TcSimplify.hs 88.7 KB
Newer Older
1 2
{-# LANGUAGE CPP #-}

3
module TcSimplify(
4
       simplifyInfer,
5
       growThetaTyVars,
Simon Peyton Jones's avatar
Simon Peyton Jones committed
6
       simplifyAmbiguityCheck,
7
       simplifyDefault,
8 9
       simplifyTop, simplifyInteractive, solveEqualities,
       simplifyWantedsTcM,
10
       tcCheckSatisfiability,
11

12 13
       -- For Rules we need these
       solveWanteds, runTcSDeriveds
14
  ) where
15

16
#include "HsVersions.h"
17

18
import Bag
19
import Class         ( Class, classKey, classTyCon )
20
import DynFlags      ( WarningFlag ( Opt_WarnMonomorphism )
21
                     , WarnReason ( Reason )
22
                     , DynFlags( solverIterations ) )
23
import Inst
24
import ListSetOps
25
import Maybes
26 27
import Name
import Outputable
28 29
import PrelInfo
import PrelNames
30 31 32
import TcErrors
import TcEvidence
import TcInteract
33
import TcCanonical   ( makeSuperClasses, mkGivensWithSuperClasses )
34
import TcMType   as TcM
35
import TcRnMonad as TcM
36 37 38
import TcSMonad  as TcS
import TcType
import TrieMap       () -- DV: for now
39
import Type
40
import TysWiredIn    ( ptrRepLiftedTy )
41 42 43 44
import Unify         ( tcMatchTy )
import Util
import Var
import VarSet
niteria's avatar
niteria committed
45
import UniqFM
46 47
import BasicTypes    ( IntWithInf, intGtLimit )
import ErrUtils      ( emptyMessages )
48
import qualified GHC.LanguageExtensions as LangExt
49

50
import Control.Monad ( when, unless )
51
import Data.List     ( partition )
52

Austin Seipp's avatar
Austin Seipp committed
53
{-
54
*********************************************************************************
55
*                                                                               *
56 57 58
*                           External interface                                  *
*                                                                               *
*********************************************************************************
Austin Seipp's avatar
Austin Seipp committed
59
-}
60

61 62
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
63 64 65
-- 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
66
simplifyTop wanteds
67
  = do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds
Simon Peyton Jones's avatar
Simon Peyton Jones committed
68 69 70 71
       ; ((final_wc, unsafe_ol), binds1) <- runTcS $
            do { final_wc <- simpl_top wanteds
               ; unsafe_ol <- getSafeOverlapFailures
               ; return (final_wc, unsafe_ol) }
72 73 74
       ; traceTc "End simplifyTop }" empty

       ; traceTc "reportUnsolved {" empty
75
       ; binds2 <- reportUnsolved final_wc
76
       ; traceTc "reportUnsolved }" empty
77

78 79 80 81
       ; traceTc "reportUnsolved (unsafe overlapping) {" empty
       ; unless (isEmptyCts unsafe_ol) $ do {
           -- grab current error messages and clear, warnAllUnsolved will
           -- update error messages which we'll grab and then restore saved
82
           -- messages.
83
           ; errs_var  <- getErrsVar
84 85
           ; saved_msg <- TcM.readTcRef errs_var
           ; TcM.writeTcRef errs_var emptyMessages
86 87 88 89 90

           ; warnAllUnsolved $ WC { wc_simple = unsafe_ol
                                  , wc_insol = emptyCts
                                  , wc_impl = emptyBag }

91 92
           ; whyUnsafe <- fst <$> TcM.readTcRef errs_var
           ; TcM.writeTcRef errs_var saved_msg
93 94 95 96
           ; recordUnsafeInfer whyUnsafe
           }
       ; traceTc "reportUnsolved (unsafe overlapping) }" empty

97 98 99
       ; return (evBindMapBinds binds1 `unionBags` binds2) }

-- | Type-check a thing that emits only equality constraints, then
100
-- solve those constraints. Fails outright if there is trouble.
101 102
solveEqualities :: TcM a -> TcM a
solveEqualities thing_inside
103 104
  = checkNoErrs $  -- See Note [Fail fast on kind errors]
    do { (result, wanted) <- captureConstraints thing_inside
105
       ; traceTc "solveEqualities {" $ text "wanted = " <+> ppr wanted
Simon Peyton Jones's avatar
Simon Peyton Jones committed
106
       ; final_wc <- runTcSEqualities $ simpl_top wanted
107 108 109 110 111 112
       ; traceTc "End solveEqualities }" empty

       ; traceTc "reportAllUnsolved {" empty
       ; reportAllUnsolved final_wc
       ; traceTc "reportAllUnsolved }" empty
       ; return result }
113

Simon Peyton Jones's avatar
Simon Peyton Jones committed
114
simpl_top :: WantedConstraints -> TcS WantedConstraints
115
    -- See Note [Top-level Defaulting Plan]
116
simpl_top wanteds
117
  = do { wc_first_go <- nestTcS (solveWantedsAndDrop wanteds)
118
                            -- This is where the main work happens
Simon Peyton Jones's avatar
Simon Peyton Jones committed
119
       ; try_tyvar_defaulting wc_first_go }
120
  where
121 122
    try_tyvar_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_tyvar_defaulting wc
123
      | isEmptyWC wc
124 125
      = return wc
      | otherwise
126 127
      = do { free_tvs <- TcS.zonkTyCoVarsAndFVList (tyCoVarsOfWCList wc)
           ; let meta_tvs = filter (isTyVar <&&> isMetaTyVar) free_tvs
128
                   -- zonkTyCoVarsAndFV: the wc_first_go is not yet zonked
129
                   -- filter isMetaTyVar: we might have runtime-skolems in GHCi,
130
                   -- and we definitely don't want to try to assign to those!
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
131
                   -- the isTyVar needs to weed out coercion variables
132

133 134 135
           ; defaulted <- mapM defaultTyVarTcS meta_tvs   -- Has unification side effects
           ; if or defaulted
             then do { wc_residual <- nestTcS (solveWanteds wc)
136
                            -- See Note [Must simplify after defaulting]
137 138
                     ; try_class_defaulting wc_residual }
             else try_class_defaulting wc }     -- No defaulting took place
139

140 141
    try_class_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_class_defaulting wc
Austin Seipp's avatar
Austin Seipp committed
142
      | isEmptyWC wc
143 144
      = return wc
      | otherwise  -- See Note [When to do type-class defaulting]
145
      = do { something_happened <- applyDefaultingRules wc
146
                                   -- See Note [Top-level Defaulting Plan]
147
           ; if something_happened
148
             then do { wc_residual <- nestTcS (solveWantedsAndDrop wc)
149
                     ; try_class_defaulting wc_residual }
Simon Peyton Jones's avatar
Simon Peyton Jones committed
150
                  -- See Note [Overview of implicit CallStacks] in TcEvidence
151 152 153 154 155 156 157 158 159 160 161
             else try_callstack_defaulting wc }

    try_callstack_defaulting :: WantedConstraints -> TcS WantedConstraints
    try_callstack_defaulting wc
      | isEmptyWC wc
      = return wc
      | otherwise
      = defaultCallStacks wc

-- | Default any remaining @CallStack@ constraints to empty @CallStack@s.
defaultCallStacks :: WantedConstraints -> TcS WantedConstraints
Simon Peyton Jones's avatar
Simon Peyton Jones committed
162
-- See Note [Overview of implicit CallStacks] in TcEvidence
163 164 165 166 167 168 169 170 171 172 173 174 175 176
defaultCallStacks wanteds
  = do simples <- handle_simples (wc_simple wanteds)
       implics <- mapBagM handle_implic (wc_impl wanteds)
       return (wanteds { wc_simple = simples, wc_impl = implics })

  where

  handle_simples simples
    = catBagMaybes <$> mapBagM defaultCallStack simples

  handle_implic implic = do
    wanteds <- defaultCallStacks (ic_wanted implic)
    return (implic { ic_wanted = wanteds })

177
  defaultCallStack ct
Eric Seidel's avatar
Eric Seidel committed
178
    | Just _ <- isCallStackPred (ctPred ct)
179
    = do { solveCallStack (cc_ev ct) EvCsEmpty
180 181 182 183 184
         ; return Nothing }

  defaultCallStack ct
    = return (Just ct)

185

186 187 188 189 190
{- Note [Fail fast on kind errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
solveEqualities is used to solve kind equalities when kind-checking
user-written types. If solving fails we should fail outright, rather
than just accumulate an error message, for two reasons:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
191

192 193 194
  * A kind-bogus type signature may cause a cascade of knock-on
    errors if we let it pass

Simon Peyton Jones's avatar
Simon Peyton Jones committed
195 196 197 198 199
  * More seriously, we don't have a convenient term-level place to add
    deferred bindings for unsolved kind-equality constraints, so we
    don't build evidence bindings (by usine reportAllUnsolved). That
    means that we'll be left with with a type that has coercion holes
    in it, something like
200 201 202 203 204 205 206
           <type> |> co-hole
    where co-hole is not filled in.  Eeek!  That un-filled-in
    hole actually causes GHC to crash with "fvProv falls into a hole"
    See Trac #11563, #11520, #11516, #11399

So it's important to use 'checkNoErrs' here!

207 208 209 210 211 212 213 214 215 216 217 218 219 220 221
Note [When to do type-class defaulting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In GHC 7.6 and 7.8.2, we did type-class defaulting only if insolubleWC
was false, on the grounds that defaulting can't help solve insoluble
constraints.  But if we *don't* do defaulting we may report a whole
lot of errors that would be solved by defaulting; these errors are
quite spurious because fixing the single insoluble error means that
defaulting happens again, which makes all the other errors go away.
This is jolly confusing: Trac #9033.

So it seems better to always do type-class defaulting.

However, always doing defaulting does mean that we'll do it in
situations like this (Trac #5934):
   run :: (forall s. GenST s) -> Int
Austin Seipp's avatar
Austin Seipp committed
222
   run = fromInteger 0
223 224 225
We don't unify the return type of fromInteger with the given function
type, because the latter involves foralls.  So we're left with
    (Num alpha, alpha ~ (forall s. GenST s) -> Int)
Austin Seipp's avatar
Austin Seipp committed
226 227
Now we do defaulting, get alpha := Integer, and report that we can't
match Integer with (forall s. GenST s) -> Int.  That's not totally
228 229 230 231 232 233
stupid, but perhaps a little strange.

Another potential alternative would be to suppress *all* non-insoluble
errors if there are *any* insoluble errors, anywhere, but that seems
too drastic.

234 235 236 237 238 239 240 241 242
Note [Must simplify after defaulting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We may have a deeply buried constraint
    (t:*) ~ (a:Open)
which we couldn't solve because of the kind incompatibility, and 'a' is free.
Then when we default 'a' we can solve the constraint.  And we want to do
that before starting in on type classes.  We MUST do it before reporting
errors, because it isn't an error!  Trac #7967 was due to this.

243 244
Note [Top-level Defaulting Plan]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
245 246
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
247
       simple constraints, maybe deep inside the context of implications.
248
       This used to be the case in GHC 7.4.1.
249
   (ii) Do it in a tight loop at simplifyTop, once all other constraints have
250 251
        finished. This is the current story.

252
Option (i) had many disadvantages:
253 254
   a) Firstly, it was deep inside the actual solver.
   b) Secondly, it was dependent on the context (Infer a type signature,
255 256
      or Check a type signature, or Interactive) since we did not want
      to always start defaulting when inferring (though there is an exception to
257
      this, see Note [Default while Inferring]).
258 259 260 261 262 263 264 265 266 267 268
   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
269
go with option (ii), implemented at SimplifyTop. Namely:
270 271
     - First, have a go at solving the residual constraint of the whole
       program
272 273
     - Try to approximate it with a simple constraint
     - Figure out derived defaulting equations for that simple constraint
274 275 276 277
     - 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:
278
     - At the top-level, once you had a go at solving the constraint, do
Gabor Greif's avatar
Gabor Greif committed
279
       figure out /all/ the touchable unification variables of the wanted constraints.
280 281 282
     - Apply defaulting to their kinds

More details in Note [DefaultTyVar].
283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325

Note [Safe Haskell Overlapping Instances]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In Safe Haskell, we apply an extra restriction to overlapping instances. The
motive is to prevent untrusted code provided by a third-party, changing the
behavior of trusted code through type-classes. This is due to the global and
implicit nature of type-classes that can hide the source of the dictionary.

Another way to state this is: if a module M compiles without importing another
module N, changing M to import N shouldn't change the behavior of M.

Overlapping instances with type-classes can violate this principle. However,
overlapping instances aren't always unsafe. They are just unsafe when the most
selected dictionary comes from untrusted code (code compiled with -XSafe) and
overlaps instances provided by other modules.

In particular, in Safe Haskell at a call site with overlapping instances, we
apply the following rule to determine if it is a 'unsafe' overlap:

 1) Most specific instance, I1, defined in an `-XSafe` compiled module.
 2) I1 is an orphan instance or a MPTC.
 3) At least one overlapped instance, Ix, is both:
    A) from a different module than I1
    B) Ix is not marked `OVERLAPPABLE`

This is a slightly involved heuristic, but captures the situation of an
imported module N changing the behavior of existing code. For example, if
condition (2) isn't violated, then the module author M must depend either on a
type-class or type defined in N.

Secondly, when should these heuristics be enforced? We enforced them when the
type-class method call site is in a module marked `-XSafe` or `-XTrustworthy`.
This allows `-XUnsafe` modules to operate without restriction, and for Safe
Haskell inferrence to infer modules with unsafe overlaps as unsafe.

One alternative design would be to also consider if an instance was imported as
a `safe` import or not and only apply the restriction to instances imported
safely. However, since instances are global and can be imported through more
than one path, this alternative doesn't work.

Note [Safe Haskell Overlapping Instances Implementation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

326
How is this implemented? It's complicated! So we'll step through it all:
327 328

 1) `InstEnv.lookupInstEnv` -- Performs instance resolution, so this is where
Simon Peyton Jones's avatar
Simon Peyton Jones committed
329 330 331 332
    we check if a particular type-class method call is safe or unsafe. We do this
    through the return type, `ClsInstLookupResult`, where the last parameter is a
    list of instances that are unsafe to overlap. When the method call is safe,
    the list is null.
333

334
 2) `TcInteract.matchClassInst` -- This module drives the instance resolution
Simon Peyton Jones's avatar
Simon Peyton Jones committed
335 336 337
    / dictionary generation. The return type is `LookupInstResult`, which either
    says no instance matched, or one found, and if it was a safe or unsafe
    overlap.
338 339

 3) `TcInteract.doTopReactDict` -- Takes a dictionary / class constraint and
Simon Peyton Jones's avatar
Simon Peyton Jones committed
340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386
     tries to resolve it by calling (in part) `matchClassInst`. The resolving
     mechanism has a work list (of constraints) that it process one at a time. If
     the constraint can't be resolved, it's added to an inert set. When compiling
     an `-XSafe` or `-XTrustworthy` module, we follow this approach as we know
     compilation should fail. These are handled as normal constraint resolution
     failures from here-on (see step 6).

     Otherwise, we may be inferring safety (or using `-Wunsafe`), and
     compilation should succeed, but print warnings and/or mark the compiled module
     as `-XUnsafe`. In this case, we call `insertSafeOverlapFailureTcS` which adds
     the unsafe (but resolved!) constraint to the `inert_safehask` field of
     `InertCans`.

 4) `TcSimplify.simplifyTop`:
       * Call simpl_top, the top-level function for driving the simplifier for
         constraint resolution.

       * Once finished, call `getSafeOverlapFailures` to retrieve the
         list of overlapping instances that were successfully resolved,
         but unsafe. Remember, this is only applicable for generating warnings
         (`-Wunsafe`) or inferring a module unsafe. `-XSafe` and `-XTrustworthy`
         cause compilation failure by not resolving the unsafe constraint at all.

       * For unresolved constraints (all types), call `TcErrors.reportUnsolved`,
         while for resolved but unsafe overlapping dictionary constraints, call
         `TcErrors.warnAllUnsolved`. Both functions convert constraints into a
         warning message for the user.

       * In the case of `warnAllUnsolved` for resolved, but unsafe
         dictionary constraints, we collect the generated warning
         message (pop it) and call `TcRnMonad.recordUnsafeInfer` to
         mark the module we are compiling as unsafe, passing the
         warning message along as the reason.

 5) `TcErrors.*Unsolved` -- Generates error messages for constraints by
    actually calling `InstEnv.lookupInstEnv` again! Yes, confusing, but all we
    know is the constraint that is unresolved or unsafe. For dictionary, all we
    know is that we need a dictionary of type C, but not what instances are
    available and how they overlap. So we once again call `lookupInstEnv` to
    figure that out so we can generate a helpful error message.

 6) `TcRnMonad.recordUnsafeInfer` -- Save the unsafe result and reason in an
      IORef called `tcg_safeInfer`.

 7) `HscMain.tcRnModule'` -- Reads `tcg_safeInfer` after type-checking, calling
    `HscMain.markUnsafeInfer` (passing the reason along) when safe-inferrence
    failed.
387 388 389 390 391 392 393 394 395 396

Note [No defaulting in the ambiguity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When simplifying constraints for the ambiguity check, we use
solveWantedsAndDrop, not simpl_top, so that we do no defaulting.
Trac #11947 was an example:
   f :: Num a => Int -> Int
This is ambiguous of course, but we don't want to default the
(Num alpha) constraint to (Num Int)!  Doing so gives a defaulting
warning, but no error.
Austin Seipp's avatar
Austin Seipp committed
397
-}
398

399
------------------
400 401 402
simplifyAmbiguityCheck :: Type -> WantedConstraints -> TcM ()
simplifyAmbiguityCheck ty wanteds
  = do { traceTc "simplifyAmbiguityCheck {" (text "type = " <+> ppr ty $$ text "wanted = " <+> ppr wanteds)
403 404 405
       ; (final_wc, _) <- runTcS $ solveWantedsAndDrop wanteds
             -- NB: no defaulting!  See Note [No defaulting in the ambiguity check]

406 407 408 409 410
       ; traceTc "End simplifyAmbiguityCheck }" empty

       -- Normally report all errors; but with -XAllowAmbiguousTypes
       -- report only insoluble ones, since they represent genuinely
       -- inaccessible code
411
       ; allow_ambiguous <- xoptM LangExt.AllowAmbiguousTypes
412
       ; traceTc "reportUnsolved(ambig) {" empty
413
       ; tc_lvl <- TcM.getTcLevel
414
       ; unless (allow_ambiguous && not (insolubleWC tc_lvl final_wc))
415
                (discardResult (reportUnsolved final_wc))
416 417 418 419
       ; traceTc "reportUnsolved(ambig) }" empty

       ; return () }

420 421
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
422
simplifyInteractive wanteds
423
  = traceTc "simplifyInteractive" empty >>
424
    simplifyTop wanteds
425 426

------------------
427
simplifyDefault :: ThetaType    -- Wanted; has no type variables in it
428
                -> TcM ()       -- Succeeds if the constraint is soluble
429
simplifyDefault theta
430
  = do { traceTc "simplifyInteractive" empty
431 432 433 434 435
       ; loc <- getCtLocM DefaultOrigin Nothing
       ; let wanted = [ CtDerived { ctev_pred = pred
                                  , ctev_loc  = loc }
                      | pred <- theta ]
       ; unsolved <- runTcSDeriveds (solveWanteds (mkSimpleWC wanted))
436
       ; traceTc "reportUnsolved {" empty
437
       ; reportAllUnsolved unsolved
438
       ; traceTc "reportUnsolved }" empty
439
       ; return () }
440

441 442 443
------------------
tcCheckSatisfiability :: Bag EvVar -> TcM Bool
-- Return True if satisfiable, False if definitely contradictory
444
tcCheckSatisfiability given_ids
445
  = do { lcl_env <- TcM.getLclEnv
446 447
       ; let given_loc = mkGivenLoc topTcLevel UnkSkol lcl_env
       ; (res, _ev_binds) <- runTcS $
448 449
             do { traceTcS "checkSatisfiability {" (ppr given_ids)
                ; given_cts <- mkGivensWithSuperClasses given_loc (bagToList given_ids)
450
                     -- See Note [Superclasses and satisfiability]
451 452 453 454 455
                ; insols <- solveSimpleGivens given_cts
                ; insols <- try_harder insols
                ; traceTcS "checkSatisfiability }" (ppr insols)
                ; return (isEmptyBag insols) }
       ; return res }
456
 where
457 458 459
    try_harder :: Cts -> TcS Cts
    -- Maybe we have to search up the superclass chain to find
    -- an unsatisfiable constraint.  Example: pmcheck/T3927b.
460
    -- At the moment we try just once
461 462 463 464 465 466
    try_harder insols
      | not (isEmptyBag insols)   -- We've found that it's definitely unsatisfiable
      = return insols             -- Hurrah -- stop now.
      | otherwise
      = do { pending_given <- getPendingScDicts
           ; new_given <- makeSuperClasses pending_given
467 468
           ; solveSimpleGivens new_given }

469
{- Note [Superclasses and satisfiability]
470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Expand superclasses before starting, because (Int ~ Bool), has
(Int ~~ Bool) as a superclass, which in turn has (Int ~N# Bool)
as a superclass, and it's the latter that is insoluble.  See
Note [The equality types story] in TysPrim.

If we fail to prove unsatisfiability we (arbitrarily) try just once to
find superclasses, using try_harder.  Reason: we might have a type
signature
   f :: F op (Implements push) => ..
where F is a type function.  This happened in Trac #3972.

We could do more than once but we'd have to have /some/ limit: in the
the recurisve case, we would go on forever in the common case where
the constraints /are/ satisfiable (Trac #10592 comment:12!).

For stratightforard situations without type functions the try_harder
step does nothing.

489

490
***********************************************************************************
491
*                                                                                 *
492 493 494
*                            Inference
*                                                                                 *
***********************************************************************************
495

496 497 498 499 500 501 502 503
Note [Inferring the type of a let-bound variable]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
   f x = rhs

To infer f's type we do the following:
 * Gather the constraints for the RHS with ambient level *one more than*
   the current one.  This is done by the call
504
        pushLevelAndCaptureConstraints (tcMonoBinds...)
505 506 507 508
   in TcBinds.tcPolyInfer

 * Call simplifyInfer to simplify the constraints and decide what to
   quantify over. We pass in the level used for the RHS constraints,
509
   here called rhs_tclvl.
510 511 512 513

This ensures that the implication constraint we generate, if any,
has a strictly-increased level compared to the ambient level outside
the let binding.
514

Austin Seipp's avatar
Austin Seipp committed
515
-}
516

517
simplifyInfer :: TcLevel               -- Used when generating the constraints
518
              -> Bool                  -- Apply monomorphism restriction
519
              -> [TcIdSigInst]         -- Any signatures (possibly partial)
520 521
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
522
              -> WantedConstraints
523
              -> TcM ([TcTyVar],    -- Quantify over these type variables
524
                      [EvVar],      -- ... and these constraints (fully zonked)
525
                      TcEvBinds)    -- ... binding these evidence variables
526
simplifyInfer rhs_tclvl apply_mr sigs name_taus wanteds
527
  | isEmptyWC wanteds
528
  = do { gbl_tvs <- tcGetGlobalTyCoVars
529
       ; dep_vars <- zonkTcTypesAndSplitDepVars (map snd name_taus)
530
       ; qtkvs <- quantifyZonkedTyVars gbl_tvs dep_vars
531
       ; traceTc "simplifyInfer: empty WC" (ppr name_taus $$ ppr qtkvs)
532
       ; return (qtkvs, [], emptyTcEvBinds) }
533

534
  | otherwise
535
  = do { traceTc "simplifyInfer {"  $ vcat
536 537 538 539 540
             [ text "sigs =" <+> ppr sigs
             , text "binds =" <+> ppr name_taus
             , text "rhs_tclvl =" <+> ppr rhs_tclvl
             , text "apply_mr =" <+> ppr apply_mr
             , text "(unzonked) wanted =" <+> ppr wanteds
541 542
             ]

543 544
       ; let partial_sigs = filter isPartialSig sigs
             psig_theta   = concatMap sig_inst_theta partial_sigs
545

546 547 548 549 550 551 552
       -- First do 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.
553

554 555 556 557 558 559 560 561 562 563 564
       ; tc_lcl_env      <- TcM.getLclEnv
       ; ev_binds_var    <- TcM.newTcEvBinds
       ; psig_theta_vars <- mapM TcM.newEvVar psig_theta
       ; wanted_transformed_incl_derivs
            <- setTcLevel rhs_tclvl $
               runTcSWithEvBinds False (Just ev_binds_var) $
               do { let loc = mkGivenLoc rhs_tclvl UnkSkol tc_lcl_env
                  ; psig_givens <- mkGivensWithSuperClasses loc psig_theta_vars
                  ; _ <- solveSimpleGivens psig_givens
                         -- See Note [Add signature contexts as givens]
                  ; solveWanteds wanteds }
565
       ; wanted_transformed_incl_derivs <- TcM.zonkWC wanted_transformed_incl_derivs
566

567 568
       -- Find quant_pred_candidates, the predicates that
       -- we'll consider quantifying over
569 570 571 572
       -- NB1: wanted_transformed does not include anything provable from
       --      the psig_theta; it's just the extra bit
       -- NB2: We do not do any defaulting when inferring a type, this can lead
       --      to less polymorphic types, see Note [Default while Inferring]
573

574
       ; let wanted_transformed = dropDerivedWC wanted_transformed_incl_derivs
575
       ; quant_pred_candidates   -- Fully zonked
576
           <- if insolubleWC rhs_tclvl wanted_transformed_incl_derivs
577
              then return []   -- See Note [Quantification with errors]
578
                               -- NB: must include derived errors in this test,
579 580 581
                               --     hence "incl_derivs"

              else do { let quant_cand = approximateWC wanted_transformed
niteria's avatar
niteria committed
582 583
                            meta_tvs   = filter isMetaTyVar $
                                         tyCoVarsOfCtsList quant_cand
584 585

                      ; gbl_tvs <- tcGetGlobalTyCoVars
586 587
                            -- Miminise quant_cand.  We are not interested in any evidence
                            -- produced, because we are going to simplify wanted_transformed
588
                            -- again later. All we want here are the predicates over which to
589
                            -- quantify.
590
                            --
591 592
                            -- If any meta-tyvar unifications take place (unlikely),
                            -- we'll pick that up later.
593

594 595 596 597 598 599 600
                      -- See Note [Promote _and_ default when inferring]
                      ; let def_tyvar tv
                              = when (not $ tv `elemVarSet` gbl_tvs) $
                                defaultTyVar tv
                      ; mapM_ def_tyvar meta_tvs
                      ; mapM_ (promoteTyVar rhs_tclvl) meta_tvs

601
                      ; WC { wc_simple = simples }
602 603
                           <- setTcLevel rhs_tclvl $
                              runTcSDeriveds       $
604 605 606 607
                              solveSimpleWanteds   $
                              mapBag toDerivedCt quant_cand
                                -- NB: we don't want evidence,
                                -- so use Derived constraints
608 609

                      ; simples <- TcM.zonkSimples simples
610

611
                      ; return [ ctEvPred ev | ct <- bagToList simples
612
                                             , let ev = ctEvidence ct ] }
613

614
       -- NB: quant_pred_candidates is already fully zonked
615

616
       -- Decide what type variables and constraints to quantify
617 618 619 620
       -- NB: bound_theta are constraints we want to quantify over,
       --     /apart from/ the psig_theta, which we always quantify over
       ; (qtvs, bound_theta) <- decideQuantification apply_mr name_taus psig_theta
                                                     quant_pred_candidates
621 622 623 624 625 626 627 628 629 630 631 632 633

         -- Promote any type variables that are free in the inferred type
         -- of the function:
         --    f :: forall qtvs. bound_theta => zonked_tau
         -- These variables now become free in the envt, and hence will show
         -- up whenever 'f' is called.  They may currently at rhs_tclvl, but
         -- they had better be unifiable at the outer_tclvl!
         -- Example:   envt mentions alpha[1]
         --            tau_ty = beta[2] -> beta[2]
         --            consraints = alpha ~ [beta]
         -- we don't quantify over beta (since it is fixed by envt)
         -- so we must promote it!  The inferred type is just
         --   f :: beta -> beta
634
       ; zonked_taus <- mapM (TcM.zonkTcType . snd) name_taus
635 636 637
              -- decideQuantification turned some meta tyvars into
              -- quantified skolems, so we have to zonk again

638
       ; let phi_tkvs = tyCoVarsOfTypes bound_theta  -- Already zonked
639
                        `unionVarSet` tyCoVarsOfTypes zonked_taus
640
             promote_tkvs = closeOverKinds phi_tkvs `delVarSetList` qtvs
641

642 643 644 645 646
       ; MASSERT2( closeOverKinds promote_tkvs `subVarSet` promote_tkvs
                 , ppr phi_tkvs $$
                   ppr (closeOverKinds phi_tkvs) $$
                   ppr promote_tkvs $$
                   ppr (closeOverKinds promote_tkvs) )
647 648 649
           -- we really don't want a type to be promoted when its kind isn't!

           -- promoteTyVar ignores coercion variables
650
       ; outer_tclvl <- TcM.getTcLevel
niteria's avatar
niteria committed
651 652 653
       ; mapM_ (promoteTyVar outer_tclvl) (nonDetEltsUFM promote_tkvs)
           -- It's OK to use nonDetEltsUFM here because promoteTyVar is
           -- commutative
654 655 656

           -- Emit an implication constraint for the
           -- remaining constraints from the RHS
657
           -- extra_qtvs: see Note [Quantification and partial signatures]
658
       ; bound_theta_vars <- mapM TcM.newEvVar bound_theta
659
       ; psig_theta_vars  <- mapM zonkId psig_theta_vars
660 661 662 663
       ; all_qtvs         <- add_psig_tvs qtvs
                             [ tv | sig <- partial_sigs
                                  , (_,tv) <- sig_inst_skols sig ]

664 665 666
       ; let full_theta      = psig_theta      ++ bound_theta
             full_theta_vars = psig_theta_vars ++ bound_theta_vars
             skol_info   = InferSkol [ (name, mkSigmaTy [] full_theta ty)
667
                                     | (name, ty) <- name_taus ]
668
                        -- Don't add the quantified variables here, because
669 670
                        -- they are also bound in ic_skols and we want them
                        -- to be tidied uniformly
671

672
             implic = Implic { ic_tclvl    = rhs_tclvl
673
                             , ic_skols    = all_qtvs
674
                             , ic_no_eqs   = False
675
                             , ic_given    = full_theta_vars
676
                             , ic_wanted   = wanted_transformed
677
                             , ic_status   = IC_Unsolved
678
                             , ic_binds    = Just ev_binds_var
679
                             , ic_info     = skol_info
680
                             , ic_env      = tc_lcl_env }
681
       ; emitImplication implic
682

683
         -- All done!
684
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
685
         vcat [ text "quant_pred_candidates =" <+> ppr quant_pred_candidates
686
              , text "promote_tvs=" <+> ppr promote_tkvs
687
              , text "psig_theta =" <+> ppr psig_theta
688
              , text "bound_theta =" <+> ppr bound_theta
689
              , text "full_theta =" <+> ppr full_theta
690 691
              , text "qtvs =" <+> ppr qtvs
              , text "implic =" <+> ppr implic ]
692

693
       ; return ( qtvs, full_theta_vars, TcEvBinds ev_binds_var ) }
694 695 696 697 698 699 700 701 702 703
  where
    add_psig_tvs qtvs [] = return qtvs
    add_psig_tvs qtvs (tv:tvs)
      = do { tv <- zonkTcTyVarToTyVar tv
           ; if tv `elem` qtvs
             then add_psig_tvs qtvs tvs
             else do { mb_tv <- zonkQuantifiedTyVar False tv
                     ; case mb_tv of
                         Nothing -> add_psig_tvs qtvs      tvs
                         Just tv -> add_psig_tvs (tv:qtvs) tvs } }
704 705 706

{- Note [Add signature contexts as givens]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
707 708 709
Consider this (Trac #11016):
  f2 :: (?x :: Int) => _
  f2 = ?x
710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725
or this
  f3 :: a ~ Bool => (a, _)
  f3 = (True, False)
or theis
  f4 :: (Ord a, _) => a -> Bool
  f4 x = x==x

We'll use plan InferGen because there are holes in the type.  But:
 * For f2 we want to have the (?x :: Int) constraint floating around
   so that the functional dependencies kick in.  Otherwise the
   occurrence of ?x on the RHS produces constraint (?x :: alpha), and
   we won't unify alpha:=Int.
 * For f3 we want the (a ~ Bool) available to solve the wanted (a ~ Bool)
   in the RHS
 * For f4 we want to use the (Ord a) in the signature to solve the Eq a
   constraint.
726 727

Solution: in simplifyInfer, just before simplifying the constraints
728 729
gathered from the RHS, add Given constraints for the context of any
type signatures.
730

Austin Seipp's avatar
Austin Seipp committed
731 732
************************************************************************
*                                                                      *
733
                Quantification
Austin Seipp's avatar
Austin Seipp committed
734 735
*                                                                      *
************************************************************************
736 737 738 739

Note [Deciding quantification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the monomorphism restriction does not apply, then we quantify as follows:
740

741 742 743
  * Take the global tyvars, and "grow" them using the equality constraints
    E.g.  if x:alpha is in the environment, and alpha ~ [beta] (which can
          happen because alpha is untouchable here) then do not quantify over
744 745
          beta, because alpha fixes beta, and beta is effectively free in
          the environment too
746 747 748
    These are the mono_tvs

  * Take the free vars of the tau-type (zonked_tau_tvs) and "grow" them
749
    using all the constraints.  These are tau_tvs_plus
750

751 752 753 754
  * Use quantifyTyVars to quantify over (tau_tvs_plus - mono_tvs), being
    careful to close over kinds, and to skolemise the quantified tyvars.
    (This actually unifies each quantifies meta-tyvar with a fresh skolem.)
    Result is qtvs.
Simon Peyton Jones's avatar
Simon Peyton Jones committed
755

756
  * Filter the constraints using pickQuantifiablePreds and the qtvs.
757 758
    We have to zonk the constraints first, so they "see" the freshly
    created skolems.
759

760 761 762
If the MR does apply, mono_tvs includes all the constrained tyvars --
including all covars -- and the quantified constraints are empty/insoluble.

Austin Seipp's avatar
Austin Seipp committed
763
-}
764

765
decideQuantification
766
  :: Bool                  -- try the MR restriction?
767
  -> [(Name, TcTauType)]   -- Variables to be generalised
768 769
  -> [PredType]            -- All annotated constraints from signatures
  -> [PredType]            -- Candidate theta
770 771
  -> TcM ( [TcTyVar]       -- Quantify over these (skolems)
         , [PredType] )    -- and this context (fully zonked)
772
-- See Note [Deciding quantification]
773
decideQuantification apply_mr name_taus psig_theta candidates
774
  = do { gbl_tvs <- tcGetGlobalTyCoVars
775 776 777
       ; zonked_taus <- mapM TcM.zonkTcType (psig_theta ++ taus)
                        -- psig_theta: see Note [Quantification and partial signatures]
       ; let DV { dv_kvs = zkvs, dv_tvs = ztvs} = splitDepVarsOfTypes zonked_taus
778 779 780 781 782 783 784 785 786 787 788 789
             (gbl_cand, quant_cand)  -- gbl_cand   = do not quantify me
                = case apply_mr of   -- quant_cand = try to quantify me
                    True  -> (candidates, [])
                    False -> ([], candidates)
             zonked_tkvs     = dVarSetToVarSet zkvs `unionVarSet` dVarSetToVarSet ztvs
             eq_constraints  = filter isEqPred quant_cand
             constrained_tvs = tyCoVarsOfTypes gbl_cand
             mono_tvs        = growThetaTyVars eq_constraints $
                               gbl_tvs `unionVarSet` constrained_tvs
             tau_tvs_plus    = growThetaTyVarsDSet quant_cand ztvs
             dvs_plus        = DV { dv_kvs = zkvs, dv_tvs = tau_tvs_plus }

790
       ; qtvs <- quantifyZonkedTyVars mono_tvs dvs_plus
791 792 793 794 795
          -- We don't grow the kvs, as there's no real need to. Recall
          -- that quantifyTyVars uses the separation between kvs and tvs
          -- only for defaulting, and we don't want (ever) to default a tv
          -- to *. So, don't grow the kvs.

796
       ; quant_cand <- TcM.zonkTcTypes quant_cand
Simon Peyton Jones's avatar
Simon Peyton Jones committed
797
                 -- quantifyTyVars turned some meta tyvars into
798 799
                 -- quantified skolems, so we have to zonk again

800
       ; let qtv_set   = mkVarSet qtvs
801
             theta     = pickQuantifiablePreds qtv_set quant_cand
802 803 804
             min_theta = mkMinimalBySCs theta
               -- See Note [Minimize by Superclasses]

805 806 807 808 809 810 811 812 813 814
           -- Warn about the monomorphism restriction
       ; warn_mono <- woptM Opt_WarnMonomorphism
       ; let mr_bites = constrained_tvs `intersectsVarSet` zonked_tkvs
       ; warnTc (Reason Opt_WarnMonomorphism) (warn_mono && mr_bites) $
         hang (text "The Monomorphism Restriction applies to the binding"
               <> plural bndrs <+> text "for" <+> pp_bndrs)
             2 (text "Consider giving a type signature for"
                <+> if isSingleton bndrs then pp_bndrs
                                         else text "these binders")

815
       ; traceTc "decideQuantification 2"
816 817
           (vcat [ text "gbl_cand:"     <+> ppr gbl_cand
                 , text "quant_cand:"   <+> ppr quant_cand
818 819 820 821 822
                 , text "gbl_tvs:"      <+> ppr gbl_tvs
                 , text "mono_tvs:"     <+> ppr mono_tvs
                 , text "tau_tvs_plus:" <+> ppr tau_tvs_plus
                 , text "qtvs:"         <+> ppr qtvs
                 , text "min_theta:"    <+> ppr min_theta ])
823
       ; return (qtvs, min_theta) }
824
  where
825
    pp_bndrs = pprWithCommas (quotes . ppr) bndrs
826
    (bndrs, taus) = unzip name_taus
827

828
------------------
829
growThetaTyVars :: ThetaType -> TyCoVarSet -> TyVarSet
830
-- See Note [Growing the tau-tvs using constraints]
831
-- NB: only returns tyvars, never covars
832
growThetaTyVars theta tvs
833 834 835
  | null theta = tvs_only
  | otherwise  = filterVarSet isTyVar $
                 transCloVarSet mk_next seed_tvs
836
  where
837 838
    tvs_only = filterVarSet isTyVar tvs
    seed_tvs = tvs `unionVarSet` tyCoVarsOfTypes ips
839
    (ips, non_ips) = partition isIPPred theta
840
                         -- See Note [Inheriting implicit parameters] in TcType
841 842 843 844 845 846

    mk_next :: VarSet -> VarSet -- Maps current set to newly-grown ones
    mk_next so_far = foldr (grow_one so_far) emptyVarSet non_ips
    grow_one so_far pred tvs
       | pred_tvs `intersectsVarSet` so_far = tvs `unionVarSet` pred_tvs
       | otherwise                          = tvs
847
       where
848
         pred_tvs = tyCoVarsOfType pred
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
------------------
growThetaTyVarsDSet :: ThetaType -> DTyCoVarSet -> DTyVarSet
-- See Note [Growing the tau-tvs using constraints]
-- NB: only returns tyvars, never covars
-- It takes a deterministic set of TyCoVars and returns a deterministic set
-- of TyVars.
-- The implementation mirrors growThetaTyVars, the only difference is that
-- it avoids unionDVarSet and uses more efficient extendDVarSetList.
growThetaTyVarsDSet theta tvs
  | null theta = tvs_only
  | otherwise  = filterDVarSet isTyVar $
                 transCloDVarSet mk_next seed_tvs
  where
    tvs_only = filterDVarSet isTyVar tvs
    seed_tvs = tvs `extendDVarSetList` tyCoVarsOfTypesList ips
    (ips, non_ips) = partition isIPPred theta
                         -- See Note [Inheriting implicit parameters] in TcType

    mk_next :: DVarSet -> DVarSet -- Maps current set to newly-grown ones
    mk_next so_far = foldr (grow_one so_far) emptyDVarSet non_ips
    grow_one so_far pred tvs
       | any (`elemDVarSet` so_far) pred_tvs = tvs `extendDVarSetList` pred_tvs
       | otherwise                           = tvs
       where
         pred_tvs = tyCoVarsOfTypeList pred

876 877
{- Note [Quantification and partial signatures]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
878 879 880 881
When choosing type variables to quantify, the basic plan is to
quantify over all type variables that are
 * free in the tau_tvs, and
 * not forced to be monomorphic (mono_tvs),
882
   for example by being free in the environment.
883

884 885
However, in the case of a partial type signature, be doing inference
*in the presence of a type signature*. For example:
886 887
   f :: _ -> a
   f x = ...
888 889
or
   g :: (Eq _a) => _b -> _b
890
In both cases we use plan InferGen, and hence call simplifyInfer.
891
But those 'a' variables are skolems, and we should be sure to quantify
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
over them, for two reasons

* In the case of a type error
     f :: _ -> Maybe a
     f x = True && x
  The inferred type of 'f' is f :: Bool -> Bool, but there's a
  left-over error of form (HoleCan (Maybe a ~ Bool)).  The error-reporting
  machine expects to find a binding site for the skolem 'a', so we
  add it to the ic_skols of the residual implication.

  Note that we /only/ do this to the residual implication. We don't
  complicate the quantified type varialbes of 'f' for downstream code;
  it's just a device to make the error message generator know what to
  report.

* Consider the partial type signature
     f :: (Eq _) => Int -> Int
     f x = x
  In normal cases that makes sense; e.g.
     g :: Eq _a => _a -> _a
     g x = x
  where the signature makes the type less general than it could
  be. But for 'f' we must therefore quantify over the user-annotated
  constraints, to get
     f :: forall a. Eq a => Int -> Int
  (thereby correctly triggering an ambiguity error later).  If we don't
  we'll end up with a strange open type
     f :: Eq alpha => Int -> Int
  which isn't ambiguous but is still very wrong.  That's why include
  psig_theta in the variables to quantify over, passed to
  decideQuantification.
923

924 925 926 927 928 929 930 931
Note [Quantifying over equality constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Should we quantify over an equality constraint (s ~ t)?  In general, we don't.
Doing so may simply postpone a type error from the function definition site to
its call site.  (At worst, imagine (Int ~ Bool)).

However, consider this
         forall a. (F [a] ~ Int) => blah
932
Should we quantify over the (F [a] ~ Int)?  Perhaps yes, because at the call
933 934 935 936
site we will know 'a', and perhaps we have instance  F [Bool] = Int.
So we *do* quantify over a type-family equality where the arguments mention
the quantified variables.

937 938 939
Note [Growing the tau-tvs using constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(growThetaTyVars insts tvs) is the result of extending the set
940
    of tyvars, tvs, using all conceivable links from pred
941 942 943 944 945 946 947 948

E.g. tvs = {a}, preds = {H [a] b, K (b,Int) c, Eq e}
Then growThetaTyVars preds tvs = {a,b,c}

Notice that
   growThetaTyVars is conservative       if v might be fixed by vs
                                         => v `elem` grow(vs,C)

949 950 951 952 953 954 955 956
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
957

958
But NB that we must include *derived* errors in the check. Example:
959 960 961
    (a::*) ~ Int#
We get an insoluble derived error *~#, and we don't want to discard
it before doing the isInsolubleWC test!  (Trac #8262)
962

963 964
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
965
Our current plan is that defaulting only happens at simplifyTop and
966
not simplifyInfer.  This may lead to some insoluble deferred constraints.
967 968
Example:

969
instance D g => C g Int b
970 971

constraint inferred = (forall b. 0 => C gamma alpha b) /\ Num alpha
Simon Peyton Jones's avatar