TcSimplify.lhs 56 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,
       solveWantedsTcM
14
  ) where
15

16
#include "HsVersions.h"
17

18
import TcRnTypes
19
import TcRnMonad
20
import TcErrors
21
import TcMType
22 23
import TcType 
import TcSMonad 
24
import TcInteract 
25
import Inst
26
import Unify	( niFixTvSubst, niSubstTvSet )
27 28
import Type     ( classifyPredType, PredTree(..), getClassPredTys_maybe )
import Class    ( Class )
29
import Var
30
import Unique
31
import VarSet
32
import VarEnv 
33
import TcEvidence
34
import TypeRep
35
import Name
36
import Bag
37 38
import ListSetOps
import Util
39 40 41
import PrelInfo
import PrelNames
import Class		( classKey )
42
import BasicTypes       ( RuleName )
43
import Control.Monad    ( when )
44
import Outputable
45
import FastString
dimitris's avatar
dimitris committed
46
import TrieMap () -- DV: for now
47
import DynFlags
48 49 50
\end{code}


51 52 53 54 55
*********************************************************************************
*                                                                               * 
*                           External interface                                  *
*                                                                               *
*********************************************************************************
56

57

58 59 60
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
61 62 63
-- 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
64
simplifyTop wanteds 
65
  = do { zonked_wanteds <- zonkWC wanteds
66 67

       ; traceTc "simplifyTop {" $ text "zonked_wc =" <+> ppr zonked_wanteds
68 69 70 71 72 73 74 75 76
       ; (final_wc, binds1) <- runTcS (simpl_top zonked_wanteds)
       ; traceTc "End simplifyTop }" empty

       ; traceTc "reportUnsolved {" empty
                 -- See Note [Deferring coercion errors to runtime]
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
       ; binds2 <- reportUnsolved runtimeCoercionErrors final_wc
       ; traceTc "reportUnsolved }" empty
       ; return (binds1 `unionBags` binds2) }
77

78 79 80 81 82 83
  where
    -- See Note [Top-level Defaulting Plan]
    simpl_top wanteds
      = do { wc_first_go <- solveWantedsTcS wanteds
           ; applyTyVarDefaulting wc_first_go 
           ; simpl_top_loop wc_first_go }
84
    
85 86 87 88 89 90 91 92 93 94 95 96
    simpl_top_loop wc
      | isEmptyWC wc 
      = return wc
      | otherwise
      = do { wc_residual <- solveWantedsTcS wc
           ; let wc_flat_approximate = approximateWC wc_residual
           ; something_happened <- applyDefaultingRules wc_flat_approximate
                                        -- See Note [Top-level Defaulting Plan]
           ; if something_happened then 
               simpl_top_loop wc_residual 
             else 
               return wc_residual }
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
\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}
141

142 143 144
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
145
  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >> 
146
    simplifyTop wanteds  -- NB: must be simplifyTop so that we
147 148
                         --     do ambiguity resolution.  
                         -- See Note [Impedence matching] in TcBinds.
149
 
150 151 152
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
153 154
  = traceTc "simplifyInteractive" empty >>
    simplifyTop wanteds 
155 156 157 158 159

------------------
simplifyDefault :: ThetaType	-- Wanted; has no type variables in it
                -> TcM ()	-- Succeeds iff the constraint is soluble
simplifyDefault theta
160 161
  = do { traceTc "simplifyInteractive" empty
       ; wanted <- newFlatWanteds DefaultOrigin theta
162 163 164 165 166 167 168 169
       ; (unsolved, _binds) <- solveWantedsTcM (mkFlatWC wanted)

       ; traceTc "reportUnsolved {" empty
       -- See Note [Deferring coercion errors to runtime]
       ; runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
       ; _ <- reportUnsolved runtimeCoercionErrors unsolved 
       ; traceTc "reportUnsolved }" empty

170 171
       ; return () }
\end{code}
172

173

174
***********************************************************************************
175
*                                                                                 * 
176
*                            Deriving                                             *
177 178
*                                                                                 *
***********************************************************************************
179

180 181
\begin{code}
simplifyDeriv :: CtOrigin
182 183 184 185
              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
186 187
-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
188
-- Fail if not possible
189
simplifyDeriv orig pred tvs theta 
190
  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
191 192 193 194
      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
195

196
       ; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
197
             skol_set   = mkVarSet tvs_skols
198
	     doc = ptext (sLit "deriving") <+> parens (ppr pred)
199 200 201

       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

202 203
       ; traceTc "simplifyDeriv" $ 
         vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
204
       ; (residual_wanted, _ev_binds1)
205
             <- solveWantedsTcM (mkFlatWC wanted)
206

207 208
       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
209
             get_good :: Ct -> Either PredType Ct
210 211 212 213 214 215
             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
216
                         where p = ctPred ct
217

218 219 220
       -- 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 })
221

222 223
       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
224
\end{code}
225

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250
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

251 252 253 254 255 256 257
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.
258

259 260 261
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)
262

263 264
Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
265

266 267 268 269
	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.  
270

271 272 273
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.
274

275 276
So for now we simply require that the derived instance context
should have only type-variable constraints.
277

278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306
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
*                                                                                 *
***********************************************************************************
307

dreixel's avatar
dreixel committed
308 309 310 311 312 313 314 315 316 317 318 319
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.

320
\begin{code}
321
simplifyInfer :: Bool
322 323 324
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
325
              -> WantedConstraints
326 327
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
328 329 330
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
331
                      TcEvBinds)    -- ... binding these evidence variables
332
simplifyInfer _top_lvl apply_mr name_taus wanteds
333 334 335
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
336
       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
dreixel's avatar
dreixel committed
337 338 339
       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
340
       ; return (qtvs, [], False, emptyTcEvBinds) }
341

342
  | otherwise
343
  = do { runtimeCoercionErrors <- doptM Opt_DeferTypeErrors
344
       ; gbl_tvs        <- tcGetGlobalTyVars
345
       ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
346
       ; zonked_wanteds <- zonkWC wanteds
347

348
       ; traceTc "simplifyInfer {"  $ vcat
349
             [ ptext (sLit "names =") <+> ppr (map fst name_taus)
350 351
             , ptext (sLit "taus =") <+> ppr (map snd name_taus)
             , ptext (sLit "tau_tvs (zonked) =") <+> ppr zonked_tau_tvs
352 353 354
             , ptext (sLit "gbl_tvs =") <+> ppr gbl_tvs
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
355
             , ptext (sLit "wanted =") <+> ppr zonked_wanteds
356 357
             ]

358 359 360 361 362
              -- 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!
363

364 365 366 367 368 369 370 371
              -- 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.
372
       ; ev_binds_var <- newTcEvBinds
373
       ; wanted_transformed <- solveWantedsWithEvBinds ev_binds_var zonked_wanteds
374 375

              -- Step 3) Fail fast if there is an insoluble constraint,
376 377 378
              -- unless we are deferring errors to runtime
       ; when (not runtimeCoercionErrors && insolubleWC wanted_transformed) $ 
         do { _ev_binds <- reportUnsolved False wanted_transformed; failM }
379 380

              -- Step 4) Candidates for quantification are an approximation of wanted_transformed
381 382 383
              -- 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]
384
 
385 386
              -- Step 5) Minimize the quantification candidates                             
              -- Step 6) Final candidates for quantification                
387 388 389 390 391 392 393 394 395 396 397
              -- We discard bindings, insolubles etc, because all we are
              -- care aout it 
       ; (quant_pred_candidates, _extra_binds)   
             <- runTcS $ do { let quant_candidates = approximateWC wanted_transformed               
                            ; promoteTyVars quant_candidates
                            ; _implics <- solveInteract quant_candidates
                            ; (flats, _insols) <- getInertUnsolved
                            ; return (map ctPred $ filter isWantedCt (bagToList flats)) }

             -- NB: quant_pred_candidates is already the fixpoint of any 
             --     unifications that may have happened
398 399
                  
       ; gbl_tvs        <- tcGetGlobalTyVars -- TODO: can we just use untch instead of gbl_tvs?
400
       ; zonked_tau_tvs <- zonkTyVarsAndFV zonked_tau_tvs
401
       
402
       ; let init_tvs  = zonked_tau_tvs `minusVarSet` gbl_tvs
403
             poly_qtvs = growThetaTyVars quant_pred_candidates init_tvs 
404
                         `minusVarSet` gbl_tvs
405
             pbound    = filter (quantifyPred poly_qtvs) quant_pred_candidates
406
             
407
	     -- Monomorphism restriction
408 409
             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
             constrained_tvs = tyVarsOfTypes quant_pred_candidates
410
	     mr_bites        = apply_mr && not (null pbound)
411

412 413
             (qtvs, bound) | mr_bites  = (mr_qtvs,   [])
                           | otherwise = (poly_qtvs, pbound)
414
             
415 416 417 418 419 420 421
       ; traceTc "simplifyWithApprox" $
         vcat [ ptext (sLit "quant_pred_candidates =") <+> ppr quant_pred_candidates
              , ptext (sLit "gbl_tvs=") <+> ppr gbl_tvs
              , ptext (sLit "zonked_tau_tvs=") <+> ppr zonked_tau_tvs
              , ptext (sLit "pbound =") <+> ppr pbound
              , ptext (sLit "init_qtvs =") <+> ppr init_tvs 
              , ptext (sLit "poly_qtvs =") <+> ppr poly_qtvs ]
422

423
       ; if isEmptyVarSet qtvs && null bound
424 425 426 427
         then do { traceTc "} simplifyInfer/no quantification" empty                   
                 ; emitConstraints wanted_transformed
                    -- Includes insolubles (if -fdefer-type-errors)
                    -- as well as flats and implications
428
                 ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
429 430
         else do

431 432 433
       { traceTc "simplifyApprox" $ 
         ptext (sLit "bound are =") <+> ppr bound 
         
434
            -- Step 4, zonk quantified variables 
435
       ; let minimal_flat_preds = mkMinimalBySCs bound
436 437
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
438 439 440 441
                        -- 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
442
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
443

444
            -- Step 7) Emit an implication
445 446
       ; minimal_bound_ev_vars <- mapM TcMType.newEvVar minimal_flat_preds
       ; lcl_env <- getLclTypeEnv
dreixel's avatar
dreixel committed
447
       ; gloc <- getCtLoc skol_info
448
       ; untch <- TcRnMonad.getUntouchables
449
       ; let implic = Implic { ic_untch    = pushUntouchables untch 
450
                             , ic_env      = lcl_env
451
                             , ic_skols    = qtvs_to_return
452 453
                             , ic_fsks     = []  -- wanted_tansformed arose only from solveWanteds
                                                 -- hence no flatten-skolems (which come from givens)
454
                             , ic_given    = minimal_bound_ev_vars
455
                             , ic_wanted   = wanted_transformed 
456 457 458 459
                             , ic_insol    = False
                             , ic_binds    = ev_binds_var
                             , ic_loc      = gloc }
       ; emitImplication implic
460
         
461 462 463
       ; traceTc "} simplifyInfer/produced residual implication for quantification" $
             vcat [ ptext (sLit "implic =") <+> ppr implic
                       -- ic_skols, ic_given give rest of result
464
                  , ptext (sLit "qtvs =") <+> ppr qtvs_to_return
465
                  , ptext (sLit "spb =") <+> ppr quant_pred_candidates
466 467
                  , ptext (sLit "bound =") <+> ppr bound ]

468 469
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
470
\end{code}
471 472


473 474
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508
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.



509 510 511 512 513 514 515 516 517
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.
518

519

520 521
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
522 523 524 525
    -------- 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.)
    ----------------------------------------
526
When inferring the type of a let-binding, with simplifyInfer,
527
try to avoid unnecessarily simplifying class constraints.
528 529
Doing so aids sharing, but it also helps with delicate 
situations like
530

531
   instance C t => C [t] where ..
532

533 534 535 536 537 538 539 540 541 542 543
   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.


544 545 546 547 548
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
549

550
See note [Simplifying RULE consraints] in TcRule
551

552 553 554 555 556 557 558 559 560 561 562 563 564 565
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.
566 567

\begin{code}
568 569 570
simplifyRule :: RuleName 
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
571 572 573
             -> TcM ([EvVar], WantedConstraints)   -- LHS evidence varaibles
-- See Note [Simplifying RULE constraints] in TcRule
simplifyRule name lhs_wanted rhs_wanted
574
  = do {      	 -- We allow ourselves to unify environment 
575
		 -- variables: runTcS runs with NoUntouchables
576
         (resid_wanted, _) <- solveWantedsTcM (lhs_wanted `andWC` rhs_wanted)
577

578 579
       ; zonked_lhs <- zonkWC lhs_wanted

580 581 582 583 584 585 586 587 588 589 590 591 592
       ; 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
             
593
       ; traceTc "simplifyRule" $
594
         vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
595
              , text "zonked_lhs" <+> ppr zonked_lhs 
596 597
              , text "q_cts"      <+> ppr q_cts ]

598 599
       ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
                , zonked_lhs { wc_flat = non_q_cts }) }
600 601 602
\end{code}


603 604 605 606 607
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
608

609 610 611 612 613 614
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
615

616 617
  a :: Int
  a = 'a'
618

619
  main = print "b"
620

621 622
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`.
623

624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646
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}
647

648
solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
649
-- Zonk the input constraints, and simplify them
650
-- Return the evidence binds in the BagEvBinds result
651
-- Discards all Derived stuff in result
652 653 654 655 656 657
solveWantedsTcM wanted 
  = do { zonked_wanted <- zonkWC wanted
       ; traceTc "solveWantedsTcM {" (ppr zonked_wanted)
       ; (wanteds', binds) <- runTcS (solve_wanteds_and_drop zonked_wanted)
       ; traceTc "solveWantedsTcM end }" (ppr wanteds') 
       ; return (wanteds', binds) }
658 659 660

solveWantedsWithEvBinds :: EvBindsVar -> WantedConstraints -> TcM WantedConstraints
-- Side-effect the EvBindsVar argument to add new bindings from solving
661
-- Discards all Derived stuff in result
662
solveWantedsWithEvBinds ev_binds_var wanted
663 664 665 666 667 668 669 670 671 672 673 674 675 676
  = runTcSWithEvBinds ev_binds_var (solve_wanteds_and_drop wanted)

solveWantedsTcS :: WantedConstraints -> TcS WantedConstraints
-- Solve, with current untouchables, augmenting the current
-- evidence bindings, ty_binds, and solved caches
-- However, revert the InertCans to the way they were at 
-- the beginning (since we are returning the residual)
solveWantedsTcS wanted = nestTcS (solve_wanteds_and_drop wanted)

solve_wanteds_and_drop :: WantedConstraints -> TcS (WantedConstraints)
-- Since solve_wanteds returns the residual WantedConstraints,
-- it should alway be called within a runTcS or something similar,
solve_wanteds_and_drop wanted = do { wc <- solve_wanteds wanted 
                                   ; return (dropDerivedWC wc) }
677 678

solve_wanteds :: WantedConstraints -> TcS WantedConstraints 
679
-- so that the inert set doesn't mindlessly propagate.
680
-- NB: wc_flats may be wanted /or/ derived now
681
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
682 683
  = do { traceTcS "solveWanteds {" (ppr wanted)

684 685
         -- Try the flat bit, including insolubles. Solving insolubles a 
         -- second time round is a bit of a waste but the code is simple 
686 687 688
         -- 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] 
689
       ; traceTcS "solveFlats {" empty
690
       ; let all_flats = flats `unionBags` insols
691 692
       ; impls_from_flats <- solveInteract all_flats
       ; traceTcS "solveFlats end }" (ppr impls_from_flats)
693

694 695
       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
696
       ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
697

698 699 700 701 702
       ; (unsolved_flats, insoluble_flats) <- getInertUnsolved

       ; wc <- unFlattenWC (WC { wc_flat  = unsolved_flats
                               , wc_impl  = unsolved_implics
                               , wc_insol = insoluble_flats })
703 704

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

713
       ; return wc }
714 715 716 717 718 719 720 721

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
722 723 724 725 726 727 728
  = 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)} }
729

730

731 732 733 734 735 736 737 738 739
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
740 741
       ; let thinner_inerts = prepareInertsForImplications inerts
                 -- See Note [Preparing inert set for implications]
742
  
743
       ; traceTcS "solveNestedImplications starting {" $ 
744
         vcat [ text "original inerts = " <+> ppr inerts
745 746
              , text "thinner_inerts  = " <+> ppr thinner_inerts ]
         
747
       ; (floated_eqs, unsolved_implics)
748
           <- flatMapBagPairM (solveImplication thinner_inerts) implics
749 750 751 752

       -- ... 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.
753
       ; traceTcS "solveNestedImplications end }" $
754
                  vcat [ text "all floated_eqs ="  <+> ppr floated_eqs
755 756
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

757
       ; return (floated_eqs, unsolved_implics) }
758

759
solveImplication :: InertSet
760 761 762 763 764
                 -> 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
765
solveImplication inerts
766
     imp@(Implic { ic_untch  = untch
767 768
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
769
                 , ic_fsks   = old_fsks
770
                 , ic_given  = givens
771
                 , ic_wanted = wanteds
772
                 , ic_loc    = loc })
773
  = 
774 775
    do { traceTcS "solveImplication {" (ppr imp) 

776
         -- Solve the nested constraints
777 778 779 780 781 782 783 784 785 786 787 788
         -- NB: 'inerts' has empty inert_fsks
       ; (new_fsks, residual_wanted) 
            <- nestImplicTcS ev_binds untch inerts $
               do { solveInteractGiven loc old_fsks givens 
                  ; residual_wanted <- solve_wanteds wanteds
                  ; more_fsks <- getFlattenSkols
                  ; return (more_fsks ++ old_fsks, residual_wanted) }

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

       ; let res_implic | isEmptyWC final_wanted 
789 790
                        = emptyBag
                        | otherwise
791 792 793
                        = unitBag (imp { ic_fsks   = new_fsks
                                       , ic_wanted = dropDerivedWC final_wanted
                                       , ic_insol  = insolubleWC final_wanted })
794

795
       ; evbinds <- getTcEvBindsMap
796
       ; traceTcS "solveImplication end }" $ vcat
797
             [ text "floated_eqs =" <+> ppr floated_eqs
798
             , text "new_fsks =" <+> ppr new_fsks
799 800
             , text "res_implic =" <+> ppr res_implic
             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]
801

802
       ; return (floated_eqs, res_implic) }
803 804 805 806
\end{code}


\begin{code}
807 808
floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints 
                -> TcS (Cts, WantedConstraints)
809 810
-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
811 812
-- Also performs some unifications, adding to monadically-carried ty_binds
-- These will be used when processing floated_eqs later
813 814
floatEqualities skols can_given wanteds@(WC { wc_flat = flats })
  | hasEqualities can_given 
815
  = return (emptyBag, wanteds)   -- Note [Float Equalities out of Implications]
816
  | otherwise 
817 818
  = do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats
       ; promoteTyVars float_eqs
819
       ; ty_binds <- getTcSTyBindsMap
820
       ; traceTcS "floatEqualities" (vcat [ text "Floated eqs =" <+> ppr float_eqs
821 822
                                          , text "Ty binds =" <+> ppr ty_binds])
       ; return (float_eqs, wanteds { wc_flat = remaining_flats }) }
823 824 825 826 827 828 829 830 831
  where 
    skol_set = growSkols wanteds (mkVarSet skols)

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

832 833 834 835 836
promoteTyVars :: Cts -> TcS ()
promoteTyVars cts
  = do { untch <- TcSMonad.getUntouchables
       ; mapM_ (promote_tv untch) (varSetElems (tyVarsOfCts cts)) }
  where
837 838 839 840 841 842 843 844
    promote_tv untch tv 
      | isFloatedTouchableMetaTyVar untch tv
      = do { cloned_tv <- TcSMonad.cloneMetaTyVar tv
           ; let rhs_tv = setMetaTyVarUntouchables cloned_tv untch
           ; setWantedTyBind tv (mkTyVarTy rhs_tv) }
      | otherwise
      = return ()

845 846 847 848 849 850 851 852 853
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
854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878

approximateWC :: WantedConstraints -> Cts
-- Postcondition: Wanted or Derived Cts 
approximateWC wc = float_wc emptyVarSet wc
  where 
    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
    float_implic skols imp
      = float_wc skols' (ic_wanted imp)
      where
        skols' = skols `extendVarSetList` ic_skols imp `extendVarSetList` ic_fsks imp
            
    float_flat :: TcTyVarSet -> Ct -> Cts
    float_flat skols ct
      | tyVarsOfCt ct `disjointVarSet` skols 
      = singleCt ct
      | otherwise = emptyCts
        
    do_bag :: (a -> Bag c) -> Bag a -> Bag c
    do_bag f = foldrBag (unionBags.f) emptyBag
\end{code}
879
\end{code}
880

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
881 882
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930
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. 

931 932 933 934 935
    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]
936 937 938 939 940 941 942

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:

943 944 945 946 947 948 949 950 951 952 953 954 955
    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 

956

957 958 959 960 961 962 963 964 965 966 967 968 969 970 971
    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
972

973 974
   (iii) Touchables that have been floated out from some nested 
         implications, see Note [Float Equalities out of Implications]. 
975

976 977 978 979 980
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):
981

982
   (F Int ~ uf)  /\  [untch=beta](forall a. C a => F Int ~ beta) 
983

984 985 986 987 988 989 990 991
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).
992

993 994 995 996
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.
997

998
\begin{code}
999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015
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
1016
     = do { untch   <- TcSMonad.getUntouchables 
1017 1018
          ; let (unsolved_can_cts, (ni_subst, cv_binds))
                    = getSolvableCTyFunEqs untch cts
1019 1020 1021 1022 1023
          ; 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
1024 1025 1026 1027 1028
                                              ])
          ; mapM_ solve_one cv_binds

          ; return (niFixTvSubst ni_subst, unsolved_can_cts) }
      where
Simon Peyton Jones's avatar
Simon Peyton Jones committed
1029
        solve_one (CtWanted { ctev_evar = cv }, tv, ty) 
1030
          = setWantedTyBind tv ty >> setEvBind cv (EvCoercion (mkTcReflCo ty))
Simon Peyton Jones's avatar
Simon Peyton Jones committed
1031
        solve_one (CtDerived {}, tv, ty)
1032 1033 1034 1035
          = setWantedTyBind tv ty
        solve_one arg
          = pprPanic "solveCTyFunEqs: can't solve a /given/ family equation!" $ ppr arg

1036
------------
1037
type FunEqBinds = (TvSubstEnv, [(CtEvidence, TcTyVar, TcType)])
1038 1039 1040 1041 1042
  -- The TvSubstEnv is not idempotent, but is loop-free
  -- See Note [Non-idempotent substitution] in Unify
emptyFunEqBinds :: FunEqBinds
emptyFunEqBinds = (emptyVarEnv, [])

1043
extendFunEqBinds :: FunEqBinds -> CtEvidence -> TcTyVar -> TcType -> FunEqBinds
dimitris's avatar
dimitris committed
1044 1045
extendFunEqBinds (tv_subst, cv_binds) fl tv ty
  = (extendVarEnv tv_subst tv ty, (fl, tv, ty):cv_binds)
1046 1047

------------
1048
getSolvableCTyFunEqs :: Untouchables
1049 1050
                     -> Cts                -- Precondition: all Wanteds or Derived!
                     -> (Cts, FunEqBinds)  -- Postcondition: returns the unsolvables
1051
getSolvableCTyFunEqs untch cts
1052
  = Bag.foldlBag dflt_funeq (emptyCts, emptyFunEqBinds) cts
1053
  where
1054 1055
    dflt_funeq :: (Cts, FunEqBinds) -> Ct
               -> (Cts, FunEqBinds)
1056
    dflt_funeq (cts_in, feb@(tv_subst, _))
1057
               (CFunEqCan { cc_ev = fl
1058 1059 1060 1061 1062
                          , cc_fun = tc
                          , cc_tyargs = xis
                          , cc_rhs = xi })