{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 The @match@ function -} {-# LANGUAGE CPP #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ViewPatterns #-} module Match ( match, matchEquations, matchWrapper, matchSimply , matchSinglePat, matchSinglePatVar ) where #include "HsVersions.h" import GhcPrelude import {-#SOURCE#-} DsExpr (dsLExpr, dsSyntaxExpr) import BasicTypes ( Origin(..) ) import DynFlags import GHC.Hs import TcHsSyn import TcEvidence import TcRnMonad import GHC.HsToCore.PmCheck import CoreSyn import Literal import CoreUtils import MkCore import DsMonad import DsBinds import DsGRHSs import DsUtils import Id import ConLike import DataCon import PatSyn import MatchCon import MatchLit import Type import Coercion ( eqCoercion ) import TyCon( isNewTyCon ) import TysWiredIn import SrcLoc import Maybes import Util import Name import Outputable import BasicTypes ( isGenerated, il_value, fl_value ) import FastString import Unique import UniqDFM import Control.Monad( when, unless ) import Data.List ( groupBy ) import qualified Data.Map as Map {- ************************************************************************ * * The main matching function * * ************************************************************************ The function @match@ is basically the same as in the Wadler chapter from "The Implementation of Functional Programming Languages", 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 <binds> in <matching-code> \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 data type, @EquationInfo@, defined in module @DsMonad@. 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} 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): \item Now {\em unmix} the equations into {\em blocks} [w\/ local function @match_groups@], in which the equations in a block all have the same match group. (see ``the mixture rule'' in SLPJ). \item Call the right match variant on each block of equations; it will do the appropriate thing for each kind of column-1 pattern. \end{enumerate} 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. This @match@ uses @tidyEqnInfo@ to get `as'- and `twiddle'-patterns out of the way (tidying), before applying ``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 patterns compatible with each other. Note [Match Ids] ~~~~~~~~~~~~~~~~ Most of the matching functions take an Id or [Id] as argument. This Id is the scrutinee(s) of the match. The desugared expression may sometimes use that Id in a local binding or as a case binder. So it should not have an External name; Lint rejects non-top-level binders with External names (#13043). See also Note [Localise pattern binders] in DsUtils -} type MatchId = Id -- See Note [Match Ids] match :: [MatchId] -- ^ Variables rep\'ing the exprs we\'re matching with -- ^ See Note [Match Ids] -> Type -- ^ Type of the case expression -> [EquationInfo] -- ^ Info about patterns, etc. (type synonym below) -> DsM MatchResult -- ^ Desugared result! match [] ty eqns = ASSERT2( not (null eqns), ppr ty ) return (foldr1 combineMatchResults match_results) where match_results = [ ASSERT( null (eqn_pats eqn) ) eqn_rhs eqn | eqn <- eqns ] match vars@(v:_) ty eqns -- Eqns *can* be empty = ASSERT2( all (isInternalName . idName) vars, ppr vars ) do { dflags <- getDynFlags -- Tidy the first pattern, generating -- auxiliary bindings if necessary ; (aux_binds, tidy_eqns) <- mapAndUnzipM (tidyEqnInfo v) eqns -- Group the equations and match each group in turn ; let grouped = groupEquations dflags tidy_eqns -- print the view patterns that are commoned up to help debug ; whenDOptM Opt_D_dump_view_pattern_commoning (debug grouped) ; match_results <- match_groups grouped ; return (adjustMatchResult (foldr (.) id aux_binds) $ foldr1 combineMatchResults match_results) } where dropGroup :: [(PatGroup,EquationInfo)] -> [EquationInfo] dropGroup = map snd match_groups :: [[(PatGroup,EquationInfo)]] -> DsM [MatchResult] -- Result list of [MatchResult] is always non-empty match_groups [] = matchEmpty v ty match_groups gs = mapM match_group gs match_group :: [(PatGroup,EquationInfo)] -> DsM MatchResult match_group [] = panic "match_group" match_group eqns@((group,_) : _) = case group of PgCon {} -> matchConFamily vars ty (subGroupUniq [(c,e) | (PgCon c, e) <- eqns]) PgSyn {} -> matchPatSyn vars ty (dropGroup eqns) PgLit {} -> matchLiterals vars ty (subGroupOrd [(l,e) | (PgLit l, e) <- eqns]) PgAny -> matchVariables vars ty (dropGroup eqns) PgN {} -> matchNPats vars ty (dropGroup eqns) PgOverS {}-> matchNPats vars ty (dropGroup eqns) PgNpK {} -> matchNPlusKPats vars ty (dropGroup eqns) PgBang -> matchBangs vars ty (dropGroup eqns) PgCo {} -> matchCoercion vars ty (dropGroup eqns) PgView {} -> matchView vars ty (dropGroup eqns) PgOverloadedList -> matchOverloadedList vars ty (dropGroup eqns) -- FIXME: we should also warn about view patterns that should be -- commoned up but are not -- print some stuff to see what's getting grouped -- use -dppr-debug to see the resolution of overloaded literals debug eqns = let gs = map (\group -> foldr (\ (p,_) -> \acc -> case p of PgView e _ -> e:acc _ -> acc) [] group) eqns maybeWarn [] = return () maybeWarn l = warnDs NoReason (vcat l) in maybeWarn $ (map (\g -> text "Putting these view expressions into the same case:" <+> (ppr g)) (filter (not . null) gs)) matchEmpty :: MatchId -> Type -> DsM [MatchResult] -- See Note [Empty case expressions] matchEmpty var res_ty = return [MatchResult CanFail mk_seq] where mk_seq fail = return $ mkWildCase (Var var) (idType var) res_ty [(DEFAULT, [], fail)] matchVariables :: [MatchId] -> 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 (_:vars) ty eqns = match vars ty (shiftEqns eqns) matchVariables [] _ _ = panic "matchVariables" matchBangs :: [MatchId] -> Type -> [EquationInfo] -> DsM MatchResult matchBangs (var:vars) ty eqns = do { match_result <- match (var:vars) ty $ map (decomposeFirstPat getBangPat) eqns ; return (mkEvalMatchResult var ty match_result) } matchBangs [] _ _ = panic "matchBangs" matchCoercion :: [MatchId] -> Type -> [EquationInfo] -> DsM MatchResult -- Apply the coercion to the match variable and then match that matchCoercion (var:vars) ty (eqns@(eqn1:_)) = do { let CoPat _ co pat _ = firstPat eqn1 ; let pat_ty' = hsPatType pat ; var' <- newUniqueId var pat_ty' ; match_result <- match (var':vars) ty $ map (decomposeFirstPat getCoPat) eqns ; core_wrap <- dsHsWrapper co ; let bind = NonRec var' (core_wrap (Var var)) ; return (mkCoLetMatchResult bind match_result) } matchCoercion _ _ _ = panic "matchCoercion" matchView :: [MatchId] -> Type -> [EquationInfo] -> DsM MatchResult -- Apply the view function to the match variable and then match that matchView (var:vars) ty (eqns@(eqn1:_)) = do { -- we could pass in the expr from the PgView, -- but this needs to extract the pat anyway -- to figure out the type of the fresh variable let ViewPat _ viewExpr (dL->L _ pat) = firstPat eqn1 -- do the rest of the compilation ; let pat_ty' = hsPatType pat ; var' <- newUniqueId var pat_ty' ; match_result <- match (var':vars) ty $ map (decomposeFirstPat getViewPat) eqns -- compile the view expressions ; viewExpr' <- dsLExpr viewExpr ; return (mkViewMatchResult var' (mkCoreAppDs (text "matchView") viewExpr' (Var var)) match_result) } matchView _ _ _ = panic "matchView" matchOverloadedList :: [MatchId] -> Type -> [EquationInfo] -> DsM MatchResult matchOverloadedList (var:vars) ty (eqns@(eqn1:_)) -- Since overloaded list patterns are treated as view patterns, -- the code is roughly the same as for matchView = do { let ListPat (ListPatTc elt_ty (Just (_,e))) _ = firstPat eqn1 ; var' <- newUniqueId var (mkListTy elt_ty) -- we construct the overall type by hand ; match_result <- match (var':vars) ty $ map (decomposeFirstPat getOLPat) eqns -- getOLPat builds the pattern inside as a non-overloaded version of the overloaded list pattern ; e' <- dsSyntaxExpr e [Var var] ; return (mkViewMatchResult var' e' match_result) } matchOverloadedList _ _ _ = panic "matchOverloadedList" -- decompose the first pattern and leave the rest alone decomposeFirstPat :: (Pat GhcTc -> Pat GhcTc) -> EquationInfo -> EquationInfo decomposeFirstPat extractpat (eqn@(EqnInfo { eqn_pats = pat : pats })) = eqn { eqn_pats = extractpat pat : pats} decomposeFirstPat _ _ = panic "decomposeFirstPat" getCoPat, getBangPat, getViewPat, getOLPat :: Pat GhcTc -> Pat GhcTc getCoPat (CoPat _ _ pat _) = pat getCoPat _ = panic "getCoPat" getBangPat (BangPat _ pat ) = unLoc pat getBangPat _ = panic "getBangPat" getViewPat (ViewPat _ _ pat) = unLoc pat getViewPat _ = panic "getViewPat" getOLPat (ListPat (ListPatTc ty (Just _)) pats) = ListPat (ListPatTc ty Nothing) pats getOLPat _ = panic "getOLPat" {- Note [Empty case alternatives] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The list of EquationInfo can be empty, arising from case x of {} or \case {} In that situation we desugar to case x of { _ -> error "pattern match failure" } The *desugarer* isn't certain whether there really should be no alternatives, so it adds a default case, as it always does. A later pass may remove it if it's inaccessible. (See also Note [Empty case alternatives] in CoreSyn.) We do *not* desugar simply to error "empty case" or some such, because 'x' might be bound to (error "hello"), in which case we want to see that "hello" exception, not (error "empty case"). See also Note [Case elimination: lifted case] in Simplify. ************************************************************************ * * Tidying patterns * * ************************************************************************ Tidy up the leftmost pattern in an @EquationInfo@, given the variable @v@ which will be scrutinised. This makes desugaring the pattern match simpler by transforming some of the patterns to simpler forms. (Tuples to Constructor Patterns) Among other things in the resulting Pattern: * Variables and irrefutable(lazy) patterns are replaced by Wildcards * As patterns are replaced by the patterns they wrap. The bindings created by the above patterns are put into the returned wrapper instead. This means a definition of the form: f x = rhs when called with v get's desugared to the equivalent of: let x = v in f _ = rhs The same principle holds for as patterns (@) and irrefutable/lazy patterns (~). In the case of irrefutable patterns the irrefutable pattern is pushed into the binding. Pattern Constructors which only represent syntactic sugar are converted into their desugared representation. This usually means converting them to Constructor patterns but for some depends on enabled extensions. (Eg OverloadedLists) GHC also tries to convert overloaded Literals into regular ones. The result of this tidying is that the column of patterns will include only these which can be assigned a PatternGroup (see patGroup). -} tidyEqnInfo :: Id -> EquationInfo -> DsM (DsWrapper, 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 -- one of these for which patGroup is defined. tidyEqnInfo _ (EqnInfo { eqn_pats = [] }) = panic "tidyEqnInfo" tidyEqnInfo v eqn@(EqnInfo { eqn_pats = pat : pats, eqn_orig = orig }) = do { (wrap, pat') <- tidy1 v orig pat ; return (wrap, eqn { eqn_pats = do pat' : pats }) } tidy1 :: Id -- The Id being scrutinised -> Origin -- Was this a pattern the user wrote? -> Pat GhcTc -- The pattern against which it is to be matched -> DsM (DsWrapper, -- Extra bindings to do before the match Pat GhcTc) -- Equivalent pattern ------------------------------------------------------- -- (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) and returns any created bindings in the wrapper. tidy1 v o (ParPat _ pat) = tidy1 v o (unLoc pat) tidy1 v o (SigPat _ pat _) = tidy1 v o (unLoc pat) tidy1 _ _ (WildPat ty) = return (idDsWrapper, WildPat ty) tidy1 v o (BangPat _ (dL->L l p)) = tidy_bang_pat v o l p -- case v of { x -> mr[] } -- = case v of { _ -> let x=v in mr[] } tidy1 v _ (VarPat _ (dL->L _ var)) = return (wrapBind var v, WildPat (idType var)) -- case v of { x@p -> mr[] } -- = case v of { p -> let x=v in mr[] } tidy1 v o (AsPat _ (dL->L _ var) pat) = do { (wrap, pat') <- tidy1 v o (unLoc pat) ; return (wrapBind var v . wrap, 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 _ (LazyPat _ pat) -- This is a convenient place to check for unlifted types under a lazy pattern. -- Doing this check during type-checking is unsatisfactory because we may -- not fully know the zonked types yet. We sure do here. = do { let unlifted_bndrs = filter (isUnliftedType . idType) (collectPatBinders pat) ; unless (null unlifted_bndrs) $ putSrcSpanDs (getLoc pat) $ errDs (hang (text "A lazy (~) pattern cannot bind variables of unlifted type." $$ text "Unlifted variables:") 2 (vcat (map (\id -> ppr id <+> dcolon <+> ppr (idType id)) unlifted_bndrs))) ; (_,sel_prs) <- mkSelectorBinds [] pat (Var v) ; let sel_binds = [NonRec b rhs | (b,rhs) <- sel_prs] ; return (mkCoreLets sel_binds, WildPat (idType v)) } tidy1 _ _ (ListPat (ListPatTc ty Nothing) pats ) = return (idDsWrapper, unLoc list_ConPat) where list_ConPat = foldr (\ x y -> mkPrefixConPat consDataCon [x, y] [ty]) (mkNilPat ty) pats tidy1 _ _ (TuplePat tys pats boxity) = return (idDsWrapper, unLoc tuple_ConPat) where arity = length pats tuple_ConPat = mkPrefixConPat (tupleDataCon boxity arity) pats tys tidy1 _ _ (SumPat tys pat alt arity) = return (idDsWrapper, unLoc sum_ConPat) where sum_ConPat = mkPrefixConPat (sumDataCon alt arity) [pat] tys -- LitPats: we *might* be able to replace these w/ a simpler form tidy1 _ o (LitPat _ lit) = do { unless (isGenerated o) $ warnAboutOverflowedLit lit ; return (idDsWrapper, tidyLitPat lit) } -- NPats: we *might* be able to replace these w/ a simpler form tidy1 _ o (NPat ty (dL->L _ lit@OverLit { ol_val = v }) mb_neg eq) = do { unless (isGenerated o) $ let lit' | Just _ <- mb_neg = lit{ ol_val = negateOverLitVal v } | otherwise = lit in warnAboutOverflowedOverLit lit' ; return (idDsWrapper, tidyNPat lit mb_neg eq ty) } -- NPlusKPat: we may want to warn about the literals tidy1 _ o n@(NPlusKPat _ _ (dL->L _ lit1) lit2 _ _) = do { unless (isGenerated o) $ do warnAboutOverflowedOverLit lit1 warnAboutOverflowedOverLit lit2 ; return (idDsWrapper, n) } -- Everything else goes through unchanged... tidy1 _ _ non_interesting_pat = return (idDsWrapper, non_interesting_pat) -------------------- tidy_bang_pat :: Id -> Origin -> SrcSpan -> Pat GhcTc -> DsM (DsWrapper, Pat GhcTc) -- Discard par/sig under a bang tidy_bang_pat v o _ (ParPat _ (dL->L l p)) = tidy_bang_pat v o l p tidy_bang_pat v o _ (SigPat _ (dL->L l p) _) = tidy_bang_pat v o l p -- Push the bang-pattern inwards, in the hope that -- it may disappear next time tidy_bang_pat v o l (AsPat x v' p) = tidy1 v o (AsPat x v' (cL l (BangPat noExtField p))) tidy_bang_pat v o l (CoPat x w p t) = tidy1 v o (CoPat x w (BangPat noExtField (cL l p)) t) -- Discard bang around strict pattern tidy_bang_pat v o _ p@(LitPat {}) = tidy1 v o p tidy_bang_pat v o _ p@(ListPat {}) = tidy1 v o p tidy_bang_pat v o _ p@(TuplePat {}) = tidy1 v o p tidy_bang_pat v o _ p@(SumPat {}) = tidy1 v o p -- Data/newtype constructors tidy_bang_pat v o l p@(ConPatOut { pat_con = (dL->L _ (RealDataCon dc)) , pat_args = args , pat_arg_tys = arg_tys }) -- Newtypes: push bang inwards (#9844) = if isNewTyCon (dataConTyCon dc) then tidy1 v o (p { pat_args = push_bang_into_newtype_arg l ty args }) else tidy1 v o p -- Data types: discard the bang where (ty:_) = dataConInstArgTys dc arg_tys ------------------- -- Default case, leave the bang there: -- VarPat, -- LazyPat, -- WildPat, -- ViewPat, -- pattern synonyms (ConPatOut with PatSynCon) -- NPat, -- NPlusKPat -- -- For LazyPat, remember that it's semantically like a VarPat -- i.e. !(~p) is not like ~p, or p! (#8952) -- -- NB: SigPatIn, ConPatIn should not happen tidy_bang_pat _ _ l p = return (idDsWrapper, BangPat noExtField (cL l p)) ------------------- push_bang_into_newtype_arg :: SrcSpan -> Type -- The type of the argument we are pushing -- onto -> HsConPatDetails GhcTc -> HsConPatDetails GhcTc -- See Note [Bang patterns and newtypes] -- We are transforming !(N p) into (N !p) push_bang_into_newtype_arg l _ty (PrefixCon (arg:args)) = ASSERT( null args) PrefixCon [cL l (BangPat noExtField arg)] push_bang_into_newtype_arg l _ty (RecCon rf) | HsRecFields { rec_flds = (dL->L lf fld) : flds } <- rf , HsRecField { hsRecFieldArg = arg } <- fld = ASSERT( null flds) RecCon (rf { rec_flds = [cL lf (fld { hsRecFieldArg = cL l (BangPat noExtField arg) })] }) push_bang_into_newtype_arg l ty (RecCon rf) -- If a user writes !(T {}) | HsRecFields { rec_flds = [] } <- rf = PrefixCon [cL l (BangPat noExtField (noLoc (WildPat ty)))] push_bang_into_newtype_arg _ _ cd = pprPanic "push_bang_into_newtype_arg" (pprConArgs cd) {- Note [Bang patterns and newtypes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For the pattern !(Just pat) we can discard the bang, because the pattern is strict anyway. But for !(N pat), where newtype NT = N Int we definitely can't discard the bang. #9844. So what we do is to push the bang inwards, in the hope that it will get discarded there. So we transform !(N pat) into (N !pat) But what if there is nothing to push the bang onto? In at least one instance a user has written !(N {}) which we translate into (N !_). See #13215 \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 Typechecked @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} -} matchWrapper :: HsMatchContext Name -- ^ For shadowing warning messages -> Maybe (LHsExpr GhcTc) -- ^ Scrutinee. (Just scrut) for a case expr -- case scrut of { p1 -> e1 ... } -- (and in this case the MatchGroup will -- have all singleton patterns) -- Nothing for a function definition -- f p1 q1 = ... -- No "scrutinee" -- f p2 q2 = ... -- in this case -> MatchGroup GhcTc (LHsExpr GhcTc) -- ^ Matches being desugared -> DsM ([Id], CoreExpr) -- ^ Results (usually passed to 'match') {- 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 -} matchWrapper ctxt mb_scr (MG { mg_alts = (dL->L _ matches) , mg_ext = MatchGroupTc arg_tys rhs_ty , mg_origin = origin }) = do { dflags <- getDynFlags ; locn <- getSrcSpanDs ; new_vars <- case matches of [] -> mapM newSysLocalDsNoLP arg_tys (m:_) -> selectMatchVars (map unLoc (hsLMatchPats m)) ; eqns_info <- mapM (mk_eqn_info new_vars) matches -- Pattern match check warnings for /this match-group/ ; when (isMatchContextPmChecked dflags origin ctxt) $ addScrutTmCs mb_scr new_vars $ -- See Note [Type and Term Equality Propagation] checkMatches dflags (DsMatchContext ctxt locn) new_vars matches ; result_expr <- handleWarnings $ matchEquations ctxt new_vars eqns_info rhs_ty ; return (new_vars, result_expr) } where -- Called once per equation in the match, or alternative in the case mk_eqn_info vars (dL->L _ (Match { m_pats = pats, m_grhss = grhss })) = do { dflags <- getDynFlags ; let upats = map (unLoc . decideBangHood dflags) pats dicts = collectEvVarsPats upats ; match_result <- -- Extend the environment with knowledge about -- the matches before desguaring the RHS -- See Note [Type and Term Equality Propagation] applyWhen (needToRunPmCheck dflags origin) (addTyCsDs dicts . addScrutTmCs mb_scr vars . addPatTmCs upats vars) (dsGRHSs ctxt grhss rhs_ty) ; return (EqnInfo { eqn_pats = upats , eqn_orig = FromSource , eqn_rhs = match_result }) } mk_eqn_info _ (dL->L _ (XMatch nec)) = noExtCon nec mk_eqn_info _ _ = panic "mk_eqn_info: Impossible Match" -- due to #15884 handleWarnings = if isGenerated origin then discardWarningsDs else id matchWrapper _ _ (XMatchGroup nec) = noExtCon nec matchEquations :: HsMatchContext Name -> [MatchId] -> [EquationInfo] -> Type -> DsM CoreExpr matchEquations ctxt vars eqns_info rhs_ty = do { let error_doc = matchContextErrString ctxt ; match_result <- match vars rhs_ty eqns_info ; fail_expr <- mkErrorAppDs pAT_ERROR_ID rhs_ty error_doc ; extractMatchResult match_result fail_expr } {- ************************************************************************ * * \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. -} matchSimply :: CoreExpr -- ^ Scrutinee -> HsMatchContext Name -- ^ Match kind -> LPat GhcTc -- ^ Pattern it should match -> CoreExpr -- ^ Return this if it matches -> CoreExpr -- ^ Return this if it doesn't -> DsM CoreExpr -- Do not warn about incomplete patterns; see matchSinglePat comments matchSimply scrut hs_ctx pat result_expr fail_expr = do 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! match_result' <- matchSinglePat scrut hs_ctx pat rhs_ty match_result extractMatchResult match_result' fail_expr matchSinglePat :: CoreExpr -> HsMatchContext Name -> LPat GhcTc -> Type -> MatchResult -> DsM MatchResult -- matchSinglePat ensures that the scrutinee is a variable -- and then calls matchSinglePatVar -- -- matchSinglePat does not warn about incomplete patterns -- Used for things like [ e | pat <- stuff ], where -- incomplete patterns are just fine matchSinglePat (Var var) ctx pat ty match_result | not (isExternalName (idName var)) = matchSinglePatVar var ctx pat ty match_result matchSinglePat scrut hs_ctx pat ty match_result = do { var <- selectSimpleMatchVarL pat ; match_result' <- matchSinglePatVar var hs_ctx pat ty match_result ; return (adjustMatchResult (bindNonRec var scrut) match_result') } matchSinglePatVar :: Id -- See Note [Match Ids] -> HsMatchContext Name -> LPat GhcTc -> Type -> MatchResult -> DsM MatchResult matchSinglePatVar var ctx pat ty match_result = ASSERT2( isInternalName (idName var), ppr var ) do { dflags <- getDynFlags ; locn <- getSrcSpanDs -- Pattern match check warnings ; checkSingle dflags (DsMatchContext ctx locn) var (unLoc pat) ; let eqn_info = EqnInfo { eqn_pats = [unLoc (decideBangHood dflags pat)] , eqn_orig = FromSource , eqn_rhs = match_result } ; match [var] ty [eqn_info] } {- ************************************************************************ * * Pattern classification * * ************************************************************************ -} data PatGroup = PgAny -- Immediate match: variables, wildcards, -- lazy patterns | PgCon DataCon -- Constructor patterns (incl list, tuple) | PgSyn PatSyn [Type] -- See Note [Pattern synonym groups] | PgLit Literal -- Literal patterns | PgN Rational -- Overloaded numeric literals; -- see Note [Don't use Literal for PgN] | PgOverS FastString -- Overloaded string literals | PgNpK Integer -- n+k patterns | PgBang -- Bang patterns | PgCo Type -- Coercion patterns; the type is the type -- of the pattern *inside* | PgView (LHsExpr GhcTc) -- view pattern (e -> p): -- the LHsExpr is the expression e Type -- the Type is the type of p (equivalently, the result type of e) | PgOverloadedList {- Note [Don't use Literal for PgN] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Previously we had, as PatGroup constructors | ... | PgN Literal -- Overloaded literals | PgNpK Literal -- n+k patterns | ... But Literal is really supposed to represent an *unboxed* literal, like Int#. We were sticking the literal from, say, an overloaded numeric literal pattern into a LitInt constructor. This didn't really make sense; and we now have the invariant that value in a LitInt must be in the range of the target machine's Int# type, and an overloaded literal could meaningfully be larger. Solution: For pattern grouping purposes, just store the literal directly in the PgN constructor as a Rational if numeric, and add a PgOverStr constructor for overloaded strings. -} groupEquations :: DynFlags -> [EquationInfo] -> [[(PatGroup, EquationInfo)]] -- If the result is of form [g1, g2, g3], -- (a) all the (pg,eq) pairs in g1 have the same pg -- (b) none of the gi are empty -- The ordering of equations is unchanged groupEquations dflags eqns = groupBy same_gp [(patGroup dflags (firstPat eqn), eqn) | eqn <- eqns] where same_gp :: (PatGroup,EquationInfo) -> (PatGroup,EquationInfo) -> Bool (pg1,_) `same_gp` (pg2,_) = pg1 `sameGroup` pg2 subGroup :: (m -> [[EquationInfo]]) -- Map.elems -> m -- Map.empty -> (a -> m -> Maybe [EquationInfo]) -- Map.lookup -> (a -> [EquationInfo] -> m -> m) -- Map.insert -> [(a, EquationInfo)] -> [[EquationInfo]] -- Input is a particular group. The result sub-groups the -- equations by with particular constructor, literal etc they match. -- Each sub-list in the result has the same PatGroup -- See Note [Take care with pattern order] -- Parameterized by map operations to allow different implementations -- and constraints, eg. types without Ord instance. subGroup elems empty lookup insert group = map reverse $ elems $ foldl' accumulate empty group where accumulate pg_map (pg, eqn) = case lookup pg pg_map of Just eqns -> insert pg (eqn:eqns) pg_map Nothing -> insert pg [eqn] pg_map -- pg_map :: Map a [EquationInfo] -- Equations seen so far in reverse order of appearance subGroupOrd :: Ord a => [(a, EquationInfo)] -> [[EquationInfo]] subGroupOrd = subGroup Map.elems Map.empty Map.lookup Map.insert subGroupUniq :: Uniquable a => [(a, EquationInfo)] -> [[EquationInfo]] subGroupUniq = subGroup eltsUDFM emptyUDFM (flip lookupUDFM) (\k v m -> addToUDFM m k v) {- Note [Pattern synonym groups] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we see f (P a) = e1 f (P b) = e2 ... where P is a pattern synonym, can we put (P a -> e1) and (P b -> e2) in the same group? We can if P is a constructor, but /not/ if P is a pattern synonym. Consider (#11224) -- readMaybe :: Read a => String -> Maybe a pattern PRead :: Read a => () => a -> String pattern PRead a <- (readMaybe -> Just a) f (PRead (x::Int)) = e1 f (PRead (y::Bool)) = e2 This is all fine: we match the string by trying to read an Int; if that fails we try to read a Bool. But clearly we can't combine the two into a single match. Conclusion: we can combine when we invoke PRead /at the same type/. Hence in PgSyn we record the instantiaing types, and use them in sameGroup. Note [Take care with pattern order] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the subGroup function we must be very careful about pattern re-ordering, Consider the patterns [ (True, Nothing), (False, x), (True, y) ] Then in bringing together the patterns for True, we must not swap the Nothing and y! -} sameGroup :: PatGroup -> PatGroup -> Bool -- Same group means that a single case expression -- or test will suffice to match both, *and* the order -- of testing within the group is insignificant. sameGroup PgAny PgAny = True sameGroup PgBang PgBang = True sameGroup (PgCon _) (PgCon _) = True -- One case expression sameGroup (PgSyn p1 t1) (PgSyn p2 t2) = p1==p2 && eqTypes t1 t2 -- eqTypes: See Note [Pattern synonym groups] sameGroup (PgLit _) (PgLit _) = True -- One case expression sameGroup (PgN l1) (PgN l2) = l1==l2 -- Order is significant sameGroup (PgOverS s1) (PgOverS s2) = s1==s2 sameGroup (PgNpK l1) (PgNpK l2) = l1==l2 -- See Note [Grouping overloaded literal patterns] sameGroup (PgCo t1) (PgCo t2) = t1 `eqType` t2 -- CoPats are in the same goup only if the type of the -- enclosed pattern is the same. The patterns outside the CoPat -- always have the same type, so this boils down to saying that -- the two coercions are identical. sameGroup (PgView e1 t1) (PgView e2 t2) = viewLExprEq (e1,t1) (e2,t2) -- ViewPats are in the same group iff the expressions -- are "equal"---conservatively, we use syntactic equality sameGroup _ _ = False -- An approximation of syntactic equality used for determining when view -- exprs are in the same group. -- This function can always safely return false; -- but doing so will result in the application of the view function being repeated. -- -- Currently: compare applications of literals and variables -- and anything else that we can do without involving other -- HsSyn types in the recursion -- -- NB we can't assume that the two view expressions have the same type. Consider -- f (e1 -> True) = ... -- f (e2 -> "hi") = ... viewLExprEq :: (LHsExpr GhcTc,Type) -> (LHsExpr GhcTc,Type) -> Bool viewLExprEq (e1,_) (e2,_) = lexp e1 e2 where lexp :: LHsExpr GhcTc -> LHsExpr GhcTc -> Bool lexp e e' = exp (unLoc e) (unLoc e') --------- exp :: HsExpr GhcTc -> HsExpr GhcTc -> Bool -- real comparison is on HsExpr's -- strip parens exp (HsPar _ (dL->L _ e)) e' = exp e e' exp e (HsPar _ (dL->L _ e')) = exp e e' -- because the expressions do not necessarily have the same type, -- we have to compare the wrappers exp (HsWrap _ h e) (HsWrap _ h' e') = wrap h h' && exp e e' exp (HsVar _ i) (HsVar _ i') = i == i' exp (HsConLikeOut _ c) (HsConLikeOut _ c') = c == c' -- the instance for IPName derives using the id, so this works if the -- above does exp (HsIPVar _ i) (HsIPVar _ i') = i == i' exp (HsOverLabel _ l x) (HsOverLabel _ l' x') = l == l' && x == x' exp (HsOverLit _ l) (HsOverLit _ l') = -- Overloaded lits are equal if they have the same type -- and the data is the same. -- this is coarser than comparing the SyntaxExpr's in l and l', -- which resolve the overloading (e.g., fromInteger 1), -- because these expressions get written as a bunch of different variables -- (presumably to improve sharing) eqType (overLitType l) (overLitType l') && l == l' exp (HsApp _ e1 e2) (HsApp _ e1' e2') = lexp e1 e1' && lexp e2 e2' -- the fixities have been straightened out by now, so it's safe -- to ignore them? exp (OpApp _ l o ri) (OpApp _ l' o' ri') = lexp l l' && lexp o o' && lexp ri ri' exp (NegApp _ e n) (NegApp _ e' n') = lexp e e' && syn_exp n n' exp (SectionL _ e1 e2) (SectionL _ e1' e2') = lexp e1 e1' && lexp e2 e2' exp (SectionR _ e1 e2) (SectionR _ e1' e2') = lexp e1 e1' && lexp e2 e2' exp (ExplicitTuple _ es1 _) (ExplicitTuple _ es2 _) = eq_list tup_arg es1 es2 exp (ExplicitSum _ _ _ e) (ExplicitSum _ _ _ e') = lexp e e' exp (HsIf _ _ e e1 e2) (HsIf _ _ e' e1' e2') = lexp e e' && lexp e1 e1' && lexp e2 e2' -- Enhancement: could implement equality for more expressions -- if it seems useful -- But no need for HsLit, ExplicitList, ExplicitTuple, -- because they cannot be functions exp _ _ = False --------- syn_exp :: SyntaxExpr GhcTc -> SyntaxExpr GhcTc -> Bool syn_exp (SyntaxExpr { syn_expr = expr1 , syn_arg_wraps = arg_wraps1 , syn_res_wrap = res_wrap1 }) (SyntaxExpr { syn_expr = expr2 , syn_arg_wraps = arg_wraps2 , syn_res_wrap = res_wrap2 }) = exp expr1 expr2 && and (zipWithEqual "viewLExprEq" wrap arg_wraps1 arg_wraps2) && wrap res_wrap1 res_wrap2 --------- tup_arg (dL->L _ (Present _ e1)) (dL->L _ (Present _ e2)) = lexp e1 e2 tup_arg (dL->L _ (Missing t1)) (dL->L _ (Missing t2)) = eqType t1 t2 tup_arg _ _ = False --------- wrap :: HsWrapper -> HsWrapper -> Bool -- Conservative, in that it demands that wrappers be -- syntactically identical and doesn't look under binders -- -- Coarser notions of equality are possible -- (e.g., reassociating compositions, -- equating different ways of writing a coercion) wrap WpHole WpHole = True wrap (WpCompose w1 w2) (WpCompose w1' w2') = wrap w1 w1' && wrap w2 w2' wrap (WpFun w1 w2 _ _) (WpFun w1' w2' _ _) = wrap w1 w1' && wrap w2 w2' wrap (WpCast co) (WpCast co') = co `eqCoercion` co' wrap (WpEvApp et1) (WpEvApp et2) = et1 `ev_term` et2 wrap (WpTyApp t) (WpTyApp t') = eqType t t' -- Enhancement: could implement equality for more wrappers -- if it seems useful (lams and lets) wrap _ _ = False --------- ev_term :: EvTerm -> EvTerm -> Bool ev_term (EvExpr (Var a)) (EvExpr (Var b)) = a==b ev_term (EvExpr (Coercion a)) (EvExpr (Coercion b)) = a `eqCoercion` b ev_term _ _ = False --------- eq_list :: (a->a->Bool) -> [a] -> [a] -> Bool eq_list _ [] [] = True eq_list _ [] (_:_) = False eq_list _ (_:_) [] = False eq_list eq (x:xs) (y:ys) = eq x y && eq_list eq xs ys patGroup :: DynFlags -> Pat GhcTc -> PatGroup patGroup _ (ConPatOut { pat_con = (dL->L _ con) , pat_arg_tys = tys }) | RealDataCon dcon <- con = PgCon dcon | PatSynCon psyn <- con = PgSyn psyn tys patGroup _ (WildPat {}) = PgAny patGroup _ (BangPat {}) = PgBang patGroup _ (NPat _ (dL->L _ (OverLit {ol_val=oval})) mb_neg _) = case (oval, isJust mb_neg) of (HsIntegral i, False) -> PgN (fromInteger (il_value i)) (HsIntegral i, True ) -> PgN (-fromInteger (il_value i)) (HsFractional r, False) -> PgN (fl_value r) (HsFractional r, True ) -> PgN (-fl_value r) (HsIsString _ s, _) -> ASSERT(isNothing mb_neg) PgOverS s patGroup _ (NPlusKPat _ _ (dL->L _ (OverLit {ol_val=oval})) _ _ _) = case oval of HsIntegral i -> PgNpK (il_value i) _ -> pprPanic "patGroup NPlusKPat" (ppr oval) patGroup _ (CoPat _ _ p _) = PgCo (hsPatType p) -- Type of innelexp pattern patGroup _ (ViewPat _ expr p) = PgView expr (hsPatType (unLoc p)) patGroup _ (ListPat (ListPatTc _ (Just _)) _) = PgOverloadedList patGroup dflags (LitPat _ lit) = PgLit (hsLitKey dflags lit) patGroup _ pat = pprPanic "patGroup" (ppr pat) {- Note [Grouping overloaded literal patterns] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ WATCH OUT! Consider f (n+1) = ... f (n+2) = ... f (n+1) = ... We can't group the first and third together, because the second may match the same thing as the first. Same goes for *overloaded* literal patterns f 1 True = ... f 2 False = ... f 1 False = ... If the first arg matches '1' but the second does not match 'True', we cannot jump to the third equation! Because the same argument might match '2'! Hence we don't regard 1 and 2, or (n+1) and (n+2), as part of the same group. -}