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
68 %************************************************************************
70 dsLocalBinds, dsValBinds
72 %************************************************************************
75 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
76 dsLocalBinds EmptyLocalBinds body = return body
77 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
78 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
80 -------------------------
81 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
82 dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
84 -------------------------
85 dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
86 dsIPBinds (IPBinds ip_binds dict_binds) body
87 = do { prs <- dsLHsBinds dict_binds
88 ; let inner = Let (Rec prs) body
89 -- The dict bindings may not be in
90 -- dependency order; hence Rec
91 ; foldrM ds_ip_bind inner ip_binds }
93 ds_ip_bind (L _ (IPBind n e)) body
95 return (Let (NonRec (ipNameName n) e') body)
97 -------------------------
98 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
99 -- Special case for bindings which bind unlifted variables
100 -- We need to do a case right away, rather than building
101 -- a tuple and doing selections.
102 -- Silently ignore INLINE and SPECIALISE pragmas...
103 ds_val_bind (NonRecursive, hsbinds) body
104 | [L _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
105 (L loc bind : null_binds) <- bagToList binds,
107 || isUnboxedTupleBind bind
108 || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
110 body_w_exports = foldr bind_export body exports
111 bind_export (tvs, g, l, _) body = ASSERT( null tvs )
112 bindNonRec g (Var l) body
114 ASSERT (null null_binds)
115 -- Non-recursive, non-overloaded bindings only come in ones
116 -- ToDo: in some bizarre case it's conceivable that there
117 -- could be dict binds in the 'binds'. (See the notes
118 -- below. Then pattern-match would fail. Urk.)
121 FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn,
122 fun_tick = tick, fun_infix = inf }
123 -> do (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
124 MASSERT( null args ) -- Functions aren't lifted
125 MASSERT( isIdHsWrapper co_fn )
126 rhs' <- mkOptTickBox tick rhs
127 return (bindNonRec fun rhs' body_w_exports)
129 PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }
130 -> -- let C x# y# = rhs in body
131 -- ==> case rhs of C x# y# -> body
133 do { rhs <- dsGuarded grhss ty
134 ; let upat = unLoc pat
135 eqn = EqnInfo { eqn_pats = [upat],
136 eqn_rhs = cantFailMatchResult body_w_exports }
137 ; var <- selectMatchVar upat
138 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
139 ; return (scrungleMatch var rhs result) }
141 _ -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
144 -- Ordinary case for bindings; none should be unlifted
145 ds_val_bind (_is_rec, binds) body
146 = do { prs <- dsLHsBinds binds
147 ; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
150 _ -> return (Let (Rec prs) body) }
151 -- Use a Rec regardless of is_rec.
152 -- Why? Because it allows the binds to be all
153 -- mixed up, which is what happens in one rare case
154 -- Namely, for an AbsBind with no tyvars and no dicts,
155 -- but which does have dictionary bindings.
156 -- See notes with TcSimplify.inferLoop [NO TYVARS]
157 -- It turned out that wrapping a Rec here was the easiest solution
159 -- NB The previous case dealt with unlifted bindings, so we
160 -- only have to deal with lifted ones now; so Rec is ok
162 isUnboxedTupleBind :: HsBind Id -> Bool
163 isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
164 isUnboxedTupleBind _ = False
166 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
167 -- Returns something like (let var = scrut in body)
168 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
169 -- Special case to handle unboxed tuple patterns; they can't appear nested
171 -- case e of (# p1, p2 #) -> rhs
173 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
175 -- let x = e in case x of ....
177 -- But there may be a big
178 -- let fail = ... in case e of ...
179 -- wrapping the whole case, which complicates matters slightly
180 -- It all seems a bit fragile. Test is dsrun013.
182 scrungleMatch var scrut body
183 | isUnboxedTupleType (idType var) = scrungle body
184 | otherwise = bindNonRec var scrut body
186 scrungle (Case (Var x) bndr ty alts)
187 | x == var = Case scrut bndr ty alts
188 scrungle (Let binds body) = Let binds (scrungle body)
189 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
193 %************************************************************************
195 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
197 %************************************************************************
200 dsLExpr :: LHsExpr Id -> DsM CoreExpr
202 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
204 dsExpr :: HsExpr Id -> DsM CoreExpr
205 dsExpr (HsPar e) = dsLExpr e
206 dsExpr (ExprWithTySigOut e _) = dsLExpr e
207 dsExpr (HsVar var) = return (Var var)
208 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
209 dsExpr (HsLit lit) = dsLit lit
210 dsExpr (HsOverLit lit) = dsOverLit lit
211 dsExpr (HsWrap co_fn e) = dsCoercion co_fn (dsExpr e)
213 dsExpr (NegApp expr neg_expr)
214 = App <$> dsExpr neg_expr <*> dsLExpr expr
216 dsExpr (HsLam a_Match)
217 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
219 dsExpr (HsApp fun arg)
220 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
223 Operator sections. At first it looks as if we can convert
232 But no! expr might be a redex, and we can lose laziness badly this
237 for example. So we convert instead to
239 let y = expr in \x -> op y x
241 If \tr{expr} is actually just a variable, say, then the simplifier
245 dsExpr (OpApp e1 op _ e2)
246 = -- for the type of y, we need the type of op's 2nd argument
247 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
249 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
250 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
252 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
253 dsExpr (SectionR op expr) = do
254 core_op <- dsLExpr op
255 -- for the type of x, we need the type of op's 2nd argument
256 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
257 -- See comment with SectionL
258 y_core <- dsLExpr expr
259 x_id <- newSysLocalDs x_ty
260 y_id <- newSysLocalDs y_ty
261 return (bindNonRec y_id y_core $
262 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
264 dsExpr (HsSCC cc expr) = do
265 mod_name <- getModuleDs
266 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
269 -- hdaume: core annotation
271 dsExpr (HsCoreAnn fs expr)
272 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
274 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
275 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
276 = -- So desugar empty HsCase to error call
277 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
280 = do { core_discrim <- dsLExpr discrim
281 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
282 ; return (scrungleMatch discrim_var core_discrim matching_code) }
284 -- Pepe: The binds are in scope in the body but NOT in the binding group
285 -- This is to avoid silliness in breakpoints
286 dsExpr (HsLet binds body) = do
287 body' <- dsLExpr body
288 dsLocalBinds binds body'
290 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
291 -- because the interpretation of `stmts' depends on what sort of thing it is.
293 dsExpr (HsDo ListComp stmts body result_ty)
294 = -- Special case for list comprehensions
295 dsListComp stmts body elt_ty
297 [elt_ty] = tcTyConAppArgs result_ty
299 dsExpr (HsDo DoExpr stmts body result_ty)
300 = dsDo stmts body result_ty
302 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
303 = dsMDo tbl stmts body result_ty
305 dsExpr (HsDo PArrComp stmts body result_ty)
306 = -- Special case for array comprehensions
307 dsPArrComp (map unLoc stmts) body elt_ty
309 [elt_ty] = tcTyConAppArgs result_ty
311 dsExpr (HsIf guard_expr then_expr else_expr)
312 = mkIfThenElse <$> dsLExpr guard_expr <*> dsLExpr then_expr <*> dsLExpr else_expr
317 \underline{\bf Various data construction things}
318 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
320 dsExpr (ExplicitList elt_ty xs)
321 = dsExplicitList elt_ty xs
323 -- We desugar [:x1, ..., xn:] as
324 -- singletonP x1 +:+ ... +:+ singletonP xn
326 dsExpr (ExplicitPArr ty []) = do
327 emptyP <- dsLookupGlobalId emptyPName
328 return (Var emptyP `App` Type ty)
329 dsExpr (ExplicitPArr ty xs) = do
330 singletonP <- dsLookupGlobalId singletonPName
331 appP <- dsLookupGlobalId appPName
332 xs' <- mapM dsLExpr xs
333 return . foldr1 (binary appP) $ map (unary singletonP) xs'
335 unary fn x = mkApps (Var fn) [Type ty, x]
336 binary fn x y = mkApps (Var fn) [Type ty, x, y]
338 dsExpr (ExplicitTuple expr_list boxity) = do
339 core_exprs <- mapM dsLExpr expr_list
340 return (mkConApp (tupleCon boxity (length expr_list))
341 (map (Type . exprType) core_exprs ++ core_exprs))
343 dsExpr (ArithSeq expr (From from))
344 = App <$> dsExpr expr <*> dsLExpr from
346 dsExpr (ArithSeq expr (FromTo from to))
347 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
349 dsExpr (ArithSeq expr (FromThen from thn))
350 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
352 dsExpr (ArithSeq expr (FromThenTo from thn to))
353 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
355 dsExpr (PArrSeq expr (FromTo from to))
356 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
358 dsExpr (PArrSeq expr (FromThenTo from thn to))
359 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
362 = panic "DsExpr.dsExpr: Infinite parallel array!"
363 -- the parser shouldn't have generated it and the renamer and typechecker
364 -- shouldn't have let it through
368 \underline{\bf Record construction and update}
369 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
370 For record construction we do this (assuming T has three arguments)
374 let err = /\a -> recConErr a
375 T (recConErr t1 "M.lhs/230/op1")
377 (recConErr t1 "M.lhs/230/op3")
379 @recConErr@ then converts its arugment string into a proper message
380 before printing it as
382 M.lhs, line 230: missing field op1 was evaluated
385 We also handle @C{}@ as valid construction syntax for an unlabelled
386 constructor @C@, setting all of @C@'s fields to bottom.
389 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
390 con_expr' <- dsExpr con_expr
392 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
393 -- A newtype in the corner should be opaque;
394 -- hence TcType.tcSplitFunTys
396 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
397 = case findField (rec_flds rbinds) lbl of
398 (rhs:rhss) -> ASSERT( null rhss )
400 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
401 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
403 labels = dataConFieldLabels (idDataCon data_con_id)
404 -- The data_con_id is guaranteed to be the wrapper id of the constructor
406 con_args <- if null labels
407 then mapM unlabelled_bottom arg_tys
408 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
410 return (mkApps con_expr' con_args)
413 Record update is a little harder. Suppose we have the decl:
415 data T = T1 {op1, op2, op3 :: Int}
416 | T2 {op4, op2 :: Int}
419 Then we translate as follows:
425 T1 op1 _ op3 -> T1 op1 op2 op3
426 T2 op4 _ -> T2 op4 op2
427 other -> recUpdError "M.lhs/230"
429 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
430 RHSs, and do not generate a Core constructor application directly, because the constructor
431 might do some argument-evaluation first; and may have to throw away some
434 Note [Update for GADTs]
435 ~~~~~~~~~~~~~~~~~~~~~~~
438 T1 { f1 :: a } :: T a Int
440 Then the wrapper function for T1 has type
442 But if x::T a b, then
443 x { f1 = v } :: T a b (not T a Int!)
444 So we need to cast (T a Int) to (T a b). Sigh.
447 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
448 cons_to_upd in_inst_tys out_inst_tys)
450 = dsLExpr record_expr
452 = ASSERT2( notNull cons_to_upd, ppr expr )
454 do { record_expr' <- dsLExpr record_expr
455 ; field_binds' <- mapM ds_field fields
456 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
457 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
459 -- It's important to generate the match with matchWrapper,
460 -- and the right hand sides with applications of the wrapper Id
461 -- so that everything works when we are doing fancy unboxing on the
462 -- constructor aguments.
463 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
464 ; ([discrim_var], matching_code)
465 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
467 ; return (add_field_binds field_binds' $
468 bindNonRec discrim_var record_expr' matching_code) }
470 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
471 -- Clone the Id in the HsRecField, because its Name is that
472 -- of the record selector, and we must not make that a lcoal binder
473 -- else we shadow other uses of the record selector
474 -- Hence 'lcl_id'. Cf Trac #2735
475 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
476 ; let fld_id = unLoc (hsRecFieldId rec_field)
477 ; lcl_id <- newSysLocalDs (idType fld_id)
478 ; return (idName fld_id, lcl_id, rhs) }
480 add_field_binds [] expr = expr
481 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
483 -- Awkwardly, for families, the match goes
484 -- from instance type to family type
485 tycon = dataConTyCon (head cons_to_upd)
486 in_ty = mkTyConApp tycon in_inst_tys
487 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
489 mk_alt upd_fld_env con
490 = do { let (univ_tvs, ex_tvs, eq_spec,
491 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
492 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
494 -- I'm not bothering to clone the ex_tvs
495 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
496 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
497 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
498 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
499 (dataConFieldLabels con) arg_ids
500 mk_val_arg field_name pat_arg_id
501 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
502 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
503 -- Reconstruct with the WrapId so that unpacking happens
504 wrap = mkWpApps theta_vars `WpCompose`
505 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
506 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
507 , isNothing (lookupTyVar wrap_subst tv) ]
508 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
510 -- Tediously wrap the application in a cast
511 -- Note [Update for GADTs]
512 wrapped_rhs | null eq_spec = rhs
513 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
514 wrap_co = mkTyConApp tycon [ lookup tv ty
515 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
516 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
519 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
520 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
522 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
523 , pat_dicts = eqs_vars ++ theta_vars
524 , pat_binds = emptyLHsBinds
525 , pat_args = PrefixCon $ map nlVarPat arg_ids
527 ; return (mkSimpleMatch [pat] wrapped_rhs) }
531 Here is where we desugar the Template Haskell brackets and escapes
534 -- Template Haskell stuff
536 #ifdef GHCI /* Only if bootstrapping */
537 dsExpr (HsBracketOut x ps) = dsBracket x ps
538 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
541 -- Arrow notation extension
542 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
548 dsExpr (HsTick ix vars e) = do
552 -- There is a problem here. The then and else branches
553 -- have no free variables, so they are open to lifting.
554 -- We need someway of stopping this.
555 -- This will make no difference to binary coverage
556 -- (did you go here: YES or NO), but will effect accurate
559 dsExpr (HsBinTick ixT ixF e) = do
561 do { ASSERT(exprType e2 `coreEqType` boolTy)
562 mkBinaryTickBox ixT ixF e2
568 -- HsSyn constructs that just shouldn't be here:
569 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
572 findField :: [HsRecField Id arg] -> Name -> [arg]
574 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
575 , lbl == idName (unLoc id) ]
578 %--------------------------------------------------------------------
580 Note [Desugaring explicit lists]
581 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
582 Explicit lists are desugared in a cleverer way to prevent some
583 fruitless allocations. Essentially, whenever we see a list literal
586 1. Find the tail of the list that can be allocated statically (say
587 [x_k, ..., x_n]) by later stages and ensure we desugar that
588 normally: this makes sure that we don't cause a code size increase
589 by having the cons in that expression fused (see later) and hence
590 being unable to statically allocate any more
592 2. For the prefix of the list which cannot be allocated statically,
593 say [x_1, ..., x_(k-1)], we turn it into an expression involving
594 build so that if we find any foldrs over it it will fuse away
597 So in this example we will desugar to:
598 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
600 If fusion fails to occur then build will get inlined and (since we
601 defined a RULE for foldr (:) []) we will get back exactly the
602 normal desugaring for an explicit list.
604 This optimisation can be worth a lot: up to 25% of the total
605 allocation in some nofib programs. Specifically
607 Program Size Allocs Runtime CompTime
608 rewrite +0.0% -26.3% 0.02 -1.8%
609 ansi -0.3% -13.8% 0.00 +0.0%
610 lift +0.0% -8.7% 0.00 -2.3%
612 Of course, if rules aren't turned on then there is pretty much no
613 point doing this fancy stuff, and it may even be harmful.
615 =======> Note by SLPJ Dec 08.
617 I'm unconvinced that we should *ever* generate a build for an explicit
618 list. See the comments in GHC.Base about the foldr/cons rule, which
619 points out that (foldr k z [a,b,c]) may generate *much* less code than
620 (a `k` b `k` c `k` z).
622 Furthermore generating builds messes up the LHS of RULES.
623 Example: the foldr/single rule in GHC.Base
625 We do not want to generate a build invocation on the LHS of this RULE!
627 To test this I've added a (static) flag -fsimple-list-literals, which
628 makes all list literals be generated via the simple route.
633 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
634 -- See Note [Desugaring explicit lists]
635 dsExplicitList elt_ty xs = do
637 xs' <- mapM dsLExpr xs
638 if opt_SimpleListLiterals || not (dopt Opt_EnableRewriteRules dflags)
639 then return $ mkListExpr elt_ty xs'
640 else mkBuildExpr elt_ty (mkSplitExplicitList (thisPackage dflags) xs')
642 mkSplitExplicitList this_package xs' (c, _) (n, n_ty) = do
643 let (dynamic_prefix, static_suffix) = spanTail (rhsIsStatic this_package) xs'
644 static_suffix' = mkListExpr elt_ty static_suffix
646 folded_static_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) static_suffix'
647 let build_body = foldr (App . App (Var c)) folded_static_suffix dynamic_prefix
650 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
651 spanTail f xs = (reverse rejected, reverse satisfying)
652 where (satisfying, rejected) = span f $ reverse xs
655 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
656 handled in DsListComp). Basically does the translation given in the
662 -> Type -- Type of the whole expression
665 dsDo stmts body _result_ty
668 goL [] = dsLExpr body
669 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go stmt lstmts)
671 go (ExprStmt rhs then_expr _) stmts
672 = do { rhs2 <- dsLExpr rhs
673 ; case tcSplitAppTy_maybe (exprType rhs2) of
674 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
676 ; then_expr2 <- dsExpr then_expr
678 ; return (mkApps then_expr2 [rhs2, rest]) }
680 go (LetStmt binds) stmts
681 = do { rest <- goL stmts
682 ; dsLocalBinds binds rest }
684 go (BindStmt pat rhs bind_op fail_op) stmts
686 do { body <- goL stmts
687 ; rhs' <- dsLExpr rhs
688 ; bind_op' <- dsExpr bind_op
689 ; var <- selectSimpleMatchVarL pat
690 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
691 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
692 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
693 res1_ty (cantFailMatchResult body)
694 ; match_code <- handle_failure pat match fail_op
695 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
697 -- In a do expression, pattern-match failure just calls
698 -- the monadic 'fail' rather than throwing an exception
699 handle_failure pat match fail_op
701 = do { fail_op' <- dsExpr fail_op
702 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
703 ; extractMatchResult match (App fail_op' fail_msg) }
705 = extractMatchResult match (error "It can't fail")
707 mk_fail_msg :: Located e -> String
708 mk_fail_msg pat = "Pattern match failure in do expression at " ++
709 showSDoc (ppr (getLoc pat))
712 Translation for RecStmt's:
713 -----------------------------
714 We turn (RecStmt [v1,..vn] stmts) into:
716 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
723 -> Type -- Type of the whole expression
726 dsMDo tbl stmts body result_ty
729 goL [] = dsLExpr body
730 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
732 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
733 mfix_id = lookupEvidence tbl mfixName
734 return_id = lookupEvidence tbl returnMName
735 bind_id = lookupEvidence tbl bindMName
736 then_id = lookupEvidence tbl thenMName
737 fail_id = lookupEvidence tbl failMName
740 go _ (LetStmt binds) stmts
741 = do { rest <- goL stmts
742 ; dsLocalBinds binds rest }
744 go _ (ExprStmt rhs _ rhs_ty) stmts
745 = do { rhs2 <- dsLExpr rhs
746 ; warnDiscardedDoBindings rhs m_ty rhs_ty
748 ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
750 go _ (BindStmt pat rhs _ _) stmts
751 = do { body <- goL stmts
752 ; var <- selectSimpleMatchVarL pat
753 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
754 result_ty (cantFailMatchResult body)
755 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
756 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
757 ; match_code <- extractMatchResult match fail_expr
759 ; rhs' <- dsLExpr rhs
760 ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
761 rhs', Lam var match_code]) }
763 go loc (RecStmt rec_stmts later_ids rec_ids rec_rets binds) stmts
764 = ASSERT( length rec_ids > 0 )
765 ASSERT( length rec_ids == length rec_rets )
766 goL (new_bind_stmt : let_stmt : stmts)
768 new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
769 let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
772 -- Remove the later_ids that appear (without fancy coercions)
773 -- in rec_rets, because there's no need to knot-tie them separately
774 -- See Note [RecStmt] in HsExpr
775 later_ids' = filter (`notElem` mono_rec_ids) later_ids
776 mono_rec_ids = [ id | HsVar id <- rec_rets ]
778 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
779 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
780 (mkFunTy tup_ty body_ty))
782 -- The rec_tup_pat must bind the rec_ids only; remember that the
783 -- trimmed_laters may share the same Names
784 -- Meanwhile, the later_pats must bind the later_vars
785 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
786 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
787 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
789 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
790 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
791 body_ty = mkAppTy m_ty tup_ty
792 tup_ty = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
793 -- mkCoreTupTy deals with singleton case
795 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
798 mk_wild_pat :: Id -> LPat Id
799 mk_wild_pat v = noLoc $ WildPat $ idType v
801 mk_later_pat :: Id -> LPat Id
802 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
803 | otherwise = nlVarPat v
805 mk_tup_pat :: [LPat Id] -> LPat Id
807 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
809 mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
811 mk_ret_tup rs = noLoc $ ExplicitTuple rs Boxed
815 %************************************************************************
817 \subsection{Errors and contexts}
819 %************************************************************************
822 -- Warn about certain types of values discarded in monadic bindings (#3263)
823 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
824 warnDiscardedDoBindings rhs container_ty returning_ty = do {
825 -- Warn about discarding non-() things in 'monadic' binding
826 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
827 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
828 then warnDs (unusedMonadBind rhs returning_ty)
830 -- Warn about discarding m a things in 'monadic' binding of the same type,
831 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
832 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
833 ; case tcSplitAppTy_maybe returning_ty of
834 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
835 warnDs (wrongMonadBind rhs returning_ty)
838 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
839 unusedMonadBind rhs returning_ty
840 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
841 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
842 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
844 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
845 wrongMonadBind rhs returning_ty
846 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
847 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
848 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")