TcSimplify.lhs 57.5 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 TcRnTypes
18
import TcRnMonad
19
import TcErrors
20
import TcMType
21 22
import TcType 
import TcSMonad 
23
import TcInteract 
24
import Inst
25
import Unify	( niFixTvSubst, niSubstTvSet )
26 27
import Type     ( classifyPredType, PredTree(..), getClassPredTys_maybe )
import Class    ( Class )
28
import Var
29
import Unique
30
import VarSet
31
import VarEnv 
32
import TcEvidence
33
import TypeRep
34
import Name
35
import Bag
36 37
import ListSetOps
import Util
38 39 40
import PrelInfo
import PrelNames
import Class		( classKey )
41
import BasicTypes       ( RuleName )
42
import Control.Monad    ( when )
43
import Outputable
44
import FastString
dimitris's avatar
dimitris committed
45
import TrieMap () -- DV: for now
46
import DynFlags
47 48 49
\end{code}


50 51 52 53 54
*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
55

56

57
\begin{code}
58 59


60 61
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
62 63 64
-- 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
65
simplifyTop wanteds 
66 67 68
  = do { ev_binds_var <- newTcEvBinds
                         
       ; zonked_wanteds <- zonkWC wanteds
69 70 71

       ; traceTc "simplifyTop {" $ text "zonked_wc =" <+> ppr zonked_wanteds

72
       ; wc_first_go <- solveWantedsWithEvBinds ev_binds_var zonked_wanteds
73 74 75 76 77
       ; cts <- applyTyVarDefaulting wc_first_go 
                -- See Note [Top-level Defaulting Plan]
                
       ; let wc_for_loop = wc_first_go { wc_flat = wc_flat wc_first_go `unionBags` cts }
                           
78
       ; traceTc "simpl_top_loop" $ text "wc_for_loop =" <+> ppr wc_for_loop
79 80 81 82 83 84 85
       ; simpl_top_loop ev_binds_var wc_for_loop }
    
  where simpl_top_loop ev_binds_var wc
          | isEmptyWC wc 
          = do { traceTc "simpl_top_loop }" empty
               ; TcRnMonad.getTcEvBinds ev_binds_var }
          | otherwise
86
          = do { wc_residual <- solveWantedsWithEvBinds ev_binds_var wc
87 88 89 90 91
               ; let wc_flat_approximate = approximateWC wc_residual
               ; (dflt_eqs,_unused_bind) <- runTcS $
                                            applyDefaultingRules wc_flat_approximate
                                            -- See Note [Top-level Defaulting Plan]
               ; if isEmptyBag dflt_eqs then 
92
                   do { traceTc "End simplifyTop }" empty
93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149
                      ; report_and_finish ev_binds_var wc_residual }
                 else
                   simpl_top_loop ev_binds_var $ 
                   wc_residual { wc_flat = wc_flat wc_residual `unionBags` dflt_eqs } }

        report_and_finish ev_binds_var wc_residual 
          = do { eb1 <- TcRnMonad.getTcEvBinds ev_binds_var
               ; traceTc "reportUnsolved {" empty
                   -- See Note [Deferring coercion errors to runtime]
               ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
               ; eb2 <- reportUnsolved runtimeCoercionErrors wc_residual
               ; traceTc "reportUnsolved }" empty
               ; return (eb1 `unionBags` eb2) }
\end{code}

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

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

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

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

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

More details in Note [DefaultTyVar].

\begin{code}
150

151 152 153
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
154
  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >> 
155 156 157
    simplifyTop wanteds  -- NB: must be simplifyTop not simplifyCheck, so that we
                         --     do ambiguity resolution.  
                         -- See Note [Impedence matching] in TcBinds.
158
 
159 160 161
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
162 163
  = traceTc "simplifyInteractive" empty >>
    simplifyTop wanteds 
164 165 166 167 168

------------------
simplifyDefault :: ThetaType	-- Wanted; has no type variables in it
                -> TcM ()	-- Succeeds iff the constraint is soluble
simplifyDefault theta
169 170 171
  = do { traceTc "simplifyInteractive" empty
       ; wanted <- newFlatWanteds DefaultOrigin theta
       ; _ignored_ev_binds <- simplifyCheck (mkFlatWC wanted)
172 173
       ; return () }
\end{code}
174

175

176
***********************************************************************************
177
*                                                                                 * 
178
*                            Deriving                                             *
179 180
*                                                                                 *
***********************************************************************************
181

182 183
\begin{code}
simplifyDeriv :: CtOrigin
184 185 186 187
              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
188 189
-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
190
-- Fail if not possible
191
simplifyDeriv orig pred tvs theta 
192
  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
193 194 195 196
      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
197

198
       ; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
199
             skol_set   = mkVarSet tvs_skols
200
	     doc = ptext (sLit "deriving") <+> parens (ppr pred)
201 202 203

       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

204 205
       ; traceTc "simplifyDeriv" $ 
         vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
206
       ; (residual_wanted, _ev_binds1)
207
             <- solveWanteds (mkFlatWC wanted)
208

209 210
       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
211
             get_good :: Ct -> Either PredType Ct
212 213 214 215 216 217
             get_good ct | validDerivPred skol_set p 
                         , isWantedCt ct  = Left p 
                         -- NB: residual_wanted may contain unsolved
                         -- Derived and we stick them into the bad set
                         -- so that reportUnsolved may decide what to do with them
                         | otherwise = Right ct
218
                         where p = ctPred ct
219

220 221 222
       -- 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 })
223

224 225
       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
226
\end{code}
227

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252
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

253 254 255 256 257 258 259
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.
260

261 262 263
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)
264

265 266
Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
267

268 269 270 271
	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.  
272

273 274 275
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.
276

277 278
So for now we simply require that the derived instance context
should have only type-variable constraints.
279

280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308
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
*                                                                                 *
***********************************************************************************
309

dreixel's avatar
dreixel committed
310 311 312 313 314 315 316 317 318 319 320 321
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.

322
\begin{code}
323
simplifyInfer :: Bool
324 325 326
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
327
              -> WantedConstraints
328 329
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
330 331 332
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
333
                      TcEvBinds)    -- ... binding these evidence variables
334
simplifyInfer _top_lvl apply_mr name_taus wanteds
335 336 337
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
338
       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
dreixel's avatar
dreixel committed
339 340 341
       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
342
       ; return (qtvs, [], False, emptyTcEvBinds) }
343

344
  | otherwise
345
  = do { runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
346
       ; gbl_tvs        <- tcGetGlobalTyVars
347
       ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
348
       ; zonked_wanteds <- zonkWC wanteds
349

350
       ; traceTc "simplifyInfer {"  $ vcat
351
             [ ptext (sLit "names =") <+> ppr (map fst name_taus)
352 353
             , ptext (sLit "taus =") <+> ppr (map snd name_taus)
             , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs
354 355 356
             , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
357
             , ptext (sLit "wanted =") <+> ppr zonked_wanteds
358 359
             ]

360 361 362 363 364
              -- Historical note: Before step 2 we used to have a
              -- HORRIBLE HACK described in Note [Avoid unecessary
              -- constraint simplification] but, as described in Trac
              -- #4361, we have taken in out now.  That's why we start
              -- with step 2!
365

366 367 368 369 370 371 372 373
              -- Step 2) First try full-blown solving 

              -- NB: we must gather up all the bindings from doing
              -- this solving; hence (runTcSWithEvBinds ev_binds_var).
              -- And note that since there are nested implications,
              -- calling solveWanteds will side-effect their evidence
              -- bindings, so we can't just revert to the input
              -- constraint.
374
       ; ev_binds_var <- newTcEvBinds
375
       ; wanted_transformed <- solveWantedsWithEvBinds ev_binds_var zonked_wanteds
376 377

              -- Step 3) Fail fast if there is an insoluble constraint,
378 379 380
              -- unless we are deferring errors to runtime
       ; when (not runtimeCoercionErrors && insolubleWC wanted_transformed) $ 
         do { _ev_binds <- reportUnsolved False wanted_transformed; failM }
381 382

              -- Step 4) Candidates for quantification are an approximation of wanted_transformed
383 384 385 386
       ; let quant_candidates = approximateWC wanted_transformed               
              -- NB: Already the fixpoint of any unifications that may have happened                                
              -- NB: We do not do any defaulting when inferring a type, this can lead
              -- to less polymorphic types, see Note [Default while Inferring]
387 388
              -- NB: quant_candidates here are wanted or derived, we filter the wanteds later, anyway
 
389
              -- Step 5) Minimize the quantification candidates                             
390
       ; (quant_candidates_transformed, _extra_binds)   
391 392 393
             <- solveWanteds $ WC { wc_flat  = quant_candidates
                                  , wc_impl  = emptyBag
                                  , wc_insol = emptyBag }
394 395

              -- Step 6) Final candidates for quantification                
396 397
       ; let final_quant_candidates :: [PredType]
             final_quant_candidates = map ctPred $ bagToList $
398
                                      wc_flat quant_candidates_transformed
399 400 401
             -- NB: Already the fixpoint of any unifications that may have happened
                  
       ; gbl_tvs        <- tcGetGlobalTyVars -- TODO: can we just use untch instead of gbl_tvs?
402
       ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs
403 404 405 406 407 408
       
       ; traceTc "simplifyWithApprox" $
         vcat [ ptext (sLit "final_quant_candidates =") <+> ppr final_quant_candidates
              , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs
              , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs ]
         
409 410 411
       ; let init_tvs  = zonked_tau_tvs `minusVarSet` gbl_tvs
             poly_qtvs = growThetaTyVars final_quant_candidates init_tvs 
                         `minusVarSet` gbl_tvs
412
             pbound    = filter (quantifyPred poly_qtvs) final_quant_candidates
413 414
             
       ; traceTc "simplifyWithApprox" $
415 416 417
         vcat [ ptext (sLit "pbound =") <+> ppr pbound
              , ptext (sLit "init_qtvs =") <+> ppr init_tvs 
              , ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs ]
418
         
419
	     -- Monomorphism restriction
420
       ; let mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
421 422
             constrained_tvs = tyVarsOfTypes final_quant_candidates
	     mr_bites        = apply_mr && not (null pbound)
423

424
             (qtvs, bound)
425
                | mr_bites  = (mr_qtvs,   [])
426 427
                | otherwise = (poly_qtvs, pbound)
             
428

429
       ; if isEmptyVarSet qtvs && null bound
430 431 432 433
         then do { traceTc "} simplifyInfer/no quantification" empty                   
                 ; emitConstraints wanted_transformed
                    -- Includes insolubles (if -fdefer-type-errors)
                    -- as well as flats and implications
434
                 ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
435 436
         else do

437 438 439
       { traceTc "simplifyApprox" $ 
         ptext (sLit "bound are =") <+> ppr bound 
         
440
            -- Step 4, zonk quantified variables 
441
       ; let minimal_flat_preds = mkMinimalBySCs bound
442 443
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
444 445 446 447
                        -- 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
448
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
449

450
            -- Step 7) Emit an implication
451 452
       ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds
       ; lcl_env <- getLclTypeEnv
dreixel's avatar
dreixel committed
453
       ; gloc <- getCtLoc skol_info
454 455 456
       ; untch <- TcRnMonad.getUntouchables
       ; uniq  <- TcRnMonad.newUnique
       ; let implic = Implic { ic_untch    = pushUntouchables uniq untch 
457
                             , ic_env      = lcl_env
458
                             , ic_skols    = qtvs_to_return
459 460
                             , ic_fsks     = []  -- wanted_tansformed arose only from solveWanteds
                                                 -- hence no flatten-skolems (which come from givens)
461
                             , ic_given    = minimal_bound_ev_vars
462
                             , ic_wanted   = wanted_transformed 
463 464 465 466
                             , ic_insol    = False
                             , ic_binds    = ev_binds_var
                             , ic_loc      = gloc }
       ; emitImplication implic
467
         
468 469 470
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ ptext (sLit "implic =") <+> ppr implic
                       -- ic_skols, ic_given give rest of result
471
                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
472
                  , ptext (sLit "spb =") <+> ppr final_quant_candidates
473 474
                  , ptext (sLit "bound =") <+> ppr bound ]

475 476
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
477
    where 
478
\end{code}
479 480


481 482
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516
Our current plan is that defaulting only happens at simplifyTop and
not simplifyInfer.  This may lead to some insoluble deferred constraints
Example:

instance D g => C g Int b 

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

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

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

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

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

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



517 518 519 520 521 522 523 524 525
Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
When we quantify over a constraint, in simplifyInfer we need to
quantify over a constraint that is minimal in some sense: For
instance, if the final wanted constraint is (Eq alpha, Ord alpha),
we'd like to quantify over Ord alpha, because we can just get Eq alpha
from superclass selection from Ord alpha. This minimization is what
mkMinimalBySCs does. Then, simplifyInfer uses the minimal constraint
to check the original wanted.
526

527 528
\begin{code}
approximateWC :: WantedConstraints -> Cts
529
-- Postcondition: Wanted or Derived Cts 
530
approximateWC wc = float_wc emptyVarSet wc
531
  where 
532 533 534 535 536 537
    float_wc :: TcTyVarSet -> WantedConstraints -> Cts
    float_wc skols (WC { wc_flat = flat, wc_impl = implic }) = floats1 `unionBags` floats2
      where floats1 = do_bag (float_flat skols) flat
            floats2 = do_bag (float_implic skols) implic
                                 
    float_implic :: TcTyVarSet -> Implication -> Cts
538
    float_implic skols imp
539 540 541
      = float_wc skols' (ic_wanted imp)
      where
        skols' = skols `extendVarSetList` ic_skols imp `extendVarSetList` ic_fsks imp
542 543 544 545
            
    float_flat :: TcTyVarSet -> Ct -> Cts
    float_flat skols ct
      | tyVarsOfCt ct `disjointVarSet` skols 
546
      = singleCt ct
547 548 549 550
      | otherwise = emptyCts
        
    do_bag :: (a -> Bag c) -> Bag a -> Bag c
    do_bag f = foldrBag (unionBags.f) emptyBag
551
\end{code}
552

553 554
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
555 556 557 558
    -------- NB NB NB (Jun 12) ------------- 
    This note not longer applies; see the notes with Trac #4361.
    But I'm leaving it in here so we remember the issue.)
    ----------------------------------------
559
When inferring the type of a let-binding, with simplifyInfer,
560
try to avoid unnecessarily simplifying class constraints.
561 562
Doing so aids sharing, but it also helps with delicate 
situations like
563

564
   instance C t => C [t] where ..
565

566 567 568 569 570 571 572 573 574 575 576
   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.


577 578 579 580 581
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
582

583
See note [Simplifying RULE consraints] in TcRule
584

585 586 587 588 589 590 591 592 593 594 595 596 597 598
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.
599 600

\begin{code}
601 602 603
simplifyRule :: RuleName 
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
604 605 606 607 608
             -> 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)
609
             
610
             	 -- We allow ourselves to unify environment 
611
		 -- variables: runTcS runs with NoUntouchables
612
       ; (resid_wanted, _) <- solveWanteds zonked_all
613

614 615
       ; zonked_lhs <- zonkWC lhs_wanted

616 617 618 619 620 621 622 623 624 625 626 627 628
       ; 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
             
629
       ; traceTc "simplifyRule" $
630 631
         vcat [ doc
              , text "zonked_lhs" <+> ppr zonked_lhs 
632 633
              , text "q_cts"      <+> ppr q_cts ]

634 635
       ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
                , zonked_lhs { wc_flat = non_q_cts }) }
636 637 638
\end{code}


639 640 641 642 643
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
644 645

\begin{code}
646
simplifyCheck :: WantedConstraints	-- Wanted
647 648 649 650 651 652 653 654 655 656 657 658 659
              -> 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
660
simplifyCheck wanteds
661
  = do { wanteds <- zonkWC wanteds
662 663 664 665

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

666
       ; (unsolved, eb1) <- solveWanteds wanteds
667 668 669

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

dimitris's avatar
dimitris committed
670
       ; traceTc "reportUnsolved {" empty
671 672 673
       -- See Note [Deferring coercion errors to runtime]
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
       ; eb2 <- reportUnsolved runtimeCoercionErrors unsolved 
dimitris's avatar
dimitris committed
674 675
       ; traceTc "reportUnsolved }" empty

676 677 678 679 680 681 682 683 684 685
       ; 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
686

687 688
  a :: Int
  a = 'a'
689

690
  main = print "b"
691

692 693
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`.
694

695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717
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}
718 719 720

solveWanteds :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
-- Return the evidence binds in the BagEvBinds result
721 722 723 724
-- Discards all Derived stuff in result
solveWanteds wanted 
  = runTcS $ do { wc <- solve_wanteds wanted 
                ; return (dropDerivedWC wc) }
725 726 727

solveWantedsWithEvBinds :: EvBindsVar -> WantedConstraints -> TcM WantedConstraints
-- Side-effect the EvBindsVar argument to add new bindings from solving
728
-- Discards all Derived stuff in result
729
solveWantedsWithEvBinds ev_binds_var wanted
730 731 732
  = runTcSWithEvBinds ev_binds_var $ 
    do { wc <- solve_wanteds wanted 
       ; return (dropDerivedWC wc) }
733 734 735

solve_wanteds :: WantedConstraints -> TcS WantedConstraints 
-- NB: wc_flats may be wanted /or/ derived now
736
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
737 738
  = do { traceTcS "solveWanteds {" (ppr wanted)

739 740
         -- Try the flat bit, including insolubles. Solving insolubles a 
         -- second time round is a bit of a waste but the code is simple 
741 742 743
         -- and the program is wrong anyway, and we don't run the danger 
         -- of adding Derived insolubles twice; see 
         -- TcSMonad Note [Do not add duplicate derived insolubles] 
744
       ; traceTcS "solveFlats {" empty
745
       ; let all_flats = flats `unionBags` insols
746 747
       ; impls_from_flats <- solveInteract all_flats
       ; traceTcS "solveFlats end }" (ppr impls_from_flats)
748

749 750
       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
751
       ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
752

753 754 755 756 757
       ; (unsolved_flats, insoluble_flats) <- getInertUnsolved

       ; wc <- unFlattenWC (WC { wc_flat  = unsolved_flats
                               , wc_impl  = unsolved_implics
                               , wc_insol = insoluble_flats })
758 759

       ; bb <- getTcEvBindsMap
760
       ; tb <- getTcSTyBindsMap
761
       ; traceTcS "solveWanteds }" $
762
                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
763
                      , text "unsolved_implics =" <+> ppr unsolved_implics
764
                      , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
765
                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
766
                      , text "final wc =" <+> ppr wc ]
767

768
       ; return wc }
769 770 771 772 773 774 775 776

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
777 778 779 780 781 782 783
  = do { (floated_eqs, unsolved_implics) <- solveNestedImplications implics
       ; if isEmptyBag floated_eqs 
         then return unsolved_implics 
         else 
    do {   -- Put floated_eqs into the current inert set before looping
         impls_from_eqs <- solveInteract floated_eqs
       ; simpl_loop (n+1) (unsolved_implics `unionBags` impls_from_eqs)} }
784

785

786 787 788 789 790 791 792 793 794
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
795 796
       ; let thinner_inerts = prepareInertsForImplications inerts
                 -- See Note [Preparing inert set for implications]
797
  
798
       ; traceTcS "solveNestedImplications starting {" $ 
799
         vcat [ text "original inerts = " <+> ppr inerts
800 801
              , text "thinner_inerts  = " <+> ppr thinner_inerts ]
         
802
       ; (floated_eqs, unsolved_implics)
803
           <- flatMapBagPairM (solveImplication thinner_inerts) implics
804

805 806 807
       ; promoteFloatedUnificationVars floated_eqs
           -- Performs some unifications, adding to monadically-carried ty_binds
           -- These will be used when processing floated_eqs later
808

809 810 811
       -- ... 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.
812
       ; traceTcS "solveNestedImplications end }" $
813
                  vcat [ text "all floated_eqs ="  <+> ppr floated_eqs
814 815
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

816
       ; return (floated_eqs, unsolved_implics) }
817

818
solveImplication :: InertSet
819 820 821 822 823
                 -> 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
824
solveImplication inerts
825
     imp@(Implic { ic_untch  = untch
826 827
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
828
                 , ic_fsks   = old_fsks
829
                 , ic_given  = givens
830
                 , ic_wanted = wanteds
831
                 , ic_loc    = loc })
832
  = nestImplicTcS ev_binds untch inerts $
833 834
    do { traceTcS "solveImplication {" (ppr imp) 

835 836 837 838
         -- Solve the nested constraints
       ; solveInteractGiven loc old_fsks givens 
       ; residual_wanted <- solve_wanteds wanteds
       ; new_fsks <- getFlattenSkols
839

840 841 842
       ; let all_fsks = new_fsks ++ old_fsks
             (floated_eqs, final_wanted)
                 = floatEqualities (skols ++ all_fsks) givens residual_wanted
843

844 845 846 847 848 849
             res_implic | isEmptyWC final_wanted 
                        = emptyBag
                        | otherwise
                        = unitBag imp { ic_fsks   = all_fsks
                                      , ic_wanted = dropDerivedWC final_wanted
                                      , ic_insol  = insolubleWC final_wanted }
850

851
       ; evbinds <- getTcEvBindsMap
852
       ; traceTcS "solveImplication end }" $ vcat
853 854 855 856
             [ text "floated_eqs =" <+> ppr floated_eqs
             , text "new_fsks =" <+> ppr all_fsks
             , text "res_implic =" <+> ppr res_implic
             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]
857

858
       ; return (floated_eqs, res_implic) }
859 860
\end{code}

861
Note [Floating equalities]
862 863

\begin{code}
864
promoteFloatedUnificationVars :: Cts -> TcS ()
865 866 867 868
promoteFloatedUnificationVars cts
  = do { untch <- TcSMonad.getUntouchables
       ; let tvs = filter (isFloatedTouchableMetaTyVar untch) $
                   varSetElems (tyVarsOfCts cts)
869 870
       ; mapM_ (promote_tv untch) tvs
       ; ty_binds <- getTcSTyBindsMap
871 872 873
       ; traceTcS "promoteFloated" (vcat [ text "Ctxt untoucables =" <+> ppr untch
                                         , text "Floated eqs =" <+> ppr cts
                                         , text "Promoted tvs =" <+> ppr tvs
874 875
                                         , text "Ty binds =" <+> ppr ty_binds])
       ; return () }
876
  where
877 878 879 880
    promote_tv untch tv 
      = do { cloned_tv <- TcSMonad.cloneMetaTyVar tv
           ; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch
           ; setWantedTyBind tv (mkTyVarTy rhs_tv) }
881

882
floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints -> (Cts, WantedConstraints)
883 884
-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908
floatEqualities skols can_given wanteds@(WC { wc_flat = flats })
  | hasEqualities can_given 
  = (emptyBag, wanteds)   -- Note [Float Equalities out of Implications]
  | otherwise 
  = (float_eqs, wanteds { wc_flat = remaining_flats })
  where 
    (float_eqs, remaining_flats) = partitionBag is_floatable flats
    skol_set = growSkols wanteds (mkVarSet skols)

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

growSkols :: WantedConstraints -> VarSet -> VarSet
-- Find all the type variables that might possibly be unified
-- with a type that mentions a skolem.  This test is very conservative.
-- I don't *think* we need look inside the implications, because any 
-- relevant unification variables in there are untouchable.
growSkols (WC { wc_flat = flats }) skols
  = growThetaTyVars theta skols
  where
    theta = foldrBag ((:) . ctPred) [] flats
909
\end{code}
910

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
911 912
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960
For ordinary pattern matches (including existentials) we float 
equalities out of implications, for instance: 
     data T where 
       MkT :: Eq a => a -> T 
     f x y = case x of MkT _ -> (y::Int)
We get the implication constraint (x::T) (y::alpha): 
     forall a. [untouchable=alpha] Eq a => alpha ~ Int
We want to float out the equality into a scope where alpha is no
longer untouchable, to solve the implication!  

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

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

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

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

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

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

Notice that, due to Note [Extra TcSTv Untouchables], the free unification variables 
of an equality that is floated out of an implication become effectively untouchables
for the leftover implication. This is absolutely necessary. Consider the following 
example. We start with two implications and a class with a functional dependency. 

961 962 963 964 965
    class C x y | x -> y
    instance C [a] [a]
          
    (I1)      [untch=beta]forall b. 0 => F Int ~ [beta]
    (I2)      [untch=beta]forall c. 0 => F Int ~ [[alpha]] /\ C beta [c]
966 967 968 969 970 971 972

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

973 974 975 976 977 978 979 980 981 982 983 984 985
    class C x y | x -> y where 
     op :: x -> y -> ()

    instance C [a] [a]

    type family F a :: *

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

    data TEx where 
      TEx :: a -> TEx 

986

987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001
    f (x::beta) = 
        let g1 :: forall b. b -> ()
            g1 _ = h [x]
            g2 z = case z of TEx y -> (h [[undefined]], op x [y])
        in (g1 '3', g2 undefined)

Note [Extra TcsTv untouchables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Whenever we are solving a bunch of flat constraints, they may contain 
the following sorts of 'touchable' unification variables:
   
   (i)   Born-touchables in that scope
 
   (ii)  Simplifier-generated unification variables, such as unification 
         flatten variables
1002

1003 1004
   (iii) Touchables that have been floated out from some nested 
         implications, see Note [Float Equalities out of Implications]. 
1005

1006 1007 1008 1009 1010
Now, once we are done with solving these flats and have to move inwards to 
the nested implications (perhaps for a second time), we must consider all the
extra variables (categories (ii) and (iii) above) as untouchables for the 
implication. Otherwise we have the danger or double unifications, as well
as the danger of not ``seeing'' some unification. Example (from Trac #4494):
1011

1012
   (F Int ~ uf)  /\  [untch=beta](forall a. C a => F Int ~ beta) 
1013

1014 1015 1016 1017 1018 1019 1020 1021
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
       uf  ~  beta  
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).
1022

1023 1024 1025 1026
The solution is to record the unification variables of 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.
1027

1028
\begin{code}
1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045
unFlattenWC :: WantedConstraints -> TcS WantedConstraints
unFlattenWC wc 
  = do { (subst, remaining_unsolved_flats) <- solveCTyFunEqs (wc_flat wc)
                -- See Note [Solving Family Equations]
                -- NB: remaining_flats has already had subst applied
       ; return $ 
         WC { wc_flat  = mapBag (substCt subst) remaining_unsolved_flats
            , wc_impl  = mapBag (substImplication subst) (wc_impl wc) 
            , wc_insol = mapBag (substCt subst) (wc_insol wc) }
       }
  where 
    solveCTyFunEqs :: Cts -> TcS (TvSubst, Cts)
    -- Default equalities (F xi ~ alpha) by setting (alpha := F xi), whenever possible
    -- See Note [Solving Family Equations]
    -- Returns: a bunch of unsolved constraints from the original Cts and implications
    --          where the newly generated equalities (alpha := F xi) have been substituted through.
    solveCTyFunEqs cts
1046
     = do { untch   <- TcSMonad.getUntouchables 
1047 1048
          ; let (unsolved_can_cts, (ni_subst, cv_binds))
                    = getSolvableCTyFunEqs untch cts
1049 1050 1051 1052 1053
          ; traceTcS "defaultCTyFunEqs" (vcat [ text "Trying to default family equations:"
                                              , text "untch" <+> ppr untch 
                                              , text "subst" <+> ppr ni_subst 
                                              , text "binds" <+> ppr cv_binds
                                              , ppr unsolved_can_cts
1054 1055 1056 1057