply ignore it
                 -- when workitem is given/solved
857 858
  , isGivenOrSolved fl2
  = irInertConsumed "FunEq/FunEq"
dimitris's avatar
dimitris committed
859 860 861
  | lhss_match
  , isSolved fl2 -- Workitem is solved and we can ignore it when
                 -- the inert is given/solved
862 863
  , isGivenOrSolved fl1                 
  = irWorkItemConsumed "FunEq/FunEq" 
864
  | fl1 `canSolve` fl2 && lhss_match
dimitris's avatar
dimitris committed
865
  = do { traceTcS "interact with inerts: FunEq/FunEq" $ 
866 867 868 869 870 871 872 873 874 875 876 877 878
         vcat [ text "workItem =" <+> ppr wi
              , text "inertItem=" <+> ppr ii ]

       ; let xev = XEvTerm xcomp xdecomp
             -- xcomp : [(xi2 ~ xi1)] -> (F args ~ xi2) 
             xcomp [x] = EvCoercion (co1 `mkTcTransCo` mk_sym_co x)
             xcomp _   = panic "No more goals!"
             -- xdecomp : (F args ~ xi2) -> [(xi2 ~ xi1)]                 
             xdecomp x = [EvCoercion (mk_sym_co x `mkTcTransCo` co1)]

       ; xCtFlavor_cache False fl2 [mkTcEqPred xi2 xi1] xev $ what_next d2
                         -- Why not simply xCtFlavor? See Note [Cache-caused loops]
                         -- Why not (mkTcEqPred xi1 xi2)? See Note [Efficient orientation]
879
       ; irWorkItemConsumed "FunEq/FunEq" }
880
  | fl2 `canSolve` fl1 && lhss_match
881 882 883 884 885 886 887 888 889 890 891 892 893 894 895
  = do { traceTcS "interact with inerts: FunEq/FunEq" $ 
         vcat [ text "workItem =" <+> ppr wi
              , text "inertItem=" <+> ppr ii ]

       ; let xev = XEvTerm xcomp xdecomp
              -- xcomp : [(xi2 ~ xi1)] -> [(F args ~ xi1)]
             xcomp [x] = EvCoercion (co2 `mkTcTransCo` mkTcCoVarCo x)
             xcomp _ = panic "No more goals!"
             -- xdecomp : (F args ~ xi1) -> [(xi2 ~ xi1)]
             xdecomp x = [EvCoercion (mkTcSymCo co2 `mkTcTransCo` mkTcCoVarCo x)]

       ; xCtFlavor_cache False fl1 [mkTcEqPred xi2 xi1] xev $ what_next d1
                          -- Why not simply xCtFlavor? See Note [Cache-caused loops]
                          -- Why not (mkTcEqPred xi1 xi2)? See Note [Efficient orientation]

896
       ; irInertConsumed "FunEq/FunEq"}
897
  where
898
    lhss_match = tc1 == tc2 && eqTypes args1 args2 
dimitris's avatar
dimitris committed
899 900 901 902 903 904 905 906
    what_next d [new_fl] 
      = updWorkListTcS $ 
        extendWorkListEq (CNonCanonical {cc_flavor=new_fl,cc_depth = d})
    what_next _ _ = return ()
    co1 = mkTcCoVarCo $ flav_evar fl1
    co2 = mkTcCoVarCo $ flav_evar fl2
    mk_sym_co x = mkTcSymCo (mkTcCoVarCo x)
    
907 908
doInteractWithInert _ _ = irKeepGoing "NOP"

909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943
\end{code}

Note [Cache-caused loops]
~~~~~~~~~~~~~~~~~~~~~~~~~
It is very dangerous to cache a rewritten wanted family equation as 'solved' in our 
solved cache (which is the default behaviour or xCtFlavor), because the interaction 
may not be contributing towards a solution. Here is an example:

Initial inert set:
  [W] g1 : F a ~ beta1
Work item:
  [W] g2 : F a ~ beta2
The work item will react with the inert yielding the _same_ inert set plus:
    i)   Will set g2 := g1 `cast` g3   
    ii)  Will add to our solved cache that [S] g2 : F a ~ beta2
    iii) Will emit [W] g3 : beta1 ~ beta2 
Now, the g3 work item will be spontaneously solved to [G] g3 : beta1 ~ beta2
and then it will react the item in the inert ([W] g1 : F a ~ beta1). So it 
will set 
      g1 := g ; sym g3 
and what is g? Well it would ideally be a new goal of type (F a ~ beta2) but
remember that we have this in our solved cache, and it is ... g2! In short we 
created the evidence loop:

        g2 := g1 ; g3 
        g3 := refl
        g1 := g2 ; sym g3 

To avoid this situation we do not cache as solved any workitems (or inert) 
which did not really made a 'step' towards proving some goal. Solved's are 
just an optimization so we don't lose anything in terms of completeness of 
solving.

\begin{code}

dimitris's avatar
dimitris committed
944 945
solveOneFromTheOther :: String    -- Info 
                     -> CtFlavor  -- Inert 
946 947 948 949 950
                     -> Ct        -- WorkItem 
                     -> TcS InteractResult
-- Preconditions: 
-- 1) inert and work item represent evidence for the /same/ predicate
-- 2) ip/class/irred evidence (no coercions) only
dimitris's avatar
dimitris committed
951
solveOneFromTheOther info ifl workItem
952
  | isDerived wfl
953
  = irWorkItemConsumed ("Solved[DW] " ++ info)
954

955 956 957
  | isDerived ifl -- The inert item is Derived, we can just throw it away, 
    	      	  -- The workItem is inert wrt earlier inert-set items, 
		  -- so it's safe to continue on from this point
958
  = irInertConsumed ("Solved[DI] " ++ info)
959
  
dimitris's avatar
dimitris committed
960
  | isSolved ifl, isGivenOrSolved wfl
dimitris's avatar
dimitris committed
961
    -- Same if the inert is a GivenSolved -- just get rid of it
962
  = irInertConsumed ("Solved[SI] " ++ info)
dimitris's avatar
dimitris committed
963

964 965 966
  | otherwise
  = ASSERT( ifl `canSolve` wfl )
      -- Because of Note [The Solver Invariant], plus Derived dealt with
dimitris's avatar
dimitris committed
967
    do { when (isWanted wfl) $ setEvBind wid (EvId iid)
968 969
           -- Overwrite the binding, if one exists
	   -- If both are Given, we already have evidence; no need to duplicate
970
       ; irWorkItemConsumed ("Solved " ++ info) }
971 972
  where 
     wfl = cc_flavor workItem
973
     wid = ctId workItem
dimitris's avatar
dimitris committed
974
     iid = flav_evar ifl
975

976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992
\end{code}

Note [Superclasses and recursive dictionaries]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Overlaps with Note [SUPERCLASS-LOOP 1]
                  Note [SUPERCLASS-LOOP 2]
                  Note [Recursive instances and superclases]
    ToDo: check overlap and delete redundant stuff

Right before adding a given into the inert set, we must
produce some more work, that will bring the superclasses 
of the given into scope. The superclass constraints go into 
our worklist. 

When we simplify a wanted constraint, if we first see a matching
instance, we may produce new wanted work. To (1) avoid doing this work 
twice in the future and (2) to handle recursive dictionaries we may ``cache'' 
993 994 995
this item as given into our inert set WITHOUT adding its superclass constraints, 
otherwise we'd be in danger of creating a loop [In fact this was the exact reason
for doing the isGoodRecEv check in an older version of the type checker]. 
996 997 998 999 1000 1001 1002 1003 1004 1005

But now we have added partially solved constraints to the worklist which may 
interact with other wanteds. Consider the example: 

Example 1: 

    class Eq b => Foo a b        --- 0-th selector
    instance Eq a => Foo [a] a   --- fooDFun

and wanted (Foo [t] t). We are first going to see that the instance matches 
1006
and create an inert set that includes the solved (Foo [t] t) but not its superclasses:
1007 1008 1009 1010
       d1 :_g Foo [t] t                 d1 := EvDFunApp fooDFun d3 
Our work list is going to contain a new *wanted* goal
       d3 :_w Eq t 

1011
Ok, so how do we get recursive dictionaries, at all: 
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 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 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 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 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 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

Example 2:

    data D r = ZeroD | SuccD (r (D r));
    
    instance (Eq (r (D r))) => Eq (D r) where
        ZeroD     == ZeroD     = True
        (SuccD a) == (SuccD b) = a == b
        _         == _         = False;
    
    equalDC :: D [] -> D [] -> Bool;
    equalDC = (==);

We need to prove (Eq (D [])). Here's how we go:

	d1 :_w Eq (D [])

by instance decl, holds if
	d2 :_w Eq [D []]
	where 	d1 = dfEqD d2

*BUT* we have an inert set which gives us (no superclasses): 
        d1 :_g Eq (D []) 
By the instance declaration of Eq we can show the 'd2' goal if 
	d3 :_w Eq (D [])
	where	d2 = dfEqList d3
		d1 = dfEqD d2
Now, however this wanted can interact with our inert d1 to set: 
        d3 := d1 
and solve the goal. Why was this interaction OK? Because, if we chase the 
evidence of d1 ~~> dfEqD d2 ~~-> dfEqList d3, so by setting d3 := d1 we 
are really setting
        d3 := dfEqD2 (dfEqList d3) 
which is FINE because the use of d3 is protected by the instance function 
applications. 

So, our strategy is to try to put solved wanted dictionaries into the
inert set along with their superclasses (when this is meaningful,
i.e. when new wanted goals are generated) but solve a wanted dictionary
from a given only in the case where the evidence variable of the
wanted is mentioned in the evidence of the given (recursively through
the evidence binds) in a protected way: more instance function applications 
than superclass selectors.

Here are some more examples from GHC's previous type checker


Example 3: 
This code arises in the context of "Scrap Your Boilerplate with Class"

    class Sat a
    class Data ctx a
    instance  Sat (ctx Char)             => Data ctx Char       -- dfunData1
    instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]        -- dfunData2

    class Data Maybe a => Foo a    

    instance Foo t => Sat (Maybe t)                             -- dfunSat

    instance Data Maybe a => Foo a                              -- dfunFoo1
    instance Foo a        => Foo [a]                            -- dfunFoo2
    instance                 Foo [Char]                         -- dfunFoo3

Consider generating the superclasses of the instance declaration
	 instance Foo a => Foo [a]

So our problem is this
    d0 :_g Foo t
    d1 :_w Data Maybe [t] 

We may add the given in the inert set, along with its superclasses
[assuming we don't fail because there is a matching instance, see 
 tryTopReact, given case ]
  Inert:
    d0 :_g Foo t 
  WorkList 
    d01 :_g Data Maybe t  -- d2 := EvDictSuperClass d0 0 
    d1 :_w Data Maybe [t] 
Then d2 can readily enter the inert, and we also do solving of the wanted
  Inert: 
    d0 :_g Foo t 
    d1 :_s Data Maybe [t]           d1 := dfunData2 d2 d3 
  WorkList
    d2 :_w Sat (Maybe [t])          
    d3 :_w Data Maybe t
    d01 :_g Data Maybe t 
Now, we may simplify d2 more: 
  Inert:
      d0 :_g Foo t 
      d1 :_s Data Maybe [t]           d1 := dfunData2 d2 d3 
      d1 :_g Data Maybe [t] 
      d2 :_g Sat (Maybe [t])          d2 := dfunSat d4 
  WorkList: 
      d3 :_w Data Maybe t 
      d4 :_w Foo [t] 
      d01 :_g Data Maybe t 

Now, we can just solve d3.
  Inert
      d0 :_g Foo t 
      d1 :_s Data Maybe [t]           d1 := dfunData2 d2 d3 
      d2 :_g Sat (Maybe [t])          d2 := dfunSat d4 
  WorkList
      d4 :_w Foo [t] 
      d01 :_g Data Maybe t 
And now we can simplify d4 again, but since it has superclasses we *add* them to the worklist:
  Inert
      d0 :_g Foo t 
      d1 :_s Data Maybe [t]           d1 := dfunData2 d2 d3 
      d2 :_g Sat (Maybe [t])          d2 := dfunSat d4 
      d4 :_g Foo [t]                  d4 := dfunFoo2 d5 
  WorkList:
      d5 :_w Foo t 
      d6 :_g Data Maybe [t]           d6 := EvDictSuperClass d4 0
      d01 :_g Data Maybe t 
Now, d5 can be solved! (and its superclass enter scope) 
  Inert
      d0 :_g Foo t 
      d1 :_s Data Maybe [t]           d1 := dfunData2 d2 d3 
      d2 :_g Sat (Maybe [t])          d2 := dfunSat d4 
      d4 :_g Foo [t]                  d4 := dfunFoo2 d5 
      d5 :_g Foo t                    d5 := dfunFoo1 d7
  WorkList:
      d7 :_w Data Maybe t
      d6 :_g Data Maybe [t]
      d8 :_g Data Maybe t            d8 := EvDictSuperClass d5 0
      d01 :_g Data Maybe t 

Now, two problems: 
   [1] Suppose we pick d8 and we react him with d01. Which of the two givens should 
       we keep? Well, we *MUST NOT* drop d01 because d8 contains recursive evidence 
       that must not be used (look at case interactInert where both inert and workitem
       are givens). So we have several options: 
       - Drop the workitem always (this will drop d8)
              This feels very unsafe -- what if the work item was the "good" one
              that should be used later to solve another wanted?
       - Don't drop anyone: the inert set may contain multiple givens! 
              [This is currently implemented] 

The "don't drop anyone" seems the most safe thing to do, so now we come to problem 2: 
  [2] We have added both d6 and d01 in the inert set, and we are interacting our wanted
      d7. Now the [isRecDictEv] function in the ineration solver 
      [case inert-given workitem-wanted] will prevent us from interacting d7 := d8 
      precisely because chasing the evidence of d8 leads us to an unguarded use of d7. 

      So, no interaction happens there. Then we meet d01 and there is no recursion 
      problem there [isRectDictEv] gives us the OK to interact and we do solve d7 := d01! 
             
Note [SUPERCLASS-LOOP 1]
~~~~~~~~~~~~~~~~~~~~~~~~
We have to be very, very careful when generating superclasses, lest we
accidentally build a loop. Here's an example:

  class S a

  class S a => C a where { opc :: a -> a }
  class S b => D b where { opd :: b -> b }
  
  instance C Int where
     opc = opd
  
  instance D Int where
     opd = opc

From (instance C Int) we get the constraint set {ds1:S Int, dd:D Int}
Simplifying, we may well get:
	$dfCInt = :C ds1 (opd dd)
	dd  = $dfDInt
	ds1 = $p1 dd
Notice that we spot that we can extract ds1 from dd.  

Alas!  Alack! We can do the same for (instance D Int):

	$dfDInt = :D ds2 (opc dc)
	dc  = $dfCInt
	ds2 = $p1 dc

And now we've defined the superclass in terms of itself.
Two more nasty cases are in
	tcrun021
	tcrun033

Solution: 
  - Satisfy the superclass context *all by itself* 
    (tcSimplifySuperClasses)
  - And do so completely; i.e. no left-over constraints
    to mix with the constraints arising from method declarations


Note [SUPERCLASS-LOOP 2]
~~~~~~~~~~~~~~~~~~~~~~~~
We need to be careful when adding "the constaint we are trying to prove".
Suppose we are *given* d1:Ord a, and want to deduce (d2:C [a]) where

	class Ord a => C a where
	instance Ord [a] => C [a] where ...

Then we'll use the instance decl to deduce C [a] from Ord [a], and then add the
superclasses of C [a] to avails.  But we must not overwrite the binding
for Ord [a] (which is obtained from Ord a) with a superclass selection or we'll just
build a loop! 

Here's another variant, immortalised in tcrun020
	class Monad m => C1 m
	class C1 m => C2 m x
	instance C2 Maybe Bool
For the instance decl we need to build (C1 Maybe), and it's no good if
we run around and add (C2 Maybe Bool) and its superclasses to the avails 
before we search for C1 Maybe.

Here's another example 
 	class Eq b => Foo a b
	instance Eq a => Foo [a] a
If we are reducing
	(Foo [t] t)

we'll first deduce that it holds (via the instance decl).  We must not
then overwrite the Eq t constraint with a superclass selection!

At first I had a gross hack, whereby I simply did not add superclass constraints
in addWanted, though I did for addGiven and addIrred.  This was sub-optimal,
becuase it lost legitimate superclass sharing, and it still didn't do the job:
I found a very obscure program (now tcrun021) in which improvement meant the
simplifier got two bites a the cherry... so something seemed to be an Stop
first time, but reducible next time.

Now we implement the Right Solution, which is to check for loops directly 
when adding superclasses.  It's a bit like the occurs check in unification.

Note [Recursive instances and superclases]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this code, which arises in the context of "Scrap Your 
Boilerplate with Class".  

    class Sat a
    class Data ctx a
    instance  Sat (ctx Char)             => Data ctx Char
    instance (Sat (ctx [a]), Data ctx a) => Data ctx [a]

    class Data Maybe a => Foo a

    instance Foo t => Sat (Maybe t)

    instance Data Maybe a => Foo a
    instance Foo a        => Foo [a]
    instance                 Foo [Char]

In the instance for Foo [a], when generating evidence for the superclasses
(ie in tcSimplifySuperClasses) we need a superclass (Data Maybe [a]).
Using the instance for Data, we therefore need
        (Sat (Maybe [a], Data Maybe a)
But we are given (Foo a), and hence its superclass (Data Maybe a).
So that leaves (Sat (Maybe [a])).  Using the instance for Sat means
we need (Foo [a]).  And that is the very dictionary we are bulding
an instance for!  So we must put that in the "givens".  So in this
case we have
	Given:  Foo a, Foo [a]
	Wanted: Data Maybe [a]

BUT we must *not not not* put the *superclasses* of (Foo [a]) in
the givens, which is what 'addGiven' would normally do. Why? Because
(Data Maybe [a]) is the superclass, so we'd "satisfy" the wanted 
by selecting a superclass from Foo [a], which simply makes a loop.

On the other hand we *must* put the superclasses of (Foo a) in
the givens, as you can see from the derivation described above.

Conclusion: in the very special case of tcSimplifySuperClasses
we have one 'given' (namely the "this" dictionary) whose superclasses
must not be added to 'givens' by addGiven.  

There is a complication though.  Suppose there are equalities
      instance (Eq a, a~b) => Num (a,b)
Then we normalise the 'givens' wrt the equalities, so the original
given "this" dictionary is cast to one of a different type.  So it's a
bit trickier than before to identify the "special" dictionary whose
superclasses must not be added. See test
   indexed-types/should_run/EqInInstance

We need a persistent property of the dictionary to record this
special-ness.  Current I'm using the InstLocOrigin (a bit of a hack,
but cool), which is maintained by dictionary normalisation.
Specifically, the InstLocOrigin is
	     NoScOrigin
then the no-superclass thing kicks in.  WATCH OUT if you fiddle
with InstLocOrigin!

Note [MATCHING-SYNONYMS]
~~~~~~~~~~~~~~~~~~~~~~~~
When trying to match a dictionary (D tau) to a top-level instance, or a 
type family equation (F taus_1 ~ tau_2) to a top-level family instance, 
we do *not* need to expand type synonyms because the matcher will do that for us.


Note [RHS-FAMILY-SYNONYMS] 
~~~~~~~~~~~~~~~~~~~~~~~~~~
The RHS of a family instance is represented as yet another constructor which is 
like a type synonym for the real RHS the programmer declared. Eg: 
    type instance F (a,a) = [a] 
Becomes: 
    :R32 a = [a]      -- internal type synonym introduced
    F (a,a) ~ :R32 a  -- instance 

When we react a family instance with a type family equation in the work list 
we keep the synonym-using RHS without expansion. 


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
%************************************************************************
%*                                                                      *
%*          Functional dependencies, instantiation of equations
%*                                                                      *
%************************************************************************

When we spot an equality arising from a functional dependency,
we now use that equality (a "wanted") to rewrite the work-item
constraint right away.  This avoids two dangers

 Danger 1: If we send the original constraint on down the pipeline
           it may react with an instance declaration, and in delicate
	   situations (when a Given overlaps with an instance) that
	   may produce new insoluble goals: see Trac #4952

 Danger 2: If we don't rewrite the constraint, it may re-react
           with the same thing later, and produce the same equality
           again --> termination worries.

To achieve this required some refactoring of FunDeps.lhs (nicer
now!).  

\begin{code}
rewriteWithFunDeps :: [Equation]
                   -> [Xi] 
                   -> WantedLoc 
                   -> TcS (Maybe ([Xi], [TcCoercion], [(EvVar,WantedLoc)])) 
                                           -- Not quite a WantedEvVar unfortunately
                                           -- Because our intention could be to make 
                                           -- it derived at the end of the day
-- NB: The flavor of the returned EvVars will be decided by the caller
-- Post: returns no trivial equalities (identities) and all EvVars returned are fresh
rewriteWithFunDeps eqn_pred_locs xis wloc
 = do { fd_ev_poss <- mapM (instFunDepEqn wloc) eqn_pred_locs
      ; let fd_ev_pos :: [(Int,(EqVar,WantedLoc))]
            fd_ev_pos = concat fd_ev_poss
            (rewritten_xis, cos) = unzip (rewriteDictParams fd_ev_pos xis)
      ; if null fd_ev_pos then return Nothing
        else return (Just (rewritten_xis, cos, map snd fd_ev_pos)) }

instFunDepEqn :: WantedLoc -> Equation -> TcS [(Int,(EvVar,WantedLoc))]
-- Post: Returns the position index as well as the corresponding FunDep equality
instFunDepEqn wl (FDEqn { fd_qtvs = qtvs, fd_eqs = eqs
                        , fd_pred1 = d1, fd_pred2 = d2 })
  = do { let tvs = varSetElems qtvs
       ; tvs' <- mapM instFlexiTcS tvs  -- IA0_TODO: we might need to do kind substitution
       ; let subst = zipTopTvSubst tvs (mkTyVarTys tvs')
       ; foldM (do_one subst) [] eqs }
  where 
    do_one subst ievs (FDEq { fd_pos = i, fd_ty_left = ty1, fd_ty_right = ty2 })
       = let sty1 = Type.substTy subst ty1 
             sty2 = Type.substTy subst ty2 
         in if eqType sty1 sty2 then return ievs -- Return no trivial equalities
dimitris's avatar
dimitris committed
1372 1373 1374 1375 1376 1377 1378 1379 1380 1381
            else do { mb_eqv <- newWantedEvVar (mkTcEqPred sty1 sty2)
                    ; case mb_eqv of
                         Fresh eqv -> return $ (i,(eqv, push_ctx wl)):ievs
                         Cached {} -> return ievs }
                           -- We are eventually going to emit FD work back in the work list so 
                           -- it is important that we only return the /freshly created/ and not 
                           -- some existing equality!
    push_ctx :: WantedLoc -> WantedLoc 
    push_ctx loc = pushErrCtxt FunDepOrigin (False, mkEqnMsg d1 d2) loc

1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417

mkEqnMsg :: (TcPredType, SDoc) 
         -> (TcPredType, SDoc) -> TidyEnv -> TcM (TidyEnv, SDoc)
mkEqnMsg (pred1,from1) (pred2,from2) tidy_env
  = do  { zpred1 <- zonkTcPredType pred1
        ; zpred2 <- zonkTcPredType pred2
	; let { tpred1 = tidyType tidy_env zpred1
              ; tpred2 = tidyType tidy_env zpred2 }
	; let msg = vcat [ptext (sLit "When using functional dependencies to combine"),
			  nest 2 (sep [ppr tpred1 <> comma, nest 2 from1]), 
			  nest 2 (sep [ppr tpred2 <> comma, nest 2 from2])]
	; return (tidy_env, msg) }

rewriteDictParams :: [(Int,(EqVar,WantedLoc))] -- A set of coercions : (pos, ty' ~ ty)
                  -> [Type]                    -- A sequence of types: tys
                  -> [(Type, TcCoercion)]      -- Returns: [(ty', co : ty' ~ ty)]
rewriteDictParams param_eqs tys
  = zipWith do_one tys [0..]
  where
    do_one :: Type -> Int -> (Type, TcCoercion)
    do_one ty n = case lookup n param_eqs of
                    Just wev -> (get_fst_ty wev, mkTcCoVarCo (fst wev))
                    Nothing  -> (ty,             mkTcReflCo ty)	-- Identity

    get_fst_ty (wev,_wloc) 
      | Just (ty1, _) <- getEqPredTys_maybe (evVarPred wev )
      = ty1
      | otherwise 
      = panic "rewriteDictParams: non equality fundep!?"

        
emitFDWorkAsDerived :: [(EvVar,WantedLoc)] 
                    -> SubGoalDepth -> TcS () 
emitFDWorkAsDerived evlocs d 
  = updWorkListTcS $ appendWorkListEqs fd_cts
  where fd_cts = map mk_fd_ct evlocs 
dimitris's avatar
dimitris committed
1418 1419 1420
        mk_fd_ct (v,wl)