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
220 dsExpr (HsHetMetBrak c e) = do { e' <- dsExpr (unLoc e)
221 ; brak <- dsLookupGlobalId hetmet_brak_name
222 ; return $ mkApps (Var brak) [ (Type c), (Type $ exprType e'), e'] }
223 dsExpr (HsHetMetEsc c t e) = do { e' <- dsExpr (unLoc e)
224 ; esc <- dsLookupGlobalId hetmet_esc_name
225 ; return $ mkApps (Var esc) [ (Type c), (Type t), e'] }
226 dsExpr (HsHetMetCSP c e) = do { e' <- dsExpr (unLoc e)
227 ; csp <- dsLookupGlobalId hetmet_csp_name
228 ; return $ mkApps (Var csp) [ (Type c), (Type $ exprType e'), e'] }
229 dsExpr (ExprWithTySigOut e _) = dsLExpr e
230 dsExpr (HsVar var) = return (Var var)
231 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
232 dsExpr (HsLit lit) = dsLit lit
233 dsExpr (HsOverLit lit) = dsOverLit lit
235 dsExpr (HsWrap co_fn e)
236 = do { co_fn' <- dsHsWrapper co_fn
238 ; warn_id <- doptDs Opt_WarnIdentities
239 ; when warn_id $ warnAboutIdentities e' co_fn'
240 ; return (co_fn' e') }
242 dsExpr (NegApp expr neg_expr)
243 = App <$> dsExpr neg_expr <*> dsLExpr expr
245 dsExpr (HsLam a_Match)
246 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
248 dsExpr (HsApp fun arg)
249 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
252 Operator sections. At first it looks as if we can convert
261 But no! expr might be a redex, and we can lose laziness badly this
266 for example. So we convert instead to
268 let y = expr in \x -> op y x
270 If \tr{expr} is actually just a variable, say, then the simplifier
274 dsExpr (OpApp e1 op _ e2)
275 = -- for the type of y, we need the type of op's 2nd argument
276 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
278 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
279 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
281 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
282 dsExpr (SectionR op expr) = do
283 core_op <- dsLExpr op
284 -- for the type of x, we need the type of op's 2nd argument
285 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
286 -- See comment with SectionL
287 y_core <- dsLExpr expr
288 x_id <- newSysLocalDs x_ty
289 y_id <- newSysLocalDs y_ty
290 return (bindNonRec y_id y_core $
291 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
293 dsExpr (ExplicitTuple tup_args boxity)
294 = do { let go (lam_vars, args) (Missing ty)
295 -- For every missing expression, we need
296 -- another lambda in the desugaring.
297 = do { lam_var <- newSysLocalDs ty
298 ; return (lam_var : lam_vars, Var lam_var : args) }
299 go (lam_vars, args) (Present expr)
300 -- Expressions that are present don't generate
301 -- lambdas, just arguments.
302 = do { core_expr <- dsLExpr expr
303 ; return (lam_vars, core_expr : args) }
305 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
306 -- The reverse is because foldM goes left-to-right
308 ; return $ mkCoreLams lam_vars $
309 mkConApp (tupleCon boxity (length tup_args))
310 (map (Type . exprType) args ++ args) }
312 dsExpr (HsSCC cc expr) = do
313 mod_name <- getModuleDs
314 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
316 dsExpr (HsCoreAnn fs expr)
317 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
319 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
320 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
321 = -- So desugar empty HsCase to error call
322 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
325 = do { core_discrim <- dsLExpr discrim
326 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
327 ; return (scrungleMatch discrim_var core_discrim matching_code) }
329 -- Pepe: The binds are in scope in the body but NOT in the binding group
330 -- This is to avoid silliness in breakpoints
331 dsExpr (HsLet binds body) = do
332 body' <- dsLExpr body
333 dsLocalBinds binds body'
335 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
336 -- because the interpretation of `stmts' depends on what sort of thing it is.
338 dsExpr (HsDo ListComp stmts res_ty) = dsListComp stmts res_ty
339 dsExpr (HsDo PArrComp stmts _) = dsPArrComp (map unLoc stmts)
340 dsExpr (HsDo DoExpr stmts _) = dsDo stmts
341 dsExpr (HsDo GhciStmt stmts _) = dsDo stmts
342 dsExpr (HsDo MDoExpr stmts _) = dsDo stmts
343 dsExpr (HsDo MonadComp stmts _) = dsMonadComp stmts
345 dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
346 = do { pred <- dsLExpr guard_expr
347 ; b1 <- dsLExpr then_expr
348 ; b2 <- dsLExpr else_expr
350 Just fun -> do { core_fun <- dsExpr fun
351 ; return (mkCoreApps core_fun [pred,b1,b2]) }
352 Nothing -> return $ mkIfThenElse pred b1 b2 }
357 \underline{\bf Various data construction things}
358 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
360 dsExpr (ExplicitList elt_ty xs)
361 = dsExplicitList elt_ty xs
363 -- We desugar [:x1, ..., xn:] as
364 -- singletonP x1 +:+ ... +:+ singletonP xn
366 dsExpr (ExplicitPArr ty []) = do
367 emptyP <- dsLookupDPHId emptyPName
368 return (Var emptyP `App` Type ty)
369 dsExpr (ExplicitPArr ty xs) = do
370 singletonP <- dsLookupDPHId singletonPName
371 appP <- dsLookupDPHId appPName
372 xs' <- mapM dsLExpr xs
373 return . foldr1 (binary appP) $ map (unary singletonP) xs'
375 unary fn x = mkApps (Var fn) [Type ty, x]
376 binary fn x y = mkApps (Var fn) [Type ty, x, y]
378 dsExpr (ArithSeq expr (From from))
379 = App <$> dsExpr expr <*> dsLExpr from
381 dsExpr (ArithSeq expr (FromTo from to))
382 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
384 dsExpr (ArithSeq expr (FromThen from thn))
385 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
387 dsExpr (ArithSeq expr (FromThenTo from thn to))
388 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
390 dsExpr (PArrSeq expr (FromTo from to))
391 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
393 dsExpr (PArrSeq expr (FromThenTo from thn to))
394 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
397 = panic "DsExpr.dsExpr: Infinite parallel array!"
398 -- the parser shouldn't have generated it and the renamer and typechecker
399 -- shouldn't have let it through
403 \underline{\bf Record construction and update}
404 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
405 For record construction we do this (assuming T has three arguments)
409 let err = /\a -> recConErr a
410 T (recConErr t1 "M.lhs/230/op1")
412 (recConErr t1 "M.lhs/230/op3")
414 @recConErr@ then converts its arugment string into a proper message
415 before printing it as
417 M.lhs, line 230: missing field op1 was evaluated
420 We also handle @C{}@ as valid construction syntax for an unlabelled
421 constructor @C@, setting all of @C@'s fields to bottom.
424 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
425 con_expr' <- dsExpr con_expr
427 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
428 -- A newtype in the corner should be opaque;
429 -- hence TcType.tcSplitFunTys
431 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
432 = case findField (rec_flds rbinds) lbl of
433 (rhs:rhss) -> ASSERT( null rhss )
435 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
436 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
438 labels = dataConFieldLabels (idDataCon data_con_id)
439 -- The data_con_id is guaranteed to be the wrapper id of the constructor
441 con_args <- if null labels
442 then mapM unlabelled_bottom arg_tys
443 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
445 return (mkApps con_expr' con_args)
448 Record update is a little harder. Suppose we have the decl:
450 data T = T1 {op1, op2, op3 :: Int}
451 | T2 {op4, op2 :: Int}
454 Then we translate as follows:
460 T1 op1 _ op3 -> T1 op1 op2 op3
461 T2 op4 _ -> T2 op4 op2
462 other -> recUpdError "M.lhs/230"
464 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
465 RHSs, and do not generate a Core constructor application directly, because the constructor
466 might do some argument-evaluation first; and may have to throw away some
469 Note [Update for GADTs]
470 ~~~~~~~~~~~~~~~~~~~~~~~
473 T1 { f1 :: a } :: T a Int
475 Then the wrapper function for T1 has type
477 But if x::T a b, then
478 x { f1 = v } :: T a b (not T a Int!)
479 So we need to cast (T a Int) to (T a b). Sigh.
482 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
483 cons_to_upd in_inst_tys out_inst_tys)
485 = dsLExpr record_expr
487 = ASSERT2( notNull cons_to_upd, ppr expr )
489 do { record_expr' <- dsLExpr record_expr
490 ; field_binds' <- mapM ds_field fields
491 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
492 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
494 -- It's important to generate the match with matchWrapper,
495 -- and the right hand sides with applications of the wrapper Id
496 -- so that everything works when we are doing fancy unboxing on the
497 -- constructor aguments.
498 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
499 ; ([discrim_var], matching_code)
500 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
502 ; return (add_field_binds field_binds' $
503 bindNonRec discrim_var record_expr' matching_code) }
505 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
506 -- Clone the Id in the HsRecField, because its Name is that
507 -- of the record selector, and we must not make that a lcoal binder
508 -- else we shadow other uses of the record selector
509 -- Hence 'lcl_id'. Cf Trac #2735
510 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
511 ; let fld_id = unLoc (hsRecFieldId rec_field)
512 ; lcl_id <- newSysLocalDs (idType fld_id)
513 ; return (idName fld_id, lcl_id, rhs) }
515 add_field_binds [] expr = expr
516 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
518 -- Awkwardly, for families, the match goes
519 -- from instance type to family type
520 tycon = dataConTyCon (head cons_to_upd)
521 in_ty = mkTyConApp tycon in_inst_tys
522 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
524 mk_alt upd_fld_env con
525 = do { let (univ_tvs, ex_tvs, eq_spec,
526 theta, arg_tys, _) = dataConFullSig con
527 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
529 -- I'm not bothering to clone the ex_tvs
530 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
531 ; theta_vars <- mapM newPredVarDs (substTheta subst theta)
532 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
533 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
534 (dataConFieldLabels con) arg_ids
535 mk_val_arg field_name pat_arg_id
536 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
537 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
538 -- Reconstruct with the WrapId so that unpacking happens
539 wrap = mkWpEvVarApps theta_vars `WpCompose`
540 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
541 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
542 , not (tv `elemVarEnv` wrap_subst) ]
543 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
545 -- Tediously wrap the application in a cast
546 -- Note [Update for GADTs]
547 wrapped_rhs | null eq_spec = rhs
548 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
549 wrap_co = mkTyConAppCo tycon [ lookup tv ty
550 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
551 lookup univ_tv ty = case lookupVarEnv wrap_subst univ_tv of
553 Nothing -> mkReflCo ty
554 wrap_subst = mkVarEnv [ (tv, mkSymCo (mkCoVarCo co_var))
555 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
557 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
558 , pat_dicts = eqs_vars ++ theta_vars
559 , pat_binds = emptyTcEvBinds
560 , pat_args = PrefixCon $ map nlVarPat arg_ids
562 ; return (mkSimpleMatch [pat] wrapped_rhs) }
566 Here is where we desugar the Template Haskell brackets and escapes
569 -- Template Haskell stuff
571 #ifdef GHCI /* Only if bootstrapping */
572 dsExpr (HsBracketOut x ps) = dsBracket x ps
573 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
576 -- Arrow notation extension
577 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
583 dsExpr (HsTick ix vars e) = do
587 -- There is a problem here. The then and else branches
588 -- have no free variables, so they are open to lifting.
589 -- We need someway of stopping this.
590 -- This will make no difference to binary coverage
591 -- (did you go here: YES or NO), but will effect accurate
594 dsExpr (HsBinTick ixT ixF e) = do
596 do { ASSERT(exprType e2 `eqType` boolTy)
597 mkBinaryTickBox ixT ixF e2
603 -- HsSyn constructs that just shouldn't be here:
604 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
607 findField :: [HsRecField Id arg] -> Name -> [arg]
609 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
610 , lbl == idName (unLoc id) ]
613 %--------------------------------------------------------------------
615 Note [Desugaring explicit lists]
616 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
617 Explicit lists are desugared in a cleverer way to prevent some
618 fruitless allocations. Essentially, whenever we see a list literal
621 1. Find the tail of the list that can be allocated statically (say
622 [x_k, ..., x_n]) by later stages and ensure we desugar that
623 normally: this makes sure that we don't cause a code size increase
624 by having the cons in that expression fused (see later) and hence
625 being unable to statically allocate any more
627 2. For the prefix of the list which cannot be allocated statically,
628 say [x_1, ..., x_(k-1)], we turn it into an expression involving
629 build so that if we find any foldrs over it it will fuse away
632 So in this example we will desugar to:
633 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
635 If fusion fails to occur then build will get inlined and (since we
636 defined a RULE for foldr (:) []) we will get back exactly the
637 normal desugaring for an explicit list.
639 This optimisation can be worth a lot: up to 25% of the total
640 allocation in some nofib programs. Specifically
642 Program Size Allocs Runtime CompTime
643 rewrite +0.0% -26.3% 0.02 -1.8%
644 ansi -0.3% -13.8% 0.00 +0.0%
645 lift +0.0% -8.7% 0.00 -2.3%
647 Of course, if rules aren't turned on then there is pretty much no
648 point doing this fancy stuff, and it may even be harmful.
650 =======> Note by SLPJ Dec 08.
652 I'm unconvinced that we should *ever* generate a build for an explicit
653 list. See the comments in GHC.Base about the foldr/cons rule, which
654 points out that (foldr k z [a,b,c]) may generate *much* less code than
655 (a `k` b `k` c `k` z).
657 Furthermore generating builds messes up the LHS of RULES.
658 Example: the foldr/single rule in GHC.Base
660 We do not want to generate a build invocation on the LHS of this RULE!
662 We fix this by disabling rules in rule LHSs, and testing that
663 flag here; see Note [Desugaring RULE left hand sides] in Desugar
665 To test this I've added a (static) flag -fsimple-list-literals, which
666 makes all list literals be generated via the simple route.
670 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
671 -- See Note [Desugaring explicit lists]
672 dsExplicitList elt_ty xs
673 = do { dflags <- getDOptsDs
674 ; xs' <- mapM dsLExpr xs
675 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
676 ; if opt_SimpleListLiterals -- -fsimple-list-literals
677 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
678 -- Don't generate a build if there are no rules to eliminate it!
679 -- See Note [Desugaring RULE left hand sides] in Desugar
680 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
681 then return $ mkListExpr elt_ty xs'
682 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
684 is_static :: CoreExpr -> Bool
685 is_static e = all is_static_var (varSetElems (exprFreeVars e))
687 is_static_var :: Var -> Bool
689 | isId v = isExternalName (idName v) -- Top-level things are given external names
690 | otherwise = False -- Type variables
692 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
693 = do { let suffix' = mkListExpr elt_ty suffix
694 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
695 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
697 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
698 spanTail f xs = (reverse rejected, reverse satisfying)
699 where (satisfying, rejected) = span f $ reverse xs
702 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
703 handled in DsListComp). Basically does the translation given in the
707 dsDo :: [LStmt Id] -> DsM CoreExpr
711 goL [] = panic "dsDo"
712 goL (L loc stmt:lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
714 go _ (LastStmt body _) stmts
715 = ASSERT( null stmts ) dsLExpr body
716 -- The 'return' op isn't used for 'do' expressions
718 go _ (ExprStmt rhs then_expr _ _) stmts
719 = do { rhs2 <- dsLExpr rhs
720 ; warnDiscardedDoBindings rhs (exprType rhs2)
721 ; then_expr2 <- dsExpr then_expr
723 ; return (mkApps then_expr2 [rhs2, rest]) }
725 go _ (LetStmt binds) stmts
726 = do { rest <- goL stmts
727 ; dsLocalBinds binds rest }
729 go _ (BindStmt pat rhs bind_op fail_op) stmts
730 = do { body <- goL stmts
731 ; rhs' <- dsLExpr rhs
732 ; bind_op' <- dsExpr bind_op
733 ; var <- selectSimpleMatchVarL pat
734 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
735 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
736 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
737 res1_ty (cantFailMatchResult body)
738 ; match_code <- handle_failure pat match fail_op
739 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
741 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
742 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
743 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
744 , recS_rec_rets = rec_rets, recS_ret_ty = body_ty }) stmts
745 = ASSERT( length rec_ids > 0 )
746 goL (new_bind_stmt : stmts)
748 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats)
750 noSyntaxExpr -- Tuple cannot fail
752 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
753 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
754 rec_tup_pats = map nlVarPat tup_ids
755 later_pats = rec_tup_pats
756 rets = map noLoc rec_rets
757 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
758 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
759 (mkFunTy tup_ty body_ty))
760 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
761 body = noLoc $ HsDo DoExpr (rec_stmts ++ [ret_stmt]) body_ty
762 ret_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
763 ret_stmt = noLoc $ mkLastStmt ret_app
764 -- This LastStmt will be desugared with dsDo,
765 -- which ignores the return_op in the LastStmt,
766 -- so we must apply the return_op explicitly
768 handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
769 -- In a do expression, pattern-match failure just calls
770 -- the monadic 'fail' rather than throwing an exception
771 handle_failure pat match fail_op
773 = do { fail_op' <- dsExpr fail_op
774 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
775 ; extractMatchResult match (App fail_op' fail_msg) }
777 = extractMatchResult match (error "It can't fail")
779 mk_fail_msg :: Located e -> String
780 mk_fail_msg pat = "Pattern match failure in do expression at " ++
781 showSDoc (ppr (getLoc pat))
785 %************************************************************************
787 Warning about identities
789 %************************************************************************
791 Warn about functions that convert between one type and another
792 when the to- and from- types are the same. Then it's probably
793 (albeit not definitely) the identity
795 warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
796 warnAboutIdentities (Var v) co_fn
797 | idName v `elem` conversionNames
798 , let fun_ty = exprType (co_fn (Var v))
799 , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
800 , arg_ty `eqType` res_ty -- So we are converting ty -> ty
801 = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
802 , nest 2 $ ptext (sLit "can probably be omitted")
803 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
805 warnAboutIdentities _ _ = return ()
807 conversionNames :: [Name]
809 = [ toIntegerName, toRationalName
810 , fromIntegralName, realToFracName ]
811 -- We can't easily add fromIntegerName, fromRationalName,
812 -- becuase they are generated by literals
815 %************************************************************************
817 \subsection{Errors and contexts}
819 %************************************************************************
822 -- Warn about certain types of values discarded in monadic bindings (#3263)
823 warnDiscardedDoBindings :: LHsExpr Id -> Type -> DsM ()
824 warnDiscardedDoBindings rhs rhs_ty
825 | Just (m_ty, elt_ty) <- tcSplitAppTy_maybe rhs_ty
826 = do { -- Warn about discarding non-() things in 'monadic' binding
827 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
828 ; if warn_unused && not (isUnitTy elt_ty)
829 then warnDs (unusedMonadBind rhs elt_ty)
831 -- Warn about discarding m a things in 'monadic' binding of the same type,
832 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
833 do { warn_wrong <- doptDs Opt_WarnWrongDoBind
834 ; case tcSplitAppTy_maybe elt_ty of
835 Just (elt_m_ty, _) | warn_wrong, m_ty `eqType` elt_m_ty
836 -> warnDs (wrongMonadBind rhs elt_ty)
839 | otherwise -- RHS does have type of form (m ty), which is wierd
840 = return () -- but at lesat this warning is irrelevant
842 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
843 unusedMonadBind rhs elt_ty
844 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
845 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
846 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
848 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
849 wrongMonadBind rhs elt_ty
850 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr elt_ty <> dot $$
851 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
852 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")