TcPat.hs 51 KB
Newer Older
Austin Seipp's avatar
Austin Seipp committed
1 2 3 4
{-
(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1992-1998

5 6

TcPat: Typechecking patterns
Austin Seipp's avatar
Austin Seipp committed
7
-}
8

9
{-# LANGUAGE CPP, RankNTypes, TupleSections #-}
10
{-# LANGUAGE FlexibleContexts #-}
Ian Lynagh's avatar
Ian Lynagh committed
11

12
module TcPat ( tcLetPat
13 14
             , TcPragEnv, lookupPragEnv, emptyPragEnv
             , LetBndrSpec(..), addInlinePrags
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
15
             , tcPat, tcPat_O, tcPats, newNoSigLetBndr
16
             , addDataConStupidTheta, badFieldCon, polyPatSig ) where
17

18
#include "HsVersions.h"
19

20
import {-# SOURCE #-}   TcExpr( tcSyntaxOp, tcSyntaxOpGen, tcInferSigma )
21 22 23

import HsSyn
import TcHsSyn
24
import TcRnMonad
25 26 27 28
import Inst
import Id
import Var
import Name
29
import NameEnv
Adam Gundry's avatar
Adam Gundry committed
30
import RdrName
31 32
import TcEnv
import TcMType
33
import TcValidity( arityErr )
34 35 36 37
import TcType
import TcUnify
import TcHsType
import TysWiredIn
38
import TcEvidence
39 40
import TyCon
import DataCon
cactus's avatar
cactus committed
41 42
import PatSyn
import ConLike
43 44
import PrelNames
import BasicTypes hiding (SuccessFlag(..))
45
import DynFlags
46
import SrcLoc
47
import VarSet
48
import Util
sof's avatar
sof committed
49
import Outputable
50
import Maybes( orElse )
51
import qualified GHC.LanguageExtensions as LangExt
Ian Lynagh's avatar
Ian Lynagh committed
52
import Control.Monad
53
import Control.Arrow  ( second )
54

Austin Seipp's avatar
Austin Seipp committed
55 56 57
{-
************************************************************************
*                                                                      *
58
                External interface
Austin Seipp's avatar
Austin Seipp committed
59 60 61
*                                                                      *
************************************************************************
-}
62

63
tcLetPat :: TcSigFun -> LetBndrSpec
64
         -> LPat Name -> ExpSigmaType
65 66
         -> TcM a
         -> TcM (LPat TcId, a)
67
tcLetPat sig_fn no_gen pat pat_ty thing_inside
68
  = tc_lpat pat pat_ty penv thing_inside
69
  where
70
    penv = PE { pe_lazy = True
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
71 72
              , pe_ctxt = LetPat sig_fn no_gen
              , pe_orig = PatOrigin }
73 74

-----------------
75
tcPats :: HsMatchContext Name
76
       -> [LPat Name]            -- Patterns,
77
       -> [ExpSigmaType]         --   and their types
78
       -> TcM a                  --   and the checker for the body
79
       -> TcM ([LPat TcId], a)
80 81 82

-- This is the externally-callable wrapper function
-- Typecheck the patterns, extend the environment to bind the variables,
83
-- do the thing inside, use any existentially-bound dictionaries to
84 85 86 87 88
-- discharge parts of the returning LIE, and deal with pattern type
-- signatures

--   1. Initialise the PatState
--   2. Check the patterns
89 90
--   3. Check the body
--   4. Check that no existentials escape
91

92
tcPats ctxt pats pat_tys thing_inside
93 94
  = tc_lpats penv pats pat_tys thing_inside
  where
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
95
    penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt, pe_orig = PatOrigin }
96

97
tcPat :: HsMatchContext Name
98
      -> LPat Name -> ExpSigmaType
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
99
      -> TcM a                     -- Checker for body
100
      -> TcM (LPat TcId, a)
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
101 102 103 104 105
tcPat ctxt = tcPat_O ctxt PatOrigin

-- | A variant of 'tcPat' that takes a custom origin
tcPat_O :: HsMatchContext Name
        -> CtOrigin              -- ^ origin to use if the type needs inst'ing
106
        -> LPat Name -> ExpSigmaType
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
107 108 109
        -> TcM a                 -- Checker for body
        -> TcM (LPat TcId, a)
tcPat_O ctxt orig pat pat_ty thing_inside
110 111
  = tc_lpat pat pat_ty penv thing_inside
  where
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
112
    penv = PE { pe_lazy = False, pe_ctxt = LamPat ctxt, pe_orig = orig }
113

114

115
-----------------
116
data PatEnv
117 118
  = PE { pe_lazy :: Bool        -- True <=> lazy context, so no existentials allowed
       , pe_ctxt :: PatCtxt     -- Context in which the whole pattern appears
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
119
       , pe_orig :: CtOrigin    -- origin to use if the pat_ty needs inst'ing
120
       }
121 122 123

data PatCtxt
  = LamPat   -- Used for lambdas, case etc
124
       (HsMatchContext Name)
125

126
  | LetPat   -- Used only for let(rec) pattern bindings
127
             -- See Note [Typing patterns in pattern bindings]
128 129 130
       TcSigFun        -- Tells type sig if any
       LetBndrSpec     -- True <=> no generalisation of this let

131 132 133
data LetBndrSpec
  = LetLclBndr            -- The binder is just a local one;
                          -- an AbsBinds will provide the global version
134

135
  | LetGblBndr TcPragEnv  -- Generalisation plan is NoGen, so there isn't going
136
                          -- to be an AbsBinds; So we must bind the global version
137
                          -- of the binder right away.
138
                          -- Oh, and here is the inline-pragma information
139

140 141 142
makeLazy :: PatEnv -> PatEnv
makeLazy penv = penv { pe_lazy = True }

143 144 145
inPatBind :: PatEnv -> Bool
inPatBind (PE { pe_ctxt = LetPat {} }) = True
inPatBind (PE { pe_ctxt = LamPat {} }) = False
146 147

---------------
148
type TcPragEnv = NameEnv [LSig Name]
149

150 151 152 153 154 155
emptyPragEnv :: TcPragEnv
emptyPragEnv = emptyNameEnv

lookupPragEnv :: TcPragEnv -> Name -> [LSig Name]
lookupPragEnv prag_fn n = lookupNameEnv prag_fn n `orElse` []

156
{- *********************************************************************
Austin Seipp's avatar
Austin Seipp committed
157
*                                                                      *
158
                Binders
Austin Seipp's avatar
Austin Seipp committed
159
*                                                                      *
160
********************************************************************* -}
161

162
tcPatBndr :: PatEnv -> Name -> ExpSigmaType -> TcM (HsWrapper, TcId)
163 164 165
-- (coi, xp) = tcPatBndr penv x pat_ty
-- Then coi : pat_ty ~ typeof(xp)
--
166 167
tcPatBndr (PE { pe_ctxt = LetPat lookup_sig no_gen
              , pe_orig = orig }) bndr_name pat_ty
168
          -- See Note [Typing patterns in pattern bindings]
169
  | LetGblBndr prags   <- no_gen
170
  , Just (TcIdSig sig) <- mb_sig
171
  , Just poly_id <- completeIdSigPolyId_maybe sig
172
  = do { bndr_id <- addInlinePrags poly_id (lookupPragEnv prags bndr_name)
173
       ; traceTc "tcPatBndr(gbl,sig)" (ppr bndr_id $$ ppr (idType bndr_id))
174
       ; co <- unifyPatType bndr_id (idType bndr_id) pat_ty
175 176 177 178 179 180 181 182 183 184 185 186
       ; return (mkWpCastN co, bndr_id) }

  -- See Note [Partial signatures for pattern bindings]
  | LetLclBndr         <- no_gen
  , Just (TcIdSig sig) <- mb_sig
  = do { mono_name <- newLocalName bndr_name
       ; (subst, _) <- newMetaSigTyVars (map snd (sig_skols sig))
       ; let tau     = substTy subst (sig_tau sig)
             mono_id = mkLocalId mono_name tau
       ; wrap <- tcSubTypeET orig pat_ty tau
       ; traceTc "tcPatBndr(lsl,sig)" (ppr mono_id $$ ppr tau $$ ppr pat_ty)
       ; return (wrap, mono_id) }
187 188

  | otherwise
189 190
  = do { pat_ty <- expTypeToType pat_ty
       ; bndr_id <- newNoSigLetBndr no_gen bndr_name pat_ty
191
       ; traceTc "tcPatBndr(no-sig)" (ppr bndr_id $$ ppr (idType bndr_id))
192 193 194
       ; return (idHsWrapper, bndr_id) }
  where
    mb_sig = lookup_sig bndr_name
195 196

tcPatBndr (PE { pe_ctxt = _lam_or_proc }) bndr_name pat_ty
197
  = do { pat_ty <- expTypeToType pat_ty
198
       ; return (idHsWrapper, mkLocalId bndr_name pat_ty) }
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
199 200
               -- whether or not there is a sig is irrelevant, as this
               -- is local
201

202 203
------------
newNoSigLetBndr :: LetBndrSpec -> Name -> TcType -> TcM TcId
204
-- In the polymorphic case (no_gen = LetLclBndr), generate a "monomorphic version"
205 206
--    of the Id; the original name will be bound to the polymorphic version
--    by the AbsBinds
207
-- In the monomorphic case (no_gen = LetBglBndr) there is no AbsBinds, and we
208
--    use the original name directly
209
newNoSigLetBndr LetLclBndr name ty
210
  =do  { mono_name <- newLocalName name
211
       ; return (mkLocalId mono_name ty) }
212
newNoSigLetBndr (LetGblBndr prags) name ty
213
  = addInlinePrags (mkLocalId name ty) (lookupPragEnv prags name)
214 215 216 217

----------
addInlinePrags :: TcId -> [LSig Name] -> TcM TcId
addInlinePrags poly_id prags
218 219 220 221 222 223
  | inl@(L _ prag) : inls <- inl_prags
  = do { traceTc "addInlinePrag" (ppr poly_id $$ ppr prag)
       ; unless (null inls) (warn_multiple_inlines inl inls)
       ; return (poly_id `setInlinePragma` prag) }
  | otherwise
  = return poly_id
224
  where
225 226 227 228 229 230 231 232 233 234 235 236
    inl_prags = [L loc prag | L loc (InlineSig _ prag) <- prags]

    warn_multiple_inlines _ [] = return ()

    warn_multiple_inlines inl1@(L loc prag1) (inl2@(L _ prag2) : inls)
       | inlinePragmaActivation prag1 == inlinePragmaActivation prag2
       , isEmptyInlineSpec (inlinePragmaSpec prag1)
       =    -- Tiresome: inl1 is put there by virtue of being in a hs-boot loop
            -- and inl2 is a user NOINLINE pragma; we don't want to complain
         warn_multiple_inlines inl2 inls
       | otherwise
       = setSrcSpan loc $
237 238
         addWarnTc NoReason
                     (hang (text "Multiple INLINE pragmas for" <+> ppr poly_id)
239
                       2 (vcat (text "Ignoring all but the first"
240 241 242
                                : map pp_inl (inl1:inl2:inls))))

    pp_inl (L loc prag) = ppr prag <+> parens (ppr loc)
243

244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
{- Note [Partial signatures for pattern bindings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider a function binding and a pattern binding, both
with a partial type signature

   f1 :: (True, _) -> Char
   f1 = \x -> x

   f2 :: (True, _) -> Char
   Just f2 = Just (\x->x)

Obviously, both should be rejected.  That happens naturally for the
function binding, f1, because we typecheck the RHS with "expected"
type '(True, apha) -> Char', which correctly fails.

But what of the pattern binding for f2?  We infer the type of the
pattern, and check tha the RHS has that type.  So we must feed in the
type of f2 when inferring the type of the pattern!  We do this right
here, in tcPatBndr, for a LetLclBndr. The signature already has fresh
unification variables for the wildcards (if any).

Extra notes

* For /complete/ type signatures, we could im principle ignore all this
  and just infer the most general type for f2, and check (in
  TcBinds.mkExport) whether it has the claimed type.

  But not so for /partial/ signatures; to get the wildcard unification
  variables into the game we really must inject them here. If we don't
  we never get /any/ value assigned to the wildcards; and programs that
  are bogus, like f2, are accepted.

  Moreover, by feeding in the expected type we do less fruitless
  creation of unification variables, and improve error messages.

* We need to take care with the skolems.  Consider
      data T a = MkT a a
      f :: forall a. a->a
      g :: forall b. b->b
      MkT f g = MkT (\x->x) (\y->y)
  Here we'll infer a type from the pattern of 'T a', but if we feed in
  the signature types for f and g, we'll end up unifying 'a' and 'b'.
  So we instantiate the skolems with SigTvs; hence newMetaSigTyVars.

  All we are doing here is getting the "shapes" right.  In tcExport
  we'll check that the Id really does have the claimed type, with
  the claimed polymorphism.

* We need to do a subsumption, not equality, check.  If
      data T = MkT (forall a. a->a)
      f :: forall b. [b]->[b]
      MkT f = blah
  Since 'blah' returns a value of type T, its payload is a polymorphic
  function of type (forall a. a->a).  And that's enough to bind the
  less-polymorphic function 'f', but we need some impedence matching
  to witness the instantiation.

301 302 303 304 305 306 307 308
Note [Typing patterns in pattern bindings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Suppose we are typing a pattern binding
    pat = rhs
Then the PatCtxt will be (LetPat sig_fn let_bndr_spec).

There can still be signatures for the binders:
     data T = MkT (forall a. a->a) Int
309
     x :: forall a. a->a
310 311 312 313 314 315 316 317 318 319
     y :: Int
     MkT x y = <rhs>

Two cases, dealt with by the LetPat case of tcPatBndr

 * If we are generalising (generalisation plan is InferGen or
   CheckGen), then the let_bndr_spec will be LetLclBndr.  In that case
   we want to bind a cloned, local version of the variable, with the
   type given by the pattern context, *not* by the signature (even if
   there is one; see Trac #7268). The mkExport part of the
Gabor Greif's avatar
Gabor Greif committed
320
   generalisation step will do the checking and impedance matching
321 322 323 324 325 326 327 328
   against the signature.

 * If for some some reason we are not generalising (plan = NoGen), the
   LetBndrSpec will be LetGblBndr.  In that case we must bind the
   global version of the Id, and do so with precisely the type given
   in the signature.  (Then we unify with the type from the pattern
   context type.

329

Austin Seipp's avatar
Austin Seipp committed
330 331
************************************************************************
*                                                                      *
332
                The main worker functions
Austin Seipp's avatar
Austin Seipp committed
333 334
*                                                                      *
************************************************************************
335

336 337
Note [Nesting]
~~~~~~~~~~~~~~
lennart@augustsson.net's avatar
lennart@augustsson.net committed
338
tcPat takes a "thing inside" over which the pattern scopes.  This is partly
339
so that tcPat can extend the environment for the thing_inside, but also
340 341 342 343
so that constraints arising in the thing_inside can be discharged by the
pattern.

This does not work so well for the ErrCtxt carried by the monad: we don't
344
want the error-context for the pattern to scope over the RHS.
345
Hence the getErrCtxt/setErrCtxt stuff in tcMultiple
Austin Seipp's avatar
Austin Seipp committed
346
-}
347 348

--------------------
349
type Checker inp out =  forall r.
350 351 352 353
                          inp
                       -> PatEnv
                       -> TcM r
                       -> TcM (out, r)
354 355

tcMultiple :: Checker inp out -> Checker [inp] [out]
356
tcMultiple tc_pat args penv thing_inside
357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372
  = do  { err_ctxt <- getErrCtxt
        ; let loop _ []
                = do { res <- thing_inside
                     ; return ([], res) }

              loop penv (arg:args)
                = do { (p', (ps', res))
                                <- tc_pat arg penv $
                                   setErrCtxt err_ctxt $
                                   loop penv args
                -- setErrCtxt: restore context before doing the next pattern
                -- See note [Nesting] above

                     ; return (p':ps', res) }

        ; loop penv args }
373 374

--------------------
375
tc_lpat :: LPat Name
376
        -> ExpSigmaType
377 378 379
        -> PatEnv
        -> TcM a
        -> TcM (LPat TcId, a)
380
tc_lpat (L span pat) pat_ty penv thing_inside
381
  = setSrcSpan span $
382
    do  { (pat', res) <- maybeWrapPatCtxt pat (tc_pat penv pat pat_ty)
383
                                          thing_inside
384
        ; return (L span pat', res) }
385 386

tc_lpats :: PatEnv
387
         -> [LPat Name] -> [ExpSigmaType]
388 389 390
         -> TcM a
         -> TcM ([LPat TcId], a)
tc_lpats penv pats tys thing_inside
Simon Peyton Jones's avatar
Simon Peyton Jones committed
391
  = ASSERT2( equalLength pats tys, ppr pats $$ ppr tys )
392
    tcMultiple (\(p,t) -> tc_lpat p t)
393
                (zipEqual "tc_lpats" pats tys)
394
                penv thing_inside
395 396

--------------------
397 398
tc_pat  :: PatEnv
        -> Pat Name
399
        -> ExpSigmaType  -- Fully refined result type
400 401 402
        -> TcM a                -- Thing inside
        -> TcM (Pat TcId,       -- Translated pattern
                a)              -- Result of thing inside
403

404
tc_pat penv (VarPat (L l name)) pat_ty thing_inside
405
  = do  { (wrap, id) <- tcPatBndr penv name pat_ty
batterseapower's avatar
batterseapower committed
406
        ; res <- tcExtendIdEnv1 name id thing_inside
407
        ; pat_ty <- readExpType pat_ty
408
        ; return (mkHsWrapPat wrap (VarPat (L l id)) pat_ty, res) }
409 410

tc_pat penv (ParPat pat) pat_ty thing_inside
411 412
  = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
        ; return (ParPat pat', res) }
413 414

tc_pat penv (BangPat pat) pat_ty thing_inside
415 416
  = do  { (pat', res) <- tc_lpat pat pat_ty penv thing_inside
        ; return (BangPat pat', res) }
417

418
tc_pat penv lpat@(LazyPat pat) pat_ty thing_inside
419 420 421 422
  = do  { (pat', (res, pat_ct))
                <- tc_lpat pat pat_ty (makeLazy penv) $
                   captureConstraints thing_inside
                -- Ignore refined penv', revert to penv
423

424 425
        ; emitConstraints pat_ct
        -- captureConstraints/extendConstraints:
426
        --   see Note [Hopping the LIE in lazy patterns]
427

428
        -- Check there are no unlifted types under the lazy pattern
429
        ; when (any (isUnliftedType . idType) $ collectPatBinders pat') $
430 431
               lazyUnliftedPatErr lpat

432
        -- Check that the expected pattern type is itself lifted
433 434
        ; pat_ty <- readExpType pat_ty
        ; _ <- unifyType noThing (typeKind pat_ty) liftedTypeKind
435

436
        ; return (LazyPat pat', res) }
437

438
tc_pat _ (WildPat _) pat_ty thing_inside
439
  = do  { res <- thing_inside
440
        ; pat_ty <- expTypeToType pat_ty
441
        ; return (WildPat pat_ty, res) }
442

443
tc_pat penv (AsPat (L nm_loc name) pat) pat_ty thing_inside
444
  = do  { (wrap, bndr_id) <- setSrcSpan nm_loc (tcPatBndr penv name pat_ty)
batterseapower's avatar
batterseapower committed
445
        ; (pat', res) <- tcExtendIdEnv1 name bndr_id $
446 447
                         tc_lpat pat (mkCheckExpType $ idType bndr_id)
                                 penv thing_inside
448 449 450 451 452 453 454
            -- NB: if we do inference on:
            --          \ (y@(x::forall a. a->a)) = e
            -- we'll fail.  The as-pattern infers a monotype for 'y', which then
            -- fails to unify with the polymorphic type for 'x'.  This could
            -- perhaps be fixed, but only with a bit more work.
            --
            -- If you fix it, don't forget the bindInstsOfPatIds!
455
        ; pat_ty <- readExpType pat_ty
456
        ; return (mkHsWrapPat wrap (AsPat (L nm_loc bndr_id) pat') pat_ty, res) }
457 458 459

tc_pat penv (ViewPat expr pat _) overall_pat_ty thing_inside
  = do  {
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
460
         -- Expr must have type `forall a1...aN. OPT' -> B`
461
         -- where overall_pat_ty is an instance of OPT'.
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
462
        ; (expr',expr'_inferred) <- tcInferSigma expr
463

464 465 466 467 468 469 470 471
         -- expression must be a function
        ; let expr_orig = exprCtOrigin (unLoc expr)
              herald    = text "A view pattern expression expects"
        ; (expr_wrap1, [inf_arg_ty], inf_res_ty)
            <- matchActualFunTys herald expr_orig (Just expr) 1 expr'_inferred
            -- expr_wrap1 :: expr'_inferred "->" (inf_arg_ty -> inf_res_ty)

         -- check that overall pattern is more polymorphic than arg type
472
        ; expr_wrap2 <- tcSubTypeET (pe_orig penv) overall_pat_ty inf_arg_ty
473 474 475 476 477 478 479 480 481 482 483
            -- expr_wrap2 :: overall_pat_ty "->" inf_arg_ty

         -- pattern must have inf_res_ty
        ; (pat', res) <- tc_lpat pat (mkCheckExpType inf_res_ty) penv thing_inside

        ; overall_pat_ty <- readExpType overall_pat_ty
        ; let expr_wrap2' = mkWpFun expr_wrap2 idHsWrapper
                                    overall_pat_ty inf_res_ty
               -- expr_wrap2' :: (inf_arg_ty -> inf_res_ty) "->"
               --                (overall_pat_ty -> inf_res_ty)
              expr_wrap = expr_wrap2' <.> expr_wrap1
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
484
        ; return (ViewPat (mkLHsWrap expr_wrap expr') pat' overall_pat_ty, res) }
485

486 487
-- Type signatures in patterns
-- See Note [Pattern coercions] below
488
tc_pat penv (SigPatIn pat sig_ty) pat_ty thing_inside
489
  = do  { (inner_ty, tv_binds, wcs, wrap) <- tcPatSig (inPatBind penv)
thomasw's avatar
thomasw committed
490
                                                            sig_ty pat_ty
491 492
        ; (pat', res) <- tcExtendTyVarEnv2 wcs      $
                         tcExtendTyVarEnv  tv_binds $
493 494
                         tc_lpat pat (mkCheckExpType inner_ty) penv thing_inside
        ; pat_ty <- readExpType pat_ty
495
        ; return (mkHsWrapPat wrap (SigPatOut pat' inner_ty) pat_ty, res) }
496 497 498

------------------------
-- Lists, tuples, arrays
499
tc_pat penv (ListPat pats _ Nothing) pat_ty thing_inside
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
500
  = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedListTy penv pat_ty
501
        ; (pats', res) <- tcMultiple (\p -> tc_lpat p (mkCheckExpType elt_ty))
502
                                     pats penv thing_inside
503
        ; pat_ty <- readExpType pat_ty
504
        ; return (mkHsWrapPat coi (ListPat pats' elt_ty Nothing) pat_ty, res)
505 506 507
        }

tc_pat penv (ListPat pats _ (Just (_,e))) pat_ty thing_inside
508 509 510 511 512 513 514 515 516
  = do  { tau_pat_ty <- expTypeToType pat_ty
        ; ((pats', res, elt_ty), e')
            <- tcSyntaxOpGen ListOrigin e [SynType (mkCheckExpType tau_pat_ty)]
                                          SynList $
                 \ [elt_ty] ->
                 do { (pats', res) <- tcMultiple (\p -> tc_lpat p (mkCheckExpType elt_ty))
                                                 pats penv thing_inside
                    ; return (pats', res, elt_ty) }
        ; return (ListPat pats' elt_ty (Just (tau_pat_ty,e')), res)
517
        }
518

519
tc_pat penv (PArrPat pats _) pat_ty thing_inside
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
520
  = do  { (coi, elt_ty) <- matchExpectedPatTy matchExpectedPArrTy penv pat_ty
521
        ; (pats', res) <- tcMultiple (\p -> tc_lpat p (mkCheckExpType elt_ty))
522
                                     pats penv thing_inside
523
        ; pat_ty <- readExpType pat_ty
524
        ; return (mkHsWrapPat coi (PArrPat pats' elt_ty) pat_ty, res)
525
        }
526

527
tc_pat penv (TuplePat pats boxity _) pat_ty thing_inside
528 529
  = do  { let arity = length pats
              tc = tupleTyCon boxity arity
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
530 531
        ; (coi, arg_tys) <- matchExpectedPatTy (matchExpectedTyConApp tc)
                                               penv pat_ty
532 533
                     -- Unboxed tuples have RuntimeRep vars, which we discard:
                     -- See Note [Unboxed tuple RuntimeRep vars] in TyCon
534 535
        ; let con_arg_tys = case boxity of Unboxed -> drop arity arg_tys
                                           Boxed   -> arg_tys
536 537
        ; (pats', res) <- tc_lpats penv pats (map mkCheckExpType con_arg_tys)
                                   thing_inside
538

539
        ; dflags <- getDynFlags
540

541 542 543 544 545
        -- Under flag control turn a pattern (x,y,z) into ~(x,y,z)
        -- so that we can experiment with lazy tuple-matching.
        -- This is a pretty odd place to make the switch, but
        -- it was easy to do.
        ; let
546
              unmangled_result = TuplePat pats' boxity con_arg_tys
547
                                 -- pat_ty /= pat_ty iff coi /= IdCo
548 549
              possibly_mangled_result
                | gopt Opt_IrrefutableTuples dflags &&
550
                  isBoxed boxity            = LazyPat (noLoc unmangled_result)
551
                | otherwise                 = unmangled_result
552

553
        ; pat_ty <- readExpType pat_ty
554
        ; ASSERT( length con_arg_tys == length pats ) -- Syntactically enforced
555
          return (mkHsWrapPat coi possibly_mangled_result pat_ty, res)
556
        }
557 558 559

------------------------
-- Data constructors
560 561
tc_pat penv (ConPatIn con arg_pats) pat_ty thing_inside
  = tcConPat penv con pat_ty arg_pats thing_inside
562 563 564

------------------------
-- Literal patterns
565
tc_pat _ (LitPat simple_lit) pat_ty thing_inside
566
  = do  { let lit_ty = hsLitType simple_lit
567
        ; co <- unifyPatType simple_lit lit_ty pat_ty
568 569
                -- coi is of kind: pat_ty ~ lit_ty
        ; res <- thing_inside
570
        ; pat_ty <- readExpType pat_ty
571
        ; return ( mkHsWrapPatCo co (LitPat simple_lit) pat_ty
572
                 , res) }
573 574 575

------------------------
-- Overloaded patterns: n, and n+k
576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592

-- In the case of a negative literal (the more complicated case),
-- we get
--
--   case v of (-5) -> blah
--
-- becoming
--
--   if v == (negate (fromInteger 5)) then blah else ...
--
-- There are two bits of rebindable syntax:
--   (==)   :: pat_ty -> neg_lit_ty -> Bool
--   negate :: lit_ty -> neg_lit_ty
-- where lit_ty is the type of the overloaded literal 5.
--
-- When there is no negation, neg_lit_ty and lit_ty are the same
tc_pat _ (NPat (L l over_lit) mb_neg eq _) pat_ty thing_inside
593
  = do  { let orig = LiteralOrigin over_lit
594 595 596 597 598 599 600 601 602 603 604 605 606 607
        ; ((lit', mb_neg'), eq')
            <- tcSyntaxOp orig eq [SynType pat_ty, SynAny]
                          (mkCheckExpType boolTy) $
               \ [neg_lit_ty] ->
               let new_over_lit lit_ty = newOverloadedLit over_lit
                                           (mkCheckExpType lit_ty)
               in case mb_neg of
                 Nothing  -> (, Nothing) <$> new_over_lit neg_lit_ty
                 Just neg -> -- Negative literal
                             -- The 'negate' is re-mappable syntax
                   second Just <$>
                   (tcSyntaxOp orig neg [SynRho] (mkCheckExpType neg_lit_ty) $
                    \ [lit_ty] -> new_over_lit lit_ty)

608
        ; res <- thing_inside
609 610
        ; pat_ty <- readExpType pat_ty
        ; return (NPat (L l lit') mb_neg' eq' pat_ty, res) }
611

612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652
{-
Note [NPlusK patterns]
~~~~~~~~~~~~~~~~~~~~~~
From

  case v of x + 5 -> blah

we get

  if v >= 5 then (\x -> blah) (v - 5) else ...

There are two bits of rebindable syntax:
  (>=) :: pat_ty -> lit1_ty -> Bool
  (-)  :: pat_ty -> lit2_ty -> var_ty

lit1_ty and lit2_ty could conceivably be different.
var_ty is the type inferred for x, the variable in the pattern.

If the pushed-down pattern type isn't a tau-type, the two pat_ty's above
could conceivably be different specializations. But this is very much
like the situation in Note [Case branches must be taus] in TcMatches.
So we tauify the pat_ty before proceeding.

Note that we need to type-check the literal twice, because it is used
twice, and may be used at different types. The second HsOverLit stored in the
AST is used for the subtraction operation.
-}

-- See Note [NPlusK patterns]
tc_pat penv (NPlusKPat (L nm_loc name) (L loc lit) _ ge minus _) pat_ty thing_inside
  = do  { pat_ty <- expTypeToType pat_ty
        ; let orig = LiteralOrigin lit
        ; (lit1', ge')
            <- tcSyntaxOp orig ge [synKnownType pat_ty, SynRho]
                                  (mkCheckExpType boolTy) $
               \ [lit1_ty] ->
               newOverloadedLit lit (mkCheckExpType lit1_ty)
        ; ((lit2', minus_wrap, bndr_id), minus')
            <- tcSyntaxOpGen orig minus [synKnownType pat_ty, SynRho] SynAny $
               \ [lit2_ty, var_ty] ->
               do { lit2' <- newOverloadedLit lit (mkCheckExpType lit2_ty)
653
                  ; (wrap, bndr_id) <- setSrcSpan nm_loc $
654 655 656 657
                                     tcPatBndr penv name (mkCheckExpType var_ty)
                           -- co :: var_ty ~ idType bndr_id

                           -- minus_wrap is applicable to minus'
658
                  ; return (lit2', wrap, bndr_id) }
659

660
        -- The Report says that n+k patterns must be in Integral
661 662 663 664
        -- but it's silly to insist on this in the RebindableSyntax case
        ; unlessM (xoptM LangExt.RebindableSyntax) $
          do { icls <- tcLookupClass integralClassName
             ; instStupidTheta orig [mkClassPred icls [pat_ty]] }
665

666
        ; res <- tcExtendIdEnv1 name bndr_id thing_inside
667 668 669 670 671 672

        ; let minus'' = minus' { syn_res_wrap =
                                    minus_wrap <.> syn_res_wrap minus' }
              pat' = NPlusKPat (L nm_loc bndr_id) (L loc lit1') lit2'
                               ge' minus'' pat_ty
        ; return (pat', res) }
673 674

tc_pat _ _other_pat _ _ = panic "tc_pat"        -- ConPatOut, SigPatOut
675 676

----------------
677
unifyPatType :: Outputable a => a -> TcType -> ExpSigmaType -> TcM TcCoercion
678 679 680 681
-- In patterns we want a coercion from the
-- context type (expected) to the actual pattern type
-- But we don't want to reverse the args to unifyType because
-- that controls the actual/expected stuff in error messages
682
unifyPatType thing actual_ty expected_ty
683
  = do { coi <- unifyExpType (Just thing) actual_ty expected_ty
684
       ; return (mkTcSymCo coi) }
685

Austin Seipp's avatar
Austin Seipp committed
686
{-
687 688 689 690
Note [Hopping the LIE in lazy patterns]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a lazy pattern, we must *not* discharge constraints from the RHS
from dictionaries bound in the pattern.  E.g.
691
        f ~(C x) = 3
692
We can't discharge the Num constraint from dictionaries bound by
693
the pattern C!
694

695
So we have to make the constraints from thing_inside "hop around"
696
the pattern.  Hence the captureConstraints and emitConstraints.
697 698 699

The same thing ensures that equality constraints in a lazy match
are not made available in the RHS of the match. For example
700 701 702
        data T a where { T1 :: Int -> T Int; ... }
        f :: T a -> Int -> a
        f ~(T1 i) y = y
703
It's obviously not sound to refine a to Int in the right
704
hand side, because the argument might not match T1 at all!
705 706 707 708

Finally, a lazy pattern should not bind any existential type variables
because they won't be in scope when we do the desugaring

709

Austin Seipp's avatar
Austin Seipp committed
710 711
************************************************************************
*                                                                      *
712 713
        Most of the work for constructors is here
        (the rest is in the ConPatIn case of tc_pat)
Austin Seipp's avatar
Austin Seipp committed
714 715
*                                                                      *
************************************************************************
716

717 718 719 720 721 722 723 724 725 726 727
[Pattern matching indexed data types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the following declarations:

  data family Map k :: * -> *
  data instance Map (a, b) v = MapPair (Map a (Pair b v))

and a case expression

  case x :: Map (Int, c) w of MapPair m -> ...

728
As explained by [Wrappers for data instance tycons] in MkIds.hs, the
729 730 731 732 733 734 735 736 737 738 739 740 741 742 743
worker/wrapper types for MapPair are

  $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v
  $wMapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v

So, the type of the scrutinee is Map (Int, c) w, but the tycon of MapPair is
:R123Map, which means the straight use of boxySplitTyConApp would give a type
error.  Hence, the smart wrapper function boxySplitTyConAppWithFamily calls
boxySplitTyConApp with the family tycon Map instead, which gives us the family
type list {(Int, c), w}.  To get the correct split for :R123Map, we need to
unify the family type list {(Int, c), w} with the instance types {(a, b), v}
(provided by tyConFamInst_maybe together with the family tycon).  This
unification yields the substitution [a -> Int, b -> c, v -> w], which gives us
the split arguments for the representation tycon :R123Map as {Int, c, w}

744
In other words, boxySplitTyConAppWithFamily implicitly takes the coercion
745

746
  Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v}
747 748 749 750 751 752

moving between representation and family type into account.  To produce type
correct Core, this coercion needs to be used to case the type of the scrutinee
from the family to the representation type.  This is achieved by
unwrapFamInstScrutinee using a CoPat around the result pattern.

753
Now it might appear seem as if we could have used the previous GADT type
754 755 756 757 758 759 760 761 762 763 764 765
refinement infrastructure of refineAlt and friends instead of the explicit
unification and CoPat generation.  However, that would be wrong.  Why?  The
whole point of GADT refinement is that the refinement is local to the case
alternative.  In contrast, the substitution generated by the unification of
the family type list and instance types needs to be propagated to the outside.
Imagine that in the above example, the type of the scrutinee would have been
(Map x w), then we would have unified {x, w} with {(a, b), v}, yielding the
substitution [x -> (a, b), v -> w].  In contrast to GADT matching, the
instantiation of x with (a, b) must be global; ie, it must be valid in *all*
alternatives of the case expression, whereas in the GADT case it might vary
between alternatives.

766 767 768
RIP GADT refinement: refinements have been replaced by the use of explicit
equality constraints that are used in conjunction with implication constraints
to express the local scope of GADT refinements.
Austin Seipp's avatar
Austin Seipp committed
769
-}
770

771
--      Running example:
772
-- MkT :: forall a b c. (a~[b]) => b -> c -> T a
773
--       with scrutinee of type (T ty)
774

775
tcConPat :: PatEnv -> Located Name
776
         -> ExpSigmaType           -- Type of the pattern
777 778
         -> HsConPatDetails Name -> TcM a
         -> TcM (Pat TcId, a)
cactus's avatar
cactus committed
779 780 781 782 783 784 785 786 787 788
tcConPat penv con_lname@(L _ con_name) pat_ty arg_pats thing_inside
  = do  { con_like <- tcLookupConLike con_name
        ; case con_like of
            RealDataCon data_con -> tcDataConPat penv con_lname data_con
                                                 pat_ty arg_pats thing_inside
            PatSynCon pat_syn -> tcPatSynPat penv con_lname pat_syn
                                             pat_ty arg_pats thing_inside
        }

tcDataConPat :: PatEnv -> Located Name -> DataCon
789
             -> ExpSigmaType               -- Type of the pattern
790 791
             -> HsConPatDetails Name -> TcM a
             -> TcM (Pat TcId, a)
cactus's avatar
cactus committed
792
tcDataConPat penv (L con_span con_name) data_con pat_ty arg_pats thing_inside
793 794 795
  = do  { let tycon = dataConTyCon data_con
                  -- For data families this is the representation tycon
              (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
796
                = dataConFullSig data_con
cactus's avatar
cactus committed
797
              header = L con_span (RealDataCon data_con)
798

799 800 801
          -- Instantiate the constructor type variables [a->ty]
          -- This may involve doing a family-instance coercion,
          -- and building a wrapper
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
802
        ; (wrap, ctxt_res_tys) <- matchExpectedConTy penv tycon pat_ty
803
        ; pat_ty <- readExpType pat_ty
804

805 806
          -- Add the stupid theta
        ; setSrcSpan con_span $ addDataConStupidTheta data_con ctxt_res_tys
807

808 809
        ; let all_arg_tys = eqSpecPreds eq_spec ++ theta ++ arg_tys
        ; checkExistentials ex_tvs all_arg_tys penv
810
        ; (tenv, ex_tvs') <- tcInstSuperSkolTyVarsX
niteria's avatar
niteria committed
811
                               (zipTvSubst univ_tvs ctxt_res_tys) ex_tvs
812 813
                     -- Get location from monad, not from ex_tvs

814
        ; let -- pat_ty' = mkTyConApp tycon ctxt_res_tys
815
              -- pat_ty' is type of the actual constructor application
816
              -- pat_ty' /= pat_ty iff coi /= IdCo
Simon Peyton Jones's avatar
Simon Peyton Jones committed
817

818
              arg_tys' = substTys tenv arg_tys
819

eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
820 821 822
        ; traceTc "tcConPat" (vcat [ ppr con_name
                                   , pprTvBndrs univ_tvs
                                   , pprTvBndrs ex_tvs
823
                                   , ppr eq_spec
eir@cis.upenn.edu's avatar
eir@cis.upenn.edu committed
824 825 826 827
                                   , ppr theta
                                   , pprTvBndrs ex_tvs'
                                   , ppr ctxt_res_tys
                                   , ppr arg_tys'
828
                                   , ppr arg_pats ])
829 830
        ; if null ex_tvs && null eq_spec && null theta
          then do { -- The common case; no class bindings etc
831
                    -- (see Note [Arrows and patterns])
832 833 834 835
                    (arg_pats', res) <- tcConArgs (RealDataCon data_con) arg_tys'
                                                  arg_pats penv thing_inside
                  ; let res_pat = ConPatOut { pat_con = header,
                                              pat_tvs = [], pat_dicts = [],
836
                                              pat_binds = emptyTcEvBinds,
837
                                              pat_args = arg_pats',
838
                                              pat_arg_tys = ctxt_res_tys,
cactus's avatar
cactus committed
839
                                              pat_wrap = idHsWrapper }
840

841
                  ; return (mkHsWrapPat wrap res_pat pat_ty, res) }
842

843
          else do   -- The general case, with existential,
844
                    -- and local equality constraints
845
        { let theta'     = substTheta tenv (eqSpecPreds eq_spec ++ theta)
846 847
                           -- order is *important* as we generate the list of
                           -- dictionary binders from theta'
848
              no_equalities = not (any isNomEqPred theta')
849
              skol_info = case pe_ctxt penv of
cactus's avatar
cactus committed
850
                            LamPat mc -> PatSkol (RealDataCon data_con) mc
851
                            LetPat {} -> UnkSkol -- Doesn't matter
852

853 854
        ; gadts_on    <- xoptM LangExt.GADTs
        ; families_on <- xoptM LangExt.TypeFamilies
855
        ; checkTc (no_equalities || gadts_on || families_on)
sivteck's avatar
sivteck committed
856 857
                  (text "A pattern match on a GADT requires the" <+>
                   text "GADTs or TypeFamilies language extension")
858 859 860
                  -- Trac #2905 decided that a *pattern-match* of a GADT
                  -- should require the GADT language flag.
                  -- Re TypeFamilies see also #7156
861

862
        ; given <- newEvVars theta'
863
        ; (ev_binds, (arg_pats', res))
864
             <- checkConstraints skol_info ex_tvs' given $
cactus's avatar
cactus committed
865
                tcConArgs (RealDataCon data_con) arg_tys' arg_pats penv thing_inside