2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[DsExpr]{Matching expressions (Exprs)}
7 module DsExpr ( dsExpr, dsLet, dsLit ) where
9 #include "HsVersions.h"
12 import Match ( matchWrapper, matchSimply )
13 import MatchLit ( dsLit )
14 import DsBinds ( dsMonoBinds, AutoScc(..) )
15 import DsGRHSs ( dsGuarded )
16 import DsCCall ( dsCCall )
17 import DsListComp ( dsListComp, dsPArrComp )
18 import DsUtils ( mkErrorAppDs, mkStringLit, mkConsExpr, mkNilExpr)
22 -- Template Haskell stuff iff bootstrapped
23 import DsMeta ( dsBracket )
26 import HsSyn ( failureFreePat,
27 HsExpr(..), Pat(..), HsLit(..), ArithSeqInfo(..),
28 Stmt(..), HsMatchContext(..), HsStmtContext(..),
29 Match(..), HsBinds(..), MonoBinds(..), HsConDetails(..),
30 mkSimpleMatch, isDoExpr
32 import TcHsSyn ( TypecheckedHsExpr, TypecheckedHsBinds, TypecheckedStmt, hsPatType )
34 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
35 -- needs to see source types (newtypes etc), and sometimes not
36 -- So WATCH OUT; check each use of split*Ty functions.
37 -- Sigh. This is a pain.
39 import TcType ( tcSplitAppTy, tcSplitFunTys, tcTyConAppArgs,
40 tcSplitTyConApp, isUnLiftedType, Type,
42 import Type ( splitFunTys )
44 import CoreUtils ( exprType, mkIfThenElse, bindNonRec )
46 import FieldLabel ( FieldLabel, fieldLabelTyCon )
47 import CostCentre ( mkUserCC )
48 import Id ( Id, idType, idName, recordSelectorFieldLabel )
49 import PrelInfo ( rEC_CON_ERROR_ID, iRREFUT_PAT_ERROR_ID )
50 import DataCon ( DataCon, dataConWrapId, dataConFieldLabels, dataConInstOrigArgTys )
51 import DataCon ( isExistentialDataCon )
53 import TyCon ( tyConDataCons )
54 import TysWiredIn ( tupleCon, mkTupleTy )
55 import BasicTypes ( RecFlag(..), Boxity(..), ipNameName )
56 import PrelNames ( toPName )
57 import SrcLoc ( noSrcLoc )
58 import Util ( zipEqual, zipWithEqual )
64 %************************************************************************
68 %************************************************************************
70 @dsLet@ is a match-result transformer, taking the @MatchResult@ for the body
71 and transforming it into one for the let-bindings enclosing the body.
73 This may seem a bit odd, but (source) let bindings can contain unboxed
78 This must be transformed to a case expression and, if the type has
79 more than one constructor, may fail.
82 dsLet :: TypecheckedHsBinds -> CoreExpr -> DsM CoreExpr
87 dsLet (ThenBinds b1 b2) body
88 = dsLet b2 body `thenDs` \ body' ->
91 -- Special case for bindings which bind unlifted variables
92 -- We need to do a case right away, rather than building
93 -- a tuple and doing selections.
94 -- Silently ignore INLINE pragmas...
95 dsLet bind@(MonoBind (AbsBinds [] [] exports inlines binds) sigs is_rec) body
96 | or [isUnLiftedType (idType g) | (_, g, l) <- exports]
97 = ASSERT (case is_rec of {NonRecursive -> True; other -> False})
98 -- Unlifted bindings are always non-recursive
99 -- and are always a Fun or Pat monobind
101 -- ToDo: in some bizarre case it's conceivable that there
102 -- could be dict binds in the 'binds'. (See the notes
103 -- below. Then pattern-match would fail. Urk.)
105 FunMonoBind fun _ matches loc
107 matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
108 ASSERT( null args ) -- Functions aren't lifted
109 returnDs (bindNonRec fun rhs body_w_exports)
111 PatMonoBind pat grhss loc
113 dsGuarded grhss `thenDs` \ rhs ->
114 mk_error_app pat `thenDs` \ error_expr ->
115 matchSimply rhs PatBindRhs pat body_w_exports error_expr
117 other -> pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
119 body_w_exports = foldr bind_export body exports
120 bind_export (tvs, g, l) body = ASSERT( null tvs )
121 bindNonRec g (Var l) body
123 mk_error_app pat = mkErrorAppDs iRREFUT_PAT_ERROR_ID
127 -- Ordinary case for bindings
128 dsLet (MonoBind binds sigs is_rec) body
129 = dsMonoBinds NoSccs binds [] `thenDs` \ prs ->
130 returnDs (Let (Rec prs) body)
131 -- Use a Rec regardless of is_rec.
132 -- Why? Because it allows the MonoBinds to be all
133 -- mixed up, which is what happens in one rare case
134 -- Namely, for an AbsBind with no tyvars and no dicts,
135 -- but which does have dictionary bindings.
136 -- See notes with TcSimplify.inferLoop [NO TYVARS]
137 -- It turned out that wrapping a Rec here was the easiest solution
139 -- NB The previous case dealt with unlifted bindings, so we
140 -- only have to deal with lifted ones now; so Rec is ok
143 %************************************************************************
145 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
147 %************************************************************************
150 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
152 dsExpr (HsPar x) = dsExpr x
153 dsExpr (HsVar var) = returnDs (Var var)
154 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
155 dsExpr (HsLit lit) = dsLit lit
156 -- HsOverLit has been gotten rid of by the type checker
158 dsExpr expr@(HsLam a_Match)
159 = matchWrapper LambdaExpr [a_Match] `thenDs` \ (binders, matching_code) ->
160 returnDs (mkLams binders matching_code)
162 dsExpr expr@(HsApp fun arg)
163 = dsExpr fun `thenDs` \ core_fun ->
164 dsExpr arg `thenDs` \ core_arg ->
165 returnDs (core_fun `App` core_arg)
168 Operator sections. At first it looks as if we can convert
177 But no! expr might be a redex, and we can lose laziness badly this
182 for example. So we convert instead to
184 let y = expr in \x -> op y x
186 If \tr{expr} is actually just a variable, say, then the simplifier
190 dsExpr (OpApp e1 op _ e2)
191 = dsExpr op `thenDs` \ core_op ->
192 -- for the type of y, we need the type of op's 2nd argument
193 dsExpr e1 `thenDs` \ x_core ->
194 dsExpr e2 `thenDs` \ y_core ->
195 returnDs (mkApps core_op [x_core, y_core])
197 dsExpr (SectionL expr op)
198 = dsExpr op `thenDs` \ core_op ->
199 -- for the type of y, we need the type of op's 2nd argument
201 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
202 -- Must look through an implicit-parameter type;
203 -- newtype impossible; hence Type.splitFunTys
205 dsExpr expr `thenDs` \ x_core ->
206 newSysLocalDs x_ty `thenDs` \ x_id ->
207 newSysLocalDs y_ty `thenDs` \ y_id ->
209 returnDs (bindNonRec x_id x_core $
210 Lam y_id (mkApps core_op [Var x_id, Var y_id]))
212 -- dsExpr (SectionR op expr) -- \ x -> op x expr
213 dsExpr (SectionR op expr)
214 = dsExpr op `thenDs` \ core_op ->
215 -- for the type of x, we need the type of op's 2nd argument
217 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
218 -- See comment with SectionL
220 dsExpr expr `thenDs` \ y_core ->
221 newSysLocalDs x_ty `thenDs` \ x_id ->
222 newSysLocalDs y_ty `thenDs` \ y_id ->
224 returnDs (bindNonRec y_id y_core $
225 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
227 dsExpr (HsCCall lbl args may_gc is_asm result_ty)
228 = mapDs dsExpr args `thenDs` \ core_args ->
229 dsCCall lbl core_args may_gc is_asm result_ty
230 -- dsCCall does all the unboxification, etc.
232 dsExpr (HsSCC cc expr)
233 = dsExpr expr `thenDs` \ core_expr ->
234 getModuleDs `thenDs` \ mod_name ->
235 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
237 -- special case to handle unboxed tuple patterns.
239 dsExpr (HsCase discrim matches src_loc)
240 | all ubx_tuple_match matches
241 = putSrcLocDs src_loc $
242 dsExpr discrim `thenDs` \ core_discrim ->
243 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
244 case matching_code of
245 Case (Var x) bndr alts | x == discrim_var ->
246 returnDs (Case core_discrim bndr alts)
247 _ -> panic ("dsExpr: tuple pattern:\n" ++ showSDoc (ppr matching_code))
249 ubx_tuple_match (Match [TuplePat ps Unboxed] _ _) = True
250 ubx_tuple_match _ = False
252 dsExpr (HsCase discrim matches src_loc)
253 = putSrcLocDs src_loc $
254 dsExpr discrim `thenDs` \ core_discrim ->
255 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
256 returnDs (bindNonRec discrim_var core_discrim matching_code)
258 dsExpr (HsLet binds body)
259 = dsExpr body `thenDs` \ body' ->
262 dsExpr (HsWith expr binds is_with)
263 = dsExpr expr `thenDs` \ expr' ->
264 foldlDs dsIPBind expr' binds
267 = dsExpr e `thenDs` \ e' ->
268 returnDs (Let (NonRec (ipNameName n) e') body)
270 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
271 -- because the interpretation of `stmts' depends on what sort of thing it is.
273 dsExpr (HsDo ListComp stmts _ result_ty src_loc)
274 = -- Special case for list comprehensions
275 putSrcLocDs src_loc $
276 dsListComp stmts elt_ty
278 (_, [elt_ty]) = tcSplitTyConApp result_ty
280 dsExpr (HsDo do_or_lc stmts ids result_ty src_loc)
282 = putSrcLocDs src_loc $
283 dsDo do_or_lc stmts ids result_ty
285 dsExpr (HsDo PArrComp stmts _ result_ty src_loc)
286 = -- Special case for array comprehensions
287 putSrcLocDs src_loc $
288 dsPArrComp stmts elt_ty
290 (_, [elt_ty]) = tcSplitTyConApp result_ty
292 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
293 = putSrcLocDs src_loc $
294 dsExpr guard_expr `thenDs` \ core_guard ->
295 dsExpr then_expr `thenDs` \ core_then ->
296 dsExpr else_expr `thenDs` \ core_else ->
297 returnDs (mkIfThenElse core_guard core_then core_else)
302 \underline{\bf Type lambda and application}
303 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
305 dsExpr (TyLam tyvars expr)
306 = dsExpr expr `thenDs` \ core_expr ->
307 returnDs (mkLams tyvars core_expr)
309 dsExpr (TyApp expr tys)
310 = dsExpr expr `thenDs` \ core_expr ->
311 returnDs (mkTyApps core_expr tys)
316 \underline{\bf Various data construction things}
317 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
319 dsExpr (ExplicitList ty xs)
322 go [] = returnDs (mkNilExpr ty)
323 go (x:xs) = dsExpr x `thenDs` \ core_x ->
324 go xs `thenDs` \ core_xs ->
325 returnDs (mkConsExpr ty core_x core_xs)
327 -- we create a list from the array elements and convert them into a list using
330 -- * the main disadvantage to this scheme is that `toP' traverses the list
331 -- twice: once to determine the length and a second time to put to elements
332 -- into the array; this inefficiency could be avoided by exposing some of
333 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
334 -- that we can exploit the fact that we already know the length of the array
335 -- here at compile time
337 dsExpr (ExplicitPArr ty xs)
338 = dsLookupGlobalId toPName `thenDs` \toP ->
339 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
340 returnDs (mkApps (Var toP) [Type ty, coreList])
342 dsExpr (ExplicitTuple expr_list boxity)
343 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
344 returnDs (mkConApp (tupleCon boxity (length expr_list))
345 (map (Type . exprType) core_exprs ++ core_exprs))
347 dsExpr (ArithSeqOut expr (From from))
348 = dsExpr expr `thenDs` \ expr2 ->
349 dsExpr from `thenDs` \ from2 ->
350 returnDs (App expr2 from2)
352 dsExpr (ArithSeqOut expr (FromTo from two))
353 = dsExpr expr `thenDs` \ expr2 ->
354 dsExpr from `thenDs` \ from2 ->
355 dsExpr two `thenDs` \ two2 ->
356 returnDs (mkApps expr2 [from2, two2])
358 dsExpr (ArithSeqOut expr (FromThen from thn))
359 = dsExpr expr `thenDs` \ expr2 ->
360 dsExpr from `thenDs` \ from2 ->
361 dsExpr thn `thenDs` \ thn2 ->
362 returnDs (mkApps expr2 [from2, thn2])
364 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
365 = dsExpr expr `thenDs` \ expr2 ->
366 dsExpr from `thenDs` \ from2 ->
367 dsExpr thn `thenDs` \ thn2 ->
368 dsExpr two `thenDs` \ two2 ->
369 returnDs (mkApps expr2 [from2, thn2, two2])
371 dsExpr (PArrSeqOut expr (FromTo from two))
372 = dsExpr expr `thenDs` \ expr2 ->
373 dsExpr from `thenDs` \ from2 ->
374 dsExpr two `thenDs` \ two2 ->
375 returnDs (mkApps expr2 [from2, two2])
377 dsExpr (PArrSeqOut expr (FromThenTo from thn two))
378 = dsExpr expr `thenDs` \ expr2 ->
379 dsExpr from `thenDs` \ from2 ->
380 dsExpr thn `thenDs` \ thn2 ->
381 dsExpr two `thenDs` \ two2 ->
382 returnDs (mkApps expr2 [from2, thn2, two2])
384 dsExpr (PArrSeqOut expr _)
385 = panic "DsExpr.dsExpr: Infinite parallel array!"
386 -- the parser shouldn't have generated it and the renamer and typechecker
387 -- shouldn't have let it through
391 \underline{\bf Record construction and update}
392 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
393 For record construction we do this (assuming T has three arguments)
397 let err = /\a -> recConErr a
398 T (recConErr t1 "M.lhs/230/op1")
400 (recConErr t1 "M.lhs/230/op3")
402 @recConErr@ then converts its arugment string into a proper message
403 before printing it as
405 M.lhs, line 230: missing field op1 was evaluated
408 We also handle @C{}@ as valid construction syntax for an unlabelled
409 constructor @C@, setting all of @C@'s fields to bottom.
412 dsExpr (RecordConOut data_con con_expr rbinds)
413 = dsExpr con_expr `thenDs` \ con_expr' ->
415 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
416 -- A newtype in the corner should be opaque;
417 -- hence TcType.tcSplitFunTys
420 = case [rhs | (sel_id,rhs) <- rbinds,
421 lbl == recordSelectorFieldLabel sel_id] of
422 (rhs:rhss) -> ASSERT( null rhss )
424 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
425 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
427 labels = dataConFieldLabels data_con
431 then mapDs unlabelled_bottom arg_tys
432 else mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
433 `thenDs` \ con_args ->
435 returnDs (mkApps con_expr' con_args)
438 Record update is a little harder. Suppose we have the decl:
440 data T = T1 {op1, op2, op3 :: Int}
441 | T2 {op4, op2 :: Int}
444 Then we translate as follows:
450 T1 op1 _ op3 -> T1 op1 op2 op3
451 T2 op4 _ -> T2 op4 op2
452 other -> recUpdError "M.lhs/230"
454 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
455 RHSs, and do not generate a Core constructor application directly, because the constructor
456 might do some argument-evaluation first; and may have to throw away some
460 dsExpr (RecordUpdOut record_expr record_in_ty record_out_ty [])
463 dsExpr expr@(RecordUpdOut record_expr record_in_ty record_out_ty rbinds)
464 = getSrcLocDs `thenDs` \ src_loc ->
465 dsExpr record_expr `thenDs` \ record_expr' ->
467 -- Desugar the rbinds, and generate let-bindings if
468 -- necessary so that we don't lose sharing
471 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
472 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
474 mk_val_arg field old_arg_id
475 = case [rhs | (sel_id, rhs) <- rbinds,
476 field == recordSelectorFieldLabel sel_id] of
477 (rhs:rest) -> ASSERT(null rest) rhs
478 [] -> HsVar old_arg_id
481 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
482 -- This call to dataConArgTys won't work for existentials
484 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
485 (dataConFieldLabels con) arg_ids
486 rhs = foldl HsApp (TyApp (HsVar (dataConWrapId con)) out_inst_tys)
489 returnDs (mkSimpleMatch [ConPatOut con (PrefixCon (map VarPat arg_ids)) record_in_ty [] []]
494 -- Record stuff doesn't work for existentials
495 -- The type checker checks for this, but we need
496 -- worry only about the constructors that are to be updated
497 ASSERT2( all (not . isExistentialDataCon) cons_to_upd, ppr expr )
499 -- It's important to generate the match with matchWrapper,
500 -- and the right hand sides with applications of the wrapper Id
501 -- so that everything works when we are doing fancy unboxing on the
502 -- constructor aguments.
503 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
504 matchWrapper RecUpd alts `thenDs` \ ([discrim_var], matching_code) ->
506 returnDs (bindNonRec discrim_var record_expr' matching_code)
509 updated_fields :: [FieldLabel]
510 updated_fields = [recordSelectorFieldLabel sel_id | (sel_id,_) <- rbinds]
512 -- Get the type constructor from the first field label,
513 -- so that we are sure it'll have all its DataCons
514 -- (In GHCI, it's possible that some TyCons may not have all
515 -- their constructors, in a module-loop situation.)
516 tycon = fieldLabelTyCon (head updated_fields)
517 data_cons = tyConDataCons tycon
518 cons_to_upd = filter has_all_fields data_cons
520 has_all_fields :: DataCon -> Bool
521 has_all_fields con_id
522 = all (`elem` con_fields) updated_fields
524 con_fields = dataConFieldLabels con_id
529 \underline{\bf Dictionary lambda and application}
530 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
531 @DictLam@ and @DictApp@ turn into the regular old things.
532 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
533 complicated; reminiscent of fully-applied constructors.
535 dsExpr (DictLam dictvars expr)
536 = dsExpr expr `thenDs` \ core_expr ->
537 returnDs (mkLams dictvars core_expr)
541 dsExpr (DictApp expr dicts) -- becomes a curried application
542 = dsExpr expr `thenDs` \ core_expr ->
543 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
546 Here is where we desugar the Template Haskell brackets and escapes
549 -- Template Haskell stuff
551 #ifdef GHCI /* Only if bootstrapping */
552 dsExpr (HsBracketOut x ps) = dsBracket x ps
553 dsExpr (HsSplice n e) = pprPanic "dsExpr:splice" (ppr e)
562 -- HsSyn constructs that just shouldn't be here:
563 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
564 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
565 dsExpr (PArrSeqIn _) = panic "dsExpr:PArrSeqIn"
570 %--------------------------------------------------------------------
572 Basically does the translation given in the Haskell~1.3 report:
575 dsDo :: HsStmtContext Name
577 -> [Id] -- id for: [return,fail,>>=,>>] and possibly mfixName
578 -> Type -- Element type; the whole expression has type (m t)
581 dsDo do_or_lc stmts ids result_ty
583 (return_id : fail_id : bind_id : then_id : _) = ids
584 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
585 is_do = isDoExpr do_or_lc -- True for both MDo and Do
587 -- For ExprStmt, see the comments near HsExpr.Stmt about
588 -- exactly what ExprStmts mean!
590 -- In dsDo we can only see DoStmt and ListComp (no guards)
592 go [ResultStmt expr locn]
593 | is_do = do_expr expr locn
594 | otherwise = do_expr expr locn `thenDs` \ expr2 ->
595 returnDs (mkApps (Var return_id) [Type b_ty, expr2])
597 go (ExprStmt expr a_ty locn : stmts)
598 | is_do -- Do expression
599 = do_expr expr locn `thenDs` \ expr2 ->
600 go stmts `thenDs` \ rest ->
601 returnDs (mkApps (Var then_id) [Type a_ty, Type b_ty, expr2, rest])
603 | otherwise -- List comprehension
604 = do_expr expr locn `thenDs` \ expr2 ->
605 go stmts `thenDs` \ rest ->
607 msg = "Pattern match failure in do expression, " ++ showSDoc (ppr locn)
609 mkStringLit msg `thenDs` \ core_msg ->
610 returnDs (mkIfThenElse expr2 rest
611 (App (App (Var fail_id) (Type b_ty)) core_msg))
613 go (LetStmt binds : stmts )
614 = go stmts `thenDs` \ rest ->
617 go (BindStmt pat expr locn : stmts)
619 dsExpr expr `thenDs` \ expr2 ->
622 fail_expr = HsApp (TyApp (HsVar fail_id) [b_ty])
623 (HsLit (HsString (mkFastString msg)))
624 msg = "Pattern match failure in do expression, " ++ showSDoc (ppr locn)
625 main_match = mkSimpleMatch [pat]
626 (HsDo do_or_lc stmts ids result_ty locn)
629 | failureFreePat pat = [main_match]
632 , mkSimpleMatch [WildPat a_ty] fail_expr result_ty locn
635 matchWrapper (StmtCtxt do_or_lc) the_matches `thenDs` \ (binders, matching_code) ->
636 returnDs (mkApps (Var bind_id) [Type a_ty, Type b_ty, expr2,
637 mkLams binders matching_code])
639 go (RecStmt rec_vars rec_stmts : stmts)
640 = go (bind_stmt : stmts)
642 bind_stmt = dsRecStmt m_ty ids rec_vars rec_stmts
648 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
651 Translation for RecStmt's:
652 -----------------------------
653 We turn (RecStmt [v1,..vn] stmts) into:
655 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
659 dsRecStmt :: Type -- Monad type constructor :: * -> *
660 -> [Id] -- Ids for: [return,fail,>>=,>>,mfix]
661 -> [Id] -> [TypecheckedStmt] -- Guts of the RecStmt
663 dsRecStmt m_ty ids@[return_id, _, _, _, mfix_id] vars stmts
664 = BindStmt tup_pat mfix_app noSrcLoc
666 (var1:rest) = vars -- Always at least one
669 mfix_app = HsApp (TyApp (HsVar mfix_id) [tup_ty]) mfix_arg
670 mfix_arg = HsLam (mkSimpleMatch [tup_pat] body tup_ty noSrcLoc)
672 tup_expr | one_var = HsVar var1
673 | otherwise = ExplicitTuple (map HsVar vars) Boxed
674 tup_ty | one_var = idType var1
675 | otherwise = mkTupleTy Boxed (length vars) (map idType vars)
676 tup_pat | one_var = VarPat var1
677 | otherwise = LazyPat (TuplePat (map VarPat vars) Boxed)
679 body = HsDo DoExpr (stmts ++ [return_stmt])
680 ids -- Don't need the mfix, but it does no harm
681 (mkAppTy m_ty tup_ty)
684 return_stmt = ResultStmt return_app noSrcLoc
685 return_app = HsApp (TyApp (HsVar return_id) [tup_ty]) tup_expr