% % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[Main_match]{The @match@ function} \begin{code} module Match ( match, matchEquations, matchWrapper, matchSimply, matchSinglePat ) where #include "HsVersions.h" import DynFlags ( DynFlag(..), dopt ) import HsSyn import TcHsSyn ( mkVanillaTuplePat ) import Check ( check, ExhaustivePat ) import CoreSyn import CoreUtils ( bindNonRec, exprType ) import DsMonad import DsBinds ( dsLHsBinds ) import DsGRHSs ( dsGRHSs ) import DsUtils import Id ( idName, idType, Id ) import DataCon ( dataConFieldLabels, dataConInstOrigArgTys, isVanillaDataCon ) import MatchCon ( matchConFamily ) import MatchLit ( matchLiterals, matchNPlusKPats, matchNPats, tidyLitPat, tidyNPat ) import PrelInfo ( pAT_ERROR_ID ) import TcType ( Type, tcTyConAppArgs ) import Type ( splitFunTysN, mkTyVarTys ) import TysWiredIn ( consDataCon, mkListTy, unitTy, tupleCon, parrFakeCon, mkPArrTy ) import BasicTypes ( Boxity(..) ) import ListSetOps ( runs ) import SrcLoc ( noLoc, unLoc, Located(..) ) import Util ( lengthExceeds, notNull ) import Name ( Name ) import Outputable \end{code} This function is a wrapper of @match@, it must be called from all the parts where it was called match, but only substitutes the firs call, .... if the associated flags are declared, warnings will be issued. It can not be called matchWrapper because this name already exists :-( JJCQ 30-Nov-1997 \begin{code} matchCheck :: DsMatchContext -> [Id] -- Vars rep'ing the exprs we're matching with -> Type -- Type of the case expression -> [EquationInfo] -- Info about patterns, etc. (type synonym below) -> DsM MatchResult -- Desugared result! matchCheck ctx vars ty qs = getDOptsDs `thenDs` \ dflags -> matchCheck_really dflags ctx vars ty qs matchCheck_really dflags ctx vars ty qs | incomplete && shadow = dsShadowWarn ctx eqns_shadow `thenDs` \ () -> dsIncompleteWarn ctx pats `thenDs` \ () -> match vars ty qs | incomplete = dsIncompleteWarn ctx pats `thenDs` \ () -> match vars ty qs | shadow = dsShadowWarn ctx eqns_shadow `thenDs` \ () -> match vars ty qs | otherwise = match vars ty qs where (pats, eqns_shadow) = check qs incomplete = want_incomplete && (notNull pats) want_incomplete = case ctx of DsMatchContext RecUpd _ -> dopt Opt_WarnIncompletePatternsRecUpd dflags _ -> dopt Opt_WarnIncompletePatterns dflags shadow = dopt Opt_WarnOverlappingPatterns dflags && not (null eqns_shadow) \end{code} This variable shows the maximum number of lines of output generated for warnings. It will limit the number of patterns/equations displayed to@ maximum_output@. (ToDo: add command-line option?) \begin{code} maximum_output = 4 \end{code} The next two functions create the warning message. \begin{code} dsShadowWarn :: DsMatchContext -> [EquationInfo] -> DsM () dsShadowWarn ctx@(DsMatchContext kind loc) qs = putSrcSpanDs loc (dsWarn warn) where warn | qs `lengthExceeds` maximum_output = pp_context ctx (ptext SLIT("are overlapped")) (\ f -> vcat (map (ppr_eqn f kind) (take maximum_output qs)) $$ ptext SLIT("...")) | otherwise = pp_context ctx (ptext SLIT("are overlapped")) (\ f -> vcat $ map (ppr_eqn f kind) qs) dsIncompleteWarn :: DsMatchContext -> [ExhaustivePat] -> DsM () dsIncompleteWarn ctx@(DsMatchContext kind loc) pats = putSrcSpanDs loc (dsWarn warn) where warn = pp_context ctx (ptext SLIT("are non-exhaustive")) (\f -> hang (ptext SLIT("Patterns not matched:")) 4 ((vcat $ map (ppr_incomplete_pats kind) (take maximum_output pats)) $$ dots)) dots | pats `lengthExceeds` maximum_output = ptext SLIT("...") | otherwise = empty pp_context (DsMatchContext kind _loc) msg rest_of_msg_fun = vcat [ptext SLIT("Pattern match(es)") <+> msg, sep [ptext SLIT("In") <+> ppr_match <> char ':', nest 4 (rest_of_msg_fun pref)]] where (ppr_match, pref) = case kind of FunRhs fun -> (pprMatchContext kind, \ pp -> ppr fun <+> pp) other -> (pprMatchContext kind, \ pp -> pp) ppr_pats pats = sep (map ppr pats) ppr_shadow_pats kind pats = sep [ppr_pats pats, matchSeparator kind, ptext SLIT("...")] ppr_incomplete_pats kind (pats,[]) = ppr_pats pats ppr_incomplete_pats kind (pats,constraints) = sep [ppr_pats pats, ptext SLIT("with"), sep (map ppr_constraint constraints)] ppr_constraint (var,pats) = sep [ppr var, ptext SLIT("`notElem`"), ppr pats] ppr_eqn prefixF kind eqn = prefixF (ppr_shadow_pats kind (eqn_pats eqn)) \end{code} The function @match@ is basically the same as in the Wadler chapter, except it is monadised, to carry around the name supply, info about annotations, etc. Notes on @match@'s arguments, assuming $m$ equations and $n$ patterns: \begin{enumerate} \item A list of $n$ variable names, those variables presumably bound to the $n$ expressions being matched against the $n$ patterns. Using the list of $n$ expressions as the first argument showed no benefit and some inelegance. \item The second argument, a list giving the ``equation info'' for each of the $m$ equations: \begin{itemize} \item the $n$ patterns for that equation, and \item a list of Core bindings [@(Id, CoreExpr)@ pairs] to be ``stuck on the front'' of the matching code, as in: \begin{verbatim} let in \end{verbatim} \item and finally: (ToDo: fill in) The right way to think about the ``after-match function'' is that it is an embryonic @CoreExpr@ with a ``hole'' at the end for the final ``else expression''. \end{itemize} There is a type synonym, @EquationInfo@, defined in module @DsUtils@. An experiment with re-ordering this information about equations (in particular, having the patterns available in column-major order) showed no benefit. \item A default expression---what to evaluate if the overall pattern-match fails. This expression will (almost?) always be a measly expression @Var@, unless we know it will only be used once (as we do in @glue_success_exprs@). Leaving out this third argument to @match@ (and slamming in lots of @Var "fail"@s) is a positively {\em bad} idea, because it makes it impossible to share the default expressions. (Also, it stands no chance of working in our post-upheaval world of @Locals@.) \end{enumerate} So, the full type signature: \begin{code} match :: [Id] -- Variables rep'ing the exprs we're matching with -> Type -- Type of the case expression -> [EquationInfo] -- Info about patterns, etc. (type synonym below) -> DsM MatchResult -- Desugared result! \end{code} Note: @match@ is often called via @matchWrapper@ (end of this module), a function that does much of the house-keeping that goes with a call to @match@. It is also worth mentioning the {\em typical} way a block of equations is desugared with @match@. At each stage, it is the first column of patterns that is examined. The steps carried out are roughly: \begin{enumerate} \item Tidy the patterns in column~1 with @tidyEqnInfo@ (this may add bindings to the second component of the equation-info): \begin{itemize} \item Remove the `as' patterns from column~1. \item Make all constructor patterns in column~1 into @ConPats@, notably @ListPats@ and @TuplePats@. \item Handle any irrefutable (or ``twiddle'') @LazyPats@. \end{itemize} \item Now {\em unmix} the equations into {\em blocks} [w/ local function @unmix_eqns@], in which the equations in a block all have variable patterns in column~1, or they all have constructor patterns in ... (see ``the mixture rule'' in SLPJ). \item Call @matchEqnBlock@ on each block of equations; it will do the appropriate thing for each kind of column-1 pattern, usually ending up in a recursive call to @match@. \end{enumerate} %************************************************************************ %* * %* match: empty rule * %* * %************************************************************************ \subsection[Match-empty-rule]{The ``empty rule''} We are a little more paranoid about the ``empty rule'' (SLPJ, p.~87) than the Wadler-chapter code for @match@ (p.~93, first @match@ clause). And gluing the ``success expressions'' together isn't quite so pretty. \begin{code} match [] ty eqns_info = ASSERT( not (null eqns_info) ) returnDs (foldr1 combineMatchResults match_results) where match_results = [ ASSERT( null (eqn_pats eqn) ) adjustMatchResult (eqn_wrap eqn) (eqn_rhs eqn) | eqn <- eqns_info ] \end{code} %************************************************************************ %* * %* match: non-empty rule * %* * %************************************************************************ \subsection[Match-nonempty]{@match@ when non-empty: unmixing} This (more interesting) clause of @match@ uses @tidy_and_unmix_eqns@ (a)~to get `as'- and `twiddle'-patterns out of the way (tidying), and (b)~to do ``the mixture rule'' (SLPJ, p.~88) [which really {\em un}mixes the equations], producing a list of equation-info blocks, each block having as its first column of patterns either all constructors, or all variables (or similar beasts), etc. @match_unmixed_eqn_blks@ simply takes the place of the @foldr@ in the Wadler-chapter @match@ (p.~93, last clause), and @match_unmixed_blk@ corresponds roughly to @matchVarCon@. \begin{code} match vars@(v:_) ty eqns_info = do { tidy_eqns <- mappM (tidyEqnInfo v) eqns_info ; let eqns_blks = runs same_family tidy_eqns ; match_results <- mappM match_block eqns_blks ; ASSERT( not (null match_results) ) return (foldr1 combineMatchResults match_results) } where same_family eqn1 eqn2 = samePatFamily (firstPat eqn1) (firstPat eqn2) match_block eqns = case firstPat (head eqns) of WildPat {} -> matchVariables vars ty eqns ConPatOut {} -> matchConFamily vars ty eqns NPlusKPat {} -> matchNPlusKPats vars ty eqns NPat {} -> matchNPats vars ty eqns LitPat {} -> matchLiterals vars ty eqns -- After tidying, there are only five kinds of patterns samePatFamily (WildPat {}) (WildPat {}) = True samePatFamily (ConPatOut {}) (ConPatOut {}) = True samePatFamily (NPlusKPat {}) (NPlusKPat {}) = True samePatFamily (NPat {}) (NPat {}) = True samePatFamily (LitPat {}) (LitPat {}) = True samePatFamily _ _ = False matchVariables :: [Id] -> Type -> [EquationInfo] -> DsM MatchResult -- Real true variables, just like in matchVar, SLPJ p 94 -- No binding to do: they'll all be wildcards by now (done in tidy) matchVariables (var:vars) ty eqns = match vars ty (shiftEqns eqns) \end{code} \end{code} Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@ which will be scrutinised. This means: \begin{itemize} \item Replace variable patterns @x@ (@x /= v@) with the pattern @_@, together with the binding @x = v@. \item Replace the `as' pattern @x@@p@ with the pattern p and a binding @x = v@. \item Removing lazy (irrefutable) patterns (you don't want to know...). \item Converting explicit tuple-, list-, and parallel-array-pats into ordinary @ConPats@. \item Convert the literal pat "" to []. \end{itemize} The result of this tidying is that the column of patterns will include {\em only}: \begin{description} \item[@WildPats@:] The @VarPat@ information isn't needed any more after this. \item[@ConPats@:] @ListPats@, @TuplePats@, etc., are all converted into @ConPats@. \item[@LitPats@ and @NPats@:] @LitPats@/@NPats@ of ``known friendly types'' (Int, Char, Float, Double, at least) are converted to unboxed form; e.g., \tr{(NPat (HsInt i) _ _)} is converted to: \begin{verbatim} (ConPat I# _ _ [LitPat (HsIntPrim i)]) \end{verbatim} \end{description} \begin{code} tidyEqnInfo :: Id -> EquationInfo -> DsM EquationInfo -- DsM'd because of internal call to dsLHsBinds -- and mkSelectorBinds. -- "tidy1" does the interesting stuff, looking at -- one pattern and fiddling the list of bindings. -- -- POST CONDITION: head pattern in the EqnInfo is -- WildPat -- ConPat -- NPat -- LitPat -- NPlusKPat -- but no other tidyEqnInfo v eqn@(EqnInfo { eqn_wrap = wrap, eqn_pats = pat : pats }) = tidy1 v wrap pat `thenDs` \ (wrap', pat') -> returnDs (eqn { eqn_wrap = wrap', eqn_pats = pat' : pats }) tidy1 :: Id -- The Id being scrutinised -> DsWrapper -- Previous wrapping bindings -> Pat Id -- The pattern against which it is to be matched -> DsM (DsWrapper, -- Extra bindings around what to do afterwards Pat Id) -- Equivalent pattern -- The extra bindings etc are all wrapped around the RHS of the match -- so they are only available when matching is complete. But that's ok -- becuase, for example, in the pattern x@(...), the x can only be -- used in the RHS, not in the nested pattern, nor subsquent patterns -- -- However this does have an awkward consequence. The bindings in -- a VarPatOut get wrapped around the result in right to left order, -- rather than left to right. This only matters if one set of -- bindings can mention things used in another, and that can happen -- if we allow equality dictionary bindings of form d1=d2. -- bindIInstsOfLocalFuns is now careful not to do this, but it's a wart. -- (Without this care in bindInstsOfLocalFuns, compiling -- Data.Generics.Schemes.hs fails in function everywhereBut.) ------------------------------------------------------- -- (pat', mr') = tidy1 v pat mr -- tidies the *outer level only* of pat, giving pat' -- It eliminates many pattern forms (as-patterns, variable patterns, -- list patterns, etc) yielding one of: -- WildPat -- ConPatOut -- LitPat -- NPat -- NPlusKPat tidy1 v wrap (ParPat pat) = tidy1 v wrap (unLoc pat) tidy1 v wrap (SigPatOut pat _) = tidy1 v wrap (unLoc pat) tidy1 v wrap (WildPat ty) = returnDs (wrap, WildPat ty) -- case v of { x -> mr[] } -- = case v of { _ -> let x=v in mr[] } tidy1 v wrap (VarPat var) = returnDs (wrap . wrapBind var v, WildPat (idType var)) tidy1 v wrap (VarPatOut var binds) = do { prs <- dsLHsBinds binds ; return (wrap . wrapBind var v . mkDsLet (Rec prs), WildPat (idType var)) } -- case v of { x@p -> mr[] } -- = case v of { p -> let x=v in mr[] } tidy1 v wrap (AsPat (L _ var) pat) = tidy1 v (wrap . wrapBind var v) (unLoc pat) tidy1 v wrap (BangPat pat) = tidy1 v (wrap . seqVar v) (unLoc pat) {- now, here we handle lazy patterns: tidy1 v ~p bs = (v, v1 = case v of p -> v1 : v2 = case v of p -> v2 : ... : bs ) where the v_i's are the binders in the pattern. ToDo: in "v_i = ... -> v_i", are the v_i's really the same thing? The case expr for v_i is just: match [v] [(p, [], \ x -> Var v_i)] any_expr -} tidy1 v wrap (LazyPat pat) = do { v' <- newSysLocalDs (idType v) ; sel_prs <- mkSelectorBinds pat (Var v) ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs] ; returnDs (wrap . wrapBind v' v . mkDsLets sel_binds, WildPat (idType v)) } -- re-express as (ConPat ...) [directly] tidy1 v wrap (ConPatOut (L loc con) ex_tvs dicts binds ps pat_ty) = returnDs (wrap, ConPatOut (L loc con) ex_tvs dicts binds tidy_ps pat_ty) where tidy_ps = PrefixCon (tidy_con con ex_tvs pat_ty ps) tidy1 v wrap (ListPat pats ty) = returnDs (wrap, unLoc list_ConPat) where list_ty = mkListTy ty list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] list_ty) (mkNilPat list_ty) pats -- Introduce fake parallel array constructors to be able to handle parallel -- arrays with the existing machinery for constructor pattern tidy1 v wrap (PArrPat pats ty) = returnDs (wrap, unLoc parrConPat) where arity = length pats parrConPat = mkPrefixConPat (parrFakeCon arity) pats (mkPArrTy ty) tidy1 v wrap (TuplePat pats boxity ty) = returnDs (wrap, unLoc tuple_ConPat) where arity = length pats tuple_ConPat = mkPrefixConPat (tupleCon boxity arity) pats ty tidy1 v wrap (DictPat dicts methods) = case num_of_d_and_ms of 0 -> tidy1 v wrap (TuplePat [] Boxed unitTy) 1 -> tidy1 v wrap (unLoc (head dict_and_method_pats)) _ -> tidy1 v wrap (mkVanillaTuplePat dict_and_method_pats Boxed) where num_of_d_and_ms = length dicts + length methods dict_and_method_pats = map nlVarPat (dicts ++ methods) -- LitPats: we *might* be able to replace these w/ a simpler form tidy1 v wrap pat@(LitPat lit) = returnDs (wrap, unLoc (tidyLitPat lit (noLoc pat))) -- NPats: we *might* be able to replace these w/ a simpler form tidy1 v wrap pat@(NPat lit mb_neg _ lit_ty) = returnDs (wrap, unLoc (tidyNPat lit mb_neg lit_ty (noLoc pat))) -- and everything else goes through unchanged... tidy1 v wrap non_interesting_pat = returnDs (wrap, non_interesting_pat) tidy_con data_con ex_tvs pat_ty (PrefixCon ps) = ps tidy_con data_con ex_tvs pat_ty (InfixCon p1 p2) = [p1,p2] tidy_con data_con ex_tvs pat_ty (RecCon rpats) | null rpats = -- Special case for C {}, which can be used for -- a constructor that isn't declared to have -- fields at all map (noLoc . WildPat) con_arg_tys' | otherwise = map mk_pat tagged_arg_tys where -- Boring stuff to find the arg-tys of the constructor inst_tys | isVanillaDataCon data_con = tcTyConAppArgs pat_ty -- Newtypes must be opaque | otherwise = mkTyVarTys ex_tvs con_arg_tys' = dataConInstOrigArgTys data_con inst_tys tagged_arg_tys = con_arg_tys' `zip` dataConFieldLabels data_con -- mk_pat picks a WildPat of the appropriate type for absent fields, -- and the specified pattern for present fields mk_pat (arg_ty, lbl) = case [ pat | (sel_id,pat) <- rpats, idName (unLoc sel_id) == lbl] of (pat:pats) -> ASSERT( null pats ) pat [] -> noLoc (WildPat arg_ty) \end{code} \noindent {\bf Previous @matchTwiddled@ stuff:} Now we get to the only interesting part; note: there are choices for translation [from Simon's notes]; translation~1: \begin{verbatim} deTwiddle [s,t] e \end{verbatim} returns \begin{verbatim} [ w = e, s = case w of [s,t] -> s t = case w of [s,t] -> t ] \end{verbatim} Here \tr{w} is a fresh variable, and the \tr{w}-binding prevents multiple evaluation of \tr{e}. An alternative translation (No.~2): \begin{verbatim} [ w = case e of [s,t] -> (s,t) s = case w of (s,t) -> s t = case w of (s,t) -> t ] \end{verbatim} %************************************************************************ %* * \subsubsection[improved-unmixing]{UNIMPLEMENTED idea for improved unmixing} %* * %************************************************************************ We might be able to optimise unmixing when confronted by only-one-constructor-possible, of which tuples are the most notable examples. Consider: \begin{verbatim} f (a,b,c) ... = ... f d ... (e:f) = ... f (g,h,i) ... = ... f j ... = ... \end{verbatim} This definition would normally be unmixed into four equation blocks, one per equation. But it could be unmixed into just one equation block, because if the one equation matches (on the first column), the others certainly will. You have to be careful, though; the example \begin{verbatim} f j ... = ... ------------------- f (a,b,c) ... = ... f d ... (e:f) = ... f (g,h,i) ... = ... \end{verbatim} {\em must} be broken into two blocks at the line shown; otherwise, you are forcing unnecessary evaluation. In any case, the top-left pattern always gives the cue. You could then unmix blocks into groups of... \begin{description} \item[all variables:] As it is now. \item[constructors or variables (mixed):] Need to make sure the right names get bound for the variable patterns. \item[literals or variables (mixed):] Presumably just a variant on the constructor case (as it is now). \end{description} %************************************************************************ %* * %* matchWrapper: a convenient way to call @match@ * %* * %************************************************************************ \subsection[matchWrapper]{@matchWrapper@: a convenient interface to @match@} Calls to @match@ often involve similar (non-trivial) work; that work is collected here, in @matchWrapper@. This function takes as arguments: \begin{itemize} \item Typchecked @Matches@ (of a function definition, or a case or lambda expression)---the main input; \item An error message to be inserted into any (runtime) pattern-matching failure messages. \end{itemize} As results, @matchWrapper@ produces: \begin{itemize} \item A list of variables (@Locals@) that the caller must ``promise'' to bind to appropriate values; and \item a @CoreExpr@, the desugared output (main result). \end{itemize} The main actions of @matchWrapper@ include: \begin{enumerate} \item Flatten the @[TypecheckedMatch]@ into a suitable list of @EquationInfo@s. \item Create as many new variables as there are patterns in a pattern-list (in any one of the @EquationInfo@s). \item Create a suitable ``if it fails'' expression---a call to @error@ using the error-string input; the {\em type} of this fail value can be found by examining one of the RHS expressions in one of the @EquationInfo@s. \item Call @match@ with all of this information! \end{enumerate} \begin{code} matchWrapper :: HsMatchContext Name -- For shadowing warning messages -> MatchGroup Id -- Matches being desugared -> DsM ([Id], CoreExpr) -- Results \end{code} There is one small problem with the Lambda Patterns, when somebody writes something similar to: \begin{verbatim} (\ (x:xs) -> ...) \end{verbatim} he/she don't want a warning about incomplete patterns, that is done with the flag @opt_WarnSimplePatterns@. This problem also appears in the: \begin{itemize} \item @do@ patterns, but if the @do@ can fail it creates another equation if the match can fail (see @DsExpr.doDo@ function) \item @let@ patterns, are treated by @matchSimply@ List Comprension Patterns, are treated by @matchSimply@ also \end{itemize} We can't call @matchSimply@ with Lambda patterns, due to the fact that lambda patterns can have more than one pattern, and match simply only accepts one pattern. JJQC 30-Nov-1997 \begin{code} matchWrapper ctxt (MatchGroup matches match_ty) = do { eqns_info <- mapM mk_eqn_info matches ; new_vars <- selectMatchVars arg_pats pat_tys ; result_expr <- matchEquations ctxt new_vars eqns_info rhs_ty ; return (new_vars, result_expr) } where arg_pats = map unLoc (hsLMatchPats (head matches)) n_pats = length arg_pats (pat_tys, rhs_ty) = splitFunTysN n_pats match_ty mk_eqn_info (L _ (Match pats _ grhss)) = do { let upats = map unLoc pats ; match_result <- dsGRHSs ctxt upats grhss rhs_ty ; return (EqnInfo { eqn_wrap = idWrapper, eqn_pats = upats, eqn_rhs = match_result}) } matchEquations :: HsMatchContext Name -> [Id] -> [EquationInfo] -> Type -> DsM CoreExpr matchEquations ctxt vars eqns_info rhs_ty = do { dflags <- getDOptsDs ; locn <- getSrcSpanDs ; let ds_ctxt = DsMatchContext ctxt locn error_string = matchContextErrString ctxt ; match_result <- match_fun dflags ds_ctxt vars rhs_ty eqns_info ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_string ; extractMatchResult match_result fail_expr } where match_fun dflags ds_ctxt = case ctxt of LambdaExpr | dopt Opt_WarnSimplePatterns dflags -> matchCheck ds_ctxt | otherwise -> match _ -> matchCheck ds_ctxt \end{code} %************************************************************************ %* * \subsection[matchSimply]{@matchSimply@: match a single expression against a single pattern} %* * %************************************************************************ @mkSimpleMatch@ is a wrapper for @match@ which deals with the situation where we want to match a single expression against a single pattern. It returns an expression. \begin{code} matchSimply :: CoreExpr -- Scrutinee -> HsMatchContext Name -- Match kind -> LPat Id -- Pattern it should match -> CoreExpr -- Return this if it matches -> CoreExpr -- Return this if it doesn't -> DsM CoreExpr matchSimply scrut hs_ctx pat result_expr fail_expr = let match_result = cantFailMatchResult result_expr rhs_ty = exprType fail_expr -- Use exprType of fail_expr, because won't refine in the case of failure! in matchSinglePat scrut hs_ctx pat rhs_ty match_result `thenDs` \ match_result' -> extractMatchResult match_result' fail_expr matchSinglePat :: CoreExpr -> HsMatchContext Name -> LPat Id -> Type -> MatchResult -> DsM MatchResult matchSinglePat (Var var) hs_ctx (L _ pat) ty match_result = getDOptsDs `thenDs` \ dflags -> getSrcSpanDs `thenDs` \ locn -> let match_fn dflags | dopt Opt_WarnSimplePatterns dflags = matchCheck ds_ctx | otherwise = match where ds_ctx = DsMatchContext hs_ctx locn in match_fn dflags [var] ty [EqnInfo { eqn_wrap = idWrapper, eqn_pats = [pat], eqn_rhs = match_result }] matchSinglePat scrut hs_ctx pat ty match_result = selectSimpleMatchVarL pat `thenDs` \ var -> matchSinglePat (Var var) hs_ctx pat ty match_result `thenDs` \ match_result' -> returnDs (adjustMatchResult (bindNonRec var scrut) match_result') \end{code}