TcSimplify.lhs 47.6 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 TcSimplify( 
10
       simplifyInfer, simplifyAmbiguityCheck,
11
       simplifyDefault, 
12 13
       simplifyRule, simplifyTop, simplifyInteractive,
       solveWantedsTcM
14
  ) where
15

16
#include "HsVersions.h"
17

18
import TcRnTypes
19
import TcRnMonad
20
import TcErrors
21
import TcMType as TcM
22
import TcType 
23
import TcSMonad as TcS
24
import TcInteract 
25
import Inst
26 27
import Type     ( classifyPredType, PredTree(..), getClassPredTys_maybe )
import Class    ( Class )
28
import Var
29
import Unique
30
import VarSet
31
import VarEnv 
32
import TcEvidence
33
import Name
34
import Bag
35 36
import ListSetOps
import Util
37 38 39
import PrelInfo
import PrelNames
import Class		( classKey )
40
import BasicTypes       ( RuleName )
41
import Outputable
42
import FastString
dimitris's avatar
dimitris committed
43
import TrieMap () -- DV: for now
44 45 46
\end{code}


47 48 49 50 51
*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
52

53 54 55
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
56 57 58
-- Usually these will be implications,
-- but when there is nothing to quantify we don't wrap
-- in a degenerate implication, so we do that here instead
59 60 61 62 63
simplifyTop wanteds
  = do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds 
       ; ev_binds_var <- newTcEvBinds
       ; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top
       ; binds1 <- TcRnMonad.getTcEvBinds ev_binds_var
64 65 66
       ; traceTc "End simplifyTop }" empty

       ; traceTc "reportUnsolved {" empty
67
       ; binds2 <- reportUnsolved zonked_final_wc
68
       ; traceTc "reportUnsolved }" empty
69
         
70
       ; return (binds1 `unionBags` binds2) }
71

72 73
  where
    -- See Note [Top-level Defaulting Plan]
Simon Peyton Jones's avatar
Simon Peyton Jones committed
74
    simpl_top :: WantedConstraints -> TcS WantedConstraints
75
    simpl_top wanteds
76
      = do { wc_first_go <- nestTcS (solve_wanteds_and_drop wanteds)
77 78 79
           ; free_tvs <- TcS.zonkTyVarsAndFV (tyVarsOfWC wc_first_go) 
           ; let meta_tvs = filterVarSet isMetaTyVar free_tvs
                   -- zonkTyVarsAndFV: the wc_first_go is not yet zonked
80 81 82
                   -- filter isMetaTyVar: we might have runtime-skolems in GHCi, 
                   -- and we definitely don't want to try to assign to those!

83
           ; mapM_ defaultTyVar (varSetElems meta_tvs)   -- Has unification side effects
84
           ; simpl_top_loop wc_first_go }
85
    
86
    simpl_top_loop wc
87 88 89
      | isEmptyWC wc || insolubleWC wc
             -- Don't do type-class defaulting if there are insolubles
             -- Doing so is not going to solve the insolubles
90 91
      = return wc
      | otherwise
92
      = do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
93 94 95 96 97 98 99
           ; let wc_flat_approximate = approximateWC wc_residual
           ; something_happened <- applyDefaultingRules wc_flat_approximate
                                        -- See Note [Top-level Defaulting Plan]
           ; if something_happened then 
               simpl_top_loop wc_residual 
             else 
               return wc_residual }
100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
\end{code}

Note [Top-level Defaulting Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We have considered two design choices for where/when to apply defaulting.   
   (i) Do it in SimplCheck mode only /whenever/ you try to solve some 
       flat constraints, maybe deep inside the context of implications.
       This used to be the case in GHC 7.4.1.
   (ii) Do it in a tight loop at simplifyTop, once all other constraint has 
        finished. This is the current story.

Option (i) had many disadvantages: 
   a) First it was deep inside the actual solver, 
   b) Second it was dependent on the context (Infer a type signature, 
      or Check a type signature, or Interactive) since we did not want 
      to always start defaulting when inferring (though there is an exception to  
      this see Note [Default while Inferring])
   c) It plainly did not work. Consider typecheck/should_compile/DfltProb2.hs:
          f :: Int -> Bool
          f x = const True (\y -> let w :: a -> a
                                      w a = const a (y+1)
                                  in w y)
      We will get an implication constraint (for beta the type of y):
               [untch=beta] forall a. 0 => Num beta
      which we really cannot default /while solving/ the implication, since beta is
      untouchable.

Instead our new defaulting story is to pull defaulting out of the solver loop and
go with option (i), implemented at SimplifyTop. Namely:
     - First have a go at solving the residual constraint of the whole program
     - Try to approximate it with a flat constraint
     - Figure out derived defaulting equations for that flat constraint
     - Go round the loop again if you did manage to get some equations

Now, that has to do with class defaulting. However there exists type variable /kind/
defaulting. Again this is done at the top-level and the plan is:
     - At the top-level, once you had a go at solving the constraint, do 
       figure out /all/ the touchable unification variables of the wanted contraints.
     - Apply defaulting to their kinds

More details in Note [DefaultTyVar].

\begin{code}
144

145 146 147
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
148
  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >> 
149
    simplifyTop wanteds  -- NB: must be simplifyTop so that we
150 151
                         --     do ambiguity resolution.  
                         -- See Note [Impedence matching] in TcBinds.
152
 
153 154 155
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
156 157
  = traceTc "simplifyInteractive" empty >>
    simplifyTop wanteds 
158 159 160 161 162

------------------
simplifyDefault :: ThetaType	-- Wanted; has no type variables in it
                -> TcM ()	-- Succeeds iff the constraint is soluble
simplifyDefault theta
163 164
  = do { traceTc "simplifyInteractive" empty
       ; wanted <- newFlatWanteds DefaultOrigin theta
165 166 167 168
       ; (unsolved, _binds) <- solveWantedsTcM (mkFlatWC wanted)

       ; traceTc "reportUnsolved {" empty
       -- See Note [Deferring coercion errors to runtime]
169
       ; reportAllUnsolved unsolved 
170 171 172
         -- Postcondition of solveWantedsTcM is that returned
         -- constraints are zonked. So Precondition of reportUnsolved
         -- is true.
173 174
       ; traceTc "reportUnsolved }" empty

175 176
       ; return () }
\end{code}
177

178

179 180 181 182 183
*********************************************************************************
*                                                                                 * 
*                            Inference
*                                                                                 *
***********************************************************************************
184

dreixel's avatar
dreixel committed
185 186 187 188 189 190 191 192 193 194 195 196
Note [Which variables to quantify]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose the inferred type of a function is
   T kappa (alpha:kappa) -> Int
where alpha is a type unification variable and 
      kappa is a kind unification variable
Then we want to quantify over *both* alpha and kappa.  But notice that
kappa appears "at top level" of the type, as well as inside the kind
of alpha.  So it should be fine to just look for the "top level"
kind/type variables of the type, without looking transitively into the
kinds of those type variables.

197
\begin{code}
198
simplifyInfer :: Bool
199 200 201
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
202
              -> WantedConstraints
203 204
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
205 206 207
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
208
                      TcEvBinds)    -- ... binding these evidence variables
209
simplifyInfer _top_lvl apply_mr name_taus wanteds
210 211 212
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
213
       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
dreixel's avatar
dreixel committed
214 215 216
       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
217
       ; return (qtvs, [], False, emptyTcEvBinds) }
218

219
  | otherwise
220 221
  = do { traceTc "simplifyInfer {"  $ vcat
             [ ptext (sLit "binds =") <+> ppr name_taus
222 223
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
224
             , ptext (sLit "(unzonked) wanted =") <+> ppr wanteds
225 226
             ]

227 228 229 230 231
              -- Historical note: Before step 2 we used to have a
              -- HORRIBLE HACK described in Note [Avoid unecessary
              -- constraint simplification] but, as described in Trac
              -- #4361, we have taken in out now.  That's why we start
              -- with step 2!
232

233 234 235 236 237 238 239 240
              -- Step 2) First try full-blown solving 

              -- NB: we must gather up all the bindings from doing
              -- this solving; hence (runTcSWithEvBinds ev_binds_var).
              -- And note that since there are nested implications,
              -- calling solveWanteds will side-effect their evidence
              -- bindings, so we can't just revert to the input
              -- constraint.
241 242

       ; ev_binds_var <- newTcEvBinds
243 244 245
       ; wanted_transformed <- solveWantedsTcMWithEvBinds ev_binds_var wanteds $
                               solve_wanteds_and_drop
                               -- Post: wanted_transformed are zonked
246 247

              -- Step 4) Candidates for quantification are an approximation of wanted_transformed
248 249 250
              -- NB: Already the fixpoint of any unifications that may have happened                                
              -- NB: We do not do any defaulting when inferring a type, this can lead
              -- to less polymorphic types, see Note [Default while Inferring]
251
 
252 253
              -- Step 5) Minimize the quantification candidates                             
              -- Step 6) Final candidates for quantification                
254
              -- We discard bindings, insolubles etc, because all we are
255 256
              -- care aout it

257 258 259 260 261 262 263 264 265 266 267 268 269
       ; tc_lcl_env <- TcRnMonad.getLclEnv
       ; let untch = tcl_untch tc_lcl_env
       ; quant_pred_candidates   
           <- if insolubleWC wanted_transformed 
              then return []   -- See Note [Quantification with errors]
              else do { gbl_tvs <- tcGetGlobalTyVars
                      ; let quant_cand = approximateWC wanted_transformed
                            meta_tvs   = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand)) 
                      ; ((flats, _insols), _extra_binds) <- runTcS $ 
                        do { mapM_ (promoteAndDefaultTyVar untch gbl_tvs) meta_tvs
                           ; _implics <- solveInteract quant_cand
                           ; getInertUnsolved }
                      ; return (map ctPred $ filter isWantedCt (bagToList flats)) }
270 271 272 273 274
                   -- NB: Dimitrios is slightly worried that we will get
                   -- family equalities (F Int ~ alpha) in the quantification
                   -- candidates, as we have performed no further unflattening
                   -- at this point. Nothing bad, but inferred contexts might
                   -- look complicated.
275

276 277 278
       -- NB: quant_pred_candidates is already the fixpoint of any 
       --     unifications that may have happened
       ; gbl_tvs        <- tcGetGlobalTyVars
279
       ; zonked_tau_tvs <- TcM.zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
280
       ; let init_tvs  = zonked_tau_tvs `minusVarSet` gbl_tvs
281
             poly_qtvs = growThetaTyVars quant_pred_candidates init_tvs 
282
                         `minusVarSet` gbl_tvs
283
             pbound    = filter (quantifyPred poly_qtvs) quant_pred_candidates
284
             
285
	     -- Monomorphism restriction
286 287
             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
             constrained_tvs = tyVarsOfTypes quant_pred_candidates
288
	     mr_bites        = apply_mr && not (null pbound)
289

290 291
             (qtvs, bound) | mr_bites  = (mr_qtvs,   [])
                           | otherwise = (poly_qtvs, pbound)
292
             
293 294 295 296 297 298 299
       ; traceTc "simplifyWithApprox" $
         vcat [ ptext (sLit "quant_pred_candidates =") <+> ppr quant_pred_candidates
              , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs
              , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs
              , ptext (sLit "pbound =") <+> ppr pbound
              , ptext (sLit "init_qtvs =") <+> ppr init_tvs 
              , ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs ]
300

301
       ; if isEmptyVarSet qtvs && null bound
302 303 304 305
         then do { traceTc "} simplifyInfer/no quantification" empty                   
                 ; emitConstraints wanted_transformed
                    -- Includes insolubles (if -fdefer-type-errors)
                    -- as well as flats and implications
306
                 ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
307 308
         else do

309 310 311
       { traceTc "simplifyApprox" $ 
         ptext (sLit "bound are =") <+> ppr bound 
         
312
            -- Step 4, zonk quantified variables 
313
       ; let minimal_flat_preds = mkMinimalBySCs bound
314 315
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
316 317 318 319
                        -- Don't add the quantified variables here, because
                        -- they are also bound in ic_skols and we want them to be
                        -- tidied uniformly

Simon Peyton Jones's avatar
Simon Peyton Jones committed
320
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
321

322
            -- Step 7) Emit an implication
323
       ; minimal_bound_ev_vars <- mapM TcM.newEvVar minimal_flat_preds
324
       ; let implic = Implic { ic_untch    = pushUntouchables untch
325
                             , ic_skols    = qtvs_to_return
326 327
                             , ic_fsks     = []  -- wanted_tansformed arose only from solveWanteds
                                                 -- hence no flatten-skolems (which come from givens)
328
                             , ic_given    = minimal_bound_ev_vars
329
                             , ic_wanted   = wanted_transformed 
330 331
                             , ic_insol    = False
                             , ic_binds    = ev_binds_var
332
                             , ic_info     = skol_info
333
                             , ic_env      = tc_lcl_env }
334
       ; emitImplication implic
335
         
336 337 338
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ ptext (sLit "implic =") <+> ppr implic
                       -- ic_skols, ic_given give rest of result
339
                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
340
                  , ptext (sLit "spb =") <+> ppr quant_pred_candidates
341 342
                  , ptext (sLit "bound =") <+> ppr bound ]

343 344
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
345
\end{code}
346

347 348 349 350 351 352 353 354 355
Note [Quantification with errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we find that the RHS of the definition has some absolutely-insoluble
constraints, we abandon all attempts to find a context to quantify
over, and instead make the function fully-polymorphic in whatever
type we have found.  For two reasons
  a) Minimise downstream errors
  b) Avoid spurious errors from this function
   
356

357 358
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
Our current plan is that defaulting only happens at simplifyTop and
not simplifyInfer.  This may lead to some insoluble deferred constraints
Example:

instance D g => C g Int b 

constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha
type inferred       = gamma -> gamma 

Now, if we try to default (alpha := Int) we will be able to refine the implication to 
  (forall b. 0 => C gamma Int b) 
which can then be simplified further to 
  (forall b. 0 => D gamma)
Finally we /can/ approximate this implication with (D gamma) and infer the quantified
type:  forall g. D g => g -> g

Instead what will currently happen is that we will get a quantified type 
(forall g. g -> g) and an implication:
       forall g. 0 => (forall b. 0 => C g alpha b) /\ Num alpha

which, even if the simplifyTop defaults (alpha := Int) we will still be left with an 
unsolvable implication:
       forall g. 0 => (forall b. 0 => D g)

The concrete example would be: 
       h :: C g a s => g -> a -> ST s a
       f (x::gamma) = (\_ -> x) (runST (h x (undefined::alpha)) + 1)

But it is quite tedious to do defaulting and resolve the implication constraints and
we have not observed code breaking because of the lack of defaulting in inference so 
we don't do it for now.



393 394 395 396 397 398 399 400 401
Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
When we quantify over a constraint, in simplifyInfer we need to
quantify over a constraint that is minimal in some sense: For
instance, if the final wanted constraint is (Eq alpha, Ord alpha),
we'd like to quantify over Ord alpha, because we can just get Eq alpha
from superclass selection from Ord alpha. This minimization is what
mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
to check the original wanted.
402

403

404 405
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
406 407 408 409
    -------- NB NB NB (Jun 12) ------------- 
    This note not longer applies; see the notes with Trac #4361.
    But I'm leaving it in here so we remember the issue.)
    ----------------------------------------
410
When inferring the type of a let-binding, with simplifyInfer,
411
try to avoid unnecessarily simplifying class constraints.
412 413
Doing so aids sharing, but it also helps with delicate 
situations like
414

415
   instance C t => C [t] where ..
416

417 418 419 420 421 422 423 424 425 426 427
   f :: C [t] => ....
   f x = let g y = ...(constraint C [t])... 
         in ...
When inferring a type for 'g', we don't want to apply the
instance decl, because then we can't satisfy (C t).  So we
just notice that g isn't quantified over 't' and partition
the contraints before simplifying.

This only half-works, but then let-generalisation only half-works.


428 429 430 431 432
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
433

434
See note [Simplifying RULE consraints] in TcRule
435

436 437 438 439 440 441 442 443 444 445 446 447 448 449
Note [RULE quanfification over equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Decideing which equalities to quantify over is tricky:
 * We do not want to quantify over insoluble equalities (Int ~ Bool)
    (a) because we prefer to report a LHS type error
    (b) because if such things end up in 'givens' we get a bogus
        "inaccessible code" error

 * But we do want to quantify over things like (a ~ F b), where
   F is a type function.

The difficulty is that it's hard to tell what is insoluble!
So we see whether the simplificaiotn step yielded any type errors,
and if so refrain from quantifying over *any* equalites.
450 451

\begin{code}
452 453 454
simplifyRule :: RuleName 
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
455 456 457
             -> TcM ([EvVar], WantedConstraints)   -- LHS evidence varaibles
-- See Note [Simplifying RULE constraints] in TcRule
simplifyRule name lhs_wanted rhs_wanted
458
  = do {      	 -- We allow ourselves to unify environment 
459
		 -- variables: runTcS runs with NoUntouchables
460
         (resid_wanted, _) <- solveWantedsTcM (lhs_wanted `andWC` rhs_wanted)
461
                              -- Post: these are zonked and unflattened
462

463 464
       ; zonked_lhs_flats <- zonkCts (wc_flat lhs_wanted)
       ; let (q_cts, non_q_cts) = partitionBag quantify_me zonked_lhs_flats
465 466 467 468 469 470 471 472 473 474 475 476
             quantify_me  -- Note [RULE quantification over equalities]
               | insolubleWC resid_wanted = quantify_insol
               | otherwise                = quantify_normal

             quantify_insol ct = not (isEqPred (ctPred ct))

             quantify_normal ct
               | EqPred t1 t2 <- classifyPredType (ctPred ct)
               = not (t1 `eqType` t2)
               | otherwise
               = True
             
477
       ; traceTc "simplifyRule" $
478
         vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
479
              , text "zonked_lhs_flats" <+> ppr zonked_lhs_flats 
480 481
              , text "q_cts"      <+> ppr q_cts ]

482
       ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
483
                , lhs_wanted { wc_flat = non_q_cts }) }
484 485 486
\end{code}


487 488 489 490 491
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
492

493 494 495 496 497 498
Note [Deferring coercion errors to runtime]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
While developing, sometimes it is desirable to allow compilation to succeed even
if there are type errors in the code. Consider the following case:

  module Main where
499

500 501
  a :: Int
  a = 'a'
502

503
  main = print "b"
504

505 506
Even though `a` is ill-typed, it is not used in the end, so if all that we're
interested in is `main` it is handy to be able to ignore the problems in `a`.
507

508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529
Since we treat type equalities as evidence, this is relatively simple. Whenever
we run into a type mismatch in TcUnify, we normally just emit an error. But it
is always safe to defer the mismatch to the main constraint solver. If we do
that, `a` will get transformed into

  co :: Int ~ Char
  co = ...

  a :: Int
  a = 'a' `cast` co

The constraint solver would realize that `co` is an insoluble constraint, and
emit an error with `reportUnsolved`. But we can also replace the right-hand side
of `co` with `error "Deferred type error: Int ~ Char"`. This allows the program
to compile, and it will run fine unless we evaluate `a`. This is what
`deferErrorsToRuntime` does.

It does this by keeping track of which errors correspond to which coercion
in TcErrors (with ErrEnv). TcErrors.reportTidyWanteds does not print the errors
and does not fail if -fwarn-type-errors is on, so that we can continue
compilation. The errors are turned into warnings in `reportUnsolved`.

530 531 532 533 534 535 536 537 538 539
Note [Zonk after solving]
~~~~~~~~~~~~~~~~~~~~~~~~~
We zonk the result immediately after constraint solving, for two reasons:

a) because zonkWC generates evidence, and this is the moment when we
   have a suitable evidence variable to hand.

Note that *after* solving the constraints are typically small, so the
overhead is not great.

540
\begin{code}
541 542 543 544
solveWantedsTcMWithEvBinds :: EvBindsVar
                           -> WantedConstraints
                           -> (WantedConstraints -> TcS WantedConstraints)
                           -> TcM WantedConstraints
545 546 547 548 549 550
-- Returns a *zonked* result
-- We zonk when we finish primarily to un-flatten out any
-- flatten-skolems etc introduced by canonicalisation of
-- types involving type funuctions.  Happily the result 
-- is typically much smaller than the input, indeed it is 
-- often empty.
551
solveWantedsTcMWithEvBinds ev_binds_var wc tcs_action
552 553
  = do { traceTc "solveWantedsTcMWithEvBinds" $ text "wanted=" <+> ppr wc
       ; wc2 <- runTcSWithEvBinds ev_binds_var (tcs_action wc)
554
       ; zonkWC ev_binds_var wc2 }
555
         -- See Note [Zonk after solving]
556

557
solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
558
-- Zonk the input constraints, and simplify them
559
-- Return the evidence binds in the BagEvBinds result
560
-- Discards all Derived stuff in result
561
-- Postcondition: fully zonked and unflattened constraints
562
solveWantedsTcM wanted 
563 564 565
  = do { ev_binds_var <- newTcEvBinds
       ; wanteds' <- solveWantedsTcMWithEvBinds ev_binds_var wanted solve_wanteds_and_drop
       ; binds <- TcRnMonad.getTcEvBinds ev_binds_var
566
       ; return (wanteds', binds) }
567

568 569 570 571 572
solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints)
-- Since solve_wanteds returns the residual WantedConstraints,
-- it should alway be called within a runTcS or something similar,
solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted 
                                   ; return (dropDerivedWC wc) }
573 574

solve_wanteds :: WantedConstraints -> TcS WantedConstraints 
575
-- so that the inert set doesn't mindlessly propagate.
576
-- NB: wc_flats may be wanted /or/ derived now
577
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
578 579
  = do { traceTcS "solveWanteds {" (ppr wanted)

580
         -- Try the flat bit, including insolubles. Solving insolubles a 
Simon Peyton Jones's avatar
Simon Peyton Jones committed
581
         -- second time round is a bit of a waste; but the code is simple 
582 583 584
         -- and the program is wrong anyway, and we don't run the danger 
         -- of adding Derived insolubles twice; see 
         -- TcSMonad Note [Do not add duplicate derived insolubles] 
585
       ; traceTcS "solveFlats {" empty
586
       ; let all_flats = flats `unionBags` insols
587 588
       ; impls_from_flats <- solveInteract all_flats
       ; traceTcS "solveFlats end }" (ppr impls_from_flats)
589

590 591
       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
592
       ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
593

594 595
       ; (unsolved_flats, insoluble_flats) <- getInertUnsolved

596 597 598 599 600 601
        -- We used to unflatten here but now we only do it once at top-level
        -- during zonking -- see Note [Unflattening while zonking] in TcMType
       ; let wc = WC { wc_flat  = unsolved_flats   
                     , wc_impl  = unsolved_implics 
                     , wc_insol = insoluble_flats }
                  
602
       ; bb <- getTcEvBindsMap
603
       ; tb <- getTcSTyBindsMap
604
       ; traceTcS "solveWanteds }" $
605
                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
606
                      , text "unsolved_implics =" <+> ppr unsolved_implics
607
                      , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
608
                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
609
                      , text "final wc =" <+> ppr wc ]
610

611
       ; return wc }
612 613 614 615 616 617 618 619

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
620 621 622 623 624 625 626
  = do { (floated_eqs, unsolved_implics) <- solveNestedImplications implics
       ; if isEmptyBag floated_eqs 
         then return unsolved_implics 
         else 
    do {   -- Put floated_eqs into the current inert set before looping
         impls_from_eqs <- solveInteract floated_eqs
       ; simpl_loop (n+1) (unsolved_implics `unionBags` impls_from_eqs)} }
627

628

629 630 631 632 633 634 635 636 637
solveNestedImplications :: Bag Implication
                        -> TcS (Cts, Bag Implication)
-- Precondition: the TcS inerts may contain unsolved flats which have 
-- to be converted to givens before we go inside a nested implication.
solveNestedImplications implics
  | isEmptyBag implics
  = return (emptyBag, emptyBag)
  | otherwise 
  = do { inerts <- getTcSInerts
638 639
       ; let thinner_inerts = prepareInertsForImplications inerts
                 -- See Note [Preparing inert set for implications]
640
  
641
       ; traceTcS "solveNestedImplications starting {" $ 
642
         vcat [ text "original inerts = " <+> ppr inerts
643 644
              , text "thinner_inerts  = " <+> ppr thinner_inerts ]
         
645
       ; (floated_eqs, unsolved_implics)
646
           <- flatMapBagPairM (solveImplication thinner_inerts) implics
647 648 649 650

       -- ... and we are back in the original TcS inerts 
       -- Notice that the original includes the _insoluble_flats so it was safe to ignore
       -- them in the beginning of this function.
651
       ; traceTcS "solveNestedImplications end }" $
652
                  vcat [ text "all floated_eqs ="  <+> ppr floated_eqs
653 654
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

655
       ; return (floated_eqs, unsolved_implics) }
656

657
solveImplication :: InertSet
658 659 660 661 662
                 -> Implication    -- Wanted
                 -> TcS (Cts,      -- All wanted or derived floated equalities: var = type
                         Bag Implication) -- Unsolved rest (always empty or singleton)
-- Precondition: The TcS monad contains an empty worklist and given-only inerts 
-- which after trying to solve this implication we must restore to their original value
663
solveImplication inerts
664
     imp@(Implic { ic_untch  = untch
665 666
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
667
                 , ic_fsks   = old_fsks
668
                 , ic_given  = givens
669
                 , ic_wanted = wanteds
670 671
                 , ic_info   = info
                 , ic_env    = env })
672
  = do { traceTcS "solveImplication {" (ppr imp) 
673

674
         -- Solve the nested constraints
675 676 677
         -- NB: 'inerts' has empty inert_fsks
       ; (new_fsks, residual_wanted) 
            <- nestImplicTcS ev_binds untch inerts $
678
               do { solveInteractGiven (mkGivenLoc info env) old_fsks givens 
679
                  ; residual_wanted <- solve_wanteds wanteds
680 681 682
                        -- solve_wanteds, *not* solve_wanteds_and_drop, because
                        -- we want to retain derived equalities so we can float
                        -- them out in floatEqualities
683 684 685 686 687 688 689
                  ; more_fsks <- getFlattenSkols
                  ; return (more_fsks ++ old_fsks, residual_wanted) }

       ; (floated_eqs, final_wanted)
             <- floatEqualities (skols ++ new_fsks) givens residual_wanted

       ; let res_implic | isEmptyWC final_wanted 
690 691
                        = emptyBag
                        | otherwise
692 693 694
                        = unitBag (imp { ic_fsks   = new_fsks
                                       , ic_wanted = dropDerivedWC final_wanted
                                       , ic_insol  = insolubleWC final_wanted })
695

696
       ; evbinds <- getTcEvBindsMap
697
       ; traceTcS "solveImplication end }" $ vcat
698
             [ text "floated_eqs =" <+> ppr floated_eqs
699
             , text "new_fsks =" <+> ppr new_fsks
700 701
             , text "res_implic =" <+> ppr res_implic
             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]
702

703
       ; return (floated_eqs, res_implic) }
704 705 706 707
\end{code}


\begin{code}
708 709
floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints 
                -> TcS (Cts, WantedConstraints)
710 711
-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
712 713
-- Also performs some unifications, adding to monadically-carried ty_binds
-- These will be used when processing floated_eqs later
714 715
floatEqualities skols can_given wanteds@(WC { wc_flat = flats })
  | hasEqualities can_given 
716
  = return (emptyBag, wanteds)   -- Note [Float Equalities out of Implications]
717
  | otherwise 
718
  = do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats
719
       ; untch <- TcS.getUntouchables
720
       ; mapM_ (promoteTyVar untch) (varSetElems (tyVarsOfCts float_eqs))
721
       ; ty_binds <- getTcSTyBindsMap
722
       ; traceTcS "floatEqualities" (vcat [ text "Floated eqs =" <+> ppr float_eqs
723 724
                                          , text "Ty binds =" <+> ppr ty_binds])
       ; return (float_eqs, wanteds { wc_flat = remaining_flats }) }
725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742
  where 
    skol_set = growSkols wanteds (mkVarSet skols)

    is_floatable :: Ct -> Bool
    is_floatable ct
       = isEqPred pred && skol_set `disjointVarSet` tyVarsOfType pred
       where
         pred = ctPred ct

growSkols :: WantedConstraints -> VarSet -> VarSet
-- Find all the type variables that might possibly be unified
-- with a type that mentions a skolem.  This test is very conservative.
-- I don't *think* we need look inside the implications, because any 
-- relevant unification variables in there are untouchable.
growSkols (WC { wc_flat = flats }) skols
  = growThetaTyVars theta skols
  where
    theta = foldrBag ((:) . ctPred) [] flats
743

744 745 746 747 748
promoteTyVar :: Untouchables -> TcTyVar  -> TcS ()
-- When we float a constraint out of an implication we must restore
-- invariant (MetaTvInv) in Note [Untouchable type variables] in TcType
promoteTyVar untch tv 
  | isFloatedTouchableMetaTyVar untch tv
749
  = do { cloned_tv <- TcS.cloneMetaTyVar tv
750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767
       ; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch
       ; setWantedTyBind tv (mkTyVarTy rhs_tv) }
  | otherwise
  = return ()

promoteAndDefaultTyVar :: Untouchables -> TcTyVarSet -> TyVar -> TcS ()
-- See Note [Promote _and_ default when inferring]
promoteAndDefaultTyVar untch gbl_tvs tv
  = do { tv1 <- if tv `elemVarSet` gbl_tvs 
                then return tv
                else defaultTyVar tv
       ; promoteTyVar untch tv1 }

defaultTyVar :: TcTyVar -> TcS TcTyVar
-- Precondition: MetaTyVars only
-- See Note [DefaultTyVar]
defaultTyVar the_tv
  | not (k `eqKind` default_k)
768
  = do { tv' <- TcS.cloneMetaTyVar the_tv
769 770 771 772 773 774 775 776 777 778 779 780 781
       ; let new_tv = setTyVarKind tv' default_k
       ; traceTcS "defaultTyVar" (ppr the_tv <+> ppr new_tv)
       ; setWantedTyBind the_tv (mkTyVarTy new_tv)
       ; return new_tv }
             -- Why not directly derived_pred = mkTcEqPred k default_k?
             -- See Note [DefaultTyVar]
             -- We keep the same Untouchables on tv'

  | otherwise = return the_tv	 -- The common case
  where
    k = tyVarKind the_tv
    default_k = defaultKind k

782 783
approximateWC :: WantedConstraints -> Cts
-- Postcondition: Wanted or Derived Cts 
784 785
approximateWC wc 
  = float_wc emptyVarSet wc
786 787
  where 
    float_wc :: TcTyVarSet -> WantedConstraints -> Cts
788 789 790
    float_wc skols (WC { wc_flat = flats, wc_impl = implics }) 
      = do_bag (float_flat skols)   flats  `unionBags` 
        do_bag (float_implic skols) implics
791 792 793
                                 
    float_implic :: TcTyVarSet -> Implication -> Cts
    float_implic skols imp
794 795 796
      | hasEqualities (ic_given imp)  -- Don't float out of equalities
      = emptyCts                      -- cf floatEqualities
      | otherwise                     -- See Note [approximateWC]
797 798 799 800 801 802 803 804 805 806 807 808 809
      = float_wc skols' (ic_wanted imp)
      where
        skols' = skols `extendVarSetList` ic_skols imp `extendVarSetList` ic_fsks imp
            
    float_flat :: TcTyVarSet -> Ct -> Cts
    float_flat skols ct
      | tyVarsOfCt ct `disjointVarSet` skols 
      = singleCt ct
      | otherwise = emptyCts
        
    do_bag :: (a -> Bag c) -> Bag a -> Bag c
    do_bag f = foldrBag (unionBags.f) emptyBag
\end{code}
810

811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834
Note [ApproximateWC]
~~~~~~~~~~~~~~~~~~~~
approximateWC takes a constraint, typically arising from the RHS of a
let-binding whose type we are *inferring*, and extracts from it some
*flat* constraints that we might plausibly abstract over.  Of course
the top-level flat constraints are plausible, but we also float constraints
out from inside, if the are not captured by skolems.

However we do *not* float anything out if the implication binds equality
constriants, because that defeats the OutsideIn story.  Consider
   data T a where
     TInt :: T Int
     MkT :: T a

   f TInt = 3::Int

We get the implication (a ~ Int => res ~ Int), where so far we've decided 
  f :: T a -> res
We don't want to float (res~Int) out because then we'll infer  
  f :: T a -> Int
which is only on of the possible types. (GHC 7.6 accidentally *did*
float out of such implications, which meant it would happily infer
non-principal types.)

835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880
Note [DefaultTyVar]
~~~~~~~~~~~~~~~~~~~
defaultTyVar is used on any un-instantiated meta type variables to
default the kind of OpenKind and ArgKind etc to *.  This is important 
to ensure that instance declarations match.  For example consider

     instance Show (a->b)
     foo x = show (\_ -> True)

Then we'll get a constraint (Show (p ->q)) where p has kind ArgKind,
and that won't match the typeKind (*) in the instance decl.  See tests
tc217 and tc175.

We look only at touchable type variables. No further constraints
are going to affect these type variables, so it's time to do it by
hand.  However we aren't ready to default them fully to () or
whatever, because the type-class defaulting rules have yet to run.

An important point is that if the type variable tv has kind k and the
default is default_k we do not simply generate [D] (k ~ default_k) because:

   (1) k may be ArgKind and default_k may be * so we will fail

   (2) We need to rewrite all occurrences of the tv to be a type
       variable with the right kind and we choose to do this by rewriting 
       the type variable /itself/ by a new variable which does have the 
       right kind.

Note [Promote _and_ default when inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we are inferring a type, we simplify the constraint, and then use
approximateWC to produce a list of candidate constraints.  Then we MUST

  a) Promote any meta-tyvars that have been floated out by 
     approximateWC, to restore invariant (MetaTvInv) described in 
     Note [Untouchable type variables] in TcType.

  b) Default the kind of any meta-tyyvars that are not mentioned in
     in the environment.

To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we
have an instance (C ((x:*) -> Int)).  The instance doesn't match -- but it
should!  If we don't solve the constraint, we'll stupidly quantify over 
(C (a->Int)) and, worse, in doing so zonkQuantifiedTyVar will quantify over
(b:*) instead of (a:OpenKind), which can lead to disaster; see Trac #7332.

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
881 882
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925
For ordinary pattern matches (including existentials) we float 
equalities out of implications, for instance: 
     data T where 
       MkT :: Eq a => a -> T 
     f x y = case x of MkT _ -> (y::Int)
We get the implication constraint (x::T) (y::alpha): 
     forall a. [untouchable=alpha] Eq a => alpha ~ Int
We want to float out the equality into a scope where alpha is no
longer untouchable, to solve the implication!  

But we cannot float equalities out of implications whose givens may
yield or contain equalities:

      data T a where 
        T1 :: T Int
        T2 :: T Bool
        T3 :: T a 
        
      h :: T a -> a -> Int
      
      f x y = case x of 
                T1 -> y::Int
                T2 -> y::Bool
                T3 -> h x y

We generate constraint, for (x::T alpha) and (y :: beta): 
   [untouchables = beta] (alpha ~ Int => beta ~ Int)   -- From 1st branch
   [untouchables = beta] (alpha ~ Bool => beta ~ Bool) -- From 2nd branch
   (alpha ~ beta)                                      -- From 3rd branch 

If we float the equality (beta ~ Int) outside of the first implication and 
the equality (beta ~ Bool) out of the second we get an insoluble constraint.
But if we just leave them inside the implications we unify alpha := beta and
solve everything.

Principle: 
    We do not want to float equalities out which may need the given *evidence*
    to become soluble.

Consequence: classes with functional dependencies don't matter (since there is 
no evidence for a fundep equality), but equality superclasses do matter (since 
they carry evidence).

926 927 928 929 930 931 932 933 934
Note [Promoting unification variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we float an equality out of an implication we must "promote" free
unification variables of the equality, in order to maintain Invariant
(MetaTvInv) from Note [Untouchable type variables] in TcType.  for the
leftover implication.

This is absolutely necessary. Consider the following example. We start
with two implications and a class with a functional dependency.
935

936 937 938 939 940
    class C x y | x -> y
    instance C [a] [a]
          
    (I1)      [untch=beta]forall b. 0 => F Int ~ [beta]
    (I2)      [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c]
941 942 943 944 945 946 947

We float (F Int ~ [beta]) out of I1, and we float (F Int ~ [[alpha]]) out of I2. 
They may react to yield that (beta := [alpha]) which can then be pushed inwards 
the leftover of I2 to get (C [alpha] [a]) which, using the FunDep, will mean that
(alpha := a). In the end we will have the skolem 'b' escaping in the untouchable
beta! Concrete example is in indexed_types/should_fail/ExtraTcsUntch.hs:

948 949 950 951 952 953 954 955 956 957 958 959 960
    class C x y | x -> y where 
     op :: x -> y -> ()

    instance C [a] [a]

    type family F a :: *

    h :: F Int -> ()
    h = undefined

    data TEx where 
      TEx :: a -> TEx 

961

962 963 964 965 966 967
    f (x::beta) = 
        let g1 :: forall b. b -> ()
            g1 _ = h [x]
            g2 z = case z of TEx y -> (h [[undefined]], op x [y])
        in (g1 '3', g2 undefined)

968

969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987

Note [Solving Family Equations] 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
After we are done with simplification we may be left with constraints of the form:
     [Wanted] F xis ~ beta 
If 'beta' is a touchable unification variable not already bound in the TyBinds 
then we'd like to create a binding for it, effectively "defaulting" it to be 'F xis'.

When is it ok to do so? 
    1) 'beta' must not already be defaulted to something. Example: 

           [Wanted] F Int  ~ beta   <~ Will default [beta := F Int]
           [Wanted] F Char ~ beta   <~ Already defaulted, can't default again. We 
                                       have to report this as unsolved.

    2) However, we must still do an occurs check when defaulting (F xis ~ beta), to 
       set [beta := F xis] only if beta is not among the free variables of xis.

    3) Notice that 'beta' can't be bound in ty binds already because we rewrite RHS 
988 989
       of type family equations. See Inert Set invariants in TcInteract.

990
This solving is now happening during zonking, see Note [Unflattening while zonking]
991
in TcMType.
992 993


994 995 996 997 998
*********************************************************************************
*                                                                               * 
*                          Defaulting and disamgiguation                        *
*                                                                               *
*********************************************************************************
999

1000
\begin{code}
1001 1002 1003
applyDefaultingRules :: Cts -> TcS Bool
  -- True <=> I did some defaulting, reflected in ty_binds
                 
1004 1005
-- Return some extra derived equalities, which express the
-- type-class default choice. 
1006
applyDefaultingRules wanteds
1007
  | isEmptyBag wanteds 
1008
  = return False
1009
  | otherwise
1010 1011
  = do { traceTcS "applyDefaultingRules { " $ 
                  text "wanteds =" <+> ppr wanteds
1012 1013 1014
                  
       ; info@(default_tys, _) <- getDefaultInfo
       ; let groups = findDefaultableGroups info wanteds
1015 1016
       ; traceTcS "findDefaultableGroups" $ vcat [ text "groups=" <+> ppr groups
                                                 , text "info=" <+> ppr info ]
1017
       ; something_happeneds <- mapM (disambigGroup default_tys) groups
1018

1019
       ; traceTcS "applyDefaultingRules }" (ppr something_happeneds)
1020

1021
       ; return (or something_happeneds) }
1022 1023 1024 1025
\end{code}



1026
\begin{code}
1027
findDefaultableGroups 
1028
    :: ( [Type]
1029
       , (Bool,Bool) )  -- (Overloaded strings, extended default rules)
1030
    -> Cts	        -- Unsolved (wanted or derived)
1031
    -> [[(Ct,Class,TcTyVar)]]
1032
findDefaultableGroups (default_tys, (ovl_strings, extended_defaults)) wanteds
1033 1034
  | null default_tys             = []
  | otherwise = filter is_defaultable_group (equivClasses cmp_tv unaries)
1035
  where 
1036
    unaries     :: [(Ct, Class, TcTyVar)]  -- (C tv) constraints
1037
    non_unaries :: [Ct]             -- and *other* constraints
1038 1039 1040
    
    (unaries, non_unaries) = partitionWith find_unary (bagToList wanteds)
        -- Finds unary type-class constraints
1041 1042 1043
    find_unary cc 
        | Just (cls,[ty]) <- getClassPredTys_maybe (ctPred cc)
        , Just tv <- tcGetTyVar_maybe ty
1044 1045
        , isMetaTyVar tv  -- We might have runtime-skolems in GHCi, and 
                          -- we definitely don't want to try to assign to those!
1046
        = Left (cc, cls, tv)
1047 1048 1049
    find_unary cc = Right cc  -- Non unary or non dictionary 

    bad_tvs :: TcTyVarSet  -- TyVars mentioned by non-unaries 
1050
    bad_tvs = foldr (unionVarSet . tyVarsOfCt) emptyVarSet non_unaries 
1051