2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[Main_match]{The @match@ function}
7 module Match ( match, matchWrapper, matchSimply, matchSinglePat ) where
9 #include "HsVersions.h"
11 import CmdLineOpts ( DynFlag(..), dopt )
13 import TcHsSyn ( hsPatType )
14 import Check ( check, ExhaustivePat )
16 import CoreUtils ( bindNonRec, exprType )
18 import DsBinds ( dsHsNestedBinds )
19 import DsGRHSs ( dsGRHSs )
21 import Id ( idName, idType, Id )
22 import DataCon ( dataConFieldLabels, dataConInstOrigArgTys, isVanillaDataCon )
23 import MatchCon ( matchConFamily )
24 import MatchLit ( matchLiterals, matchNPlusKPats, matchNPats, tidyLitPat, tidyNPat )
25 import PrelInfo ( pAT_ERROR_ID )
26 import TcType ( Type, tcTyConAppArgs )
27 import Type ( splitFunTysN )
28 import TysWiredIn ( consDataCon, mkTupleTy, mkListTy,
29 tupleCon, parrFakeCon, mkPArrTy )
30 import BasicTypes ( Boxity(..) )
31 import ListSetOps ( runs )
32 import SrcLoc ( noSrcSpan, noLoc, unLoc, Located(..) )
33 import Util ( lengthExceeds, notNull )
38 This function is a wrapper of @match@, it must be called from all the parts where
39 it was called match, but only substitutes the firs call, ....
40 if the associated flags are declared, warnings will be issued.
41 It can not be called matchWrapper because this name already exists :-(
46 matchCheck :: DsMatchContext
47 -> [Id] -- Vars rep'ing the exprs we're matching with
48 -> Type -- Type of the case expression
49 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
50 -> DsM MatchResult -- Desugared result!
52 matchCheck ctx vars ty qs
53 = getDOptsDs `thenDs` \ dflags ->
54 matchCheck_really dflags ctx vars ty qs
56 matchCheck_really dflags ctx vars ty qs
57 | incomplete && shadow =
58 dsShadowWarn ctx eqns_shadow `thenDs` \ () ->
59 dsIncompleteWarn ctx pats `thenDs` \ () ->
62 dsIncompleteWarn ctx pats `thenDs` \ () ->
65 dsShadowWarn ctx eqns_shadow `thenDs` \ () ->
69 where (pats, eqns_shadow) = check qs
70 incomplete = want_incomplete && (notNull pats)
71 want_incomplete = case ctx of
72 DsMatchContext RecUpd _ _ ->
73 dopt Opt_WarnIncompletePatternsRecUpd dflags
75 dopt Opt_WarnIncompletePatterns dflags
76 shadow = dopt Opt_WarnOverlappingPatterns dflags
77 && not (null eqns_shadow)
80 This variable shows the maximum number of lines of output generated for warnings.
81 It will limit the number of patterns/equations displayed to@ maximum_output@.
83 (ToDo: add command-line option?)
89 The next two functions create the warning message.
92 dsShadowWarn :: DsMatchContext -> [EquationInfo] -> DsM ()
93 dsShadowWarn ctx@(DsMatchContext kind _ _) qs = dsWarn warn
95 warn | qs `lengthExceeds` maximum_output
96 = pp_context ctx (ptext SLIT("are overlapped"))
97 (\ f -> vcat (map (ppr_eqn f kind) (take maximum_output qs)) $$
100 = pp_context ctx (ptext SLIT("are overlapped"))
101 (\ f -> vcat $ map (ppr_eqn f kind) qs)
104 dsIncompleteWarn :: DsMatchContext -> [ExhaustivePat] -> DsM ()
105 dsIncompleteWarn ctx@(DsMatchContext kind _ _) pats = dsWarn warn
107 warn = pp_context ctx (ptext SLIT("are non-exhaustive"))
108 (\f -> hang (ptext SLIT("Patterns not matched:"))
109 4 ((vcat $ map (ppr_incomplete_pats kind)
110 (take maximum_output pats))
113 dots | pats `lengthExceeds` maximum_output = ptext SLIT("...")
116 pp_context NoMatchContext msg rest_of_msg_fun
117 = (noSrcSpan, ptext SLIT("Some match(es)") <+> hang msg 8 (rest_of_msg_fun id))
119 pp_context (DsMatchContext kind pats loc) msg rest_of_msg_fun
120 = (loc, vcat [ptext SLIT("Pattern match(es)") <+> msg,
121 sep [ptext SLIT("In") <+> ppr_match <> char ':', nest 4 (rest_of_msg_fun pref)]])
125 FunRhs fun -> (pprMatchContext kind, \ pp -> ppr fun <+> pp)
126 other -> (pprMatchContext kind, \ pp -> pp)
128 ppr_pats pats = sep (map ppr pats)
130 ppr_shadow_pats kind pats
131 = sep [ppr_pats pats, matchSeparator kind, ptext SLIT("...")]
133 ppr_incomplete_pats kind (pats,[]) = ppr_pats pats
134 ppr_incomplete_pats kind (pats,constraints) =
135 sep [ppr_pats pats, ptext SLIT("with"),
136 sep (map ppr_constraint constraints)]
139 ppr_constraint (var,pats) = sep [ppr var, ptext SLIT("`notElem`"), ppr pats]
141 ppr_eqn prefixF kind eqn = prefixF (ppr_shadow_pats kind (eqn_pats eqn))
145 The function @match@ is basically the same as in the Wadler chapter,
146 except it is monadised, to carry around the name supply, info about
149 Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns:
152 A list of $n$ variable names, those variables presumably bound to the
153 $n$ expressions being matched against the $n$ patterns. Using the
154 list of $n$ expressions as the first argument showed no benefit and
158 The second argument, a list giving the ``equation info'' for each of
162 the $n$ patterns for that equation, and
164 a list of Core bindings [@(Id, CoreExpr)@ pairs] to be ``stuck on
165 the front'' of the matching code, as in:
171 and finally: (ToDo: fill in)
173 The right way to think about the ``after-match function'' is that it
174 is an embryonic @CoreExpr@ with a ``hole'' at the end for the
175 final ``else expression''.
178 There is a type synonym, @EquationInfo@, defined in module @DsUtils@.
180 An experiment with re-ordering this information about equations (in
181 particular, having the patterns available in column-major order)
185 A default expression---what to evaluate if the overall pattern-match
186 fails. This expression will (almost?) always be
187 a measly expression @Var@, unless we know it will only be used once
188 (as we do in @glue_success_exprs@).
190 Leaving out this third argument to @match@ (and slamming in lots of
191 @Var "fail"@s) is a positively {\em bad} idea, because it makes it
192 impossible to share the default expressions. (Also, it stands no
193 chance of working in our post-upheaval world of @Locals@.)
195 So, the full type signature:
197 match :: [Id] -- Variables rep'ing the exprs we're matching with
198 -> Type -- Type of the case expression
199 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
200 -> DsM MatchResult -- Desugared result!
203 Note: @match@ is often called via @matchWrapper@ (end of this module),
204 a function that does much of the house-keeping that goes with a call
207 It is also worth mentioning the {\em typical} way a block of equations
208 is desugared with @match@. At each stage, it is the first column of
209 patterns that is examined. The steps carried out are roughly:
212 Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add
213 bindings to the second component of the equation-info):
216 Remove the `as' patterns from column~1.
218 Make all constructor patterns in column~1 into @ConPats@, notably
219 @ListPats@ and @TuplePats@.
221 Handle any irrefutable (or ``twiddle'') @LazyPats@.
224 Now {\em unmix} the equations into {\em blocks} [w/ local function
225 @unmix_eqns@], in which the equations in a block all have variable
226 patterns in column~1, or they all have constructor patterns in ...
227 (see ``the mixture rule'' in SLPJ).
229 Call @matchEqnBlock@ on each block of equations; it will do the
230 appropriate thing for each kind of column-1 pattern, usually ending up
231 in a recursive call to @match@.
234 %************************************************************************
236 %* match: empty rule *
238 %************************************************************************
239 \subsection[Match-empty-rule]{The ``empty rule''}
241 We are a little more paranoid about the ``empty rule'' (SLPJ, p.~87)
242 than the Wadler-chapter code for @match@ (p.~93, first @match@ clause).
243 And gluing the ``success expressions'' together isn't quite so pretty.
246 match [] ty eqns_info
247 = ASSERT( not (null eqns_info) )
248 returnDs (foldr1 combineMatchResults match_results)
250 match_results = [ ASSERT( null (eqn_pats eqn) )
251 adjustMatchResult (eqn_wrap eqn) (eqn_rhs eqn)
256 %************************************************************************
258 %* match: non-empty rule *
260 %************************************************************************
261 \subsection[Match-nonempty]{@match@ when non-empty: unmixing}
263 This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@
264 (a)~to get `as'- and `twiddle'-patterns out of the way (tidying), and
265 (b)~to do ``the mixture rule'' (SLPJ, p.~88) [which really {\em
266 un}mixes the equations], producing a list of equation-info
267 blocks, each block having as its first column of patterns either all
268 constructors, or all variables (or similar beasts), etc.
270 @match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the
271 Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@
272 corresponds roughly to @matchVarCon@.
275 match vars@(v:_) ty eqns_info
276 = do { tidy_eqns <- mappM (tidyEqnInfo v) eqns_info
277 ; let eqns_blks = runs same_family tidy_eqns
278 ; match_results <- mappM match_block eqns_blks
279 ; ASSERT( not (null match_results) )
280 return (foldr1 combineMatchResults match_results) }
282 same_family eqn1 eqn2
283 = samePatFamily (firstPat eqn1) (firstPat eqn2)
286 = case firstPat (head eqns) of
287 WildPat {} -> matchVariables vars ty eqns
288 ConPatOut {} -> matchConFamily vars ty eqns
289 NPlusKPatOut {} -> matchNPlusKPats vars ty eqns
290 NPatOut {} -> matchNPats vars ty eqns
291 LitPat {} -> matchLiterals vars ty eqns
293 -- After tidying, there are only five kinds of patterns
294 samePatFamily (WildPat {}) (WildPat {}) = True
295 samePatFamily (ConPatOut {}) (ConPatOut {}) = True
296 samePatFamily (NPlusKPatOut {}) (NPlusKPatOut {}) = True
297 samePatFamily (NPatOut {}) (NPatOut {}) = True
298 samePatFamily (LitPat {}) (LitPat {}) = True
299 samePatFamily _ _ = False
301 matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
302 -- Real true variables, just like in matchVar, SLPJ p 94
303 -- No binding to do: they'll all be wildcards by now (done in tidy)
304 matchVariables (var:vars) ty eqns = match vars ty (shiftEqns eqns)
310 Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@
311 which will be scrutinised. This means:
314 Replace variable patterns @x@ (@x /= v@) with the pattern @_@,
315 together with the binding @x = v@.
317 Replace the `as' pattern @x@@p@ with the pattern p and a binding @x = v@.
319 Removing lazy (irrefutable) patterns (you don't want to know...).
321 Converting explicit tuple-, list-, and parallel-array-pats into ordinary
324 Convert the literal pat "" to [].
327 The result of this tidying is that the column of patterns will include
331 The @VarPat@ information isn't needed any more after this.
334 @ListPats@, @TuplePats@, etc., are all converted into @ConPats@.
336 \item[@LitPats@ and @NPats@:]
337 @LitPats@/@NPats@ of ``known friendly types'' (Int, Char,
338 Float, Double, at least) are converted to unboxed form; e.g.,
339 \tr{(NPat (HsInt i) _ _)} is converted to:
341 (ConPat I# _ _ [LitPat (HsIntPrim i)])
346 tidyEqnInfo :: Id -> EquationInfo -> DsM EquationInfo
347 -- DsM'd because of internal call to dsHsNestedBinds
348 -- and mkSelectorBinds.
349 -- "tidy1" does the interesting stuff, looking at
350 -- one pattern and fiddling the list of bindings.
352 -- POST CONDITION: head pattern in the EqnInfo is
360 tidyEqnInfo v eqn@(EqnInfo { eqn_wrap = wrap, eqn_pats = pat : pats })
361 = tidy1 v wrap pat `thenDs` \ (wrap', pat') ->
362 returnDs (eqn { eqn_wrap = wrap', eqn_pats = pat' : pats })
364 tidy1 :: Id -- The Id being scrutinised
365 -> DsWrapper -- Previous wrapping bindings
366 -> Pat Id -- The pattern against which it is to be matched
367 -> DsM (DsWrapper, -- Extra bindings around what to do afterwards
368 Pat Id) -- Equivalent pattern
370 -- The extra bindings etc are all wrapped around the RHS of the match
371 -- so they are only available when matching is complete. But that's ok
372 -- becuase, for example, in the pattern x@(...), the x can only be
373 -- used in the RHS, not in the nested pattern, nor subsquent patterns
375 -- However this does have an awkward consequence. The bindings in
376 -- a VarPatOut get wrapped around the result in right to left order,
377 -- rather than left to right. This only matters if one set of
378 -- bindings can mention things used in another, and that can happen
379 -- if we allow equality dictionary bindings of form d1=d2.
380 -- bindIInstsOfLocalFuns is now careful not to do this, but it's a wart.
381 -- (Without this care in bindInstsOfLocalFuns, compiling
382 -- Data.Generics.Schemes.hs fails in function everywhereBut.)
384 -------------------------------------------------------
385 -- (pat', mr') = tidy1 v pat mr
386 -- tidies the *outer level only* of pat, giving pat'
387 -- It eliminates many pattern forms (as-patterns, variable patterns,
388 -- list patterns, etc) yielding one of:
395 tidy1 v wrap (ParPat pat) = tidy1 v wrap (unLoc pat)
396 tidy1 v wrap (SigPatOut pat _) = tidy1 v wrap (unLoc pat)
397 tidy1 v wrap (WildPat ty) = returnDs (wrap, WildPat ty)
399 -- case v of { x -> mr[] }
400 -- = case v of { _ -> let x=v in mr[] }
401 tidy1 v wrap (VarPat var)
402 = returnDs (wrap . wrapBind var v, WildPat (idType var))
404 tidy1 v wrap (VarPatOut var binds)
405 = do { prs <- dsHsNestedBinds binds
406 ; return (wrap . wrapBind var v . mkDsLet (Rec prs),
407 WildPat (idType var)) }
409 -- case v of { x@p -> mr[] }
410 -- = case v of { p -> let x=v in mr[] }
411 tidy1 v wrap (AsPat (L _ var) pat)
412 = tidy1 v (wrap . wrapBind var v) (unLoc 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
426 tidy1 v wrap (LazyPat pat)
427 = do { v' <- newSysLocalDs (idType v)
428 ; sel_prs <- mkSelectorBinds pat (Var v)
429 ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs]
430 ; returnDs (wrap . wrapBind v' v . mkDsLets sel_binds,
431 WildPat (idType v)) }
433 -- re-express <con-something> as (ConPat ...) [directly]
435 tidy1 v wrap (ConPatOut (L loc con) ex_tvs dicts binds ps pat_ty)
436 = returnDs (wrap, ConPatOut (L loc con) ex_tvs dicts binds tidy_ps pat_ty)
438 tidy_ps = PrefixCon (tidy_con con pat_ty ps)
440 tidy1 v wrap (ListPat pats ty)
441 = returnDs (wrap, unLoc list_ConPat)
443 list_ty = mkListTy ty
444 list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] list_ty)
448 -- Introduce fake parallel array constructors to be able to handle parallel
449 -- arrays with the existing machinery for constructor pattern
450 tidy1 v wrap (PArrPat pats ty)
451 = returnDs (wrap, unLoc parrConPat)
454 parrConPat = mkPrefixConPat (parrFakeCon arity) pats (mkPArrTy ty)
456 tidy1 v wrap (TuplePat pats boxity)
457 = returnDs (wrap, unLoc tuple_ConPat)
460 tuple_ConPat = mkPrefixConPat (tupleCon boxity arity) pats
461 (mkTupleTy boxity arity (map hsPatType pats))
463 tidy1 v wrap (DictPat dicts methods)
464 = case num_of_d_and_ms of
465 0 -> tidy1 v wrap (TuplePat [] Boxed)
466 1 -> tidy1 v wrap (unLoc (head dict_and_method_pats))
467 _ -> tidy1 v wrap (TuplePat dict_and_method_pats Boxed)
469 num_of_d_and_ms = length dicts + length methods
470 dict_and_method_pats = map nlVarPat (dicts ++ methods)
472 -- LitPats: we *might* be able to replace these w/ a simpler form
473 tidy1 v wrap pat@(LitPat lit)
474 = returnDs (wrap, unLoc (tidyLitPat lit (noLoc pat)))
476 -- NPats: we *might* be able to replace these w/ a simpler form
477 tidy1 v wrap pat@(NPatOut lit lit_ty _)
478 = returnDs (wrap, unLoc (tidyNPat lit lit_ty (noLoc pat)))
480 -- and everything else goes through unchanged...
482 tidy1 v wrap non_interesting_pat
483 = returnDs (wrap, non_interesting_pat)
486 tidy_con data_con pat_ty (PrefixCon ps) = ps
487 tidy_con data_con pat_ty (InfixCon p1 p2) = [p1,p2]
488 tidy_con data_con pat_ty (RecCon rpats)
490 = -- Special case for C {}, which can be used for
491 -- a constructor that isn't declared to have
493 map (noLoc . WildPat) con_arg_tys'
496 = ASSERT( isVanillaDataCon data_con )
497 -- We're in a record case, so the data con must be vanilla
498 -- and hence no existentials to worry about
499 map mk_pat tagged_arg_tys
501 -- Boring stuff to find the arg-tys of the constructor
503 inst_tys = tcTyConAppArgs pat_ty -- Newtypes must be opaque
504 con_arg_tys' = dataConInstOrigArgTys data_con inst_tys
505 tagged_arg_tys = con_arg_tys' `zip` dataConFieldLabels data_con
507 -- mk_pat picks a WildPat of the appropriate type for absent fields,
508 -- and the specified pattern for present fields
509 mk_pat (arg_ty, lbl) =
510 case [ pat | (sel_id,pat) <- rpats, idName (unLoc sel_id) == lbl] of
511 (pat:pats) -> ASSERT( null pats ) pat
512 [] -> noLoc (WildPat arg_ty)
516 {\bf Previous @matchTwiddled@ stuff:}
518 Now we get to the only interesting part; note: there are choices for
519 translation [from Simon's notes]; translation~1:
526 s = case w of [s,t] -> s
527 t = case w of [s,t] -> t
531 Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple
532 evaluation of \tr{e}. An alternative translation (No.~2):
534 [ w = case e of [s,t] -> (s,t)
535 s = case w of (s,t) -> s
536 t = case w of (s,t) -> t
540 %************************************************************************
542 \subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing}
544 %************************************************************************
546 We might be able to optimise unmixing when confronted by
547 only-one-constructor-possible, of which tuples are the most notable
555 This definition would normally be unmixed into four equation blocks,
556 one per equation. But it could be unmixed into just one equation
557 block, because if the one equation matches (on the first column),
558 the others certainly will.
560 You have to be careful, though; the example
568 {\em must} be broken into two blocks at the line shown; otherwise, you
569 are forcing unnecessary evaluation. In any case, the top-left pattern
570 always gives the cue. You could then unmix blocks into groups of...
572 \item[all variables:]
574 \item[constructors or variables (mixed):]
575 Need to make sure the right names get bound for the variable patterns.
576 \item[literals or variables (mixed):]
577 Presumably just a variant on the constructor case (as it is now).
580 %************************************************************************
582 %* matchWrapper: a convenient way to call @match@ *
584 %************************************************************************
585 \subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@}
587 Calls to @match@ often involve similar (non-trivial) work; that work
588 is collected here, in @matchWrapper@. This function takes as
592 Typchecked @Matches@ (of a function definition, or a case or lambda
593 expression)---the main input;
595 An error message to be inserted into any (runtime) pattern-matching
599 As results, @matchWrapper@ produces:
602 A list of variables (@Locals@) that the caller must ``promise'' to
603 bind to appropriate values; and
605 a @CoreExpr@, the desugared output (main result).
608 The main actions of @matchWrapper@ include:
611 Flatten the @[TypecheckedMatch]@ into a suitable list of
614 Create as many new variables as there are patterns in a pattern-list
615 (in any one of the @EquationInfo@s).
617 Create a suitable ``if it fails'' expression---a call to @error@ using
618 the error-string input; the {\em type} of this fail value can be found
619 by examining one of the RHS expressions in one of the @EquationInfo@s.
621 Call @match@ with all of this information!
625 matchWrapper :: HsMatchContext Name -- For shadowing warning messages
626 -> MatchGroup Id -- Matches being desugared
627 -> DsM ([Id], CoreExpr) -- Results
630 There is one small problem with the Lambda Patterns, when somebody
631 writes something similar to:
635 he/she don't want a warning about incomplete patterns, that is done with
636 the flag @opt_WarnSimplePatterns@.
637 This problem also appears in the:
639 \item @do@ patterns, but if the @do@ can fail
640 it creates another equation if the match can fail
641 (see @DsExpr.doDo@ function)
642 \item @let@ patterns, are treated by @matchSimply@
643 List Comprension Patterns, are treated by @matchSimply@ also
646 We can't call @matchSimply@ with Lambda patterns,
647 due to the fact that lambda patterns can have more than
648 one pattern, and match simply only accepts one pattern.
653 matchWrapper ctxt (MatchGroup matches match_ty)
654 = do { eqns_info <- mapM mk_eqn_info matches
655 ; dflags <- getDOptsDs
656 ; locn <- getSrcSpanDs
657 ; let ds_ctxt = DsMatchContext ctxt arg_pats locn
658 error_string = matchContextErrString ctxt
660 ; new_vars <- selectMatchVars arg_pats pat_tys
661 ; match_result <- match_fun dflags ds_ctxt new_vars rhs_ty eqns_info
663 ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_string
664 ; result_expr <- extractMatchResult match_result fail_expr
665 ; return (new_vars, result_expr) }
667 arg_pats = map unLoc (hsLMatchPats (head matches))
668 n_pats = length arg_pats
669 (pat_tys, rhs_ty) = splitFunTysN n_pats match_ty
671 mk_eqn_info (L _ (Match pats _ grhss))
672 = do { let upats = map unLoc pats
673 ; match_result <- dsGRHSs ctxt upats grhss rhs_ty
674 ; return (EqnInfo { eqn_wrap = idWrapper,
676 eqn_rhs = match_result}) }
678 match_fun dflags ds_ctxt
680 LambdaExpr | dopt Opt_WarnSimplePatterns dflags -> matchCheck ds_ctxt
682 _ -> matchCheck ds_ctxt
685 %************************************************************************
687 \subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern}
689 %************************************************************************
691 @mkSimpleMatch@ is a wrapper for @match@ which deals with the
692 situation where we want to match a single expression against a single
693 pattern. It returns an expression.
696 matchSimply :: CoreExpr -- Scrutinee
697 -> HsMatchContext Name -- Match kind
698 -> LPat Id -- Pattern it should match
699 -> CoreExpr -- Return this if it matches
700 -> CoreExpr -- Return this if it doesn't
703 matchSimply scrut kind pat result_expr fail_expr
704 = getSrcSpanDs `thenDs` \ locn ->
706 ctx = DsMatchContext kind [unLoc pat] locn
707 match_result = cantFailMatchResult result_expr
708 rhs_ty = exprType fail_expr
709 -- Use exprType of fail_expr, because won't refine in the case of failure!
711 matchSinglePat scrut ctx pat rhs_ty match_result `thenDs` \ match_result' ->
712 extractMatchResult match_result' fail_expr
715 matchSinglePat :: CoreExpr -> DsMatchContext -> LPat Id
716 -> Type -> MatchResult -> DsM MatchResult
717 matchSinglePat (Var var) ctx pat ty match_result
718 = getDOptsDs `thenDs` \ dflags ->
719 match_fn dflags [var] ty [EqnInfo { eqn_wrap = idWrapper,
720 eqn_pats = [unLoc pat],
721 eqn_rhs = match_result }]
724 | dopt Opt_WarnSimplePatterns dflags = matchCheck ctx
727 matchSinglePat scrut ctx pat ty match_result
728 = selectSimpleMatchVarL pat `thenDs` \ var ->
729 matchSinglePat (Var var) ctx pat ty match_result `thenDs` \ match_result' ->
730 returnDs (adjustMatchResult (bindNonRec var scrut) match_result')