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
225 dsExpr (HsWrap co_fn e) = do { co_fn' <- dsHsWrapper co_fn
227 ; return (co_fn' e') }
229 dsExpr (NegApp expr neg_expr)
230 = App <$> dsExpr neg_expr <*> dsLExpr expr
232 dsExpr (HsLam a_Match)
233 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
235 dsExpr (HsApp fun arg)
236 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
239 Operator sections. At first it looks as if we can convert
248 But no! expr might be a redex, and we can lose laziness badly this
253 for example. So we convert instead to
255 let y = expr in \x -> op y x
257 If \tr{expr} is actually just a variable, say, then the simplifier
261 dsExpr (OpApp e1 op _ e2)
262 = -- for the type of y, we need the type of op's 2nd argument
263 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
265 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
266 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
268 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
269 dsExpr (SectionR op expr) = do
270 core_op <- dsLExpr op
271 -- for the type of x, we need the type of op's 2nd argument
272 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
273 -- See comment with SectionL
274 y_core <- dsLExpr expr
275 x_id <- newSysLocalDs x_ty
276 y_id <- newSysLocalDs y_ty
277 return (bindNonRec y_id y_core $
278 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
280 dsExpr (ExplicitTuple tup_args boxity)
281 = do { let go (lam_vars, args) (Missing ty)
282 -- For every missing expression, we need
283 -- another lambda in the desugaring.
284 = do { lam_var <- newSysLocalDs ty
285 ; return (lam_var : lam_vars, Var lam_var : args) }
286 go (lam_vars, args) (Present expr)
287 -- Expressions that are present don't generate
288 -- lambdas, just arguments.
289 = do { core_expr <- dsLExpr expr
290 ; return (lam_vars, core_expr : args) }
292 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
293 -- The reverse is because foldM goes left-to-right
295 ; return $ mkCoreLams lam_vars $
296 mkConApp (tupleCon boxity (length tup_args))
297 (map (Type . exprType) args ++ args) }
299 dsExpr (HsSCC cc expr) = do
300 mod_name <- getModuleDs
301 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
303 dsExpr (HsCoreAnn fs expr)
304 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
306 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
307 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
308 = -- So desugar empty HsCase to error call
309 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
312 = do { core_discrim <- dsLExpr discrim
313 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
314 ; return (scrungleMatch discrim_var core_discrim matching_code) }
316 -- Pepe: The binds are in scope in the body but NOT in the binding group
317 -- This is to avoid silliness in breakpoints
318 dsExpr (HsLet binds body) = do
319 body' <- dsLExpr body
320 dsLocalBinds binds body'
322 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
323 -- because the interpretation of `stmts' depends on what sort of thing it is.
325 dsExpr (HsDo ListComp stmts body result_ty)
326 = -- Special case for list comprehensions
327 dsListComp stmts body elt_ty
329 [elt_ty] = tcTyConAppArgs result_ty
331 dsExpr (HsDo DoExpr stmts body result_ty)
332 = dsDo stmts body result_ty
334 dsExpr (HsDo GhciStmt stmts body result_ty)
335 = dsDo stmts body result_ty
337 dsExpr (HsDo ctxt@(MDoExpr tbl) stmts body result_ty)
338 = do { (meth_binds, tbl') <- dsSyntaxTable tbl
339 ; core_expr <- dsMDo ctxt tbl' stmts body result_ty
340 ; return (mkLets meth_binds core_expr) }
342 dsExpr (HsDo PArrComp stmts body result_ty)
343 = -- Special case for array comprehensions
344 dsPArrComp (map unLoc stmts) body elt_ty
346 [elt_ty] = tcTyConAppArgs result_ty
348 dsExpr (HsIf guard_expr then_expr else_expr)
349 = mkIfThenElse <$> dsLExpr guard_expr <*> dsLExpr then_expr <*> dsLExpr else_expr
354 \underline{\bf Various data construction things}
355 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
357 dsExpr (ExplicitList elt_ty xs)
358 = dsExplicitList elt_ty xs
360 -- We desugar [:x1, ..., xn:] as
361 -- singletonP x1 +:+ ... +:+ singletonP xn
363 dsExpr (ExplicitPArr ty []) = do
364 emptyP <- dsLookupGlobalId emptyPName
365 return (Var emptyP `App` Type ty)
366 dsExpr (ExplicitPArr ty xs) = do
367 singletonP <- dsLookupGlobalId singletonPName
368 appP <- dsLookupGlobalId appPName
369 xs' <- mapM dsLExpr xs
370 return . foldr1 (binary appP) $ map (unary singletonP) xs'
372 unary fn x = mkApps (Var fn) [Type ty, x]
373 binary fn x y = mkApps (Var fn) [Type ty, x, y]
375 dsExpr (ArithSeq expr (From from))
376 = App <$> dsExpr expr <*> dsLExpr from
378 dsExpr (ArithSeq expr (FromTo from to))
379 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
381 dsExpr (ArithSeq expr (FromThen from thn))
382 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
384 dsExpr (ArithSeq expr (FromThenTo from thn to))
385 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
387 dsExpr (PArrSeq expr (FromTo from to))
388 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
390 dsExpr (PArrSeq expr (FromThenTo from thn to))
391 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
394 = panic "DsExpr.dsExpr: Infinite parallel array!"
395 -- the parser shouldn't have generated it and the renamer and typechecker
396 -- shouldn't have let it through
400 \underline{\bf Record construction and update}
401 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
402 For record construction we do this (assuming T has three arguments)
406 let err = /\a -> recConErr a
407 T (recConErr t1 "M.lhs/230/op1")
409 (recConErr t1 "M.lhs/230/op3")
411 @recConErr@ then converts its arugment string into a proper message
412 before printing it as
414 M.lhs, line 230: missing field op1 was evaluated
417 We also handle @C{}@ as valid construction syntax for an unlabelled
418 constructor @C@, setting all of @C@'s fields to bottom.
421 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
422 con_expr' <- dsExpr con_expr
424 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
425 -- A newtype in the corner should be opaque;
426 -- hence TcType.tcSplitFunTys
428 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
429 = case findField (rec_flds rbinds) lbl of
430 (rhs:rhss) -> ASSERT( null rhss )
432 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
433 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
435 labels = dataConFieldLabels (idDataCon data_con_id)
436 -- The data_con_id is guaranteed to be the wrapper id of the constructor
438 con_args <- if null labels
439 then mapM unlabelled_bottom arg_tys
440 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
442 return (mkApps con_expr' con_args)
445 Record update is a little harder. Suppose we have the decl:
447 data T = T1 {op1, op2, op3 :: Int}
448 | T2 {op4, op2 :: Int}
451 Then we translate as follows:
457 T1 op1 _ op3 -> T1 op1 op2 op3
458 T2 op4 _ -> T2 op4 op2
459 other -> recUpdError "M.lhs/230"
461 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
462 RHSs, and do not generate a Core constructor application directly, because the constructor
463 might do some argument-evaluation first; and may have to throw away some
466 Note [Update for GADTs]
467 ~~~~~~~~~~~~~~~~~~~~~~~
470 T1 { f1 :: a } :: T a Int
472 Then the wrapper function for T1 has type
474 But if x::T a b, then
475 x { f1 = v } :: T a b (not T a Int!)
476 So we need to cast (T a Int) to (T a b). Sigh.
479 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
480 cons_to_upd in_inst_tys out_inst_tys)
482 = dsLExpr record_expr
484 = ASSERT2( notNull cons_to_upd, ppr expr )
486 do { record_expr' <- dsLExpr record_expr
487 ; field_binds' <- mapM ds_field fields
488 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
489 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
491 -- It's important to generate the match with matchWrapper,
492 -- and the right hand sides with applications of the wrapper Id
493 -- so that everything works when we are doing fancy unboxing on the
494 -- constructor aguments.
495 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
496 ; ([discrim_var], matching_code)
497 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
499 ; return (add_field_binds field_binds' $
500 bindNonRec discrim_var record_expr' matching_code) }
502 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
503 -- Clone the Id in the HsRecField, because its Name is that
504 -- of the record selector, and we must not make that a lcoal binder
505 -- else we shadow other uses of the record selector
506 -- Hence 'lcl_id'. Cf Trac #2735
507 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
508 ; let fld_id = unLoc (hsRecFieldId rec_field)
509 ; lcl_id <- newSysLocalDs (idType fld_id)
510 ; return (idName fld_id, lcl_id, rhs) }
512 add_field_binds [] expr = expr
513 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
515 -- Awkwardly, for families, the match goes
516 -- from instance type to family type
517 tycon = dataConTyCon (head cons_to_upd)
518 in_ty = mkTyConApp tycon in_inst_tys
519 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
521 mk_alt upd_fld_env con
522 = do { let (univ_tvs, ex_tvs, eq_spec,
523 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
524 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
526 -- I'm not bothering to clone the ex_tvs
527 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
528 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
529 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
530 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
531 (dataConFieldLabels con) arg_ids
532 mk_val_arg field_name pat_arg_id
533 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
534 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
535 -- Reconstruct with the WrapId so that unpacking happens
536 wrap = mkWpEvVarApps theta_vars `WpCompose`
537 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
538 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
539 , isNothing (lookupTyVar wrap_subst tv) ]
540 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
542 -- Tediously wrap the application in a cast
543 -- Note [Update for GADTs]
544 wrapped_rhs | null eq_spec = rhs
545 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
546 wrap_co = mkTyConApp tycon [ lookup tv ty
547 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
548 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
551 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
552 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
554 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
555 , pat_dicts = eqs_vars ++ theta_vars
556 , pat_binds = emptyTcEvBinds
557 , pat_args = PrefixCon $ map nlVarPat arg_ids
559 ; return (mkSimpleMatch [pat] wrapped_rhs) }
563 Here is where we desugar the Template Haskell brackets and escapes
566 -- Template Haskell stuff
568 #ifdef GHCI /* Only if bootstrapping */
569 dsExpr (HsBracketOut x ps) = dsBracket x ps
570 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
573 -- Arrow notation extension
574 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
580 dsExpr (HsTick ix vars e) = do
584 -- There is a problem here. The then and else branches
585 -- have no free variables, so they are open to lifting.
586 -- We need someway of stopping this.
587 -- This will make no difference to binary coverage
588 -- (did you go here: YES or NO), but will effect accurate
591 dsExpr (HsBinTick ixT ixF e) = do
593 do { ASSERT(exprType e2 `coreEqType` boolTy)
594 mkBinaryTickBox ixT ixF e2
600 -- HsSyn constructs that just shouldn't be here:
601 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
604 findField :: [HsRecField Id arg] -> Name -> [arg]
606 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
607 , lbl == idName (unLoc id) ]
610 %--------------------------------------------------------------------
612 Note [Desugaring explicit lists]
613 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
614 Explicit lists are desugared in a cleverer way to prevent some
615 fruitless allocations. Essentially, whenever we see a list literal
618 1. Find the tail of the list that can be allocated statically (say
619 [x_k, ..., x_n]) by later stages and ensure we desugar that
620 normally: this makes sure that we don't cause a code size increase
621 by having the cons in that expression fused (see later) and hence
622 being unable to statically allocate any more
624 2. For the prefix of the list which cannot be allocated statically,
625 say [x_1, ..., x_(k-1)], we turn it into an expression involving
626 build so that if we find any foldrs over it it will fuse away
629 So in this example we will desugar to:
630 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
632 If fusion fails to occur then build will get inlined and (since we
633 defined a RULE for foldr (:) []) we will get back exactly the
634 normal desugaring for an explicit list.
636 This optimisation can be worth a lot: up to 25% of the total
637 allocation in some nofib programs. Specifically
639 Program Size Allocs Runtime CompTime
640 rewrite +0.0% -26.3% 0.02 -1.8%
641 ansi -0.3% -13.8% 0.00 +0.0%
642 lift +0.0% -8.7% 0.00 -2.3%
644 Of course, if rules aren't turned on then there is pretty much no
645 point doing this fancy stuff, and it may even be harmful.
647 =======> Note by SLPJ Dec 08.
649 I'm unconvinced that we should *ever* generate a build for an explicit
650 list. See the comments in GHC.Base about the foldr/cons rule, which
651 points out that (foldr k z [a,b,c]) may generate *much* less code than
652 (a `k` b `k` c `k` z).
654 Furthermore generating builds messes up the LHS of RULES.
655 Example: the foldr/single rule in GHC.Base
657 We do not want to generate a build invocation on the LHS of this RULE!
659 We fix this by disabling rules in rule LHSs, and testing that
660 flag here; see Note [Desugaring RULE left hand sides] in Desugar
662 To test this I've added a (static) flag -fsimple-list-literals, which
663 makes all list literals be generated via the simple route.
667 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
668 -- See Note [Desugaring explicit lists]
669 dsExplicitList elt_ty xs
670 = do { dflags <- getDOptsDs
671 ; xs' <- mapM dsLExpr xs
672 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
673 ; if opt_SimpleListLiterals -- -fsimple-list-literals
674 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
675 -- Don't generate a build if there are no rules to eliminate it!
676 -- See Note [Desugaring RULE left hand sides] in Desugar
677 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
678 then return $ mkListExpr elt_ty xs'
679 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
681 is_static :: CoreExpr -> Bool
682 is_static e = all is_static_var (varSetElems (exprFreeVars e))
684 is_static_var :: Var -> Bool
686 | isId v = isExternalName (idName v) -- Top-level things are given external names
687 | otherwise = False -- Type variables
689 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
690 = do { let suffix' = mkListExpr elt_ty suffix
691 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
692 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
694 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
695 spanTail f xs = (reverse rejected, reverse satisfying)
696 where (satisfying, rejected) = span f $ reverse xs
699 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
700 handled in DsListComp). Basically does the translation given in the
706 -> Type -- Type of the whole expression
709 dsDo stmts body result_ty
712 -- result_ty must be of the form (m b)
713 (m_ty, _b_ty) = tcSplitAppTy result_ty
715 goL [] = dsLExpr body
716 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
718 go _ (ExprStmt rhs then_expr _) stmts
719 = do { rhs2 <- dsLExpr rhs
720 ; case tcSplitAppTy_maybe (exprType rhs2) of
721 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
723 ; then_expr2 <- dsExpr then_expr
725 ; return (mkApps then_expr2 [rhs2, rest]) }
727 go _ (LetStmt binds) stmts
728 = do { rest <- goL stmts
729 ; dsLocalBinds binds rest }
731 go _ (BindStmt pat rhs bind_op fail_op) stmts
732 = do { body <- goL stmts
733 ; rhs' <- dsLExpr rhs
734 ; bind_op' <- dsExpr bind_op
735 ; var <- selectSimpleMatchVarL pat
736 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
737 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
738 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
739 res1_ty (cantFailMatchResult body)
740 ; match_code <- handle_failure pat match fail_op
741 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
743 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
744 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
745 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
746 , recS_rec_rets = rec_rets, recS_dicts = _ev_binds }) stmts
747 = ASSERT( length rec_ids > 0 )
748 ASSERT( isEmptyTcEvBinds _ev_binds ) -- No method binds
749 goL (new_bind_stmt : stmts)
751 -- returnE <- dsExpr return_id
752 -- mfixE <- dsExpr mfix_id
753 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
755 noSyntaxExpr -- Tuple cannot fail
757 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
758 rec_tup_pats = map nlVarPat tup_ids
759 later_pats = rec_tup_pats
760 rets = map noLoc rec_rets
762 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
763 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
764 (mkFunTy tup_ty body_ty))
765 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
766 body = noLoc $ HsDo DoExpr rec_stmts return_app body_ty
767 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
768 body_ty = mkAppTy m_ty tup_ty
769 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
771 -- In a do expression, pattern-match failure just calls
772 -- the monadic 'fail' rather than throwing an exception
773 handle_failure pat match fail_op
775 = do { fail_op' <- dsExpr fail_op
776 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
777 ; extractMatchResult match (App fail_op' fail_msg) }
779 = extractMatchResult match (error "It can't fail")
781 mk_fail_msg :: Located e -> String
782 mk_fail_msg pat = "Pattern match failure in do expression at " ++
783 showSDoc (ppr (getLoc pat))
786 Translation for RecStmt's:
787 -----------------------------
788 We turn (RecStmt [v1,..vn] stmts) into:
790 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
794 dsMDo :: HsStmtContext Name
798 -> Type -- Type of the whole expression
801 dsMDo ctxt tbl stmts body result_ty
804 goL [] = dsLExpr body
805 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
807 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
808 mfix_id = lookupEvidence tbl mfixName
809 return_id = lookupEvidence tbl returnMName
810 bind_id = lookupEvidence tbl bindMName
811 then_id = lookupEvidence tbl thenMName
812 fail_id = lookupEvidence tbl failMName
814 go _ (LetStmt binds) stmts
815 = do { rest <- goL stmts
816 ; dsLocalBinds binds rest }
818 go _ (ExprStmt rhs _ rhs_ty) stmts
819 = do { rhs2 <- dsLExpr rhs
820 ; warnDiscardedDoBindings rhs m_ty rhs_ty
822 ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
824 go _ (BindStmt pat rhs _ _) stmts
825 = do { body <- goL stmts
826 ; var <- selectSimpleMatchVarL pat
827 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
828 result_ty (cantFailMatchResult body)
829 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
830 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
831 ; match_code <- extractMatchResult match fail_expr
833 ; rhs' <- dsLExpr rhs
834 ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
835 rhs', Lam var match_code]) }
837 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
838 , recS_rec_ids = rec_ids, recS_rec_rets = rec_rets
839 , recS_dicts = _ev_binds }) stmts
840 = ASSERT( length rec_ids > 0 )
841 ASSERT( length rec_ids == length rec_rets )
842 ASSERT( isEmptyTcEvBinds _ev_binds )
843 pprTrace "dsMDo" (ppr later_ids) $
844 goL (new_bind_stmt : stmts)
846 new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
848 -- Remove the later_ids that appear (without fancy coercions)
849 -- in rec_rets, because there's no need to knot-tie them separately
850 -- See Note [RecStmt] in HsExpr
851 later_ids' = filter (`notElem` mono_rec_ids) later_ids
852 mono_rec_ids = [ id | HsVar id <- rec_rets ]
854 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
855 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
856 (mkFunTy tup_ty body_ty))
858 -- The rec_tup_pat must bind the rec_ids only; remember that the
859 -- trimmed_laters may share the same Names
860 -- Meanwhile, the later_pats must bind the later_vars
861 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
862 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
863 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
865 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
866 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
867 body_ty = mkAppTy m_ty tup_ty
868 tup_ty = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids)) -- Deals with singleton case
870 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
871 (mkLHsTupleExpr rets)
873 mk_wild_pat :: Id -> LPat Id
874 mk_wild_pat v = noLoc $ WildPat $ idType v
876 mk_later_pat :: Id -> LPat Id
877 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
878 | otherwise = nlVarPat v
880 mk_tup_pat :: [LPat Id] -> LPat Id
882 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
886 %************************************************************************
888 \subsection{Errors and contexts}
890 %************************************************************************
893 -- Warn about certain types of values discarded in monadic bindings (#3263)
894 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
895 warnDiscardedDoBindings rhs container_ty returning_ty = do {
896 -- Warn about discarding non-() things in 'monadic' binding
897 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
898 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
899 then warnDs (unusedMonadBind rhs returning_ty)
901 -- Warn about discarding m a things in 'monadic' binding of the same type,
902 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
903 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
904 ; case tcSplitAppTy_maybe returning_ty of
905 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
906 warnDs (wrongMonadBind rhs returning_ty)
909 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
910 unusedMonadBind rhs returning_ty
911 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
912 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
913 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
915 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
916 wrongMonadBind rhs returning_ty
917 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
918 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
919 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")