{- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 Utilities for desugaring This module exports some utility functions of no great interest. -} {-# LANGUAGE CPP #-} -- | Utility functions for constructing Core syntax, principally for desugaring module DsUtils ( EquationInfo(..), firstPat, shiftEqns, MatchResult(..), CanItFail(..), CaseAlt(..), cantFailMatchResult, alwaysFailMatchResult, extractMatchResult, combineMatchResults, adjustMatchResult, adjustMatchResultDs, mkCoLetMatchResult, mkViewMatchResult, mkGuardedMatchResult, matchCanFail, mkEvalMatchResult, mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult, mkCoSynCaseMatchResult, wrapBind, wrapBinds, mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs, mkCastDs, seqVar, -- LHs tuples mkLHsVarPatTup, mkLHsPatTup, mkVanillaTuplePat, mkBigLHsVarTupId, mkBigLHsTupId, mkBigLHsVarPatTupId, mkBigLHsPatTupId, mkSelectorBinds, selectSimpleMatchVarL, selectMatchVars, selectMatchVar, mkOptTickBox, mkBinaryTickBox, decideBangHood ) where #include "HsVersions.h" import {-# SOURCE #-} Match ( matchSimply ) import HsSyn import TcHsSyn import TcType( tcSplitTyConApp ) import CoreSyn import DsMonad import {-# SOURCE #-} DsExpr ( dsLExpr ) import CoreUtils import MkCore import MkId import Id import Literal import TyCon -- import ConLike import DataCon import PatSyn import Type import Coercion import TysPrim import TysWiredIn import BasicTypes import ConLike import UniqSet import UniqSupply import Module import PrelNames import Outputable import SrcLoc import Util import DynFlags import FastString import qualified GHC.LanguageExtensions as LangExt import TcEvidence import Control.Monad ( zipWithM ) {- ************************************************************************ * * \subsection{ Selecting match variables} * * ************************************************************************ We're about to match against some patterns. We want to make some @Ids@ to use as match variables. If a pattern has an @Id@ readily at hand, which should indeed be bound to the pattern as a whole, then use it; otherwise, make one up. -} selectSimpleMatchVarL :: LPat Id -> DsM Id selectSimpleMatchVarL pat = selectMatchVar (unLoc pat) -- (selectMatchVars ps tys) chooses variables of type tys -- to use for matching ps against. If the pattern is a variable, -- we try to use that, to save inventing lots of fresh variables. -- -- OLD, but interesting note: -- But even if it is a variable, its type might not match. Consider -- data T a where -- T1 :: Int -> T Int -- T2 :: a -> T a -- -- f :: T a -> a -> Int -- f (T1 i) (x::Int) = x -- f (T2 i) (y::a) = 0 -- Then we must not choose (x::Int) as the matching variable! -- And nowadays we won't, because the (x::Int) will be wrapped in a CoPat selectMatchVars :: [Pat Id] -> DsM [Id] selectMatchVars ps = mapM selectMatchVar ps selectMatchVar :: Pat Id -> DsM Id selectMatchVar (BangPat pat) = selectMatchVar (unLoc pat) selectMatchVar (LazyPat pat) = selectMatchVar (unLoc pat) selectMatchVar (ParPat pat) = selectMatchVar (unLoc pat) selectMatchVar (VarPat var) = return (localiseId (unLoc var)) -- Note [Localise pattern binders] selectMatchVar (AsPat var _) = return (unLoc var) selectMatchVar other_pat = newSysLocalDs (hsPatType other_pat) -- OK, better make up one... {- Note [Localise pattern binders] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider module M where [Just a] = e After renaming it looks like module M where [Just M.a] = e We don't generalise, since it's a pattern binding, monomorphic, etc, so after desugaring we may get something like M.a = case e of (v:_) -> case v of Just M.a -> M.a Notice the "M.a" in the pattern; after all, it was in the original pattern. However, after optimisation those pattern binders can become let-binders, and then end up floated to top level. They have a different *unique* by then (the simplifier is good about maintaining proper scoping), but it's BAD to have two top-level bindings with the External Name M.a, because that turns into two linker symbols for M.a. It's quite rare for this to actually *happen* -- the only case I know of is tc003 compiled with the 'hpc' way -- but that only makes it all the more annoying. To avoid this, we craftily call 'localiseId' in the desugarer, which simply turns the External Name for the Id into an Internal one, but doesn't change the unique. So the desugarer produces this: M.a{r8} = case e of (v:_) -> case v of Just a{r8} -> M.a{r8} The unique is still 'r8', but the binding site in the pattern is now an Internal Name. Now the simplifier's usual mechanisms will propagate that Name to all the occurrence sites, as well as un-shadowing it, so we'll get M.a{r8} = case e of (v:_) -> case v of Just a{s77} -> a{s77} In fact, even CoreSubst.simplOptExpr will do this, and simpleOptExpr runs on the output of the desugarer, so all is well by the end of the desugaring pass. ************************************************************************ * * * type synonym EquationInfo and access functions for its pieces * * * ************************************************************************ \subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym} The ``equation info'' used by @match@ is relatively complicated and worthy of a type synonym and a few handy functions. -} firstPat :: EquationInfo -> Pat Id firstPat eqn = ASSERT( notNull (eqn_pats eqn) ) head (eqn_pats eqn) shiftEqns :: [EquationInfo] -> [EquationInfo] -- Drop the first pattern in each equation shiftEqns eqns = [ eqn { eqn_pats = tail (eqn_pats eqn) } | eqn <- eqns ] -- Functions on MatchResults matchCanFail :: MatchResult -> Bool matchCanFail (MatchResult CanFail _) = True matchCanFail (MatchResult CantFail _) = False alwaysFailMatchResult :: MatchResult alwaysFailMatchResult = MatchResult CanFail (\fail -> return fail) cantFailMatchResult :: CoreExpr -> MatchResult cantFailMatchResult expr = MatchResult CantFail (\_ -> return expr) extractMatchResult :: MatchResult -> CoreExpr -> DsM CoreExpr extractMatchResult (MatchResult CantFail match_fn) _ = match_fn (error "It can't fail!") extractMatchResult (MatchResult CanFail match_fn) fail_expr = do (fail_bind, if_it_fails) <- mkFailurePair fail_expr body <- match_fn if_it_fails return (mkCoreLet fail_bind body) combineMatchResults :: MatchResult -> MatchResult -> MatchResult combineMatchResults (MatchResult CanFail body_fn1) (MatchResult can_it_fail2 body_fn2) = MatchResult can_it_fail2 body_fn where body_fn fail = do body2 <- body_fn2 fail (fail_bind, duplicatable_expr) <- mkFailurePair body2 body1 <- body_fn1 duplicatable_expr return (Let fail_bind body1) combineMatchResults match_result1@(MatchResult CantFail _) _ = match_result1 adjustMatchResult :: DsWrapper -> MatchResult -> MatchResult adjustMatchResult encl_fn (MatchResult can_it_fail body_fn) = MatchResult can_it_fail (\fail -> encl_fn <$> body_fn fail) adjustMatchResultDs :: (CoreExpr -> DsM CoreExpr) -> MatchResult -> MatchResult adjustMatchResultDs encl_fn (MatchResult can_it_fail body_fn) = MatchResult can_it_fail (\fail -> encl_fn =<< body_fn fail) wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr wrapBinds [] e = e wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e) wrapBind :: Var -> Var -> CoreExpr -> CoreExpr wrapBind new old body -- NB: this function must deal with term | new==old = body -- variables, type variables or coercion variables | otherwise = Let (NonRec new (varToCoreExpr old)) body seqVar :: Var -> CoreExpr -> CoreExpr seqVar var body = Case (Var var) var (exprType body) [(DEFAULT, [], body)] mkCoLetMatchResult :: CoreBind -> MatchResult -> MatchResult mkCoLetMatchResult bind = adjustMatchResult (mkCoreLet bind) -- (mkViewMatchResult var' viewExpr mr) makes the expression -- let var' = viewExpr in mr mkViewMatchResult :: Id -> CoreExpr -> MatchResult -> MatchResult mkViewMatchResult var' viewExpr = adjustMatchResult (mkCoreLet (NonRec var' viewExpr)) mkEvalMatchResult :: Id -> Type -> MatchResult -> MatchResult mkEvalMatchResult var ty = adjustMatchResult (\e -> Case (Var var) var ty [(DEFAULT, [], e)]) mkGuardedMatchResult :: CoreExpr -> MatchResult -> MatchResult mkGuardedMatchResult pred_expr (MatchResult _ body_fn) = MatchResult CanFail (\fail -> do body <- body_fn fail return (mkIfThenElse pred_expr body fail)) mkCoPrimCaseMatchResult :: Id -- Scrutinee -> Type -- Type of the case -> [(Literal, MatchResult)] -- Alternatives -> MatchResult -- Literals are all unlifted mkCoPrimCaseMatchResult var ty match_alts = MatchResult CanFail mk_case where mk_case fail = do alts <- mapM (mk_alt fail) sorted_alts return (Case (Var var) var ty ((DEFAULT, [], fail) : alts)) sorted_alts = sortWith fst match_alts -- Right order for a Case mk_alt fail (lit, MatchResult _ body_fn) = ASSERT( not (litIsLifted lit) ) do body <- body_fn fail return (LitAlt lit, [], body) data CaseAlt a = MkCaseAlt{ alt_pat :: a, alt_bndrs :: [Var], alt_wrapper :: HsWrapper, alt_result :: MatchResult } mkCoAlgCaseMatchResult :: DynFlags -> Id -- Scrutinee -> Type -- Type of exp -> [CaseAlt DataCon] -- Alternatives (bndrs *include* tyvars, dicts) -> MatchResult mkCoAlgCaseMatchResult dflags var ty match_alts | isNewtype -- Newtype case; use a let = ASSERT( null (tail match_alts) && null (tail arg_ids1) ) mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1 | isPArrFakeAlts match_alts = MatchResult CanFail $ mkPArrCase dflags var ty (sort_alts match_alts) | otherwise = mkDataConCase var ty match_alts where isNewtype = isNewTyCon (dataConTyCon (alt_pat alt1)) -- [Interesting: because of GADTs, we can't rely on the type of -- the scrutinised Id to be sufficiently refined to have a TyCon in it] alt1@MkCaseAlt{ alt_bndrs = arg_ids1, alt_result = match_result1 } = ASSERT( notNull match_alts ) head match_alts -- Stuff for newtype arg_id1 = ASSERT( notNull arg_ids1 ) head arg_ids1 var_ty = idType var (tc, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes -- (not that splitTyConApp does, these days) newtype_rhs = unwrapNewTypeBody tc ty_args (Var var) --- Stuff for parallel arrays -- -- Concerning `isPArrFakeAlts': -- -- * it is *not* sufficient to just check the type of the type -- constructor, as we have to be careful not to confuse the real -- representation of parallel arrays with the fake constructors; -- moreover, a list of alternatives must not mix fake and real -- constructors (this is checked earlier on) -- -- FIXME: We actually go through the whole list and make sure that -- either all or none of the constructors are fake parallel -- array constructors. This is to spot equations that mix fake -- constructors with the real representation defined in -- `PrelPArr'. It would be nicer to spot this situation -- earlier and raise a proper error message, but it can really -- only happen in `PrelPArr' anyway. -- isPArrFakeAlts :: [CaseAlt DataCon] -> Bool isPArrFakeAlts [alt] = isPArrFakeCon (alt_pat alt) isPArrFakeAlts (alt:alts) = case (isPArrFakeCon (alt_pat alt), isPArrFakeAlts alts) of (True , True ) -> True (False, False) -> False _ -> panic "DsUtils: you may not mix `[:...:]' with `PArr' patterns" isPArrFakeAlts [] = panic "DsUtils: unexpectedly found an empty list of PArr fake alternatives" mkCoSynCaseMatchResult :: Id -> Type -> CaseAlt PatSyn -> MatchResult mkCoSynCaseMatchResult var ty alt = MatchResult CanFail $ mkPatSynCase var ty alt sort_alts :: [CaseAlt DataCon] -> [CaseAlt DataCon] sort_alts = sortWith (dataConTag . alt_pat) mkPatSynCase :: Id -> Type -> CaseAlt PatSyn -> CoreExpr -> DsM CoreExpr mkPatSynCase var ty alt fail = do matcher <- dsLExpr $ mkLHsWrap wrapper $ nlHsTyApp matcher [getRuntimeRep "mkPatSynCase" ty, ty] let MatchResult _ mkCont = match_result cont <- mkCoreLams bndrs <$> mkCont fail return $ mkCoreAppsDs (text "patsyn" <+> ppr var) matcher [Var var, ensure_unstrict cont, Lam voidArgId fail] where MkCaseAlt{ alt_pat = psyn, alt_bndrs = bndrs, alt_wrapper = wrapper, alt_result = match_result} = alt (matcher, needs_void_lam) = patSynMatcher psyn -- See Note [Matchers and builders for pattern synonyms] in PatSyns -- on these extra Void# arguments ensure_unstrict cont | needs_void_lam = Lam voidArgId cont | otherwise = cont mkDataConCase :: Id -> Type -> [CaseAlt DataCon] -> MatchResult mkDataConCase _ _ [] = panic "mkDataConCase: no alternatives" mkDataConCase var ty alts@(alt1:_) = MatchResult fail_flag mk_case where con1 = alt_pat alt1 tycon = dataConTyCon con1 data_cons = tyConDataCons tycon match_results = map alt_result alts sorted_alts :: [CaseAlt DataCon] sorted_alts = sort_alts alts var_ty = idType var (_, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes -- (not that splitTyConApp does, these days) mk_case :: CoreExpr -> DsM CoreExpr mk_case fail = do alts <- mapM (mk_alt fail) sorted_alts return $ mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts) mk_alt :: CoreExpr -> CaseAlt DataCon -> DsM CoreAlt mk_alt fail MkCaseAlt{ alt_pat = con, alt_bndrs = args, alt_result = MatchResult _ body_fn } = do { body <- body_fn fail ; case dataConBoxer con of { Nothing -> return (DataAlt con, args, body) ; Just (DCB boxer) -> do { us <- newUniqueSupply ; let (rep_ids, binds) = initUs_ us (boxer ty_args args) ; return (DataAlt con, rep_ids, mkLets binds body) } } } mk_default :: CoreExpr -> [CoreAlt] mk_default fail | exhaustive_case = [] | otherwise = [(DEFAULT, [], fail)] fail_flag :: CanItFail fail_flag | exhaustive_case = foldr orFail CantFail [can_it_fail | MatchResult can_it_fail _ <- match_results] | otherwise = CanFail mentioned_constructors = mkUniqSet $ map alt_pat alts un_mentioned_constructors = mkUniqSet data_cons `minusUniqSet` mentioned_constructors exhaustive_case = isEmptyUniqSet un_mentioned_constructors --- Stuff for parallel arrays -- -- * the following is to desugar cases over fake constructors for -- parallel arrays, which are introduced by `tidy1' in the `PArrPat' -- case -- mkPArrCase :: DynFlags -> Id -> Type -> [CaseAlt DataCon] -> CoreExpr -> DsM CoreExpr mkPArrCase dflags var ty sorted_alts fail = do lengthP <- dsDPHBuiltin lengthPVar alt <- unboxAlt return (mkWildCase (len lengthP) intTy ty [alt]) where elemTy = case splitTyConApp (idType var) of (_, [elemTy]) -> elemTy _ -> panic panicMsg panicMsg = "DsUtils.mkCoAlgCaseMatchResult: not a parallel array?" len lengthP = mkApps (Var lengthP) [Type elemTy, Var var] -- unboxAlt = do l <- newSysLocalDs intPrimTy indexP <- dsDPHBuiltin indexPVar alts <- mapM (mkAlt indexP) sorted_alts return (DataAlt intDataCon, [l], mkWildCase (Var l) intPrimTy ty (dft : alts)) where dft = (DEFAULT, [], fail) -- -- each alternative matches one array length (corresponding to one -- fake array constructor), so the match is on a literal; each -- alternative's body is extended by a local binding for each -- constructor argument, which are bound to array elements starting -- with the first -- mkAlt indexP alt@MkCaseAlt{alt_result = MatchResult _ bodyFun} = do body <- bodyFun fail return (LitAlt lit, [], mkCoreLets binds body) where lit = MachInt $ toInteger (dataConSourceArity (alt_pat alt)) binds = [NonRec arg (indexExpr i) | (i, arg) <- zip [1..] (alt_bndrs alt)] -- indexExpr i = mkApps (Var indexP) [Type elemTy, Var var, mkIntExpr dflags i] {- ************************************************************************ * * \subsection{Desugarer's versions of some Core functions} * * ************************************************************************ -} mkErrorAppDs :: Id -- The error function -> Type -- Type to which it should be applied -> SDoc -- The error message string to pass -> DsM CoreExpr mkErrorAppDs err_id ty msg = do src_loc <- getSrcSpanDs dflags <- getDynFlags let full_msg = showSDoc dflags (hcat [ppr src_loc, vbar, msg]) core_msg = Lit (mkMachString full_msg) -- mkMachString returns a result of type String# return (mkApps (Var err_id) [Type (getRuntimeRep "mkErrorAppDs" ty), Type ty, core_msg]) {- 'mkCoreAppDs' and 'mkCoreAppsDs' hand the special-case desugaring of 'seq'. Note [Desugaring seq (1)] cf Trac #1031 ~~~~~~~~~~~~~~~~~~~~~~~~~ f x y = x `seq` (y `seq` (# x,y #)) The [CoreSyn let/app invariant] means that, other things being equal, because the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus: f x y = case (y `seq` (# x,y #)) of v -> x `seq` v But that is bad for two reasons: (a) we now evaluate y before x, and (b) we can't bind v to an unboxed pair Seq is very, very special! So we recognise it right here, and desugar to case x of _ -> case y of _ -> (# x,y #) Note [Desugaring seq (2)] cf Trac #2273 ~~~~~~~~~~~~~~~~~~~~~~~~~ Consider let chp = case b of { True -> fst x; False -> 0 } in chp `seq` ...chp... Here the seq is designed to plug the space leak of retaining (snd x) for too long. If we rely on the ordinary inlining of seq, we'll get let chp = case b of { True -> fst x; False -> 0 } case chp of _ { I# -> ...chp... } But since chp is cheap, and the case is an alluring contet, we'll inline chp into the case scrutinee. Now there is only one use of chp, so we'll inline a second copy. Alas, we've now ruined the purpose of the seq, by re-introducing the space leak: case (case b of {True -> fst x; False -> 0}) of I# _ -> ...case b of {True -> fst x; False -> 0}... We can try to avoid doing this by ensuring that the binder-swap in the case happens, so we get his at an early stage: case chp of chp2 { I# -> ...chp2... } But this is fragile. The real culprit is the source program. Perhaps we should have said explicitly let !chp2 = chp in ...chp2... But that's painful. So the code here does a little hack to make seq more robust: a saturated application of 'seq' is turned *directly* into the case expression, thus: x `seq` e2 ==> case x of x -> e2 -- Note shadowing! e1 `seq` e2 ==> case x of _ -> e2 So we desugar our example to: let chp = case b of { True -> fst x; False -> 0 } case chp of chp { I# -> ...chp... } And now all is well. The reason it's a hack is because if you define mySeq=seq, the hack won't work on mySeq. Note [Desugaring seq (3)] cf Trac #2409 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The isLocalId ensures that we don't turn True `seq` e into case True of True { ... } which stupidly tries to bind the datacon 'True'. -} mkCoreAppDs :: SDoc -> CoreExpr -> CoreExpr -> CoreExpr mkCoreAppDs _ (Var f `App` Type ty1 `App` Type ty2 `App` arg1) arg2 | f `hasKey` seqIdKey -- Note [Desugaring seq (1), (2)] = Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)] where case_bndr = case arg1 of Var v1 | isLocalId v1 -> v1 -- Note [Desugaring seq (2) and (3)] _ -> mkWildValBinder ty1 mkCoreAppDs s fun arg = mkCoreApp s fun arg -- The rest is done in MkCore mkCoreAppsDs :: SDoc -> CoreExpr -> [CoreExpr] -> CoreExpr mkCoreAppsDs s fun args = foldl (mkCoreAppDs s) fun args mkCastDs :: CoreExpr -> Coercion -> CoreExpr -- We define a desugarer-specific verison of CoreUtils.mkCast, -- because in the immediate output of the desugarer, we can have -- apparently-mis-matched coercions: E.g. -- let a = b -- in (x :: a) |> (co :: b ~ Int) -- Lint know about type-bindings for let and does not complain -- So here we do not make the assertion checks that we make in -- CoreUtils.mkCast; and we do less peephole optimisation too mkCastDs e co | isReflCo co = e | otherwise = Cast e co {- ************************************************************************ * * Tuples and selector bindings * * ************************************************************************ This is used in various places to do with lazy patterns. For each binder $b$ in the pattern, we create a binding: \begin{verbatim} b = case v of pat' -> b' \end{verbatim} where @pat'@ is @pat@ with each binder @b@ cloned into @b'@. ToDo: making these bindings should really depend on whether there's much work to be done per binding. If the pattern is complex, it should be de-mangled once, into a tuple (and then selected from). Otherwise the demangling can be in-line in the bindings (as here). Boring! Boring! One error message per binder. The above ToDo is even more helpful. Something very similar happens for pattern-bound expressions. Note [mkSelectorBinds] ~~~~~~~~~~~~~~~~~~~~~~ mkSelectorBinds is used to desugar a pattern binding {p = e}, in a binding group: let { ...; p = e; ... } in body where p binds x,y (this list of binders can be empty). There are two cases. ------ Special case (A) ------- For a pattern that is just a variable, let !x = e in body ==> let x = e in x `seq` body So we return the binding, with 'x' as the variable to seq. ------ Special case (B) ------- For a pattern that is essentially just a tuple: * A product type, so cannot fail * Only one level, so that - generating multiple matches is fine - seq'ing it evaluates the same as matching it Then instead we generate { v = e ; x = case v of p -> x ; y = case v of p -> y } with 'v' as the variable to force ------ General case (C) ------- In the general case we generate these bindings: let { ...; p = e; ... } in body ==> let { t = case e of p -> (x,y) ; x = case t of (x,y) -> x ; y = case t of (x,y) -> y } in t `seq` body Note that we return 't' as the variable to force if the pattern is strict (i.e. with -XStrict or an outermost-bang-pattern) Note that (A) /includes/ the situation where * The pattern binds exactly one variable let !(Just (Just x) = e in body ==> let { t = case e of Just (Just v) -> Unit v ; v = case t of Unit v -> v } in t `seq` body The 'Unit' is a one-tuple; see Note [One-tuples] in TysWiredIn Note that forcing 't' makes the pattern match happen, but does not force 'v'. * The pattern binds no variables let !(True,False) = e in body ==> let t = case e of (True,False) -> () in t `seq` body ------ Examples ---------- * !(_, (_, a)) = e ==> t = case e of (_, (_, a)) -> Unit a a = case t of Unit a -> a Note that - Forcing 't' will force the pattern to match fully; e.g. will diverge if (snd e) is bottom - But 'a' itself is not forced; it is wrapped in a one-tuple (see Note [One-tuples] in TysWiredIn) * !(Just x) = e ==> t = case e of Just x -> Unit x x = case t of Unit x -> x Again, forcing 't' will fail if 'e' yields Nothing. Note that even though this is rather general, the special cases work out well: * One binder, not -XStrict: let Just (Just v) = e in body ==> let t = case e of Just (Just v) -> Unit v v = case t of Unit v -> v in body ==> let v = case (case e of Just (Just v) -> Unit v) of Unit v -> v in body ==> let v = case e of Just (Just v) -> v in body * Non-recursive, -XStrict let p = e in body ==> let { t = case e of p -> (x,y) ; x = case t of (x,y) -> x ; y = case t of (x,y) -> x } in t `seq` body ==> {inline seq, float x,y bindings inwards} let t = case e of p -> (x,y) in case t of t' -> let { x = case t' of (x,y) -> x ; y = case t' of (x,y) -> x } in body ==> {inline t, do case of case} case e of p -> let t = (x,y) in let { x = case t' of (x,y) -> x ; y = case t' of (x,y) -> x } in body ==> {case-cancellation, drop dead code} case e of p -> body * Special case (B) is there to avoid fruitlessly taking the tuple apart and rebuilding it. For example, consider { K x y = e } where K is a product constructor. Then general case (A) does: { t = case e of K x y -> (x,y) ; x = case t of (x,y) -> x ; y = case t of (x,y) -> y } In the lazy case we can't optimise out this fruitless taking apart and rebuilding. Instead (B) builds { v = e ; x = case v of K x y -> x ; y = case v of K x y -> y } which is better. -} mkSelectorBinds :: [[Tickish Id]] -- ^ ticks to add, possibly -> LPat Id -- ^ The pattern -> CoreExpr -- ^ Expression to which the pattern is bound -> DsM (Id,[(Id,CoreExpr)]) -- ^ Id the rhs is bound to, for desugaring strict -- binds (see Note [Desugar Strict binds] in DsBinds) -- and all the desugared binds mkSelectorBinds ticks pat val_expr | L _ (VarPat (L _ v)) <- pat' -- Special case (A) = return (v, [(v, val_expr)]) | is_flat_prod_lpat pat' -- Special case (B) = do { let pat_ty = hsLPatType pat' ; val_var <- newSysLocalDs pat_ty ; let mk_bind tick bndr_var -- (mk_bind sv bv) generates bv = case sv of { pat -> bv } -- Remember, 'pat' binds 'bv' = do { rhs_expr <- matchSimply (Var val_var) PatBindRhs pat' (Var bndr_var) (Var bndr_var) -- Neat hack -- Neat hack: since 'pat' can't fail, the -- "fail-expr" passed to matchSimply is not -- used. But it /is/ used for its type, and for -- that bndr_var is just the ticket. ; return (bndr_var, mkOptTickBox tick rhs_expr) } ; binds <- zipWithM mk_bind ticks' binders ; return ( val_var, (val_var, val_expr) : binds) } | otherwise -- General case (C) = do { tuple_var <- newSysLocalDs tuple_ty ; error_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID tuple_ty (ppr pat') ; tuple_expr <- matchSimply val_expr PatBindRhs pat local_tuple error_expr ; let mk_tup_bind tick binder = (binder, mkOptTickBox tick $ mkTupleSelector1 local_binders binder tuple_var (Var tuple_var)) tup_binds = zipWith mk_tup_bind ticks' binders ; return (tuple_var, (tuple_var, tuple_expr) : tup_binds) } where pat' = strip_bangs pat -- Strip the bangs before looking for case (A) or (B) -- The incoming pattern may well have a bang on it binders = collectPatBinders pat' ticks' = ticks ++ repeat [] local_binders = map localiseId binders -- See Note [Localise pattern binders] local_tuple = mkBigCoreVarTup1 binders tuple_ty = exprType local_tuple strip_bangs :: LPat a -> LPat a -- Remove outermost bangs and parens strip_bangs (L _ (ParPat p)) = strip_bangs p strip_bangs (L _ (BangPat p)) = strip_bangs p strip_bangs lp = lp is_flat_prod_lpat :: LPat a -> Bool is_flat_prod_lpat p = is_flat_prod_pat (unLoc p) is_flat_prod_pat :: Pat a -> Bool is_flat_prod_pat (ParPat p) = is_flat_prod_lpat p is_flat_prod_pat (TuplePat ps Boxed _) = all is_triv_lpat ps is_flat_prod_pat (ConPatOut { pat_con = L _ pcon, pat_args = ps}) | RealDataCon con <- pcon , isProductTyCon (dataConTyCon con) = all is_triv_lpat (hsConPatArgs ps) is_flat_prod_pat _ = False is_triv_lpat :: LPat a -> Bool is_triv_lpat p = is_triv_pat (unLoc p) is_triv_pat :: Pat a -> Bool is_triv_pat (VarPat _) = True is_triv_pat (WildPat _) = True is_triv_pat (ParPat p) = is_triv_lpat p is_triv_pat _ = False {- ********************************************************************* * * Creating big tuples and their types for full Haskell expressions. They work over *Ids*, and create tuples replete with their types, which is whey they are not in HsUtils. * * ********************************************************************* -} mkLHsPatTup :: [LPat Id] -> LPat Id mkLHsPatTup [] = noLoc $ mkVanillaTuplePat [] Boxed mkLHsPatTup [lpat] = lpat mkLHsPatTup lpats = L (getLoc (head lpats)) $ mkVanillaTuplePat lpats Boxed mkLHsVarPatTup :: [Id] -> LPat Id mkLHsVarPatTup bs = mkLHsPatTup (map nlVarPat bs) mkVanillaTuplePat :: [OutPat Id] -> Boxity -> Pat Id -- A vanilla tuple pattern simply gets its type from its sub-patterns mkVanillaTuplePat pats box = TuplePat pats box (map hsLPatType pats) -- The Big equivalents for the source tuple expressions mkBigLHsVarTupId :: [Id] -> LHsExpr Id mkBigLHsVarTupId ids = mkBigLHsTupId (map nlHsVar ids) mkBigLHsTupId :: [LHsExpr Id] -> LHsExpr Id mkBigLHsTupId = mkChunkified mkLHsTupleExpr -- The Big equivalents for the source tuple patterns mkBigLHsVarPatTupId :: [Id] -> LPat Id mkBigLHsVarPatTupId bs = mkBigLHsPatTupId (map nlVarPat bs) mkBigLHsPatTupId :: [LPat Id] -> LPat Id mkBigLHsPatTupId = mkChunkified mkLHsPatTup {- ************************************************************************ * * Code for pattern-matching and other failures * * ************************************************************************ Generally, we handle pattern matching failure like this: let-bind a fail-variable, and use that variable if the thing fails: \begin{verbatim} let fail.33 = error "Help" in case x of p1 -> ... p2 -> fail.33 p3 -> fail.33 p4 -> ... \end{verbatim} Then \begin{itemize} \item If the case can't fail, then there'll be no mention of @fail.33@, and the simplifier will later discard it. \item If it can fail in only one way, then the simplifier will inline it. \item Only if it is used more than once will the let-binding remain. \end{itemize} There's a problem when the result of the case expression is of unboxed type. Then the type of @fail.33@ is unboxed too, and there is every chance that someone will change the let into a case: \begin{verbatim} case error "Help" of fail.33 -> case .... \end{verbatim} which is of course utterly wrong. Rather than drop the condition that only boxed types can be let-bound, we just turn the fail into a function for the primitive case: \begin{verbatim} let fail.33 :: Void -> Int# fail.33 = \_ -> error "Help" in case x of p1 -> ... p2 -> fail.33 void p3 -> fail.33 void p4 -> ... \end{verbatim} Now @fail.33@ is a function, so it can be let-bound. -} mkFailurePair :: CoreExpr -- Result type of the whole case expression -> DsM (CoreBind, -- Binds the newly-created fail variable -- to \ _ -> expression CoreExpr) -- Fail variable applied to realWorld# -- See Note [Failure thunks and CPR] mkFailurePair expr = do { fail_fun_var <- newFailLocalDs (voidPrimTy `mkFunTy` ty) ; fail_fun_arg <- newSysLocalDs voidPrimTy ; let real_arg = setOneShotLambda fail_fun_arg ; return (NonRec fail_fun_var (Lam real_arg expr), App (Var fail_fun_var) (Var voidPrimId)) } where ty = exprType expr {- Note [Failure thunks and CPR] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we make a failure point we ensure that it does not look like a thunk. Example: let fail = \rw -> error "urk" in case x of [] -> fail realWorld# (y:ys) -> case ys of [] -> fail realWorld# (z:zs) -> (y,z) Reason: we know that a failure point is always a "join point" and is entered at most once. Adding a dummy 'realWorld' token argument makes it clear that sharing is not an issue. And that in turn makes it more CPR-friendly. This matters a lot: if you don't get it right, you lose the tail call property. For example, see Trac #3403. ************************************************************************ * * Ticks * * ********************************************************************* -} mkOptTickBox :: [Tickish Id] -> CoreExpr -> CoreExpr mkOptTickBox = flip (foldr Tick) mkBinaryTickBox :: Int -> Int -> CoreExpr -> DsM CoreExpr mkBinaryTickBox ixT ixF e = do uq <- newUnique this_mod <- getModule let bndr1 = mkSysLocal (fsLit "t1") uq boolTy let falseBox = Tick (HpcTick this_mod ixF) (Var falseDataConId) trueBox = Tick (HpcTick this_mod ixT) (Var trueDataConId) -- return $ Case e bndr1 boolTy [ (DataAlt falseDataCon, [], falseBox) , (DataAlt trueDataCon, [], trueBox) ] -- ******************************************************************* -- | Use -XStrict to add a ! or remove a ~ -- -- Examples: -- ~pat => pat -- when -XStrict (even if pat = ~pat') -- !pat => !pat -- always -- pat => !pat -- when -XStrict -- pat => pat -- otherwise decideBangHood :: DynFlags -> LPat id -- ^ Original pattern -> LPat id -- Pattern with bang if necessary decideBangHood dflags lpat | not (xopt LangExt.Strict dflags) = lpat | otherwise -- -XStrict = go lpat where go lp@(L l p) = case p of ParPat p -> L l (ParPat (go p)) LazyPat lp' -> lp' BangPat _ -> lp _ -> L l (BangPat lp)