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 = dopt Opt_WarnIncompletePatterns dflags
72 shadow = dopt Opt_WarnOverlappingPatterns dflags
73 && not (null eqns_shadow)
76 This variable shows the maximum number of lines of output generated for warnings.
77 It will limit the number of patterns/equations displayed to@ maximum_output@.
79 (ToDo: add command-line option?)
85 The next two functions create the warning message.
88 dsShadowWarn :: DsMatchContext -> [EquationInfo] -> DsM ()
89 dsShadowWarn ctx@(DsMatchContext kind _ _) qs = dsWarn warn
91 warn | qs `lengthExceeds` maximum_output
92 = pp_context ctx (ptext SLIT("are overlapped"))
93 (\ f -> vcat (map (ppr_eqn f kind) (take maximum_output qs)) $$
96 = pp_context ctx (ptext SLIT("are overlapped"))
97 (\ f -> vcat $ map (ppr_eqn f kind) qs)
100 dsIncompleteWarn :: DsMatchContext -> [ExhaustivePat] -> DsM ()
101 dsIncompleteWarn ctx@(DsMatchContext kind _ _) pats = dsWarn warn
103 warn = pp_context ctx (ptext SLIT("are non-exhaustive"))
104 (\f -> hang (ptext SLIT("Patterns not matched:"))
105 4 ((vcat $ map (ppr_incomplete_pats kind)
106 (take maximum_output pats))
109 dots | pats `lengthExceeds` maximum_output = ptext SLIT("...")
112 pp_context NoMatchContext msg rest_of_msg_fun
113 = (noSrcSpan, ptext SLIT("Some match(es)") <+> hang msg 8 (rest_of_msg_fun id))
115 pp_context (DsMatchContext kind pats loc) msg rest_of_msg_fun
116 = (loc, vcat [ptext SLIT("Pattern match(es)") <+> msg,
117 sep [ptext SLIT("In") <+> ppr_match <> char ':', nest 4 (rest_of_msg_fun pref)]])
121 FunRhs fun -> (pprMatchContext kind, \ pp -> ppr fun <+> pp)
122 other -> (pprMatchContext kind, \ pp -> pp)
124 ppr_pats pats = sep (map ppr pats)
126 ppr_shadow_pats kind pats
127 = sep [ppr_pats pats, matchSeparator kind, ptext SLIT("...")]
129 ppr_incomplete_pats kind (pats,[]) = ppr_pats pats
130 ppr_incomplete_pats kind (pats,constraints) =
131 sep [ppr_pats pats, ptext SLIT("with"),
132 sep (map ppr_constraint constraints)]
135 ppr_constraint (var,pats) = sep [ppr var, ptext SLIT("`notElem`"), ppr pats]
137 ppr_eqn prefixF kind eqn = prefixF (ppr_shadow_pats kind (eqn_pats eqn))
141 The function @match@ is basically the same as in the Wadler chapter,
142 except it is monadised, to carry around the name supply, info about
145 Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns:
148 A list of $n$ variable names, those variables presumably bound to the
149 $n$ expressions being matched against the $n$ patterns. Using the
150 list of $n$ expressions as the first argument showed no benefit and
154 The second argument, a list giving the ``equation info'' for each of
158 the $n$ patterns for that equation, and
160 a list of Core bindings [@(Id, CoreExpr)@ pairs] to be ``stuck on
161 the front'' of the matching code, as in:
167 and finally: (ToDo: fill in)
169 The right way to think about the ``after-match function'' is that it
170 is an embryonic @CoreExpr@ with a ``hole'' at the end for the
171 final ``else expression''.
174 There is a type synonym, @EquationInfo@, defined in module @DsUtils@.
176 An experiment with re-ordering this information about equations (in
177 particular, having the patterns available in column-major order)
181 A default expression---what to evaluate if the overall pattern-match
182 fails. This expression will (almost?) always be
183 a measly expression @Var@, unless we know it will only be used once
184 (as we do in @glue_success_exprs@).
186 Leaving out this third argument to @match@ (and slamming in lots of
187 @Var "fail"@s) is a positively {\em bad} idea, because it makes it
188 impossible to share the default expressions. (Also, it stands no
189 chance of working in our post-upheaval world of @Locals@.)
191 So, the full type signature:
193 match :: [Id] -- Variables rep'ing the exprs we're matching with
194 -> Type -- Type of the case expression
195 -> [EquationInfo] -- Info about patterns, etc. (type synonym below)
196 -> DsM MatchResult -- Desugared result!
199 Note: @match@ is often called via @matchWrapper@ (end of this module),
200 a function that does much of the house-keeping that goes with a call
203 It is also worth mentioning the {\em typical} way a block of equations
204 is desugared with @match@. At each stage, it is the first column of
205 patterns that is examined. The steps carried out are roughly:
208 Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add
209 bindings to the second component of the equation-info):
212 Remove the `as' patterns from column~1.
214 Make all constructor patterns in column~1 into @ConPats@, notably
215 @ListPats@ and @TuplePats@.
217 Handle any irrefutable (or ``twiddle'') @LazyPats@.
220 Now {\em unmix} the equations into {\em blocks} [w/ local function
221 @unmix_eqns@], in which the equations in a block all have variable
222 patterns in column~1, or they all have constructor patterns in ...
223 (see ``the mixture rule'' in SLPJ).
225 Call @matchEqnBlock@ on each block of equations; it will do the
226 appropriate thing for each kind of column-1 pattern, usually ending up
227 in a recursive call to @match@.
230 %************************************************************************
232 %* match: empty rule *
234 %************************************************************************
235 \subsection[Match-empty-rule]{The ``empty rule''}
237 We are a little more paranoid about the ``empty rule'' (SLPJ, p.~87)
238 than the Wadler-chapter code for @match@ (p.~93, first @match@ clause).
239 And gluing the ``success expressions'' together isn't quite so pretty.
242 match [] ty eqns_info
243 = ASSERT( not (null eqns_info) )
244 returnDs (foldr1 combineMatchResults match_results)
246 match_results = [ ASSERT( null (eqn_pats eqn) )
252 %************************************************************************
254 %* match: non-empty rule *
256 %************************************************************************
257 \subsection[Match-nonempty]{@match@ when non-empty: unmixing}
259 This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@
260 (a)~to get `as'- and `twiddle'-patterns out of the way (tidying), and
261 (b)~to do ``the mixture rule'' (SLPJ, p.~88) [which really {\em
262 un}mixes the equations], producing a list of equation-info
263 blocks, each block having as its first column of patterns either all
264 constructors, or all variables (or similar beasts), etc.
266 @match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the
267 Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@
268 corresponds roughly to @matchVarCon@.
271 match vars@(v:_) ty eqns_info
272 = do { tidy_eqns <- mappM (tidyEqnInfo v) eqns_info
273 ; let eqns_blks = runs same_family tidy_eqns
274 ; match_results <- mappM match_block eqns_blks
275 ; ASSERT( not (null match_results) )
276 return (foldr1 combineMatchResults match_results) }
278 same_family eqn1 eqn2
279 = samePatFamily (firstPat eqn1) (firstPat eqn2)
282 = case firstPat (head eqns) of
283 WildPat {} -> matchVariables vars ty eqns
284 ConPatOut {} -> matchConFamily vars ty eqns
285 NPlusKPatOut {} -> matchNPlusKPats vars ty eqns
286 NPatOut {} -> matchNPats vars ty eqns
287 LitPat {} -> matchLiterals vars ty eqns
289 -- After tidying, there are only five kinds of patterns
290 samePatFamily (WildPat {}) (WildPat {}) = True
291 samePatFamily (ConPatOut {}) (ConPatOut {}) = True
292 samePatFamily (NPlusKPatOut {}) (NPlusKPatOut {}) = True
293 samePatFamily (NPatOut {}) (NPatOut {}) = True
294 samePatFamily (LitPat {}) (LitPat {}) = True
295 samePatFamily _ _ = False
297 matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult
298 -- Real true variables, just like in matchVar, SLPJ p 94
299 -- No binding to do: they'll all be wildcards by now (done in tidy)
300 matchVariables (var:vars) ty eqns = match vars ty (shiftEqns eqns)
306 Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@
307 which will be scrutinised. This means:
310 Replace variable patterns @x@ (@x /= v@) with the pattern @_@,
311 together with the binding @x = v@.
313 Replace the `as' pattern @x@@p@ with the pattern p and a binding @x = v@.
315 Removing lazy (irrefutable) patterns (you don't want to know...).
317 Converting explicit tuple-, list-, and parallel-array-pats into ordinary
320 Convert the literal pat "" to [].
323 The result of this tidying is that the column of patterns will include
327 The @VarPat@ information isn't needed any more after this.
330 @ListPats@, @TuplePats@, etc., are all converted into @ConPats@.
332 \item[@LitPats@ and @NPats@:]
333 @LitPats@/@NPats@ of ``known friendly types'' (Int, Char,
334 Float, Double, at least) are converted to unboxed form; e.g.,
335 \tr{(NPat (HsInt i) _ _)} is converted to:
337 (ConPat I# _ _ [LitPat (HsIntPrim i)])
342 tidyEqnInfo :: Id -> EquationInfo -> DsM EquationInfo
343 -- DsM'd because of internal call to dsHsNestedBinds
344 -- and mkSelectorBinds.
345 -- "tidy1" does the interesting stuff, looking at
346 -- one pattern and fiddling the list of bindings.
348 -- POST CONDITION: head pattern in the EqnInfo is
356 tidyEqnInfo v eqn@(EqnInfo { eqn_pats = pat : pats, eqn_rhs = rhs })
357 = tidy1 v pat rhs `thenDs` \ (pat', rhs') ->
358 returnDs (eqn { eqn_pats = pat' : pats, eqn_rhs = rhs' })
360 tidy1 :: Id -- The Id being scrutinised
361 -> Pat Id -- The pattern against which it is to be matched
362 -> MatchResult -- What to do afterwards
363 -> DsM (Pat Id, -- Equivalent pattern
364 MatchResult) -- Extra bindings around what to do afterwards
366 -- The extra bindings etc are all wrapped around the RHS of the match
367 -- so they are only available when matching is complete. But that's ok
368 -- becuase, for example, in the pattern x@(...), the x can only be
369 -- used in the RHS, not in the nested pattern, nor subsquent patterns
371 -- However this does have an awkward consequence. The bindings in
372 -- a VarPatOut get wrapped around the result in right to left order,
373 -- rather than left to right. This only matters if one set of
374 -- bindings can mention things used in another, and that can happen
375 -- if we allow equality dictionary bindings of form d1=d2.
376 -- bindIInstsOfLocalFuns is now careful not to do this, but it's a wart.
377 -- (Without this care in bindInstsOfLocalFuns, compiling
378 -- Data.Generics.Schemes.hs fails in function everywhereBut.)
380 -------------------------------------------------------
381 -- (pat', mr') = tidy1 v pat mr
382 -- tidies the *outer level only* of pat, giving pat'
383 -- It eliminates many pattern forms (as-patterns, variable patterns,
384 -- list patterns, etc) yielding one of:
391 tidy1 v (ParPat pat) wrap = tidy1 v (unLoc pat) wrap
392 tidy1 v (SigPatOut pat _) wrap = tidy1 v (unLoc pat) wrap
393 tidy1 v (WildPat ty) wrap = returnDs (WildPat ty, wrap)
395 -- case v of { x -> mr[] }
396 -- = case v of { _ -> let x=v in mr[] }
397 tidy1 v (VarPat var) rhs
398 = returnDs (WildPat (idType var), bindOneInMatchResult var v rhs)
400 tidy1 v (VarPatOut var binds) rhs
401 = do { prs <- dsHsNestedBinds binds
402 ; return (WildPat (idType var),
403 bindOneInMatchResult var v $
404 mkCoLetMatchResult (Rec prs) rhs) }
406 -- case v of { x@p -> mr[] }
407 -- = case v of { p -> let x=v in mr[] }
408 tidy1 v (AsPat (L _ var) pat) rhs
409 = tidy1 v (unLoc pat) (bindOneInMatchResult var v rhs)
412 {- now, here we handle lazy patterns:
413 tidy1 v ~p bs = (v, v1 = case v of p -> v1 :
414 v2 = case v of p -> v2 : ... : bs )
416 where the v_i's are the binders in the pattern.
418 ToDo: in "v_i = ... -> v_i", are the v_i's really the same thing?
420 The case expr for v_i is just: match [v] [(p, [], \ x -> Var v_i)] any_expr
423 tidy1 v (LazyPat pat) rhs
424 = do { v' <- newSysLocalDs (idType v)
425 ; sel_prs <- mkSelectorBinds pat (Var v)
426 ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs]
427 ; returnDs (WildPat (idType v),
428 bindOneInMatchResult v' v $
429 mkCoLetsMatchResult sel_binds rhs) }
431 -- re-express <con-something> as (ConPat ...) [directly]
433 tidy1 v (ConPatOut con ex_tvs dicts binds ps pat_ty) rhs
434 = returnDs (ConPatOut con ex_tvs dicts binds tidy_ps pat_ty, rhs)
436 tidy_ps = PrefixCon (tidy_con con pat_ty ps)
438 tidy1 v (ListPat pats ty) rhs
439 = returnDs (unLoc list_ConPat, rhs)
441 list_ty = mkListTy ty
442 list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] list_ty)
446 -- Introduce fake parallel array constructors to be able to handle parallel
447 -- arrays with the existing machinery for constructor pattern
448 tidy1 v (PArrPat pats ty) rhs
449 = returnDs (unLoc parrConPat, rhs)
452 parrConPat = mkPrefixConPat (parrFakeCon arity) pats (mkPArrTy ty)
454 tidy1 v (TuplePat pats boxity) rhs
455 = returnDs (unLoc tuple_ConPat, rhs)
458 tuple_ConPat = mkPrefixConPat (tupleCon boxity arity) pats
459 (mkTupleTy boxity arity (map hsPatType pats))
461 tidy1 v (DictPat dicts methods) rhs
462 = case num_of_d_and_ms of
463 0 -> tidy1 v (TuplePat [] Boxed) rhs
464 1 -> tidy1 v (unLoc (head dict_and_method_pats)) rhs
465 _ -> tidy1 v (TuplePat dict_and_method_pats Boxed) rhs
467 num_of_d_and_ms = length dicts + length methods
468 dict_and_method_pats = map nlVarPat (dicts ++ methods)
470 -- LitPats: we *might* be able to replace these w/ a simpler form
471 tidy1 v pat@(LitPat lit) rhs
472 = returnDs (unLoc (tidyLitPat lit (noLoc pat)), rhs)
474 -- NPats: we *might* be able to replace these w/ a simpler form
475 tidy1 v pat@(NPatOut lit lit_ty _) rhs
476 = returnDs (unLoc (tidyNPat lit lit_ty (noLoc pat)), rhs)
478 -- and everything else goes through unchanged...
480 tidy1 v non_interesting_pat rhs
481 = returnDs (non_interesting_pat, rhs)
484 tidy_con data_con pat_ty (PrefixCon ps) = ps
485 tidy_con data_con pat_ty (InfixCon p1 p2) = [p1,p2]
486 tidy_con data_con pat_ty (RecCon rpats)
488 = -- Special case for C {}, which can be used for
489 -- a constructor that isn't declared to have
491 map (noLoc . WildPat) con_arg_tys'
494 = ASSERT( isVanillaDataCon data_con )
495 -- We're in a record case, so the data con must be vanilla
496 -- and hence no existentials to worry about
497 map mk_pat tagged_arg_tys
499 -- Boring stuff to find the arg-tys of the constructor
501 inst_tys = tcTyConAppArgs pat_ty -- Newtypes must be opaque
502 con_arg_tys' = dataConInstOrigArgTys data_con inst_tys
503 tagged_arg_tys = con_arg_tys' `zip` dataConFieldLabels data_con
505 -- mk_pat picks a WildPat of the appropriate type for absent fields,
506 -- and the specified pattern for present fields
507 mk_pat (arg_ty, lbl) =
508 case [ pat | (sel_id,pat) <- rpats, idName (unLoc sel_id) == lbl] of
509 (pat:pats) -> ASSERT( null pats ) pat
510 [] -> noLoc (WildPat arg_ty)
514 {\bf Previous @matchTwiddled@ stuff:}
516 Now we get to the only interesting part; note: there are choices for
517 translation [from Simon's notes]; translation~1:
524 s = case w of [s,t] -> s
525 t = case w of [s,t] -> t
529 Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple
530 evaluation of \tr{e}. An alternative translation (No.~2):
532 [ w = case e of [s,t] -> (s,t)
533 s = case w of (s,t) -> s
534 t = case w of (s,t) -> t
538 %************************************************************************
540 \subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing}
542 %************************************************************************
544 We might be able to optimise unmixing when confronted by
545 only-one-constructor-possible, of which tuples are the most notable
553 This definition would normally be unmixed into four equation blocks,
554 one per equation. But it could be unmixed into just one equation
555 block, because if the one equation matches (on the first column),
556 the others certainly will.
558 You have to be careful, though; the example
566 {\em must} be broken into two blocks at the line shown; otherwise, you
567 are forcing unnecessary evaluation. In any case, the top-left pattern
568 always gives the cue. You could then unmix blocks into groups of...
570 \item[all variables:]
572 \item[constructors or variables (mixed):]
573 Need to make sure the right names get bound for the variable patterns.
574 \item[literals or variables (mixed):]
575 Presumably just a variant on the constructor case (as it is now).
578 %************************************************************************
580 %* matchWrapper: a convenient way to call @match@ *
582 %************************************************************************
583 \subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@}
585 Calls to @match@ often involve similar (non-trivial) work; that work
586 is collected here, in @matchWrapper@. This function takes as
590 Typchecked @Matches@ (of a function definition, or a case or lambda
591 expression)---the main input;
593 An error message to be inserted into any (runtime) pattern-matching
597 As results, @matchWrapper@ produces:
600 A list of variables (@Locals@) that the caller must ``promise'' to
601 bind to appropriate values; and
603 a @CoreExpr@, the desugared output (main result).
606 The main actions of @matchWrapper@ include:
609 Flatten the @[TypecheckedMatch]@ into a suitable list of
612 Create as many new variables as there are patterns in a pattern-list
613 (in any one of the @EquationInfo@s).
615 Create a suitable ``if it fails'' expression---a call to @error@ using
616 the error-string input; the {\em type} of this fail value can be found
617 by examining one of the RHS expressions in one of the @EquationInfo@s.
619 Call @match@ with all of this information!
623 matchWrapper :: HsMatchContext Name -- For shadowing warning messages
624 -> MatchGroup Id -- Matches being desugared
625 -> DsM ([Id], CoreExpr) -- Results
628 There is one small problem with the Lambda Patterns, when somebody
629 writes something similar to:
633 he/she don't want a warning about incomplete patterns, that is done with
634 the flag @opt_WarnSimplePatterns@.
635 This problem also appears in the:
637 \item @do@ patterns, but if the @do@ can fail
638 it creates another equation if the match can fail
639 (see @DsExpr.doDo@ function)
640 \item @let@ patterns, are treated by @matchSimply@
641 List Comprension Patterns, are treated by @matchSimply@ also
644 We can't call @matchSimply@ with Lambda patterns,
645 due to the fact that lambda patterns can have more than
646 one pattern, and match simply only accepts one pattern.
651 matchWrapper ctxt (MatchGroup matches match_ty)
652 = do { eqns_info <- mapM mk_eqn_info matches
653 ; dflags <- getDOptsDs
654 ; locn <- getSrcSpanDs
655 ; let ds_ctxt = DsMatchContext ctxt arg_pats locn
656 error_string = matchContextErrString ctxt
658 ; new_vars <- selectMatchVars arg_pats pat_tys
659 ; match_result <- match_fun dflags ds_ctxt new_vars rhs_ty eqns_info
661 ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_string
662 ; result_expr <- extractMatchResult match_result fail_expr
663 ; return (new_vars, result_expr) }
665 arg_pats = map unLoc (hsLMatchPats (head matches))
666 n_pats = length arg_pats
667 (pat_tys, rhs_ty) = splitFunTysN n_pats match_ty
669 mk_eqn_info (L _ (Match pats _ grhss))
670 = do { let upats = map unLoc pats
671 ; match_result <- dsGRHSs ctxt upats grhss rhs_ty
672 ; return (EqnInfo { eqn_pats = upats,
673 eqn_rhs = match_result}) }
675 match_fun dflags ds_ctxt
677 LambdaExpr | dopt Opt_WarnSimplePatterns dflags -> matchCheck ds_ctxt
679 _ -> matchCheck ds_ctxt
682 %************************************************************************
684 \subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern}
686 %************************************************************************
688 @mkSimpleMatch@ is a wrapper for @match@ which deals with the
689 situation where we want to match a single expression against a single
690 pattern. It returns an expression.
693 matchSimply :: CoreExpr -- Scrutinee
694 -> HsMatchContext Name -- Match kind
695 -> LPat Id -- Pattern it should match
696 -> CoreExpr -- Return this if it matches
697 -> CoreExpr -- Return this if it doesn't
700 matchSimply scrut kind pat result_expr fail_expr
701 = getSrcSpanDs `thenDs` \ locn ->
703 ctx = DsMatchContext kind [unLoc pat] locn
704 match_result = cantFailMatchResult result_expr
705 rhs_ty = exprType fail_expr
706 -- Use exprType of fail_expr, because won't refine in the case of failure!
708 matchSinglePat scrut ctx pat rhs_ty match_result `thenDs` \ match_result' ->
709 extractMatchResult match_result' fail_expr
712 matchSinglePat :: CoreExpr -> DsMatchContext -> LPat Id
713 -> Type -> MatchResult -> DsM MatchResult
714 matchSinglePat (Var var) ctx pat ty match_result
715 = getDOptsDs `thenDs` \ dflags ->
716 match_fn dflags [var] ty [EqnInfo { eqn_pats = [unLoc pat],
717 eqn_rhs = match_result }]
720 | dopt Opt_WarnSimplePatterns dflags = matchCheck ctx
723 matchSinglePat scrut ctx pat ty match_result
724 = selectSimpleMatchVarL pat `thenDs` \ var ->
725 matchSinglePat (Var var) ctx pat ty match_result `thenDs` \ match_result' ->
726 returnDs (adjustMatchResult (bindNonRec var scrut) match_result')