1 {-
2 (c) The University of Glasgow 2006
3 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 The @match@ function
7 -}
9 {-# LANGUAGE CPP #-}
11 module Match ( match, matchEquations, matchWrapper, matchSimply, matchSinglePat ) where
13 #include "HsVersions.h"
15 import {-#SOURCE#-} DsExpr (dsLExpr, dsExpr)
17 import DynFlags
18 import HsSyn
19 import TcHsSyn
20 import TcEvidence
22 import Check
23 import CoreSyn
24 import Literal
25 import CoreUtils
26 import MkCore
28 import DsBinds
29 import DsGRHSs
30 import DsUtils
31 import Id
32 import ConLike
33 import DataCon
34 import PatSyn
35 import MatchCon
36 import MatchLit
37 import Type
38 import TcType ( toTcTypeBag )
39 import TyCon( isNewTyCon )
40 import TysWiredIn
41 import ListSetOps
42 import SrcLoc
43 import Maybes
44 import Util
45 import Name
46 import Outputable
47 import BasicTypes ( isGenerated )
50 import qualified Data.Map as Map
52 {-
53 ************************************************************************
54 * *
55 The main matching function
56 * *
57 ************************************************************************
59 The function @match@ is basically the same as in the Wadler chapter,
60 except it is monadised, to carry around the name supply, info about
61 annotations, etc.
63 Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns:
64 \begin{enumerate}
65 \item
66 A list of $n$ variable names, those variables presumably bound to the
67 $n$ expressions being matched against the $n$ patterns. Using the
68 list of $n$ expressions as the first argument showed no benefit and
69 some inelegance.
71 \item
72 The second argument, a list giving the equation info'' for each of
73 the $m$ equations:
74 \begin{itemize}
75 \item
76 the $n$ patterns for that equation, and
77 \item
78 a list of Core bindings [@(Id, CoreExpr)@ pairs] to be stuck on
79 the front'' of the matching code, as in:
80 \begin{verbatim}
81 let <binds>
82 in <matching-code>
83 \end{verbatim}
84 \item
85 and finally: (ToDo: fill in)
87 The right way to think about the after-match function'' is that it
88 is an embryonic @CoreExpr@ with a hole'' at the end for the
89 final else expression''.
90 \end{itemize}
92 There is a type synonym, @EquationInfo@, defined in module @DsUtils@.
94 An experiment with re-ordering this information about equations (in
95 particular, having the patterns available in column-major order)
96 showed no benefit.
98 \item
99 A default expression---what to evaluate if the overall pattern-match
100 fails. This expression will (almost?) always be
101 a measly expression @Var@, unless we know it will only be used once
102 (as we do in @glue_success_exprs@).
104 Leaving out this third argument to @match@ (and slamming in lots of
105 @Var "fail"@s) is a positively {\em bad} idea, because it makes it
106 impossible to share the default expressions. (Also, it stands no
107 chance of working in our post-upheaval world of @Locals@.)
108 \end{enumerate}
110 Note: @match@ is often called via @matchWrapper@ (end of this module),
111 a function that does much of the house-keeping that goes with a call
112 to @match@.
114 It is also worth mentioning the {\em typical} way a block of equations
115 is desugared with @match@. At each stage, it is the first column of
116 patterns that is examined. The steps carried out are roughly:
117 \begin{enumerate}
118 \item
119 Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add
120 bindings to the second component of the equation-info):
121 \begin{itemize}
122 \item
123 Remove the as' patterns from column~1.
124 \item
125 Make all constructor patterns in column~1 into @ConPats@, notably
126 @ListPats@ and @TuplePats@.
127 \item
128 Handle any irrefutable (or twiddle'') @LazyPats@.
129 \end{itemize}
130 \item
131 Now {\em unmix} the equations into {\em blocks} [w\/ local function
132 @unmix_eqns@], in which the equations in a block all have variable
133 patterns in column~1, or they all have constructor patterns in ...
134 (see the mixture rule'' in SLPJ).
135 \item
136 Call @matchEqnBlock@ on each block of equations; it will do the
137 appropriate thing for each kind of column-1 pattern, usually ending up
138 in a recursive call to @match@.
139 \end{enumerate}
141 We are a little more paranoid about the empty rule'' (SLPJ, p.~87)
142 than the Wadler-chapter code for @match@ (p.~93, first @match@ clause).
143 And gluing the success expressions'' together isn't quite so pretty.
145 This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@
146 (a)~to get as'- and twiddle'-patterns out of the way (tidying), and
147 (b)~to do the mixture rule'' (SLPJ, p.~88) [which really {\em
148 un}mixes the equations], producing a list of equation-info
149 blocks, each block having as its first column of patterns either all
150 constructors, or all variables (or similar beasts), etc.
152 @match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the
153 Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@
154 corresponds roughly to @matchVarCon@.
155 -}
157 match :: [Id] -- Variables rep\'ing the exprs we\'re matching with
158 -> Type -- Type of the case expression
159 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
160 -> DsM MatchResult -- Desugared result!
162 match [] ty eqns
163 = ASSERT2( not (null eqns), ppr ty )
164 return (foldr1 combineMatchResults match_results)
165 where
166 match_results = [ ASSERT( null (eqn_pats eqn) )
167 eqn_rhs eqn
168 | eqn <- eqns ]
170 match vars@(v:_) ty eqns -- Eqns *can* be empty
171 = do { dflags <- getDynFlags
172 -- Tidy the first pattern, generating
173 -- auxiliary bindings if necessary
174 ; (aux_binds, tidy_eqns) <- mapAndUnzipM (tidyEqnInfo v) eqns
176 -- Group the equations and match each group in turn
177 ; let grouped = groupEquations dflags tidy_eqns
179 -- print the view patterns that are commoned up to help debug
180 ; whenDOptM Opt_D_dump_view_pattern_commoning (debug grouped)
182 ; match_results <- match_groups grouped
183 ; return (adjustMatchResult (foldr (.) id aux_binds) $184 foldr1 combineMatchResults match_results) } 185 where 186 dropGroup :: [(PatGroup,EquationInfo)] -> [EquationInfo] 187 dropGroup = map snd 189 match_groups :: [[(PatGroup,EquationInfo)]] -> DsM [MatchResult] 190 -- Result list of [MatchResult] is always non-empty 191 match_groups [] = matchEmpty v ty 192 match_groups gs = mapM match_group gs 194 match_group :: [(PatGroup,EquationInfo)] -> DsM MatchResult 195 match_group [] = panic "match_group" 196 match_group eqns@((group,_) : _) 197 = case group of 198 PgCon _ -> matchConFamily vars ty (subGroup [(c,e) | (PgCon c, e) <- eqns]) 199 PgSyn _ -> matchPatSyn vars ty (dropGroup eqns) 200 PgLit _ -> matchLiterals vars ty (subGroup [(l,e) | (PgLit l, e) <- eqns]) 201 PgAny -> matchVariables vars ty (dropGroup eqns) 202 PgN _ -> matchNPats vars ty (dropGroup eqns) 203 PgNpK _ -> matchNPlusKPats vars ty (dropGroup eqns) 204 PgBang -> matchBangs vars ty (dropGroup eqns) 205 PgCo _ -> matchCoercion vars ty (dropGroup eqns) 206 PgView _ _ -> matchView vars ty (dropGroup eqns) 207 PgOverloadedList -> matchOverloadedList vars ty (dropGroup eqns) 209 -- FIXME: we should also warn about view patterns that should be 210 -- commoned up but are not 212 -- print some stuff to see what's getting grouped 213 -- use -dppr-debug to see the resolution of overloaded literals 214 debug eqns = 215 let gs = map (\group -> foldr (\ (p,_) -> \acc -> 216 case p of PgView e _ -> e:acc 217 _ -> acc) [] group) eqns 218 maybeWarn [] = return () 219 maybeWarn l = warnDs (vcat l) 220 in 221 maybeWarn$ (map (\g -> text "Putting these view expressions into the same case:" <+> (ppr g))
222 (filter (not . null) gs))
224 matchEmpty :: Id -> Type -> DsM [MatchResult]
225 -- See Note [Empty case expressions]
226 matchEmpty var res_ty
227 = return [MatchResult CanFail mk_seq]
228 where
229 mk_seq fail = return $mkWildCase (Var var) (idType var) res_ty 230 [(DEFAULT, [], fail)] 232 matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult 233 -- Real true variables, just like in matchVar, SLPJ p 94 234 -- No binding to do: they'll all be wildcards by now (done in tidy) 235 matchVariables (_:vars) ty eqns = match vars ty (shiftEqns eqns) 236 matchVariables [] _ _ = panic "matchVariables" 238 matchBangs :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult 239 matchBangs (var:vars) ty eqns 240 = do { match_result <- match (var:vars) ty$
241 map (decomposeFirstPat getBangPat) eqns
242 ; return (mkEvalMatchResult var ty match_result) }
243 matchBangs [] _ _ = panic "matchBangs"
245 matchCoercion :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
246 -- Apply the coercion to the match variable and then match that
247 matchCoercion (var:vars) ty (eqns@(eqn1:_))
248 = do { let CoPat co pat _ = firstPat eqn1
249 ; var' <- newUniqueId var (hsPatType pat)
250 ; match_result <- match (var':vars) ty $251 map (decomposeFirstPat getCoPat) eqns 252 ; rhs' <- dsHsWrapper co (Var var) 253 ; return (mkCoLetMatchResult (NonRec var' rhs') match_result) } 254 matchCoercion _ _ _ = panic "matchCoercion" 256 matchView :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult 257 -- Apply the view function to the match variable and then match that 258 matchView (var:vars) ty (eqns@(eqn1:_)) 259 = do { -- we could pass in the expr from the PgView, 260 -- but this needs to extract the pat anyway 261 -- to figure out the type of the fresh variable 262 let ViewPat viewExpr (L _ pat) _ = firstPat eqn1 263 -- do the rest of the compilation 264 ; var' <- newUniqueId var (hsPatType pat) 265 ; match_result <- match (var':vars) ty$
266 map (decomposeFirstPat getViewPat) eqns
267 -- compile the view expressions
268 ; viewExpr' <- dsLExpr viewExpr
269 ; return (mkViewMatchResult var' viewExpr' var match_result) }
270 matchView _ _ _ = panic "matchView"
272 matchOverloadedList :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
274 -- Since overloaded list patterns are treated as view patterns,
275 -- the code is roughly the same as for matchView
276 = do { let ListPat _ elt_ty (Just (_,e)) = firstPat eqn1
277 ; var' <- newUniqueId var (mkListTy elt_ty) -- we construct the overall type by hand
278 ; match_result <- match (var':vars) ty $279 map (decomposeFirstPat getOLPat) eqns -- getOLPat builds the pattern inside as a non-overloaded version of the overloaded list pattern 280 ; e' <- dsExpr e 281 ; return (mkViewMatchResult var' e' var match_result) } 282 matchOverloadedList _ _ _ = panic "matchOverloadedList" 284 -- decompose the first pattern and leave the rest alone 285 decomposeFirstPat :: (Pat Id -> Pat Id) -> EquationInfo -> EquationInfo 286 decomposeFirstPat extractpat (eqn@(EqnInfo { eqn_pats = pat : pats })) 287 = eqn { eqn_pats = extractpat pat : pats} 288 decomposeFirstPat _ _ = panic "decomposeFirstPat" 290 getCoPat, getBangPat, getViewPat, getOLPat :: Pat Id -> Pat Id 291 getCoPat (CoPat _ pat _) = pat 292 getCoPat _ = panic "getCoPat" 293 getBangPat (BangPat pat ) = unLoc pat 294 getBangPat _ = panic "getBangPat" 295 getViewPat (ViewPat _ pat _) = unLoc pat 296 getViewPat _ = panic "getViewPat" 297 getOLPat (ListPat pats ty (Just _)) = ListPat pats ty Nothing 298 getOLPat _ = panic "getOLPat" 300 {- 301 Note [Empty case alternatives] 302 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 303 The list of EquationInfo can be empty, arising from 304 case x of {} or \case {} 305 In that situation we desugar to 306 case x of { _ -> error "pattern match failure" } 307 The *desugarer* isn't certain whether there really should be no 308 alternatives, so it adds a default case, as it always does. A later 309 pass may remove it if it's inaccessible. (See also Note [Empty case 310 alternatives] in CoreSyn.) 312 We do *not* desugar simply to 313 error "empty case" 314 or some such, because 'x' might be bound to (error "hello"), in which 315 case we want to see that "hello" exception, not (error "empty case"). 316 See also Note [Case elimination: lifted case] in Simplify. 319 ************************************************************************ 320 * * 321 Tidying patterns 322 * * 323 ************************************************************************ 325 Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@ 326 which will be scrutinised. This means: 327 \begin{itemize} 328 \item 329 Replace variable patterns @x@ (@x /= v@) with the pattern @_@, 330 together with the binding @x = v@. 331 \item 332 Replace the as' pattern @x@@p@ with the pattern p and a binding @x = v@. 333 \item 334 Removing lazy (irrefutable) patterns (you don't want to know...). 335 \item 336 Converting explicit tuple-, list-, and parallel-array-pats into ordinary 337 @ConPats@. 338 \item 339 Convert the literal pat "" to []. 340 \end{itemize} 342 The result of this tidying is that the column of patterns will include 343 {\em only}: 344 \begin{description} 345 \item[@WildPats@:] 346 The @VarPat@ information isn't needed any more after this. 348 \item[@ConPats@:] 349 @ListPats@, @TuplePats@, etc., are all converted into @ConPats@. 351 \item[@LitPats@ and @NPats@:] 352 @LitPats@/@NPats@ of known friendly types'' (Int, Char, 353 Float, Double, at least) are converted to unboxed form; e.g., 354 \tr{(NPat (HsInt i) _ _)} is converted to: 355 \begin{verbatim} 356 (ConPat I# _ _ [LitPat (HsIntPrim i)]) 357 \end{verbatim} 358 \end{description} 359 -} 361 tidyEqnInfo :: Id -> EquationInfo 362 -> DsM (DsWrapper, EquationInfo) 363 -- DsM'd because of internal call to dsLHsBinds 364 -- and mkSelectorBinds. 365 -- "tidy1" does the interesting stuff, looking at 366 -- one pattern and fiddling the list of bindings. 367 -- 368 -- POST CONDITION: head pattern in the EqnInfo is 369 -- WildPat 370 -- ConPat 371 -- NPat 372 -- LitPat 373 -- NPlusKPat 374 -- but no other 376 tidyEqnInfo _ (EqnInfo { eqn_pats = [] }) 377 = panic "tidyEqnInfo" 379 tidyEqnInfo v eqn@(EqnInfo { eqn_pats = pat : pats }) 380 = do { (wrap, pat') <- tidy1 v pat 381 ; return (wrap, eqn { eqn_pats = do pat' : pats }) } 383 tidy1 :: Id -- The Id being scrutinised 384 -> Pat Id -- The pattern against which it is to be matched 385 -> DsM (DsWrapper, -- Extra bindings to do before the match 386 Pat Id) -- Equivalent pattern 388 ------------------------------------------------------- 389 -- (pat', mr') = tidy1 v pat mr 390 -- tidies the *outer level only* of pat, giving pat' 391 -- It eliminates many pattern forms (as-patterns, variable patterns, 392 -- list patterns, etc) yielding one of: 393 -- WildPat 394 -- ConPatOut 395 -- LitPat 396 -- NPat 397 -- NPlusKPat 399 tidy1 v (ParPat pat) = tidy1 v (unLoc pat) 400 tidy1 v (SigPatOut pat _) = tidy1 v (unLoc pat) 401 tidy1 _ (WildPat ty) = return (idDsWrapper, WildPat ty) 402 tidy1 v (BangPat (L l p)) = tidy_bang_pat v l p 404 -- case v of { x -> mr[] } 405 -- = case v of { _ -> let x=v in mr[] } 406 tidy1 v (VarPat (L _ var)) 407 = return (wrapBind var v, WildPat (idType var)) 409 -- case v of { x@p -> mr[] } 410 -- = case v of { p -> let x=v in mr[] } 411 tidy1 v (AsPat (L _ var) pat) 412 = do { (wrap, pat') <- tidy1 v (unLoc pat) 413 ; return (wrapBind var v . wrap, pat') } 415 {- now, here we handle lazy patterns: 416 tidy1 v ~p bs = (v, v1 = case v of p -> v1 : 417 v2 = case v of p -> v2 : ... : bs ) 419 where the v_i's are the binders in the pattern. 421 ToDo: in "v_i = ... -> v_i", are the v_i's really the same thing? 423 The case expr for v_i is just: match [v] [(p, [], \ x -> Var v_i)] any_expr 424 -} 426 tidy1 v (LazyPat pat) 427 = do { (_,sel_prs) <- mkSelectorBinds False [] pat (Var v) 428 ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs] 429 ; return (mkCoreLets sel_binds, WildPat (idType v)) } 431 tidy1 _ (ListPat pats ty Nothing) 432 = return (idDsWrapper, unLoc list_ConPat) 433 where 434 list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] [ty]) 435 (mkNilPat ty) 436 pats 438 -- Introduce fake parallel array constructors to be able to handle parallel 439 -- arrays with the existing machinery for constructor pattern 440 tidy1 _ (PArrPat pats ty) 441 = return (idDsWrapper, unLoc parrConPat) 442 where 443 arity = length pats 444 parrConPat = mkPrefixConPat (parrFakeCon arity) pats [ty] 446 tidy1 _ (TuplePat pats boxity tys) 447 = return (idDsWrapper, unLoc tuple_ConPat) 448 where 449 arity = length pats 450 tuple_ConPat = mkPrefixConPat (tupleDataCon boxity arity) pats tys 452 -- LitPats: we *might* be able to replace these w/ a simpler form 453 tidy1 _ (LitPat lit) 454 = return (idDsWrapper, tidyLitPat lit) 456 -- NPats: we *might* be able to replace these w/ a simpler form 457 tidy1 _ (NPat (L _ lit) mb_neg eq) 458 = return (idDsWrapper, tidyNPat tidyLitPat lit mb_neg eq) 460 -- Everything else goes through unchanged... 462 tidy1 _ non_interesting_pat 463 = return (idDsWrapper, non_interesting_pat) 465 -------------------- 466 tidy_bang_pat :: Id -> SrcSpan -> Pat Id -> DsM (DsWrapper, Pat Id) 468 -- Discard par/sig under a bang 469 tidy_bang_pat v _ (ParPat (L l p)) = tidy_bang_pat v l p 470 tidy_bang_pat v _ (SigPatOut (L l p) _) = tidy_bang_pat v l p 472 -- Push the bang-pattern inwards, in the hope that 473 -- it may disappear next time 474 tidy_bang_pat v l (AsPat v' p) = tidy1 v (AsPat v' (L l (BangPat p))) 475 tidy_bang_pat v l (CoPat w p t) = tidy1 v (CoPat w (BangPat (L l p)) t) 477 -- Discard bang around strict pattern 478 tidy_bang_pat v _ p@(LitPat {}) = tidy1 v p 479 tidy_bang_pat v _ p@(ListPat {}) = tidy1 v p 480 tidy_bang_pat v _ p@(TuplePat {}) = tidy1 v p 481 tidy_bang_pat v _ p@(PArrPat {}) = tidy1 v p 483 -- Data/newtype constructors 484 tidy_bang_pat v l p@(ConPatOut { pat_con = L _ (RealDataCon dc), pat_args = args }) 485 | isNewTyCon (dataConTyCon dc) -- Newtypes: push bang inwards (Trac #9844) 486 = tidy1 v (p { pat_args = push_bang_into_newtype_arg l args }) 487 | otherwise -- Data types: discard the bang 488 = tidy1 v p 490 ------------------- 491 -- Default case, leave the bang there: 492 -- VarPat, 493 -- LazyPat, 494 -- WildPat, 495 -- ViewPat, 496 -- pattern synonyms (ConPatOut with PatSynCon) 497 -- NPat, 498 -- NPlusKPat 499 -- 500 -- For LazyPat, remember that it's semantically like a VarPat 501 -- i.e. !(~p) is not like ~p, or p! (Trac #8952) 502 -- 503 -- NB: SigPatIn, ConPatIn should not happen 505 tidy_bang_pat _ l p = return (idDsWrapper, BangPat (L l p)) 507 ------------------- 508 push_bang_into_newtype_arg :: SrcSpan -> HsConPatDetails Id -> HsConPatDetails Id 509 -- See Note [Bang patterns and newtypes] 510 -- We are transforming !(N p) into (N !p) 511 push_bang_into_newtype_arg l (PrefixCon (arg:args)) 512 = ASSERT( null args) 513 PrefixCon [L l (BangPat arg)] 514 push_bang_into_newtype_arg l (RecCon rf) 515 | HsRecFields { rec_flds = L lf fld : flds } <- rf 516 , HsRecField { hsRecFieldArg = arg } <- fld 517 = ASSERT( null flds) 518 RecCon (rf { rec_flds = [L lf (fld { hsRecFieldArg = L l (BangPat arg) })] }) 519 push_bang_into_newtype_arg _ cd 520 = pprPanic "push_bang_into_newtype_arg" (pprConArgs cd) 522 {- 523 Note [Bang patterns and newtypes] 524 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 525 For the pattern !(Just pat) we can discard the bang, because 526 the pattern is strict anyway. But for !(N pat), where 527 newtype NT = N Int 528 we definitely can't discard the bang. Trac #9844. 530 So what we do is to push the bang inwards, in the hope that it will 531 get discarded there. So we transform 532 !(N pat) into (N !pat) 535 \noindent 536 {\bf Previous @matchTwiddled@ stuff:} 538 Now we get to the only interesting part; note: there are choices for 539 translation [from Simon's notes]; translation~1: 540 \begin{verbatim} 541 deTwiddle [s,t] e 542 \end{verbatim} 543 returns 544 \begin{verbatim} 545 [ w = e, 546 s = case w of [s,t] -> s 547 t = case w of [s,t] -> t 548 ] 549 \end{verbatim} 551 Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple 552 evaluation of \tr{e}. An alternative translation (No.~2): 553 \begin{verbatim} 554 [ w = case e of [s,t] -> (s,t) 555 s = case w of (s,t) -> s 556 t = case w of (s,t) -> t 557 ] 558 \end{verbatim} 560 ************************************************************************ 561 * * 562 \subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing} 563 * * 564 ************************************************************************ 566 We might be able to optimise unmixing when confronted by 567 only-one-constructor-possible, of which tuples are the most notable 568 examples. Consider: 569 \begin{verbatim} 570 f (a,b,c) ... = ... 571 f d ... (e:f) = ... 572 f (g,h,i) ... = ... 573 f j ... = ... 574 \end{verbatim} 575 This definition would normally be unmixed into four equation blocks, 576 one per equation. But it could be unmixed into just one equation 577 block, because if the one equation matches (on the first column), 578 the others certainly will. 580 You have to be careful, though; the example 581 \begin{verbatim} 582 f j ... = ... 583 ------------------- 584 f (a,b,c) ... = ... 585 f d ... (e:f) = ... 586 f (g,h,i) ... = ... 587 \end{verbatim} 588 {\em must} be broken into two blocks at the line shown; otherwise, you 589 are forcing unnecessary evaluation. In any case, the top-left pattern 590 always gives the cue. You could then unmix blocks into groups of... 591 \begin{description} 592 \item[all variables:] 593 As it is now. 594 \item[constructors or variables (mixed):] 595 Need to make sure the right names get bound for the variable patterns. 596 \item[literals or variables (mixed):] 597 Presumably just a variant on the constructor case (as it is now). 598 \end{description} 600 ************************************************************************ 601 * * 602 * matchWrapper: a convenient way to call @match@ * 603 * * 604 ************************************************************************ 605 \subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@} 607 Calls to @match@ often involve similar (non-trivial) work; that work 608 is collected here, in @matchWrapper@. This function takes as 609 arguments: 610 \begin{itemize} 611 \item 612 Typchecked @Matches@ (of a function definition, or a case or lambda 613 expression)---the main input; 614 \item 615 An error message to be inserted into any (runtime) pattern-matching 616 failure messages. 617 \end{itemize} 619 As results, @matchWrapper@ produces: 620 \begin{itemize} 621 \item 622 A list of variables (@Locals@) that the caller must promise'' to 623 bind to appropriate values; and 624 \item 625 a @CoreExpr@, the desugared output (main result). 626 \end{itemize} 628 The main actions of @matchWrapper@ include: 629 \begin{enumerate} 630 \item 631 Flatten the @[TypecheckedMatch]@ into a suitable list of 632 @EquationInfo@s. 633 \item 634 Create as many new variables as there are patterns in a pattern-list 635 (in any one of the @EquationInfo@s). 636 \item 637 Create a suitable if it fails'' expression---a call to @error@ using 638 the error-string input; the {\em type} of this fail value can be found 639 by examining one of the RHS expressions in one of the @EquationInfo@s. 640 \item 641 Call @match@ with all of this information! 642 \end{enumerate} 643 -} 645 matchWrapper :: HsMatchContext Name -- For shadowing warning messages 646 -> Maybe (LHsExpr Id) -- The scrutinee, if we check a case expr 647 -> MatchGroup Id (LHsExpr Id) -- Matches being desugared 648 -> DsM ([Id], CoreExpr) -- Results 650 {- 651 There is one small problem with the Lambda Patterns, when somebody 652 writes something similar to: 653 \begin{verbatim} 654 (\ (x:xs) -> ...) 655 \end{verbatim} 656 he/she don't want a warning about incomplete patterns, that is done with 657 the flag @opt_WarnSimplePatterns@. 658 This problem also appears in the: 659 \begin{itemize} 660 \item @do@ patterns, but if the @do@ can fail 661 it creates another equation if the match can fail 662 (see @DsExpr.doDo@ function) 663 \item @let@ patterns, are treated by @matchSimply@ 664 List Comprension Patterns, are treated by @matchSimply@ also 665 \end{itemize} 667 We can't call @matchSimply@ with Lambda patterns, 668 due to the fact that lambda patterns can have more than 669 one pattern, and match simply only accepts one pattern. 671 JJQC 30-Nov-1997 672 -} 674 matchWrapper ctxt mb_scr (MG { mg_alts = L _ matches 675 , mg_arg_tys = arg_tys 676 , mg_res_ty = rhs_ty 677 , mg_origin = origin }) 678 = do { dflags <- getDynFlags 679 ; locn <- getSrcSpanDs 681 ; new_vars <- case matches of 682 [] -> mapM newSysLocalDs arg_tys 683 (m:_) -> selectMatchVars (map unLoc (hsLMatchPats m)) 685 ; eqns_info <- mapM (mk_eqn_info new_vars) matches 687 -- pattern match check warnings 688 ; unless (isGenerated origin)$
689 -- See Note [Type and Term Equality Propagation]
690 addTmCsDs (genCaseTmCs1 mb_scr new_vars) $691 dsPmWarn dflags (DsMatchContext ctxt locn)$
692 checkMatches new_vars matches
694 ; result_expr <- handleWarnings $695 matchEquations ctxt new_vars eqns_info rhs_ty 696 ; return (new_vars, result_expr) } 697 where 698 mk_eqn_info vars (L _ (Match _ pats _ grhss)) 699 = do { dflags <- getDynFlags 700 ; let upats = map (strictify dflags) pats 701 dicts = toTcTypeBag (collectEvVarsPats upats) -- Only TcTyVars 702 ; tm_cs <- genCaseTmCs2 mb_scr upats vars 703 ; match_result <- addDictsDs dicts$ -- See Note [Type and Term Equality Propagation]
704 addTmCsDs tm_cs $-- See Note [Type and Term Equality Propagation] 705 dsGRHSs ctxt upats grhss rhs_ty 706 ; return (EqnInfo { eqn_pats = upats, eqn_rhs = match_result}) } 708 strictify dflags pat = 709 let (is_strict, pat') = getUnBangedLPat dflags pat 710 in if is_strict then BangPat pat' else unLoc pat' 712 handleWarnings = if isGenerated origin 713 then discardWarningsDs 714 else id 717 matchEquations :: HsMatchContext Name 718 -> [Id] -> [EquationInfo] -> Type 719 -> DsM CoreExpr 720 matchEquations ctxt vars eqns_info rhs_ty 721 = do { let error_doc = matchContextErrString ctxt 723 ; match_result <- match vars rhs_ty eqns_info 725 ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_doc 726 ; extractMatchResult match_result fail_expr } 728 {- 729 ************************************************************************ 730 * * 731 \subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern} 732 * * 733 ************************************************************************ 735 @mkSimpleMatch@ is a wrapper for @match@ which deals with the 736 situation where we want to match a single expression against a single 737 pattern. It returns an expression. 738 -} 740 matchSimply :: CoreExpr -- Scrutinee 741 -> HsMatchContext Name -- Match kind 742 -> LPat Id -- Pattern it should match 743 -> CoreExpr -- Return this if it matches 744 -> CoreExpr -- Return this if it doesn't 745 -> DsM CoreExpr 746 -- Do not warn about incomplete patterns; see matchSinglePat comments 747 matchSimply scrut hs_ctx pat result_expr fail_expr = do 748 let 749 match_result = cantFailMatchResult result_expr 750 rhs_ty = exprType fail_expr 751 -- Use exprType of fail_expr, because won't refine in the case of failure! 752 match_result' <- matchSinglePat scrut hs_ctx pat rhs_ty match_result 753 extractMatchResult match_result' fail_expr 755 matchSinglePat :: CoreExpr -> HsMatchContext Name -> LPat Id 756 -> Type -> MatchResult -> DsM MatchResult 757 -- Do not warn about incomplete patterns 758 -- Used for things like [ e | pat <- stuff ], where 759 -- incomplete patterns are just fine 760 matchSinglePat (Var var) ctx (L _ pat) ty match_result 761 = do { dflags <- getDynFlags 762 ; locn <- getSrcSpanDs 764 -- pattern match check warnings 765 ; dsPmWarn dflags (DsMatchContext ctx locn) (checkSingle var pat) 767 ; match [var] ty 768 [EqnInfo { eqn_pats = [pat], eqn_rhs = match_result }] } 770 matchSinglePat scrut hs_ctx pat ty match_result 771 = do { var <- selectSimpleMatchVarL pat 772 ; match_result' <- matchSinglePat (Var var) hs_ctx pat ty match_result 773 ; return (adjustMatchResult (bindNonRec var scrut) match_result') } 775 {- 776 ************************************************************************ 777 * * 778 Pattern classification 779 * * 780 ************************************************************************ 781 -} 783 data PatGroup 784 = PgAny -- Immediate match: variables, wildcards, 785 -- lazy patterns 786 | PgCon DataCon -- Constructor patterns (incl list, tuple) 787 | PgSyn PatSyn 788 | PgLit Literal -- Literal patterns 789 | PgN Literal -- Overloaded literals 790 | PgNpK Literal -- n+k patterns 791 | PgBang -- Bang patterns 792 | PgCo Type -- Coercion patterns; the type is the type 793 -- of the pattern *inside* 794 | PgView (LHsExpr Id) -- view pattern (e -> p): 795 -- the LHsExpr is the expression e 796 Type -- the Type is the type of p (equivalently, the result type of e) 797 | PgOverloadedList 799 groupEquations :: DynFlags -> [EquationInfo] -> [[(PatGroup, EquationInfo)]] 800 -- If the result is of form [g1, g2, g3], 801 -- (a) all the (pg,eq) pairs in g1 have the same pg 802 -- (b) none of the gi are empty 803 -- The ordering of equations is unchanged 804 groupEquations dflags eqns 805 = runs same_gp [(patGroup dflags (firstPat eqn), eqn) | eqn <- eqns] 806 where 807 same_gp :: (PatGroup,EquationInfo) -> (PatGroup,EquationInfo) -> Bool 808 (pg1,_) same_gp (pg2,_) = pg1 sameGroup pg2 810 subGroup :: Ord a => [(a, EquationInfo)] -> [[EquationInfo]] 811 -- Input is a particular group. The result sub-groups the 812 -- equations by with particular constructor, literal etc they match. 813 -- Each sub-list in the result has the same PatGroup 814 -- See Note [Take care with pattern order] 815 subGroup group 816 = map reverse$ Map.elems \$ foldl accumulate Map.empty group
817 where
818 accumulate pg_map (pg, eqn)
819 = case Map.lookup pg pg_map of
820 Just eqns -> Map.insert pg (eqn:eqns) pg_map
821 Nothing -> Map.insert pg [eqn] pg_map
823 -- pg_map :: Map a [EquationInfo]
824 -- Equations seen so far in reverse order of appearance
826 {-
827 Note [Take care with pattern order]
828 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
829 In the subGroup function we must be very careful about pattern re-ordering,
830 Consider the patterns [ (True, Nothing), (False, x), (True, y) ]
831 Then in bringing together the patterns for True, we must not
832 swap the Nothing and y!
833 -}
835 sameGroup :: PatGroup -> PatGroup -> Bool
836 -- Same group means that a single case expression
837 -- or test will suffice to match both, *and* the order
838 -- of testing within the group is insignificant.
839 sameGroup PgAny PgAny = True
840 sameGroup PgBang PgBang = True
841 sameGroup (PgCon _) (PgCon _) = True -- One case expression
842 sameGroup (PgSyn p1) (PgSyn p2) = p1==p2
843 sameGroup (PgLit _) (PgLit _) = True -- One case expression
844 sameGroup (PgN l1) (PgN l2) = l1==l2 -- Order is significant
845 sameGroup (PgNpK l1) (PgNpK l2) = l1==l2 -- See Note [Grouping overloaded literal patterns]
846 sameGroup (PgCo t1) (PgCo t2) = t1 eqType t2
847 -- CoPats are in the same goup only if the type of the
848 -- enclosed pattern is the same. The patterns outside the CoPat
849 -- always have the same type, so this boils down to saying that
850 -- the two coercions are identical.
851 sameGroup (PgView e1 t1) (PgView e2 t2) = viewLExprEq (e1,t1) (e2,t2)
852 -- ViewPats are in the same group iff the expressions
853 -- are "equal"---conservatively, we use syntactic equality
854 sameGroup _ _ = False
856 -- An approximation of syntactic equality used for determining when view
857 -- exprs are in the same group.
858 -- This function can always safely return false;
859 -- but doing so will result in the application of the view function being repeated.
860 --
861 -- Currently: compare applications of literals and variables
862 -- and anything else that we can do without involving other
863 -- HsSyn types in the recursion
864 --
865 -- NB we can't assume that the two view expressions have the same type. Consider
866 -- f (e1 -> True) = ...
867 -- f (e2 -> "hi") = ...
868 viewLExprEq :: (LHsExpr Id,Type) -> (LHsExpr Id,Type) -> Bool
869 viewLExprEq (e1,_) (e2,_) = lexp e1 e2
870 where
871 lexp :: LHsExpr Id -> LHsExpr Id -> Bool
872 lexp e e' = exp (unLoc e) (unLoc e')
874 ---------
875 exp :: HsExpr Id -> HsExpr Id -> Bool
876 -- real comparison is on HsExpr's
877 -- strip parens
878 exp (HsPar (L _ e)) e' = exp e e'
879 exp e (HsPar (L _ e')) = exp e e'
880 -- because the expressions do not necessarily have the same type,
881 -- we have to compare the wrappers
882 exp (HsWrap h e) (HsWrap h' e') = wrap h h' && exp e e'
883 exp (HsVar i) (HsVar i') = i == i'
884 -- the instance for IPName derives using the id, so this works if the
885 -- above does
886 exp (HsIPVar i) (HsIPVar i') = i == i'
887 exp (HsOverLabel l) (HsOverLabel l') = l == l'
888 exp (HsOverLit l) (HsOverLit l') =
889 -- Overloaded lits are equal if they have the same type
890 -- and the data is the same.
891 -- this is coarser than comparing the SyntaxExpr's in l and l',
893 -- because these expressions get written as a bunch of different variables
894 -- (presumably to improve sharing)
895 eqType (overLitType l) (overLitType l') && l == l'
896 exp (HsApp e1 e2) (HsApp e1' e2') = lexp e1 e1' && lexp e2 e2'
897 -- the fixities have been straightened out by now, so it's safe
898 -- to ignore them?
899 exp (OpApp l o _ ri) (OpApp l' o' _ ri') =
900 lexp l l' && lexp o o' && lexp ri ri'
901 exp (NegApp e n) (NegApp e' n') = lexp e e' && exp n n'
902 exp (SectionL e1 e2) (SectionL e1' e2') =
903 lexp e1 e1' && lexp e2 e2'
904 exp (SectionR e1 e2) (SectionR e1' e2') =
905 lexp e1 e1' && lexp e2 e2'
906 exp (ExplicitTuple es1 _) (ExplicitTuple es2 _) =
907 eq_list tup_arg es1 es2
908 exp (HsIf _ e e1 e2) (HsIf _ e' e1' e2') =
909 lexp e e' && lexp e1 e1' && lexp e2 e2'
911 -- Enhancement: could implement equality for more expressions
912 -- if it seems useful
913 -- But no need for HsLit, ExplicitList, ExplicitTuple,
914 -- because they cannot be functions
915 exp _ _ = False
917 ---------
918 tup_arg (L _ (Present e1)) (L _ (Present e2)) = lexp e1 e2
919 tup_arg (L _ (Missing t1)) (L _ (Missing t2)) = eqType t1 t2
920 tup_arg _ _ = False
922 ---------
923 wrap :: HsWrapper -> HsWrapper -> Bool
924 -- Conservative, in that it demands that wrappers be
925 -- syntactically identical and doesn't look under binders
926 --
927 -- Coarser notions of equality are possible
928 -- (e.g., reassociating compositions,
929 -- equating different ways of writing a coercion)
930 wrap WpHole WpHole = True
931 wrap (WpCompose w1 w2) (WpCompose w1' w2') = wrap w1 w1' && wrap w2 w2'
932 wrap (WpFun w1 w2 _ _) (WpFun w1' w2' _ _) = wrap w1 w1' && wrap w2 w2'
933 wrap (WpCast co) (WpCast co') = co eq_co co'
934 wrap (WpEvApp et1) (WpEvApp et2) = et1 ev_term et2
935 wrap (WpTyApp t) (WpTyApp t') = eqType t t'
936 -- Enhancement: could implement equality for more wrappers
937 -- if it seems useful (lams and lets)
938 wrap _ _ = False
940 ---------
941 ev_term :: EvTerm -> EvTerm -> Bool
942 ev_term (EvId a) (EvId b) = a==b
943 ev_term (EvCoercion a) (EvCoercion b) = a eq_co b
944 ev_term _ _ = False
946 ---------
947 eq_list :: (a->a->Bool) -> [a] -> [a] -> Bool
948 eq_list _ [] [] = True
949 eq_list _ [] (_:_) = False
950 eq_list _ (_:_) [] = False
951 eq_list eq (x:xs) (y:ys) = eq x y && eq_list eq xs ys
953 ---------
954 eq_co :: TcCoercion -> TcCoercion -> Bool
955 -- Just some simple cases (should the r1 == r2 rather be an ASSERT?)
956 eq_co (TcRefl r1 t1) (TcRefl r2 t2) = r1 == r2 && eqType t1 t2
957 eq_co (TcCoVarCo v1) (TcCoVarCo v2) = v1==v2
958 eq_co (TcSymCo co1) (TcSymCo co2) = co1 eq_co co2
959 eq_co (TcTyConAppCo r1 tc1 cos1) (TcTyConAppCo r2 tc2 cos2) = r1 == r2 && tc1==tc2 && eq_list eq_co cos1 cos2
960 eq_co _ _ = False
962 patGroup :: DynFlags -> Pat Id -> PatGroup
963 patGroup _ (WildPat {}) = PgAny
964 patGroup _ (BangPat {}) = PgBang
965 patGroup _ (ConPatOut { pat_con = con }) = case unLoc con of
966 RealDataCon dcon -> PgCon dcon
967 PatSynCon psyn -> PgSyn psyn
968 patGroup dflags (LitPat lit) = PgLit (hsLitKey dflags lit)
969 patGroup _ (NPat (L _ olit) mb_neg _)
970 = PgN (hsOverLitKey olit (isJust mb_neg))
971 patGroup _ (NPlusKPat _ (L _ olit) _ _) = PgNpK (hsOverLitKey olit False)
972 patGroup _ (CoPat _ p _) = PgCo (hsPatType p) -- Type of innelexp pattern
973 patGroup _ (ViewPat expr p _) = PgView expr (hsPatType (unLoc p))
974 patGroup _ (ListPat _ _ (Just _)) = PgOverloadedList
975 patGroup _ pat = pprPanic "patGroup" (ppr pat)
977 {-
978 Note [Grouping overloaded literal patterns]
979 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
980 WATCH OUT! Consider
982 f (n+1) = ...
983 f (n+2) = ...
984 f (n+1) = ...
986 We can't group the first and third together, because the second may match
987 the same thing as the first. Same goes for *overloaded* literal patterns
988 f 1 True = ...
989 f 2 False = ...
990 f 1 False = ...
991 If the first arg matches '1' but the second does not match 'True', we
992 cannot jump to the third equation! Because the same argument might
993 match '2'!
994 Hence we don't regard 1 and 2, or (n+1) and (n+2), as part of the same group.
995 -}