2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
9 {-# OPTIONS -fno-warn-incomplete-patterns #-}
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module Match ( match, matchEquations, matchWrapper, matchSimply, matchSinglePat ) where
18 #include "HsVersions.h"
20 import {-#SOURCE#-} DsExpr (dsLExpr)
51 This function is a wrapper of @match@, it must be called from all the parts where
52 it was called match, but only substitutes the firs call, ....
53 if the associated flags are declared, warnings will be issued.
54 It can not be called matchWrapper because this name already exists :-(
59 matchCheck :: DsMatchContext
60 -> [Id] -- Vars rep'ing the exprs we're matching with
61 -> Type -- Type of the case expression
62 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
63 -> DsM MatchResult -- Desugared result!
65 matchCheck ctx vars ty qs = do
67 matchCheck_really dflags ctx vars ty qs
69 matchCheck_really :: DynFlags
75 matchCheck_really dflags ctx vars ty qs
76 | incomplete && shadow = do
77 dsShadowWarn ctx eqns_shadow
78 dsIncompleteWarn ctx pats
81 dsIncompleteWarn ctx pats
84 dsShadowWarn ctx eqns_shadow
88 where (pats, eqns_shadow) = check qs
89 incomplete = want_incomplete && (notNull pats)
90 want_incomplete = case ctx of
91 DsMatchContext RecUpd _ ->
92 dopt Opt_WarnIncompletePatternsRecUpd dflags
94 dopt Opt_WarnIncompletePatterns dflags
95 shadow = dopt Opt_WarnOverlappingPatterns dflags
96 && not (null eqns_shadow)
99 This variable shows the maximum number of lines of output generated for warnings.
100 It will limit the number of patterns/equations displayed to@ maximum_output@.
102 (ToDo: add command-line option?)
105 maximum_output :: Int
109 The next two functions create the warning message.
112 dsShadowWarn :: DsMatchContext -> [EquationInfo] -> DsM ()
113 dsShadowWarn ctx@(DsMatchContext kind loc) qs
114 = putSrcSpanDs loc (warnDs warn)
116 warn | qs `lengthExceeds` maximum_output
117 = pp_context ctx (ptext (sLit "are overlapped"))
118 (\ f -> vcat (map (ppr_eqn f kind) (take maximum_output qs)) $$
121 = pp_context ctx (ptext (sLit "are overlapped"))
122 (\ f -> vcat $ map (ppr_eqn f kind) qs)
125 dsIncompleteWarn :: DsMatchContext -> [ExhaustivePat] -> DsM ()
126 dsIncompleteWarn ctx@(DsMatchContext kind loc) pats
127 = putSrcSpanDs loc (warnDs warn)
129 warn = pp_context ctx (ptext (sLit "are non-exhaustive"))
130 (\_ -> hang (ptext (sLit "Patterns not matched:"))
131 4 ((vcat $ map (ppr_incomplete_pats kind)
132 (take maximum_output pats))
135 dots | pats `lengthExceeds` maximum_output = ptext (sLit "...")
138 pp_context :: DsMatchContext -> SDoc -> ((SDoc -> SDoc) -> SDoc) -> SDoc
139 pp_context (DsMatchContext kind _loc) msg rest_of_msg_fun
140 = vcat [ptext (sLit "Pattern match(es)") <+> msg,
141 sep [ptext (sLit "In") <+> ppr_match <> char ':', nest 4 (rest_of_msg_fun pref)]]
145 FunRhs fun _ -> (pprMatchContext kind, \ pp -> ppr fun <+> pp)
146 _ -> (pprMatchContext kind, \ pp -> pp)
148 ppr_pats :: Outputable a => [a] -> SDoc
149 ppr_pats pats = sep (map ppr pats)
151 ppr_shadow_pats :: HsMatchContext Name -> [Pat Id] -> SDoc
152 ppr_shadow_pats kind pats
153 = sep [ppr_pats pats, matchSeparator kind, ptext (sLit "...")]
155 ppr_incomplete_pats :: HsMatchContext Name -> ExhaustivePat -> SDoc
156 ppr_incomplete_pats _ (pats,[]) = ppr_pats pats
157 ppr_incomplete_pats _ (pats,constraints) =
158 sep [ppr_pats pats, ptext (sLit "with"),
159 sep (map ppr_constraint constraints)]
161 ppr_constraint :: (Name,[HsLit]) -> SDoc
162 ppr_constraint (var,pats) = sep [ppr var, ptext (sLit "`notElem`"), ppr pats]
164 ppr_eqn :: (SDoc -> SDoc) -> HsMatchContext Name -> EquationInfo -> SDoc
165 ppr_eqn prefixF kind eqn = prefixF (ppr_shadow_pats kind (eqn_pats eqn))
169 %************************************************************************
171 The main matching function
173 %************************************************************************
175 The function @match@ is basically the same as in the Wadler chapter,
176 except it is monadised, to carry around the name supply, info about
179 Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns:
182 A list of $n$ variable names, those variables presumably bound to the
183 $n$ expressions being matched against the $n$ patterns. Using the
184 list of $n$ expressions as the first argument showed no benefit and
188 The second argument, a list giving the ``equation info'' for each of
192 the $n$ patterns for that equation, and
194 a list of Core bindings [@(Id, CoreExpr)@ pairs] to be ``stuck on
195 the front'' of the matching code, as in:
201 and finally: (ToDo: fill in)
203 The right way to think about the ``after-match function'' is that it
204 is an embryonic @CoreExpr@ with a ``hole'' at the end for the
205 final ``else expression''.
208 There is a type synonym, @EquationInfo@, defined in module @DsUtils@.
210 An experiment with re-ordering this information about equations (in
211 particular, having the patterns available in column-major order)
215 A default expression---what to evaluate if the overall pattern-match
216 fails. This expression will (almost?) always be
217 a measly expression @Var@, unless we know it will only be used once
218 (as we do in @glue_success_exprs@).
220 Leaving out this third argument to @match@ (and slamming in lots of
221 @Var "fail"@s) is a positively {\em bad} idea, because it makes it
222 impossible to share the default expressions. (Also, it stands no
223 chance of working in our post-upheaval world of @Locals@.)
226 Note: @match@ is often called via @matchWrapper@ (end of this module),
227 a function that does much of the house-keeping that goes with a call
230 It is also worth mentioning the {\em typical} way a block of equations
231 is desugared with @match@. At each stage, it is the first column of
232 patterns that is examined. The steps carried out are roughly:
235 Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add
236 bindings to the second component of the equation-info):
239 Remove the `as' patterns from column~1.
241 Make all constructor patterns in column~1 into @ConPats@, notably
242 @ListPats@ and @TuplePats@.
244 Handle any irrefutable (or ``twiddle'') @LazyPats@.
247 Now {\em unmix} the equations into {\em blocks} [w\/ local function
248 @unmix_eqns@], in which the equations in a block all have variable
249 patterns in column~1, or they all have constructor patterns in ...
250 (see ``the mixture rule'' in SLPJ).
252 Call @matchEqnBlock@ on each block of equations; it will do the
253 appropriate thing for each kind of column-1 pattern, usually ending up
254 in a recursive call to @match@.
257 We are a little more paranoid about the ``empty rule'' (SLPJ, p.~87)
258 than the Wadler-chapter code for @match@ (p.~93, first @match@ clause).
259 And gluing the ``success expressions'' together isn't quite so pretty.
261 This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@
262 (a)~to get `as'- and `twiddle'-patterns out of the way (tidying), and
263 (b)~to do ``the mixture rule'' (SLPJ, p.~88) [which really {\em
264 un}mixes the equations], producing a list of equation-info
265 blocks, each block having as its first column of patterns either all
266 constructors, or all variables (or similar beasts), etc.
268 @match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the
269 Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@
270 corresponds roughly to @matchVarCon@.
273 match :: [Id] -- Variables rep\'ing the exprs we\'re matching with
274 -> Type -- Type of the case expression
275 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
276 -> DsM MatchResult -- Desugared result!
279 = ASSERT2( not (null eqns), ppr ty )
280 return (foldr1 combineMatchResults match_results)
282 match_results = [ ASSERT( null (eqn_pats eqn) )
286 match vars@(v:_) ty eqns
287 = ASSERT( not (null eqns ) )
288 do { -- Tidy the first pattern, generating
289 -- auxiliary bindings if necessary
290 (aux_binds, tidy_eqns) <- mapAndUnzipM (tidyEqnInfo v) eqns
292 -- Group the equations and match each group in turn
293 ; let grouped = groupEquations tidy_eqns
295 -- print the view patterns that are commoned up to help debug
296 ; ifOptM Opt_D_dump_view_pattern_commoning (debug grouped)
298 ; match_results <- mapM match_group grouped
299 ; return (adjustMatchResult (foldr1 (.) aux_binds) $
300 foldr1 combineMatchResults match_results) }
302 dropGroup :: [(PatGroup,EquationInfo)] -> [EquationInfo]
305 match_group :: [(PatGroup,EquationInfo)] -> DsM MatchResult
306 match_group eqns@((group,_) : _)
308 PgCon _ -> matchConFamily vars ty (subGroup [(c,e) | (PgCon c, e) <- eqns])
309 PgLit _ -> matchLiterals vars ty (subGroup [(l,e) | (PgLit l, e) <- eqns])
311 PgAny -> matchVariables vars ty (dropGroup eqns)
312 PgN _ -> matchNPats vars ty (dropGroup eqns)
313 PgNpK _ -> matchNPlusKPats vars ty (dropGroup eqns)
314 PgBang -> matchBangs vars ty (dropGroup eqns)
315 PgCo _ -> matchCoercion vars ty (dropGroup eqns)
316 PgView _ _ -> matchView vars ty (dropGroup eqns)
318 -- FIXME: we should also warn about view patterns that should be
319 -- commoned up but are not
321 -- print some stuff to see what's getting grouped
322 -- use -dppr-debug to see the resolution of overloaded lits
324 let gs = map (\group -> foldr (\ (p,_) -> \acc ->
325 case p of PgView e _ -> e:acc
326 _ -> acc) [] group) eqns
327 maybeWarn [] = return ()
328 maybeWarn l = warnDs (vcat l)
330 maybeWarn $ (map (\g -> text "Putting these view expressions into the same case:" <+> (ppr g))
331 (filter (not . null) gs))
333 matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
334 -- Real true variables, just like in matchVar, SLPJ p 94
335 -- No binding to do: they'll all be wildcards by now (done in tidy)
336 matchVariables (_:vars) ty eqns = match vars ty (shiftEqns eqns)
338 matchBangs :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
339 matchBangs (var:vars) ty eqns
340 = do { match_result <- match (var:vars) ty (map decomposeFirst_Bang eqns)
341 ; return (mkEvalMatchResult var ty match_result) }
343 matchCoercion :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
344 -- Apply the coercion to the match variable and then match that
345 matchCoercion (var:vars) ty (eqns@(eqn1:_))
346 = do { let CoPat co pat _ = firstPat eqn1
347 ; var' <- newUniqueId var (hsPatType pat)
348 ; match_result <- match (var':vars) ty (map decomposeFirst_Coercion eqns)
349 ; co' <- dsCoercion co
350 ; let rhs' = co' (Var var)
351 ; return (mkCoLetMatchResult (NonRec var' rhs') match_result) }
353 matchView :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
354 -- Apply the view function to the match variable and then match that
355 matchView (var:vars) ty (eqns@(eqn1:_))
356 = do { -- we could pass in the expr from the PgView,
357 -- but this needs to extract the pat anyway
358 -- to figure out the type of the fresh variable
359 let ViewPat viewExpr (L _ pat) _ = firstPat eqn1
360 -- do the rest of the compilation
361 ; var' <- newUniqueId var (hsPatType pat)
362 ; match_result <- match (var':vars) ty (map decomposeFirst_View eqns)
363 -- compile the view expressions
364 ; viewExpr' <- dsLExpr viewExpr
365 ; return (mkViewMatchResult var' viewExpr' var match_result) }
367 -- decompose the first pattern and leave the rest alone
368 decomposeFirstPat :: (Pat Id -> Pat Id) -> EquationInfo -> EquationInfo
369 decomposeFirstPat extractpat (eqn@(EqnInfo { eqn_pats = pat : pats }))
370 = eqn { eqn_pats = extractpat pat : pats}
372 decomposeFirst_Coercion, decomposeFirst_Bang, decomposeFirst_View :: EquationInfo -> EquationInfo
374 decomposeFirst_Coercion = decomposeFirstPat (\ (CoPat _ pat _) -> pat)
375 decomposeFirst_Bang = decomposeFirstPat (\ (BangPat pat ) -> unLoc pat)
376 decomposeFirst_View = decomposeFirstPat (\ (ViewPat _ pat _) -> unLoc pat)
380 %************************************************************************
384 %************************************************************************
386 Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@
387 which will be scrutinised. This means:
390 Replace variable patterns @x@ (@x /= v@) with the pattern @_@,
391 together with the binding @x = v@.
393 Replace the `as' pattern @x@@p@ with the pattern p and a binding @x = v@.
395 Removing lazy (irrefutable) patterns (you don't want to know...).
397 Converting explicit tuple-, list-, and parallel-array-pats into ordinary
400 Convert the literal pat "" to [].
403 The result of this tidying is that the column of patterns will include
407 The @VarPat@ information isn't needed any more after this.
410 @ListPats@, @TuplePats@, etc., are all converted into @ConPats@.
412 \item[@LitPats@ and @NPats@:]
413 @LitPats@/@NPats@ of ``known friendly types'' (Int, Char,
414 Float, Double, at least) are converted to unboxed form; e.g.,
415 \tr{(NPat (HsInt i) _ _)} is converted to:
417 (ConPat I# _ _ [LitPat (HsIntPrim i)])
422 tidyEqnInfo :: Id -> EquationInfo
423 -> DsM (DsWrapper, EquationInfo)
424 -- DsM'd because of internal call to dsLHsBinds
425 -- and mkSelectorBinds.
426 -- "tidy1" does the interesting stuff, looking at
427 -- one pattern and fiddling the list of bindings.
429 -- POST CONDITION: head pattern in the EqnInfo is
437 tidyEqnInfo v eqn@(EqnInfo { eqn_pats = pat : pats }) = do
438 (wrap, pat') <- tidy1 v pat
439 return (wrap, eqn { eqn_pats = do pat' : pats })
441 tidy1 :: Id -- The Id being scrutinised
442 -> Pat Id -- The pattern against which it is to be matched
443 -> DsM (DsWrapper, -- Extra bindings to do before the match
444 Pat Id) -- Equivalent pattern
446 -------------------------------------------------------
447 -- (pat', mr') = tidy1 v pat mr
448 -- tidies the *outer level only* of pat, giving pat'
449 -- It eliminates many pattern forms (as-patterns, variable patterns,
450 -- list patterns, etc) yielding one of:
457 tidy1 v (ParPat pat) = tidy1 v (unLoc pat)
458 tidy1 v (SigPatOut pat _) = tidy1 v (unLoc pat)
459 tidy1 _ (WildPat ty) = return (idDsWrapper, WildPat ty)
461 -- case v of { x -> mr[] }
462 -- = case v of { _ -> let x=v in mr[] }
464 = return (wrapBind var v, WildPat (idType var))
466 tidy1 v (VarPatOut var binds)
467 = do { prs <- dsLHsBinds binds
468 ; return (wrapBind var v . mkCoreLet (Rec prs),
469 WildPat (idType var)) }
471 -- case v of { x@p -> mr[] }
472 -- = case v of { p -> let x=v in mr[] }
473 tidy1 v (AsPat (L _ var) pat)
474 = do { (wrap, pat') <- tidy1 v (unLoc pat)
475 ; return (wrapBind var v . wrap, pat') }
477 {- now, here we handle lazy patterns:
478 tidy1 v ~p bs = (v, v1 = case v of p -> v1 :
479 v2 = case v of p -> v2 : ... : bs )
481 where the v_i's are the binders in the pattern.
483 ToDo: in "v_i = ... -> v_i", are the v_i's really the same thing?
485 The case expr for v_i is just: match [v] [(p, [], \ x -> Var v_i)] any_expr
488 tidy1 v (LazyPat pat)
489 = do { sel_prs <- mkSelectorBinds pat (Var v)
490 ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs]
491 ; return (mkCoreLets sel_binds, WildPat (idType v)) }
493 tidy1 _ (ListPat pats ty)
494 = return (idDsWrapper, unLoc list_ConPat)
496 list_ty = mkListTy ty
497 list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] list_ty)
501 -- Introduce fake parallel array constructors to be able to handle parallel
502 -- arrays with the existing machinery for constructor pattern
503 tidy1 _ (PArrPat pats ty)
504 = return (idDsWrapper, unLoc parrConPat)
507 parrConPat = mkPrefixConPat (parrFakeCon arity) pats (mkPArrTy ty)
509 tidy1 _ (TuplePat pats boxity ty)
510 = return (idDsWrapper, unLoc tuple_ConPat)
513 tuple_ConPat = mkPrefixConPat (tupleCon boxity arity) pats ty
515 -- LitPats: we *might* be able to replace these w/ a simpler form
517 = return (idDsWrapper, tidyLitPat lit)
519 -- NPats: we *might* be able to replace these w/ a simpler form
520 tidy1 _ (NPat lit mb_neg eq)
521 = return (idDsWrapper, tidyNPat lit mb_neg eq)
523 -- BangPatterns: Pattern matching is already strict in constructors,
524 -- tuples etc, so the last case strips off the bang for thoses patterns.
525 tidy1 v (BangPat (L _ (LazyPat p))) = tidy1 v (BangPat p)
526 tidy1 v (BangPat (L _ (ParPat p))) = tidy1 v (BangPat p)
527 tidy1 _ p@(BangPat (L _(VarPat _))) = return (idDsWrapper, p)
528 tidy1 _ p@(BangPat (L _(VarPatOut _ _))) = return (idDsWrapper, p)
529 tidy1 _ p@(BangPat (L _ (WildPat _))) = return (idDsWrapper, p)
530 tidy1 _ p@(BangPat (L _ (CoPat _ _ _))) = return (idDsWrapper, p)
531 tidy1 _ p@(BangPat (L _ (SigPatIn _ _))) = return (idDsWrapper, p)
532 tidy1 _ p@(BangPat (L _ (SigPatOut _ _))) = return (idDsWrapper, p)
533 tidy1 v (BangPat (L _ (AsPat (L _ var) pat)))
534 = do { (wrap, pat') <- tidy1 v (BangPat pat)
535 ; return (wrapBind var v . wrap, pat') }
536 tidy1 v (BangPat (L _ p)) = tidy1 v p
538 -- Everything else goes through unchanged...
540 tidy1 _ non_interesting_pat
541 = return (idDsWrapper, non_interesting_pat)
545 {\bf Previous @matchTwiddled@ stuff:}
547 Now we get to the only interesting part; note: there are choices for
548 translation [from Simon's notes]; translation~1:
555 s = case w of [s,t] -> s
556 t = case w of [s,t] -> t
560 Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple
561 evaluation of \tr{e}. An alternative translation (No.~2):
563 [ w = case e of [s,t] -> (s,t)
564 s = case w of (s,t) -> s
565 t = case w of (s,t) -> t
569 %************************************************************************
571 \subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing}
573 %************************************************************************
575 We might be able to optimise unmixing when confronted by
576 only-one-constructor-possible, of which tuples are the most notable
584 This definition would normally be unmixed into four equation blocks,
585 one per equation. But it could be unmixed into just one equation
586 block, because if the one equation matches (on the first column),
587 the others certainly will.
589 You have to be careful, though; the example
597 {\em must} be broken into two blocks at the line shown; otherwise, you
598 are forcing unnecessary evaluation. In any case, the top-left pattern
599 always gives the cue. You could then unmix blocks into groups of...
601 \item[all variables:]
603 \item[constructors or variables (mixed):]
604 Need to make sure the right names get bound for the variable patterns.
605 \item[literals or variables (mixed):]
606 Presumably just a variant on the constructor case (as it is now).
609 %************************************************************************
611 %* matchWrapper: a convenient way to call @match@ *
613 %************************************************************************
614 \subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@}
616 Calls to @match@ often involve similar (non-trivial) work; that work
617 is collected here, in @matchWrapper@. This function takes as
621 Typchecked @Matches@ (of a function definition, or a case or lambda
622 expression)---the main input;
624 An error message to be inserted into any (runtime) pattern-matching
628 As results, @matchWrapper@ produces:
631 A list of variables (@Locals@) that the caller must ``promise'' to
632 bind to appropriate values; and
634 a @CoreExpr@, the desugared output (main result).
637 The main actions of @matchWrapper@ include:
640 Flatten the @[TypecheckedMatch]@ into a suitable list of
643 Create as many new variables as there are patterns in a pattern-list
644 (in any one of the @EquationInfo@s).
646 Create a suitable ``if it fails'' expression---a call to @error@ using
647 the error-string input; the {\em type} of this fail value can be found
648 by examining one of the RHS expressions in one of the @EquationInfo@s.
650 Call @match@ with all of this information!
654 matchWrapper :: HsMatchContext Name -- For shadowing warning messages
655 -> MatchGroup Id -- Matches being desugared
656 -> DsM ([Id], CoreExpr) -- Results
659 There is one small problem with the Lambda Patterns, when somebody
660 writes something similar to:
664 he/she don't want a warning about incomplete patterns, that is done with
665 the flag @opt_WarnSimplePatterns@.
666 This problem also appears in the:
668 \item @do@ patterns, but if the @do@ can fail
669 it creates another equation if the match can fail
670 (see @DsExpr.doDo@ function)
671 \item @let@ patterns, are treated by @matchSimply@
672 List Comprension Patterns, are treated by @matchSimply@ also
675 We can't call @matchSimply@ with Lambda patterns,
676 due to the fact that lambda patterns can have more than
677 one pattern, and match simply only accepts one pattern.
682 matchWrapper ctxt (MatchGroup matches match_ty)
683 = ASSERT( notNull matches )
684 do { eqns_info <- mapM mk_eqn_info matches
685 ; new_vars <- selectMatchVars arg_pats
686 ; result_expr <- matchEquations ctxt new_vars eqns_info rhs_ty
687 ; return (new_vars, result_expr) }
689 arg_pats = map unLoc (hsLMatchPats (head matches))
690 n_pats = length arg_pats
691 (_, rhs_ty) = splitFunTysN n_pats match_ty
693 mk_eqn_info (L _ (Match pats _ grhss))
694 = do { let upats = map unLoc pats
695 ; match_result <- dsGRHSs ctxt upats grhss rhs_ty
696 ; return (EqnInfo { eqn_pats = upats, eqn_rhs = match_result}) }
699 matchEquations :: HsMatchContext Name
700 -> [Id] -> [EquationInfo] -> Type
702 matchEquations ctxt vars eqns_info rhs_ty
703 = do { dflags <- getDOptsDs
704 ; locn <- getSrcSpanDs
705 ; let ds_ctxt = DsMatchContext ctxt locn
706 error_doc = matchContextErrString ctxt
708 ; match_result <- match_fun dflags ds_ctxt vars rhs_ty eqns_info
710 ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_doc
711 ; extractMatchResult match_result fail_expr }
713 match_fun dflags ds_ctxt
715 LambdaExpr | dopt Opt_WarnSimplePatterns dflags -> matchCheck ds_ctxt
717 _ -> matchCheck ds_ctxt
720 %************************************************************************
722 \subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern}
724 %************************************************************************
726 @mkSimpleMatch@ is a wrapper for @match@ which deals with the
727 situation where we want to match a single expression against a single
728 pattern. It returns an expression.
731 matchSimply :: CoreExpr -- Scrutinee
732 -> HsMatchContext Name -- Match kind
733 -> LPat Id -- Pattern it should match
734 -> CoreExpr -- Return this if it matches
735 -> CoreExpr -- Return this if it doesn't
738 matchSimply scrut hs_ctx pat result_expr fail_expr = do
740 match_result = cantFailMatchResult result_expr
741 rhs_ty = exprType fail_expr
742 -- Use exprType of fail_expr, because won't refine in the case of failure!
743 match_result' <- matchSinglePat scrut hs_ctx pat rhs_ty match_result
744 extractMatchResult match_result' fail_expr
747 matchSinglePat :: CoreExpr -> HsMatchContext Name -> LPat Id
748 -> Type -> MatchResult -> DsM MatchResult
749 matchSinglePat (Var var) hs_ctx (L _ pat) ty match_result = do
754 | dopt Opt_WarnSimplePatterns dflags = matchCheck ds_ctx
757 ds_ctx = DsMatchContext hs_ctx locn
758 match_fn dflags [var] ty [EqnInfo { eqn_pats = [pat], eqn_rhs = match_result }]
760 matchSinglePat scrut hs_ctx pat ty match_result = do
761 var <- selectSimpleMatchVarL pat
762 match_result' <- matchSinglePat (Var var) hs_ctx pat ty match_result
763 return (adjustMatchResult (bindNonRec var scrut) match_result')
767 %************************************************************************
769 Pattern classification
771 %************************************************************************
775 = PgAny -- Immediate match: variables, wildcards,
777 | PgCon DataCon -- Constructor patterns (incl list, tuple)
778 | PgLit Literal -- Literal patterns
779 | PgN Literal -- Overloaded literals
780 | PgNpK Literal -- n+k patterns
781 | PgBang -- Bang patterns
782 | PgCo Type -- Coercion patterns; the type is the type
783 -- of the pattern *inside*
784 | PgView (LHsExpr Id) -- view pattern (e -> p):
785 -- the LHsExpr is the expression e
786 Type -- the Type is the type of p (equivalently, the result type of e)
788 groupEquations :: [EquationInfo] -> [[(PatGroup, EquationInfo)]]
789 -- If the result is of form [g1, g2, g3],
790 -- (a) all the (pg,eq) pairs in g1 have the same pg
791 -- (b) none of the gi are empty
792 -- The ordering of equations is unchanged
794 = runs same_gp [(patGroup (firstPat eqn), eqn) | eqn <- eqns]
796 same_gp :: (PatGroup,EquationInfo) -> (PatGroup,EquationInfo) -> Bool
797 (pg1,_) `same_gp` (pg2,_) = pg1 `sameGroup` pg2
799 subGroup :: Ord a => [(a, EquationInfo)] -> [[EquationInfo]]
800 -- Input is a particular group. The result sub-groups the
801 -- equations by with particular constructor, literal etc they match.
802 -- Each sub-list in the result has the same PatGroup
803 -- See Note [Take care with pattern order]
805 = map reverse $ eltsFM $ foldl accumulate emptyFM group
807 accumulate pg_map (pg, eqn)
808 = case lookupFM pg_map pg of
809 Just eqns -> addToFM pg_map pg (eqn:eqns)
810 Nothing -> addToFM pg_map pg [eqn]
812 -- pg_map :: FiniteMap a [EquationInfo]
813 -- Equations seen so far in reverse order of appearance
816 Note [Take care with pattern order]
817 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
818 In the subGroup function we must be very careful about pattern re-ordering,
819 Consider the patterns [ (True, Nothing), (False, x), (True, y) ]
820 Then in bringing together the patterns for True, we must not
821 swap the Nothing and y!
825 sameGroup :: PatGroup -> PatGroup -> Bool
826 -- Same group means that a single case expression
827 -- or test will suffice to match both, *and* the order
828 -- of testing within the group is insignificant.
829 sameGroup PgAny PgAny = True
830 sameGroup PgBang PgBang = True
831 sameGroup (PgCon _) (PgCon _) = True -- One case expression
832 sameGroup (PgLit _) (PgLit _) = True -- One case expression
833 sameGroup (PgN l1) (PgN l2) = l1==l2 -- Order is significant
834 sameGroup (PgNpK l1) (PgNpK l2) = l1==l2 -- See Note [Grouping overloaded literal patterns]
835 sameGroup (PgCo t1) (PgCo t2) = t1 `coreEqType` t2
836 -- CoPats are in the same goup only if the type of the
837 -- enclosed pattern is the same. The patterns outside the CoPat
838 -- always have the same type, so this boils down to saying that
839 -- the two coercions are identical.
840 sameGroup (PgView e1 t1) (PgView e2 t2) = viewLExprEq (e1,t1) (e2,t2)
841 -- ViewPats are in the same gorup iff the expressions
842 -- are "equal"---conservatively, we use syntactic equality
843 sameGroup _ _ = False
845 -- An approximation of syntactic equality used for determining when view
846 -- exprs are in the same group.
847 -- This function can always safely return false;
848 -- but doing so will result in the application of the view function being repeated.
850 -- Currently: compare applications of literals and variables
851 -- and anything else that we can do without involving other
852 -- HsSyn types in the recursion
854 -- NB we can't assume that the two view expressions have the same type. Consider
855 -- f (e1 -> True) = ...
856 -- f (e2 -> "hi") = ...
857 viewLExprEq :: (LHsExpr Id,Type) -> (LHsExpr Id,Type) -> Bool
858 viewLExprEq (e1,_) (e2,_) =
860 -- short name for recursive call on unLoc
861 lexp e e' = exp (unLoc e) (unLoc e')
863 eq_list :: (a->a->Bool) -> [a] -> [a] -> Bool
864 eq_list _ [] [] = True
865 eq_list _ [] (_:_) = False
866 eq_list _ (_:_) [] = False
867 eq_list eq (x:xs) (y:ys) = eq x y && eq_list eq xs ys
869 -- conservative, in that it demands that wrappers be
870 -- syntactically identical and doesn't look under binders
872 -- coarser notions of equality are possible
873 -- (e.g., reassociating compositions,
874 -- equating different ways of writing a coercion)
875 wrap WpHole WpHole = True
876 wrap (WpCompose w1 w2) (WpCompose w1' w2') = wrap w1 w1' && wrap w2 w2'
877 wrap (WpCast c) (WpCast c') = tcEqType c c'
878 wrap (WpApp d) (WpApp d') = d == d'
879 wrap (WpTyApp t) (WpTyApp t') = tcEqType t t'
880 -- Enhancement: could implement equality for more wrappers
881 -- if it seems useful (lams and lets)
884 -- real comparison is on HsExpr's
886 exp (HsPar (L _ e)) e' = exp e e'
887 exp e (HsPar (L _ e')) = exp e e'
888 -- because the expressions do not necessarily have the same type,
889 -- we have to compare the wrappers
890 exp (HsWrap h e) (HsWrap h' e') = wrap h h' && exp e e'
891 exp (HsVar i) (HsVar i') = i == i'
892 -- the instance for IPName derives using the id, so this works if the
894 exp (HsIPVar i) (HsIPVar i') = i == i'
895 exp (HsOverLit l) (HsOverLit l') =
896 -- Overloaded lits are equal if they have the same type
897 -- and the data is the same.
898 -- this is coarser than comparing the SyntaxExpr's in l and l',
899 -- which resolve the overloading (e.g., fromInteger 1),
900 -- because these expressions get written as a bunch of different variables
901 -- (presumably to improve sharing)
902 tcEqType (overLitType l) (overLitType l') && l == l'
903 exp (HsApp e1 e2) (HsApp e1' e2') = lexp e1 e1' && lexp e2 e2'
904 -- the fixities have been straightened out by now, so it's safe
906 exp (OpApp l o _ ri) (OpApp l' o' _ ri') =
907 lexp l l' && lexp o o' && lexp ri ri'
908 exp (NegApp e n) (NegApp e' n') = lexp e e' && exp n n'
909 exp (SectionL e1 e2) (SectionL e1' e2') =
910 lexp e1 e1' && lexp e2 e2'
911 exp (SectionR e1 e2) (SectionR e1' e2') =
912 lexp e1 e1' && lexp e2 e2'
913 exp (ExplicitTuple es1 _) (ExplicitTuple es2 _) =
914 eq_list tup_arg es1 es2
915 exp (HsIf e e1 e2) (HsIf e' e1' e2') =
916 lexp e e' && lexp e1 e1' && lexp e2 e2'
918 -- Enhancement: could implement equality for more expressions
919 -- if it seems useful
920 -- But no need for HsLit, ExplicitList, ExplicitTuple,
921 -- because they cannot be functions
924 tup_arg (Present e1) (Present e2) = lexp e1 e2
925 tup_arg (Missing t1) (Missing t2) = tcEqType t1 t2
930 patGroup :: Pat Id -> PatGroup
931 patGroup (WildPat {}) = PgAny
932 patGroup (BangPat {}) = PgBang
933 patGroup (ConPatOut { pat_con = dc }) = PgCon (unLoc dc)
934 patGroup (LitPat lit) = PgLit (hsLitKey lit)
935 patGroup (NPat olit mb_neg _) = PgN (hsOverLitKey olit (isJust mb_neg))
936 patGroup (NPlusKPat _ olit _ _) = PgNpK (hsOverLitKey olit False)
937 patGroup (CoPat _ p _) = PgCo (hsPatType p) -- Type of innelexp pattern
938 patGroup (ViewPat expr p _) = PgView expr (hsPatType (unLoc p))
939 patGroup pat = pprPanic "patGroup" (ppr pat)
942 Note [Grouping overloaded literal patterns]
943 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
950 We can't group the first and third together, because the second may match
951 the same thing as the first. Same goes for *overloaded* literal patterns
955 If the first arg matches '1' but the second does not match 'True', we
956 cannot jump to the third equation! Because the same argument might
958 Hence we don't regard 1 and 2, or (n+1) and (n+2), as part of the same group.