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,
19 mkCoreTupTy, selectMatchVar,
20 dsReboundNames, lookupReboundName )
21 import DsArrows ( dsProcExpr )
25 -- Template Haskell stuff iff bootstrapped
26 import DsMeta ( dsBracket, dsReify )
29 import HsSyn ( HsExpr(..), Pat(..), ArithSeqInfo(..),
30 Stmt(..), HsMatchContext(..), HsStmtContext(..),
31 Match(..), HsBinds(..), MonoBinds(..), HsConDetails(..),
33 mkSimpleMatch, isDoExpr
35 import TcHsSyn ( TypecheckedHsExpr, TypecheckedHsBinds, TypecheckedStmt, hsPatType )
37 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
38 -- needs to see source types (newtypes etc), and sometimes not
39 -- So WATCH OUT; check each use of split*Ty functions.
40 -- Sigh. This is a pain.
42 import TcType ( tcSplitAppTy, tcSplitFunTys, tcTyConAppArgs,
43 tcSplitTyConApp, isUnLiftedType, Type,
45 import Type ( splitFunTys )
47 import CoreUtils ( exprType, mkIfThenElse, bindNonRec )
49 import FieldLabel ( FieldLabel, fieldLabelTyCon )
50 import CostCentre ( mkUserCC )
51 import Id ( Id, idType, idName, recordSelectorFieldLabel )
52 import PrelInfo ( rEC_CON_ERROR_ID, iRREFUT_PAT_ERROR_ID )
53 import DataCon ( DataCon, dataConWrapId, dataConFieldLabels, dataConInstOrigArgTys )
54 import DataCon ( isExistentialDataCon )
56 import TyCon ( tyConDataCons )
57 import TysWiredIn ( tupleCon )
58 import BasicTypes ( RecFlag(..), Boxity(..), ipNameName )
59 import PrelNames ( toPName,
60 returnMName, bindMName, thenMName, failMName,
62 import SrcLoc ( noSrcLoc )
63 import Util ( zipEqual, zipWithEqual )
69 %************************************************************************
73 %************************************************************************
75 @dsLet@ is a match-result transformer, taking the @MatchResult@ for the body
76 and transforming it into one for the let-bindings enclosing the body.
78 This may seem a bit odd, but (source) let bindings can contain unboxed
83 This must be transformed to a case expression and, if the type has
84 more than one constructor, may fail.
87 dsLet :: TypecheckedHsBinds -> CoreExpr -> DsM CoreExpr
92 dsLet (ThenBinds b1 b2) body
93 = dsLet b2 body `thenDs` \ body' ->
96 dsLet (IPBinds binds is_with) body
97 = foldlDs dsIPBind body binds
100 = dsExpr e `thenDs` \ e' ->
101 returnDs (Let (NonRec (ipNameName n) e') body)
103 -- Special case for bindings which bind unlifted variables
104 -- We need to do a case right away, rather than building
105 -- a tuple and doing selections.
106 -- Silently ignore INLINE pragmas...
107 dsLet bind@(MonoBind (AbsBinds [] [] exports inlines binds) sigs is_rec) body
108 | or [isUnLiftedType (idType g) | (_, g, l) <- exports]
109 = ASSERT (case is_rec of {NonRecursive -> True; other -> False})
110 -- Unlifted bindings are always non-recursive
111 -- and are always a Fun or Pat monobind
113 -- ToDo: in some bizarre case it's conceivable that there
114 -- could be dict binds in the 'binds'. (See the notes
115 -- below. Then pattern-match would fail. Urk.)
117 FunMonoBind fun _ matches loc
119 matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
120 ASSERT( null args ) -- Functions aren't lifted
121 returnDs (bindNonRec fun rhs body_w_exports)
123 PatMonoBind pat grhss loc
125 dsGuarded grhss `thenDs` \ rhs ->
126 mk_error_app pat `thenDs` \ error_expr ->
127 matchSimply rhs PatBindRhs pat body_w_exports error_expr
129 other -> pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)
131 body_w_exports = foldr bind_export body exports
132 bind_export (tvs, g, l) body = ASSERT( null tvs )
133 bindNonRec g (Var l) body
135 mk_error_app pat = mkErrorAppDs iRREFUT_PAT_ERROR_ID
139 -- Ordinary case for bindings
140 dsLet (MonoBind binds sigs is_rec) body
141 = dsMonoBinds NoSccs binds [] `thenDs` \ prs ->
142 returnDs (Let (Rec prs) body)
143 -- Use a Rec regardless of is_rec.
144 -- Why? Because it allows the MonoBinds to be all
145 -- mixed up, which is what happens in one rare case
146 -- Namely, for an AbsBind with no tyvars and no dicts,
147 -- but which does have dictionary bindings.
148 -- See notes with TcSimplify.inferLoop [NO TYVARS]
149 -- It turned out that wrapping a Rec here was the easiest solution
151 -- NB The previous case dealt with unlifted bindings, so we
152 -- only have to deal with lifted ones now; so Rec is ok
155 %************************************************************************
157 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
159 %************************************************************************
162 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
164 dsExpr (HsPar x) = dsExpr x
165 dsExpr (HsVar var) = returnDs (Var var)
166 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
167 dsExpr (HsLit lit) = dsLit lit
168 -- HsOverLit has been gotten rid of by the type checker
170 dsExpr expr@(HsLam a_Match)
171 = matchWrapper LambdaExpr [a_Match] `thenDs` \ (binders, matching_code) ->
172 returnDs (mkLams binders matching_code)
174 dsExpr expr@(HsApp fun arg)
175 = dsExpr fun `thenDs` \ core_fun ->
176 dsExpr arg `thenDs` \ core_arg ->
177 returnDs (core_fun `App` core_arg)
180 Operator sections. At first it looks as if we can convert
189 But no! expr might be a redex, and we can lose laziness badly this
194 for example. So we convert instead to
196 let y = expr in \x -> op y x
198 If \tr{expr} is actually just a variable, say, then the simplifier
202 dsExpr (OpApp e1 op _ e2)
203 = dsExpr op `thenDs` \ core_op ->
204 -- for the type of y, we need the type of op's 2nd argument
205 dsExpr e1 `thenDs` \ x_core ->
206 dsExpr e2 `thenDs` \ y_core ->
207 returnDs (mkApps core_op [x_core, y_core])
209 dsExpr (SectionL expr op)
210 = dsExpr op `thenDs` \ core_op ->
211 -- for the type of y, we need the type of op's 2nd argument
213 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
214 -- Must look through an implicit-parameter type;
215 -- newtype impossible; hence Type.splitFunTys
217 dsExpr expr `thenDs` \ x_core ->
218 newSysLocalDs x_ty `thenDs` \ x_id ->
219 newSysLocalDs y_ty `thenDs` \ y_id ->
221 returnDs (bindNonRec x_id x_core $
222 Lam y_id (mkApps core_op [Var x_id, Var y_id]))
224 -- dsExpr (SectionR op expr) -- \ x -> op x expr
225 dsExpr (SectionR op expr)
226 = dsExpr op `thenDs` \ core_op ->
227 -- for the type of x, we need the type of op's 2nd argument
229 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
230 -- See comment with SectionL
232 dsExpr expr `thenDs` \ y_core ->
233 newSysLocalDs x_ty `thenDs` \ x_id ->
234 newSysLocalDs y_ty `thenDs` \ y_id ->
236 returnDs (bindNonRec y_id y_core $
237 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
239 dsExpr (HsCCall lbl args may_gc is_asm result_ty)
240 = mapDs dsExpr args `thenDs` \ core_args ->
241 dsCCall lbl core_args may_gc is_asm result_ty
242 -- dsCCall does all the unboxification, etc.
244 dsExpr (HsSCC cc expr)
245 = dsExpr expr `thenDs` \ core_expr ->
246 getModuleDs `thenDs` \ mod_name ->
247 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
250 -- hdaume: core annotation
252 dsExpr (HsCoreAnn fs expr)
253 = dsExpr expr `thenDs` \ core_expr ->
254 returnDs (Note (CoreNote $ unpackFS fs) core_expr)
256 -- special case to handle unboxed tuple patterns.
258 dsExpr (HsCase discrim matches src_loc)
259 | all ubx_tuple_match matches
260 = putSrcLocDs src_loc $
261 dsExpr discrim `thenDs` \ core_discrim ->
262 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
263 case matching_code of
264 Case (Var x) bndr alts | x == discrim_var ->
265 returnDs (Case core_discrim bndr alts)
266 _ -> panic ("dsExpr: tuple pattern:\n" ++ showSDoc (ppr matching_code))
268 ubx_tuple_match (Match [TuplePat ps Unboxed] _ _) = True
269 ubx_tuple_match _ = False
271 dsExpr (HsCase discrim matches src_loc)
272 = putSrcLocDs src_loc $
273 dsExpr discrim `thenDs` \ core_discrim ->
274 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
275 returnDs (bindNonRec discrim_var core_discrim matching_code)
277 dsExpr (HsLet binds body)
278 = dsExpr body `thenDs` \ body' ->
281 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
282 -- because the interpretation of `stmts' depends on what sort of thing it is.
284 dsExpr (HsDo ListComp stmts _ result_ty src_loc)
285 = -- Special case for list comprehensions
286 putSrcLocDs src_loc $
287 dsListComp stmts elt_ty
289 (_, [elt_ty]) = tcSplitTyConApp result_ty
291 dsExpr (HsDo do_or_lc stmts ids result_ty src_loc)
293 = putSrcLocDs src_loc $
294 dsDo do_or_lc stmts ids result_ty
296 dsExpr (HsDo PArrComp stmts _ result_ty src_loc)
297 = -- Special case for array comprehensions
298 putSrcLocDs src_loc $
299 dsPArrComp stmts elt_ty
301 (_, [elt_ty]) = tcSplitTyConApp result_ty
303 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
304 = putSrcLocDs src_loc $
305 dsExpr guard_expr `thenDs` \ core_guard ->
306 dsExpr then_expr `thenDs` \ core_then ->
307 dsExpr else_expr `thenDs` \ core_else ->
308 returnDs (mkIfThenElse core_guard core_then core_else)
313 \underline{\bf Type lambda and application}
314 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
316 dsExpr (TyLam tyvars expr)
317 = dsExpr expr `thenDs` \ core_expr ->
318 returnDs (mkLams tyvars core_expr)
320 dsExpr (TyApp expr tys)
321 = dsExpr expr `thenDs` \ core_expr ->
322 returnDs (mkTyApps core_expr tys)
327 \underline{\bf Various data construction things}
328 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
330 dsExpr (ExplicitList ty xs)
333 go [] = returnDs (mkNilExpr ty)
334 go (x:xs) = dsExpr x `thenDs` \ core_x ->
335 go xs `thenDs` \ core_xs ->
336 returnDs (mkConsExpr ty core_x core_xs)
338 -- we create a list from the array elements and convert them into a list using
341 -- * the main disadvantage to this scheme is that `toP' traverses the list
342 -- twice: once to determine the length and a second time to put to elements
343 -- into the array; this inefficiency could be avoided by exposing some of
344 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
345 -- that we can exploit the fact that we already know the length of the array
346 -- here at compile time
348 dsExpr (ExplicitPArr ty xs)
349 = dsLookupGlobalId toPName `thenDs` \toP ->
350 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
351 returnDs (mkApps (Var toP) [Type ty, coreList])
353 dsExpr (ExplicitTuple expr_list boxity)
354 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
355 returnDs (mkConApp (tupleCon boxity (length expr_list))
356 (map (Type . exprType) core_exprs ++ core_exprs))
358 dsExpr (ArithSeqOut expr (From from))
359 = dsExpr expr `thenDs` \ expr2 ->
360 dsExpr from `thenDs` \ from2 ->
361 returnDs (App expr2 from2)
363 dsExpr (ArithSeqOut expr (FromTo from two))
364 = dsExpr expr `thenDs` \ expr2 ->
365 dsExpr from `thenDs` \ from2 ->
366 dsExpr two `thenDs` \ two2 ->
367 returnDs (mkApps expr2 [from2, two2])
369 dsExpr (ArithSeqOut expr (FromThen from thn))
370 = dsExpr expr `thenDs` \ expr2 ->
371 dsExpr from `thenDs` \ from2 ->
372 dsExpr thn `thenDs` \ thn2 ->
373 returnDs (mkApps expr2 [from2, thn2])
375 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
376 = dsExpr expr `thenDs` \ expr2 ->
377 dsExpr from `thenDs` \ from2 ->
378 dsExpr thn `thenDs` \ thn2 ->
379 dsExpr two `thenDs` \ two2 ->
380 returnDs (mkApps expr2 [from2, thn2, two2])
382 dsExpr (PArrSeqOut expr (FromTo from two))
383 = dsExpr expr `thenDs` \ expr2 ->
384 dsExpr from `thenDs` \ from2 ->
385 dsExpr two `thenDs` \ two2 ->
386 returnDs (mkApps expr2 [from2, two2])
388 dsExpr (PArrSeqOut expr (FromThenTo from thn two))
389 = dsExpr expr `thenDs` \ expr2 ->
390 dsExpr from `thenDs` \ from2 ->
391 dsExpr thn `thenDs` \ thn2 ->
392 dsExpr two `thenDs` \ two2 ->
393 returnDs (mkApps expr2 [from2, thn2, two2])
395 dsExpr (PArrSeqOut expr _)
396 = panic "DsExpr.dsExpr: Infinite parallel array!"
397 -- the parser shouldn't have generated it and the renamer and typechecker
398 -- shouldn't have let it through
402 \underline{\bf Record construction and update}
403 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
404 For record construction we do this (assuming T has three arguments)
408 let err = /\a -> recConErr a
409 T (recConErr t1 "M.lhs/230/op1")
411 (recConErr t1 "M.lhs/230/op3")
413 @recConErr@ then converts its arugment string into a proper message
414 before printing it as
416 M.lhs, line 230: missing field op1 was evaluated
419 We also handle @C{}@ as valid construction syntax for an unlabelled
420 constructor @C@, setting all of @C@'s fields to bottom.
423 dsExpr (RecordConOut data_con con_expr rbinds)
424 = dsExpr con_expr `thenDs` \ con_expr' ->
426 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
427 -- A newtype in the corner should be opaque;
428 -- hence TcType.tcSplitFunTys
431 = case [rhs | (sel_id,rhs) <- rbinds,
432 lbl == recordSelectorFieldLabel sel_id] of
433 (rhs:rhss) -> ASSERT( null rhss )
435 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
436 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
438 labels = dataConFieldLabels data_con
442 then mapDs unlabelled_bottom arg_tys
443 else mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
444 `thenDs` \ con_args ->
446 returnDs (mkApps con_expr' con_args)
449 Record update is a little harder. Suppose we have the decl:
451 data T = T1 {op1, op2, op3 :: Int}
452 | T2 {op4, op2 :: Int}
455 Then we translate as follows:
461 T1 op1 _ op3 -> T1 op1 op2 op3
462 T2 op4 _ -> T2 op4 op2
463 other -> recUpdError "M.lhs/230"
465 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
466 RHSs, and do not generate a Core constructor application directly, because the constructor
467 might do some argument-evaluation first; and may have to throw away some
471 dsExpr (RecordUpdOut record_expr record_in_ty record_out_ty [])
474 dsExpr expr@(RecordUpdOut record_expr record_in_ty record_out_ty rbinds)
475 = getSrcLocDs `thenDs` \ src_loc ->
476 dsExpr record_expr `thenDs` \ record_expr' ->
478 -- Desugar the rbinds, and generate let-bindings if
479 -- necessary so that we don't lose sharing
482 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
483 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
485 mk_val_arg field old_arg_id
486 = case [rhs | (sel_id, rhs) <- rbinds,
487 field == recordSelectorFieldLabel sel_id] of
488 (rhs:rest) -> ASSERT(null rest) rhs
489 [] -> HsVar old_arg_id
492 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
493 -- This call to dataConArgTys won't work for existentials
495 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
496 (dataConFieldLabels con) arg_ids
497 rhs = foldl HsApp (TyApp (HsVar (dataConWrapId con)) out_inst_tys)
500 returnDs (mkSimpleMatch [ConPatOut con (PrefixCon (map VarPat arg_ids)) record_in_ty [] []]
505 -- Record stuff doesn't work for existentials
506 -- The type checker checks for this, but we need
507 -- worry only about the constructors that are to be updated
508 ASSERT2( all (not . isExistentialDataCon) cons_to_upd, ppr expr )
510 -- It's important to generate the match with matchWrapper,
511 -- and the right hand sides with applications of the wrapper Id
512 -- so that everything works when we are doing fancy unboxing on the
513 -- constructor aguments.
514 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
515 matchWrapper RecUpd alts `thenDs` \ ([discrim_var], matching_code) ->
517 returnDs (bindNonRec discrim_var record_expr' matching_code)
520 updated_fields :: [FieldLabel]
521 updated_fields = [recordSelectorFieldLabel sel_id | (sel_id,_) <- rbinds]
523 -- Get the type constructor from the first field label,
524 -- so that we are sure it'll have all its DataCons
525 -- (In GHCI, it's possible that some TyCons may not have all
526 -- their constructors, in a module-loop situation.)
527 tycon = fieldLabelTyCon (head updated_fields)
528 data_cons = tyConDataCons tycon
529 cons_to_upd = filter has_all_fields data_cons
531 has_all_fields :: DataCon -> Bool
532 has_all_fields con_id
533 = all (`elem` con_fields) updated_fields
535 con_fields = dataConFieldLabels con_id
540 \underline{\bf Dictionary lambda and application}
541 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
542 @DictLam@ and @DictApp@ turn into the regular old things.
543 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
544 complicated; reminiscent of fully-applied constructors.
546 dsExpr (DictLam dictvars expr)
547 = dsExpr expr `thenDs` \ core_expr ->
548 returnDs (mkLams dictvars core_expr)
552 dsExpr (DictApp expr dicts) -- becomes a curried application
553 = dsExpr expr `thenDs` \ core_expr ->
554 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
557 Here is where we desugar the Template Haskell brackets and escapes
560 -- Template Haskell stuff
562 #ifdef GHCI /* Only if bootstrapping */
563 dsExpr (HsBracketOut x ps) = dsBracket x ps
564 dsExpr (HsReify r) = dsReify r
565 dsExpr (HsSplice n e _) = pprPanic "dsExpr:splice" (ppr e)
568 -- Arrow notation extension
569 dsExpr (HsProc pat cmd src_loc) = dsProcExpr pat cmd src_loc
576 -- HsSyn constructs that just shouldn't be here:
577 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
578 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
579 dsExpr (PArrSeqIn _) = panic "dsExpr:PArrSeqIn"
584 %--------------------------------------------------------------------
586 Basically does the translation given in the Haskell~1.3 report:
589 dsDo :: HsStmtContext Name
591 -> ReboundNames Id -- id for: [return,fail,>>=,>>] and possibly mfixName
592 -> Type -- Element type; the whole expression has type (m t)
595 dsDo do_or_lc stmts ids result_ty
596 = dsReboundNames ids `thenDs` \ (meth_binds, ds_meths) ->
598 return_id = lookupReboundName ds_meths returnMName
599 fail_id = lookupReboundName ds_meths failMName
600 bind_id = lookupReboundName ds_meths bindMName
601 then_id = lookupReboundName ds_meths thenMName
603 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
604 is_do = isDoExpr do_or_lc -- True for both MDo and Do
606 -- For ExprStmt, see the comments near HsExpr.Stmt about
607 -- exactly what ExprStmts mean!
609 -- In dsDo we can only see DoStmt and ListComp (no guards)
611 go [ResultStmt expr locn]
612 | is_do = do_expr expr locn
613 | otherwise = do_expr expr locn `thenDs` \ expr2 ->
614 returnDs (mkApps return_id [Type b_ty, expr2])
616 go (ExprStmt expr a_ty locn : stmts)
617 | is_do -- Do expression
618 = do_expr expr locn `thenDs` \ expr2 ->
619 go stmts `thenDs` \ rest ->
620 returnDs (mkApps then_id [Type a_ty, Type b_ty, expr2, rest])
622 | otherwise -- List comprehension
623 = do_expr expr locn `thenDs` \ expr2 ->
624 go stmts `thenDs` \ rest ->
626 msg = "Pattern match failure in do expression, " ++ showSDoc (ppr locn)
628 mkStringLit msg `thenDs` \ core_msg ->
629 returnDs (mkIfThenElse expr2 rest
630 (App (App fail_id (Type b_ty)) core_msg))
632 go (LetStmt binds : stmts)
633 = go stmts `thenDs` \ rest ->
636 go (BindStmt pat expr locn : stmts)
637 = go stmts `thenDs` \ body ->
638 putSrcLocDs locn $ -- Rest is associated with this location
639 dsExpr expr `thenDs` \ rhs ->
640 mkStringLit (mk_msg locn) `thenDs` \ core_msg ->
642 -- In a do expression, pattern-match failure just calls
643 -- the monadic 'fail' rather than throwing an exception
644 fail_expr = mkApps fail_id [Type b_ty, core_msg]
647 selectMatchVar pat `thenDs` \ var ->
648 matchSimply (Var var) (StmtCtxt do_or_lc) pat
649 body fail_expr `thenDs` \ match_code ->
650 returnDs (mkApps bind_id [Type a_ty, Type b_ty, rhs, Lam var match_code])
652 go (RecStmt rec_stmts later_vars rec_vars rec_rets : stmts)
653 = go (bind_stmt : stmts)
655 bind_stmt = dsRecStmt m_ty ds_meths rec_stmts later_vars rec_vars rec_rets
658 go stmts `thenDs` \ stmts_code ->
659 returnDs (foldr Let stmts_code meth_binds)
662 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
663 mk_msg locn = "Pattern match failure in do expression at " ++ showSDoc (ppr locn)
666 Translation for RecStmt's:
667 -----------------------------
668 We turn (RecStmt [v1,..vn] stmts) into:
670 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
674 dsRecStmt :: Type -- Monad type constructor :: * -> *
675 -> [(Name,Id)] -- Rebound Ids
677 -> [Id] -> [Id] -> [TypecheckedHsExpr]
679 dsRecStmt m_ty ds_meths stmts later_vars rec_vars rec_rets
680 = ASSERT( length vars == length rets )
681 BindStmt tup_pat mfix_app noSrcLoc
683 vars@(var1:rest) = later_vars ++ rec_vars -- Always at least one
684 rets@(ret1:_) = map HsVar later_vars ++ rec_rets
687 mfix_app = HsApp (TyApp (HsVar mfix_id) [tup_ty]) mfix_arg
688 mfix_arg = HsLam (mkSimpleMatch [tup_pat] body tup_ty noSrcLoc)
690 tup_expr | one_var = ret1
691 | otherwise = ExplicitTuple rets Boxed
692 tup_ty = mkCoreTupTy (map idType vars)
693 -- Deals with singleton case
694 tup_pat | one_var = VarPat var1
695 | otherwise = LazyPat (TuplePat (map VarPat vars) Boxed)
697 body = HsDo DoExpr (stmts ++ [return_stmt])
698 [(n, HsVar id) | (n,id) <- ds_meths] -- A bit of a hack
699 (mkAppTy m_ty tup_ty)
702 Var return_id = lookupReboundName ds_meths returnMName
703 Var mfix_id = lookupReboundName ds_meths mfixName
705 return_stmt = ResultStmt return_app noSrcLoc
706 return_app = HsApp (TyApp (HsVar return_id) [tup_ty]) tup_expr