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
39 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
40 -- needs to see source types
70 %************************************************************************
72 dsLocalBinds, dsValBinds
74 %************************************************************************
77 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
78 dsLocalBinds EmptyLocalBinds body = return body
79 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
80 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
82 -------------------------
83 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
84 dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
86 -------------------------
87 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
88 dsIPBinds (IPBinds ip_binds ev_binds) body
89 = do { ds_ev_binds <- dsTcEvBinds ev_binds
90 ; let inner = wrapDsEvBinds ds_ev_binds body
91 -- The dict bindings may not be in
92 -- dependency order; hence Rec
93 ; foldrM ds_ip_bind inner ip_binds }
95 ds_ip_bind (L _ (IPBind n e)) body
97 return (Let (NonRec (ipNameName n) e') body)
99 -------------------------
100 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
101 -- Special case for bindings which bind unlifted variables
102 -- We need to do a case right away, rather than building
103 -- a tuple and doing selections.
104 -- Silently ignore INLINE and SPECIALISE pragmas...
105 ds_val_bind (NonRecursive, hsbinds) body
106 | [L loc bind] <- bagToList hsbinds,
107 -- Non-recursive, non-overloaded bindings only come in ones
108 -- ToDo: in some bizarre case it's conceivable that there
109 -- could be dict binds in the 'binds'. (See the notes
110 -- below. Then pattern-match would fail. Urk.)
112 = putSrcSpanDs loc (dsStrictBind bind body)
114 -- Ordinary case for bindings; none should be unlifted
115 ds_val_bind (_is_rec, binds) body
116 = do { prs <- dsLHsBinds binds
117 ; ASSERT2( not (any (isUnLiftedType . idType . fst) prs), ppr _is_rec $$ ppr binds )
120 _ -> return (Let (Rec prs) body) }
121 -- Use a Rec regardless of is_rec.
122 -- Why? Because it allows the binds to be all
123 -- mixed up, which is what happens in one rare case
124 -- Namely, for an AbsBind with no tyvars and no dicts,
125 -- but which does have dictionary bindings.
126 -- See notes with TcSimplify.inferLoop [NO TYVARS]
127 -- It turned out that wrapping a Rec here was the easiest solution
129 -- NB The previous case dealt with unlifted bindings, so we
130 -- only have to deal with lifted ones now; so Rec is ok
133 dsStrictBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
134 dsStrictBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
135 , abs_exports = exports
136 , abs_ev_binds = ev_binds
137 , abs_binds = binds }) body
138 = do { ds_ev_binds <- dsTcEvBinds ev_binds
139 ; let body1 = foldr bind_export body exports
140 bind_export (_, g, l, _) b = bindNonRec g (Var l) b
141 ; body2 <- foldlBagM (\body bind -> dsStrictBind (unLoc bind) body)
143 ; return (wrapDsEvBinds ds_ev_binds body2) }
145 dsStrictBind (FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn
146 , fun_tick = tick, fun_infix = inf }) body
147 -- Can't be a bang pattern (that looks like a PatBind)
148 -- so must be simply unboxed
149 = do { (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
150 ; MASSERT( null args ) -- Functions aren't lifted
151 ; MASSERT( isIdHsWrapper co_fn )
152 ; rhs' <- mkOptTickBox tick rhs
153 ; return (bindNonRec fun rhs' body) }
155 dsStrictBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
156 = -- let C x# y# = rhs in body
157 -- ==> case rhs of C x# y# -> body
158 do { rhs <- dsGuarded grhss ty
159 ; let upat = unLoc pat
160 eqn = EqnInfo { eqn_pats = [upat],
161 eqn_rhs = cantFailMatchResult body }
162 ; var <- selectMatchVar upat
163 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
164 ; return (scrungleMatch var rhs result) }
166 dsStrictBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
168 ----------------------
169 strictMatchOnly :: HsBind Id -> Bool
170 strictMatchOnly (AbsBinds { abs_binds = binds })
171 = anyBag (strictMatchOnly . unLoc) binds
172 strictMatchOnly (PatBind { pat_lhs = lpat, pat_rhs_ty = ty })
173 = isUnboxedTupleType ty
175 || any (isUnLiftedType . idType) (collectPatBinders lpat)
176 strictMatchOnly (FunBind { fun_id = L _ id })
177 = isUnLiftedType (idType id)
178 strictMatchOnly _ = False -- I hope! Checked immediately by caller in fact
180 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
181 -- Returns something like (let var = scrut in body)
182 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
183 -- Special case to handle unboxed tuple patterns; they can't appear nested
185 -- case e of (# p1, p2 #) -> rhs
187 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
189 -- let x = e in case x of ....
191 -- But there may be a big
192 -- let fail = ... in case e of ...
193 -- wrapping the whole case, which complicates matters slightly
194 -- It all seems a bit fragile. Test is dsrun013.
196 scrungleMatch var scrut body
197 | isUnboxedTupleType (idType var) = scrungle body
198 | otherwise = bindNonRec var scrut body
200 scrungle (Case (Var x) bndr ty alts)
201 | x == var = Case scrut bndr ty alts
202 scrungle (Let binds body) = Let binds (scrungle body)
203 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
207 %************************************************************************
209 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
211 %************************************************************************
214 dsLExpr :: LHsExpr Id -> DsM CoreExpr
216 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
218 dsExpr :: HsExpr Id -> DsM CoreExpr
219 dsExpr (HsPar e) = dsLExpr e
220 dsExpr (ExprWithTySigOut e _) = dsLExpr e
221 dsExpr (HsVar var) = return (Var var)
222 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
223 dsExpr (HsLit lit) = dsLit lit
224 dsExpr (HsOverLit lit) = dsOverLit lit
226 dsExpr (HsWrap co_fn e)
227 = do { co_fn' <- dsHsWrapper co_fn
229 ; warn_id <- doptDs Opt_WarnIdentities
230 ; when warn_id $ warnAboutIdentities e' co_fn'
231 ; return (co_fn' e') }
233 dsExpr (NegApp expr neg_expr)
234 = App <$> dsExpr neg_expr <*> dsLExpr expr
236 dsExpr (HsLam a_Match)
237 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
239 dsExpr (HsApp fun arg)
240 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
243 Operator sections. At first it looks as if we can convert
252 But no! expr might be a redex, and we can lose laziness badly this
257 for example. So we convert instead to
259 let y = expr in \x -> op y x
261 If \tr{expr} is actually just a variable, say, then the simplifier
265 dsExpr (OpApp e1 op _ e2)
266 = -- for the type of y, we need the type of op's 2nd argument
267 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
269 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
270 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
272 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
273 dsExpr (SectionR op expr) = do
274 core_op <- dsLExpr op
275 -- for the type of x, we need the type of op's 2nd argument
276 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
277 -- See comment with SectionL
278 y_core <- dsLExpr expr
279 x_id <- newSysLocalDs x_ty
280 y_id <- newSysLocalDs y_ty
281 return (bindNonRec y_id y_core $
282 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
284 dsExpr (ExplicitTuple tup_args boxity)
285 = do { let go (lam_vars, args) (Missing ty)
286 -- For every missing expression, we need
287 -- another lambda in the desugaring.
288 = do { lam_var <- newSysLocalDs ty
289 ; return (lam_var : lam_vars, Var lam_var : args) }
290 go (lam_vars, args) (Present expr)
291 -- Expressions that are present don't generate
292 -- lambdas, just arguments.
293 = do { core_expr <- dsLExpr expr
294 ; return (lam_vars, core_expr : args) }
296 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
297 -- The reverse is because foldM goes left-to-right
299 ; return $ mkCoreLams lam_vars $
300 mkConApp (tupleCon boxity (length tup_args))
301 (map (Type . exprType) args ++ args) }
303 dsExpr (HsSCC cc expr) = do
304 mod_name <- getModuleDs
305 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
307 dsExpr (HsCoreAnn fs expr)
308 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
310 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
311 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
312 = -- So desugar empty HsCase to error call
313 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
316 = do { core_discrim <- dsLExpr discrim
317 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
318 ; return (scrungleMatch discrim_var core_discrim matching_code) }
320 -- Pepe: The binds are in scope in the body but NOT in the binding group
321 -- This is to avoid silliness in breakpoints
322 dsExpr (HsLet binds body) = do
323 body' <- dsLExpr body
324 dsLocalBinds binds body'
326 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
327 -- because the interpretation of `stmts' depends on what sort of thing it is.
329 dsExpr (HsDo ListComp stmts body result_ty)
330 = -- Special case for list comprehensions
331 dsListComp stmts body elt_ty
333 [elt_ty] = tcTyConAppArgs result_ty
335 dsExpr (HsDo DoExpr stmts body result_ty)
336 = dsDo stmts body result_ty
338 dsExpr (HsDo GhciStmt stmts body result_ty)
339 = dsDo stmts body result_ty
341 dsExpr (HsDo ctxt@(MDoExpr tbl) stmts body result_ty)
342 = do { (meth_binds, tbl') <- dsSyntaxTable tbl
343 ; core_expr <- dsMDo ctxt tbl' stmts body result_ty
344 ; return (mkLets meth_binds core_expr) }
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 <- dsLookupGlobalId emptyPName
375 return (Var emptyP `App` Type ty)
376 dsExpr (ExplicitPArr ty xs) = do
377 singletonP <- dsLookupGlobalId singletonPName
378 appP <- dsLookupGlobalId 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, recS_dicts = _ev_binds }) stmts
757 = ASSERT( length rec_ids > 0 )
758 ASSERT( isEmptyTcEvBinds _ev_binds ) -- No method binds
759 goL (new_bind_stmt : stmts)
761 -- returnE <- dsExpr return_id
762 -- mfixE <- dsExpr mfix_id
763 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
765 noSyntaxExpr -- Tuple cannot fail
767 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
768 rec_tup_pats = map nlVarPat tup_ids
769 later_pats = rec_tup_pats
770 rets = map noLoc rec_rets
772 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
773 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
774 (mkFunTy tup_ty body_ty))
775 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
776 body = noLoc $ HsDo DoExpr rec_stmts return_app body_ty
777 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
778 body_ty = mkAppTy m_ty tup_ty
779 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
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
804 dsMDo :: HsStmtContext Name
808 -> Type -- Type of the whole expression
811 dsMDo ctxt tbl stmts body result_ty
814 goL [] = dsLExpr body
815 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
817 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
818 mfix_id = lookupEvidence tbl mfixName
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 _ rhs_ty) stmts
829 = do { rhs2 <- dsLExpr rhs
830 ; warnDiscardedDoBindings rhs m_ty rhs_ty
832 ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
834 go _ (BindStmt pat rhs _ _) stmts
835 = do { body <- goL stmts
836 ; var <- selectSimpleMatchVarL pat
837 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
838 result_ty (cantFailMatchResult body)
839 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
840 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
841 ; match_code <- extractMatchResult match fail_expr
843 ; rhs' <- dsLExpr rhs
844 ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
845 rhs', Lam var match_code]) }
847 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
848 , recS_rec_ids = rec_ids, recS_rec_rets = rec_rets
849 , recS_dicts = _ev_binds }) stmts
850 = ASSERT( length rec_ids > 0 )
851 ASSERT( length rec_ids == length rec_rets )
852 ASSERT( isEmptyTcEvBinds _ev_binds )
853 pprTrace "dsMDo" (ppr later_ids) $
854 goL (new_bind_stmt : stmts)
856 new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
858 -- Remove the later_ids that appear (without fancy coercions)
859 -- in rec_rets, because there's no need to knot-tie them separately
860 -- See Note [RecStmt] in HsExpr
861 later_ids' = filter (`notElem` mono_rec_ids) later_ids
862 mono_rec_ids = [ id | HsVar id <- rec_rets ]
864 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
865 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
866 (mkFunTy tup_ty body_ty))
868 -- The rec_tup_pat must bind the rec_ids only; remember that the
869 -- trimmed_laters may share the same Names
870 -- Meanwhile, the later_pats must bind the later_vars
871 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
872 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
873 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
875 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
876 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
877 body_ty = mkAppTy m_ty tup_ty
878 tup_ty = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids)) -- Deals with singleton case
880 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
881 (mkLHsTupleExpr rets)
883 mk_wild_pat :: Id -> LPat Id
884 mk_wild_pat v = noLoc $ WildPat $ idType v
886 mk_later_pat :: Id -> LPat Id
887 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
888 | otherwise = nlVarPat v
890 mk_tup_pat :: [LPat Id] -> LPat Id
892 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
896 %************************************************************************
898 Warning about identities
900 %************************************************************************
902 Warn about functions that convert between one type and another
903 when the to- and from- types are the same. Then it's probably
904 (albeit not definitely) the identity
906 warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
907 warnAboutIdentities (Var v) co_fn
908 | idName v `elem` conversionNames
909 , let fun_ty = exprType (co_fn (Var v))
910 , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
911 , arg_ty `tcEqType` res_ty -- So we are converting ty -> ty
912 = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
913 , nest 2 $ ptext (sLit "can probably be omitted")
914 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
916 warnAboutIdentities _ _ = return ()
918 conversionNames :: [Name]
920 = [ toIntegerName, toRationalName
921 , fromIntegralName, realToFracName ]
922 -- We can't easily add fromIntegerName, fromRationalName,
923 -- becuase they are generated by literals
926 %************************************************************************
928 \subsection{Errors and contexts}
930 %************************************************************************
933 -- Warn about certain types of values discarded in monadic bindings (#3263)
934 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
935 warnDiscardedDoBindings rhs container_ty returning_ty = do {
936 -- Warn about discarding non-() things in 'monadic' binding
937 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
938 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
939 then warnDs (unusedMonadBind rhs returning_ty)
941 -- Warn about discarding m a things in 'monadic' binding of the same type,
942 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
943 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
944 ; case tcSplitAppTy_maybe returning_ty of
945 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
946 warnDs (wrongMonadBind rhs returning_ty)
949 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
950 unusedMonadBind rhs returning_ty
951 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
952 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
953 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
955 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
956 wrongMonadBind rhs returning_ty
957 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
958 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
959 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")