1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
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
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
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
599
600
601
602
603
604
605
606
607
608
609
610
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
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
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
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
|
%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Utilities for desugaring
This module exports some utility functions of no great interest.
\begin{code}
{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
-- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details
-- | Utility functions for constructing Core syntax, principally for desugaring
module DsUtils (
EquationInfo(..),
firstPat, shiftEqns,
MatchResult(..), CanItFail(..),
cantFailMatchResult, alwaysFailMatchResult,
extractMatchResult, combineMatchResults,
adjustMatchResult, adjustMatchResultDs,
mkCoLetMatchResult, mkViewMatchResult, mkGuardedMatchResult,
matchCanFail, mkEvalMatchResult,
mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult,
wrapBind, wrapBinds,
mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs,
seqVar,
-- LHs tuples
mkLHsVarPatTup, mkLHsPatTup, mkVanillaTuplePat,
mkBigLHsVarTup, mkBigLHsTup, mkBigLHsVarPatTup, mkBigLHsPatTup,
mkSelectorBinds,
selectSimpleMatchVarL, selectMatchVars, selectMatchVar,
mkOptTickBox, mkBinaryTickBox
) where
#include "HsVersions.h"
import {-# SOURCE #-} Match ( matchSimply )
import HsSyn
import TcHsSyn
import TcType( tcSplitTyConApp )
import CoreSyn
import DsMonad
import CoreUtils
import MkCore
import MkId
import Id
import Literal
import TyCon
import DataCon
import Type
import Coercion
import TysPrim
import TysWiredIn
import BasicTypes
import UniqSet
import UniqSupply
import PrelNames
import Outputable
import SrcLoc
import Util
import DynFlags
import FastString
import Control.Monad ( zipWithM )
\end{code}
%************************************************************************
%* *
\subsection{ Selecting match variables}
%* *
%************************************************************************
We're about to match against some patterns. We want to make some
@Ids@ to use as match variables. If a pattern has an @Id@ readily at
hand, which should indeed be bound to the pattern as a whole, then use it;
otherwise, make one up.
\begin{code}
selectSimpleMatchVarL :: LPat Id -> DsM Id
selectSimpleMatchVarL pat = selectMatchVar (unLoc pat)
-- (selectMatchVars ps tys) chooses variables of type tys
-- to use for matching ps against. If the pattern is a variable,
-- we try to use that, to save inventing lots of fresh variables.
--
-- OLD, but interesting note:
-- But even if it is a variable, its type might not match. Consider
-- data T a where
-- T1 :: Int -> T Int
-- T2 :: a -> T a
--
-- f :: T a -> a -> Int
-- f (T1 i) (x::Int) = x
-- f (T2 i) (y::a) = 0
-- Then we must not choose (x::Int) as the matching variable!
-- And nowadays we won't, because the (x::Int) will be wrapped in a CoPat
selectMatchVars :: [Pat Id] -> DsM [Id]
selectMatchVars ps = mapM selectMatchVar ps
selectMatchVar :: Pat Id -> DsM Id
selectMatchVar (BangPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (LazyPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (ParPat pat) = selectMatchVar (unLoc pat)
selectMatchVar (VarPat var) = return (localiseId var) -- Note [Localise pattern binders]
selectMatchVar (AsPat var _) = return (unLoc var)
selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat)
-- OK, better make up one...
\end{code}
Note [Localise pattern binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider module M where
[Just a] = e
After renaming it looks like
module M where
[Just M.a] = e
We don't generalise, since it's a pattern binding, monomorphic, etc,
so after desugaring we may get something like
M.a = case e of (v:_) ->
case v of Just M.a -> M.a
Notice the "M.a" in the pattern; after all, it was in the original
pattern. However, after optimisation those pattern binders can become
let-binders, and then end up floated to top level. They have a
different *unique* by then (the simplifier is good about maintaining
proper scoping), but it's BAD to have two top-level bindings with the
External Name M.a, because that turns into two linker symbols for M.a.
It's quite rare for this to actually *happen* -- the only case I know
of is tc003 compiled with the 'hpc' way -- but that only makes it
all the more annoying.
To avoid this, we craftily call 'localiseId' in the desugarer, which
simply turns the External Name for the Id into an Internal one, but
doesn't change the unique. So the desugarer produces this:
M.a{r8} = case e of (v:_) ->
case v of Just a{r8} -> M.a{r8}
The unique is still 'r8', but the binding site in the pattern
is now an Internal Name. Now the simplifier's usual mechanisms
will propagate that Name to all the occurrence sites, as well as
un-shadowing it, so we'll get
M.a{r8} = case e of (v:_) ->
case v of Just a{s77} -> a{s77}
In fact, even CoreSubst.simplOptExpr will do this, and simpleOptExpr
runs on the output of the desugarer, so all is well by the end of
the desugaring pass.
%************************************************************************
%* *
%* type synonym EquationInfo and access functions for its pieces *
%* *
%************************************************************************
\subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym}
The ``equation info'' used by @match@ is relatively complicated and
worthy of a type synonym and a few handy functions.
\begin{code}
firstPat :: EquationInfo -> Pat Id
firstPat eqn = ASSERT( notNull (eqn_pats eqn) ) head (eqn_pats eqn)
shiftEqns :: [EquationInfo] -> [EquationInfo]
-- Drop the first pattern in each equation
shiftEqns eqns = [ eqn { eqn_pats = tail (eqn_pats eqn) } | eqn <- eqns ]
\end{code}
Functions on MatchResults
\begin{code}
matchCanFail :: MatchResult -> Bool
matchCanFail (MatchResult CanFail _) = True
matchCanFail (MatchResult CantFail _) = False
alwaysFailMatchResult :: MatchResult
alwaysFailMatchResult = MatchResult CanFail (\fail -> return fail)
cantFailMatchResult :: CoreExpr -> MatchResult
cantFailMatchResult expr = MatchResult CantFail (\_ -> return expr)
extractMatchResult :: MatchResult -> CoreExpr -> DsM CoreExpr
extractMatchResult (MatchResult CantFail match_fn) _
= match_fn (error "It can't fail!")
extractMatchResult (MatchResult CanFail match_fn) fail_expr = do
(fail_bind, if_it_fails) <- mkFailurePair fail_expr
body <- match_fn if_it_fails
return (mkCoreLet fail_bind body)
combineMatchResults :: MatchResult -> MatchResult -> MatchResult
combineMatchResults (MatchResult CanFail body_fn1)
(MatchResult can_it_fail2 body_fn2)
= MatchResult can_it_fail2 body_fn
where
body_fn fail = do body2 <- body_fn2 fail
(fail_bind, duplicatable_expr) <- mkFailurePair body2
body1 <- body_fn1 duplicatable_expr
return (Let fail_bind body1)
combineMatchResults match_result1@(MatchResult CantFail _) _
= match_result1
adjustMatchResult :: DsWrapper -> MatchResult -> MatchResult
adjustMatchResult encl_fn (MatchResult can_it_fail body_fn)
= MatchResult can_it_fail (\fail -> encl_fn <$> body_fn fail)
adjustMatchResultDs :: (CoreExpr -> DsM CoreExpr) -> MatchResult -> MatchResult
adjustMatchResultDs encl_fn (MatchResult can_it_fail body_fn)
= MatchResult can_it_fail (\fail -> encl_fn =<< body_fn fail)
wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr
wrapBinds [] e = e
wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e)
wrapBind :: Var -> Var -> CoreExpr -> CoreExpr
wrapBind new old body -- NB: this function must deal with term
| new==old = body -- variables, type variables or coercion variables
| otherwise = Let (NonRec new (varToCoreExpr old)) body
seqVar :: Var -> CoreExpr -> CoreExpr
seqVar var body = Case (Var var) var (exprType body)
[(DEFAULT, [], body)]
mkCoLetMatchResult :: CoreBind -> MatchResult -> MatchResult
mkCoLetMatchResult bind = adjustMatchResult (mkCoreLet bind)
-- (mkViewMatchResult var' viewExpr var mr) makes the expression
-- let var' = viewExpr var in mr
mkViewMatchResult :: Id -> CoreExpr -> Id -> MatchResult -> MatchResult
mkViewMatchResult var' viewExpr var =
adjustMatchResult (mkCoreLet (NonRec var' (mkCoreAppDs viewExpr (Var var))))
mkEvalMatchResult :: Id -> Type -> MatchResult -> MatchResult
mkEvalMatchResult var ty
= adjustMatchResult (\e -> Case (Var var) var ty [(DEFAULT, [], e)])
mkGuardedMatchResult :: CoreExpr -> MatchResult -> MatchResult
mkGuardedMatchResult pred_expr (MatchResult _ body_fn)
= MatchResult CanFail (\fail -> do body <- body_fn fail
return (mkIfThenElse pred_expr body fail))
mkCoPrimCaseMatchResult :: Id -- Scrutinee
-> Type -- Type of the case
-> [(Literal, MatchResult)] -- Alternatives
-> MatchResult -- Literals are all unlifted
mkCoPrimCaseMatchResult var ty match_alts
= MatchResult CanFail mk_case
where
mk_case fail = do
alts <- mapM (mk_alt fail) sorted_alts
return (Case (Var var) var ty ((DEFAULT, [], fail) : alts))
sorted_alts = sortWith fst match_alts -- Right order for a Case
mk_alt fail (lit, MatchResult _ body_fn)
= ASSERT( not (litIsLifted lit) )
do body <- body_fn fail
return (LitAlt lit, [], body)
mkCoAlgCaseMatchResult
:: DynFlags
-> Id -- Scrutinee
-> Type -- Type of exp
-> [(DataCon, [CoreBndr], MatchResult)] -- Alternatives (bndrs *include* tyvars, dicts)
-> MatchResult
mkCoAlgCaseMatchResult dflags var ty match_alts
| isNewTyCon tycon -- Newtype case; use a let
= ASSERT( null (tail match_alts) && null (tail arg_ids1) )
mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1
| isPArrFakeAlts match_alts -- Sugared parallel array; use a literal case
= MatchResult CanFail mk_parrCase
| otherwise -- Datatype case; use a case
= MatchResult fail_flag mk_case
where
tycon = dataConTyCon con1
-- [Interesting: becuase of GADTs, we can't rely on the type of
-- the scrutinised Id to be sufficiently refined to have a TyCon in it]
-- Stuff for newtype
(con1, arg_ids1, match_result1) = ASSERT( notNull match_alts ) head match_alts
arg_id1 = ASSERT( notNull arg_ids1 ) head arg_ids1
var_ty = idType var
(tc, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes
-- (not that splitTyConApp does, these days)
newtype_rhs = unwrapNewTypeBody tc ty_args (Var var)
-- Stuff for data types
data_cons = tyConDataCons tycon
match_results = [match_result | (_,_,match_result) <- match_alts]
fail_flag | exhaustive_case
= foldr1 orFail [can_it_fail | MatchResult can_it_fail _ <- match_results]
| otherwise
= CanFail
sorted_alts = sortWith get_tag match_alts
get_tag (con, _, _) = dataConTag con
mk_case fail = do alts <- mapM (mk_alt fail) sorted_alts
return (mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts))
mk_alt fail (con, args, MatchResult _ body_fn) = do
body <- body_fn fail
us <- newUniqueSupply
return (mkReboxingAlt (uniqsFromSupply us) con args body)
mk_default fail | exhaustive_case = []
| otherwise = [(DEFAULT, [], fail)]
un_mentioned_constructors
= mkUniqSet data_cons `minusUniqSet` mkUniqSet [ con | (con, _, _) <- match_alts]
exhaustive_case = isEmptyUniqSet un_mentioned_constructors
-- Stuff for parallel arrays
--
-- * the following is to desugar cases over fake constructors for
-- parallel arrays, which are introduced by `tidy1' in the `PArrPat'
-- case
--
-- Concerning `isPArrFakeAlts':
--
-- * it is *not* sufficient to just check the type of the type
-- constructor, as we have to be careful not to confuse the real
-- representation of parallel arrays with the fake constructors;
-- moreover, a list of alternatives must not mix fake and real
-- constructors (this is checked earlier on)
--
-- FIXME: We actually go through the whole list and make sure that
-- either all or none of the constructors are fake parallel
-- array constructors. This is to spot equations that mix fake
-- constructors with the real representation defined in
-- `PrelPArr'. It would be nicer to spot this situation
-- earlier and raise a proper error message, but it can really
-- only happen in `PrelPArr' anyway.
--
isPArrFakeAlts [(dcon, _, _)] = isPArrFakeCon dcon
isPArrFakeAlts ((dcon, _, _):alts) =
case (isPArrFakeCon dcon, isPArrFakeAlts alts) of
(True , True ) -> True
(False, False) -> False
_ -> panic "DsUtils: you may not mix `[:...:]' with `PArr' patterns"
isPArrFakeAlts [] = panic "DsUtils: unexpectedly found an empty list of PArr fake alternatives"
--
mk_parrCase fail = do
lengthP <- dsDPHBuiltin lengthPVar
alt <- unboxAlt
return (mkWildCase (len lengthP) intTy ty [alt])
where
elemTy = case splitTyConApp (idType var) of
(_, [elemTy]) -> elemTy
_ -> panic panicMsg
panicMsg = "DsUtils.mkCoAlgCaseMatchResult: not a parallel array?"
len lengthP = mkApps (Var lengthP) [Type elemTy, Var var]
--
unboxAlt = do
l <- newSysLocalDs intPrimTy
indexP <- dsDPHBuiltin indexPVar
alts <- mapM (mkAlt indexP) sorted_alts
return (DataAlt intDataCon, [l], mkWildCase (Var l) intPrimTy ty (dft : alts))
where
dft = (DEFAULT, [], fail)
--
-- each alternative matches one array length (corresponding to one
-- fake array constructor), so the match is on a literal; each
-- alternative's body is extended by a local binding for each
-- constructor argument, which are bound to array elements starting
-- with the first
--
mkAlt indexP (con, args, MatchResult _ bodyFun) = do
body <- bodyFun fail
return (LitAlt lit, [], mkCoreLets binds body)
where
lit = MachInt $ toInteger (dataConSourceArity con)
binds = [NonRec arg (indexExpr i) | (i, arg) <- zip [1..] args]
--
indexExpr i = mkApps (Var indexP) [Type elemTy, Var var, mkIntExpr dflags i]
\end{code}
%************************************************************************
%* *
\subsection{Desugarer's versions of some Core functions}
%* *
%************************************************************************
\begin{code}
mkErrorAppDs :: Id -- The error function
-> Type -- Type to which it should be applied
-> SDoc -- The error message string to pass
-> DsM CoreExpr
mkErrorAppDs err_id ty msg = do
src_loc <- getSrcSpanDs
dflags <- getDynFlags
let
full_msg = showSDoc dflags (hcat [ppr src_loc, text "|", msg])
core_msg = Lit (mkMachString full_msg)
-- mkMachString returns a result of type String#
return (mkApps (Var err_id) [Type ty, core_msg])
\end{code}
'mkCoreAppDs' and 'mkCoreAppsDs' hand the special-case desugaring of 'seq'.
Note [Desugaring seq (1)] cf Trac #1031
~~~~~~~~~~~~~~~~~~~~~~~~~
f x y = x `seq` (y `seq` (# x,y #))
The [CoreSyn let/app invariant] means that, other things being equal, because
the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:
f x y = case (y `seq` (# x,y #)) of v -> x `seq` v
But that is bad for two reasons:
(a) we now evaluate y before x, and
(b) we can't bind v to an unboxed pair
Seq is very, very special! So we recognise it right here, and desugar to
case x of _ -> case y of _ -> (# x,y #)
Note [Desugaring seq (2)] cf Trac #2273
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
let chp = case b of { True -> fst x; False -> 0 }
in chp `seq` ...chp...
Here the seq is designed to plug the space leak of retaining (snd x)
for too long.
If we rely on the ordinary inlining of seq, we'll get
let chp = case b of { True -> fst x; False -> 0 }
case chp of _ { I# -> ...chp... }
But since chp is cheap, and the case is an alluring contet, we'll
inline chp into the case scrutinee. Now there is only one use of chp,
so we'll inline a second copy. Alas, we've now ruined the purpose of
the seq, by re-introducing the space leak:
case (case b of {True -> fst x; False -> 0}) of
I# _ -> ...case b of {True -> fst x; False -> 0}...
We can try to avoid doing this by ensuring that the binder-swap in the
case happens, so we get his at an early stage:
case chp of chp2 { I# -> ...chp2... }
But this is fragile. The real culprit is the source program. Perhaps we
should have said explicitly
let !chp2 = chp in ...chp2...
But that's painful. So the code here does a little hack to make seq
more robust: a saturated application of 'seq' is turned *directly* into
the case expression, thus:
x `seq` e2 ==> case x of x -> e2 -- Note shadowing!
e1 `seq` e2 ==> case x of _ -> e2
So we desugar our example to:
let chp = case b of { True -> fst x; False -> 0 }
case chp of chp { I# -> ...chp... }
And now all is well.
The reason it's a hack is because if you define mySeq=seq, the hack
won't work on mySeq.
Note [Desugaring seq (3)] cf Trac #2409
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The isLocalId ensures that we don't turn
True `seq` e
into
case True of True { ... }
which stupidly tries to bind the datacon 'True'.
\begin{code}
mkCoreAppDs :: CoreExpr -> CoreExpr -> CoreExpr
mkCoreAppDs (Var f `App` Type ty1 `App` Type ty2 `App` arg1) arg2
| f `hasKey` seqIdKey -- Note [Desugaring seq (1), (2)]
= Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
where
case_bndr = case arg1 of
Var v1 | isLocalId v1 -> v1 -- Note [Desugaring seq (2) and (3)]
_ -> mkWildValBinder ty1
mkCoreAppDs fun arg = mkCoreApp fun arg -- The rest is done in MkCore
mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
mkCoreAppsDs fun args = foldl mkCoreAppDs fun args
\end{code}
%************************************************************************
%* *
\subsection[mkSelectorBind]{Make a selector bind}
%* *
%************************************************************************
This is used in various places to do with lazy patterns.
For each binder $b$ in the pattern, we create a binding:
\begin{verbatim}
b = case v of pat' -> b'
\end{verbatim}
where @pat'@ is @pat@ with each binder @b@ cloned into @b'@.
ToDo: making these bindings should really depend on whether there's
much work to be done per binding. If the pattern is complex, it
should be de-mangled once, into a tuple (and then selected from).
Otherwise the demangling can be in-line in the bindings (as here).
Boring! Boring! One error message per binder. The above ToDo is
even more helpful. Something very similar happens for pattern-bound
expressions.
Note [mkSelectorBinds]
~~~~~~~~~~~~~~~~~~~~~~
Given p = e, where p binds x,y
we are going to make EITHER
EITHER (A) v = e (where v is fresh)
x = case v of p -> x
y = case v of p -> y
OR (B) t = case e of p -> (x,y)
x = case t of (x,_) -> x
y = case t of (_,y) -> y
We do (A) when
* Matching the pattern is cheap so we don't mind
doing it twice.
* Or if the pattern binds only one variable (so we'll only
match once)
* AND the pattern can't fail (else we tiresomely get two inexhaustive
pattern warning messages)
Otherwise we do (B). Really (A) is just an optimisation for very common
cases like
Just x = e
(p,q) = e
\begin{code}
mkSelectorBinds :: [Maybe (Tickish Id)] -- ticks to add, possibly
-> LPat Id -- The pattern
-> CoreExpr -- Expression to which the pattern is bound
-> DsM [(Id,CoreExpr)]
mkSelectorBinds ticks (L _ (VarPat v)) val_expr
= return [(v, case ticks of
[t] -> mkOptTickBox t val_expr
_ -> val_expr)]
mkSelectorBinds ticks pat val_expr
| null binders
= return []
| isSingleton binders || is_simple_lpat pat
-- See Note [mkSelectorBinds]
= do { val_var <- newSysLocalDs (hsLPatType pat)
-- Make up 'v' in Note [mkSelectorBinds]
-- NB: give it the type of *pattern* p, not the type of the *rhs* e.
-- This does not matter after desugaring, but there's a subtle
-- issue with implicit parameters. Consider
-- (x,y) = ?i
-- Then, ?i is given type {?i :: Int}, a PredType, which is opaque
-- to the desugarer. (Why opaque? Because newtypes have to be. Why
-- does it get that type? So that when we abstract over it we get the
-- right top-level type (?i::Int) => ...)
--
-- So to get the type of 'v', use the pattern not the rhs. Often more
-- efficient too.
-- For the error message we make one error-app, to avoid duplication.
-- But we need it at different types... so we use coerce for that
; err_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID unitTy (ppr pat)
; err_var <- newSysLocalDs unitTy
; binds <- zipWithM (mk_bind val_var err_var) ticks' binders
; return ( (val_var, val_expr) :
(err_var, err_expr) :
binds ) }
| otherwise
= do { error_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID tuple_ty (ppr pat)
; tuple_expr <- matchSimply val_expr PatBindRhs pat local_tuple error_expr
; tuple_var <- newSysLocalDs tuple_ty
; let mk_tup_bind tick binder
= (binder, mkOptTickBox tick $
mkTupleSelector local_binders binder
tuple_var (Var tuple_var))
; return ( (tuple_var, tuple_expr) : zipWith mk_tup_bind ticks' binders ) }
where
binders = collectPatBinders pat
ticks' = ticks ++ repeat Nothing
local_binders = map localiseId binders -- See Note [Localise pattern binders]
local_tuple = mkBigCoreVarTup binders
tuple_ty = exprType local_tuple
mk_bind scrut_var err_var tick bndr_var = do
-- (mk_bind sv err_var) generates
-- bv = case sv of { pat -> bv; other -> coerce (type-of-bv) err_var }
-- Remember, pat binds bv
rhs_expr <- matchSimply (Var scrut_var) PatBindRhs pat
(Var bndr_var) error_expr
return (bndr_var, mkOptTickBox tick rhs_expr)
where
error_expr = mkCast (Var err_var) co
co = mkUnsafeCo (exprType (Var err_var)) (idType bndr_var)
is_simple_lpat p = is_simple_pat (unLoc p)
is_simple_pat (TuplePat ps Boxed _) = all is_triv_lpat ps
is_simple_pat pat@(ConPatOut{}) = isProductTyCon (dataConTyCon (unLoc (pat_con pat)))
&& all is_triv_lpat (hsConPatArgs (pat_args pat))
is_simple_pat (VarPat _) = True
is_simple_pat (ParPat p) = is_simple_lpat p
is_simple_pat _ = False
is_triv_lpat p = is_triv_pat (unLoc p)
is_triv_pat (VarPat _) = True
is_triv_pat (WildPat _) = True
is_triv_pat (ParPat p) = is_triv_lpat p
is_triv_pat _ = False
\end{code}
Creating big tuples and their types for full Haskell expressions.
They work over *Ids*, and create tuples replete with their types,
which is whey they are not in HsUtils.
\begin{code}
mkLHsPatTup :: [LPat Id] -> LPat Id
mkLHsPatTup [] = noLoc $ mkVanillaTuplePat [] Boxed
mkLHsPatTup [lpat] = lpat
mkLHsPatTup lpats = L (getLoc (head lpats)) $
mkVanillaTuplePat lpats Boxed
mkLHsVarPatTup :: [Id] -> LPat Id
mkLHsVarPatTup bs = mkLHsPatTup (map nlVarPat bs)
mkVanillaTuplePat :: [OutPat Id] -> Boxity -> Pat Id
-- A vanilla tuple pattern simply gets its type from its sub-patterns
mkVanillaTuplePat pats box
= TuplePat pats box (mkTupleTy (boxityNormalTupleSort box) (map hsLPatType pats))
-- The Big equivalents for the source tuple expressions
mkBigLHsVarTup :: [Id] -> LHsExpr Id
mkBigLHsVarTup ids = mkBigLHsTup (map nlHsVar ids)
mkBigLHsTup :: [LHsExpr Id] -> LHsExpr Id
mkBigLHsTup = mkChunkified mkLHsTupleExpr
-- The Big equivalents for the source tuple patterns
mkBigLHsVarPatTup :: [Id] -> LPat Id
mkBigLHsVarPatTup bs = mkBigLHsPatTup (map nlVarPat bs)
mkBigLHsPatTup :: [LPat Id] -> LPat Id
mkBigLHsPatTup = mkChunkified mkLHsPatTup
\end{code}
%************************************************************************
%* *
\subsection[mkFailurePair]{Code for pattern-matching and other failures}
%* *
%************************************************************************
Generally, we handle pattern matching failure like this: let-bind a
fail-variable, and use that variable if the thing fails:
\begin{verbatim}
let fail.33 = error "Help"
in
case x of
p1 -> ...
p2 -> fail.33
p3 -> fail.33
p4 -> ...
\end{verbatim}
Then
\begin{itemize}
\item
If the case can't fail, then there'll be no mention of @fail.33@, and the
simplifier will later discard it.
\item
If it can fail in only one way, then the simplifier will inline it.
\item
Only if it is used more than once will the let-binding remain.
\end{itemize}
There's a problem when the result of the case expression is of
unboxed type. Then the type of @fail.33@ is unboxed too, and
there is every chance that someone will change the let into a case:
\begin{verbatim}
case error "Help" of
fail.33 -> case ....
\end{verbatim}
which is of course utterly wrong. Rather than drop the condition that
only boxed types can be let-bound, we just turn the fail into a function
for the primitive case:
\begin{verbatim}
let fail.33 :: Void -> Int#
fail.33 = \_ -> error "Help"
in
case x of
p1 -> ...
p2 -> fail.33 void
p3 -> fail.33 void
p4 -> ...
\end{verbatim}
Now @fail.33@ is a function, so it can be let-bound.
\begin{code}
mkFailurePair :: CoreExpr -- Result type of the whole case expression
-> DsM (CoreBind, -- Binds the newly-created fail variable
-- to \ _ -> expression
CoreExpr) -- Fail variable applied to realWorld#
-- See Note [Failure thunks and CPR]
mkFailurePair expr
= do { fail_fun_var <- newFailLocalDs (realWorldStatePrimTy `mkFunTy` ty)
; fail_fun_arg <- newSysLocalDs realWorldStatePrimTy
; return (NonRec fail_fun_var (Lam fail_fun_arg expr),
App (Var fail_fun_var) (Var realWorldPrimId)) }
where
ty = exprType expr
\end{code}
Note [Failure thunks and CPR]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we make a failure point we ensure that it
does not look like a thunk. Example:
let fail = \rw -> error "urk"
in case x of
[] -> fail realWorld#
(y:ys) -> case ys of
[] -> fail realWorld#
(z:zs) -> (y,z)
Reason: we know that a failure point is always a "join point" and is
entered at most once. Adding a dummy 'realWorld' token argument makes
it clear that sharing is not an issue. And that in turn makes it more
CPR-friendly. This matters a lot: if you don't get it right, you lose
the tail call property. For example, see Trac #3403.
\begin{code}
mkOptTickBox :: Maybe (Tickish Id) -> CoreExpr -> CoreExpr
mkOptTickBox Nothing e = e
mkOptTickBox (Just tickish) e = Tick tickish e
mkBinaryTickBox :: Int -> Int -> CoreExpr -> DsM CoreExpr
mkBinaryTickBox ixT ixF e = do
uq <- newUnique
this_mod <- getModuleDs
let bndr1 = mkSysLocal (fsLit "t1") uq boolTy
let
falseBox = Tick (HpcTick this_mod ixF) (Var falseDataConId)
trueBox = Tick (HpcTick this_mod ixT) (Var trueDataConId)
--
return $ Case e bndr1 boolTy
[ (DataAlt falseDataCon, [], falseBox)
, (DataAlt trueDataCon, [], trueBox)
]
\end{code}
|