TcSimplify.lhs 58 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, simplifyDeriv, 
12 13
       simplifyRule, simplifyTop, simplifyInteractive
  ) where
14

15
#include "HsVersions.h"
16

17
import TcRnMonad
18
import TcErrors
19
import TcMType
20 21
import TcType 
import TcSMonad 
22
import TcInteract 
23
import Inst
24
import Unify	( niFixTvSubst, niSubstTvSet )
25
import Type     ( classifyPredType, PredTree(..) )
26
import Var
27
import VarSet
28
import VarEnv 
29
import TcEvidence
30
import TypeRep
31
import Name
32
import Bag
33 34
import ListSetOps
import Util
35 36 37
import PrelInfo
import PrelNames
import Class		( classKey )
38
import BasicTypes       ( RuleName )
39
import Control.Monad    ( when )
40
import Outputable
41
import FastString
dimitris's avatar
dimitris committed
42
import TrieMap () -- DV: for now
43
import DynFlags
44

45 46 47
\end{code}


48 49 50 51 52
*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
53

54 55 56
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
57 58 59
-- 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
60
simplifyTop wanteds 
61
  = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds
62

63 64 65 66
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
  = simplifyCheck (SimplCheck (ptext (sLit "ambiguity check for") <+> ppr name)) wanteds
67
 
68 69 70 71 72 73 74 75 76
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
  = simplifyCheck SimplInteractive wanteds

------------------
simplifyDefault :: ThetaType	-- Wanted; has no type variables in it
                -> TcM ()	-- Succeeds iff the constraint is soluble
simplifyDefault theta
77
  = do { wanted <- newFlatWanteds DefaultOrigin theta
78 79
       ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) 
                                            (mkFlatWC wanted)
80 81
       ; return () }
\end{code}
82

83

84
***********************************************************************************
85
*                                                                                 * 
86
*                            Deriving                                             *
87 88
*                                                                                 *
***********************************************************************************
89

90 91
\begin{code}
simplifyDeriv :: CtOrigin
92 93 94 95
              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
96 97
-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
98
-- Fail if not possible
99
simplifyDeriv orig pred tvs theta 
100
  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
101 102 103 104
      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
105

106
       ; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
107
             skol_set   = mkVarSet tvs_skols
108
	     doc = parens $ ptext (sLit "deriving") <+> parens (ppr pred)
109 110 111

       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

112
       ; traceTc "simplifyDeriv" (pprTvBndrs tvs $$ ppr theta $$ ppr wanted)
113 114 115
       ; (residual_wanted, _ev_binds1)
             <- runTcS (SimplInfer doc) NoUntouchables emptyInert emptyWorkList $
                solveWanteds $ mkFlatWC wanted
116

117 118
       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
119 120 121
             get_good :: Ct -> Either PredType Ct
             get_good ct | validDerivPred skol_set p = Left p
                         | otherwise                 = Right ct
122
                         where p = ctPred ct
123

124 125 126
       -- We never want to defer these errors because they are errors in the
       -- compiler! Hence the `False` below
       ; _ev_binds2 <- reportUnsolved False (residual_wanted { wc_flat = bad })
127

128 129
       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
130
\end{code}
131

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156
Note [Overlap and deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider some overlapping instances:
  data Show a => Show [a] where ..
  data Show [Char] where ...

Now a data type with deriving:
  data T a = MkT [a] deriving( Show )

We want to get the derived instance
  instance Show [a] => Show (T a) where...
and NOT
  instance Show a => Show (T a) where...
so that the (Show (T Char)) instance does the Right Thing

It's very like the situation when we're inferring the type
of a function
   f x = show [x]
and we want to infer
   f :: Show [a] => a -> String

BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
             the context for the derived instance. 
	     Hence tcInstSkolTyVars not tcInstSuperSkolTyVars

157 158 159 160 161 162 163
Note [Exotic derived instance contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a 'derived' instance declaration, we *infer* the context.  It's a
bit unclear what rules we should apply for this; the Haskell report is
silent.  Obviously, constraints like (Eq a) are fine, but what about
	data T f a = MkT (f a) deriving( Eq )
where we'd get an Eq (f a) constraint.  That's probably fine too.
164

165 166 167
One could go further: consider
	data T a b c = MkT (Foo a b c) deriving( Eq )
	instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
168

169 170
Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
171

172 173 174 175
	instance (C Int a, Eq b, Eq c) => Eq (T a b c) 
even though there is no instance for (C Int a), because there just
*might* be an instance for, say, (C Int Bool) at a site where we
need the equality instance for T's.  
176

177 178 179
However, this seems pretty exotic, and it's quite tricky to allow
this, and yet give sensible error messages in the (much more common)
case where we really want that instance decl for C.
180

181 182
So for now we simply require that the derived instance context
should have only type-variable constraints.
183

184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212
Here is another example:
	data Fix f = In (f (Fix f)) deriving( Eq )
Here, if we are prepared to allow -XUndecidableInstances we
could derive the instance
	instance Eq (f (Fix f)) => Eq (Fix f)
but this is so delicate that I don't think it should happen inside
'deriving'. If you want this, write it yourself!

NB: if you want to lift this condition, make sure you still meet the
termination conditions!  If not, the deriving mechanism generates
larger and larger constraints.  Example:
  data Succ a = S a
  data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show

Note the lack of a Show instance for Succ.  First we'll generate
  instance (Show (Succ a), Show a) => Show (Seq a)
and then
  instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
and so on.  Instead we want to complain of no instance for (Show (Succ a)).

The bottom line
~~~~~~~~~~~~~~~
Allow constraints which consist only of type variables, with no repeats.

*********************************************************************************
*                                                                                 * 
*                            Inference
*                                                                                 *
***********************************************************************************
213

dreixel's avatar
dreixel committed
214 215 216 217 218 219 220 221 222 223 224 225
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.

226
\begin{code}
227
simplifyInfer :: Bool
228 229 230
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
231 232 233
              -> WantedConstraints
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
234 235 236
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
237
                      TcEvBinds)    -- ... binding these evidence variables
238
simplifyInfer _top_lvl apply_mr name_taus wanteds
239 240 241
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
242
       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
dreixel's avatar
dreixel committed
243 244 245
       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
246
       ; return (qtvs, [], False, emptyTcEvBinds) }
247

248
  | otherwise
249 250
  = do { zonked_wanteds <- zonkWC wanteds
       ; gbl_tvs        <- tcGetGlobalTyVars
251
       ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
252
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
253

254
       ; traceTc "simplifyInfer {"  $ vcat
255
             [ ptext (sLit "names =") <+> ppr (map fst name_taus)
256 257
             , ptext (sLit "taus =") <+> ppr (map snd name_taus)
             , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs
258 259 260
             , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
261
             , ptext (sLit "wanted =") <+> ppr zonked_wanteds
262 263
             ]

264 265
             -- Step 1
             -- Make a guess at the quantified type variables
266 267 268
	     -- Then split the constraints on the baisis of those tyvars
	     -- to avoid unnecessarily simplifying a class constraint
	     -- See Note [Avoid unecessary constraint simplification]
269
       ; let proto_qtvs = growWanteds gbl_tvs zonked_wanteds $
270
                          zonked_tau_tvs `minusVarSet` gbl_tvs
271 272 273 274 275 276 277 278 279
             (perhaps_bound, surely_free)
                        = partitionBag (quantifyMe proto_qtvs) (wc_flat zonked_wanteds)

       ; traceTc "simplifyInfer proto"  $ vcat
             [ ptext (sLit "zonked_tau_tvs =") <+> ppr zonked_tau_tvs
             , ptext (sLit "proto_qtvs =") <+> ppr proto_qtvs
             , ptext (sLit "surely_fref =") <+> ppr surely_free
             ]

280
       ; emitFlats surely_free
281 282 283 284
       ; traceTc "sinf"  $ vcat
             [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound
             , ptext (sLit "surely_free   =") <+> ppr surely_free
             ]
285

286
            -- Step 2 
287 288 289 290 291 292 293 294 295 296 297
            -- Now simplify the possibly-bound constraints
       ; let ctxt = SimplInfer (ppr (map fst name_taus))
       ; (simpl_results, tc_binds)
             <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $ 
                simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound })

            -- Fail fast if there is an insoluble constraint,
            -- unless we are deferring errors to runtime
       ; when (not runtimeCoercionErrors && insolubleWC simpl_results) $ 
         do { _ev_binds <- reportUnsolved False simpl_results 
            ; failM }
298 299 300 301 302

            -- Step 3 
            -- Split again simplified_perhaps_bound, because some unifications 
            -- may have happened, and emit the free constraints. 
       ; gbl_tvs        <- tcGetGlobalTyVars
303
       ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs
304
       ; zonked_flats <- zonkCts (wc_flat simpl_results)
305
       ; let init_tvs 	     = zonked_tau_tvs `minusVarSet` gbl_tvs
306 307
             poly_qtvs       = growWantedEVs gbl_tvs zonked_flats init_tvs
	     (pbound, pfree) = partitionBag (quantifyMe poly_qtvs) zonked_flats
308 309

	     -- Monomorphism restriction
310
             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
311
             constrained_tvs = tyVarsOfCts zonked_flats
312 313 314
	     mr_bites        = apply_mr && not (isEmptyBag pbound)

             (qtvs, (bound, free))
315
                | mr_bites  = (mr_qtvs,   (emptyBag, zonked_flats))
316
                | otherwise = (poly_qtvs, (pbound,   pfree))
317
       ; emitFlats free
318

319
       ; if isEmptyVarSet qtvs && isEmptyBag bound
320 321 322
         then ASSERT( isEmptyBag (wc_insol simpl_results) )
              do { traceTc "} simplifyInfer/no quantification" empty
                 ; emitImplications (wc_impl simpl_results)
323
                 ; return ([], [], mr_bites, EvBinds tc_binds) }
324 325 326
         else do

            -- Step 4, zonk quantified variables 
327
       { let minimal_flat_preds = mkMinimalBySCs $ 
328
                                  map ctPred $ bagToList bound
329 330
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
331 332 333 334
                        -- 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
335
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
336 337 338 339 340

            -- Step 5
            -- Minimize `bound' and emit an implication
       ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds
       ; ev_binds_var <- newTcEvBinds
341 342
       ; mapBagM_ (\(EvBind evar etrm) -> addTcEvBind ev_binds_var evar etrm) 
           tc_binds
343
       ; lcl_env <- getLclTypeEnv
dreixel's avatar
dreixel committed
344
       ; gloc <- getCtLoc skol_info
345 346
       ; let implic = Implic { ic_untch    = NoUntouchables
                             , ic_env      = lcl_env
347
                             , ic_skols    = qtvs_to_return
348 349 350 351 352 353 354 355 356
                             , ic_given    = minimal_bound_ev_vars
                             , ic_wanted   = simpl_results { wc_flat = bound }
                             , ic_insol    = False
                             , ic_binds    = ev_binds_var
                             , ic_loc      = gloc }
       ; emitImplication implic
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ ptext (sLit "implic =") <+> ppr implic
                       -- ic_skols, ic_given give rest of result
357
                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
358
                  , ptext (sLit "spb =") <+> ppr zonked_flats
359 360 361 362
                  , ptext (sLit "bound =") <+> ppr bound ]



363 364
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
365
\end{code}
366 367


368 369
Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
370

371 372 373 374 375 376 377
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.
378

379
\begin{code}
380

381
simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints
382
-- Post: returns only wanteds (no deriveds)
383 384
simplifyWithApprox wanted
 = do { traceTcS "simplifyApproxLoop" (ppr wanted)
385

386
      ; let all_flats = wc_flat wanted `unionBags` keepWanted (wc_insol wanted) 
387 388 389
      ; implics_from_flats <- solveInteractCts $ bagToList all_flats
      ; unsolved_implics <- simpl_loop 1 (wc_impl wanted `unionBags` 
                                               implics_from_flats)
390 391 392 393 394 395

      ; let (residual_implics,floats) = approximateImplications unsolved_implics

      -- Solve extra stuff for real: notice that all the extra unsolved constraints will 
      -- be in the inerts of the monad, so we are OK
      ; traceTcS "simplifyApproxLoop" $ text "Calling solve_wanteds!"
396 397 398 399 400
      ; wants_or_ders <- solve_wanteds (WC { wc_flat  = floats -- They are floated so they are not in the evvar cache
                                           , wc_impl  = residual_implics
                                           , wc_insol = emptyBag })
      ; return $ 
        wants_or_ders { wc_flat = keepWanted (wc_flat wants_or_ders) } }
401

402 403

approximateImplications :: Bag Implication -> (Bag Implication, Cts)
404 405 406
-- Extracts any nested constraints that don't mention the skolems
approximateImplications impls
  = do_bag (float_implic emptyVarSet) impls
407
  where 
408 409 410 411 412
    do_bag :: forall a b c. (a -> (Bag b, Bag c)) -> Bag a -> (Bag b, Bag c)
    do_bag f = foldrBag (plus . f) (emptyBag, emptyBag)
    plus :: forall b c. (Bag b, Bag c) -> (Bag b, Bag c) -> (Bag b, Bag c)
    plus (a1,b1) (a2,b2) = (a1 `unionBags` a2, b1 `unionBags` b2)

413
    float_implic :: TyVarSet -> Implication -> (Bag Implication, Cts)
414 415 416
    float_implic skols imp
      = (unitBag (imp { ic_wanted = wanted' }), floats)
      where
417
        (wanted', floats) = float_wc (skols `extendVarSetList` ic_skols imp) (ic_wanted imp)
418 419 420 421 422 423 424

    float_wc skols wc@(WC { wc_flat = flat, wc_impl = implic })
      = (wc { wc_flat = flat', wc_impl = implic' }, floats1 `unionBags` floats2)
      where
        (flat',   floats1) = do_bag (float_flat   skols) flat
        (implic', floats2) = do_bag (float_implic skols) implic

425 426 427 428
    float_flat :: TcTyVarSet -> Ct -> (Cts, Cts)
    float_flat skols ct
      | tyVarsOfCt ct `disjointVarSet` skols = (emptyBag, unitBag ct)
      | otherwise                            = (unitBag ct, emptyBag)
429
\end{code}
430

431
\begin{code}
432 433
-- (growX gbls wanted tvs) grows a seed 'tvs' against the 
-- X-constraint 'wanted', nuking the 'gbls' at each stage
434 435
-- It's conservative in that if the seed could *possibly*
-- grow to include a type variable, then it does
436

437 438 439
growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)

440
growWantedEVs :: TyVarSet -> Cts -> TyVarSet -> TyVarSet
441 442
growWantedEVs gbl_tvs ws tvs
  | isEmptyBag ws = tvs
443
  | otherwise     = fixVarSet (growPreds gbl_tvs ctPred ws) tvs
444

445 446 447
--------  Helper functions, do not do fixpoint ------------------------
growWC :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWC gbl_tvs wc = growImplics gbl_tvs             (wc_impl wc) .
448 449
                    growPreds   gbl_tvs ctPred (wc_flat wc) .
                    growPreds   gbl_tvs ctPred (wc_insol wc)
450

451 452 453 454 455
growImplics :: TyVarSet -> Bag Implication -> TyVarSet -> TyVarSet
growImplics gbl_tvs implics tvs
  = foldrBag grow_implic tvs implics
  where
    grow_implic implic tvs
456
      = grow tvs `delVarSetList` ic_skols implic
457 458 459 460 461 462 463 464
      where
        grow = growWC gbl_tvs (ic_wanted implic) .
               growPreds gbl_tvs evVarPred (listToBag (ic_given implic))
               -- We must grow from givens too; see test IPRun

growPreds :: TyVarSet -> (a -> PredType) -> Bag a -> TyVarSet -> TyVarSet
growPreds gbl_tvs get_pred items tvs
  = foldrBag extend tvs items
465
  where
466 467
    extend item tvs = tvs `unionVarSet`
                      (growPredTyVars (get_pred item) tvs `minusVarSet` gbl_tvs)
468 469 470

--------------------
quantifyMe :: TyVarSet      -- Quantifying over these
471
	   -> Ct
472
	   -> Bool	    -- True <=> quantify over this wanted
473
quantifyMe qtvs ct
474
  | isIPPred pred = True  -- Note [Inheriting implicit parameters]
batterseapower's avatar
batterseapower committed
475
  | otherwise	  = tyVarsOfType pred `intersectsVarSet` qtvs
476
  where
477
    pred = ctPred ct
478
\end{code}
479

480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When inferring the type of a let-binding, with simplifyInfer,
try to avoid unnecessariliy simplifying class constraints.
Doing so aids sharing, but it also helps with delicate 
situations like
   instance C t => C [t] where ..
   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.


498 499
Note [Inheriting implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
500 501 502
Consider this:

	f x = (x::Int) + ?y
503

504 505 506
where f is *not* a top-level binding.
From the RHS of f we'll get the constraint (?y::Int).
There are two types we might infer for f:
507

508 509 510
	f :: Int -> Int

(so we get ?y from the context of f's definition), or
511 512 513

	f :: (?y::Int) => Int -> Int

514 515 516 517 518 519
At first you might think the first was better, becuase then
?y behaves like a free variable of the definition, rather than
having to be passed at each call site.  But of course, the WHOLE
IDEA is that ?y should be passed at each call site (that's what
dynamic binding means) so we'd better infer the second.

520 521
BOTTOM LINE: when *inferring types* you *must* quantify 
over implicit parameters. See the predicate isFreeWhenInferring.
522

523

524 525 526 527 528
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
529

530
See note [Simplifying RULE consraints] in TcRule
531

532 533 534 535 536 537 538 539 540 541 542 543 544 545
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.
546 547

\begin{code}
548 549 550
simplifyRule :: RuleName 
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
551 552 553 554 555 556 557 558 559 560 561 562
             -> TcM ([EvVar], WantedConstraints)   -- LHS evidence varaibles
-- See Note [Simplifying RULE constraints] in TcRule
simplifyRule name lhs_wanted rhs_wanted
  = do { zonked_all <- zonkWC (lhs_wanted `andWC` rhs_wanted)
       ; let doc = ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
	     untch = NoUntouchables
             	 -- We allow ourselves to unify environment 
		 -- variables; hence NoUntouchables

       ; (resid_wanted, _) <- runTcS (SimplInfer doc) untch emptyInert emptyWorkList $
              solveWanteds zonked_all

563 564
       ; zonked_lhs <- zonkWC lhs_wanted

565 566 567 568 569 570 571 572 573 574 575 576 577
       ; let (q_cts, non_q_cts) = partitionBag quantify_me (wc_flat zonked_lhs)
             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
             
578
       ; traceTc "simplifyRule" $
579 580 581 582
         vcat [ text "zonked_lhs" <+> ppr zonked_lhs 
              , text "q_cts"      <+> ppr q_cts ]

       ; return (map ctId (bagToList q_cts), zonked_lhs { wc_flat = non_q_cts }) }
583 584 585
\end{code}


586 587 588 589 590
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
591 592

\begin{code}
593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608
simplifyCheck :: SimplContext
	      -> WantedConstraints	-- Wanted
              -> TcM (Bag EvBind)
-- Solve a single, top-level implication constraint
-- e.g. typically one created from a top-level type signature
-- 	    f :: forall a. [a] -> [a]
--          f x = rhs
-- We do this even if the function has no polymorphism:
--    	    g :: Int -> Int

--          g y = rhs
-- (whereas for *nested* bindings we would not create
--  an implication constraint for g at all.)
--
-- Fails if can't solve something in the input wanteds
simplifyCheck ctxt wanteds
609
  = do { wanteds <- zonkWC wanteds
610 611 612 613

       ; traceTc "simplifyCheck {" (vcat
             [ ptext (sLit "wanted =") <+> ppr wanteds ])

614 615 616 617 618 619
       ; (unsolved, eb1)
           <- runTcS ctxt NoUntouchables emptyInert emptyWorkList $ 
              solveWanteds wanteds

       ; traceTc "simplifyCheck }" $ ptext (sLit "unsolved =") <+> ppr unsolved

dimitris's avatar
dimitris committed
620
       ; traceTc "reportUnsolved {" empty
621 622 623
       -- See Note [Deferring coercion errors to runtime]
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
       ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved 
dimitris's avatar
dimitris committed
624 625
       ; traceTc "reportUnsolved }" empty

626 627 628 629 630 631 632 633 634 635
       ; return (eb1 `unionBags` eb2) }
\end{code}

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
636

637 638
  a :: Int
  a = 'a'
639

640
  main = print "b"
641

642 643
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`.
644

645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668
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`.

\begin{code}
solveWanteds :: WantedConstraints -> TcS WantedConstraints
669 670
-- Returns: residual constraints, plus evidence bindings 
-- NB: When we are called from TcM there are no inerts to pass down to TcS
671 672
solveWanteds wanted
  = do { wc_out <- solve_wanteds wanted
673 674
       ; let wc_ret = wc_out { wc_flat = keepWanted (wc_flat wc_out) } 
                      -- Discard Derived
675
       ; return wc_ret }
676 677 678 679

solve_wanteds :: WantedConstraints
              -> TcS WantedConstraints  -- NB: wc_flats may be wanted *or* derived now
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
680 681 682
  = do { traceTcS "solveWanteds {" (ppr wanted)

                 -- Try the flat bit
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
683 684 685 686 687
                 -- Discard from insols all the derived/given constraints
                 -- because they will show up again when we try to solve
                 -- everything else.  Solving them a second time is a bit
                 -- of a waste, but the code is simple, and the program is
                 -- wrong anyway!
688

689
       ; let all_flats = flats `unionBags` keepWanted insols
690
       ; impls_from_flats <- solveInteractCts $ bagToList all_flats
691

692 693
       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
694
       ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
695

696 697 698
       ; (insoluble_flats,unsolved_flats) <- extractUnsolvedTcS 

       ; bb <- getTcEvBindsMap
699
       ; tb <- getTcSTyBindsMap
700

701
       ; traceTcS "solveWanteds }" $
702
                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
703
                      , text "unsolved_implics =" <+> ppr unsolved_implics
704
                      , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
705 706 707
                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
                      ]

708
       ; (subst, remaining_unsolved_flats) <- solveCTyFunEqs unsolved_flats
709
                -- See Note [Solving Family Equations]
710 711
                -- NB: remaining_flats has already had subst applied

712 713 714 715 716
       ; traceTcS "solveWanteds finished with" $
                 vcat [ text "remaining_unsolved_flats =" <+> ppr remaining_unsolved_flats
                      , text "subst =" <+> ppr subst
                      ]

717 718 719 720 721 722 723 724 725 726 727 728 729 730
       ; return $ 
         WC { wc_flat  = mapBag (substCt subst) remaining_unsolved_flats
            , wc_impl  = mapBag (substImplication subst) unsolved_implics
            , wc_insol = mapBag (substCt subst) insoluble_flats }
       }

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
  = do { (implic_eqs, unsolved_implics) <- solveNestedImplications implics
731

732 733
       ; inerts <- getTcSInerts
       ; let ((_,unsolved_flats),_) = extractUnsolved inerts
dimitris's avatar
dimitris committed
734
                                      
735 736 737
       ; improve_eqs <- if not (isEmptyBag implic_eqs)
                        then return implic_eqs
                        else applyDefaultingRules unsolved_flats
738

739 740 741 742
       ; traceTcS "solveWanteds: simpl_loop end" $
             vcat [ text "improve_eqs      =" <+> ppr improve_eqs
                  , text "unsolved_flats   =" <+> ppr unsolved_flats
                  , text "unsolved_implics =" <+> ppr unsolved_implics ]
743

744
       ; if isEmptyBag improve_eqs then return unsolved_implics 
745 746 747
         else do { impls_from_eqs <- solveInteractCts $ bagToList improve_eqs
                 ; simpl_loop (n+1) (unsolved_implics `unionBags` 
                                                 impls_from_eqs)} }
748

749 750 751 752 753 754 755 756 757 758
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
       ; let ((_insoluble_flats, unsolved_flats),thinner_inerts) = extractUnsolved inerts 
759

760 761 762 763 764 765 766 767 768
       ; (implic_eqs, unsolved_implics)
           <- doWithInert thinner_inerts $ 
              do { let pushed_givens = givens_from_wanteds unsolved_flats
                       tcs_untouchables = filterVarSet isFlexiTcsTv $ 
                                          tyVarsOfCts unsolved_flats
                 -- See Note [Preparing inert set for implications]
	         -- Push the unsolved wanteds inwards, but as givens
                 ; traceTcS "solveWanteds: preparing inerts for implications {" $ 
                   vcat [ppr tcs_untouchables, ppr pushed_givens]
769 770 771
                 ; impls_from_givens <- solveInteractCts pushed_givens 
                 ; MASSERT (isEmptyBag impls_from_givens)
                   
772 773 774 775 776 777
                 ; traceTcS "solveWanteds: } now doing nested implications {" empty
                 ; flatMapBagPairM (solveImplication tcs_untouchables) implics }

       -- ... 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.
778 779 780 781 782 783
       ; traceTcS "solveWanteds: done nested implications }" $
                  vcat [ text "implic_eqs ="       <+> ppr implic_eqs
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

       ; return (implic_eqs, unsolved_implics) }

784 785 786
  where givens_from_wanteds = foldrBag get_wanted []
        get_wanted cc rest_givens
            | pushable_wanted cc
dimitris's avatar
dimitris committed
787 788 789 790
            = let fl = cc_flavor cc
                  wloc = flav_wloc fl
                  gfl = Given (mkGivenLoc wloc UnkSkol) (flav_evar fl) 
                  this_given = cc { cc_flavor = gfl }
791 792 793 794 795 796
              in this_given : rest_givens
            | otherwise = rest_givens 

        pushable_wanted :: Ct -> Bool 
        pushable_wanted cc 
         | isWantedCt cc 
797
         = isEqPred (ctPred cc) -- see Note [Preparing inert set for implications]
798 799 800 801 802 803 804 805 806 807
         | otherwise = False 

solveImplication :: TcTyVarSet     -- Untouchable TcS unification variables
                 -> 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
solveImplication tcs_untouchables
     imp@(Implic { ic_untch  = untch
808 809 810
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
                 , ic_given  = givens
811
                 , ic_wanted = wanteds
812
                 , ic_loc    = loc })
813
  = nestImplicTcS ev_binds (untch, tcs_untouchables) $
814 815
    recoverTcS (return (emptyBag, emptyBag)) $
       -- Recover from nested failures.  Even the top level is
816
       -- just a bunch of implications, so failing at the first one is bad
817 818 819
    do { traceTcS "solveImplication {" (ppr imp) 

         -- Solve flat givens
820 821 822
       ; impls_from_givens <- solveInteractGiven loc givens 
       ; MASSERT (isEmptyBag impls_from_givens)
         
823
         -- Simplify the wanteds
824 825 826
       ; WC { wc_flat = unsolved_flats
            , wc_impl = unsolved_implics
            , wc_insol = insols } <- solve_wanteds wanteds
827 828 829 830

       ; let (res_flat_free, res_flat_bound)
                 = floatEqualities skols givens unsolved_flats
             final_flat = keepWanted res_flat_bound
831

832 833
       ; let res_wanted = WC { wc_flat  = final_flat
                             , wc_impl  = unsolved_implics
834
                             , wc_insol = insols }
835

836 837 838
             res_implic = unitImplication $
                          imp { ic_wanted = res_wanted
                              , ic_insol  = insolubleWC res_wanted }
839

840 841
       ; evbinds <- getTcEvBindsMap

842 843
       ; traceTcS "solveImplication end }" $ vcat
             [ text "res_flat_free =" <+> ppr res_flat_free
844
             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds)
845
             , text "res_implic =" <+> ppr res_implic ]
846

847
       ; return (res_flat_free, res_implic) }
848
    -- and we are back to the original inerts
849 850


851
floatEqualities :: [TcTyVar] -> [EvVar] -> Cts -> (Cts, Cts)
852 853 854 855
-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
floatEqualities skols can_given wantders
  | hasEqualities can_given = (emptyBag, wantders)
856
          -- Note [Float Equalities out of Implications]
857
  | otherwise = partitionBag is_floatable wantders
858 859 860 861

-- TODO: Maybe we should try out /not/ floating constraints that contain touchables only, 
-- since they are inert and not going to interact with anything more in a more global scope.

862 863
  where skol_set = mkVarSet skols
        is_floatable :: Ct -> Bool
864
        is_floatable ct
865
          | ct_predty <- ctPred ct
866
          , isEqPred ct_predty
867
          = skol_set `disjointVarSet` tvs_under_fsks ct_predty
868
        is_floatable _ct = False
869 870 871 872 873 874 875 876

        tvs_under_fsks :: Type -> TyVarSet
        -- ^ NB: for type synonyms tvs_under_fsks does /not/ expand the synonym
        tvs_under_fsks (TyVarTy tv)     
          | not (isTcTyVar tv)               = unitVarSet tv
          | FlatSkol ty <- tcTyVarDetails tv = tvs_under_fsks ty
          | otherwise                        = unitVarSet tv
        tvs_under_fsks (TyConApp _ tys) = unionVarSets (map tvs_under_fsks tys)
877
        tvs_under_fsks (LitTy {})       = emptyVarSet
878 879 880
        tvs_under_fsks (FunTy arg res)  = tvs_under_fsks arg `unionVarSet` tvs_under_fsks res
        tvs_under_fsks (AppTy fun arg)  = tvs_under_fsks fun `unionVarSet` tvs_under_fsks arg
        tvs_under_fsks (ForAllTy tv ty) -- The kind of a coercion binder 
881
        	     	       	        -- can mention type variables!
882 883 884 885 886
          | isTyVar tv		      = inner_tvs `delVarSet` tv
          | otherwise  {- Coercion -} = -- ASSERT( not (tv `elemVarSet` inner_tvs) )
                                        inner_tvs `unionVarSet` tvs_under_fsks (tyVarKind tv)
          where
            inner_tvs = tvs_under_fsks ty
887
\end{code}
888

889 890 891 892
Note [Preparing inert set for implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Before solving the nested implications, we convert any unsolved flat wanteds
to givens, and add them to the inert set.  Reasons:
893 894

  a) In checking mode, suppresses unnecessary errors.  We already have
895
     on unsolved-wanted error; adding it to the givens prevents any 
896
     consequential errors from showing up
897

898 899 900 901
  b) More importantly, in inference mode, we are going to quantify over this
     constraint, and we *don't* want to quantify over any constraints that
     are deducible from it.

902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924
  c) Flattened type-family equalities must be exposed to the nested
     constraints.  Consider
	F b ~ alpha, (forall c.  F b ~ alpha)
     Obviously this is soluble with [alpha := F b].  But the
     unification is only done by solveCTyFunEqs, right at the end of
     solveWanteds, and if we aren't careful we'll end up with an
     unsolved goal inside the implication.  We need to "push" the
     as-yes-unsolved (F b ~ alpha) inwards, as a *given*, so that it
     can be used to solve the inner (F b
     ~ alpha).  See Trac #4935.

  d) There are other cases where interactions between wanteds that can help
     to solve a constraint. For example

  	class C a b | a -> b

  	(C Int alpha), (forall d. C d blah => C Int a)

     If we push the (C Int alpha) inwards, as a given, it can produce
     a fundep (alpha~a) and this can float out again and be used to
     fix alpha.  (In general we can't float class constraints out just
     in case (C d blah) might help to solve (C Int a).)

925 926 927 928 929 930 931 932
The unsolved wanteds are *canonical* but they may not be *inert*,
because when made into a given they might interact with other givens.
Hence the call to solveInteract.  Example:

 Original inert set = (d :_g D a) /\ (co :_w  a ~ [beta]) 

We were not able to solve (a ~w [beta]) but we can't just assume it as
given because the resulting set is not inert. Hence we have to do a
933 934
'solveInteract' step first. 

dimitris's avatar
dimitris committed
935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970
Finally, note that we convert them to [Given] and NOT [Given/Solved].
The reason is that Given/Solved are weaker than Givens and may be discarded.
As an example consider the inference case, where we may have, the following 
original constraints: 
     [Wanted] F Int ~ Int
             (F Int ~ a => F Int ~ a)
If we convert F Int ~ Int to [Given/Solved] instead of Given, then the next 
given (F Int ~ a) is going to cause the Given/Solved to be ignored, casting 
the (F Int ~ a) insoluble. Hence we should really convert the residual 
wanteds to plain old Given. 

We need only push in unsolved equalities both in checking mode and inference mode: 

  (1) In checking mode we should not push given dictionaries in because of
example LongWayOverlapping.hs, where we might get strange overlap
errors between far-away constraints in the program.  But even in
checking mode, we must still push type family equations. Consider:

   type instance F True a b = a 
   type instance F False a b = b

   [w] F c a b ~ gamma 
   (c ~ True) => a ~ gamma 
   (c ~ False) => b ~ gamma

Since solveCTyFunEqs happens at the very end of solving, the only way to solve
the two implications is temporarily consider (F c a b ~ gamma) as Given (NB: not 
merely Given/Solved because it has to interact with the top-level instance 
environment) and push it inside the implications. Now, when we come out again at
the end, having solved the implications solveCTyFunEqs will solve this equality.

  (2) In inference mode, we recheck the final constraint in checking mode and
hence we will be able to solve inner implications from top-level quantified
constraints nonetheless.


971 972
Note [Extra TcsTv untouchables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
973 974 975 976 977
Furthemore, we record the inert set simplifier-generated unification
variables of the TcsTv kind (such as variables from instance that have
been applied, or unification flattens). These variables must be passed
to the implications as extra untouchable variables. Otherwise we have
the danger of double unifications. Example (from trac ticket #4494):
978 979 980

   (F Int ~ uf)  /\  (forall a. C a => F Int ~ beta) 

981 982 983
In this example, beta is touchable inside the implication. The first
solveInteract step leaves 'uf' ununified. Then we move inside the
implication where a new constraint
984
       uf  ~  beta  
985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000
emerges. We may spontaneously solve it to get uf := beta, so the whole
implication disappears but when we pop out again we are left with (F
Int ~ uf) which will be unified by our final solveCTyFunEqs stage and
uf will get unified *once more* to (F Int).

The solution is to record the TcsTvs (i.e. the simplifier-generated
unification variables) that are generated when solving the flats, and
make them untouchables for the nested implication. In the example
above uf would become untouchable, so beta would be forced to be
unified as beta := uf.

NB: A consequence is that every simplifier-generated TcsTv variable
    that gets floated out of an implication becomes now untouchable
    next time we go inside that implication to solve any residual
    constraints. In effect, by floating an equality out of the
    implication we are committing to have it solved in the outside.
1001

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1002 1003 1004 1005
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
We want to float equalities out of vanilla existentials, but *not* out 
of GADT pattern matches. 
1006

1007 1008
---> TODO Expand in accordance to our discussion

1009

1010 1011
\begin{code}

1012
solveCTyFunEqs :: Cts -> TcS (TvSubst, Cts)
1013 1014
-- Default equalities (F xi ~ alpha) by setting (alpha := F xi), whenever possible
-- See Note [Solving Family Equations]
1015
-- Returns: a bunch of unsolved constraints from the original Cts and implications
1016
--          where the newly generated equalities (alpha := F xi) have been substituted through.
1017
solveCTyFunEqs cts
1018
 = do { untch   <- getUntouchables 
1019 1020
      ; let (unsolved_can_cts, (ni_subst, cv_binds))
                = getSolvableCTyFunEqs untch cts
1021
      ; traceTcS "defaultCTyFunEqs" (vcat [text "Trying to default family equations:"
1022
                                          , ppr ni_subst, ppr cv_binds
1023
                                          ])
1024 1025 1026 1027
      ; mapM_ solve_one cv_binds

      ; return (niFixTvSubst ni_subst, unsolved_can_cts) }
  where
dimitris's avatar
dimitris committed
1028 1029 1030 1031 1032 1033
    solve_one (Wanted _ cv,tv,ty) 
      = setWantedTyBind tv ty >> setEvBind cv (EvCoercion (mkTcReflCo ty))
    solve_one (Derived {}, tv, ty)
      = setWantedTyBind tv ty
    solve_one arg
      = pprPanic "solveCTyFunEqs: can't solve a /given/ family equation!" $ ppr arg