TcSimplify.lhs 52.5 KB
Newer Older
1
\begin{code}
Ian Lynagh's avatar
Ian Lynagh committed
2 3 4 5 6 7 8
{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

9
module TcSimplify( 
10
       simplifyInfer, simplifyAmbiguityCheck,
11
       simplifyDefault, simplifyDeriv, 
12 13
       simplifyRule, simplifyTop, simplifyInteractive,
       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 27
import Type     ( classifyPredType, PredTree(..), getClassPredTys_maybe )
import Class    ( Class )
28
import Var
29
import Unique
30
import VarSet
31
import VarEnv 
32
import TcEvidence
33
import TypeRep
34
import Name
35
import Bag
36 37
import ListSetOps
import Util
38 39 40
import PrelInfo
import PrelNames
import Class		( classKey )
41
import BasicTypes       ( RuleName )
42
import Outputable
43
import FastString
dimitris's avatar
dimitris committed
44
import TrieMap () -- DV: for now
45 46 47
\end{code}


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

54 55 56
\begin{code}
simplifyTop :: WantedConstraints -> TcM (Bag EvBind)
-- Simplify top-level constraints
57 58 59
-- Usually these will be implications,
-- but when there is nothing to quantify we don't wrap
-- in a degenerate implication, so we do that here instead
60 61 62 63 64
simplifyTop wanteds
  = do { traceTc "simplifyTop {" $ text "wanted = " <+> ppr wanteds 
       ; ev_binds_var <- newTcEvBinds
       ; zonked_final_wc <- solveWantedsTcMWithEvBinds ev_binds_var wanteds simpl_top
       ; binds1 <- TcRnMonad.getTcEvBinds ev_binds_var
65 66 67
       ; traceTc "End simplifyTop }" empty

       ; traceTc "reportUnsolved {" empty
68
       ; binds2 <- reportUnsolved zonked_final_wc
69
       ; traceTc "reportUnsolved }" empty
70
         
71
       ; return (binds1 `unionBags` binds2) }
72

73 74
  where
    -- See Note [Top-level Defaulting Plan]
Simon Peyton Jones's avatar
Simon Peyton Jones committed
75
    simpl_top :: WantedConstraints -> TcS WantedConstraints
76
    simpl_top wanteds
77
      = do { wc_first_go <- nestTcS (solve_wanteds_and_drop wanteds)
78 79 80 81 82 83 84
           ; let meta_tvs = filter isMetaTyVar (varSetElems (tyVarsOfWC wc_first_go))
                   -- tyVarsOfWC: post-simplification the WC should reflect
                   --             all unifications that have happened
                   -- filter isMetaTyVar: we might have runtime-skolems in GHCi, 
                   -- and we definitely don't want to try to assign to those!

           ; mapM_ defaultTyVar meta_tvs   -- Has unification side effects
85
           ; simpl_top_loop wc_first_go }
86
    
87
    simpl_top_loop wc
88 89 90
      | isEmptyWC wc || insolubleWC wc
             -- Don't do type-class defaulting if there are insolubles
             -- Doing so is not going to solve the insolubles
91 92
      = return wc
      | otherwise
93
      = do { wc_residual <- nestTcS (solve_wanteds_and_drop wc)
94 95 96 97 98 99 100
           ; 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 }
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144
\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}
145

146 147 148
------------------
simplifyAmbiguityCheck :: Name -> WantedConstraints -> TcM (Bag EvBind)
simplifyAmbiguityCheck name wanteds
149
  = traceTc "simplifyAmbiguityCheck" (text "name =" <+> ppr name) >> 
150
    simplifyTop wanteds  -- NB: must be simplifyTop so that we
151 152
                         --     do ambiguity resolution.  
                         -- See Note [Impedence matching] in TcBinds.
153
 
154 155 156
------------------
simplifyInteractive :: WantedConstraints -> TcM (Bag EvBind)
simplifyInteractive wanteds 
157 158
  = traceTc "simplifyInteractive" empty >>
    simplifyTop wanteds 
159 160 161 162 163

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

       ; traceTc "reportUnsolved {" empty
       -- See Note [Deferring coercion errors to runtime]
170
       ; reportAllUnsolved unsolved 
171 172 173
         -- Postcondition of solveWantedsTcM is that returned
         -- constraints are zonked. So Precondition of reportUnsolved
         -- is true.
174 175
       ; traceTc "reportUnsolved }" empty

176 177
       ; return () }
\end{code}
178

179

180
***********************************************************************************
181
*                                                                                 * 
182
*                            Deriving                                             *
183 184
*                                                                                 *
***********************************************************************************
185

186 187
\begin{code}
simplifyDeriv :: CtOrigin
188 189 190 191
              -> PredType
	      -> [TyVar]	
	      -> ThetaType		-- Wanted
	      -> TcM ThetaType	-- Needed
192 193
-- Given  instance (wanted) => C inst_ty 
-- Simplify 'wanted' as much as possibles
194
-- Fail if not possible
195
simplifyDeriv orig pred tvs theta 
196
  = do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs -- Skolemize
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
197 198 199 200
      	 	-- The constraint solving machinery 
		-- expects *TcTyVars* not TyVars.  
		-- We use *non-overlappable* (vanilla) skolems
		-- See Note [Overlap and deriving]
201

202
       ; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
203
             skol_set   = mkVarSet tvs_skols
204
	     doc = ptext (sLit "deriving") <+> parens (ppr pred)
205 206 207

       ; wanted <- newFlatWanteds orig (substTheta skol_subst theta)

208 209
       ; traceTc "simplifyDeriv" $ 
         vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
210
       ; (residual_wanted, _ev_binds1)
211
             <- solveWantedsTcM (mkFlatWC wanted)
212
                -- Post: residual_wanted are already zonked
213

214 215
       ; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
                         -- See Note [Exotic derived instance contexts]
216
             get_good :: Ct -> Either PredType Ct
217 218 219 220 221 222
             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
223
                         where p = ctPred ct
224

225 226
       -- We never want to defer these errors because they are errors in the
       -- compiler! Hence the `False` below
227
       ; reportAllUnsolved (residual_wanted { wc_flat = bad })
228

229 230
       ; let min_theta = mkMinimalBySCs (bagToList good)
       ; return (substTheta subst_skol min_theta) }
231
\end{code}
232

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257
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

258 259 260 261 262 263 264
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.
265

266 267 268
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)
269

270 271
Notice that this instance (just) satisfies the Paterson termination 
conditions.  Then we *could* derive an instance decl like this:
272

273 274 275 276
	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.  
277

278 279 280
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.
281

282 283
So for now we simply require that the derived instance context
should have only type-variable constraints.
284

285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313
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
*                                                                                 *
***********************************************************************************
314

dreixel's avatar
dreixel committed
315 316 317 318 319 320 321 322 323 324 325 326
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.

327
\begin{code}
328
simplifyInfer :: Bool
329 330 331
              -> Bool                  -- Apply monomorphism restriction
              -> [(Name, TcTauType)]   -- Variables to be generalised,
                                       -- and their tau-types
332
              -> WantedConstraints
333 334
              -> TcM ([TcTyVar],    -- Quantify over these type variables
                      [EvVar],      -- ... and these constraints
335 336 337
		      Bool,	    -- The monomorphism restriction did something
		      		    --   so the results type is not as general as
				    --   it could be
338
                      TcEvBinds)    -- ... binding these evidence variables
339
simplifyInfer _top_lvl apply_mr name_taus wanteds
340 341 342
  | isEmptyWC wanteds
  = do { gbl_tvs     <- tcGetGlobalTyVars            -- Already zonked
       ; zonked_taus <- zonkTcTypes (map snd name_taus)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
343
       ; let tvs_to_quantify = varSetElems (tyVarsOfTypes zonked_taus `minusVarSet` gbl_tvs)
dreixel's avatar
dreixel committed
344 345 346
       	     		       -- tvs_to_quantify can contain both kind and type vars
       	                       -- See Note [Which variables to quantify]
       ; qtvs <- zonkQuantifiedTyVars tvs_to_quantify
347
       ; return (qtvs, [], False, emptyTcEvBinds) }
348

349
  | otherwise
350 351
  = do { traceTc "simplifyInfer {"  $ vcat
             [ ptext (sLit "binds =") <+> ppr name_taus
352 353
             , ptext (sLit "closed =") <+> ppr _top_lvl
             , ptext (sLit "apply_mr =") <+> ppr apply_mr
354
             , ptext (sLit "(unzonked) wanted =") <+> ppr wanteds
355 356
             ]

357 358 359 360 361
              -- 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!
362

363 364 365 366 367 368 369 370
              -- 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.
371 372

       ; ev_binds_var <- newTcEvBinds
373 374 375
       ; wanted_transformed <- solveWantedsTcMWithEvBinds ev_binds_var wanteds $
                               solve_wanteds_and_drop
                               -- Post: wanted_transformed are zonked
376 377

              -- Step 4) Candidates for quantification are an approximation of wanted_transformed
378 379 380
              -- 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]
381
 
382 383
              -- Step 5) Minimize the quantification candidates                             
              -- Step 6) Final candidates for quantification                
384
              -- We discard bindings, insolubles etc, because all we are
385 386
              -- care aout it

387 388 389 390 391 392 393 394 395 396 397 398 399
       ; tc_lcl_env <- TcRnMonad.getLclEnv
       ; let untch = tcl_untch tc_lcl_env
       ; quant_pred_candidates   
           <- if insolubleWC wanted_transformed 
              then return []   -- See Note [Quantification with errors]
              else do { gbl_tvs <- tcGetGlobalTyVars
                      ; let quant_cand = approximateWC wanted_transformed
                            meta_tvs   = filter isMetaTyVar (varSetElems (tyVarsOfCts quant_cand)) 
                      ; ((flats, _insols), _extra_binds) <- runTcS $ 
                        do { mapM_ (promoteAndDefaultTyVar untch gbl_tvs) meta_tvs
                           ; _implics <- solveInteract quant_cand
                           ; getInertUnsolved }
                      ; return (map ctPred $ filter isWantedCt (bagToList flats)) }
400 401 402 403 404
                   -- NB: Dimitrios is slightly worried that we will get
                   -- family equalities (F Int ~ alpha) in the quantification
                   -- candidates, as we have performed no further unflattening
                   -- at this point. Nothing bad, but inferred contexts might
                   -- look complicated.
405

406 407 408 409
       -- NB: quant_pred_candidates is already the fixpoint of any 
       --     unifications that may have happened
       ; gbl_tvs        <- tcGetGlobalTyVars
       ; zonked_tau_tvs <- zonkTyVarsAndFV (tyVarsOfTypes (map snd name_taus))
410
       ; let init_tvs  = zonked_tau_tvs `minusVarSet` gbl_tvs
411
             poly_qtvs = growThetaTyVars quant_pred_candidates init_tvs 
412
                         `minusVarSet` gbl_tvs
413
             pbound    = filter (quantifyPred poly_qtvs) quant_pred_candidates
414
             
415
	     -- Monomorphism restriction
416 417
             mr_qtvs  	     = init_tvs `minusVarSet` constrained_tvs
             constrained_tvs = tyVarsOfTypes quant_pred_candidates
418
	     mr_bites        = apply_mr && not (null pbound)
419

420 421
             (qtvs, bound) | mr_bites  = (mr_qtvs,   [])
                           | otherwise = (poly_qtvs, pbound)
422
             
423 424 425 426 427 428 429
       ; 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 ]
430

431
       ; if isEmptyVarSet qtvs && null bound
432 433 434 435
         then do { traceTc "} simplifyInfer/no quantification" empty                   
                 ; emitConstraints wanted_transformed
                    -- Includes insolubles (if -fdefer-type-errors)
                    -- as well as flats and implications
436
                 ; return ([], [], mr_bites, TcEvBinds ev_binds_var) }
437 438
         else do

439 440 441
       { traceTc "simplifyApprox" $ 
         ptext (sLit "bound are =") <+> ppr bound 
         
442
            -- Step 4, zonk quantified variables 
443
       ; let minimal_flat_preds = mkMinimalBySCs bound
444 445
             skol_info = InferSkol [ (name, mkSigmaTy [] minimal_flat_preds ty)
                                   | (name, ty) <- name_taus ]
446 447 448 449
                        -- 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
450
       ; qtvs_to_return <- zonkQuantifiedTyVars (varSetElems qtvs)
451

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

473 474
       ; return ( qtvs_to_return, minimal_bound_ev_vars
                , mr_bites,  TcEvBinds ev_binds_var) } }
475
\end{code}
476

477 478 479 480 481 482 483 484 485
Note [Quantification with errors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we find that the RHS of the definition has some absolutely-insoluble
constraints, we abandon all attempts to find a context to quantify
over, and instead make the function fully-polymorphic in whatever
type we have found.  For two reasons
  a) Minimise downstream errors
  b) Avoid spurious errors from this function
   
486

487 488
Note [Default while Inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522
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.



523 524 525 526 527 528 529 530 531
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.
532

533

534 535
Note [Avoid unecessary constraint simplification]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
536 537 538 539
    -------- 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.)
    ----------------------------------------
540
When inferring the type of a let-binding, with simplifyInfer,
541
try to avoid unnecessarily simplifying class constraints.
542 543
Doing so aids sharing, but it also helps with delicate 
situations like
544

545
   instance C t => C [t] where ..
546

547 548 549 550 551 552 553 554 555 556 557
   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.


558 559 560 561 562
*********************************************************************************
*                                                                                 * 
*                             RULES                                               *
*                                                                                 *
***********************************************************************************
563

564
See note [Simplifying RULE consraints] in TcRule
565

566 567 568 569 570 571 572 573 574 575 576 577 578 579
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.
580 581

\begin{code}
582 583 584
simplifyRule :: RuleName 
             -> WantedConstraints	-- Constraints from LHS
             -> WantedConstraints	-- Constraints from RHS
585 586 587
             -> TcM ([EvVar], WantedConstraints)   -- LHS evidence varaibles
-- See Note [Simplifying RULE constraints] in TcRule
simplifyRule name lhs_wanted rhs_wanted
588
  = do {      	 -- We allow ourselves to unify environment 
589
		 -- variables: runTcS runs with NoUntouchables
590
         (resid_wanted, _) <- solveWantedsTcM (lhs_wanted `andWC` rhs_wanted)
591
                              -- Post: these are zonked and unflattened
592

593 594
       ; zonked_lhs_flats <- zonkCts (wc_flat lhs_wanted)
       ; let (q_cts, non_q_cts) = partitionBag quantify_me zonked_lhs_flats
595 596 597 598 599 600 601 602 603 604 605 606
             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
             
607
       ; traceTc "simplifyRule" $
608
         vcat [ ptext (sLit "LHS of rule") <+> doubleQuotes (ftext name)
609
              , text "zonked_lhs_flats" <+> ppr zonked_lhs_flats 
610 611
              , text "q_cts"      <+> ppr q_cts ]

612
       ; return ( map (ctEvId . ctEvidence) (bagToList q_cts)
613
                , lhs_wanted { wc_flat = non_q_cts }) }
614 615 616
\end{code}


617 618 619 620 621
*********************************************************************************
*                                                                                 * 
*                                 Main Simplifier                                 *
*                                                                                 *
***********************************************************************************
622

623 624 625 626 627 628
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
629

630 631
  a :: Int
  a = 'a'
632

633
  main = print "b"
634

635 636
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`.
637

638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659
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`.

660 661 662 663 664 665 666 667 668 669
Note [Zonk after solving]
~~~~~~~~~~~~~~~~~~~~~~~~~
We zonk the result immediately after constraint solving, for two reasons:

a) because zonkWC generates evidence, and this is the moment when we
   have a suitable evidence variable to hand.

Note that *after* solving the constraints are typically small, so the
overhead is not great.

670
\begin{code}
671 672 673 674
solveWantedsTcMWithEvBinds :: EvBindsVar
                           -> WantedConstraints
                           -> (WantedConstraints -> TcS WantedConstraints)
                           -> TcM WantedConstraints
675 676 677 678 679 680
-- Returns a *zonked* result
-- We zonk when we finish primarily to un-flatten out any
-- flatten-skolems etc introduced by canonicalisation of
-- types involving type funuctions.  Happily the result 
-- is typically much smaller than the input, indeed it is 
-- often empty.
681
solveWantedsTcMWithEvBinds ev_binds_var wc tcs_action
682 683
  = do { traceTc "solveWantedsTcMWithEvBinds" $ text "wanted=" <+> ppr wc
       ; wc2 <- runTcSWithEvBinds ev_binds_var (tcs_action wc)
684
       ; zonkWC ev_binds_var wc2 }
685
         -- See Note [Zonk after solving]
686

687
solveWantedsTcM :: WantedConstraints -> TcM (WantedConstraints, Bag EvBind)
688
-- Zonk the input constraints, and simplify them
689
-- Return the evidence binds in the BagEvBinds result
690
-- Discards all Derived stuff in result
691
-- Postcondition: fully zonked and unflattened constraints
692
solveWantedsTcM wanted 
693 694 695
  = do { ev_binds_var <- newTcEvBinds
       ; wanteds' <- solveWantedsTcMWithEvBinds ev_binds_var wanted solve_wanteds_and_drop
       ; binds <- TcRnMonad.getTcEvBinds ev_binds_var
696
       ; return (wanteds', binds) }
697

698 699 700 701 702
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) }
703 704

solve_wanteds :: WantedConstraints -> TcS WantedConstraints 
705
-- so that the inert set doesn't mindlessly propagate.
706
-- NB: wc_flats may be wanted /or/ derived now
707
solve_wanteds wanted@(WC { wc_flat = flats, wc_impl = implics, wc_insol = insols }) 
708 709
  = do { traceTcS "solveWanteds {" (ppr wanted)

710
         -- Try the flat bit, including insolubles. Solving insolubles a 
Simon Peyton Jones's avatar
Simon Peyton Jones committed
711
         -- second time round is a bit of a waste; but the code is simple 
712 713 714
         -- 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] 
715
       ; traceTcS "solveFlats {" empty
716
       ; let all_flats = flats `unionBags` insols
717 718
       ; impls_from_flats <- solveInteract all_flats
       ; traceTcS "solveFlats end }" (ppr impls_from_flats)
719

720 721
       -- solve_wanteds iterates when it is able to float equalities 
       -- out of one or more of the implications. 
722
       ; unsolved_implics <- simpl_loop 1 (implics `unionBags` impls_from_flats)
723

724 725
       ; (unsolved_flats, insoluble_flats) <- getInertUnsolved

726 727 728 729 730 731
        -- We used to unflatten here but now we only do it once at top-level
        -- during zonking -- see Note [Unflattening while zonking] in TcMType
       ; let wc = WC { wc_flat  = unsolved_flats   
                     , wc_impl  = unsolved_implics 
                     , wc_insol = insoluble_flats }
                  
732
       ; bb <- getTcEvBindsMap
733
       ; tb <- getTcSTyBindsMap
734
       ; traceTcS "solveWanteds }" $
735
                 vcat [ text "unsolved_flats   =" <+> ppr unsolved_flats
736
                      , text "unsolved_implics =" <+> ppr unsolved_implics
737
                      , text "current evbinds  =" <+> ppr (evBindMapBinds bb)
738
                      , text "current tybinds  =" <+> vcat (map ppr (varEnvElts tb))
739
                      , text "final wc =" <+> ppr wc ]
740

741
       ; return wc }
742 743 744 745 746 747 748 749

simpl_loop :: Int
           -> Bag Implication
           -> TcS (Bag Implication)
simpl_loop n implics
  | n > 10 
  = traceTcS "solveWanteds: loop!" empty >> return implics
  | otherwise 
750 751 752 753 754 755 756
  = 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)} }
757

758

759 760 761 762 763 764 765 766 767
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
768 769
       ; let thinner_inerts = prepareInertsForImplications inerts
                 -- See Note [Preparing inert set for implications]
770
  
771
       ; traceTcS "solveNestedImplications starting {" $ 
772
         vcat [ text "original inerts = " <+> ppr inerts
773 774
              , text "thinner_inerts  = " <+> ppr thinner_inerts ]
         
775
       ; (floated_eqs, unsolved_implics)
776
           <- flatMapBagPairM (solveImplication thinner_inerts) implics
777 778 779 780

       -- ... 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.
781
       ; traceTcS "solveNestedImplications end }" $
782
                  vcat [ text "all floated_eqs ="  <+> ppr floated_eqs
783 784
                       , text "unsolved_implics =" <+> ppr unsolved_implics ]

785
       ; return (floated_eqs, unsolved_implics) }
786

787
solveImplication :: InertSet
788 789 790 791 792
                 -> 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
793
solveImplication inerts
794
     imp@(Implic { ic_untch  = untch
795 796
                 , ic_binds  = ev_binds
                 , ic_skols  = skols 
797
                 , ic_fsks   = old_fsks
798
                 , ic_given  = givens
799
                 , ic_wanted = wanteds
800 801
                 , ic_info   = info
                 , ic_env    = env })
802
  = 
803 804
    do { traceTcS "solveImplication {" (ppr imp) 

805
         -- Solve the nested constraints
806 807 808
         -- NB: 'inerts' has empty inert_fsks
       ; (new_fsks, residual_wanted) 
            <- nestImplicTcS ev_binds untch inerts $
809
               do { solveInteractGiven (mkGivenLoc info env) old_fsks givens 
810
                  ; residual_wanted <- solve_wanteds wanteds
811 812 813
                        -- solve_wanteds, *not* solve_wanteds_and_drop, because
                        -- we want to retain derived equalities so we can float
                        -- them out in floatEqualities
814 815 816 817 818 819 820
                  ; 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 
821 822
                        = emptyBag
                        | otherwise
823 824 825
                        = unitBag (imp { ic_fsks   = new_fsks
                                       , ic_wanted = dropDerivedWC final_wanted
                                       , ic_insol  = insolubleWC final_wanted })
826

827
       ; evbinds <- getTcEvBindsMap
828
       ; traceTcS "solveImplication end }" $ vcat
829
             [ text "floated_eqs =" <+> ppr floated_eqs
830
             , text "new_fsks =" <+> ppr new_fsks
831 832
             , text "res_implic =" <+> ppr res_implic
             , text "implication evbinds = " <+> ppr (evBindMapBinds evbinds) ]
833

834
       ; return (floated_eqs, res_implic) }
835 836 837 838
\end{code}


\begin{code}
839 840
floatEqualities :: [TcTyVar] -> [EvVar] -> WantedConstraints 
                -> TcS (Cts, WantedConstraints)
841 842
-- Post: The returned FlavoredEvVar's are only Wanted or Derived
-- and come from the input wanted ev vars or deriveds 
843 844
-- Also performs some unifications, adding to monadically-carried ty_binds
-- These will be used when processing floated_eqs later
845 846
floatEqualities skols can_given wanteds@(WC { wc_flat = flats })
  | hasEqualities can_given 
847
  = return (emptyBag, wanteds)   -- Note [Float Equalities out of Implications]
848
  | otherwise 
849
  = do { let (float_eqs, remaining_flats) = partitionBag is_floatable flats
850 851
       ; untch <- TcSMonad.getUntouchables
       ; mapM_ (promoteTyVar untch) (varSetElems (tyVarsOfCts float_eqs))
852
       ; ty_binds <- getTcSTyBindsMap
853
       ; traceTcS "floatEqualities" (vcat [ text "Floated eqs =" <+> ppr float_eqs
854 855
                                          , text "Ty binds =" <+> ppr ty_binds])
       ; return (float_eqs, wanteds { wc_flat = remaining_flats }) }
856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873
  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

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
874

875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912
promoteTyVar :: Untouchables -> TcTyVar  -> TcS ()
-- When we float a constraint out of an implication we must restore
-- invariant (MetaTvInv) in Note [Untouchable type variables] in TcType
promoteTyVar 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 ()

promoteAndDefaultTyVar :: Untouchables -> TcTyVarSet -> TyVar -> TcS ()
-- See Note [Promote _and_ default when inferring]
promoteAndDefaultTyVar untch gbl_tvs tv
  = do { tv1 <- if tv `elemVarSet` gbl_tvs 
                then return tv
                else defaultTyVar tv
       ; promoteTyVar untch tv1 }

defaultTyVar :: TcTyVar -> TcS TcTyVar
-- Precondition: MetaTyVars only
-- See Note [DefaultTyVar]
defaultTyVar the_tv
  | not (k `eqKind` default_k)
  = do { tv' <- TcSMonad.cloneMetaTyVar the_tv
       ; let new_tv = setTyVarKind tv' default_k
       ; traceTcS "defaultTyVar" (ppr the_tv <+> ppr new_tv)
       ; setWantedTyBind the_tv (mkTyVarTy new_tv)
       ; return new_tv }
             -- Why not directly derived_pred = mkTcEqPred k default_k?
             -- See Note [DefaultTyVar]
             -- We keep the same Untouchables on tv'

  | otherwise = return the_tv	 -- The common case
  where
    k = tyVarKind the_tv
    default_k = defaultKind k

913 914
approximateWC :: WantedConstraints -> Cts
-- Postcondition: Wanted or Derived Cts 
915 916
approximateWC wc 
  = float_wc emptyVarSet wc
917 918
  where 
    float_wc :: TcTyVarSet -> WantedConstraints -> Cts
919 920 921
    float_wc skols (WC { wc_flat = flats, wc_impl = implics }) 
      = do_bag (float_flat skols)   flats  `unionBags` 
        do_bag (float_implic skols) implics
922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937
                                 
    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}
938

939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984
Note [DefaultTyVar]
~~~~~~~~~~~~~~~~~~~
defaultTyVar is used on any un-instantiated meta type variables to
default the kind of OpenKind and ArgKind etc to *.  This is important 
to ensure that instance declarations match.  For example consider

     instance Show (a->b)
     foo x = show (\_ -> True)

Then we'll get a constraint (Show (p ->q)) where p has kind ArgKind,
and that won't match the typeKind (*) in the instance decl.  See tests
tc217 and tc175.

We look only at touchable type variables. No further constraints
are going to affect these type variables, so it's time to do it by
hand.  However we aren't ready to default them fully to () or
whatever, because the type-class defaulting rules have yet to run.

An important point is that if the type variable tv has kind k and the
default is default_k we do not simply generate [D] (k ~ default_k) because:

   (1) k may be ArgKind and default_k may be * so we will fail

   (2) We need to rewrite all occurrences of the tv to be a type
       variable with the right kind and we choose to do this by rewriting 
       the type variable /itself/ by a new variable which does have the 
       right kind.

Note [Promote _and_ default when inferring]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we are inferring a type, we simplify the constraint, and then use
approximateWC to produce a list of candidate constraints.  Then we MUST

  a) Promote any meta-tyvars that have been floated out by 
     approximateWC, to restore invariant (MetaTvInv) described in 
     Note [Untouchable type variables] in TcType.

  b) Default the kind of any meta-tyyvars that are not mentioned in
     in the environment.

To see (b), suppose the constraint is (C ((a :: OpenKind) -> Int)), and we
have an instance (C ((x:*) -> Int)).  The instance doesn't match -- but it
should!  If we don't solve the constraint, we'll stupidly quantify over 
(C (a->Int)) and, worse, in doing so zonkQuantifiedTyVar will quantify over
(b:*) instead of (a:OpenKind), which can lead to disaster; see Trac #7332.

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
985 986
Note [Float Equalities out of Implications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029
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).

1030 1031 1032 1033 1034 1035 1036 1037 1038
Note [Promoting unification variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we float an equality out of an implication we must "promote" free
unification variables of the equality, in order to maintain Invariant
(MetaTvInv) from Note [Untouchable type variables] in TcType.  for the
leftover implication.

This is absolutely necessary. Consider the following example. We start
with two implications and a class with a functional dependency.
1039

1040 1041 1042 1043 1044
    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]
1045 1046 1047 1048 1049 1050 1051

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:

1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064
    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 

1065

1066 1067 1068 1069 1070 1071
    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)

1072

1073 1074 1075