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 res_ty) = dsListComp stmts res_ty
329 dsExpr (HsDo PArrComp stmts _) = dsPArrComp (map unLoc stmts)
330 dsExpr (HsDo DoExpr stmts _) = dsDo stmts
331 dsExpr (HsDo GhciStmt stmts _) = dsDo stmts
332 dsExpr (HsDo MDoExpr stmts _) = dsDo stmts
333 dsExpr (HsDo MonadComp stmts _) = dsMonadComp stmts
335 dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
336 = do { pred <- dsLExpr guard_expr
337 ; b1 <- dsLExpr then_expr
338 ; b2 <- dsLExpr else_expr
340 Just fun -> do { core_fun <- dsExpr fun
341 ; return (mkCoreApps core_fun [pred,b1,b2]) }
342 Nothing -> return $ mkIfThenElse pred b1 b2 }
347 \underline{\bf Various data construction things}
348 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
350 dsExpr (ExplicitList elt_ty xs)
351 = dsExplicitList elt_ty xs
353 -- We desugar [:x1, ..., xn:] as
354 -- singletonP x1 +:+ ... +:+ singletonP xn
356 dsExpr (ExplicitPArr ty []) = do
357 emptyP <- dsLookupDPHId emptyPName
358 return (Var emptyP `App` Type ty)
359 dsExpr (ExplicitPArr ty xs) = do
360 singletonP <- dsLookupDPHId singletonPName
361 appP <- dsLookupDPHId appPName
362 xs' <- mapM dsLExpr xs
363 return . foldr1 (binary appP) $ map (unary singletonP) xs'
365 unary fn x = mkApps (Var fn) [Type ty, x]
366 binary fn x y = mkApps (Var fn) [Type ty, x, y]
368 dsExpr (ArithSeq expr (From from))
369 = App <$> dsExpr expr <*> dsLExpr from
371 dsExpr (ArithSeq expr (FromTo from to))
372 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
374 dsExpr (ArithSeq expr (FromThen from thn))
375 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
377 dsExpr (ArithSeq expr (FromThenTo from thn to))
378 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
380 dsExpr (PArrSeq expr (FromTo from to))
381 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
383 dsExpr (PArrSeq expr (FromThenTo from thn to))
384 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
387 = panic "DsExpr.dsExpr: Infinite parallel array!"
388 -- the parser shouldn't have generated it and the renamer and typechecker
389 -- shouldn't have let it through
393 \underline{\bf Record construction and update}
394 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
395 For record construction we do this (assuming T has three arguments)
399 let err = /\a -> recConErr a
400 T (recConErr t1 "M.lhs/230/op1")
402 (recConErr t1 "M.lhs/230/op3")
404 @recConErr@ then converts its arugment string into a proper message
405 before printing it as
407 M.lhs, line 230: missing field op1 was evaluated
410 We also handle @C{}@ as valid construction syntax for an unlabelled
411 constructor @C@, setting all of @C@'s fields to bottom.
414 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
415 con_expr' <- dsExpr con_expr
417 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
418 -- A newtype in the corner should be opaque;
419 -- hence TcType.tcSplitFunTys
421 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
422 = case findField (rec_flds rbinds) lbl of
423 (rhs:rhss) -> ASSERT( null rhss )
425 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
426 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
428 labels = dataConFieldLabels (idDataCon data_con_id)
429 -- The data_con_id is guaranteed to be the wrapper id of the constructor
431 con_args <- if null labels
432 then mapM unlabelled_bottom arg_tys
433 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
435 return (mkApps con_expr' con_args)
438 Record update is a little harder. Suppose we have the decl:
440 data T = T1 {op1, op2, op3 :: Int}
441 | T2 {op4, op2 :: Int}
444 Then we translate as follows:
450 T1 op1 _ op3 -> T1 op1 op2 op3
451 T2 op4 _ -> T2 op4 op2
452 other -> recUpdError "M.lhs/230"
454 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
455 RHSs, and do not generate a Core constructor application directly, because the constructor
456 might do some argument-evaluation first; and may have to throw away some
459 Note [Update for GADTs]
460 ~~~~~~~~~~~~~~~~~~~~~~~
463 T1 { f1 :: a } :: T a Int
465 Then the wrapper function for T1 has type
467 But if x::T a b, then
468 x { f1 = v } :: T a b (not T a Int!)
469 So we need to cast (T a Int) to (T a b). Sigh.
472 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
473 cons_to_upd in_inst_tys out_inst_tys)
475 = dsLExpr record_expr
477 = ASSERT2( notNull cons_to_upd, ppr expr )
479 do { record_expr' <- dsLExpr record_expr
480 ; field_binds' <- mapM ds_field fields
481 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
482 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
484 -- It's important to generate the match with matchWrapper,
485 -- and the right hand sides with applications of the wrapper Id
486 -- so that everything works when we are doing fancy unboxing on the
487 -- constructor aguments.
488 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
489 ; ([discrim_var], matching_code)
490 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
492 ; return (add_field_binds field_binds' $
493 bindNonRec discrim_var record_expr' matching_code) }
495 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
496 -- Clone the Id in the HsRecField, because its Name is that
497 -- of the record selector, and we must not make that a lcoal binder
498 -- else we shadow other uses of the record selector
499 -- Hence 'lcl_id'. Cf Trac #2735
500 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
501 ; let fld_id = unLoc (hsRecFieldId rec_field)
502 ; lcl_id <- newSysLocalDs (idType fld_id)
503 ; return (idName fld_id, lcl_id, rhs) }
505 add_field_binds [] expr = expr
506 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
508 -- Awkwardly, for families, the match goes
509 -- from instance type to family type
510 tycon = dataConTyCon (head cons_to_upd)
511 in_ty = mkTyConApp tycon in_inst_tys
512 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
514 mk_alt upd_fld_env con
515 = do { let (univ_tvs, ex_tvs, eq_spec,
516 theta, arg_tys, _) = dataConFullSig con
517 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
519 -- I'm not bothering to clone the ex_tvs
520 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
521 ; theta_vars <- mapM newPredVarDs (substTheta subst theta)
522 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
523 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
524 (dataConFieldLabels con) arg_ids
525 mk_val_arg field_name pat_arg_id
526 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
527 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
528 -- Reconstruct with the WrapId so that unpacking happens
529 wrap = mkWpEvVarApps theta_vars `WpCompose`
530 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
531 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
532 , not (tv `elemVarEnv` wrap_subst) ]
533 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
535 -- Tediously wrap the application in a cast
536 -- Note [Update for GADTs]
537 wrapped_rhs | null eq_spec = rhs
538 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
539 wrap_co = mkTyConAppCo tycon [ lookup tv ty
540 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
541 lookup univ_tv ty = case lookupVarEnv wrap_subst univ_tv of
543 Nothing -> mkReflCo ty
544 wrap_subst = mkVarEnv [ (tv, mkSymCo (mkCoVarCo co_var))
545 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
547 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
548 , pat_dicts = eqs_vars ++ theta_vars
549 , pat_binds = emptyTcEvBinds
550 , pat_args = PrefixCon $ map nlVarPat arg_ids
552 ; return (mkSimpleMatch [pat] wrapped_rhs) }
556 Here is where we desugar the Template Haskell brackets and escapes
559 -- Template Haskell stuff
561 #ifdef GHCI /* Only if bootstrapping */
562 dsExpr (HsBracketOut x ps) = dsBracket x ps
563 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
566 -- Arrow notation extension
567 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
573 dsExpr (HsTick ix vars e) = do
577 -- There is a problem here. The then and else branches
578 -- have no free variables, so they are open to lifting.
579 -- We need someway of stopping this.
580 -- This will make no difference to binary coverage
581 -- (did you go here: YES or NO), but will effect accurate
584 dsExpr (HsBinTick ixT ixF e) = do
586 do { ASSERT(exprType e2 `eqType` boolTy)
587 mkBinaryTickBox ixT ixF e2
593 -- HsSyn constructs that just shouldn't be here:
594 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
597 findField :: [HsRecField Id arg] -> Name -> [arg]
599 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
600 , lbl == idName (unLoc id) ]
603 %--------------------------------------------------------------------
605 Note [Desugaring explicit lists]
606 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
607 Explicit lists are desugared in a cleverer way to prevent some
608 fruitless allocations. Essentially, whenever we see a list literal
611 1. Find the tail of the list that can be allocated statically (say
612 [x_k, ..., x_n]) by later stages and ensure we desugar that
613 normally: this makes sure that we don't cause a code size increase
614 by having the cons in that expression fused (see later) and hence
615 being unable to statically allocate any more
617 2. For the prefix of the list which cannot be allocated statically,
618 say [x_1, ..., x_(k-1)], we turn it into an expression involving
619 build so that if we find any foldrs over it it will fuse away
622 So in this example we will desugar to:
623 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
625 If fusion fails to occur then build will get inlined and (since we
626 defined a RULE for foldr (:) []) we will get back exactly the
627 normal desugaring for an explicit list.
629 This optimisation can be worth a lot: up to 25% of the total
630 allocation in some nofib programs. Specifically
632 Program Size Allocs Runtime CompTime
633 rewrite +0.0% -26.3% 0.02 -1.8%
634 ansi -0.3% -13.8% 0.00 +0.0%
635 lift +0.0% -8.7% 0.00 -2.3%
637 Of course, if rules aren't turned on then there is pretty much no
638 point doing this fancy stuff, and it may even be harmful.
640 =======> Note by SLPJ Dec 08.
642 I'm unconvinced that we should *ever* generate a build for an explicit
643 list. See the comments in GHC.Base about the foldr/cons rule, which
644 points out that (foldr k z [a,b,c]) may generate *much* less code than
645 (a `k` b `k` c `k` z).
647 Furthermore generating builds messes up the LHS of RULES.
648 Example: the foldr/single rule in GHC.Base
650 We do not want to generate a build invocation on the LHS of this RULE!
652 We fix this by disabling rules in rule LHSs, and testing that
653 flag here; see Note [Desugaring RULE left hand sides] in Desugar
655 To test this I've added a (static) flag -fsimple-list-literals, which
656 makes all list literals be generated via the simple route.
660 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
661 -- See Note [Desugaring explicit lists]
662 dsExplicitList elt_ty xs
663 = do { dflags <- getDOptsDs
664 ; xs' <- mapM dsLExpr xs
665 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
666 ; if opt_SimpleListLiterals -- -fsimple-list-literals
667 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
668 -- Don't generate a build if there are no rules to eliminate it!
669 -- See Note [Desugaring RULE left hand sides] in Desugar
670 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
671 then return $ mkListExpr elt_ty xs'
672 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
674 is_static :: CoreExpr -> Bool
675 is_static e = all is_static_var (varSetElems (exprFreeVars e))
677 is_static_var :: Var -> Bool
679 | isId v = isExternalName (idName v) -- Top-level things are given external names
680 | otherwise = False -- Type variables
682 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
683 = do { let suffix' = mkListExpr elt_ty suffix
684 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
685 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
687 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
688 spanTail f xs = (reverse rejected, reverse satisfying)
689 where (satisfying, rejected) = span f $ reverse xs
692 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
693 handled in DsListComp). Basically does the translation given in the
697 dsDo :: [LStmt Id] -> DsM CoreExpr
701 goL [] = panic "dsDo"
702 goL (L loc stmt:lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
704 go _ (LastStmt body _) stmts
705 = ASSERT( null stmts ) dsLExpr body
706 -- The 'return' op isn't used for 'do' expressions
708 go _ (ExprStmt rhs then_expr _ _) stmts
709 = do { rhs2 <- dsLExpr rhs
710 ; warnDiscardedDoBindings rhs (exprType rhs2)
711 ; then_expr2 <- dsExpr then_expr
713 ; return (mkApps then_expr2 [rhs2, rest]) }
715 go _ (LetStmt binds) stmts
716 = do { rest <- goL stmts
717 ; dsLocalBinds binds rest }
719 go _ (BindStmt pat rhs bind_op fail_op) stmts
720 = do { body <- goL stmts
721 ; rhs' <- dsLExpr rhs
722 ; bind_op' <- dsExpr bind_op
723 ; var <- selectSimpleMatchVarL pat
724 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
725 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
726 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
727 res1_ty (cantFailMatchResult body)
728 ; match_code <- handle_failure pat match fail_op
729 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
731 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
732 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
733 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
734 , recS_rec_rets = rec_rets, recS_ret_ty = body_ty }) stmts
735 = ASSERT( length rec_ids > 0 )
736 goL (new_bind_stmt : stmts)
738 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats)
740 noSyntaxExpr -- Tuple cannot fail
742 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
743 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
744 rec_tup_pats = map nlVarPat tup_ids
745 later_pats = rec_tup_pats
746 rets = map noLoc rec_rets
747 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
748 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
749 (mkFunTy tup_ty body_ty))
750 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
751 body = noLoc $ HsDo DoExpr (rec_stmts ++ [ret_stmt]) body_ty
752 ret_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
753 ret_stmt = noLoc $ mkLastStmt ret_app
754 -- This LastStmt will be desugared with dsDo,
755 -- which ignores the return_op in the LastStmt,
756 -- so we must apply the return_op explicitly
758 handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
759 -- In a do expression, pattern-match failure just calls
760 -- the monadic 'fail' rather than throwing an exception
761 handle_failure pat match fail_op
763 = do { fail_op' <- dsExpr fail_op
764 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
765 ; extractMatchResult match (App fail_op' fail_msg) }
767 = extractMatchResult match (error "It can't fail")
769 mk_fail_msg :: Located e -> String
770 mk_fail_msg pat = "Pattern match failure in do expression at " ++
771 showSDoc (ppr (getLoc pat))
775 %************************************************************************
777 Warning about identities
779 %************************************************************************
781 Warn about functions that convert between one type and another
782 when the to- and from- types are the same. Then it's probably
783 (albeit not definitely) the identity
785 warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
786 warnAboutIdentities (Var v) co_fn
787 | idName v `elem` conversionNames
788 , let fun_ty = exprType (co_fn (Var v))
789 , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
790 , arg_ty `eqType` res_ty -- So we are converting ty -> ty
791 = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
792 , nest 2 $ ptext (sLit "can probably be omitted")
793 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
795 warnAboutIdentities _ _ = return ()
797 conversionNames :: [Name]
799 = [ toIntegerName, toRationalName
800 , fromIntegralName, realToFracName ]
801 -- We can't easily add fromIntegerName, fromRationalName,
802 -- becuase they are generated by literals
805 %************************************************************************
807 \subsection{Errors and contexts}
809 %************************************************************************
812 -- Warn about certain types of values discarded in monadic bindings (#3263)
813 warnDiscardedDoBindings :: LHsExpr Id -> Type -> DsM ()
814 warnDiscardedDoBindings rhs rhs_ty
815 | Just (m_ty, elt_ty) <- tcSplitAppTy_maybe rhs_ty
816 = do { -- Warn about discarding non-() things in 'monadic' binding
817 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
818 ; if warn_unused && not (isUnitTy elt_ty)
819 then warnDs (unusedMonadBind rhs elt_ty)
821 -- Warn about discarding m a things in 'monadic' binding of the same type,
822 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
823 do { warn_wrong <- doptDs Opt_WarnWrongDoBind
824 ; case tcSplitAppTy_maybe elt_ty of
825 Just (elt_m_ty, _) | warn_wrong, m_ty `eqType` elt_m_ty
826 -> warnDs (wrongMonadBind rhs elt_ty)
829 | otherwise -- RHS does have type of form (m ty), which is wierd
830 = return () -- but at lesat this warning is irrelevant
832 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
833 unusedMonadBind rhs elt_ty
834 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
835 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
836 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
838 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
839 wrongMonadBind rhs elt_ty
840 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
841 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
842 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")