TcCanonical.hs 67.3 KB
Newer Older
1 2
{-# LANGUAGE CPP #-}

3
module TcCanonical(
4 5 6 7 8
     canonicalize,
     unifyDerived,

     StopOrContinue(..), stopWith, continueWith
  ) where
9 10 11 12 13

#include "HsVersions.h"

import TcRnTypes
import TcType
14
import Type
dreixel's avatar
dreixel committed
15
import Kind
16 17
import TcFlatten
import TcSMonad
18
import TcEvidence
19 20 21
import Class
import TyCon
import TypeRep
22 23 24
import Coercion
import FamInstEnv ( FamInstEnvs )
import FamInst ( tcTopNormaliseNewTypeTF_maybe )
25
import Var
26
import Name( isSystemName )
27
import OccName( OccName )
28
import Outputable
29
import DynFlags( DynFlags )
30
import VarSet
31
import RdrName
32

33
import Pair
34
import Util
35
import Bag
36 37
import MonadUtils ( zipWith3M, zipWith3M_ )
import Data.List  ( zip4 )
38
import BasicTypes
39
import FastString
40

Austin Seipp's avatar
Austin Seipp committed
41 42 43 44 45 46
{-
************************************************************************
*                                                                      *
*                      The Canonicaliser                               *
*                                                                      *
************************************************************************
47

48 49
Note [Canonicalization]
~~~~~~~~~~~~~~~~~~~~~~~
50

51
Canonicalization converts a simple constraint to a canonical form. It is
52 53 54
unary (i.e. treats individual constraints one at a time), does not do
any zonking, but lives in TcS monad because it needs to create fresh
variables (for flattening) and consult the inerts (for efficiency).
55

56
The execution plan for canonicalization is the following:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
57 58

  1) Decomposition of equalities happens as necessary until we reach a
59
     variable or type family in one side. There is no decomposition step
Simon Peyton Jones's avatar
Simon Peyton Jones committed
60
     for other forms of constraints.
61

Simon Peyton Jones's avatar
Simon Peyton Jones committed
62 63 64 65
  2) If, when we decompose, we discover a variable on the head then we
     look at inert_eqs from the current inert for a substitution for this
     variable and contine decomposing. Hence we lazily apply the inert
     substitution if it is needed.
66

67 68
  3) If no more decomposition is possible, we deeply apply the substitution
     from the inert_eqs and continue with flattening.
69

Simon Peyton Jones's avatar
Simon Peyton Jones committed
70 71 72 73 74
  4) During flattening, we examine whether we have already flattened some
     function application by looking at all the CTyFunEqs with the same
     function in the inert set. The reason for deeply applying the inert
     substitution at step (3) is to maximise our chances of matching an
     already flattened family application in the inert.
75

Simon Peyton Jones's avatar
Simon Peyton Jones committed
76 77
The net result is that a constraint coming out of the canonicalization
phase cannot be rewritten any further from the inerts (but maybe /it/ can
78 79
rewrite an inert or still interact with an inert in a further phase in the
simplifier.
dimitris's avatar
dimitris committed
80

81
Note [Caching for canonicals]
Simon Peyton Jones's avatar
Simon Peyton Jones committed
82
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
83 84 85 86
Our plan with pre-canonicalization is to be able to solve a constraint
really fast from existing bindings in TcEvBinds. So one may think that
the condition (isCNonCanonical) is not necessary.  However consider
the following setup:
87

Simon Peyton Jones's avatar
Simon Peyton Jones committed
88 89
InertSet = { [W] d1 : Num t }
WorkList = { [W] d2 : Num t, [W] c : t ~ Int}
90

91 92 93 94 95
Now, we prioritize equalities, but in our concrete example
(should_run/mc17.hs) the first (d2) constraint is dealt with first,
because (t ~ Int) is an equality that only later appears in the
worklist since it is pulled out from a nested implication
constraint. So, let's examine what happens:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
96

97 98
   - We encounter work item (d2 : Num t)

Simon Peyton Jones's avatar
Simon Peyton Jones committed
99
   - Nothing is yet in EvBinds, so we reach the interaction with inerts
100
     and set:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
101
              d2 := d1
102 103
    and we discard d2 from the worklist. The inert set remains unaffected.

104 105 106
   - Now the equation ([W] c : t ~ Int) is encountered and kicks-out
     (d1 : Num t) from the inerts.  Then that equation gets
     spontaneously solved, perhaps. We end up with:
107
        InertSet : { [G] c : t ~ Int }
Simon Peyton Jones's avatar
Simon Peyton Jones committed
108
        WorkList : { [W] d1 : Num t}
109

110 111
   - Now we examine (d1), we observe that there is a binding for (Num
     t) in the evidence binds and we set:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
112
             d1 := d2
113 114
     and end up in a loop!

115 116 117 118 119 120 121 122
Now, the constraints that get kicked out from the inert set are always
Canonical, so by restricting the use of the pre-canonicalizer to
NonCanonical constraints we eliminate this danger. Moreover, for
canonical constraints we already have good caching mechanisms
(effectively the interaction solver) and we are interested in reducing
things like superclasses of the same non-canonical constraint being
generated hence I don't expect us to lose a lot by introducing the
(isCNonCanonical) restriction.
123

124 125 126 127 128 129 130
A similar situation can arise in TcSimplify, at the end of the
solve_wanteds function, where constraints from the inert set are
returned as new work -- our substCt ensures however that if they are
not rewritten by subst, they remain canonical and hence we will not
attempt to solve them from the EvBinds. If on the other hand they did
get rewritten and are now non-canonical they will still not match the
EvBinds, so we are again good.
Austin Seipp's avatar
Austin Seipp committed
131
-}
132

133 134 135
-- Top-level canonicalization
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

136
canonicalize :: Ct -> TcS (StopOrContinue Ct)
137
canonicalize ct@(CNonCanonical { cc_ev = ev })
138
  = do { traceTcS "canonicalize (non-canonical)" (ppr ct)
139
       ; {-# SCC "canEvVar" #-}
140
         canEvNC ev }
141

142
canonicalize (CDictCan { cc_ev = ev
143 144
                       , cc_class  = cls
                       , cc_tyargs = xis })
145
  = {-# SCC "canClass" #-}
146 147
    canClass ev cls xis -- Do not add any superclasses
canonicalize (CTyEqCan { cc_ev = ev
148
                       , cc_tyvar  = tv
149 150
                       , cc_rhs    = xi
                       , cc_eq_rel = eq_rel })
151
  = {-# SCC "canEqLeafTyVarEq" #-}
152 153 154
    canEqNC ev eq_rel (mkTyVarTy tv) xi
      -- NB: Don't use canEqTyVar because that expects flattened types,
      -- and tv and xi may not be flat w.r.t. an updated inert set
155

156
canonicalize (CFunEqCan { cc_ev = ev
157 158
                        , cc_fun    = fn
                        , cc_tyargs = xis1
159
                        , cc_fsk    = fsk })
Simon Peyton Jones's avatar
Simon Peyton Jones committed
160
  = {-# SCC "canEqLeafFunEq" #-}
161
    canCFunEqCan ev fn xis1 fsk
162

163 164
canonicalize (CIrredEvCan { cc_ev = ev })
  = canIrred ev
thomasw's avatar
thomasw committed
165 166
canonicalize (CHoleCan { cc_ev = ev, cc_occ = occ, cc_hole = hole })
  = canHole ev occ hole
167

168
canEvNC :: CtEvidence -> TcS (StopOrContinue Ct)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
169
-- Called only for non-canonical EvVars
170
canEvNC ev
171
  = case classifyPredType (ctEvPred ev) of
172 173 174 175 176 177
      ClassPred cls tys     -> do traceTcS "canEvNC:cls" (ppr cls <+> ppr tys)
                                  canClassNC ev cls tys
      EqPred eq_rel ty1 ty2 -> do traceTcS "canEvNC:eq" (ppr ty1 $$ ppr ty2)
                                  canEqNC    ev eq_rel ty1 ty2
      IrredPred {}          -> do traceTcS "canEvNC:irred" (ppr (ctEvPred ev))
                                  canIrred   ev
Austin Seipp's avatar
Austin Seipp committed
178 179 180 181 182 183 184
{-
************************************************************************
*                                                                      *
*                      Class Canonicalization
*                                                                      *
************************************************************************
-}
185

Simon Peyton Jones's avatar
Simon Peyton Jones committed
186
canClass, canClassNC
187
   :: CtEvidence
188
   -> Class -> [Type] -> TcS (StopOrContinue Ct)
Simon Peyton Jones's avatar
Simon Peyton Jones committed
189
-- Precondition: EvVar is class evidence
190 191 192 193 194 195

-- The canClassNC version is used on non-canonical constraints
-- and adds superclasses.  The plain canClass version is used
-- for already-canonical class constraints (but which might have
-- been subsituted or somthing), and hence do not need superclasses

196 197
canClassNC ev cls tys
  = canClass ev cls tys
198 199
    `andWhenContinue` emitSuperclasses

200
canClass ev cls tys
201 202
  =   -- all classes do *nominal* matching
    ASSERT2( ctEvRole ev == Nominal, ppr ev $$ ppr cls $$ ppr tys )
203
    do { (xis, cos) <- flattenManyNom ev tys
Joachim Breitner's avatar
Joachim Breitner committed
204
       ; let co = mkTcTyConAppCo Nominal (classTyCon cls) cos
205
             xi = mkClassPred cls xis
206 207
             mk_ct new_ev = CDictCan { cc_ev = new_ev
                                     , cc_tyargs = xis, cc_class = cls }
208
       ; mb <- rewriteEvidence ev xi co
Simon Peyton Jones's avatar
Simon Peyton Jones committed
209
       ; traceTcS "canClass" (vcat [ ppr ev <+> ppr cls <+> ppr tys
Simon Peyton Jones's avatar
Simon Peyton Jones committed
210
                                   , ppr xi, ppr mb ])
211
       ; return (fmap mk_ct mb) }
dimitris's avatar
dimitris committed
212

213
emitSuperclasses :: Ct -> TcS (StopOrContinue Ct)
214
emitSuperclasses ct@(CDictCan { cc_ev = ev , cc_tyargs = xis_new, cc_class = cls })
Simon Peyton Jones's avatar
Simon Peyton Jones committed
215 216
            -- Add superclasses of this one here, See Note [Adding superclasses].
            -- But only if we are not simplifying the LHS of a rule.
217
 = do { newSCWorkFromFlavored ev cls xis_new
Simon Peyton Jones's avatar
Simon Peyton Jones committed
218
      -- Arguably we should "seq" the coercions if they are derived,
219
      -- as we do below for emit_kind_constraint, to allow errors in
Simon Peyton Jones's avatar
Simon Peyton Jones committed
220
      -- superclasses to be executed if deferred to runtime!
221 222
      ; continueWith ct }
emitSuperclasses _ = panic "emit_superclasses of non-class!"
223

224 225
{- Note [Adding superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
226 227 228 229 230
Since dictionaries are canonicalized only once in their lifetime, the
place to add their superclasses is canonicalisation.  See Note [Add
superclasses only during canonicalisation].  Here is what we do:

  Givens:   Add all their superclasses as Givens.
231
            They may be needed to prove Wanteds.
232

233 234
  Wanteds/Derived:
            Add all their superclasses as Derived.
235 236
            The sole reason is to expose functional dependencies
            in superclasses or equality superclasses.
237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256

Examples of how adding superclasses as Derived is useful

    --- Example 1
        class C a b | a -> b
    Suppose we want to solve
         [G] C a b
         [W] C a beta
    Then adding [D] beta~b will let us solve it.

    -- Example 2 (similar but using a type-equality superclass)
        class (F a ~ b) => C a b
    And try to sllve:
         [G] C a b
         [W] C a beta
    Follow the superclass rules to add
         [G] F a ~ b
         [D] F a ~ beta
    Now we we get [D] beta ~ b, and can solve that.

257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274
    -- Example (tcfail138)
      class L a b | a -> b
      class (G a, L a b) => C a b

      instance C a b' => G (Maybe a)
      instance C a b  => C (Maybe a) a
      instance L (Maybe a) a

    When solving the superclasses of the (C (Maybe a) a) instance, we get
      [G] C a b, and hance by superclasses, [G] G a, [G] L a b
      [W] G (Maybe a)
    Use the instance decl to get
      [W] C a beta
    Generate its derived superclass
      [D] L a beta.  Now using fundeps, combine with [G] L a b to get
      [D] beta ~ b
    which is what we want.

275
---------- Historical note -----------
276 277 278 279 280 281 282 283
Example of why adding superclass of a Wanted as a Given would
be terrible, see Note [Do not add superclasses of solved dictionaries]
in TcSMonad, which has this example:
        class Ord a => C a where
        instance Ord [a] => C [a] where ...
Suppose we are trying to solve
  [G] d1 : Ord a
  [W] d2 : C [a]
Jan Stolarek's avatar
Jan Stolarek committed
284
If we (bogusly) added the superclass of d2 as Given we'd have
285 286 287 288 289 290 291 292 293
  [G] d1 : Ord a
  [W] d2 : C [a]
  [G] d3 : Ord [a]   -- Superclass of d2, bogus

Then we'll use the instance decl to give
  [G] d1 : Ord a     Solved: d2 : C [a] = $dfCList d4
  [G] d3 : Ord [a]   -- Superclass of d2, bogus
  [W] d4: Ord [a]

Jan Stolarek's avatar
Jan Stolarek committed
294
And now we could bogusly solve d4 from d3.
295
---------- End of historical note -----------
296 297 298 299 300 301 302 303

Note [Add superclasses only during canonicalisation]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We add superclasses only during canonicalisation, on the passage
from CNonCanonical to CDictCan.  A class constraint can be repeatedly
rewritten, and there's no point in repeatedly adding its superclasses.

Here's a serious, but now out-dated example, from Trac #4497:
Simon Peyton Jones's avatar
Simon Peyton Jones committed
304

305 306 307
   class Num (RealOf t) => Normed t
   type family RealOf x

Simon Peyton Jones's avatar
Simon Peyton Jones committed
308
Assume the generated wanted constraint is:
309 310 311
   [W] RealOf e ~ e
   [W] Normed e

Simon Peyton Jones's avatar
Simon Peyton Jones committed
312
If we were to be adding the superclasses during simplification we'd get:
313 314 315 316
   [W] RealOf e ~ e
   [W] Normed e
   [D] RealOf e ~ fuv
   [D] Num fuv
Simon Peyton Jones's avatar
Simon Peyton Jones committed
317
==>
318
   e := fuv, Num fuv, Normed fuv, RealOf fuv ~ fuv
Simon Peyton Jones's avatar
Simon Peyton Jones committed
319

320 321 322
While looks exactly like our original constraint. If we add the
superclass of (Normed fuv) again we'd loop.  By adding superclasses
definitely only once, during canonicalisation, this situation can't
323
happen.
324 325 326 327

Mind you, now that Wanteds cannot rewrite Derived, I think this particular
situation can't happen.
  -}
328

329
newSCWorkFromFlavored :: CtEvidence -> Class -> [Xi] -> TcS ()
330
-- Returns superclasses, see Note [Adding superclasses]
331
newSCWorkFromFlavored flavor cls xis
332
  | CtGiven { ctev_evar = evar, ctev_loc = loc } <- flavor
333 334
  = do { given_evs <- newGivenEvVars (mk_given_loc loc)
                                     (mkEvScSelectors (EvId evar) cls xis)
335
       ; emitWorkNC given_evs }
dimitris's avatar
dimitris committed
336 337

  | isEmptyVarSet (tyVarsOfTypes xis)
338
  = return () -- Wanteds with no variables yield no deriveds.
339
              -- See Note [Improvement from Ground Wanteds]
340

341
  | otherwise -- Wanted/Derived case, just add those SC that can lead to improvement.
Simon Peyton Jones's avatar
Simon Peyton Jones committed
342
  = do { let sc_rec_theta = transSuperClasses cls xis
343
             impr_theta   = filter isImprovementPred sc_rec_theta
344
             loc          = ctEvLoc flavor
345
       ; traceTcS "newSCWork/Derived" $ text "impr_theta =" <+> ppr impr_theta
346
       ; emitNewDeriveds loc impr_theta }
347

348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366
  where
    size = sizeTypes xis
    mk_given_loc loc
       | isCTupleClass cls
       = loc   -- For tuple predicates, just take them apart, without
               -- adding their (large) size into the chain.  When we
               -- get down to a base predicate, we'll include its size.
               -- Trac #10335

       | GivenOrigin skol_info <- ctLocOrigin loc
         -- See Note [Solving superclass constraints] in TcInstDcls
         -- for explantation of this transformation for givens
       = case skol_info of
            InstSkol -> loc { ctl_origin = GivenOrigin (InstSC size) }
            InstSC n -> loc { ctl_origin = GivenOrigin (InstSC (n `max` size)) }
            _        -> loc

       | otherwise  -- Probably doesn't happen, since this function
       = loc        -- is only used for Givens, but does no harm
367

Austin Seipp's avatar
Austin Seipp committed
368 369 370 371 372 373 374
{-
************************************************************************
*                                                                      *
*                      Irreducibles canonicalization
*                                                                      *
************************************************************************
-}
375

376
canIrred :: CtEvidence -> TcS (StopOrContinue Ct)
377
-- Precondition: ty not a tuple and no other evidence form
378
canIrred old_ev
Simon Peyton Jones's avatar
Simon Peyton Jones committed
379 380
  = do { let old_ty = ctEvPred old_ev
       ; traceTcS "can_pred" (text "IrredPred = " <+> ppr old_ty)
381
       ; (xi,co) <- flatten FM_FlattenAll old_ev old_ty -- co :: xi ~ old_ty
382
       ; rewriteEvidence old_ev xi co `andWhenContinue` \ new_ev ->
383 384
    do { -- Re-classify, in case flattening has improved its shape
       ; case classifyPredType (ctEvPred new_ev) of
385 386 387 388
           ClassPred cls tys     -> canClassNC new_ev cls tys
           EqPred eq_rel ty1 ty2 -> canEqNC new_ev eq_rel ty1 ty2
           _                     -> continueWith $
                                    CIrredEvCan { cc_ev = new_ev } } }
389

thomasw's avatar
thomasw committed
390 391
canHole :: CtEvidence -> OccName -> HoleSort -> TcS (StopOrContinue Ct)
canHole ev occ hole_sort
392 393
  = do { let ty = ctEvPred ev
       ; (xi,co) <- flatten FM_SubstOnly ev ty -- co :: xi ~ ty
394 395 396 397 398
       ; rewriteEvidence ev xi co `andWhenContinue` \ new_ev ->
    do { emitInsoluble (CHoleCan { cc_ev = new_ev
                                 , cc_occ = occ
                                 , cc_hole = hole_sort })
       ; stopWith new_ev "Emit insoluble hole" } }
399

Austin Seipp's avatar
Austin Seipp committed
400 401 402 403 404 405
{-
************************************************************************
*                                                                      *
*        Equalities
*                                                                      *
************************************************************************
406 407 408 409 410 411 412 413 414 415 416 417 418

Note [Canonicalising equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to canonicalise an equality, we look at the structure of the
two types at hand, looking for similarities. A difficulty is that the
types may look dissimilar before flattening but similar after flattening.
However, we don't just want to jump in and flatten right away, because
this might be wasted effort. So, after looking for similarities and failing,
we flatten and then try again. Of course, we don't want to loop, so we
track whether or not we've already flattened.

It is conceivable to do a better job at tracking whether or not a type
is flattened, but this is left as future work. (Mar '15)
Austin Seipp's avatar
Austin Seipp committed
419
-}
420

421 422
canEqNC :: CtEvidence -> EqRel -> Type -> Type -> TcS (StopOrContinue Ct)
canEqNC ev eq_rel ty1 ty2
423
  = can_eq_nc False ev eq_rel ty1 ty1 ty2 ty2
424

425
can_eq_nc
426 427
   :: Bool            -- True => both types are flat
   -> CtEvidence
428
   -> EqRel
Austin Seipp's avatar
Austin Seipp committed
429 430
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
431
   -> TcS (StopOrContinue Ct)
432
can_eq_nc flat ev eq_rel ty1 ps_ty1 ty2 ps_ty2
Austin Seipp's avatar
Austin Seipp committed
433
  = do { traceTcS "can_eq_nc" $
434 435 436
         vcat [ ppr ev, ppr eq_rel, ppr ty1, ppr ps_ty1, ppr ty2, ppr ps_ty2 ]
       ; rdr_env <- getGlobalRdrEnvTcS
       ; fam_insts <- getFamInstEnvs
437
       ; can_eq_nc' flat rdr_env fam_insts ev eq_rel ty1 ps_ty1 ty2 ps_ty2 }
438 439

can_eq_nc'
440 441
   :: Bool           -- True => both input types are flattened
   -> GlobalRdrEnv   -- needed to see which newtypes are in scope
442 443 444 445 446 447
   -> FamInstEnvs    -- needed to unwrap data instances
   -> CtEvidence
   -> EqRel
   -> Type -> Type    -- LHS, after and before type-synonym expansion, resp
   -> Type -> Type    -- RHS, after and before type-synonym expansion, resp
   -> TcS (StopOrContinue Ct)
448 449

-- Expand synonyms first; see Note [Type synonyms and canonicalization]
450
can_eq_nc' flat _rdr_env _envs ev eq_rel ty1 ps_ty1 ty2 ps_ty2
451 452
  | Just ty1' <- coreView ty1 = can_eq_nc flat ev eq_rel ty1' ps_ty1 ty2  ps_ty2
  | Just ty2' <- coreView ty2 = can_eq_nc flat ev eq_rel ty1  ps_ty1 ty2' ps_ty2
453 454

-- need to check for reflexivity in the ReprEq case.
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
455
-- See Note [Eager reflexivity check]
456 457 458 459 460 461
can_eq_nc' _flat _rdr_env _envs ev ReprEq ty1 _ ty2 _
  | ty1 `eqType` ty2
  = canEqReflexive ev ReprEq ty1

-- When working with ReprEq, unwrap newtypes.
can_eq_nc' _flat rdr_env envs ev ReprEq ty1 _ ty2 ps_ty2
462 463
  | Just (co, ty1') <- tcTopNormaliseNewTypeTF_maybe envs rdr_env ty1
  = can_eq_newtype_nc rdr_env ev NotSwapped co ty1 ty1' ty2 ps_ty2
464
can_eq_nc' _flat rdr_env envs ev ReprEq ty1 ps_ty1 ty2 _
465 466
  | Just (co, ty2') <- tcTopNormaliseNewTypeTF_maybe envs rdr_env ty2
  = can_eq_newtype_nc rdr_env ev IsSwapped  co ty2 ty2' ty1 ps_ty1
467 468 469 470 471 472

----------------------
-- Otherwise try to decompose
----------------------

-- Literals
473
can_eq_nc' _flat _rdr_env _envs ev eq_rel ty1@(LitTy l1) _ (LitTy l2) _
474
 | l1 == l2
475 476
  = do { setEvBindIfWanted ev (EvCoercion $
                               mkTcReflCo (eqRelRole eq_rel) ty1)
477
       ; stopWith ev "Equal LitTy" }
478

Simon Peyton Jones's avatar
Simon Peyton Jones committed
479 480
-- Try to decompose type constructor applications
-- Including FunTy (s -> t)
481 482 483 484 485
can_eq_nc' _flat _rdr_env _envs ev eq_rel ty1 _ ty2 _
  | Just (tc1, tys1) <- tcSplitTyConApp_maybe ty1
  , Just (tc2, tys2) <- tcSplitTyConApp_maybe ty2
  , not (isTypeFamilyTyCon tc1)
  , not (isTypeFamilyTyCon tc2)
486
  = canTyConApp ev eq_rel tc1 tys1 tc2 tys2
487

488 489
can_eq_nc' _flat _rdr_env _envs ev eq_rel
           s1@(ForAllTy {}) _ s2@(ForAllTy {}) _
490
 | CtWanted { ctev_loc = loc, ctev_evar = orig_ev } <- ev
491 492
 = do { let (tvs1,body1) = tcSplitForAllTys s1
            (tvs2,body2) = tcSplitForAllTys s2
Simon Peyton Jones's avatar
Simon Peyton Jones committed
493
      ; if not (equalLength tvs1 tvs2) then
494
          canEqHardFailure ev eq_rel s1 s2
495
        else
496
          do { traceTcS "Creating implication for polytype equality" $ ppr ev
497 498
             ; ev_term <- deferTcSForAllEq (eqRelRole eq_rel)
                                           loc (tvs1,body1) (tvs2,body2)
499
             ; setWantedEvBind orig_ev ev_term
500
             ; stopWith ev "Deferred polytype equality" } }
501
 | otherwise
Simon Peyton Jones's avatar
Simon Peyton Jones committed
502
 = do { traceTcS "Ommitting decomposition of given polytype equality" $
503
        pprEq s1 s2    -- See Note [Do not decompose given polytype equalities]
504
      ; stopWith ev "Discard given polytype equality" }
505

506
-- See Note [Canonicalising type applications] about why we require flat types
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
507
can_eq_nc' True _rdr_env _envs ev eq_rel (AppTy t1 s1) _ ty2 _
508
  | Just (t2, s2) <- tcSplitAppTy_maybe ty2
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
509 510
  = can_eq_app ev eq_rel t1 s1 t2 s2
can_eq_nc' True _rdr_env _envs ev eq_rel ty1 _ (AppTy t2 s2) _
511
  | Just (t1, s1) <- tcSplitAppTy_maybe ty1
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
512
  = can_eq_app ev eq_rel t1 s1 t2 s2
513 514 515 516 517 518

-- No similarity in type structure detected. Flatten and try again!
can_eq_nc' False rdr_env envs ev eq_rel _ ps_ty1 _ ps_ty2
  = do { (xi1, co1) <- flatten FM_FlattenAll ev ps_ty1
       ; (xi2, co2) <- flatten FM_FlattenAll ev ps_ty2
       ; rewriteEqEvidence ev eq_rel NotSwapped xi1 xi2 co1 co2
519
         `andWhenContinue` \ new_ev ->
520 521 522 523 524 525 526 527 528 529 530 531 532
         can_eq_nc' True rdr_env envs new_ev eq_rel xi1 xi1 xi2 xi2 }

-- Type variable on LHS or RHS are last. We want only flat types sent
-- to canEqTyVar.
-- See also Note [No top-level newtypes on RHS of representational equalities]
can_eq_nc' True _rdr_env _envs ev eq_rel (TyVarTy tv1) _ _ ps_ty2
  = canEqTyVar ev eq_rel NotSwapped tv1 ps_ty2
can_eq_nc' True _rdr_env _envs ev eq_rel _ ps_ty1 (TyVarTy tv2) _
  = canEqTyVar ev eq_rel IsSwapped  tv2 ps_ty1

-- We've flattened and the types don't match. Give up.
can_eq_nc' True _rdr_env _envs ev eq_rel _ ps_ty1 _ ps_ty2
  = canEqHardFailure ev eq_rel ps_ty1 ps_ty2
533

534
{-
535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552
Note [Newtypes can blow the stack]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)
  newtype Y = MkY (Int -> Y)

and now wish to prove

  [W] X ~R Y

This Wanted will loop, expanding out the newtypes ever deeper looking
for a solid match or a solid discrepancy. Indeed, there is something
appropriate to this looping, because X and Y *do* have the same representation,
in the limit -- they're both (Fix ((->) Int)). However, no finitely-sized
coercion will ever witness it. This loop won't actually cause GHC to hang,
though, because we check our depth when unwrapping newtypes.

553 554 555 556 557 558 559 560 561
Note [Eager reflexivity check]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we have

  newtype X = MkX (Int -> X)

and

  [W] X ~R X
562

563 564 565 566 567
Naively, we would start unwrapping X and end up in a loop. Instead,
we do this eager reflexivity check. This is necessary only for representational
equality because the flattener technology deals with the similar case
(recursive type families) for nominal equality.

568 569
Note that this check does not catch all cases, but it will catch the cases
we're most worried about, types like X above that are actually inhabited.
570

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
571
Here's another place where this reflexivity check is key:
572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598
Consider trying to prove (f a) ~R (f a). The AppTys in there can't
be decomposed, because representational equality isn't congruent with respect
to AppTy. So, when canonicalising the equality above, we get stuck and
would normally produce a CIrredEvCan. However, we really do want to
be able to solve (f a) ~R (f a). So, in the representational case only,
we do a reflexivity check.

(This would be sound in the nominal case, but unnecessary, and I [Richard
E.] am worried that it would slow down the common case.)
-}

------------------------
-- | We're able to unwrap a newtype. Update the bits accordingly.
can_eq_newtype_nc :: GlobalRdrEnv
                  -> CtEvidence           -- ^ :: ty1 ~ ty2
                  -> SwapFlag
                  -> TcCoercion           -- ^ :: ty1 ~ ty1'
                  -> TcType               -- ^ ty1
                  -> TcType               -- ^ ty1'
                  -> TcType               -- ^ ty2
                  -> TcType               -- ^ ty2, with type synonyms
                  -> TcS (StopOrContinue Ct)
can_eq_newtype_nc rdr_env ev swapped co ty1 ty1' ty2 ps_ty2
  = do { traceTcS "can_eq_newtype_nc" $
         vcat [ ppr ev, ppr swapped, ppr co, ppr ty1', ppr ty2 ]

         -- check for blowing our stack:
599 600
         -- See Note [Newtypes can blow the stack]
       ; checkReductionDepth (ctEvLoc ev) ty1
601
       ; addUsedDataCons rdr_env (tyConAppTyCon ty1)
602 603 604 605 606 607
           -- we have actually used the newtype constructor here, so
           -- make sure we don't warn about importing it!

       ; rewriteEqEvidence ev ReprEq swapped ty1' ps_ty2
                           (mkTcSymCo co) (mkTcReflCo Representational ps_ty2)
         `andWhenContinue` \ new_ev ->
608
         can_eq_nc False new_ev ReprEq ty1' ty1' ty2 ps_ty2 }
609

610
---------
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
611
-- ^ Decompose a type application.
612
-- All input types must be flat. See Note [Canonicalising type applications]
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
613 614
can_eq_app :: CtEvidence       -- :: s1 t1 ~r s2 t2
           -> EqRel            -- r
615 616 617
           -> Xi -> Xi         -- s1 t1
           -> Xi -> Xi         -- s2 t2
           -> TcS (StopOrContinue Ct)
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
618 619

-- AppTys only decompose for nominal equality, so this case just leads
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
620
-- to an irreducible constraint; see typecheck/should_compile/T10494
Simon Peyton Jones's avatar
Simon Peyton Jones committed
621
-- See Note [Decomposing equality], note {4}
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
622 623 624
can_eq_app ev ReprEq _ _ _ _
  = do { traceTcS "failing to decompose representational AppTy equality" (ppr ev)
       ; continueWith (CIrredEvCan { cc_ev = ev }) }
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
625 626
          -- no need to call canEqFailure, because that flattens, and the
          -- types involved here are already flat
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
627 628

can_eq_app ev NomEq s1 t1 s2 t2
629
  | CtDerived { ctev_loc = loc } <- ev
630
  = do { emitNewDerivedEq loc (mkTcEqPred t1 t2)
631 632 633 634 635 636 637
       ; canEqNC ev NomEq s1 s2 }
  | CtWanted { ctev_evar = evar, ctev_loc = loc } <- ev
  = do { ev_s <- newWantedEvVarNC loc (mkTcEqPred s1 s2)
       ; co_t <- unifyWanted loc Nominal t1 t2
       ; let co = mkTcAppCo (ctEvCoercion ev_s) co_t
       ; setWantedEvBind evar (EvCoercion co)
       ; canEqNC ev_s NomEq s1 s2 }
638 639
  | CtGiven { ctev_evar = evar, ctev_loc = loc } <- ev
  = do { let co   = mkTcCoVarCo evar
640 641 642 643 644 645 646 647
             co_s = mkTcLRCo CLeft  co
             co_t = mkTcLRCo CRight co
       ; evar_s <- newGivenEvVar loc (mkTcEqPred s1 s2, EvCoercion co_s)
       ; evar_t <- newGivenEvVar loc (mkTcEqPred t1 t2, EvCoercion co_t)
       ; emitWorkNC [evar_t]
       ; canEqNC evar_s NomEq s1 s2 }
  | otherwise  -- Can't happen
  = error "can_eq_app"
648

649
------------------------
650 651 652 653
canTyConApp :: CtEvidence -> EqRel
            -> TyCon -> [TcType]
            -> TyCon -> [TcType]
            -> TcS (StopOrContinue Ct)
654
-- See Note [Decomposing TyConApps]
655
canTyConApp ev eq_rel tc1 tys1 tc2 tys2
656
  | tc1 == tc2
657
  , length tys1 == length tys2
658
  = do { inerts <- getTcSInerts
659 660
       ; if can_decompose inerts
         then do { traceTcS "canTyConApp"
661 662 663 664
                       (ppr ev $$ ppr eq_rel $$ ppr tc1 $$ ppr tys1 $$ ppr tys2)
                 ; canDecomposableTyConAppOK ev eq_rel tc1 tys1 tys2
                 ; stopWith ev "Decomposed TyConApp" }
         else canEqFailure ev eq_rel ty1 ty2 }
665

666 667
  -- Fail straight away for better error messages
  -- See Note [Use canEqFailure in canDecomposableTyConApp]
668 669
  | eq_rel == ReprEq && not (isGenerativeTyCon tc1 Representational &&
                             isGenerativeTyCon tc2 Representational)
670 671 672 673 674 675 676
  = canEqFailure ev eq_rel ty1 ty2
  | otherwise
  = canEqHardFailure ev eq_rel ty1 ty2
  where
    ty1 = mkTyConApp tc1 tys1
    ty2 = mkTyConApp tc2 tys2

677 678 679
    loc  = ctEvLoc ev
    pred = ctEvPred ev

680 681 682 683 684
     -- See Note [Decomposing equality]
    can_decompose inerts
      =  isInjectiveTyCon tc1 (eqRelRole eq_rel)
      || (ctEvFlavour ev /= Given && isEmptyBag (matchableGivens loc pred inerts))

685 686 687 688 689
{-
Note [Use canEqFailure in canDecomposableTyConApp]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We must use canEqFailure, not canEqHardFailure here, because there is
the possibility of success if working with a representational equality.
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
690
Here is one case:
691 692 693 694 695

  type family TF a where TF Char = Bool
  data family DF a
  newtype instance DF Bool = MkDF Int

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
696
Suppose we are canonicalising (Int ~R DF (TF a)), where we don't yet
697 698
know `a`. This is *not* a hard failure, because we might soon learn
that `a` is, in fact, Char, and then the equality succeeds.
699

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
700 701
Here is another case:

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
702
  [G] Age ~R Int
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
703 704 705 706

where Age's constructor is not in scope. We don't want to report
an "inaccessible code" error in the context of this Given!

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
707 708 709 710 711 712 713 714 715 716
For example, see typecheck/should_compile/T10493, repeated here:

  import Data.Ord (Down)  -- no constructor

  foo :: Coercible (Down Int) Int => Down Int -> Int
  foo = coerce

That should compile, but only because we use canEqFailure and not
canEqHardFailure.

717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755
Note [Decomposing equality]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we have a constraint (of any flavour and role) that looks like
T tys1 ~ T tys2, what can we conclude about tys1 and tys2? The answer,
of course, is "it depends". This Note spells it all out.

In this Note, "decomposition" refers to taking the constraint
  [fl] (T tys1 ~X T tys2)
(for some flavour fl and some role X) and replacing it with
  [fls'] (tys1 ~Xs' tys2)
where that notation indicates a list of new constraints, where the
new constraints may have different flavours and different roles.

The key property to consider is injectivity. When decomposing a Given the
decomposition is sound if and only if T is injective in all of its type
arguments. When decomposing a Wanted, the decomposition is sound (assuming the
correct roles in the produced equality constraints), but it may be a guess --
that is, an unforced decision by the constraint solver. Decomposing Wanteds
over injective TyCons does not entail guessing. But sometimes we want to
decompose a Wanted even when the TyCon involved is not injective! (See below.)

So, in broad strokes, we want this rule:

(*) Decompose a constraint (T tys1 ~X T tys2) if and only if T is injective
at role X.

Pursuing the details requires exploring three axes:
* Flavour: Given vs. Derived vs. Wanted
* Role: Nominal vs. Representational
* TyCon species: datatype vs. newtype vs. data family vs. type family vs. type variable

(So a type variable isn't a TyCon, but it's convenient to put the AppTy case
in the same table.)

Right away, we can say that Derived behaves just as Wanted for the purposes
of decomposition. The difference between Derived and Wanted is the handling of
evidence. Since decomposition in these cases isn't a matter of soundness but of
guessing, we want the same behavior regardless of evidence.

Simon Peyton Jones's avatar
Simon Peyton Jones committed
756 757 758 759 760 761
Here is a table (discussion following) detailing where decomposition of
   (T s1 ... sn) ~r (T t1 .. tn)
is allowed.  The first four lines (Data types ... type family) refer
to TyConApps with various TyCons T; the last line is for AppTy, where
there is presumably a type variable at the head, so it's actually
   (s s1 ... sn) ~r (t t1 .. tn)
762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788

NOMINAL               GIVEN                       WANTED

Datatype               YES                         YES
Newtype                YES                         YES
Data family            YES                         YES
Type family            YES, in injective args{1}   YES, in injective args{1}
Type variable          YES                         YES

REPRESENTATIONAL      GIVEN                       WANTED

Datatype               YES                         YES
Newtype                NO{2}                      MAYBE{2}
Data family            NO{3}                      MAYBE{3}
Type family             NO                          NO
Type variable          NO{4}                       NO{4}

{1}: Type families can be injective in some, but not all, of their arguments,
so we want to do partial decomposition. This is quite different than the way
other decomposition is done, where the decomposed equalities replace the original
one. We thus proceed much like we do with superclasses: emitting new Givens
when "decomposing" a partially-injective type family Given and new Deriveds
when "decomposing" a partially-injective type family Wanted. (As of the time of
writing, 13 June 2015, the implementation of injective type families has not
been merged, but it should be soon. Please delete this parenthetical if the
implementation is indeed merged.)

Simon Peyton Jones's avatar
Simon Peyton Jones committed
789
{2}: See Note [Decomposing newtypes at representational role]
790

Simon Peyton Jones's avatar
Simon Peyton Jones committed
791 792 793
{3}: Because of the possibility of newtype instances, we must treat
data families like newtypes. See also Note [Decomposing newtypes at
representational role]. See #10534 and test case
794
typecheck/should_fail/T10534.
795 796 797 798 799 800 801 802 803 804 805

{4}: Because type variables can stand in for newtypes, we conservatively do not
decompose AppTys over representational equality.

In the implementation of can_eq_nc and friends, we don't directly pattern
match using lines like in the tables above, as those tables don't cover
all cases (what about PrimTyCon? tuples?). Instead we just ask about injectivity,
boiling the tables above down to rule (*). The exceptions to rule (*) are for
injective type families, which are handled separately from other decompositions,
and the MAYBE entries above.

Simon Peyton Jones's avatar
Simon Peyton Jones committed
806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865
Note [Decomposing newtypes at representational role]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This note discusses the 'newtype' line in the REPRESENTATIONAL table
in Note [Decomposing equality]. (At nominal role, newtypes are fully
decomposable.)

Here is a representative example of why representational equality over
newtypes is tricky:

  newtype Nt a = Mk Bool         -- NB: a is not used in the RHS,
  type role Nt representational  -- but the user gives it an R role anyway

If we have [W] Nt alpha ~R Nt beta, we *don't* want to decompose to
[W] alpha ~R beta, because it's possible that alpha and beta aren't
representationally equal. Here's another example.

  newtype Nt a = MkNt (Id a)
  type family Id a where Id a = a

  [W] Nt Int ~R Nt Age

Because of its use of a type family, Nt's parameter will get inferred to have
a nominal role. Thus, decomposing the wanted will yield [W] Int ~N Age, which
is unsatisfiable. Unwrapping, though, leads to a solution.

Conclusion:
 * Unwrap newtypes before attempting to decompose them.
   This is done in can_eq_nc'.

It all comes from the fact that newtypes aren't necessarily injective
w.r.t. representational equality.

Furthermore, as explained in Note [NthCo and newtypes] in Coercion, we can't use
NthCo on representational coercions over newtypes. NthCo comes into play
only when decomposing givens.

Conclusion:
 * Do not decompose [G] N s ~R N t

Is it sensible to decompose *Wanted* constraints over newtypes?  Yes!
It's the only way we could ever prove (IO Int ~R IO Age), recalling
that IO is a newtype.

However we must be careful.  Consider

  type role Nt representational

  [G] Nt a ~R Nt b       (1)
  [W] NT alpha ~R Nt b   (2)
  [W] alpha ~ a          (3)

If we focus on (3) first, we'll substitute in (2), and now it's
identical to the given (1), so we succeed.  But if we focus on (2)
first, and decompose it, we'll get (alpha ~R b), which is not soluble.
This is exactly like the question of overlapping Givens for class
constraints: see Note [Instance and Given overlap] in TcInteract.

Conclusion:
  * Decompose [W] N s ~R N t  iff there no given constraint that could
    later solve it.
866 867 868
-}

canDecomposableTyConAppOK :: CtEvidence -> EqRel
869
                          -> TyCon -> [TcType] -> [TcType]
870 871
                          -> TcS ()
-- Precondition: tys1 and tys2 are the same length, hence "OK"
872
canDecomposableTyConAppOK ev eq_rel tc tys1 tys2
873 874
  = case ev of
     CtDerived { ctev_loc = loc }
875
        -> unifyDeriveds loc tc_roles tys1 tys2
876 877

     CtWanted { ctev_evar = evar, ctev_loc = loc }
878
        -> do { cos <- zipWith3M (unifyWanted loc) tc_roles tys1 tys2
879
              ; setWantedEvBind evar (EvCoercion (mkTcTyConAppCo role tc cos)) }
880

881 882
     CtGiven { ctev_evar = evar, ctev_loc = loc }
        -> do { let ev_co = mkTcCoVarCo evar
883 884 885 886 887
              ; given_evs <- newGivenEvVars loc $
                             [ ( mkTcEqPredRole r ty1 ty2
                               , EvCoercion (mkTcNthCo i ev_co) )
                             | (r, ty1, ty2, i) <- zip4 tc_roles tys1 tys2 [0..]
                             , r /= Phantom ]
888 889
              ; emitWorkNC given_evs }
  where
890 891 892 893 894
    role     = eqRelRole eq_rel
    tc_roles = tyConRolesX role tc

-- | Call when canonicalizing an equality fails, but if the equality is
-- representational, there is some hope for the future.
895
-- Examples in Note [Use canEqFailure in canDecomposableTyConApp]
896 897
canEqFailure :: CtEvidence -> EqRel
             -> TcType -> TcType -> TcS (StopOrContinue Ct)
898 899
canEqFailure ev NomEq ty1 ty2
  = canEqHardFailure ev NomEq ty1 ty2
900
canEqFailure ev ReprEq ty1 ty2
901
  = do { (xi1, co1) <- flatten FM_FlattenAll ev ty1
902
       ; (xi2, co2) <- flatten FM_FlattenAll ev ty2
903 904 905
            -- We must flatten the types before putting them in the
            -- inert set, so that we are sure to kick them out when
            -- new equalities become available
906 907
       ; traceTcS "canEqFailure with ReprEq" $
         vcat [ ppr ev, ppr ty1, ppr ty2, ppr xi1, ppr xi2 ]
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
908 909 910
       ; rewriteEqEvidence ev ReprEq NotSwapped xi1 xi2 co1 co2
         `andWhenContinue` \ new_ev ->
         continueWith (CIrredEvCan { cc_ev = new_ev }) }
911 912 913 914

-- | Call when canonicalizing an equality fails with utterly no hope.
canEqHardFailure :: CtEvidence -> EqRel
                 -> TcType -> TcType -> TcS (StopOrContinue Ct)
915
-- See Note [Make sure that insolubles are fully rewritten]
916
canEqHardFailure ev eq_rel ty1 ty2
917 918
  = do { (s1, co1) <- flatten FM_SubstOnly ev ty1
       ; (s2, co2) <- flatten FM_SubstOnly ev ty2
919 920 921 922
       ; rewriteEqEvidence ev eq_rel NotSwapped s1 s2 co1 co2
         `andWhenContinue` \ new_ev ->
    do { emitInsoluble (mkNonCanonical new_ev)
       ; stopWith new_ev "Definitely not equal" }}
923

Austin Seipp's avatar
Austin Seipp committed
924
{-
925 926 927 928 929 930
Note [Decomposing TyConApps]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we see (T s1 t1 ~ T s2 t2), then we can just decompose to
  (s1 ~ s2, t1 ~ t2)
and push those back into the work list.  But if
  s1 = K k1    s2 = K k2
Jan Stolarek's avatar
Jan Stolarek committed
931
then we will just decomopose s1~s2, and it might be better to
932 933 934 935 936 937
do so on the spot.  An important special case is where s1=s2,
and we get just Refl.

So canDecomposableTyCon is a fast-path decomposition that uses
unifyWanted etc to short-cut that work.

938 939 940
Note [Canonicalising type applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Given (s1 t1) ~ ty2, how should we proceed?
Austin Seipp's avatar
Austin Seipp committed
941
The simple things is to see if ty2 is of form (s2 t2), and
942
decompose.  By this time s1 and s2 can't be saturated type
Austin Seipp's avatar
Austin Seipp committed
943 944
function applications, because those have been dealt with
by an earlier equation in can_eq_nc, so it is always sound to
945 946
decompose.

Austin Seipp's avatar
Austin Seipp committed
947
However, over-eager decomposition gives bad error messages
948 949 950 951 952 953 954
for things like
   a b ~ Maybe c
   e f ~ p -> q
Suppose (in the first example) we already know a~Array.  Then if we
decompose the application eagerly, yielding
   a ~ Maybe
   b ~ c
Austin Seipp's avatar
Austin Seipp committed
955
we get an error        "Can't match Array ~ Maybe",
956 957
but we'd prefer to get "Can't match Array b ~ Maybe c".

958 959 960
So instead can_eq_wanted_app flattens the LHS and RHS, in the hope of
replacing (a b) by (Array b), before using try_decompose_app to
decompose it.
961

962 963
Note [Make sure that insolubles are fully rewritten]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Simon Peyton Jones's avatar
Simon Peyton Jones committed
964 965
When an equality fails, we still want to rewrite the equality
all the way down, so that it accurately reflects
966 967
 (a) the mutable reference substitution in force at start of solving
 (b) any ty-binds in force at this point in solving
968
See Note [Kick out insolubles] in TcSMonad.
Simon Peyton Jones's avatar
Simon Peyton Jones committed
969
And if we don't do this there is a bad danger that
970 971 972
TcSimplify.applyTyVarDefaulting will find a variable
that has in fact been substituted.

973
Note [Do not decompose Given polytype equalities]
974 975
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider [G] (forall a. t1 ~ forall a. t2).  Can we decompose this?
976
No -- what would the evidence look like?  So instead we simply discard
Simon Peyton Jones's avatar
Simon Peyton Jones committed
977
this given evidence.
978 979


980 981 982 983 984 985 986 987 988 989 990 991 992
Note [Combining insoluble constraints]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As this point we have an insoluble constraint, like Int~Bool.

 * If it is Wanted, delete it from the cache, so that subsequent
   Int~Bool constraints give rise to separate error messages

 * But if it is Derived, DO NOT delete from cache.  A class constraint
   may get kicked out of the inert set, and then have its functional
   dependency Derived constraints generated a second time. In that
   case we don't want to get two (or more) error messages by
   generating two (or more) insoluble fundep constraints from the same
   class constraint.
993 994 995 996 997 998 999 1000 1001 1002 1003

Note [No top-level newtypes on RHS of representational equalities]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

1004 1005 1006 1007
We want to make sure canEqTyVar sees [W] a ~R a, after b is flattened
and the Id newtype is unwrapped. This is assured by requiring only flat
types in canEqTyVar *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.
1008

1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025
Note [Occurs check error]
~~~~~~~~~~~~~~~~~~~~~~~~~
If we have an occurs check error, are we necessarily hosed? Say our
tyvar is tv1 and the type it appears in is xi2. Because xi2 is function
free, then if we're computing w.r.t. nominal equality, then, yes, we're
hosed. Nothing good can come from (a ~ [a]). If we're computing w.r.t.
representational equality, this is a little subtler. Once again, (a ~R [a])
is a bad thing, but (a ~R N a) for a newtype N might be just fine. This
means also that (a ~ b a) might be fine, because `b` might become a newtype.

So, we must check: does tv1 appear in xi2 under any type constructor that
is generative w.r.t. representational equality? That's what isTyVarUnderDatatype
does. (The other name I considered, isTyVarUnderTyConGenerativeWrtReprEq was
a bit verbose. And the shorter name gets the point across.)

See also #10715, which induced this addition.

Austin Seipp's avatar
Austin Seipp committed
1026
-}
Simon Peyton Jones's avatar
Simon Peyton Jones committed
1027

Austin Seipp's avatar
Austin Seipp committed
1028
canCFunEqCan :: CtEvidence
1029
             -> TyCon -> [TcType]   -- LHS
1030 1031
             -> TcTyVar             -- RHS
             -> TcS (StopOrContinue Ct)
Austin Seipp's avatar
Austin Seipp committed
1032 1033
-- ^ Canonicalise a CFunEqCan.  We know that
--     the arg types are already flat,
1034 1035 1036
-- and the RHS is a fsk, which we must *not* substitute.
-- So just substitute in the LHS
canCFunEqCan ev fn tys fsk
1037
  = do { (tys', cos) <- flattenManyNom ev tys
1038 1039 1040 1041 1042
                        -- cos :: tys' ~ tys
       ; let lhs_co  = mkTcTyConAppCo Nominal fn cos
                        -- :: F tys' ~ F tys
             new_lhs = mkTyConApp fn tys'
             fsk_ty  = mkTyVarTy fsk
1043 1044 1045 1046
       ; rewriteEqEvidence ev NomEq NotSwapped new_lhs fsk_ty
                           lhs_co (mkTcNomReflCo fsk_ty)
         `andWhenContinue` \ ev' ->
    do { extendFlatCache fn tys' (ctEvCoercion ev', fsk_ty, ctEvFlavour ev')
1047
       ; continueWith (CFunEqCan { cc_ev = ev', cc_fun = fn
1048
                                 , cc_tyargs = tys', cc_fsk = fsk }) } }
1049 1050

---------------------
1051
canEqTyVar :: CtEvidence -> EqRel -> SwapFlag
1052 1053
           -> TcTyVar             -- already flat
           -> TcType              -- already flat
1054
           -> TcS (StopOrContinue Ct)
1055
-- A TyVar on LHS, but so far un-zonked
1056 1057 1058
canEqTyVar ev eq_rel swapped tv1 ps_ty2              -- ev :: tv ~ s2
  = do { traceTcS "canEqTyVar" (ppr tv1 $$ ppr ps_ty2 $$ ppr swapped)
         -- FM_Avoid commented out: see Note [Lazy flattening] in TcFlatten
1059 1060 1061 1062
         -- let fmode = FE { fe_ev = ev, fe_mode = FM_Avoid tv1' True }
         -- Flatten the RHS less vigorously, to avoid gratuitous flattening
         -- True <=> xi2 should not itself be a type-function application
       ; dflags <- getDynFlags
1063
       ; canEqTyVar2 dflags ev eq_rel swapped tv1 ps_ty2 }
1064 1065

canEqTyVar2 :: DynFlags
1066
            -> CtEvidence   -- lhs ~ rhs (or, if swapped, orhs ~ olhs)
1067
            -> EqRel
1068
            -> SwapFlag
1069 1070
            -> TcTyVar      -- lhs, flat
            -> TcType       -- rhs, flat
1071
            -> TcS (StopOrContinue Ct)
Austin Seipp's avatar
Austin Seipp committed
1072
-- LHS is an inert type variable,
1073
-- and RHS is fully rewritten, but with type synonyms
1074
-- preserved as much as possible
1075

1076
canEqTyVar2 dflags ev eq_rel swapped tv1 xi2
1077
  | Just tv2 <- getTyVar_maybe xi2
1078
  = canEqTyVarTyVar ev eq_rel swapped tv1 tv2
1079

1080
  | OC_OK xi2' <- occurCheckExpand dflags tv1 xi2  -- No occurs check
1081 1082 1083 1084
     -- We use xi2' on the RHS of the new CTyEqCan, a ~ xi2'
     -- to establish the invariant that a does not appear in the
     -- rhs of the CTyEqCan. This is guaranteed by occurCheckExpand;
     -- see Note [Occurs check expansion] in TcType
1085 1086
  = do { let k1 = tyVarKind tv1
             k2 = typeKind xi2'
1087
       ; rewriteEqEvidence ev eq_rel swapped xi1 xi2' co1 (mkTcReflCo role xi2')
1088 1089 1090
         `andWhenContinue` \ new_ev ->
         if k2 `isSubKind` k1
         then   -- Establish CTyEqCan kind invariant
1091 1092
                -- Reorientation has done its best, but the kinds might
                -- simply be incompatible
1093 1094 1095 1096
               continueWith (CTyEqCan { cc_ev = new_ev
                                      , cc_tyvar  = tv1, cc_rhs = xi2'
                                      , cc_eq_rel = eq_rel })
         else incompatibleKind new_ev xi1 k1 xi2' k2 }
1097 1098

  | otherwise  -- Occurs check error
1099 1100
  = rewriteEqEvidence ev eq_rel swapped xi1 xi2 co1 co2
    `andWhenContinue` \ new_ev ->
1101 1102 1103 1104
    if eq_rel == NomEq || isTyVarUnderDatatype tv1 xi2
      -- See Note [Occurs check error]

    then do { emitInsoluble (mkNonCanonical new_ev)
1105 1106 1107
              -- If we have a ~ [a], it is not canonical, and in particular
              -- we don't want to rewrite existing inerts with it, otherwise
              -- we'd risk divergence in the constraint solver
1108
            ; stopWith new_ev "Occurs check" }
1109 1110 1111 1112 1113 1114 1115

        -- A representational equality with an occurs-check problem isn't
        -- insoluble! For example:
        --   a ~R b a
        -- We might learn that b is the newtype Id.
        -- But, the occurs-check certainly prevents the equality from being
        -- canonical, and we might loop if we were to use it in rewriting.
1116
    else do { traceTcS "Occurs-check in representational equality"
1117
                              (ppr xi1 $$ ppr xi2)
1118
            ; continueWith (CIrredEvCan { cc_ev = new_ev }) }
1119
  where
1120 1121 1122 1123
    role = eqRelRole eq_rel
    xi1  = mkTyVarTy tv1
    co1  = mkTcReflCo role xi1
    co2  = mkTcReflCo role xi2
1124

1125
canEqTyVarTyVar :: CtEvidence           -- tv1 ~ rhs (or rhs ~ tv1, if swapped)
1126
                -> EqRel
1127
                -> SwapFlag
1128
                -> TcTyVar -> TcTyVar   -- tv1, tv2
1129
                -> TcS (StopOrContinue Ct)
1130
-- Both LHS and RHS rewrote to a type variable
1131
-- See Note [Canonical orientation for tyvar/tyvar equality constraints]
1132
canEqTyVarTyVar ev eq_rel swapped tv1 tv2
1133
  | tv1 == tv2
1134
  = do { setEvBindIfWanted ev (EvCoercion $ mkTcReflCo role xi1)
1135 1136
       ; stopWith ev "Equal tyvars" }

1137 1138 1139
  | incompat_kind   = incompatibleKind ev xi1 k1 xi2 k2

-- We don't do this any more
Simon Peyton Jones's avatar
Simon Peyton Jones committed
1140
-- See Note [Orientation of equalities with fmvs] in TcFlatten
1141 1142 1143
--  | isFmvTyVar tv1  = do_fmv swapped            tv1 xi1 xi2 co1 co2
--  | isFmvTyVar tv2  = do_fmv (flipSwap swapped) tv2 xi2 xi1 co2 co1

1144 1145 1146
  | same_kind       = if swap_over then do_swap else no_swap
  | k1_sub_k2       = do_swap   -- Note [Kind orientation for CTyEqCan]
  | otherwise       = no_swap   -- k2_sub_k1
1147
  where
1148
    role = eqRelRole eq_rel
1149
    xi1 = mkTyVarTy tv1
1150
    co1 = mkTcReflCo role xi1
1151
    xi2 = mkTyVarTy tv2