2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Desugaring exporessions.
9 {-# OPTIONS -fno-warn-incomplete-patterns #-}
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module DsExpr ( dsExpr, dsLExpr, dsLocalBinds, dsValBinds, dsLit ) where
18 #include "HsVersions.h"
32 -- Template Haskell stuff iff bootstrapped
38 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
39 -- needs to see source types
69 %************************************************************************
71 dsLocalBinds, dsValBinds
73 %************************************************************************
76 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
77 dsLocalBinds EmptyLocalBinds body = return body
78 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
79 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
81 -------------------------
82 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
83 dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
85 -------------------------
86 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
87 dsIPBinds (IPBinds ip_binds ev_binds) body
88 = do { ds_ev_binds <- dsTcEvBinds ev_binds
89 ; let inner = wrapDsEvBinds ds_ev_binds body
90 -- The dict bindings may not be in
91 -- dependency order; hence Rec
92 ; foldrM ds_ip_bind inner ip_binds }
94 ds_ip_bind (L _ (IPBind n e)) body
96 return (Let (NonRec (ipNameName n) e') body)
98 -------------------------
99 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
100 -- Special case for bindings which bind unlifted variables
101 -- We need to do a case right away, rather than building
102 -- a tuple and doing selections.
103 -- Silently ignore INLINE and SPECIALISE pragmas...
104 ds_val_bind (NonRecursive, hsbinds) body
105 | [L loc bind] <- bagToList hsbinds,
106 -- Non-recursive, non-overloaded bindings only come in ones
107 -- ToDo: in some bizarre case it's conceivable that there
108 -- could be dict binds in the 'binds'. (See the notes
109 -- below. Then pattern-match would fail. Urk.)
111 = putSrcSpanDs loc (dsStrictBind bind body)
113 -- Ordinary case for bindings; none should be unlifted
114 ds_val_bind (_is_rec, binds) body
115 = do { prs <- dsLHsBinds binds
116 ; ASSERT2( not (any (isUnLiftedType . idType . fst) prs), ppr _is_rec $$ ppr binds )
119 _ -> return (Let (Rec prs) body) }
120 -- Use a Rec regardless of is_rec.
121 -- Why? Because it allows the binds to be all
122 -- mixed up, which is what happens in one rare case
123 -- Namely, for an AbsBind with no tyvars and no dicts,
124 -- but which does have dictionary bindings.
125 -- See notes with TcSimplify.inferLoop [NO TYVARS]
126 -- It turned out that wrapping a Rec here was the easiest solution
128 -- NB The previous case dealt with unlifted bindings, so we
129 -- only have to deal with lifted ones now; so Rec is ok
132 dsStrictBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
133 dsStrictBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
134 , abs_exports = exports
135 , abs_ev_binds = ev_binds
136 , abs_binds = binds }) body
137 = do { ds_ev_binds <- dsTcEvBinds ev_binds
138 ; let body1 = foldr bind_export body exports
139 bind_export (_, g, l, _) b = bindNonRec g (Var l) b
140 ; body2 <- foldlBagM (\body bind -> dsStrictBind (unLoc bind) body)
142 ; return (wrapDsEvBinds ds_ev_binds body2) }
144 dsStrictBind (FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn
145 , fun_tick = tick, fun_infix = inf }) body
146 -- Can't be a bang pattern (that looks like a PatBind)
147 -- so must be simply unboxed
148 = do { (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
149 ; MASSERT( null args ) -- Functions aren't lifted
150 ; MASSERT( isIdHsWrapper co_fn )
151 ; rhs' <- mkOptTickBox tick rhs
152 ; return (bindNonRec fun rhs' body) }
154 dsStrictBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
155 = -- let C x# y# = rhs in body
156 -- ==> case rhs of C x# y# -> body
157 do { rhs <- dsGuarded grhss ty
158 ; let upat = unLoc pat
159 eqn = EqnInfo { eqn_pats = [upat],
160 eqn_rhs = cantFailMatchResult body }
161 ; var <- selectMatchVar upat
162 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
163 ; return (scrungleMatch var rhs result) }
165 dsStrictBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
167 ----------------------
168 strictMatchOnly :: HsBind Id -> Bool
169 strictMatchOnly (AbsBinds { abs_binds = binds })
170 = anyBag (strictMatchOnly . unLoc) binds
171 strictMatchOnly (PatBind { pat_lhs = lpat, pat_rhs_ty = ty })
172 = isUnboxedTupleType ty
174 || any (isUnLiftedType . idType) (collectPatBinders lpat)
175 strictMatchOnly (FunBind { fun_id = L _ id })
176 = isUnLiftedType (idType id)
177 strictMatchOnly _ = False -- I hope! Checked immediately by caller in fact
179 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
180 -- Returns something like (let var = scrut in body)
181 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
182 -- Special case to handle unboxed tuple patterns; they can't appear nested
184 -- case e of (# p1, p2 #) -> rhs
186 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
188 -- let x = e in case x of ....
190 -- But there may be a big
191 -- let fail = ... in case e of ...
192 -- wrapping the whole case, which complicates matters slightly
193 -- It all seems a bit fragile. Test is dsrun013.
195 scrungleMatch var scrut body
196 | isUnboxedTupleType (idType var) = scrungle body
197 | otherwise = bindNonRec var scrut body
199 scrungle (Case (Var x) bndr ty alts)
200 | x == var = Case scrut bndr ty alts
201 scrungle (Let binds body) = Let binds (scrungle body)
202 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
206 %************************************************************************
208 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
210 %************************************************************************
213 dsLExpr :: LHsExpr Id -> DsM CoreExpr
215 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
217 dsExpr :: HsExpr Id -> DsM CoreExpr
218 dsExpr (HsPar e) = dsLExpr e
219 dsExpr (ExprWithTySigOut e _) = dsLExpr e
220 dsExpr (HsVar var) = return (Var var)
221 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
222 dsExpr (HsLit lit) = dsLit lit
223 dsExpr (HsOverLit lit) = dsOverLit lit
225 dsExpr (HsWrap co_fn e)
226 = do { co_fn' <- dsHsWrapper co_fn
228 ; warn_id <- doptDs Opt_WarnIdentities
229 ; when warn_id $ warnAboutIdentities e' co_fn'
230 ; return (co_fn' e') }
232 dsExpr (NegApp expr neg_expr)
233 = App <$> dsExpr neg_expr <*> dsLExpr expr
235 dsExpr (HsLam a_Match)
236 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
238 dsExpr (HsApp fun arg)
239 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
242 Operator sections. At first it looks as if we can convert
251 But no! expr might be a redex, and we can lose laziness badly this
256 for example. So we convert instead to
258 let y = expr in \x -> op y x
260 If \tr{expr} is actually just a variable, say, then the simplifier
264 dsExpr (OpApp e1 op _ e2)
265 = -- for the type of y, we need the type of op's 2nd argument
266 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
268 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
269 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
271 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
272 dsExpr (SectionR op expr) = do
273 core_op <- dsLExpr op
274 -- for the type of x, we need the type of op's 2nd argument
275 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
276 -- See comment with SectionL
277 y_core <- dsLExpr expr
278 x_id <- newSysLocalDs x_ty
279 y_id <- newSysLocalDs y_ty
280 return (bindNonRec y_id y_core $
281 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
283 dsExpr (ExplicitTuple tup_args boxity)
284 = do { let go (lam_vars, args) (Missing ty)
285 -- For every missing expression, we need
286 -- another lambda in the desugaring.
287 = do { lam_var <- newSysLocalDs ty
288 ; return (lam_var : lam_vars, Var lam_var : args) }
289 go (lam_vars, args) (Present expr)
290 -- Expressions that are present don't generate
291 -- lambdas, just arguments.
292 = do { core_expr <- dsLExpr expr
293 ; return (lam_vars, core_expr : args) }
295 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
296 -- The reverse is because foldM goes left-to-right
298 ; return $ mkCoreLams lam_vars $
299 mkConApp (tupleCon boxity (length tup_args))
300 (map (Type . exprType) args ++ args) }
302 dsExpr (HsSCC cc expr) = do
303 mod_name <- getModuleDs
304 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
306 dsExpr (HsCoreAnn fs expr)
307 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
309 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
310 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
311 = -- So desugar empty HsCase to error call
312 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
315 = do { core_discrim <- dsLExpr discrim
316 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
317 ; return (scrungleMatch discrim_var core_discrim matching_code) }
319 -- Pepe: The binds are in scope in the body but NOT in the binding group
320 -- This is to avoid silliness in breakpoints
321 dsExpr (HsLet binds body) = do
322 body' <- dsLExpr body
323 dsLocalBinds binds body'
325 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
326 -- because the interpretation of `stmts' depends on what sort of thing it is.
328 dsExpr (HsDo ListComp stmts body result_ty)
329 = -- Special case for list comprehensions
330 dsListComp stmts body elt_ty
332 [elt_ty] = tcTyConAppArgs result_ty
334 dsExpr (HsDo DoExpr stmts body result_ty)
335 = dsDo stmts body result_ty
337 dsExpr (HsDo GhciStmt stmts body result_ty)
338 = dsDo stmts body result_ty
340 dsExpr (HsDo MDoExpr stmts body result_ty)
341 = dsDo stmts body result_ty
343 dsExpr (HsDo PArrComp stmts body result_ty)
344 = -- Special case for array comprehensions
345 dsPArrComp (map unLoc stmts) body elt_ty
347 [elt_ty] = tcTyConAppArgs result_ty
349 dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
350 = do { pred <- dsLExpr guard_expr
351 ; b1 <- dsLExpr then_expr
352 ; b2 <- dsLExpr else_expr
354 Just fun -> do { core_fun <- dsExpr fun
355 ; return (mkCoreApps core_fun [pred,b1,b2]) }
356 Nothing -> return $ mkIfThenElse pred b1 b2 }
361 \underline{\bf Various data construction things}
362 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
364 dsExpr (ExplicitList elt_ty xs)
365 = dsExplicitList elt_ty xs
367 -- We desugar [:x1, ..., xn:] as
368 -- singletonP x1 +:+ ... +:+ singletonP xn
370 dsExpr (ExplicitPArr ty []) = do
371 emptyP <- dsLookupDPHId emptyPName
372 return (Var emptyP `App` Type ty)
373 dsExpr (ExplicitPArr ty xs) = do
374 singletonP <- dsLookupDPHId singletonPName
375 appP <- dsLookupDPHId appPName
376 xs' <- mapM dsLExpr xs
377 return . foldr1 (binary appP) $ map (unary singletonP) xs'
379 unary fn x = mkApps (Var fn) [Type ty, x]
380 binary fn x y = mkApps (Var fn) [Type ty, x, y]
382 dsExpr (ArithSeq expr (From from))
383 = App <$> dsExpr expr <*> dsLExpr from
385 dsExpr (ArithSeq expr (FromTo from to))
386 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
388 dsExpr (ArithSeq expr (FromThen from thn))
389 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
391 dsExpr (ArithSeq expr (FromThenTo from thn to))
392 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
394 dsExpr (PArrSeq expr (FromTo from to))
395 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
397 dsExpr (PArrSeq expr (FromThenTo from thn to))
398 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
401 = panic "DsExpr.dsExpr: Infinite parallel array!"
402 -- the parser shouldn't have generated it and the renamer and typechecker
403 -- shouldn't have let it through
407 \underline{\bf Record construction and update}
408 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
409 For record construction we do this (assuming T has three arguments)
413 let err = /\a -> recConErr a
414 T (recConErr t1 "M.lhs/230/op1")
416 (recConErr t1 "M.lhs/230/op3")
418 @recConErr@ then converts its arugment string into a proper message
419 before printing it as
421 M.lhs, line 230: missing field op1 was evaluated
424 We also handle @C{}@ as valid construction syntax for an unlabelled
425 constructor @C@, setting all of @C@'s fields to bottom.
428 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
429 con_expr' <- dsExpr con_expr
431 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
432 -- A newtype in the corner should be opaque;
433 -- hence TcType.tcSplitFunTys
435 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
436 = case findField (rec_flds rbinds) lbl of
437 (rhs:rhss) -> ASSERT( null rhss )
439 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
440 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
442 labels = dataConFieldLabels (idDataCon data_con_id)
443 -- The data_con_id is guaranteed to be the wrapper id of the constructor
445 con_args <- if null labels
446 then mapM unlabelled_bottom arg_tys
447 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
449 return (mkApps con_expr' con_args)
452 Record update is a little harder. Suppose we have the decl:
454 data T = T1 {op1, op2, op3 :: Int}
455 | T2 {op4, op2 :: Int}
458 Then we translate as follows:
464 T1 op1 _ op3 -> T1 op1 op2 op3
465 T2 op4 _ -> T2 op4 op2
466 other -> recUpdError "M.lhs/230"
468 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
469 RHSs, and do not generate a Core constructor application directly, because the constructor
470 might do some argument-evaluation first; and may have to throw away some
473 Note [Update for GADTs]
474 ~~~~~~~~~~~~~~~~~~~~~~~
477 T1 { f1 :: a } :: T a Int
479 Then the wrapper function for T1 has type
481 But if x::T a b, then
482 x { f1 = v } :: T a b (not T a Int!)
483 So we need to cast (T a Int) to (T a b). Sigh.
486 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
487 cons_to_upd in_inst_tys out_inst_tys)
489 = dsLExpr record_expr
491 = ASSERT2( notNull cons_to_upd, ppr expr )
493 do { record_expr' <- dsLExpr record_expr
494 ; field_binds' <- mapM ds_field fields
495 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
496 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
498 -- It's important to generate the match with matchWrapper,
499 -- and the right hand sides with applications of the wrapper Id
500 -- so that everything works when we are doing fancy unboxing on the
501 -- constructor aguments.
502 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
503 ; ([discrim_var], matching_code)
504 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
506 ; return (add_field_binds field_binds' $
507 bindNonRec discrim_var record_expr' matching_code) }
509 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
510 -- Clone the Id in the HsRecField, because its Name is that
511 -- of the record selector, and we must not make that a lcoal binder
512 -- else we shadow other uses of the record selector
513 -- Hence 'lcl_id'. Cf Trac #2735
514 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
515 ; let fld_id = unLoc (hsRecFieldId rec_field)
516 ; lcl_id <- newSysLocalDs (idType fld_id)
517 ; return (idName fld_id, lcl_id, rhs) }
519 add_field_binds [] expr = expr
520 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
522 -- Awkwardly, for families, the match goes
523 -- from instance type to family type
524 tycon = dataConTyCon (head cons_to_upd)
525 in_ty = mkTyConApp tycon in_inst_tys
526 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
528 mk_alt upd_fld_env con
529 = do { let (univ_tvs, ex_tvs, eq_spec,
530 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
531 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
533 -- I'm not bothering to clone the ex_tvs
534 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
535 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
536 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
537 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
538 (dataConFieldLabels con) arg_ids
539 mk_val_arg field_name pat_arg_id
540 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
541 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
542 -- Reconstruct with the WrapId so that unpacking happens
543 wrap = mkWpEvVarApps theta_vars `WpCompose`
544 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
545 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
546 , isNothing (lookupTyVar wrap_subst tv) ]
547 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
549 -- Tediously wrap the application in a cast
550 -- Note [Update for GADTs]
551 wrapped_rhs | null eq_spec = rhs
552 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
553 wrap_co = mkTyConApp tycon [ lookup tv ty
554 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
555 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
558 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
559 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
561 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
562 , pat_dicts = eqs_vars ++ theta_vars
563 , pat_binds = emptyTcEvBinds
564 , pat_args = PrefixCon $ map nlVarPat arg_ids
566 ; return (mkSimpleMatch [pat] wrapped_rhs) }
570 Here is where we desugar the Template Haskell brackets and escapes
573 -- Template Haskell stuff
575 #ifdef GHCI /* Only if bootstrapping */
576 dsExpr (HsBracketOut x ps) = dsBracket x ps
577 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
580 -- Arrow notation extension
581 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
587 dsExpr (HsTick ix vars e) = do
591 -- There is a problem here. The then and else branches
592 -- have no free variables, so they are open to lifting.
593 -- We need someway of stopping this.
594 -- This will make no difference to binary coverage
595 -- (did you go here: YES or NO), but will effect accurate
598 dsExpr (HsBinTick ixT ixF e) = do
600 do { ASSERT(exprType e2 `coreEqType` boolTy)
601 mkBinaryTickBox ixT ixF e2
607 -- HsSyn constructs that just shouldn't be here:
608 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
611 findField :: [HsRecField Id arg] -> Name -> [arg]
613 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
614 , lbl == idName (unLoc id) ]
617 %--------------------------------------------------------------------
619 Note [Desugaring explicit lists]
620 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
621 Explicit lists are desugared in a cleverer way to prevent some
622 fruitless allocations. Essentially, whenever we see a list literal
625 1. Find the tail of the list that can be allocated statically (say
626 [x_k, ..., x_n]) by later stages and ensure we desugar that
627 normally: this makes sure that we don't cause a code size increase
628 by having the cons in that expression fused (see later) and hence
629 being unable to statically allocate any more
631 2. For the prefix of the list which cannot be allocated statically,
632 say [x_1, ..., x_(k-1)], we turn it into an expression involving
633 build so that if we find any foldrs over it it will fuse away
636 So in this example we will desugar to:
637 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
639 If fusion fails to occur then build will get inlined and (since we
640 defined a RULE for foldr (:) []) we will get back exactly the
641 normal desugaring for an explicit list.
643 This optimisation can be worth a lot: up to 25% of the total
644 allocation in some nofib programs. Specifically
646 Program Size Allocs Runtime CompTime
647 rewrite +0.0% -26.3% 0.02 -1.8%
648 ansi -0.3% -13.8% 0.00 +0.0%
649 lift +0.0% -8.7% 0.00 -2.3%
651 Of course, if rules aren't turned on then there is pretty much no
652 point doing this fancy stuff, and it may even be harmful.
654 =======> Note by SLPJ Dec 08.
656 I'm unconvinced that we should *ever* generate a build for an explicit
657 list. See the comments in GHC.Base about the foldr/cons rule, which
658 points out that (foldr k z [a,b,c]) may generate *much* less code than
659 (a `k` b `k` c `k` z).
661 Furthermore generating builds messes up the LHS of RULES.
662 Example: the foldr/single rule in GHC.Base
664 We do not want to generate a build invocation on the LHS of this RULE!
666 We fix this by disabling rules in rule LHSs, and testing that
667 flag here; see Note [Desugaring RULE left hand sides] in Desugar
669 To test this I've added a (static) flag -fsimple-list-literals, which
670 makes all list literals be generated via the simple route.
674 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
675 -- See Note [Desugaring explicit lists]
676 dsExplicitList elt_ty xs
677 = do { dflags <- getDOptsDs
678 ; xs' <- mapM dsLExpr xs
679 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
680 ; if opt_SimpleListLiterals -- -fsimple-list-literals
681 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
682 -- Don't generate a build if there are no rules to eliminate it!
683 -- See Note [Desugaring RULE left hand sides] in Desugar
684 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
685 then return $ mkListExpr elt_ty xs'
686 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
688 is_static :: CoreExpr -> Bool
689 is_static e = all is_static_var (varSetElems (exprFreeVars e))
691 is_static_var :: Var -> Bool
693 | isId v = isExternalName (idName v) -- Top-level things are given external names
694 | otherwise = False -- Type variables
696 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
697 = do { let suffix' = mkListExpr elt_ty suffix
698 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
699 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
701 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
702 spanTail f xs = (reverse rejected, reverse satisfying)
703 where (satisfying, rejected) = span f $ reverse xs
706 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
707 handled in DsListComp). Basically does the translation given in the
713 -> Type -- Type of the whole expression
716 dsDo stmts body result_ty
719 -- result_ty must be of the form (m b)
720 (m_ty, _b_ty) = tcSplitAppTy result_ty
722 goL [] = dsLExpr body
723 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
725 go _ (ExprStmt rhs then_expr _) stmts
726 = do { rhs2 <- dsLExpr rhs
727 ; case tcSplitAppTy_maybe (exprType rhs2) of
728 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
730 ; then_expr2 <- dsExpr then_expr
732 ; return (mkApps then_expr2 [rhs2, rest]) }
734 go _ (LetStmt binds) stmts
735 = do { rest <- goL stmts
736 ; dsLocalBinds binds rest }
738 go _ (BindStmt pat rhs bind_op fail_op) stmts
739 = do { body <- goL stmts
740 ; rhs' <- dsLExpr rhs
741 ; bind_op' <- dsExpr bind_op
742 ; var <- selectSimpleMatchVarL pat
743 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
744 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
745 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
746 res1_ty (cantFailMatchResult body)
747 ; match_code <- handle_failure pat match fail_op
748 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
750 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
751 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
752 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
753 , recS_rec_rets = rec_rets }) stmts
754 = ASSERT( length rec_ids > 0 )
755 goL (new_bind_stmt : stmts)
757 -- returnE <- dsExpr return_id
758 -- mfixE <- dsExpr mfix_id
759 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
761 noSyntaxExpr -- Tuple cannot fail
763 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
764 rec_tup_pats = map nlVarPat tup_ids
765 later_pats = rec_tup_pats
766 rets = map noLoc rec_rets
768 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
769 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
770 (mkFunTy tup_ty body_ty))
771 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
772 body = noLoc $ HsDo DoExpr rec_stmts return_app body_ty
773 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
774 body_ty = mkAppTy m_ty tup_ty
775 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
777 handle_failure :: LPat Id -> MatchResult -> SyntaxExpr Id -> DsM CoreExpr
778 -- In a do expression, pattern-match failure just calls
779 -- the monadic 'fail' rather than throwing an exception
780 handle_failure pat match fail_op
782 = do { fail_op' <- dsExpr fail_op
783 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
784 ; extractMatchResult match (App fail_op' fail_msg) }
786 = extractMatchResult match (error "It can't fail")
788 mk_fail_msg :: Located e -> String
789 mk_fail_msg pat = "Pattern match failure in do expression at " ++
790 showSDoc (ppr (getLoc pat))
793 Translation for RecStmt's:
794 -----------------------------
795 We turn (RecStmt [v1,..vn] stmts) into:
797 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
802 dsMDo :: HsStmtContext Name
806 -> Type -- Type of the whole expression
809 dsMDo ctxt tbl stmts body result_ty
812 goL [] = dsLExpr body
813 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
815 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
816 return_id = lookupEvidence tbl returnMName
817 bind_id = lookupEvidence tbl bindMName
818 then_id = lookupEvidence tbl thenMName
819 fail_id = lookupEvidence tbl failMName
821 go _ (LetStmt binds) stmts
822 = do { rest <- goL stmts
823 ; dsLocalBinds binds rest }
825 go _ (ExprStmt rhs then_expr rhs_ty) stmts
826 = do { rhs2 <- dsLExpr rhs
827 ; warnDiscardedDoBindings rhs m_ty rhs_ty
828 ; then_expr2 <- dsExpr then_expr
830 ; return (mkApps then_expr2 [rhs2, rest]) }
832 go _ (BindStmt pat rhs bind_op _) stmts
833 = do { body <- goL stmts
834 ; rhs' <- dsLExpr rhs
835 ; bind_op' <- dsExpr bind_op
836 ; var <- selectSimpleMatchVarL pat
837 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
838 result_ty (cantFailMatchResult body)
839 ; match_code <- handle_failure pat match fail_op
840 ; return (mkApps bind_op [rhs', Lam var match_code]) }
842 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
843 , recS_rec_ids = rec_ids, recS_rec_rets = rec_rets
844 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op }) stmts
845 = ASSERT( length rec_ids > 0 )
846 ASSERT( length rec_ids == length rec_rets )
847 ASSERT( isEmptyTcEvBinds _ev_binds )
848 pprTrace "dsMDo" (ppr later_ids) $
849 goL (new_bind_stmt : stmts)
851 new_bind_stmt = L loc $ BindStmt (mk_tup_pat later_pats) mfix_app
854 -- Remove the later_ids that appear (without fancy coercions)
855 -- in rec_rets, because there's no need to knot-tie them separately
856 -- See Note [RecStmt] in HsExpr
857 later_ids' = filter (`notElem` mono_rec_ids) later_ids
858 mono_rec_ids = [ id | HsVar id <- rec_rets ]
860 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
861 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
862 (mkFunTy tup_ty body_ty))
864 -- The rec_tup_pat must bind the rec_ids only; remember that the
865 -- trimmed_laters may share the same Names
866 -- Meanwhile, the later_pats must bind the later_vars
867 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
868 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
869 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
871 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
872 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
873 body_ty = mkAppTy m_ty tup_ty
874 tup_ty = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids)) -- Deals with singleton case
876 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
878 mk_wild_pat :: Id -> LPat Id
879 mk_wild_pat v = noLoc $ WildPat $ idType v
881 mk_later_pat :: Id -> LPat Id
882 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
883 | otherwise = nlVarPat v
885 mk_tup_pat :: [LPat Id] -> LPat Id
887 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
892 %************************************************************************
894 Warning about identities
896 %************************************************************************
898 Warn about functions that convert between one type and another
899 when the to- and from- types are the same. Then it's probably
900 (albeit not definitely) the identity
902 warnAboutIdentities :: CoreExpr -> (CoreExpr -> CoreExpr) -> DsM ()
903 warnAboutIdentities (Var v) co_fn
904 | idName v `elem` conversionNames
905 , let fun_ty = exprType (co_fn (Var v))
906 , Just (arg_ty, res_ty) <- splitFunTy_maybe fun_ty
907 , arg_ty `tcEqType` res_ty -- So we are converting ty -> ty
908 = warnDs (vcat [ ptext (sLit "Call of") <+> ppr v <+> dcolon <+> ppr fun_ty
909 , nest 2 $ ptext (sLit "can probably be omitted")
910 , parens (ptext (sLit "Use -fno-warn-identities to suppress this messsage)"))
912 warnAboutIdentities _ _ = return ()
914 conversionNames :: [Name]
916 = [ toIntegerName, toRationalName
917 , fromIntegralName, realToFracName ]
918 -- We can't easily add fromIntegerName, fromRationalName,
919 -- becuase they are generated by literals
922 %************************************************************************
924 \subsection{Errors and contexts}
926 %************************************************************************
929 -- Warn about certain types of values discarded in monadic bindings (#3263)
930 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
931 warnDiscardedDoBindings rhs container_ty returning_ty = do {
932 -- Warn about discarding non-() things in 'monadic' binding
933 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
934 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
935 then warnDs (unusedMonadBind rhs returning_ty)
937 -- Warn about discarding m a things in 'monadic' binding of the same type,
938 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
939 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
940 ; case tcSplitAppTy_maybe returning_ty of
941 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
942 warnDs (wrongMonadBind rhs returning_ty)
945 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
946 unusedMonadBind rhs returning_ty
947 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
948 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
949 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
951 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
952 wrongMonadBind rhs returning_ty
953 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
954 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
955 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")