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
71 %************************************************************************
73 dsLocalBinds, dsValBinds
75 %************************************************************************
78 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
79 dsLocalBinds EmptyLocalBinds body = return body
80 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
81 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
83 -------------------------
84 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
85 dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
87 -------------------------
88 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
89 dsIPBinds (IPBinds ip_binds dict_binds) body
90 = do { prs <- dsLHsBinds dict_binds
91 ; let inner = Let (Rec prs) body
92 -- The dict bindings may not be in
93 -- dependency order; hence Rec
94 ; foldrM ds_ip_bind inner ip_binds }
96 ds_ip_bind (L _ (IPBind n e)) body
98 return (Let (NonRec (ipNameName n) e') body)
100 -------------------------
101 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
102 -- Special case for bindings which bind unlifted variables
103 -- We need to do a case right away, rather than building
104 -- a tuple and doing selections.
105 -- Silently ignore INLINE and SPECIALISE pragmas...
106 ds_val_bind (NonRecursive, hsbinds) body
107 | [L _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
108 (L loc bind : null_binds) <- bagToList binds,
110 || isUnboxedTupleBind bind
111 || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
113 body_w_exports = foldr bind_export body exports
114 bind_export (tvs, g, l, _) body = ASSERT( null tvs )
115 bindNonRec g (Var l) body
117 ASSERT (null null_binds)
118 -- Non-recursive, non-overloaded bindings only come in ones
119 -- ToDo: in some bizarre case it's conceivable that there
120 -- could be dict binds in the 'binds'. (See the notes
121 -- below. Then pattern-match would fail. Urk.)
124 FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn,
125 fun_tick = tick, fun_infix = inf }
126 -> do (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
127 MASSERT( null args ) -- Functions aren't lifted
128 MASSERT( isIdHsWrapper co_fn )
129 rhs' <- mkOptTickBox tick rhs
130 return (bindNonRec fun rhs' body_w_exports)
132 PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }
133 -> -- let C x# y# = rhs in body
134 -- ==> case rhs of C x# y# -> body
136 do { rhs <- dsGuarded grhss ty
137 ; let upat = unLoc pat
138 eqn = EqnInfo { eqn_pats = [upat],
139 eqn_rhs = cantFailMatchResult body_w_exports }
140 ; var <- selectMatchVar upat
141 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
142 ; return (scrungleMatch var rhs result) }
144 _ -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
147 -- Ordinary case for bindings; none should be unlifted
148 ds_val_bind (_is_rec, binds) body
149 = do { prs <- dsLHsBinds binds
150 ; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
153 _ -> return (Let (Rec prs) body) }
154 -- Use a Rec regardless of is_rec.
155 -- Why? Because it allows the binds to be all
156 -- mixed up, which is what happens in one rare case
157 -- Namely, for an AbsBind with no tyvars and no dicts,
158 -- but which does have dictionary bindings.
159 -- See notes with TcSimplify.inferLoop [NO TYVARS]
160 -- It turned out that wrapping a Rec here was the easiest solution
162 -- NB The previous case dealt with unlifted bindings, so we
163 -- only have to deal with lifted ones now; so Rec is ok
165 isUnboxedTupleBind :: HsBind Id -> Bool
166 isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
167 isUnboxedTupleBind _ = False
169 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
170 -- Returns something like (let var = scrut in body)
171 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
172 -- Special case to handle unboxed tuple patterns; they can't appear nested
174 -- case e of (# p1, p2 #) -> rhs
176 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
178 -- let x = e in case x of ....
180 -- But there may be a big
181 -- let fail = ... in case e of ...
182 -- wrapping the whole case, which complicates matters slightly
183 -- It all seems a bit fragile. Test is dsrun013.
185 scrungleMatch var scrut body
186 | isUnboxedTupleType (idType var) = scrungle body
187 | otherwise = bindNonRec var scrut body
189 scrungle (Case (Var x) bndr ty alts)
190 | x == var = Case scrut bndr ty alts
191 scrungle (Let binds body) = Let binds (scrungle body)
192 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
196 %************************************************************************
198 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
200 %************************************************************************
203 dsLExpr :: LHsExpr Id -> DsM CoreExpr
205 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
207 dsExpr :: HsExpr Id -> DsM CoreExpr
208 dsExpr (HsPar e) = dsLExpr e
209 dsExpr (ExprWithTySigOut e _) = dsLExpr e
210 dsExpr (HsVar var) = return (Var var)
211 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
212 dsExpr (HsLit lit) = dsLit lit
213 dsExpr (HsOverLit lit) = dsOverLit lit
214 dsExpr (HsWrap co_fn e) = do { co_fn' <- dsCoercion co_fn
216 ; return (co_fn' e') }
218 dsExpr (NegApp expr neg_expr)
219 = App <$> dsExpr neg_expr <*> dsLExpr expr
221 dsExpr (HsLam a_Match)
222 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
224 dsExpr (HsApp fun arg)
225 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
228 Operator sections. At first it looks as if we can convert
237 But no! expr might be a redex, and we can lose laziness badly this
242 for example. So we convert instead to
244 let y = expr in \x -> op y x
246 If \tr{expr} is actually just a variable, say, then the simplifier
250 dsExpr (OpApp e1 op _ e2)
251 = -- for the type of y, we need the type of op's 2nd argument
252 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
254 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
255 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
257 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
258 dsExpr (SectionR op expr) = do
259 core_op <- dsLExpr op
260 -- for the type of x, we need the type of op's 2nd argument
261 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
262 -- See comment with SectionL
263 y_core <- dsLExpr expr
264 x_id <- newSysLocalDs x_ty
265 y_id <- newSysLocalDs y_ty
266 return (bindNonRec y_id y_core $
267 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
269 dsExpr (ExplicitTuple tup_args boxity)
270 = do { let go (lam_vars, args) (Missing ty)
271 -- For every missing expression, we need
272 -- another lambda in the desugaring.
273 = do { lam_var <- newSysLocalDs ty
274 ; return (lam_var : lam_vars, Var lam_var : args) }
275 go (lam_vars, args) (Present expr)
276 -- Expressions that are present don't generate
277 -- lambdas, just arguments.
278 = do { core_expr <- dsLExpr expr
279 ; return (lam_vars, core_expr : args) }
281 ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
282 -- The reverse is because foldM goes left-to-right
284 ; return $ mkCoreLams lam_vars $
285 mkConApp (tupleCon boxity (length tup_args))
286 (map (Type . exprType) args ++ args) }
288 dsExpr (HsSCC cc expr) = do
289 mod_name <- getModuleDs
290 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
293 -- hdaume: core annotation
295 dsExpr (HsCoreAnn fs expr)
296 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
298 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
299 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
300 = -- So desugar empty HsCase to error call
301 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
304 = do { core_discrim <- dsLExpr discrim
305 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
306 ; return (scrungleMatch discrim_var core_discrim matching_code) }
308 -- Pepe: The binds are in scope in the body but NOT in the binding group
309 -- This is to avoid silliness in breakpoints
310 dsExpr (HsLet binds body) = do
311 body' <- dsLExpr body
312 dsLocalBinds binds body'
314 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
315 -- because the interpretation of `stmts' depends on what sort of thing it is.
317 dsExpr (HsDo ListComp stmts body result_ty)
318 = -- Special case for list comprehensions
319 dsListComp stmts body elt_ty
321 [elt_ty] = tcTyConAppArgs result_ty
323 dsExpr (HsDo DoExpr stmts body result_ty)
324 = dsDo stmts body result_ty
326 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
327 = dsMDo tbl stmts body result_ty
329 dsExpr (HsDo PArrComp stmts body result_ty)
330 = -- Special case for array comprehensions
331 dsPArrComp (map unLoc stmts) body elt_ty
333 [elt_ty] = tcTyConAppArgs result_ty
335 dsExpr (HsIf guard_expr then_expr else_expr)
336 = mkIfThenElse <$> dsLExpr guard_expr <*> dsLExpr then_expr <*> dsLExpr else_expr
341 \underline{\bf Various data construction things}
342 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
344 dsExpr (ExplicitList elt_ty xs)
345 = dsExplicitList elt_ty xs
347 -- We desugar [:x1, ..., xn:] as
348 -- singletonP x1 +:+ ... +:+ singletonP xn
350 dsExpr (ExplicitPArr ty []) = do
351 emptyP <- dsLookupGlobalId emptyPName
352 return (Var emptyP `App` Type ty)
353 dsExpr (ExplicitPArr ty xs) = do
354 singletonP <- dsLookupGlobalId singletonPName
355 appP <- dsLookupGlobalId appPName
356 xs' <- mapM dsLExpr xs
357 return . foldr1 (binary appP) $ map (unary singletonP) xs'
359 unary fn x = mkApps (Var fn) [Type ty, x]
360 binary fn x y = mkApps (Var fn) [Type ty, x, y]
362 dsExpr (ArithSeq expr (From from))
363 = App <$> dsExpr expr <*> dsLExpr from
365 dsExpr (ArithSeq expr (FromTo from to))
366 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
368 dsExpr (ArithSeq expr (FromThen from thn))
369 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
371 dsExpr (ArithSeq expr (FromThenTo from thn to))
372 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
374 dsExpr (PArrSeq expr (FromTo from to))
375 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
377 dsExpr (PArrSeq expr (FromThenTo from thn to))
378 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
381 = panic "DsExpr.dsExpr: Infinite parallel array!"
382 -- the parser shouldn't have generated it and the renamer and typechecker
383 -- shouldn't have let it through
387 \underline{\bf Record construction and update}
388 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
389 For record construction we do this (assuming T has three arguments)
393 let err = /\a -> recConErr a
394 T (recConErr t1 "M.lhs/230/op1")
396 (recConErr t1 "M.lhs/230/op3")
398 @recConErr@ then converts its arugment string into a proper message
399 before printing it as
401 M.lhs, line 230: missing field op1 was evaluated
404 We also handle @C{}@ as valid construction syntax for an unlabelled
405 constructor @C@, setting all of @C@'s fields to bottom.
408 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
409 con_expr' <- dsExpr con_expr
411 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
412 -- A newtype in the corner should be opaque;
413 -- hence TcType.tcSplitFunTys
415 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
416 = case findField (rec_flds rbinds) lbl of
417 (rhs:rhss) -> ASSERT( null rhss )
419 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
420 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
422 labels = dataConFieldLabels (idDataCon data_con_id)
423 -- The data_con_id is guaranteed to be the wrapper id of the constructor
425 con_args <- if null labels
426 then mapM unlabelled_bottom arg_tys
427 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
429 return (mkApps con_expr' con_args)
432 Record update is a little harder. Suppose we have the decl:
434 data T = T1 {op1, op2, op3 :: Int}
435 | T2 {op4, op2 :: Int}
438 Then we translate as follows:
444 T1 op1 _ op3 -> T1 op1 op2 op3
445 T2 op4 _ -> T2 op4 op2
446 other -> recUpdError "M.lhs/230"
448 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
449 RHSs, and do not generate a Core constructor application directly, because the constructor
450 might do some argument-evaluation first; and may have to throw away some
453 Note [Update for GADTs]
454 ~~~~~~~~~~~~~~~~~~~~~~~
457 T1 { f1 :: a } :: T a Int
459 Then the wrapper function for T1 has type
461 But if x::T a b, then
462 x { f1 = v } :: T a b (not T a Int!)
463 So we need to cast (T a Int) to (T a b). Sigh.
466 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
467 cons_to_upd in_inst_tys out_inst_tys)
469 = dsLExpr record_expr
471 = ASSERT2( notNull cons_to_upd, ppr expr )
473 do { record_expr' <- dsLExpr record_expr
474 ; field_binds' <- mapM ds_field fields
475 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
476 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
478 -- It's important to generate the match with matchWrapper,
479 -- and the right hand sides with applications of the wrapper Id
480 -- so that everything works when we are doing fancy unboxing on the
481 -- constructor aguments.
482 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
483 ; ([discrim_var], matching_code)
484 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
486 ; return (add_field_binds field_binds' $
487 bindNonRec discrim_var record_expr' matching_code) }
489 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
490 -- Clone the Id in the HsRecField, because its Name is that
491 -- of the record selector, and we must not make that a lcoal binder
492 -- else we shadow other uses of the record selector
493 -- Hence 'lcl_id'. Cf Trac #2735
494 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
495 ; let fld_id = unLoc (hsRecFieldId rec_field)
496 ; lcl_id <- newSysLocalDs (idType fld_id)
497 ; return (idName fld_id, lcl_id, rhs) }
499 add_field_binds [] expr = expr
500 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
502 -- Awkwardly, for families, the match goes
503 -- from instance type to family type
504 tycon = dataConTyCon (head cons_to_upd)
505 in_ty = mkTyConApp tycon in_inst_tys
506 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
508 mk_alt upd_fld_env con
509 = do { let (univ_tvs, ex_tvs, eq_spec,
510 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
511 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
513 -- I'm not bothering to clone the ex_tvs
514 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
515 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
516 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
517 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
518 (dataConFieldLabels con) arg_ids
519 mk_val_arg field_name pat_arg_id
520 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
521 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
522 -- Reconstruct with the WrapId so that unpacking happens
523 wrap = mkWpApps theta_vars `WpCompose`
524 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
525 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
526 , isNothing (lookupTyVar wrap_subst tv) ]
527 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
529 -- Tediously wrap the application in a cast
530 -- Note [Update for GADTs]
531 wrapped_rhs | null eq_spec = rhs
532 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
533 wrap_co = mkTyConApp tycon [ lookup tv ty
534 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
535 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
538 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
539 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
541 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
542 , pat_dicts = eqs_vars ++ theta_vars
543 , pat_binds = emptyLHsBinds
544 , pat_args = PrefixCon $ map nlVarPat arg_ids
546 ; return (mkSimpleMatch [pat] wrapped_rhs) }
550 Here is where we desugar the Template Haskell brackets and escapes
553 -- Template Haskell stuff
555 #ifdef GHCI /* Only if bootstrapping */
556 dsExpr (HsBracketOut x ps) = dsBracket x ps
557 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
560 -- Arrow notation extension
561 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
567 dsExpr (HsTick ix vars e) = do
571 -- There is a problem here. The then and else branches
572 -- have no free variables, so they are open to lifting.
573 -- We need someway of stopping this.
574 -- This will make no difference to binary coverage
575 -- (did you go here: YES or NO), but will effect accurate
578 dsExpr (HsBinTick ixT ixF e) = do
580 do { ASSERT(exprType e2 `coreEqType` boolTy)
581 mkBinaryTickBox ixT ixF e2
587 -- HsSyn constructs that just shouldn't be here:
588 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
591 findField :: [HsRecField Id arg] -> Name -> [arg]
593 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
594 , lbl == idName (unLoc id) ]
597 %--------------------------------------------------------------------
599 Note [Desugaring explicit lists]
600 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
601 Explicit lists are desugared in a cleverer way to prevent some
602 fruitless allocations. Essentially, whenever we see a list literal
605 1. Find the tail of the list that can be allocated statically (say
606 [x_k, ..., x_n]) by later stages and ensure we desugar that
607 normally: this makes sure that we don't cause a code size increase
608 by having the cons in that expression fused (see later) and hence
609 being unable to statically allocate any more
611 2. For the prefix of the list which cannot be allocated statically,
612 say [x_1, ..., x_(k-1)], we turn it into an expression involving
613 build so that if we find any foldrs over it it will fuse away
616 So in this example we will desugar to:
617 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
619 If fusion fails to occur then build will get inlined and (since we
620 defined a RULE for foldr (:) []) we will get back exactly the
621 normal desugaring for an explicit list.
623 This optimisation can be worth a lot: up to 25% of the total
624 allocation in some nofib programs. Specifically
626 Program Size Allocs Runtime CompTime
627 rewrite +0.0% -26.3% 0.02 -1.8%
628 ansi -0.3% -13.8% 0.00 +0.0%
629 lift +0.0% -8.7% 0.00 -2.3%
631 Of course, if rules aren't turned on then there is pretty much no
632 point doing this fancy stuff, and it may even be harmful.
634 =======> Note by SLPJ Dec 08.
636 I'm unconvinced that we should *ever* generate a build for an explicit
637 list. See the comments in GHC.Base about the foldr/cons rule, which
638 points out that (foldr k z [a,b,c]) may generate *much* less code than
639 (a `k` b `k` c `k` z).
641 Furthermore generating builds messes up the LHS of RULES.
642 Example: the foldr/single rule in GHC.Base
644 We do not want to generate a build invocation on the LHS of this RULE!
646 We fix this by disabling rules in rule LHSs, and testing that
647 flag here; see Note [Desugaring RULE left hand sides] in Desugar
649 To test this I've added a (static) flag -fsimple-list-literals, which
650 makes all list literals be generated via the simple route.
654 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
655 -- See Note [Desugaring explicit lists]
656 dsExplicitList elt_ty xs
657 = do { dflags <- getDOptsDs
658 ; xs' <- mapM dsLExpr xs
659 ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
660 ; if opt_SimpleListLiterals -- -fsimple-list-literals
661 || not (dopt Opt_EnableRewriteRules dflags) -- Rewrite rules off
662 -- Don't generate a build if there are no rules to eliminate it!
663 -- See Note [Desugaring RULE left hand sides] in Desugar
664 || null dynamic_prefix -- Avoid build (\c n. foldr c n xs)!
665 then return $ mkListExpr elt_ty xs'
666 else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }
668 is_static :: CoreExpr -> Bool
669 is_static e = all is_static_var (varSetElems (exprFreeVars e))
671 is_static_var :: Var -> Bool
673 | isId v = isExternalName (idName v) -- Top-level things are given external names
674 | otherwise = False -- Type variables
676 mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
677 = do { let suffix' = mkListExpr elt_ty suffix
678 ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
679 ; return (foldr (App . App (Var c)) folded_suffix prefix) }
681 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
682 spanTail f xs = (reverse rejected, reverse satisfying)
683 where (satisfying, rejected) = span f $ reverse xs
686 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
687 handled in DsListComp). Basically does the translation given in the
693 -> Type -- Type of the whole expression
696 dsDo stmts body result_ty
699 -- result_ty must be of the form (m b)
700 (m_ty, _b_ty) = tcSplitAppTy result_ty
702 goL [] = dsLExpr body
703 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
705 go _ (ExprStmt rhs then_expr _) stmts
706 = do { rhs2 <- dsLExpr rhs
707 ; case tcSplitAppTy_maybe (exprType rhs2) of
708 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
710 ; then_expr2 <- dsExpr then_expr
712 ; return (mkApps then_expr2 [rhs2, rest]) }
714 go _ (LetStmt binds) stmts
715 = do { rest <- goL stmts
716 ; dsLocalBinds binds rest }
718 go _ (BindStmt pat rhs bind_op fail_op) stmts
719 = do { body <- goL stmts
720 ; rhs' <- dsLExpr rhs
721 ; bind_op' <- dsExpr bind_op
722 ; var <- selectSimpleMatchVarL pat
723 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
724 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
725 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
726 res1_ty (cantFailMatchResult body)
727 ; match_code <- handle_failure pat match fail_op
728 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
730 go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
731 , recS_rec_ids = rec_ids, recS_ret_fn = return_op
732 , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
733 , recS_rec_rets = rec_rets, recS_dicts = binds }) stmts
734 = ASSERT( length rec_ids > 0 )
735 goL (new_bind_stmt : let_stmt : stmts)
737 -- returnE <- dsExpr return_id
738 -- mfixE <- dsExpr mfix_id
739 new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
741 noSyntaxExpr -- Tuple cannot fail
743 let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
745 tup_ids = rec_ids ++ filterOut (`elem` rec_ids) later_ids
746 rec_tup_pats = map nlVarPat tup_ids
747 later_pats = rec_tup_pats
748 rets = map noLoc rec_rets
750 mfix_app = nlHsApp (noLoc mfix_op) mfix_arg
751 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
752 (mkFunTy tup_ty body_ty))
753 mfix_pat = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
754 body = noLoc $ HsDo DoExpr rec_stmts return_app body_ty
755 return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
756 body_ty = mkAppTy m_ty tup_ty
757 tup_ty = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
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))
774 Translation for RecStmt's:
775 -----------------------------
776 We turn (RecStmt [v1,..vn] stmts) into:
778 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
785 -> Type -- Type of the whole expression
788 dsMDo tbl stmts body result_ty
791 goL [] = dsLExpr body
792 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
794 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
795 mfix_id = lookupEvidence tbl mfixName
796 return_id = lookupEvidence tbl returnMName
797 bind_id = lookupEvidence tbl bindMName
798 then_id = lookupEvidence tbl thenMName
799 fail_id = lookupEvidence tbl failMName
802 go _ (LetStmt binds) stmts
803 = do { rest <- goL stmts
804 ; dsLocalBinds binds rest }
806 go _ (ExprStmt rhs _ rhs_ty) stmts
807 = do { rhs2 <- dsLExpr rhs
808 ; warnDiscardedDoBindings rhs m_ty rhs_ty
810 ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
812 go _ (BindStmt pat rhs _ _) stmts
813 = do { body <- goL stmts
814 ; var <- selectSimpleMatchVarL pat
815 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
816 result_ty (cantFailMatchResult body)
817 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
818 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
819 ; match_code <- extractMatchResult match fail_expr
821 ; rhs' <- dsLExpr rhs
822 ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
823 rhs', Lam var match_code]) }
825 go loc (RecStmt rec_stmts later_ids rec_ids _ _ _ rec_rets binds) stmts
826 = ASSERT( length rec_ids > 0 )
827 ASSERT( length rec_ids == length rec_rets )
828 pprTrace "dsMDo" (ppr later_ids) $
829 goL (new_bind_stmt : let_stmt : stmts)
831 new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
832 let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
835 -- Remove the later_ids that appear (without fancy coercions)
836 -- in rec_rets, because there's no need to knot-tie them separately
837 -- See Note [RecStmt] in HsExpr
838 later_ids' = filter (`notElem` mono_rec_ids) later_ids
839 mono_rec_ids = [ id | HsVar id <- rec_rets ]
841 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
842 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
843 (mkFunTy tup_ty body_ty))
845 -- The rec_tup_pat must bind the rec_ids only; remember that the
846 -- trimmed_laters may share the same Names
847 -- Meanwhile, the later_pats must bind the later_vars
848 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
849 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
850 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
852 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
853 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
854 body_ty = mkAppTy m_ty tup_ty
855 tup_ty = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids)) -- Deals with singleton case
857 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
858 (mkLHsTupleExpr rets)
860 mk_wild_pat :: Id -> LPat Id
861 mk_wild_pat v = noLoc $ WildPat $ idType v
863 mk_later_pat :: Id -> LPat Id
864 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
865 | otherwise = nlVarPat v
867 mk_tup_pat :: [LPat Id] -> LPat Id
869 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
873 %************************************************************************
875 \subsection{Errors and contexts}
877 %************************************************************************
880 -- Warn about certain types of values discarded in monadic bindings (#3263)
881 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
882 warnDiscardedDoBindings rhs container_ty returning_ty = do {
883 -- Warn about discarding non-() things in 'monadic' binding
884 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
885 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
886 then warnDs (unusedMonadBind rhs returning_ty)
888 -- Warn about discarding m a things in 'monadic' binding of the same type,
889 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
890 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
891 ; case tcSplitAppTy_maybe returning_ty of
892 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
893 warnDs (wrongMonadBind rhs returning_ty)
896 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
897 unusedMonadBind rhs returning_ty
898 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
899 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
900 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
902 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
903 wrongMonadBind rhs returning_ty
904 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
905 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
906 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")