2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Desugaring exporessions.
9 {-# OPTIONS -fno-warn-incomplete-patterns #-}
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module DsExpr ( dsExpr, dsLExpr, dsLocalBinds, dsValBinds, dsLit ) where
18 #include "HsVersions.h"
32 -- Template Haskell stuff iff bootstrapped
38 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
39 -- needs to see source types
69 %************************************************************************
71 dsLocalBinds, dsValBinds
73 %************************************************************************
76 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
77 dsLocalBinds EmptyLocalBinds body = return body
78 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
79 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
81 -------------------------
82 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
83 dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
85 -------------------------
86 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
87 dsIPBinds (IPBinds ip_binds ev_binds) body
88 = do { ds_ev_binds <- dsTcEvBinds ev_binds
89 ; let inner = wrapDsEvBinds ds_ev_binds body
90 -- The dict bindings may not be in
91 -- dependency order; hence Rec
92 ; foldrM ds_ip_bind inner ip_binds }
94 ds_ip_bind (L _ (IPBind n e)) body
96 return (Let (NonRec (ipNameName n) e') body)
98 -------------------------
99 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
100 -- Special case for bindings which bind unlifted variables
101 -- We need to do a case right away, rather than building
102 -- a tuple and doing selections.
103 -- Silently ignore INLINE and SPECIALISE pragmas...
104 ds_val_bind (NonRecursive, hsbinds) body
105 | [L loc bind] <- bagToList hsbinds,
106 -- Non-recursive, non-overloaded bindings only come in ones
107 -- ToDo: in some bizarre case it's conceivable that there
108 -- could be dict binds in the 'binds'. (See the notes
109 -- below. Then pattern-match would fail. Urk.)
111 = putSrcSpanDs loc (dsStrictBind bind body)
113 -- Ordinary case for bindings; none should be unlifted
114 ds_val_bind (_is_rec, binds) body
115 = do { prs <- dsLHsBinds binds
116 ; ASSERT2( not (any (isUnLiftedType . idType . fst) prs), ppr _is_rec $$ ppr binds )
119 _ -> return (Let (Rec prs) body) }
120 -- Use a Rec regardless of is_rec.
121 -- Why? Because it allows the binds to be all
122 -- mixed up, which is what happens in one rare case
123 -- Namely, for an AbsBind with no tyvars and no dicts,
124 -- but which does have dictionary bindings.
125 -- See notes with TcSimplify.inferLoop [NO TYVARS]
126 -- It turned out that wrapping a Rec here was the easiest solution
128 -- NB The previous case dealt with unlifted bindings, so we
129 -- only have to deal with lifted ones now; so Rec is ok
132 dsStrictBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
133 dsStrictBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
134 , abs_exports = exports
135 , abs_ev_binds = ev_binds
136 , abs_binds = binds }) body
137 = do { ds_ev_binds <- dsTcEvBinds ev_binds
138 ; let body1 = foldr bind_export body exports
139 bind_export (_, g, l, _) b = bindNonRec g (Var l) b
140 ; body2 <- foldlBagM (\body bind -> dsStrictBind (unLoc bind) body)
142 ; return (wrapDsEvBinds ds_ev_binds body2) }
144 dsStrictBind (FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn
145 , fun_tick = tick, fun_infix = inf }) body
146 -- Can't be a bang pattern (that looks like a PatBind)
147 -- so must be simply unboxed
148 = do { (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
149 ; MASSERT( null args ) -- Functions aren't lifted
150 ; MASSERT( isIdHsWrapper co_fn )
151 ; rhs' <- mkOptTickBox tick rhs
152 ; return (bindNonRec fun rhs' body) }
154 dsStrictBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
155 = -- let C x# y# = rhs in body
156 -- ==> case rhs of C x# y# -> body
157 do { rhs <- dsGuarded grhss ty
158 ; let upat = unLoc pat
159 eqn = EqnInfo { eqn_pats = [upat],
160 eqn_rhs = cantFailMatchResult body }
161 ; var <- selectMatchVar upat
162 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
163 ; return (scrungleMatch var rhs result) }
165 dsStrictBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
167 ----------------------
168 strictMatchOnly :: HsBind Id -> Bool
169 strictMatchOnly (AbsBinds { abs_binds = binds })
170 = anyBag (strictMatchOnly . unLoc) binds
171 strictMatchOnly (PatBind { pat_lhs = lpat, pat_rhs_ty = ty })
172 = isUnboxedTupleType ty
174 || any (isUnLiftedType . idType) (collectPatBinders lpat)
175 strictMatchOnly (FunBind { fun_id = L _ id })
176 = isUnLiftedType (idType id)
177 strictMatchOnly _ = False -- I hope! Checked immediately by caller in fact
179 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
180 -- Returns something like (let var = scrut in body)
181 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
182 -- Special case to handle unboxed tuple patterns; they can't appear nested
184 -- case e of (# p1, p2 #) -> rhs
186 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
188 -- let x = e in case x of ....
190 -- But there may be a big
191 -- let fail = ... in case e of ...
192 -- wrapping the whole case, which complicates matters slightly
193 -- It all seems a bit fragile. Test is dsrun013.
195 scrungleMatch var scrut body
196 | isUnboxedTupleType (idType var) = scrungle body
197 | otherwise = bindNonRec var scrut body
199 scrungle (Case (Var x) bndr ty alts)
200 | x == var = Case scrut bndr ty alts
201 scrungle (Let binds body) = Let binds (scrungle body)
202 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
206 %************************************************************************
208 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
210 %************************************************************************
213 dsLExpr :: LHsExpr Id -> DsM CoreExpr
215 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
217 dsExpr :: HsExpr Id -> DsM CoreExpr
218 dsExpr (HsPar e) = dsLExpr e
219 dsExpr (ExprWithTySigOut e _) = dsLExpr e
220 dsExpr (HsVar var) = return (Var var)
221 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
222 dsExpr (HsLit lit) = dsLit lit
223 dsExpr (HsOverLit lit) = dsOverLit lit
225 dsExpr (HsWrap co_fn e)
226 = do { co_fn' <- dsHsWrapper co_fn
228 ; warn_id <- doptDs Opt_WarnIdentities
229 ; when warn_id $ warnAboutIdentities e' co_fn'
230 ; return (co_fn' e') }
232 dsExpr (NegApp expr neg_expr)
233 = App <$> dsExpr neg_expr <*> dsLExpr expr
235 dsExpr (HsLam a_Match)
236 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
238 dsExpr (HsApp fun arg)
239 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
242 Operator sections. At first it looks as if we can convert
251 But no! expr might be a redex, and we can lose laziness badly this
256 for example. So we convert instead to
258 let y = expr in \x -> op y x
260 If \tr{expr} is actually just a variable, say, then the simplifier
264 dsExpr (OpApp e1 op _ e2)
265 = -- for the type of y, we need the type of op's 2nd argument
266 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
268 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
269 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
271 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
272 dsExpr (SectionR op expr) = do
273 core_op <- dsLExpr op
274 -- for the type of x, we need the type of op's 2nd argument
275 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
276 -- See comment with SectionL
277 y_core <- dsLExpr expr
278 x_id <- newSysLocalDs x_ty
279 y_id <- newSysLocalDs y_ty
280 return (bindNonRec y_id y_core $
281 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
283 dsExpr (ExplicitTuple tup_args boxity)
284 = do { let go (lam_vars, args) (Missing ty)
285 -- For every missing expression, we need
286 -- another lambda in the desugaring.
287 = do { lam_var <- newSysLocalDs ty
288 ; return (lam_var : lam_vars, Var lam_var : args) }
289 go (lam_vars, args) (Present expr)
290 -- Expressions that are present don't generate
291 -- lambdas, just arguments.
292 = do { core_expr <- dsLExpr expr
293 ; return (lam_vars, core_expr : args) }
295 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
296 -- The reverse is because foldM goes left-to-right
298 ; return $ mkCoreLams lam_vars $
299 mkConApp (tupleCon boxity (length tup_args))
300 (map (Type . exprType) args ++ args) }
302 dsExpr (HsSCC cc expr) = do
303 mod_name <- getModuleDs
304 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
306 dsExpr (HsCoreAnn fs expr)
307 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
309 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
310 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
311 = -- So desugar empty HsCase to error call
312 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
315 = do { core_discrim <- dsLExpr discrim
316 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
317 ; return (scrungleMatch discrim_var core_discrim matching_code) }
319 -- Pepe: The binds are in scope in the body but NOT in the binding group
320 -- This is to avoid silliness in breakpoints
321 dsExpr (HsLet binds body) = do
322 body' <- dsLExpr body
323 dsLocalBinds binds body'
325 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
326 -- because the interpretation of `stmts' depends on what sort of thing it is.
328 dsExpr (HsDo ListComp stmts body _ result_ty)
329 = -- Special case for list comprehensions
330 dsListComp stmts body elt_ty
332 [elt_ty] = tcTyConAppArgs result_ty
334 dsExpr (HsDo DoExpr stmts body _ result_ty)
335 = dsDo stmts body result_ty
337 dsExpr (HsDo GhciStmt stmts body _ result_ty)
338 = dsDo stmts body result_ty
340 dsExpr (HsDo MDoExpr stmts body _ result_ty)
341 = dsDo stmts body result_ty
343 dsExpr (HsDo MonadComp stmts body return_op result_ty)
344 = dsMonadComp stmts return_op body result_ty
346 dsExpr (HsDo PArrComp stmts body _ result_ty)
347 = -- Special case for array comprehensions
348 dsPArrComp (map unLoc stmts) body elt_ty
350 [elt_ty] = tcTyConAppArgs result_ty
352 dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
353 = do { pred <- dsLExpr guard_expr
354 ; b1 <- dsLExpr then_expr
355 ; b2 <- dsLExpr else_expr
357 Just fun -> do { core_fun <- dsExpr fun
358 ; return (mkCoreApps core_fun [pred,b1,b2]) }
359 Nothing -> return $ mkIfThenElse pred b1 b2 }
364 \underline{\bf Various data construction things}
365 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
367 dsExpr (ExplicitList elt_ty xs)
368 = dsExplicitList elt_ty xs
370 -- We desugar [:x1, ..., xn:] as
371 -- singletonP x1 +:+ ... +:+ singletonP xn
373 dsExpr (ExplicitPArr ty []) = do
374 emptyP <- dsLookupDPHId emptyPName
375 return (Var emptyP `App` Type ty)
376 dsExpr (ExplicitPArr ty xs) = do
377 singletonP <- dsLookupDPHId singletonPName
378 appP <- dsLookupDPHId appPName
379 xs' <- mapM dsLExpr xs
380 return . foldr1 (binary appP) $ map (unary singletonP) xs'
382 unary fn x = mkApps (Var fn) [Type ty, x]
383 binary fn x y = mkApps (Var fn) [Type ty, x, y]
385 dsExpr (ArithSeq expr (From from))
386 = App <$> dsExpr expr <*> dsLExpr from
388 dsExpr (ArithSeq expr (FromTo from to))
389 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
391 dsExpr (ArithSeq expr (FromThen from thn))
392 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
394 dsExpr (ArithSeq expr (FromThenTo from thn to))
395 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
397 dsExpr (PArrSeq expr (FromTo from to))
398 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
400 dsExpr (PArrSeq expr (FromThenTo from thn to))
401 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
404 = panic "DsExpr.dsExpr: Infinite parallel array!"
405 -- the parser shouldn't have generated it and the renamer and typechecker
406 -- shouldn't have let it through
410 \underline{\bf Record construction and update}
411 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
412 For record construction we do this (assuming T has three arguments)
416 let err = /\a -> recConErr a
417 T (recConErr t1 "M.lhs/230/op1")
419 (recConErr t1 "M.lhs/230/op3")
421 @recConErr@ then converts its arugment string into a proper message
422 before printing it as
424 M.lhs, line 230: missing field op1 was evaluated
427 We also handle @C{}@ as valid construction syntax for an unlabelled
428 constructor @C@, setting all of @C@'s fields to bottom.
431 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
432 con_expr' <- dsExpr con_expr
434 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
435 -- A newtype in the corner should be opaque;
436 -- hence TcType.tcSplitFunTys
438 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
439 = case findField (rec_flds rbinds) lbl of
440 (rhs:rhss) -> ASSERT( null rhss )
442 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
443 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
445 labels = dataConFieldLabels (idDataCon data_con_id)
446 -- The data_con_id is guaranteed to be the wrapper id of the constructor
448 con_args <- if null labels
449 then mapM unlabelled_bottom arg_tys
450 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
452 return (mkApps con_expr' con_args)
455 Record update is a little harder. Suppose we have the decl:
457 data T = T1 {op1, op2, op3 :: Int}
458 | T2 {op4, op2 :: Int}
461 Then we translate as follows:
467 T1 op1 _ op3 -> T1 op1 op2 op3
468 T2 op4 _ -> T2 op4 op2
469 other -> recUpdError "M.lhs/230"
471 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
472 RHSs, and do not generate a Core constructor application directly, because the constructor
473 might do some argument-evaluation first; and may have to throw away some
476 Note [Update for GADTs]
477 ~~~~~~~~~~~~~~~~~~~~~~~
480 T1 { f1 :: a } :: T a Int
482 Then the wrapper function for T1 has type
484 But if x::T a b, then
485 x { f1 = v } :: T a b (not T a Int!)
486 So we need to cast (T a Int) to (T a b). Sigh.
489 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
490 cons_to_upd in_inst_tys out_inst_tys)
492 = dsLExpr record_expr
494 = ASSERT2( notNull cons_to_upd, ppr expr )
496 do { record_expr' <- dsLExpr record_expr
497 ; field_binds' <- mapM ds_field fields
498 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
499 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
501 -- It's important to generate the match with matchWrapper,
502 -- and the right hand sides with applications of the wrapper Id
503 -- so that everything works when we are doing fancy unboxing on the
504 -- constructor aguments.
505 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
506 ; ([discrim_var], matching_code)
507 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
509 ; return (add_field_binds field_binds' $
510 bindNonRec discrim_var record_expr' matching_code) }
512 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
513 -- Clone the Id in the HsRecField, because its Name is that
514 -- of the record selector, and we must not make that a lcoal binder
515 -- else we shadow other uses of the record selector
516 -- Hence 'lcl_id'. Cf Trac #2735
517 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
518 ; let fld_id = unLoc (hsRecFieldId rec_field)
519 ; lcl_id <- newSysLocalDs (idType fld_id)
520 ; return (idName fld_id, lcl_id, rhs) }
522 add_field_binds [] expr = expr
523 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
525 -- Awkwardly, for families, the match goes
526 -- from instance type to family type
527 tycon = dataConTyCon (head cons_to_upd)
528 in_ty = mkTyConApp tycon in_inst_tys
529 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
531 mk_alt upd_fld_env con
532 = do { let (univ_tvs, ex_tvs, eq_spec,
533 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
534 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
536 -- I'm not bothering to clone the ex_tvs
537 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
538 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
539 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
540 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
541 (dataConFieldLabels con) arg_ids
542 mk_val_arg field_name pat_arg_id
543 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
544 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
545 -- Reconstruct with the WrapId so that unpacking happens
546 wrap = mkWpEvVarApps theta_vars `WpCompose`
547 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
548 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
549 , isNothing (lookupTyVar wrap_subst tv) ]
550 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
552 -- Tediously wrap the application in a cast
553 -- Note [Update for GADTs]
554 wrapped_rhs | null eq_spec = rhs
555 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
556 wrap_co = mkTyConApp tycon [ lookup tv ty
557 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
558 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
561 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
562 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
564 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
565 , pat_dicts = eqs_vars ++ theta_vars
566 , pat_binds = emptyTcEvBinds
567 , pat_args = PrefixCon $ map nlVarPat arg_ids
569 ; return (mkSimpleMatch [pat] wrapped_rhs) }
573 Here is where we desugar the Template Haskell brackets and escapes
576 -- Template Haskell stuff
578 #ifdef GHCI /* Only if bootstrapping */
579 dsExpr (HsBracketOut x ps) = dsBracket x ps
580 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
583 -- Arrow notation extension
584 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
590 dsExpr (HsTick ix vars e) = do
594 -- There is a problem here. The then and else branches
595 -- have no free variables, so they are open to lifting.
596 -- We need someway of stopping this.
597 -- This will make no difference to binary coverage
598 -- (did you go here: YES or NO), but will effect accurate
601 dsExpr (HsBinTick ixT ixF e) = do
603 do { ASSERT(exprType e2 `coreEqType` boolTy)
604 mkBinaryTickBox ixT ixF e2
610 -- HsSyn constructs that just shouldn't be here:
611 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
614 findField :: [HsRecField Id arg] -> Name -> [arg]
616 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
617 , lbl == idName (unLoc id) ]
620 %--------------------------------------------------------------------
622 Note [Desugaring explicit lists]
623 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
624 Explicit lists are desugared in a cleverer way to prevent some
625 fruitless allocations. Essentially, whenever we see a list literal
628 1. Find the tail of the list that can be allocated statically (say
629 [x_k, ..., x_n]) by later stages and ensure we desugar that
630 normally: this makes sure that we don't cause a code size increase
631 by having the cons in that expression fused (see later) and hence
632 being unable to statically allocate any more
634 2. For the prefix of the list which cannot be allocated statically,
635 say [x_1, ..., x_(k-1)], we turn it into an expression involving
636 build so that if we find any foldrs over it it will fuse away
639 So in this example we will desugar to:
640 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
642 If fusion fails to occur then build will get inlined and (since we
643 defined a RULE for foldr (:) []) we will get back exactly the
644 normal desugaring for an explicit list.
646 This optimisation can be worth a lot: up to 25% of the total
647 allocation in some nofib programs. Specifically
649 Program Size Allocs Runtime CompTime
650 rewrite +0.0% -26.3% 0.02 -1.8%
651 ansi -0.3% -13.8% 0.00 +0.0%
652 lift +0.0% -8.7% 0.00 -2.3%
654 Of course, if rules aren't turned on then there is pretty much no
655 point doing this fancy stuff, and it may even be harmful.
657 =======> Note by SLPJ Dec 08.
659 I'm unconvinced that we should *ever* generate a build for an explicit
660 list. See the comments in GHC.Base about the foldr/cons rule, which
661 points out that (foldr k z [a,b,c]) may generate *much* less code than
662 (a `k` b `k` c `k` z).
664 Furthermore generating builds messes up the LHS of RULES.
665 Example: the foldr/single rule in GHC.Base
667 We do not want to generate a build invocation on the LHS of this RULE!
669 We fix this by disabling rules in rule LHSs, and testing that
670 flag here; see Note [Desugaring RULE left hand sides] in Desugar
672 To test this I've added a (static) flag -fsimple-list-literals, which
673 makes all list literals be generated via the simple route.
677 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
678 -- See Note [Desugaring explicit lists]
679 dsExplicitList elt_ty xs
680 = do { dflags <- getDOptsDs
681 ; xs' <- mapM dsLExpr xs
682 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
683 ; if opt_SimpleListLiterals -- -fsimple-list-literals
684 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
685 -- Don't generate a build if there are no rules to eliminate it!
686 -- See Note [Desugaring RULE left hand sides] in Desugar
687 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
688 then return $ mkListExpr elt_ty xs'
689 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
691 is_static :: CoreExpr -> Bool
692 is_static e = all is_static_var (varSetElems (exprFreeVars e))
694 is_static_var :: Var -> Bool
696 | isId v = isExternalName (idName v) -- Top-level things are given external names
697 | otherwise = False -- Type variables
699 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
700 = do { let suffix' = mkListExpr elt_ty suffix
701 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
702 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
704 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
705 spanTail f xs = (reverse rejected, reverse satisfying)
706 where (satisfying, rejected) = span f $ reverse xs
709 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
710 handled in DsListComp). Basically does the translation given in the
716 -> Type -- Type of the whole expression
719 dsDo stmts body result_ty
722 -- result_ty must be of the form (m b)
723 (m_ty, _b_ty) = tcSplitAppTy result_ty
725 goL [] = dsLExpr body
726 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
728 go _ (ExprStmt rhs then_expr _ _) stmts
729 = do { rhs2 <- dsLExpr rhs
730 ; case tcSplitAppTy_maybe (exprType rhs2) of
731 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
733 ; then_expr2 <- dsExpr then_expr
735 ; return (mkApps then_expr2 [rhs2, rest]) }
737 go _ (LetStmt binds) stmts
738 = do { rest <- goL stmts
739 ; dsLocalBinds binds rest }
741 go _ (BindStmt pat rhs bind_op fail_op) stmts
742 = do { body <- goL stmts
743 ; rhs' <- dsLExpr rhs
744 ; bind_op' <- dsExpr bind_op
745 ; var <- selectSimpleMatchVarL pat
746 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
747 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
748 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
749 res1_ty (cantFailMatchResult body)
750 ; match_code <- handle_failure pat match fail_op
751 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
753 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
754 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
755 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
756 , recS_rec_rets = rec_rets }) stmts
757 = ASSERT( length rec_ids > 0 )
758 goL (new_bind_stmt : stmts)
760 -- returnE <- dsExpr return_id
761 -- mfixE <- dsExpr mfix_id
762 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
764 noSyntaxExpr -- Tuple cannot fail
766 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
767 rec_tup_pats = map nlVarPat tup_ids
768 later_pats = rec_tup_pats
769 rets = map noLoc rec_rets
771 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
772 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
773 (mkFunTy tup_ty body_ty))
774 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
775 body = noLoc $ HsDo DoExpr rec_stmts return_app noSyntaxExpr body_ty
776 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
777 body_ty = mkAppTy m_ty tup_ty
778 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
780 handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
781 -- In a do expression, pattern-match failure just calls
782 -- the monadic 'fail' rather than throwing an exception
783 handle_failure pat match fail_op
785 = do { fail_op' <- dsExpr fail_op
786 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
787 ; extractMatchResult match (App fail_op' fail_msg) }
789 = extractMatchResult match (error "It can't fail")
791 mk_fail_msg :: Located e -> String
792 mk_fail_msg pat = "Pattern match failure in do expression at " ++
793 showSDoc (ppr (getLoc pat))
796 Translation for RecStmt's:
797 -----------------------------
798 We turn (RecStmt [v1,..vn] stmts) into:
800 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
805 dsMDo :: HsStmtContext Name
809 -> Type -- Type of the whole expression
812 dsMDo ctxt tbl stmts body result_ty
815 goL [] = dsLExpr body
816 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
818 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
819 return_id = lookupEvidence tbl returnMName
820 bind_id = lookupEvidence tbl bindMName
821 then_id = lookupEvidence tbl thenMName
822 fail_id = lookupEvidence tbl failMName
824 go _ (LetStmt binds) stmts
825 = do { rest <- goL stmts
826 ; dsLocalBinds binds rest }
828 go _ (ExprStmt rhs then_expr rhs_ty) stmts
829 = do { rhs2 <- dsLExpr rhs
830 ; warnDiscardedDoBindings rhs m_ty rhs_ty
831 ; then_expr2 <- dsExpr then_expr
833 ; return (mkApps then_expr2 [rhs2, rest]) }
835 go _ (BindStmt pat rhs bind_op _) stmts
836 = do { body <- goL stmts
837 ; rhs' <- dsLExpr rhs
838 ; bind_op' <- dsExpr bind_op
839 ; var <- selectSimpleMatchVarL pat
840 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
841 result_ty (cantFailMatchResult body)
842 ; match_code <- handle_failure pat match fail_op
843 ; return (mkApps bind_op [rhs', Lam var match_code]) }
845 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
846 , recS_rec_ids = rec_ids, recS_rec_rets = rec_rets
847 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op }) stmts
848 = ASSERT( length rec_ids > 0 )
849 ASSERT( length rec_ids == length rec_rets )
850 ASSERT( isEmptyTcEvBinds _ev_binds )
851 pprTrace "dsMDo" (ppr later_ids) $
852 goL (new_bind_stmt : stmts)
854 new_bind_stmt = L loc $ BindStmt (mk_tup_pat later_pats) mfix_app
857 -- Remove the later_ids that appear (without fancy coercions)
858 -- in rec_rets, because there's no need to knot-tie them separately
859 -- See Note [RecStmt] in HsExpr
860 later_ids' = filter (`notElem` mono_rec_ids) later_ids
861 mono_rec_ids = [ id | HsVar id <- rec_rets ]
863 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
864 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
865 (mkFunTy tup_ty body_ty))
867 -- The rec_tup_pat must bind the rec_ids only; remember that the
868 -- trimmed_laters may share the same Names
869 -- Meanwhile, the later_pats must bind the later_vars
870 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
871 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
872 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
874 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
875 body = noLoc $ HsDo ctxt rec_stmts return_app noSyntaxExpr body_ty
876 body_ty = mkAppTy m_ty tup_ty
877 tup_ty = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids)) -- Deals with singleton case
879 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
881 mk_wild_pat :: Id -> LPat Id
882 mk_wild_pat v = noLoc $ WildPat $ idType v
884 mk_later_pat :: Id -> LPat Id
885 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
886 | otherwise = nlVarPat v
888 mk_tup_pat :: [LPat Id] -> LPat Id
890 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
894 %************************************************************************
896 Warning about identities
898 %************************************************************************
900 Warn about functions that convert between one type and another
901 when the to- and from- types are the same. Then it's probably
902 (albeit not definitely) the identity
904 warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
905 warnAboutIdentities (Var v) co_fn
906 | idName v `elem` conversionNames
907 , let fun_ty = exprType (co_fn (Var v))
908 , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
909 , arg_ty `tcEqType` res_ty -- So we are converting ty -> ty
910 = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
911 , nest 2 $ ptext (sLit "can probably be omitted")
912 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
914 warnAboutIdentities _ _ = return ()
916 conversionNames :: [Name]
918 = [ toIntegerName, toRationalName
919 , fromIntegralName, realToFracName ]
920 -- We can't easily add fromIntegerName, fromRationalName,
921 -- becuase they are generated by literals
924 %************************************************************************
926 \subsection{Errors and contexts}
928 %************************************************************************
931 -- Warn about certain types of values discarded in monadic bindings (#3263)
932 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
933 warnDiscardedDoBindings rhs container_ty returning_ty = do {
934 -- Warn about discarding non-() things in 'monadic' binding
935 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
936 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
937 then warnDs (unusedMonadBind rhs returning_ty)
939 -- Warn about discarding m a things in 'monadic' binding of the same type,
940 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
941 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
942 ; case tcSplitAppTy_maybe returning_ty of
943 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
944 warnDs (wrongMonadBind rhs returning_ty)
947 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
948 unusedMonadBind rhs returning_ty
949 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
950 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
951 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
953 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
954 wrongMonadBind rhs returning_ty
955 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
956 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
957 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")