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) 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 (HsSCC cc expr)
240 = dsExpr expr `thenDs` \ core_expr ->
241 getModuleDs `thenDs` \ mod_name ->
242 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
245 -- hdaume: core annotation
247 dsExpr (HsCoreAnn fs expr)
248 = dsExpr expr `thenDs` \ core_expr ->
249 returnDs (Note (CoreNote $ unpackFS fs) core_expr)
251 -- special case to handle unboxed tuple patterns.
253 dsExpr (HsCase discrim matches src_loc)
254 | all ubx_tuple_match matches
255 = putSrcLocDs src_loc $
256 dsExpr discrim `thenDs` \ core_discrim ->
257 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
258 case matching_code of
259 Case (Var x) bndr alts | x == discrim_var ->
260 returnDs (Case core_discrim bndr alts)
261 _ -> panic ("dsExpr: tuple pattern:\n" ++ showSDoc (ppr matching_code))
263 ubx_tuple_match (Match [TuplePat ps Unboxed] _ _) = True
264 ubx_tuple_match _ = False
266 dsExpr (HsCase discrim matches src_loc)
267 = putSrcLocDs src_loc $
268 dsExpr discrim `thenDs` \ core_discrim ->
269 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
270 returnDs (bindNonRec discrim_var core_discrim matching_code)
272 dsExpr (HsLet binds body)
273 = dsExpr body `thenDs` \ body' ->
276 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
277 -- because the interpretation of `stmts' depends on what sort of thing it is.
279 dsExpr (HsDo ListComp stmts _ result_ty src_loc)
280 = -- Special case for list comprehensions
281 putSrcLocDs src_loc $
282 dsListComp stmts elt_ty
284 (_, [elt_ty]) = tcSplitTyConApp result_ty
286 dsExpr (HsDo do_or_lc stmts ids result_ty src_loc)
288 = putSrcLocDs src_loc $
289 dsDo do_or_lc stmts ids result_ty
291 dsExpr (HsDo PArrComp stmts _ result_ty src_loc)
292 = -- Special case for array comprehensions
293 putSrcLocDs src_loc $
294 dsPArrComp stmts elt_ty
296 (_, [elt_ty]) = tcSplitTyConApp result_ty
298 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
299 = putSrcLocDs src_loc $
300 dsExpr guard_expr `thenDs` \ core_guard ->
301 dsExpr then_expr `thenDs` \ core_then ->
302 dsExpr else_expr `thenDs` \ core_else ->
303 returnDs (mkIfThenElse core_guard core_then core_else)
308 \underline{\bf Type lambda and application}
309 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
311 dsExpr (TyLam tyvars expr)
312 = dsExpr expr `thenDs` \ core_expr ->
313 returnDs (mkLams tyvars core_expr)
315 dsExpr (TyApp expr tys)
316 = dsExpr expr `thenDs` \ core_expr ->
317 returnDs (mkTyApps core_expr tys)
322 \underline{\bf Various data construction things}
323 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
325 dsExpr (ExplicitList ty xs)
328 go [] = returnDs (mkNilExpr ty)
329 go (x:xs) = dsExpr x `thenDs` \ core_x ->
330 go xs `thenDs` \ core_xs ->
331 returnDs (mkConsExpr ty core_x core_xs)
333 -- we create a list from the array elements and convert them into a list using
336 -- * the main disadvantage to this scheme is that `toP' traverses the list
337 -- twice: once to determine the length and a second time to put to elements
338 -- into the array; this inefficiency could be avoided by exposing some of
339 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
340 -- that we can exploit the fact that we already know the length of the array
341 -- here at compile time
343 dsExpr (ExplicitPArr ty xs)
344 = dsLookupGlobalId toPName `thenDs` \toP ->
345 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
346 returnDs (mkApps (Var toP) [Type ty, coreList])
348 dsExpr (ExplicitTuple expr_list boxity)
349 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
350 returnDs (mkConApp (tupleCon boxity (length expr_list))
351 (map (Type . exprType) core_exprs ++ core_exprs))
353 dsExpr (ArithSeqOut expr (From from))
354 = dsExpr expr `thenDs` \ expr2 ->
355 dsExpr from `thenDs` \ from2 ->
356 returnDs (App expr2 from2)
358 dsExpr (ArithSeqOut expr (FromTo from two))
359 = dsExpr expr `thenDs` \ expr2 ->
360 dsExpr from `thenDs` \ from2 ->
361 dsExpr two `thenDs` \ two2 ->
362 returnDs (mkApps expr2 [from2, two2])
364 dsExpr (ArithSeqOut expr (FromThen from thn))
365 = dsExpr expr `thenDs` \ expr2 ->
366 dsExpr from `thenDs` \ from2 ->
367 dsExpr thn `thenDs` \ thn2 ->
368 returnDs (mkApps expr2 [from2, thn2])
370 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
371 = dsExpr expr `thenDs` \ expr2 ->
372 dsExpr from `thenDs` \ from2 ->
373 dsExpr thn `thenDs` \ thn2 ->
374 dsExpr two `thenDs` \ two2 ->
375 returnDs (mkApps expr2 [from2, thn2, two2])
377 dsExpr (PArrSeqOut expr (FromTo from two))
378 = dsExpr expr `thenDs` \ expr2 ->
379 dsExpr from `thenDs` \ from2 ->
380 dsExpr two `thenDs` \ two2 ->
381 returnDs (mkApps expr2 [from2, two2])
383 dsExpr (PArrSeqOut expr (FromThenTo from thn two))
384 = dsExpr expr `thenDs` \ expr2 ->
385 dsExpr from `thenDs` \ from2 ->
386 dsExpr thn `thenDs` \ thn2 ->
387 dsExpr two `thenDs` \ two2 ->
388 returnDs (mkApps expr2 [from2, thn2, two2])
390 dsExpr (PArrSeqOut expr _)
391 = panic "DsExpr.dsExpr: Infinite parallel array!"
392 -- the parser shouldn't have generated it and the renamer and typechecker
393 -- shouldn't have let it through
397 \underline{\bf Record construction and update}
398 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
399 For record construction we do this (assuming T has three arguments)
403 let err = /\a -> recConErr a
404 T (recConErr t1 "M.lhs/230/op1")
406 (recConErr t1 "M.lhs/230/op3")
408 @recConErr@ then converts its arugment string into a proper message
409 before printing it as
411 M.lhs, line 230: missing field op1 was evaluated
414 We also handle @C{}@ as valid construction syntax for an unlabelled
415 constructor @C@, setting all of @C@'s fields to bottom.
418 dsExpr (RecordConOut data_con con_expr rbinds)
419 = dsExpr con_expr `thenDs` \ con_expr' ->
421 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
422 -- A newtype in the corner should be opaque;
423 -- hence TcType.tcSplitFunTys
426 = case [rhs | (sel_id,rhs) <- rbinds,
427 lbl == recordSelectorFieldLabel sel_id] of
428 (rhs:rhss) -> ASSERT( null rhss )
430 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
431 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
433 labels = dataConFieldLabels data_con
437 then mapDs unlabelled_bottom arg_tys
438 else mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
439 `thenDs` \ con_args ->
441 returnDs (mkApps con_expr' con_args)
444 Record update is a little harder. Suppose we have the decl:
446 data T = T1 {op1, op2, op3 :: Int}
447 | T2 {op4, op2 :: Int}
450 Then we translate as follows:
456 T1 op1 _ op3 -> T1 op1 op2 op3
457 T2 op4 _ -> T2 op4 op2
458 other -> recUpdError "M.lhs/230"
460 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
461 RHSs, and do not generate a Core constructor application directly, because the constructor
462 might do some argument-evaluation first; and may have to throw away some
466 dsExpr (RecordUpdOut record_expr record_in_ty record_out_ty [])
469 dsExpr expr@(RecordUpdOut record_expr record_in_ty record_out_ty rbinds)
470 = getSrcLocDs `thenDs` \ src_loc ->
471 dsExpr record_expr `thenDs` \ record_expr' ->
473 -- Desugar the rbinds, and generate let-bindings if
474 -- necessary so that we don't lose sharing
477 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
478 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
480 mk_val_arg field old_arg_id
481 = case [rhs | (sel_id, rhs) <- rbinds,
482 field == recordSelectorFieldLabel sel_id] of
483 (rhs:rest) -> ASSERT(null rest) rhs
484 [] -> HsVar old_arg_id
487 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
488 -- This call to dataConArgTys won't work for existentials
490 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
491 (dataConFieldLabels con) arg_ids
492 rhs = foldl HsApp (TyApp (HsVar (dataConWrapId con)) out_inst_tys)
495 returnDs (mkSimpleMatch [ConPatOut con (PrefixCon (map VarPat arg_ids)) record_in_ty [] []]
500 -- Record stuff doesn't work for existentials
501 -- The type checker checks for this, but we need
502 -- worry only about the constructors that are to be updated
503 ASSERT2( all (not . isExistentialDataCon) cons_to_upd, ppr expr )
505 -- It's important to generate the match with matchWrapper,
506 -- and the right hand sides with applications of the wrapper Id
507 -- so that everything works when we are doing fancy unboxing on the
508 -- constructor aguments.
509 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
510 matchWrapper RecUpd alts `thenDs` \ ([discrim_var], matching_code) ->
512 returnDs (bindNonRec discrim_var record_expr' matching_code)
515 updated_fields :: [FieldLabel]
516 updated_fields = [recordSelectorFieldLabel sel_id | (sel_id,_) <- rbinds]
518 -- Get the type constructor from the first field label,
519 -- so that we are sure it'll have all its DataCons
520 -- (In GHCI, it's possible that some TyCons may not have all
521 -- their constructors, in a module-loop situation.)
522 tycon = fieldLabelTyCon (head updated_fields)
523 data_cons = tyConDataCons tycon
524 cons_to_upd = filter has_all_fields data_cons
526 has_all_fields :: DataCon -> Bool
527 has_all_fields con_id
528 = all (`elem` con_fields) updated_fields
530 con_fields = dataConFieldLabels con_id
535 \underline{\bf Dictionary lambda and application}
536 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
537 @DictLam@ and @DictApp@ turn into the regular old things.
538 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
539 complicated; reminiscent of fully-applied constructors.
541 dsExpr (DictLam dictvars expr)
542 = dsExpr expr `thenDs` \ core_expr ->
543 returnDs (mkLams dictvars core_expr)
547 dsExpr (DictApp expr dicts) -- becomes a curried application
548 = dsExpr expr `thenDs` \ core_expr ->
549 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
552 Here is where we desugar the Template Haskell brackets and escapes
555 -- Template Haskell stuff
557 #ifdef GHCI /* Only if bootstrapping */
558 dsExpr (HsBracketOut x ps) = dsBracket x ps
559 dsExpr (HsReify r) = dsReify r
560 dsExpr (HsSplice n e _) = pprPanic "dsExpr:splice" (ppr e)
563 -- Arrow notation extension
564 dsExpr (HsProc pat cmd src_loc) = dsProcExpr pat cmd src_loc
571 -- HsSyn constructs that just shouldn't be here:
572 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
573 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
574 dsExpr (PArrSeqIn _) = panic "dsExpr:PArrSeqIn"
579 %--------------------------------------------------------------------
581 Basically does the translation given in the Haskell~1.3 report:
584 dsDo :: HsStmtContext Name
586 -> ReboundNames Id -- id for: [return,fail,>>=,>>] and possibly mfixName
587 -> Type -- Element type; the whole expression has type (m t)
590 dsDo do_or_lc stmts ids result_ty
591 = dsReboundNames ids `thenDs` \ (meth_binds, ds_meths) ->
593 return_id = lookupReboundName ds_meths returnMName
594 fail_id = lookupReboundName ds_meths failMName
595 bind_id = lookupReboundName ds_meths bindMName
596 then_id = lookupReboundName ds_meths thenMName
598 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
599 is_do = isDoExpr do_or_lc -- True for both MDo and Do
601 -- For ExprStmt, see the comments near HsExpr.Stmt about
602 -- exactly what ExprStmts mean!
604 -- In dsDo we can only see DoStmt and ListComp (no guards)
606 go [ResultStmt expr locn]
607 | is_do = do_expr expr locn
608 | otherwise = do_expr expr locn `thenDs` \ expr2 ->
609 returnDs (mkApps return_id [Type b_ty, expr2])
611 go (ExprStmt expr a_ty locn : stmts)
612 | is_do -- Do expression
613 = do_expr expr locn `thenDs` \ expr2 ->
614 go stmts `thenDs` \ rest ->
615 returnDs (mkApps then_id [Type a_ty, Type b_ty, expr2, rest])
617 | otherwise -- List comprehension
618 = do_expr expr locn `thenDs` \ expr2 ->
619 go stmts `thenDs` \ rest ->
621 msg = "Pattern match failure in do expression, " ++ showSDoc (ppr locn)
623 mkStringLit msg `thenDs` \ core_msg ->
624 returnDs (mkIfThenElse expr2 rest
625 (App (App fail_id (Type b_ty)) core_msg))
627 go (LetStmt binds : stmts)
628 = go stmts `thenDs` \ rest ->
631 go (BindStmt pat expr locn : stmts)
632 = go stmts `thenDs` \ body ->
633 putSrcLocDs locn $ -- Rest is associated with this location
634 dsExpr expr `thenDs` \ rhs ->
635 mkStringLit (mk_msg locn) `thenDs` \ core_msg ->
637 -- In a do expression, pattern-match failure just calls
638 -- the monadic 'fail' rather than throwing an exception
639 fail_expr = mkApps fail_id [Type b_ty, core_msg]
642 selectMatchVar pat `thenDs` \ var ->
643 matchSimply (Var var) (StmtCtxt do_or_lc) pat
644 body fail_expr `thenDs` \ match_code ->
645 returnDs (mkApps bind_id [Type a_ty, Type b_ty, rhs, Lam var match_code])
647 go (RecStmt rec_stmts later_vars rec_vars rec_rets : stmts)
648 = go (bind_stmt : stmts)
650 bind_stmt = dsRecStmt m_ty ds_meths rec_stmts later_vars rec_vars rec_rets
653 go stmts `thenDs` \ stmts_code ->
654 returnDs (foldr Let stmts_code meth_binds)
657 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
658 mk_msg locn = "Pattern match failure in do expression at " ++ showSDoc (ppr locn)
661 Translation for RecStmt's:
662 -----------------------------
663 We turn (RecStmt [v1,..vn] stmts) into:
665 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
669 dsRecStmt :: Type -- Monad type constructor :: * -> *
670 -> [(Name,Id)] -- Rebound Ids
672 -> [Id] -> [Id] -> [TypecheckedHsExpr]
674 dsRecStmt m_ty ds_meths stmts later_vars rec_vars rec_rets
675 = ASSERT( length vars == length rets )
676 BindStmt tup_pat mfix_app noSrcLoc
678 vars@(var1:rest) = later_vars ++ rec_vars -- Always at least one
679 rets@(ret1:_) = map HsVar later_vars ++ rec_rets
682 mfix_app = HsApp (TyApp (HsVar mfix_id) [tup_ty]) mfix_arg
683 mfix_arg = HsLam (mkSimpleMatch [tup_pat] body tup_ty noSrcLoc)
685 tup_expr | one_var = ret1
686 | otherwise = ExplicitTuple rets Boxed
687 tup_ty = mkCoreTupTy (map idType vars)
688 -- Deals with singleton case
689 tup_pat | one_var = VarPat var1
690 | otherwise = LazyPat (TuplePat (map VarPat vars) Boxed)
692 body = HsDo DoExpr (stmts ++ [return_stmt])
693 [(n, HsVar id) | (n,id) <- ds_meths] -- A bit of a hack
694 (mkAppTy m_ty tup_ty)
697 Var return_id = lookupReboundName ds_meths returnMName
698 Var mfix_id = lookupReboundName ds_meths mfixName
700 return_stmt = ResultStmt return_app noSrcLoc
701 return_app = HsApp (TyApp (HsVar return_id) [tup_ty]) tup_expr