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, selectMatchVar )
22 -- Template Haskell stuff iff bootstrapped
23 import DsMeta ( dsBracket, dsReify )
26 import HsSyn ( HsExpr(..), Pat(..), HsLit(..), ArithSeqInfo(..),
27 Stmt(..), HsMatchContext(..), HsStmtContext(..),
28 Match(..), HsBinds(..), MonoBinds(..), HsConDetails(..),
29 mkSimpleMatch, isDoExpr
31 import TcHsSyn ( TypecheckedHsExpr, TypecheckedHsBinds, TypecheckedStmt, hsPatType )
33 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
34 -- needs to see source types (newtypes etc), and sometimes not
35 -- So WATCH OUT; check each use of split*Ty functions.
36 -- Sigh. This is a pain.
38 import TcType ( tcSplitAppTy, tcSplitFunTys, tcTyConAppArgs,
39 tcSplitTyConApp, isUnLiftedType, Type,
41 import Type ( splitFunTys )
43 import Literal ( Literal(..) )
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 dsLet (IPBinds binds is_with) body
92 = foldlDs dsIPBind body binds
95 = dsExpr e `thenDs` \ e' ->
96 returnDs (Let (NonRec (ipNameName n) e') body)
98 -- Special case for bindings which bind unlifted variables
99 -- We need to do a case right away, rather than building
100 -- a tuple and doing selections.
101 -- Silently ignore INLINE pragmas...
102 dsLet bind@(MonoBind (AbsBinds [] [] exports inlines binds) sigs is_rec) body
103 | or [isUnLiftedType (idType g) | (_, g, l) <- exports]
104 = ASSERT (case is_rec of {NonRecursive -> True; other -> False})
105 -- Unlifted bindings are always non-recursive
106 -- and are always a Fun or Pat monobind
108 -- ToDo: in some bizarre case it's conceivable that there
109 -- could be dict binds in the 'binds'. (See the notes
110 -- below. Then pattern-match would fail. Urk.)
112 FunMonoBind fun _ matches loc
114 matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
115 ASSERT( null args ) -- Functions aren't lifted
116 returnDs (bindNonRec fun rhs body_w_exports)
118 PatMonoBind pat grhss loc
120 dsGuarded grhss `thenDs` \ rhs ->
121 mk_error_app pat `thenDs` \ error_expr ->
122 matchSimply rhs PatBindRhs pat body_w_exports error_expr
124 other -> pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
126 body_w_exports = foldr bind_export body exports
127 bind_export (tvs, g, l) body = ASSERT( null tvs )
128 bindNonRec g (Var l) body
130 mk_error_app pat = mkErrorAppDs iRREFUT_PAT_ERROR_ID
134 -- Ordinary case for bindings
135 dsLet (MonoBind binds sigs is_rec) body
136 = dsMonoBinds NoSccs binds [] `thenDs` \ prs ->
137 returnDs (Let (Rec prs) body)
138 -- Use a Rec regardless of is_rec.
139 -- Why? Because it allows the MonoBinds to be all
140 -- mixed up, which is what happens in one rare case
141 -- Namely, for an AbsBind with no tyvars and no dicts,
142 -- but which does have dictionary bindings.
143 -- See notes with TcSimplify.inferLoop [NO TYVARS]
144 -- It turned out that wrapping a Rec here was the easiest solution
146 -- NB The previous case dealt with unlifted bindings, so we
147 -- only have to deal with lifted ones now; so Rec is ok
150 %************************************************************************
152 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
154 %************************************************************************
157 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
159 dsExpr (HsPar x) = dsExpr x
160 dsExpr (HsVar var) = returnDs (Var var)
161 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
162 dsExpr (HsLit lit) = dsLit lit
163 -- HsOverLit has been gotten rid of by the type checker
165 dsExpr expr@(HsLam a_Match)
166 = matchWrapper LambdaExpr [a_Match] `thenDs` \ (binders, matching_code) ->
167 returnDs (mkLams binders matching_code)
169 dsExpr expr@(HsApp fun arg)
170 = dsExpr fun `thenDs` \ core_fun ->
171 dsExpr arg `thenDs` \ core_arg ->
172 returnDs (core_fun `App` core_arg)
175 Operator sections. At first it looks as if we can convert
184 But no! expr might be a redex, and we can lose laziness badly this
189 for example. So we convert instead to
191 let y = expr in \x -> op y x
193 If \tr{expr} is actually just a variable, say, then the simplifier
197 dsExpr (OpApp e1 op _ e2)
198 = dsExpr op `thenDs` \ core_op ->
199 -- for the type of y, we need the type of op's 2nd argument
200 dsExpr e1 `thenDs` \ x_core ->
201 dsExpr e2 `thenDs` \ y_core ->
202 returnDs (mkApps core_op [x_core, y_core])
204 dsExpr (SectionL expr op)
205 = dsExpr op `thenDs` \ core_op ->
206 -- for the type of y, we need the type of op's 2nd argument
208 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
209 -- Must look through an implicit-parameter type;
210 -- newtype impossible; hence Type.splitFunTys
212 dsExpr expr `thenDs` \ x_core ->
213 newSysLocalDs x_ty `thenDs` \ x_id ->
214 newSysLocalDs y_ty `thenDs` \ y_id ->
216 returnDs (bindNonRec x_id x_core $
217 Lam y_id (mkApps core_op [Var x_id, Var y_id]))
219 -- dsExpr (SectionR op expr) -- \ x -> op x expr
220 dsExpr (SectionR op expr)
221 = dsExpr op `thenDs` \ core_op ->
222 -- for the type of x, we need the type of op's 2nd argument
224 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
225 -- See comment with SectionL
227 dsExpr expr `thenDs` \ y_core ->
228 newSysLocalDs x_ty `thenDs` \ x_id ->
229 newSysLocalDs y_ty `thenDs` \ y_id ->
231 returnDs (bindNonRec y_id y_core $
232 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
234 dsExpr (HsCCall lbl args may_gc is_asm result_ty)
235 = mapDs dsExpr args `thenDs` \ core_args ->
236 dsCCall lbl core_args may_gc is_asm result_ty
237 -- dsCCall does all the unboxification, etc.
239 dsExpr (HsSCC cc expr)
240 = dsExpr expr `thenDs` \ core_expr ->
241 getModuleDs `thenDs` \ mod_name ->
242 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
244 -- special case to handle unboxed tuple patterns.
246 dsExpr (HsCase discrim matches src_loc)
247 | all ubx_tuple_match matches
248 = putSrcLocDs src_loc $
249 dsExpr discrim `thenDs` \ core_discrim ->
250 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
251 case matching_code of
252 Case (Var x) bndr alts | x == discrim_var ->
253 returnDs (Case core_discrim bndr alts)
254 _ -> panic ("dsExpr: tuple pattern:\n" ++ showSDoc (ppr matching_code))
256 ubx_tuple_match (Match [TuplePat ps Unboxed] _ _) = True
257 ubx_tuple_match _ = False
259 dsExpr (HsCase discrim matches src_loc)
260 = putSrcLocDs src_loc $
261 dsExpr discrim `thenDs` \ core_discrim ->
262 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
263 returnDs (bindNonRec discrim_var core_discrim matching_code)
265 dsExpr (HsLet binds body)
266 = dsExpr body `thenDs` \ body' ->
269 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
270 -- because the interpretation of `stmts' depends on what sort of thing it is.
272 dsExpr (HsDo ListComp stmts _ result_ty src_loc)
273 = -- Special case for list comprehensions
274 putSrcLocDs src_loc $
275 dsListComp stmts elt_ty
277 (_, [elt_ty]) = tcSplitTyConApp result_ty
279 dsExpr (HsDo do_or_lc stmts ids result_ty src_loc)
281 = putSrcLocDs src_loc $
282 dsDo do_or_lc stmts ids result_ty
284 dsExpr (HsDo PArrComp stmts _ result_ty src_loc)
285 = -- Special case for array comprehensions
286 putSrcLocDs src_loc $
287 dsPArrComp stmts elt_ty
289 (_, [elt_ty]) = tcSplitTyConApp result_ty
291 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
292 = putSrcLocDs src_loc $
293 dsExpr guard_expr `thenDs` \ core_guard ->
294 dsExpr then_expr `thenDs` \ core_then ->
295 dsExpr else_expr `thenDs` \ core_else ->
296 returnDs (mkIfThenElse core_guard core_then core_else)
301 \underline{\bf Type lambda and application}
302 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
304 dsExpr (TyLam tyvars expr)
305 = dsExpr expr `thenDs` \ core_expr ->
306 returnDs (mkLams tyvars core_expr)
308 dsExpr (TyApp expr tys)
309 = dsExpr expr `thenDs` \ core_expr ->
310 returnDs (mkTyApps core_expr tys)
315 \underline{\bf Various data construction things}
316 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
318 dsExpr (ExplicitList ty xs)
321 go [] = returnDs (mkNilExpr ty)
322 go (x:xs) = dsExpr x `thenDs` \ core_x ->
323 go xs `thenDs` \ core_xs ->
324 returnDs (mkConsExpr ty core_x core_xs)
326 -- we create a list from the array elements and convert them into a list using
329 -- * the main disadvantage to this scheme is that `toP' traverses the list
330 -- twice: once to determine the length and a second time to put to elements
331 -- into the array; this inefficiency could be avoided by exposing some of
332 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
333 -- that we can exploit the fact that we already know the length of the array
334 -- here at compile time
336 dsExpr (ExplicitPArr ty xs)
337 = dsLookupGlobalId toPName `thenDs` \toP ->
338 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
339 returnDs (mkApps (Var toP) [Type ty, coreList])
341 dsExpr (ExplicitTuple expr_list boxity)
342 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
343 returnDs (mkConApp (tupleCon boxity (length expr_list))
344 (map (Type . exprType) core_exprs ++ core_exprs))
346 dsExpr (ArithSeqOut expr (From from))
347 = dsExpr expr `thenDs` \ expr2 ->
348 dsExpr from `thenDs` \ from2 ->
349 returnDs (App expr2 from2)
351 dsExpr (ArithSeqOut expr (FromTo from two))
352 = dsExpr expr `thenDs` \ expr2 ->
353 dsExpr from `thenDs` \ from2 ->
354 dsExpr two `thenDs` \ two2 ->
355 returnDs (mkApps expr2 [from2, two2])
357 dsExpr (ArithSeqOut expr (FromThen from thn))
358 = dsExpr expr `thenDs` \ expr2 ->
359 dsExpr from `thenDs` \ from2 ->
360 dsExpr thn `thenDs` \ thn2 ->
361 returnDs (mkApps expr2 [from2, thn2])
363 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
364 = dsExpr expr `thenDs` \ expr2 ->
365 dsExpr from `thenDs` \ from2 ->
366 dsExpr thn `thenDs` \ thn2 ->
367 dsExpr two `thenDs` \ two2 ->
368 returnDs (mkApps expr2 [from2, thn2, two2])
370 dsExpr (PArrSeqOut expr (FromTo from two))
371 = dsExpr expr `thenDs` \ expr2 ->
372 dsExpr from `thenDs` \ from2 ->
373 dsExpr two `thenDs` \ two2 ->
374 returnDs (mkApps expr2 [from2, two2])
376 dsExpr (PArrSeqOut expr (FromThenTo from thn two))
377 = dsExpr expr `thenDs` \ expr2 ->
378 dsExpr from `thenDs` \ from2 ->
379 dsExpr thn `thenDs` \ thn2 ->
380 dsExpr two `thenDs` \ two2 ->
381 returnDs (mkApps expr2 [from2, thn2, two2])
383 dsExpr (PArrSeqOut expr _)
384 = panic "DsExpr.dsExpr: Infinite parallel array!"
385 -- the parser shouldn't have generated it and the renamer and typechecker
386 -- shouldn't have let it through
390 \underline{\bf Record construction and update}
391 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
392 For record construction we do this (assuming T has three arguments)
396 let err = /\a -> recConErr a
397 T (recConErr t1 "M.lhs/230/op1")
399 (recConErr t1 "M.lhs/230/op3")
401 @recConErr@ then converts its arugment string into a proper message
402 before printing it as
404 M.lhs, line 230: missing field op1 was evaluated
407 We also handle @C{}@ as valid construction syntax for an unlabelled
408 constructor @C@, setting all of @C@'s fields to bottom.
411 dsExpr (RecordConOut data_con con_expr rbinds)
412 = dsExpr con_expr `thenDs` \ con_expr' ->
414 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
415 -- A newtype in the corner should be opaque;
416 -- hence TcType.tcSplitFunTys
419 = case [rhs | (sel_id,rhs) <- rbinds,
420 lbl == recordSelectorFieldLabel sel_id] of
421 (rhs:rhss) -> ASSERT( null rhss )
423 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
424 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
426 labels = dataConFieldLabels data_con
430 then mapDs unlabelled_bottom arg_tys
431 else mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
432 `thenDs` \ con_args ->
434 returnDs (mkApps con_expr' con_args)
437 Record update is a little harder. Suppose we have the decl:
439 data T = T1 {op1, op2, op3 :: Int}
440 | T2 {op4, op2 :: Int}
443 Then we translate as follows:
449 T1 op1 _ op3 -> T1 op1 op2 op3
450 T2 op4 _ -> T2 op4 op2
451 other -> recUpdError "M.lhs/230"
453 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
454 RHSs, and do not generate a Core constructor application directly, because the constructor
455 might do some argument-evaluation first; and may have to throw away some
459 dsExpr (RecordUpdOut record_expr record_in_ty record_out_ty [])
462 dsExpr expr@(RecordUpdOut record_expr record_in_ty record_out_ty rbinds)
463 = getSrcLocDs `thenDs` \ src_loc ->
464 dsExpr record_expr `thenDs` \ record_expr' ->
466 -- Desugar the rbinds, and generate let-bindings if
467 -- necessary so that we don't lose sharing
470 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
471 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
473 mk_val_arg field old_arg_id
474 = case [rhs | (sel_id, rhs) <- rbinds,
475 field == recordSelectorFieldLabel sel_id] of
476 (rhs:rest) -> ASSERT(null rest) rhs
477 [] -> HsVar old_arg_id
480 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
481 -- This call to dataConArgTys won't work for existentials
483 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
484 (dataConFieldLabels con) arg_ids
485 rhs = foldl HsApp (TyApp (HsVar (dataConWrapId con)) out_inst_tys)
488 returnDs (mkSimpleMatch [ConPatOut con (PrefixCon (map VarPat arg_ids)) record_in_ty [] []]
493 -- Record stuff doesn't work for existentials
494 -- The type checker checks for this, but we need
495 -- worry only about the constructors that are to be updated
496 ASSERT2( all (not . isExistentialDataCon) cons_to_upd, ppr expr )
498 -- It's important to generate the match with matchWrapper,
499 -- and the right hand sides with applications of the wrapper Id
500 -- so that everything works when we are doing fancy unboxing on the
501 -- constructor aguments.
502 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
503 matchWrapper RecUpd alts `thenDs` \ ([discrim_var], matching_code) ->
505 returnDs (bindNonRec discrim_var record_expr' matching_code)
508 updated_fields :: [FieldLabel]
509 updated_fields = [recordSelectorFieldLabel sel_id | (sel_id,_) <- rbinds]
511 -- Get the type constructor from the first field label,
512 -- so that we are sure it'll have all its DataCons
513 -- (In GHCI, it's possible that some TyCons may not have all
514 -- their constructors, in a module-loop situation.)
515 tycon = fieldLabelTyCon (head updated_fields)
516 data_cons = tyConDataCons tycon
517 cons_to_upd = filter has_all_fields data_cons
519 has_all_fields :: DataCon -> Bool
520 has_all_fields con_id
521 = all (`elem` con_fields) updated_fields
523 con_fields = dataConFieldLabels con_id
528 \underline{\bf Dictionary lambda and application}
529 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
530 @DictLam@ and @DictApp@ turn into the regular old things.
531 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
532 complicated; reminiscent of fully-applied constructors.
534 dsExpr (DictLam dictvars expr)
535 = dsExpr expr `thenDs` \ core_expr ->
536 returnDs (mkLams dictvars core_expr)
540 dsExpr (DictApp expr dicts) -- becomes a curried application
541 = dsExpr expr `thenDs` \ core_expr ->
542 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
545 Here is where we desugar the Template Haskell brackets and escapes
548 -- Template Haskell stuff
550 #ifdef GHCI /* Only if bootstrapping */
551 dsExpr (HsBracketOut x ps) = dsBracket x ps
552 dsExpr (HsReify r) = dsReify r
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)
618 = go stmts `thenDs` \ body ->
619 putSrcLocDs locn $ -- Rest is associated with this location
620 dsExpr expr `thenDs` \ rhs ->
621 mkStringLit (mk_msg locn) `thenDs` \ core_msg ->
623 -- In a do expression, pattern-match failure just calls
624 -- the monadic 'fail' rather than throwing an exception
625 fail_expr = mkApps (Var fail_id) [Type b_ty, core_msg]
628 selectMatchVar pat `thenDs` \ var ->
629 matchSimply (Var var) (StmtCtxt do_or_lc) pat
630 body fail_expr `thenDs` \ match_code ->
631 returnDs (mkApps (Var bind_id) [Type a_ty, Type b_ty, rhs, Lam var match_code])
633 go (RecStmt rec_vars rec_stmts rec_rets : stmts)
634 = go (bind_stmt : stmts)
636 bind_stmt = dsRecStmt m_ty ids rec_vars rec_stmts rec_rets
642 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
643 mk_msg locn = "Pattern match failure in do expression at " ++ showSDoc (ppr locn)
646 Translation for RecStmt's:
647 -----------------------------
648 We turn (RecStmt [v1,..vn] stmts) into:
650 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
654 dsRecStmt :: Type -- Monad type constructor :: * -> *
655 -> [Id] -- Ids for: [return,fail,>>=,>>,mfix]
656 -> [Id] -> [TypecheckedStmt] -> [TypecheckedHsExpr] -- Guts of the RecStmt
658 dsRecStmt m_ty ids@[return_id, _, _, _, mfix_id] vars stmts rets
659 = ASSERT( length vars == length rets )
660 BindStmt tup_pat mfix_app noSrcLoc
662 (var1:rest) = vars -- Always at least one
666 mfix_app = HsApp (TyApp (HsVar mfix_id) [tup_ty]) mfix_arg
667 mfix_arg = HsLam (mkSimpleMatch [tup_pat] body tup_ty noSrcLoc)
669 tup_expr | one_var = ret1
670 | otherwise = ExplicitTuple rets Boxed
671 tup_ty | one_var = idType var1
672 | otherwise = mkTupleTy Boxed (length vars) (map idType vars)
673 tup_pat | one_var = VarPat var1
674 | otherwise = LazyPat (TuplePat (map VarPat vars) Boxed)
676 body = HsDo DoExpr (stmts ++ [return_stmt])
677 ids -- Don't need the mfix, but it does no harm
678 (mkAppTy m_ty tup_ty)
681 return_stmt = ResultStmt return_app noSrcLoc
682 return_app = HsApp (TyApp (HsVar return_id) [tup_ty]) tup_expr