TcSimplify.lhs 55.6 KB
Newer Older
1
\begin{code}
2
module TcSimplify( 
3
       simplifyInfer, simplifyAmbiguityCheck,
4
       simplifyDefault, simplifyDeriv, 
5 6
       simplifyRule, simplifyTop, simplifyInteractive
  ) where
7

8
#include "HsVersions.h"
9

10
import HsSyn	       
11
import TcRnMonad
12
import TcErrors
13
import TcMType
14 15 16 17
import TcType 
import TcSMonad 
import TcInteract
import Inst
18
import Unify	( niFixTvSubst, niSubstTvSet )
19
import Var
20
import VarSet
21
import VarEnv 
22
import Coercion
23
import TypeRep
24

25 26
import Name
import NameEnv	( emptyNameEnv )
27
import Bag
28 29
import ListSetOps
import Util
30 31 32
import PrelInfo
import PrelNames
import Class		( classKey )
33
import BasicTypes       ( RuleName )
34
import Control.Monad    ( when )
35
import Outputable
36
import FastString
37 38 39
\end{code}


40 41 42 43 44
*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
45

46 47 48
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
49 50 51
-- 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
52
simplifyTop wanteds 
53
  = simplifyCheck (SimplCheck (ptext (sLit "top level"))) wanteds
54

55 56 57 58 59
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
  = simplifyCheck (SimplCheck (ptext (sLit "ambiguity check for") <+> ppr name)) wanteds

60 61 62 63 64 65 66 67 68
------------------
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
69
  = do { wanted <- newFlatWanteds DefaultOrigin theta
70 71
       ; _ignored_ev_binds <- simplifyCheck (SimplCheck (ptext (sLit "defaults"))) 
                                            (mkFlatWC wanted)
72 73
       ; return () }
\end{code}
74

75

76

77 78 79 80 81
*********************************************************************************
*                                                                                 * 
*                            Deriving
*                                                                                 *
***********************************************************************************
82

83 84
\begin{code}
simplifyDeriv :: CtOrigin
85 86 87 88
              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
89 90
-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
91
-- Fail if not possible
92
simplifyDeriv orig pred tvs theta 
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
93 94 95 96 97
  = do { tvs_skols <- tcInstSkolTyVars tvs -- Skolemize
      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
98 99

       ; let skol_subst = zipTopTvSubst tvs $ map mkTyVarTy tvs_skols
100
             subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
101
             skol_set   = mkVarSet tvs_skols
102
	     doc = parens $ ptext (sLit "deriving") <+> parens (ppr pred)
103 104 105 106 107

       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

       ; traceTc "simplifyDeriv" (ppr tvs $$ ppr theta $$ ppr wanted)
       ; (residual_wanted, _binds)
108
             <- runTcS (SimplInfer doc) NoUntouchables $
109
                solveWanteds emptyInert (mkFlatWC wanted)
110

111 112 113
       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
             get_good :: WantedEvVar -> Either PredType WantedEvVar
114 115
             get_good wev | validDerivPred skol_set p = Left p
                          | otherwise                 = Right wev
116
                          where p = evVarOfPred wev
117

118
       ; reportUnsolved (residual_wanted { wc_flat = bad })
119

120 121
       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
122
\end{code}
123

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
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
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

149 150 151 152 153 154 155
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.
156

157 158 159
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)
160

161 162
Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
163

164 165 166 167
	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.  
168

169 170 171
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.
172

173 174
So for now we simply require that the derived instance context
should have only type-variable constraints.
175

176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204
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
*                                                                                 *
***********************************************************************************
205

206
\begin{code}
207
simplifyInfer :: Bool
208 209 210
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
211 212 213
              -> WantedConstraints
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
214 215 216
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
217
                      TcEvBinds)    -- ... binding these evidence variables
218
simplifyInfer _top_lvl apply_mr name_taus wanteds
219 220 221 222 223
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
       ; let tvs_to_quantify = get_tau_tvs zonked_taus `minusVarSet` gbl_tvs
       ; qtvs <- zonkQuantifiedTyVars (varSetElems tvs_to_quantify)
224
       ; return (qtvs, [], False, emptyTcEvBinds) }
225

226
  | otherwise
227 228 229 230
  = do { zonked_wanteds <- zonkWC wanteds
       ; zonked_taus    <- zonkTcTypes (map snd name_taus)
       ; gbl_tvs        <- tcGetGlobalTyVars

231
       ; traceTc "simplifyInfer {"  $ vcat
232 233 234 235 236
             [ ptext (sLit "names =") <+> ppr (map fst name_taus)
             , ptext (sLit "taus (zonked) =") <+> ppr zonked_taus
             , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
237
             , ptext (sLit "wanted =") <+> ppr zonked_wanteds
238 239
             ]

240 241
             -- Step 1
             -- Make a guess at the quantified type variables
242 243 244
	     -- Then split the constraints on the baisis of those tyvars
	     -- to avoid unnecessarily simplifying a class constraint
	     -- See Note [Avoid unecessary constraint simplification]
245 246
       ; let zonked_tau_tvs = get_tau_tvs zonked_taus
             proto_qtvs = growWanteds gbl_tvs zonked_wanteds $
247
                          zonked_tau_tvs `minusVarSet` gbl_tvs
248 249 250 251 252 253 254 255 256 257
             (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
             ]

       ; emitFlats surely_free
258 259 260 261
       ; traceTc "sinf"  $ vcat
             [ ptext (sLit "perhaps_bound =") <+> ppr perhaps_bound
             , ptext (sLit "surely_free   =") <+> ppr surely_free
             ]
262

263 264 265
            -- Step 2 
       	    -- Now simplify the possibly-bound constraints
       ; (simpl_results, tc_binds0)
266
           <- runTcS (SimplInfer (ppr (map fst name_taus))) NoUntouchables $
267 268 269 270 271 272 273 274 275 276 277
              simplifyWithApprox (zonked_wanteds { wc_flat = perhaps_bound })

       ; when (insolubleWC simpl_results)  -- Fail fast if there is an insoluble constraint
              (do { reportUnsolved simpl_results; failM })

            -- Step 3 
            -- Split again simplified_perhaps_bound, because some unifications 
            -- may have happened, and emit the free constraints. 
       ; gbl_tvs        <- tcGetGlobalTyVars
       ; zonked_tau_tvs <- zonkTcTyVarsAndFV zonked_tau_tvs
       ; zonked_simples <- zonkWantedEvVars (wc_flat simpl_results)
278
       ; let init_tvs 	     = zonked_tau_tvs `minusVarSet` gbl_tvs
279 280 281 282
             poly_qtvs       = growWantedEVs gbl_tvs zonked_simples init_tvs
	     (pbound, pfree) = partitionBag (quantifyMe poly_qtvs) zonked_simples

	     -- Monomorphism restriction
283
             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
284
             constrained_tvs = tyVarsOfEvVarXs zonked_simples
285 286 287 288 289
	     mr_bites        = apply_mr && not (isEmptyBag pbound)

             (qtvs, (bound, free))
                | mr_bites  = (mr_qtvs,   (emptyBag, zonked_simples))
                | otherwise = (poly_qtvs, (pbound,   pfree))
290 291
       ; emitFlats free

292
       ; if isEmptyVarSet qtvs && isEmptyBag bound
293 294 295
         then ASSERT( isEmptyBag (wc_insol simpl_results) )
              do { traceTc "} simplifyInfer/no quantification" empty
                 ; emitImplications (wc_impl simpl_results)
296
                 ; return ([], [], mr_bites, EvBinds tc_binds0) }
297 298 299 300
         else do

            -- Step 4, zonk quantified variables 
       { let minimal_flat_preds = mkMinimalBySCs $ map evVarOfPred $ bagToList bound
301 302
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
303 304 305 306 307
                        -- Don't add the quantified variables here, because
                        -- they are also bound in ic_skols and we want them to be
                        -- tidied uniformly

       ; gloc <- getCtLoc skol_info
308
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327

            -- Step 5
            -- Minimize `bound' and emit an implication
       ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds
       ; ev_binds_var <- newTcEvBinds
       ; mapBagM_ (\(EvBind evar etrm) -> addTcEvBind ev_binds_var evar etrm) tc_binds0
       ; lcl_env <- getLclTypeEnv
       ; let implic = Implic { ic_untch    = NoUntouchables
                             , ic_env      = lcl_env
                             , ic_skols    = mkVarSet qtvs_to_return
                             , 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
328
                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
329 330 331 332 333
                  , ptext (sLit "spb =") <+> ppr zonked_simples
                  , ptext (sLit "bound =") <+> ppr bound ]



334 335
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
336
  where
337 338
    get_tau_tvs = tyVarsOfTypes	-- I think this stuff is out of date
{-
339 340 341
    get_tau_tvs | isTopLevel top_lvl = tyVarsOfTypes
                | otherwise          = exactTyVarsOfTypes
     -- See Note [Silly type synonym] in TcType
342
-}
343
\end{code}
344 345


346 347
Note [Minimize by Superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
348

349 350 351 352 353 354 355
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.
356

357 358 359 360
\begin{code}
simplifyWithApprox :: WantedConstraints -> TcS WantedConstraints
simplifyWithApprox wanted
 = do { traceTcS "simplifyApproxLoop" (ppr wanted)
361

362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378
      ; results <- solveWanteds emptyInert wanted

      ; let (residual_implics, floats) = approximateImplications (wc_impl results)

        -- If no new work was produced then we are done with simplifyApproxLoop
      ; if insolubleWC results || isEmptyBag floats
        then return results

        else solveWanteds emptyInert
                (WC { wc_flat = floats `unionBags` wc_flat results
                    , wc_impl = residual_implics
                    , wc_insol = emptyBag }) }

approximateImplications :: Bag Implication -> (Bag Implication, Bag WantedEvVar)
-- Extracts any nested constraints that don't mention the skolems
approximateImplications impls
  = do_bag (float_implic emptyVarSet) impls
379
  where 
380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400
    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)

    float_implic :: TyVarSet -> Implication -> (Bag Implication, Bag WantedEvVar)
    float_implic skols imp
      = (unitBag (imp { ic_wanted = wanted' }), floats)
      where
        (wanted', floats) = float_wc (skols `unionVarSet` ic_skols imp) (ic_wanted imp)

    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

    float_flat :: TcTyVarSet -> WantedEvVar -> (Bag WantedEvVar, Bag WantedEvVar)
    float_flat skols wev
      | tyVarsOfEvVarX wev `disjointVarSet` skols = (emptyBag, unitBag wev)
      | otherwise                                 = (unitBag wev, emptyBag)
401
\end{code}
402

403
\begin{code}
404 405
-- (growX gbls wanted tvs) grows a seed 'tvs' against the 
-- X-constraint 'wanted', nuking the 'gbls' at each stage
406 407
-- It's conservative in that if the seed could *possibly*
-- grow to include a type variable, then it does
408

409 410 411 412 413 414 415
growWanteds :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWanteds gbl_tvs wc = fixVarSet (growWC gbl_tvs wc)

growWantedEVs :: TyVarSet -> Bag WantedEvVar -> TyVarSet -> TyVarSet
growWantedEVs gbl_tvs ws tvs
  | isEmptyBag ws = tvs
  | otherwise     = fixVarSet (growPreds gbl_tvs evVarOfPred ws) tvs
416

417 418 419 420 421
--------  Helper functions, do not do fixpoint ------------------------
growWC :: TyVarSet -> WantedConstraints -> TyVarSet -> TyVarSet
growWC gbl_tvs wc = growImplics gbl_tvs             (wc_impl wc) .
                    growPreds   gbl_tvs evVarOfPred (wc_flat wc) .
                    growPreds   gbl_tvs evVarOfPred (wc_insol wc)
422

423 424 425 426 427 428 429 430 431 432 433 434 435 436
growImplics :: TyVarSet -> Bag Implication -> TyVarSet -> TyVarSet
growImplics gbl_tvs implics tvs
  = foldrBag grow_implic tvs implics
  where
    grow_implic implic tvs
      = grow tvs `minusVarSet` ic_skols implic
      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
437
  where
438 439
    extend item tvs = tvs `unionVarSet`
                      (growPredTyVars (get_pred item) tvs `minusVarSet` gbl_tvs)
440 441 442 443 444 445 446

--------------------
quantifyMe :: TyVarSet      -- Quantifying over these
	   -> WantedEvVar
	   -> Bool	    -- True <=> quantify over this wanted
quantifyMe qtvs wev
  | isIPPred pred = True  -- Note [Inheriting implicit parameters]
batterseapower's avatar
batterseapower committed
447
  | otherwise	  = tyVarsOfType pred `intersectsVarSet` qtvs
448
  where
449
    pred = evVarOfPred wev
450
\end{code}
451

452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469
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.


470 471
Note [Inheriting implicit parameters]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
472 473 474
Consider this:

	f x = (x::Int) + ?y
475

476 477 478
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:
479

480 481 482
	f :: Int -> Int

(so we get ?y from the context of f's definition), or
483 484 485

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

486 487 488 489 490 491
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.

492 493
BOTTOM LINE: when *inferring types* you *must* quantify 
over implicit parameters. See the predicate isFreeWhenInferring.
494

495

496 497 498 499 500
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
501

502 503
Note [Simplifying RULE lhs constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
504
On the LHS of transformation rules we only simplify only equalities,
505 506 507 508
but not dictionaries.  We want to keep dictionaries unsimplified, to
serve as the available stuff for the RHS of the rule.  We *do* want to
simplify equalities, however, to detect ill-typed rules that cannot be
applied.
509

510 511 512
Implementation: the TcSFlags carried by the TcSMonad controls the
amount of simplification, so simplifyRuleLhs just sets the flag
appropriately.
513

514 515 516 517 518 519 520 521 522 523
Example.  Consider the following left-hand side of a rule
	f (x == y) (y > z) = ...
If we typecheck this expression we get constraints
	d1 :: Ord a, d2 :: Eq a
We do NOT want to "simplify" to the LHS
	forall x::a, y::a, z::a, d1::Ord a.
	  f ((==) (eqFromOrd d1) x y) ((>) d1 y z) = ...
Instead we want	
	forall x::a, y::a, z::a, d1::Ord a, d2::Eq a.
	  f ((==) d2 x y) ((>) d1 y z) = ...
524

525
Here is another example:
526 527
	fromIntegral :: (Integral a, Num b) => a -> b
	{-# RULES "foo"  fromIntegral = id :: Int -> Int #-}
528 529
In the rule, a=b=Int, and Num Int is a superclass of Integral Int. But
we *dont* want to get
530
	forall dIntegralInt.
531
	   fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int
532
because the scsel will mess up RULE matching.  Instead we want
533
	forall dIntegralInt, dNumInt.
534
	  fromIntegral Int Int dIntegralInt dNumInt = id Int
535

536 537 538 539 540 541 542
Even if we have 
	g (x == y) (y == z) = ..
where the two dictionaries are *identical*, we do NOT WANT
	forall x::a, y::a, z::a, d1::Eq a
	  f ((==) d1 x y) ((>) d1 y z) = ...
because that will only match if the dict args are (visibly) equal.
Instead we want to quantify over the dictionaries separately.
543

544 545
In short, simplifyRuleLhs must *only* squash equalities, leaving
all dicts unchanged, with absolutely no sharing.  
546

547 548 549 550 551 552 553 554 555
HOWEVER, under a nested implication things are different
Consider
  f :: (forall a. Eq a => a->a) -> Bool -> ...
  {-# RULES "foo" forall (v::forall b. Eq b => b->b).
       f b True = ...
    #=}
Here we *must* solve the wanted (Eq a) from the given (Eq a)
resulting from skolemising the agument type of g.  So we 
revert to SimplCheck when going under an implication.  
556 557

\begin{code}
558 559 560 561 562 563 564 565 566
simplifyRule :: RuleName 
             -> [TcTyVar]		-- Explicit skolems
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
             -> TcM ([EvVar], 		-- LHS dicts
                     TcEvBinds,		-- Evidence for LHS
                     TcEvBinds)		-- Evidence for RHS
-- See Note [Simplifying RULE lhs constraints]
simplifyRule name tv_bndrs lhs_wanted rhs_wanted
567 568 569 570 571 572 573
  = do { loc        <- getCtLoc (RuleSkol name)
       ; zonked_lhs <- zonkWC lhs_wanted
       ; let untch = NoUntouchables
	     	 -- We allow ourselves to unify environment 
		 -- variables; hence *no untouchables*

       ; (lhs_results, lhs_binds)
574
              <- runTcS (SimplRuleLhs name) untch $
575
                 solveWanteds emptyInert zonked_lhs
576 577 578 579 580 581 582

       ; traceTc "simplifyRule" $
         vcat [ text "zonked_lhs"   <+> ppr zonked_lhs 
              , text "lhs_results" <+> ppr lhs_results
              , text "lhs_binds"    <+> ppr lhs_binds 
              , text "rhs_wanted"   <+> ppr rhs_wanted ]

583 584

       -- Don't quantify over equalities (judgement call here)
585 586 587 588 589 590 591 592 593 594 595 596 597 598 599
       ; let (eqs, dicts) = partitionBag (isEqPred . evVarOfPred)
                                         (wc_flat lhs_results)
             lhs_dicts    = map evVarOf (bagToList dicts)
                                 -- Dicts and implicit parameters

           -- Fail if we have not got down to unsolved flats
       ; ev_binds_var <- newTcEvBinds
       ; emitImplication $ Implic { ic_untch  = untch
                                  , ic_env    = emptyNameEnv
                                  , ic_skols  = mkVarSet tv_bndrs
                                  , ic_given  = lhs_dicts
                                  , ic_wanted = lhs_results { wc_flat = eqs }
                                  , ic_insol  = insolubleWC lhs_results
                                  , ic_binds  = ev_binds_var
                                  , ic_loc    = loc }
600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615

	     -- Notice that we simplify the RHS with only the explicitly
	     -- introduced skolems, allowing the RHS to constrain any 
	     -- unification variables.
	     -- Then, and only then, we call zonkQuantifiedTypeVariables
	     -- Example   foo :: Ord a => a -> a
	     --		  foo_spec :: Int -> Int
	     --		  {-# RULE "foo"  foo = foo_spec #-}
	     --	    Here, it's the RHS that fixes the type variable

	     -- So we don't want to make untouchable the type
	     -- variables in the envt of the RHS, because they include
	     -- the template variables of the RULE

	     -- Hence the rather painful ad-hoc treatement here
       ; rhs_binds_var@(EvBindsVar evb_ref _)  <- newTcEvBinds
616 617
       ; let doc = ptext (sLit "rhs of rule") <+> doubleQuotes (ftext name)
       ; rhs_binds1 <- simplifyCheck (SimplCheck doc) $
618 619 620 621 622 623 624 625 626 627 628
            WC { wc_flat = emptyBag
               , wc_insol = emptyBag
               , wc_impl = unitBag $
                    Implic { ic_untch   = NoUntouchables
                            , ic_env    = emptyNameEnv
                            , ic_skols  = mkVarSet tv_bndrs
                            , ic_given  = lhs_dicts
                            , ic_wanted = rhs_wanted
                            , ic_insol  = insolubleWC rhs_wanted
                            , ic_binds  = rhs_binds_var
                            , ic_loc    = loc } }
629 630 631 632 633
       ; rhs_binds2 <- readTcRef evb_ref

       ; return ( lhs_dicts
                , EvBinds lhs_binds 
                , EvBinds (rhs_binds1 `unionBags` evBindMapBinds rhs_binds2)) }
634 635 636
\end{code}


637 638 639 640 641
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
642 643

\begin{code}
644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659
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
660
  = do { wanteds <- zonkWC wanteds
661 662 663 664

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

665 666
       ; (unsolved, ev_binds) <- runTcS ctxt NoUntouchables $
                                 solveWanteds emptyInert wanteds
667 668

       ; traceTc "simplifyCheck }" $
669
         ptext (sLit "unsolved =") <+> ppr unsolved
670

671
       ; reportUnsolved unsolved
672 673 674 675

       ; return ev_binds }

----------------
676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694
solveWanteds :: InertSet                            -- Given
             -> WantedConstraints
             -> TcS WantedConstraints
solveWanteds inert wanted
  = do { (unsolved_flats, unsolved_implics, insols)
             <- solve_wanteds inert wanted
       ; return (WC { wc_flat = keepWanted unsolved_flats   -- Discard Derived
                    , wc_impl = unsolved_implics
                    , wc_insol = insols }) }

solve_wanteds :: InertSet                            -- Given
              -> WantedConstraints
              -> TcS (Bag FlavoredEvVar, Bag Implication, Bag FlavoredEvVar)
-- solve_wanteds iterates when it is able to float equalities
-- out of one or more of the implications
solve_wanteds inert wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols })
  = do { traceTcS "solveWanteds {" (ppr wanted)

                 -- Try the flat bit
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
695 696 697 698 699
                 -- 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!
700 701 702 703 704
       ; let all_flats = flats `unionBags` keepWanted insols
       ; inert1 <- solveInteractWanted inert (bagToList all_flats)

       ; (unsolved_flats, unsolved_implics) <- simpl_loop 1 inert1 implics

705 706
       ; bb <- getTcEvBindsBag
       ; tb <- getTcSTyBindsMap
707
       ; traceTcS "solveWanteds }" $
708
                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
709
                      , text "unsolved_implics =" <+> ppr unsolved_implics
710 711 712 713
                      , text "current evbinds  =" <+> vcat (map ppr (varEnvElts bb))
                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
                      ]

714
       ; (subst, remaining_flats) <- solveCTyFunEqs unsolved_flats
715
                -- See Note [Solving Family Equations]
716 717 718 719 720 721 722 723
                -- NB: remaining_flats has already had subst applied

       ; let (insoluble_flats, unsolved_flats) = partitionBag isCFrozenErr remaining_flats

       ; return ( mapBag (substFlavoredEvVar subst . deCanonicalise) unsolved_flats
                , mapBag (substImplication subst) unsolved_implics
                , mapBag (substFlavoredEvVar subst . deCanonicalise) insoluble_flats ) }

724
  where
725 726
    simpl_loop :: Int
               -> InertSet
727
               -> Bag Implication
728 729
               -> TcS (CanonicalCts, Bag Implication) -- CanonicalCts are Wanted or Derived
    simpl_loop n inert implics
730
      | n>10
731
      = trace "solveWanteds: loop" $	                -- Always bleat
732
        do { traceTcS "solveWanteds: loop" (ppr inert)  -- Bleat more informatively
733
           ; let (_, unsolved_cans) = extractUnsolved inert
734
           ; return (unsolved_cans, implics) }
735 736

      | otherwise
737 738 739
      = do { traceTcS "solveWanteds: simpl_loop start {" $
                 vcat [ text "n =" <+> ppr n
                      , text "implics =" <+> ppr implics
740 741 742
                      , text "inert   =" <+> ppr inert ]
           
           ; let (just_given_inert, unsolved_cans) = extractUnsolved inert
743
                     -- unsolved_cans contains either Wanted or Derived!
744

745
           ; (implic_eqs, unsolved_implics) 
746
                  <- solveNestedImplications just_given_inert unsolved_cans implics
747 748

                -- Apply defaulting rules if and only if there
749 750
		-- no floated equalities.  If there are, they may
		-- solve the remaining wanteds, so don't do defaulting.
751 752 753
           ; improve_eqs <- if not (isEmptyBag implic_eqs)
			    then return implic_eqs
                            else applyDefaultingRules just_given_inert unsolved_cans
754

755
           ; traceTcS "solveWanteds: simpl_loop end }" $
756 757
                 vcat [ text "improve_eqs      =" <+> ppr improve_eqs
                      , text "unsolved_flats   =" <+> ppr unsolved_cans
758 759
                      , text "unsolved_implics =" <+> ppr unsolved_implics ]

760 761 762 763
           ; (improve_eqs_already_in_inert, inert_with_improvement)
               <- solveInteract inert improve_eqs 

           ; if improve_eqs_already_in_inert then
764
                 return (unsolved_cans, unsolved_implics)
765
             else 
766 767 768
                 simpl_loop (n+1) inert_with_improvement 
                                         -- Contain unsolved_cans and the improve_eqs
                                  unsolved_implics
769 770
           }

dimitris's avatar
dimitris committed
771 772 773 774
givensFromWanteds :: SimplContext -> CanonicalCts -> Bag FlavoredEvVar
-- Extract the Wanted ones from CanonicalCts and conver to
-- Givens; not Given/Solved, see Note [Preparing inert set for implications]
givensFromWanteds _ctxt = foldrBag getWanted emptyBag
775 776 777
  where
    getWanted :: CanonicalCt -> Bag FlavoredEvVar -> Bag FlavoredEvVar
    getWanted cc givens
dimitris's avatar
dimitris committed
778 779 780 781 782 783 784 785 786 787 788 789 790
      | pushable_wanted cc
      = let given = mkEvVarX (cc_id cc) (mkGivenFlavor (cc_flavor cc) UnkSkol)
        in given `consBag` givens     -- and not mkSolvedFlavor,
                                      -- see Note [Preparing inert set for implications]
      | otherwise = givens

    pushable_wanted :: CanonicalCt -> Bool 
    pushable_wanted cc 
      | not (isCFrozenErr cc) 
      , isWantedCt cc 
      = isEqPred (evVarPred (cc_id cc)) -- see Note [Preparing inert set for implications]
      | otherwise = False 
 
791 792
solveNestedImplications :: InertSet -> CanonicalCts
                        -> Bag Implication
793
                        -> TcS (Bag FlavoredEvVar, Bag Implication)
794
solveNestedImplications just_given_inert unsolved_cans implics
795 796 797
  | isEmptyBag implics
  = return (emptyBag, emptyBag)
  | otherwise 
798 799
  = do {  -- See Note [Preparing inert set for implications]
	  -- Push the unsolved wanteds inwards, but as givens
dimitris's avatar
dimitris committed
800 801 802 803
             
       ; simpl_ctx <- getTcSContext 

       ; let pushed_givens    = givensFromWanteds simpl_ctx unsolved_cans
804 805 806
             tcs_untouchables = filterVarSet isFlexiTcsTv $
                                tyVarsOfEvVarXs pushed_givens
             -- See Note [Extra TcsTv untouchables]
807

808 809
       ; traceTcS "solveWanteds: preparing inerts for implications {"  
                  (vcat [ppr tcs_untouchables, ppr pushed_givens])
dimitris's avatar
dimitris committed
810 811

       ; (_, inert_for_implics) <- solveInteract just_given_inert pushed_givens 
812

813
       ; traceTcS "solveWanteds: } now doing nested implications {" $
814 815 816 817 818 819 820 821 822 823 824 825
         vcat [ text "inerts_for_implics =" <+> ppr inert_for_implics
              , text "implics =" <+> ppr implics ]

       ; (implic_eqs, unsolved_implics)
           <- flatMapBagPairM (solveImplication tcs_untouchables inert_for_implics) implics

       ; traceTcS "solveWanteds: done nested implications }" $
                  vcat [ text "implic_eqs ="       <+> ppr implic_eqs
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

       ; return (implic_eqs, unsolved_implics) }

826 827 828 829 830
solveImplication :: TcTyVarSet                -- Untouchable TcS unification variables
                 -> InertSet                  -- Given
                 -> Implication               -- Wanted
                 -> TcS (Bag FlavoredEvVar, -- All wanted or derived unifications: var = type
                         Bag Implication)     -- Unsolved rest (always empty or singleton)
831 832 833 834 835 836 837
-- Returns: 
--  1. A bag of floatable wanted constraints, not mentioning any skolems, 
--     that are of the form unification var = type
-- 
--  2. Maybe a unsolved implication, empty if entirely solved! 
-- 
-- Precondition: everything is zonked by now
838
solveImplication tcs_untouchables inert
839 840 841 842
     imp@(Implic { ic_untch  = untch 
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
                 , ic_given  = givens
843
                 , ic_wanted = wanteds
844
                 , ic_loc    = loc })
845
  = nestImplicTcS ev_binds (untch, tcs_untouchables) $
846 847 848 849
    recoverTcS (return (emptyBag, emptyBag)) $
       -- Recover from nested failures.  Even the top level is
       -- just a bunch of implications, so failing at the first
       -- one is bad
850 851 852
    do { traceTcS "solveImplication {" (ppr imp) 

         -- Solve flat givens
853
       ; given_inert <- solveInteractGiven inert loc givens 
854 855

         -- Simplify the wanteds
856 857 858 859 860 861
       ; (unsolved_flats, unsolved_implics, insols)
             <- solve_wanteds given_inert wanteds

       ; let (res_flat_free, res_flat_bound)
                 = floatEqualities skols givens unsolved_flats
             final_flat = keepWanted res_flat_bound
862

863 864 865 866 867 868
       ; let res_wanted = WC { wc_flat = final_flat
                             , wc_impl = unsolved_implics
                             , wc_insol = insols }
             res_implic = unitImplication $
                          imp { ic_wanted = res_wanted
                              , ic_insol  = insolubleWC res_wanted }
869 870 871

       ; traceTcS "solveImplication end }" $ vcat
             [ text "res_flat_free =" <+> ppr res_flat_free
872
             , text "res_implic =" <+> ppr res_implic ]
873

874
       ; return (res_flat_free, res_implic) }
875 876


877 878 879 880 881 882
floatEqualities :: TcTyVarSet -> [EvVar]
                -> Bag FlavoredEvVar -> (Bag FlavoredEvVar, Bag FlavoredEvVar)
-- 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)
883
          -- Note [Float Equalities out of Implications]
884 885 886 887
  | otherwise = partitionBag is_floatable wantders
  

  where is_floatable :: FlavoredEvVar -> Bool
batterseapower's avatar
batterseapower committed
888 889
        is_floatable (EvVarX eqv _fl)
          | isEqPred (evVarPred eqv) = skols `disjointVarSet` tvs_under_fsks (evVarPred eqv)
890
        is_floatable _flev = False
891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907

        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)
        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 
        	     	       	      -- can mention type variables!
          | 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
908
\end{code}
909

910 911 912 913
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:
914 915

  a) In checking mode, suppresses unnecessary errors.  We already have
916
     on unsolved-wanted error; adding it to the givens prevents any 
917
     consequential errors from showing up
918

919 920 921 922
  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.

923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945
  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).)

946 947 948 949 950 951 952 953
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
954 955
'solveInteract' step first. 

dimitris's avatar
dimitris committed
956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991
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.


992 993
Note [Extra TcsTv untouchables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
994 995 996 997 998
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):
999 1000 1001

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

1002 1003 1004
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
1005
       uf  ~  beta  
1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021
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.
1022

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1023 1024 1025 1026
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
We want to float equalities out of vanilla existentials, but *not* out 
of GADT pattern matches. 
1027

1028

1029 1030
\begin{code}

1031
solveCTyFunEqs :: CanonicalCts -> TcS (TvSubst, CanonicalCts)
1032 1033 1034 1035
-- 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 CanonicalCts and implications
--          where the newly generated equalities (alpha := F xi) have been substituted through.
1036
solveCTyFunEqs cts
1037
 = do { untch   <- getUntouchables 
1038 1039
      ; let (unsolved_can_cts, (ni_subst, cv_binds))
                = getSolvableCTyFunEqs untch cts
1040
      ; traceTcS "defaultCTyFunEqs" (vcat [text "Trying to default family equations:"
1041
                                          , ppr ni_subst, ppr cv_binds
1042
                                          ])
1043 1044 1045 1046
      ; mapM_ solve_one cv_binds

      ; return (niFixTvSubst ni_subst, unsolved_can_cts) }
  where
1047
    solve_one (cv,tv,ty) = do { setWantedTyBind tv ty
batterseapower's avatar
batterseapower committed
1048
                              ; setEqBind cv (mkReflCo ty) }
1049

1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061
------------
type FunEqBinds = (TvSubstEnv, [(CoVar, TcTyVar, TcType)])
  -- The TvSubstEnv is not idempotent, but is loop-free
  -- See Note [Non-idempotent substitution] in Unify
emptyFunEqBinds :: FunEqBinds
emptyFunEqBinds = (emptyVarEnv, [])

extendFunEqBinds :: FunEqBinds -> CoVar -> TcTyVar -> TcType -> FunEqBinds
extendFunEqBinds (tv_subst, cv_binds) cv tv ty
  = (extendVarEnv tv_subst tv ty, (cv, tv, ty):cv_binds)

------------
1062
getSolvableCTyFunEqs :: TcsUntouchables
1063
                     -> CanonicalCts                -- Precondition: all Wanteds or Derived!
1064 1065
                     -> (CanonicalCts, FunEqBinds)  -- Postcondition: returns the unsolvables
getSolvableCTyFunEqs untch cts
1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090
  = Bag.foldlBag dflt_funeq (emptyCCan, emptyFunEqBinds) cts
  where
    dflt_funeq :: (CanonicalCts, FunEqBinds) -> CanonicalCt
               -> (CanonicalCts, FunEqBinds)
    dflt_funeq (cts_in, feb@(tv_subst, _))
               (CFunEqCan { cc_id = cv
                          , cc_flavor = fl
                          , cc_fun = tc
                          , cc_tyargs = xis
                          , cc_rhs = xi })
      | Just tv <- tcGetTyVar_maybe xi      -- RHS is a type variable

      , isTouchableMetaTyVar_InRange untch tv
           -- And it's a *touchable* unification variable

      , typeKind xi `isSubKind` tyVarKind tv
         -- Must do a small kind check since TcCanonical invariants 
         -- on family equations only impose compatibility, not subkinding

      , not (tv `elemVarEnv` tv_subst)
           -- Check not in extra_binds
           -- See Note [Solving Family Equations], Point 1

      , not (tv `elemVarSet` niSubstTvSet tv_subst (tyVarsOfTypes xis))
           -- Occurs check: see Note [Solving Family Equations], Point 2
dimitris's avatar
dimitris committed
1091
      = ASSERT ( not (isGivenOrSolved fl) )
1092 1093 1094 1095
        (cts_in, extendFunEqBinds feb cv tv (mkTyConApp tc xis))

    dflt_funeq (cts_in, fun_eq_binds) ct
      = (cts_in `extendCCans` ct, fun_eq_binds)
1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118
\end{code}

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

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

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

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

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


1119 1120 1121 1122 1123 1124 1125
*********************************************************************************
*                                                                               * 
*                          Defaulting and disamgiguation                        *
*                                                                               *
*********************************************************************************

Basic plan behind applyDefaulting rules: 
1126
 
1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152
 Step 1:  
    Split wanteds into defaultable groups, `groups' and the rest `rest_wanted' 
    For each defaultable group, do: 
      For each possible substitution for [alpha |-> tau] where `alpha' is the 
      group's variable, do: 
        1) Make up new TcEvBinds
        2) Extend TcS with (groupVariable 
        3) given_inert <- solveOne inert (given : a ~ tau) 
        4) (final_inert,unsolved) <- solveWanted (given_inert) (group_constraints)
        5) if unsolved == empty then 
                 sneakyUnify a |-> tau 
                 write the evidence bins
                 return (final_inert ++ group_constraints,[]) 
                      -- will contain the info (alpha |-> tau)!!
                 goto next defaultable group 
           if unsolved <> empty then 
                 throw away evidence binds
                 try next substitution 
     If you've run out of substitutions for this group, too bad, you failed 
                 return (inert,group) 
                 goto next defaultable group
 
 Step 2: 
   Collect all the (canonical-cts, wanteds) gathered this way. 
   - Do a solveGiven over the canonical-cts to make sure they are inert 
------------------------------------------------------------------------------------------
1153

1154 1155

\begin{code}
1156
applyDefaultingRules :: InertSet
1157 1158
                     -> CanonicalCts             -- All wanteds
                     -> TcS (Bag FlavoredEvVar)  -- All wanteds again!