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"
33 -- Template Haskell stuff iff bootstrapped
40 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
41 -- 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 dict_binds) body
88 = do { prs <- dsLHsBinds dict_binds
89 ; let inner = Let (Rec prs) 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 _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
106 (L loc bind : null_binds) <- bagToList binds,
108 || isUnboxedTupleBind bind
109 || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
111 body_w_exports = foldr bind_export body exports
112 bind_export (tvs, g, l, _) body = ASSERT( null tvs )
113 bindNonRec g (Var l) body
115 ASSERT (null null_binds)
116 -- Non-recursive, non-overloaded bindings only come in ones
117 -- ToDo: in some bizarre case it's conceivable that there
118 -- could be dict binds in the 'binds'. (See the notes
119 -- below. Then pattern-match would fail. Urk.)
122 FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn,
123 fun_tick = tick, fun_infix = inf }
124 -> do (args, rhs) <- matchWrapper (FunRhs (idName fun ) inf) matches
125 MASSERT( null args ) -- Functions aren't lifted
126 MASSERT( isIdHsWrapper co_fn )
127 rhs' <- mkOptTickBox tick rhs
128 return (bindNonRec fun rhs' body_w_exports)
130 PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }
131 -> -- let C x# y# = rhs in body
132 -- ==> case rhs of C x# y# -> body
134 do { rhs <- dsGuarded grhss ty
135 ; let upat = unLoc pat
136 eqn = EqnInfo { eqn_pats = [upat],
137 eqn_rhs = cantFailMatchResult body_w_exports }
138 ; var <- selectMatchVar upat
139 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
140 ; return (scrungleMatch var rhs result) }
142 _ -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
145 -- Ordinary case for bindings; none should be unlifted
146 ds_val_bind (_is_rec, binds) body
147 = do { prs <- dsLHsBinds binds
148 ; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
151 _ -> return (Let (Rec prs) body) }
152 -- Use a Rec regardless of is_rec.
153 -- Why? Because it allows the binds to be all
154 -- mixed up, which is what happens in one rare case
155 -- Namely, for an AbsBind with no tyvars and no dicts,
156 -- but which does have dictionary bindings.
157 -- See notes with TcSimplify.inferLoop [NO TYVARS]
158 -- It turned out that wrapping a Rec here was the easiest solution
160 -- NB The previous case dealt with unlifted bindings, so we
161 -- only have to deal with lifted ones now; so Rec is ok
163 isUnboxedTupleBind :: HsBind Id -> Bool
164 isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
165 isUnboxedTupleBind _ = False
167 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
168 -- Returns something like (let var = scrut in body)
169 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
170 -- Special case to handle unboxed tuple patterns; they can't appear nested
172 -- case e of (# p1, p2 #) -> rhs
174 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
176 -- let x = e in case x of ....
178 -- But there may be a big
179 -- let fail = ... in case e of ...
180 -- wrapping the whole case, which complicates matters slightly
181 -- It all seems a bit fragile. Test is dsrun013.
183 scrungleMatch var scrut body
184 | isUnboxedTupleType (idType var) = scrungle body
185 | otherwise = bindNonRec var scrut body
187 scrungle (Case (Var x) bndr ty alts)
188 | x == var = Case scrut bndr ty alts
189 scrungle (Let binds body) = Let binds (scrungle body)
190 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
194 %************************************************************************
196 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
198 %************************************************************************
201 dsLExpr :: LHsExpr Id -> DsM CoreExpr
203 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
205 dsExpr :: HsExpr Id -> DsM CoreExpr
206 dsExpr (HsPar e) = dsLExpr e
207 dsExpr (ExprWithTySigOut e _) = dsLExpr e
208 dsExpr (HsVar var) = return (Var var)
209 dsExpr (HsIPVar ip) = return (Var (ipNameName ip))
210 dsExpr (HsLit lit) = dsLit lit
211 dsExpr (HsOverLit lit) = dsOverLit lit
212 dsExpr (HsWrap co_fn e) = dsCoercion co_fn (dsExpr e)
214 dsExpr (NegApp expr neg_expr)
215 = App <$> dsExpr neg_expr <*> dsLExpr expr
217 dsExpr (HsLam a_Match)
218 = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
220 dsExpr (HsApp fun arg)
221 = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
224 Operator sections. At first it looks as if we can convert
233 But no! expr might be a redex, and we can lose laziness badly this
238 for example. So we convert instead to
240 let y = expr in \x -> op y x
242 If \tr{expr} is actually just a variable, say, then the simplifier
246 dsExpr (OpApp e1 op _ e2)
247 = -- for the type of y, we need the type of op's 2nd argument
248 mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
250 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
251 = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
253 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
254 dsExpr (SectionR op expr) = do
255 core_op <- dsLExpr op
256 -- for the type of x, we need the type of op's 2nd argument
257 let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
258 -- See comment with SectionL
259 y_core <- dsLExpr expr
260 x_id <- newSysLocalDs x_ty
261 y_id <- newSysLocalDs y_ty
262 return (bindNonRec y_id y_core $
263 Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
265 dsExpr (HsSCC cc expr) = do
266 mod_name <- getModuleDs
267 Note (SCC (mkUserCC cc mod_name)) <$> dsLExpr expr
270 -- hdaume: core annotation
272 dsExpr (HsCoreAnn fs expr)
273 = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
275 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
276 | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
277 = -- So desugar empty HsCase to error call
278 mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
281 = do { core_discrim <- dsLExpr discrim
282 ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
283 ; return (scrungleMatch discrim_var core_discrim matching_code) }
285 -- Pepe: The binds are in scope in the body but NOT in the binding group
286 -- This is to avoid silliness in breakpoints
287 dsExpr (HsLet binds body) = do
288 body' <- dsLExpr body
289 dsLocalBinds binds body'
291 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
292 -- because the interpretation of `stmts' depends on what sort of thing it is.
294 dsExpr (HsDo ListComp stmts body result_ty)
295 = -- Special case for list comprehensions
296 dsListComp stmts body elt_ty
298 [elt_ty] = tcTyConAppArgs result_ty
300 dsExpr (HsDo DoExpr stmts body result_ty)
301 = dsDo stmts body result_ty
303 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
304 = dsMDo tbl stmts body result_ty
306 dsExpr (HsDo PArrComp stmts body result_ty)
307 = -- Special case for array comprehensions
308 dsPArrComp (map unLoc stmts) body elt_ty
310 [elt_ty] = tcTyConAppArgs result_ty
312 dsExpr (HsIf guard_expr then_expr else_expr)
313 = mkIfThenElse <$> dsLExpr guard_expr <*> dsLExpr then_expr <*> dsLExpr else_expr
318 \underline{\bf Various data construction things}
319 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
321 dsExpr (ExplicitList elt_ty xs)
322 = dsExplicitList elt_ty xs
324 -- We desugar [:x1, ..., xn:] as
325 -- singletonP x1 +:+ ... +:+ singletonP xn
327 dsExpr (ExplicitPArr ty []) = do
328 emptyP <- dsLookupGlobalId emptyPName
329 return (Var emptyP `App` Type ty)
330 dsExpr (ExplicitPArr ty xs) = do
331 singletonP <- dsLookupGlobalId singletonPName
332 appP <- dsLookupGlobalId appPName
333 xs' <- mapM dsLExpr xs
334 return . foldr1 (binary appP) $ map (unary singletonP) xs'
336 unary fn x = mkApps (Var fn) [Type ty, x]
337 binary fn x y = mkApps (Var fn) [Type ty, x, y]
339 dsExpr (ExplicitTuple expr_list boxity) = do
340 core_exprs <- mapM dsLExpr expr_list
341 return (mkConApp (tupleCon boxity (length expr_list))
342 (map (Type . exprType) core_exprs ++ core_exprs))
344 dsExpr (ArithSeq expr (From from))
345 = App <$> dsExpr expr <*> dsLExpr from
347 dsExpr (ArithSeq expr (FromTo from to))
348 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
350 dsExpr (ArithSeq expr (FromThen from thn))
351 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn]
353 dsExpr (ArithSeq expr (FromThenTo from thn to))
354 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
356 dsExpr (PArrSeq expr (FromTo from to))
357 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, to]
359 dsExpr (PArrSeq expr (FromThenTo from thn to))
360 = mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
363 = panic "DsExpr.dsExpr: Infinite parallel array!"
364 -- the parser shouldn't have generated it and the renamer and typechecker
365 -- shouldn't have let it through
369 \underline{\bf Record construction and update}
370 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
371 For record construction we do this (assuming T has three arguments)
375 let err = /\a -> recConErr a
376 T (recConErr t1 "M.lhs/230/op1")
378 (recConErr t1 "M.lhs/230/op3")
380 @recConErr@ then converts its arugment string into a proper message
381 before printing it as
383 M.lhs, line 230: missing field op1 was evaluated
386 We also handle @C{}@ as valid construction syntax for an unlabelled
387 constructor @C@, setting all of @C@'s fields to bottom.
390 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
391 con_expr' <- dsExpr con_expr
393 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
394 -- A newtype in the corner should be opaque;
395 -- hence TcType.tcSplitFunTys
397 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
398 = case findField (rec_flds rbinds) lbl of
399 (rhs:rhss) -> ASSERT( null rhss )
401 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
402 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty
404 labels = dataConFieldLabels (idDataCon data_con_id)
405 -- The data_con_id is guaranteed to be the wrapper id of the constructor
407 con_args <- if null labels
408 then mapM unlabelled_bottom arg_tys
409 else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)
411 return (mkApps con_expr' con_args)
414 Record update is a little harder. Suppose we have the decl:
416 data T = T1 {op1, op2, op3 :: Int}
417 | T2 {op4, op2 :: Int}
420 Then we translate as follows:
426 T1 op1 _ op3 -> T1 op1 op2 op3
427 T2 op4 _ -> T2 op4 op2
428 other -> recUpdError "M.lhs/230"
430 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
431 RHSs, and do not generate a Core constructor application directly, because the constructor
432 might do some argument-evaluation first; and may have to throw away some
435 Note [Update for GADTs]
436 ~~~~~~~~~~~~~~~~~~~~~~~
439 T1 { f1 :: a } :: T a Int
441 Then the wrapper function for T1 has type
443 But if x::T a b, then
444 x { f1 = v } :: T a b (not T a Int!)
445 So we need to cast (T a Int) to (T a b). Sigh.
448 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
449 cons_to_upd in_inst_tys out_inst_tys)
451 = dsLExpr record_expr
453 = ASSERT2( notNull cons_to_upd, ppr expr )
455 do { record_expr' <- dsLExpr record_expr
456 ; field_binds' <- mapM ds_field fields
457 ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
458 upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
460 -- It's important to generate the match with matchWrapper,
461 -- and the right hand sides with applications of the wrapper Id
462 -- so that everything works when we are doing fancy unboxing on the
463 -- constructor aguments.
464 ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
465 ; ([discrim_var], matching_code)
466 <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
468 ; return (add_field_binds field_binds' $
469 bindNonRec discrim_var record_expr' matching_code) }
471 ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
472 -- Clone the Id in the HsRecField, because its Name is that
473 -- of the record selector, and we must not make that a lcoal binder
474 -- else we shadow other uses of the record selector
475 -- Hence 'lcl_id'. Cf Trac #2735
476 ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
477 ; let fld_id = unLoc (hsRecFieldId rec_field)
478 ; lcl_id <- newSysLocalDs (idType fld_id)
479 ; return (idName fld_id, lcl_id, rhs) }
481 add_field_binds [] expr = expr
482 add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
484 -- Awkwardly, for families, the match goes
485 -- from instance type to family type
486 tycon = dataConTyCon (head cons_to_upd)
487 in_ty = mkTyConApp tycon in_inst_tys
488 in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
490 mk_alt upd_fld_env con
491 = do { let (univ_tvs, ex_tvs, eq_spec,
492 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
493 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
495 -- I'm not bothering to clone the ex_tvs
496 ; eqs_vars <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
497 ; theta_vars <- mapM newPredVarDs (substTheta subst (eq_theta ++ dict_theta))
498 ; arg_ids <- newSysLocalsDs (substTys subst arg_tys)
499 ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
500 (dataConFieldLabels con) arg_ids
501 mk_val_arg field_name pat_arg_id
502 = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
503 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
504 -- Reconstruct with the WrapId so that unpacking happens
505 wrap = mkWpApps theta_vars `WpCompose`
506 mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
507 mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
508 , isNothing (lookupTyVar wrap_subst tv) ]
509 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
511 -- Tediously wrap the application in a cast
512 -- Note [Update for GADTs]
513 wrapped_rhs | null eq_spec = rhs
514 | otherwise = mkLHsWrap (WpCast wrap_co) rhs
515 wrap_co = mkTyConApp tycon [ lookup tv ty
516 | (tv,ty) <- univ_tvs `zip` out_inst_tys]
517 lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
520 wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
521 | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
523 pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
524 , pat_dicts = eqs_vars ++ theta_vars
525 , pat_binds = emptyLHsBinds
526 , pat_args = PrefixCon $ map nlVarPat arg_ids
528 ; return (mkSimpleMatch [pat] wrapped_rhs) }
532 Here is where we desugar the Template Haskell brackets and escapes
535 -- Template Haskell stuff
537 #ifdef GHCI /* Only if bootstrapping */
538 dsExpr (HsBracketOut x ps) = dsBracket x ps
539 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
542 -- Arrow notation extension
543 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
549 dsExpr (HsTick ix vars e) = do
553 -- There is a problem here. The then and else branches
554 -- have no free variables, so they are open to lifting.
555 -- We need someway of stopping this.
556 -- This will make no difference to binary coverage
557 -- (did you go here: YES or NO), but will effect accurate
560 dsExpr (HsBinTick ixT ixF e) = do
562 do { ASSERT(exprType e2 `coreEqType` boolTy)
563 mkBinaryTickBox ixT ixF e2
569 -- HsSyn constructs that just shouldn't be here:
570 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
573 findField :: [HsRecField Id arg] -> Name -> [arg]
575 = [rhs | HsRecField { hsRecFieldId = id, hsRecFieldArg = rhs } <- rbinds
576 , lbl == idName (unLoc id) ]
579 %--------------------------------------------------------------------
581 Note [Desugaring explicit lists]
582 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
583 Explicit lists are desugared in a cleverer way to prevent some
584 fruitless allocations. Essentially, whenever we see a list literal
587 1. Find the tail of the list that can be allocated statically (say
588 [x_k, ..., x_n]) by later stages and ensure we desugar that
589 normally: this makes sure that we don't cause a code size increase
590 by having the cons in that expression fused (see later) and hence
591 being unable to statically allocate any more
593 2. For the prefix of the list which cannot be allocated statically,
594 say [x_1, ..., x_(k-1)], we turn it into an expression involving
595 build so that if we find any foldrs over it it will fuse away
598 So in this example we will desugar to:
599 build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
601 If fusion fails to occur then build will get inlined and (since we
602 defined a RULE for foldr (:) []) we will get back exactly the
603 normal desugaring for an explicit list.
605 This optimisation can be worth a lot: up to 25% of the total
606 allocation in some nofib programs. Specifically
608 Program Size Allocs Runtime CompTime
609 rewrite +0.0% -26.3% 0.02 -1.8%
610 ansi -0.3% -13.8% 0.00 +0.0%
611 lift +0.0% -8.7% 0.00 -2.3%
613 Of course, if rules aren't turned on then there is pretty much no
614 point doing this fancy stuff, and it may even be harmful.
616 =======> Note by SLPJ Dec 08.
618 I'm unconvinced that we should *ever* generate a build for an explicit
619 list. See the comments in GHC.Base about the foldr/cons rule, which
620 points out that (foldr k z [a,b,c]) may generate *much* less code than
621 (a `k` b `k` c `k` z).
623 Furthermore generating builds messes up the LHS of RULES.
624 Example: the foldr/single rule in GHC.Base
626 We do not want to generate a build invocation on the LHS of this RULE!
628 To test this I've added a (static) flag -fsimple-list-literals, which
629 makes all list literals be generated via the simple route.
634 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
635 -- See Note [Desugaring explicit lists]
636 dsExplicitList elt_ty xs = do
638 xs' <- mapM dsLExpr xs
639 if opt_SimpleListLiterals || not (dopt Opt_EnableRewriteRules dflags)
640 then return $ mkListExpr elt_ty xs'
641 else mkBuildExpr elt_ty (mkSplitExplicitList (thisPackage dflags) xs')
643 mkSplitExplicitList this_package xs' (c, _) (n, n_ty) = do
644 let (dynamic_prefix, static_suffix) = spanTail (rhsIsStatic this_package) xs'
645 static_suffix' = mkListExpr elt_ty static_suffix
647 folded_static_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) static_suffix'
648 let build_body = foldr (App . App (Var c)) folded_static_suffix dynamic_prefix
651 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
652 spanTail f xs = (reverse rejected, reverse satisfying)
653 where (satisfying, rejected) = span f $ reverse xs
656 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
657 handled in DsListComp). Basically does the translation given in the
663 -> Type -- Type of the whole expression
666 dsDo stmts body _result_ty
669 goL [] = dsLExpr body
670 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go stmt lstmts)
672 go (ExprStmt rhs then_expr _) stmts
673 = do { rhs2 <- dsLExpr rhs
674 ; case tcSplitAppTy_maybe (exprType rhs2) of
675 Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
677 ; then_expr2 <- dsExpr then_expr
679 ; return (mkApps then_expr2 [rhs2, rest]) }
681 go (LetStmt binds) stmts
682 = do { rest <- goL stmts
683 ; dsLocalBinds binds rest }
685 go (BindStmt pat rhs bind_op fail_op) stmts
687 do { body <- goL stmts
688 ; rhs' <- dsLExpr rhs
689 ; bind_op' <- dsExpr bind_op
690 ; var <- selectSimpleMatchVarL pat
691 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
692 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
693 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
694 res1_ty (cantFailMatchResult body)
695 ; match_code <- handle_failure pat match fail_op
696 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
698 -- In a do expression, pattern-match failure just calls
699 -- the monadic 'fail' rather than throwing an exception
700 handle_failure pat match fail_op
702 = do { fail_op' <- dsExpr fail_op
703 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
704 ; extractMatchResult match (App fail_op' fail_msg) }
706 = extractMatchResult match (error "It can't fail")
708 mk_fail_msg :: Located e -> String
709 mk_fail_msg pat = "Pattern match failure in do expression at " ++
710 showSDoc (ppr (getLoc pat))
713 Translation for RecStmt's:
714 -----------------------------
715 We turn (RecStmt [v1,..vn] stmts) into:
717 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
724 -> Type -- Type of the whole expression
727 dsMDo tbl stmts body result_ty
730 goL [] = dsLExpr body
731 goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
733 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
734 mfix_id = lookupEvidence tbl mfixName
735 return_id = lookupEvidence tbl returnMName
736 bind_id = lookupEvidence tbl bindMName
737 then_id = lookupEvidence tbl thenMName
738 fail_id = lookupEvidence tbl failMName
741 go _ (LetStmt binds) stmts
742 = do { rest <- goL stmts
743 ; dsLocalBinds binds rest }
745 go _ (ExprStmt rhs _ rhs_ty) stmts
746 = do { rhs2 <- dsLExpr rhs
747 ; warnDiscardedDoBindings rhs m_ty rhs_ty
749 ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
751 go _ (BindStmt pat rhs _ _) stmts
752 = do { body <- goL stmts
753 ; var <- selectSimpleMatchVarL pat
754 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
755 result_ty (cantFailMatchResult body)
756 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
757 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
758 ; match_code <- extractMatchResult match fail_expr
760 ; rhs' <- dsLExpr rhs
761 ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
762 rhs', Lam var match_code]) }
764 go loc (RecStmt rec_stmts later_ids rec_ids rec_rets binds) stmts
765 = ASSERT( length rec_ids > 0 )
766 ASSERT( length rec_ids == length rec_rets )
767 goL (new_bind_stmt : let_stmt : stmts)
769 new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
770 let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
773 -- Remove the later_ids that appear (without fancy coercions)
774 -- in rec_rets, because there's no need to knot-tie them separately
775 -- See Note [RecStmt] in HsExpr
776 later_ids' = filter (`notElem` mono_rec_ids) later_ids
777 mono_rec_ids = [ id | HsVar id <- rec_rets ]
779 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
780 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
781 (mkFunTy tup_ty body_ty))
783 -- The rec_tup_pat must bind the rec_ids only; remember that the
784 -- trimmed_laters may share the same Names
785 -- Meanwhile, the later_pats must bind the later_vars
786 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
787 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
788 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
790 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
791 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
792 body_ty = mkAppTy m_ty tup_ty
793 tup_ty = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
794 -- mkCoreTupTy deals with singleton case
796 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
799 mk_wild_pat :: Id -> LPat Id
800 mk_wild_pat v = noLoc $ WildPat $ idType v
802 mk_later_pat :: Id -> LPat Id
803 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
804 | otherwise = nlVarPat v
806 mk_tup_pat :: [LPat Id] -> LPat Id
808 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
810 mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
812 mk_ret_tup rs = noLoc $ ExplicitTuple rs Boxed
816 %************************************************************************
818 \subsection{Errors and contexts}
820 %************************************************************************
823 -- Warn about certain types of values discarded in monadic bindings (#3263)
824 warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
825 warnDiscardedDoBindings rhs container_ty returning_ty = do {
826 -- Warn about discarding non-() things in 'monadic' binding
827 ; warn_unused <- doptDs Opt_WarnUnusedDoBind
828 ; if warn_unused && not (returning_ty `tcEqType` unitTy)
829 then warnDs (unusedMonadBind rhs returning_ty)
831 -- Warn about discarding m a things in 'monadic' binding of the same type,
832 -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
833 ; warn_wrong <- doptDs Opt_WarnWrongDoBind
834 ; case tcSplitAppTy_maybe returning_ty of
835 Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
836 warnDs (wrongMonadBind rhs returning_ty)
839 unusedMonadBind :: LHsExpr Id -> Type -> SDoc
840 unusedMonadBind rhs returning_ty
841 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
842 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
843 ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
845 wrongMonadBind :: LHsExpr Id -> Type -> SDoc
846 wrongMonadBind rhs returning_ty
847 = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
848 ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
849 ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")