2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
4 \section[DsExpr]{Matching expressions (Exprs)}
7 #include "HsVersions.h"
9 module DsExpr ( dsExpr ) where
12 IMPORT_DELOOPER(DsLoop) -- partly to get dsBinds, partly to chk dsExpr
14 import HsSyn ( failureFreePat,
15 HsExpr(..), OutPat(..), HsLit(..), ArithSeqInfo(..),
16 Stmt(..), Match(..), Qualifier, HsBinds, PolyType,
19 import TcHsSyn ( SYN_IE(TypecheckedHsExpr), SYN_IE(TypecheckedHsBinds),
20 SYN_IE(TypecheckedRecordBinds), SYN_IE(TypecheckedPat),
21 SYN_IE(TypecheckedStmt)
26 import DsCCall ( dsCCall )
27 import DsHsSyn ( outPatType )
28 import DsListComp ( dsListComp )
29 import DsUtils ( mkAppDs, mkConDs, mkPrimDs, dsExprToAtom,
30 mkErrorAppDs, showForErr, EquationInfo,
31 MatchResult, SYN_IE(DsCoreArg)
33 import Match ( matchWrapper )
35 import CoreUnfold ( Unfolding )
36 import CoreUtils ( coreExprType, substCoreExpr, argToExpr,
37 mkCoreIfThenElse, unTagBinders )
38 import CostCentre ( mkUserCC )
39 import FieldLabel ( fieldLabelType, FieldLabel )
40 import Id ( mkTupleCon, idType, nullIdEnv, addOneToIdEnv,
41 getIdUnfolding, dataConArgTys, dataConFieldLabels,
42 recordSelectorFieldLabel
44 import Literal ( mkMachInt, Literal(..) )
45 import MagicUFs ( MagicUnfoldingFun )
46 import Name ( Name{--O only-} )
47 import PprStyle ( PprStyle(..) )
48 import PprType ( GenType )
49 import PrelVals ( rEC_CON_ERROR_ID, rEC_UPD_ERROR_ID, voidId )
50 import Pretty ( ppShow, ppBesides, ppPStr, ppStr )
51 import TyCon ( isDataTyCon, isNewTyCon )
52 import Type ( splitSigmaTy, splitFunTy, typePrimRep,
53 getAppDataTyConExpandingDicts, getAppTyCon, applyTy,
56 import TysPrim ( voidTy )
57 import TysWiredIn ( mkTupleTy, nilDataCon, consDataCon,
60 import TyVar ( nullTyVarEnv, addOneToTyVarEnv, GenTyVar{-instance Eq-} )
61 import Usage ( SYN_IE(UVar) )
62 import Util ( zipEqual, pprError, panic, assertPanic )
64 mk_nil_con ty = mkCon nilDataCon [] [ty] [] -- micro utility...
67 The funny business to do with variables is that we look them up in the
68 Id-to-Id and Id-to-Id maps that the monadery is carrying
69 around; if we get hits, we use the value accordingly.
71 %************************************************************************
73 \subsection[DsExpr-vars-and-cons]{Variables and constructors}
75 %************************************************************************
78 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
80 dsExpr e@(HsVar var) = dsApp e []
83 %************************************************************************
85 \subsection[DsExpr-literals]{Literals}
87 %************************************************************************
89 We give int/float literals type Integer and Rational, respectively.
90 The typechecker will (presumably) have put \tr{from{Integer,Rational}s}
93 ToDo: put in range checks for when converting "i"
94 (or should that be in the typechecker?)
96 For numeric literals, we try to detect there use at a standard type
97 (Int, Float, etc.) are directly put in the right constructor.
98 [NB: down with the @App@ conversion.]
99 Otherwise, we punt, putting in a "NoRep" Core literal (where the
100 representation decisions are delayed)...
102 See also below where we look for @DictApps@ for \tr{plusInt}, etc.
105 dsExpr (HsLitOut (HsString s) _)
107 = returnDs (mk_nil_con charTy)
111 the_char = mkCon charDataCon [] [] [LitArg (MachChar (_HEAD_ s))]
112 the_nil = mk_nil_con charTy
114 mkConDs consDataCon [TyArg charTy, VarArg the_char, VarArg the_nil]
116 -- "_" => build (\ c n -> c 'c' n) -- LATER
118 -- "str" ==> build (\ c n -> foldr charTy T c n "str")
121 dsExpr (HsLitOut (HsString str) _)
122 = newTyVarsDs [alphaTyVar] `thenDs` \ [new_tyvar] ->
124 new_ty = mkTyVarTy new_tyvar
127 charTy `mkFunTy` (new_ty `mkFunTy` new_ty),
129 mkForallTy [alphaTyVar]
130 ((charTy `mkFunTy` (alphaTy `mkFunTy` alphaTy))
131 `mkFunTy` (alphaTy `mkFunTy` alphaTy))
132 ] `thenDs` \ [c,n,g] ->
133 returnDs (mkBuild charTy new_tyvar c n g (
135 (CoTyApp (CoTyApp (Var foldrId) charTy) new_ty) *** ensure non-prim type ***
136 [VarArg c,VarArg n,LitArg (NoRepStr str)]))
139 -- otherwise, leave it as a NoRepStr;
140 -- the Core-to-STG pass will wrap it in an application of "unpackCStringId".
142 dsExpr (HsLitOut (HsString str) _)
143 = returnDs (Lit (NoRepStr str))
145 dsExpr (HsLitOut (HsLitLit s) ty)
146 = returnDs ( mkCon data_con [] [] [LitArg (MachLitLit s kind)] )
149 = case (maybeBoxedPrimType ty) of
150 Just (boxing_data_con, prim_ty)
151 -> (boxing_data_con, typePrimRep prim_ty)
153 -> pprError "ERROR: ``literal-literal'' not a single-constructor type: "
154 (ppBesides [ppPStr s, ppStr "; type: ", ppr PprDebug ty])
156 dsExpr (HsLitOut (HsInt i) ty)
157 = returnDs (Lit (NoRepInteger i ty))
159 dsExpr (HsLitOut (HsFrac r) ty)
160 = returnDs (Lit (NoRepRational r ty))
162 -- others where we know what to do:
164 dsExpr (HsLitOut (HsIntPrim i) _)
165 = if (i >= toInteger minInt && i <= toInteger maxInt) then
166 returnDs (Lit (mkMachInt i))
168 error ("ERROR: Int constant " ++ show i ++ out_of_range_msg)
170 dsExpr (HsLitOut (HsFloatPrim f) _)
171 = returnDs (Lit (MachFloat f))
172 -- ToDo: range checking needed!
174 dsExpr (HsLitOut (HsDoublePrim d) _)
175 = returnDs (Lit (MachDouble d))
176 -- ToDo: range checking needed!
178 dsExpr (HsLitOut (HsChar c) _)
179 = returnDs ( mkCon charDataCon [] [] [LitArg (MachChar c)] )
181 dsExpr (HsLitOut (HsCharPrim c) _)
182 = returnDs (Lit (MachChar c))
184 dsExpr (HsLitOut (HsStringPrim s) _)
185 = returnDs (Lit (MachStr s))
187 -- end of literals magic. --
189 dsExpr expr@(HsLam a_Match)
190 = matchWrapper LambdaMatch [a_Match] "lambda" `thenDs` \ (binders, matching_code) ->
191 returnDs ( mkValLam binders matching_code )
193 dsExpr expr@(HsApp e1 e2) = dsApp expr []
194 dsExpr expr@(OpApp e1 op e2) = dsApp expr []
197 Operator sections. At first it looks as if we can convert
206 But no! expr might be a redex, and we can lose laziness badly this
211 for example. So we convert instead to
213 let y = expr in \x -> op y x
215 If \tr{expr} is actually just a variable, say, then the simplifier
219 dsExpr (SectionL expr op)
220 = dsExpr op `thenDs` \ core_op ->
221 dsExpr expr `thenDs` \ core_expr ->
222 dsExprToAtom (VarArg core_expr) $ \ y_atom ->
224 -- for the type of x, we need the type of op's 2nd argument
226 x_ty = case (splitSigmaTy (coreExprType core_op)) of { (_, _, tau_ty) ->
227 case (splitFunTy tau_ty) of {
228 ((_:arg2_ty:_), _) -> arg2_ty;
229 _ -> panic "dsExpr:SectionL:arg 2 ty" }}
231 newSysLocalDs x_ty `thenDs` \ x_id ->
232 returnDs (mkValLam [x_id] (core_op `App` y_atom `App` VarArg x_id))
234 -- dsExpr (SectionR op expr) -- \ x -> op x expr
235 dsExpr (SectionR op expr)
236 = dsExpr op `thenDs` \ core_op ->
237 dsExpr expr `thenDs` \ core_expr ->
238 dsExprToAtom (VarArg core_expr) $ \ y_atom ->
240 -- for the type of x, we need the type of op's 1st argument
242 x_ty = case (splitSigmaTy (coreExprType core_op)) of { (_, _, tau_ty) ->
243 case (splitFunTy tau_ty) of {
244 ((arg1_ty:_), _) -> arg1_ty;
245 _ -> panic "dsExpr:SectionR:arg 1 ty" }}
247 newSysLocalDs x_ty `thenDs` \ x_id ->
248 returnDs (mkValLam [x_id] (core_op `App` VarArg x_id `App` y_atom))
250 dsExpr (CCall label args may_gc is_asm result_ty)
251 = mapDs dsExpr args `thenDs` \ core_args ->
252 dsCCall label core_args may_gc is_asm result_ty
253 -- dsCCall does all the unboxification, etc.
255 dsExpr (HsSCC cc expr)
256 = dsExpr expr `thenDs` \ core_expr ->
257 getModuleAndGroupDs `thenDs` \ (mod_name, group_name) ->
258 returnDs ( SCC (mkUserCC cc mod_name group_name) core_expr)
260 dsExpr expr@(HsCase discrim matches src_loc)
261 = putSrcLocDs src_loc $
262 dsExpr discrim `thenDs` \ core_discrim ->
263 matchWrapper CaseMatch matches "case" `thenDs` \ ([discrim_var], matching_code) ->
264 returnDs ( mkCoLetAny (NonRec discrim_var core_discrim) matching_code )
266 dsExpr (ListComp expr quals)
267 = dsExpr expr `thenDs` \ core_expr ->
268 dsListComp core_expr quals
270 dsExpr (HsLet binds expr)
271 = dsBinds False binds `thenDs` \ core_binds ->
272 dsExpr expr `thenDs` \ core_expr ->
273 returnDs ( mkCoLetsAny core_binds core_expr )
275 dsExpr (HsDoOut stmts then_id zero_id src_loc)
276 = putSrcLocDs src_loc $
277 dsDo then_id zero_id stmts
279 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
280 = putSrcLocDs src_loc $
281 dsExpr guard_expr `thenDs` \ core_guard ->
282 dsExpr then_expr `thenDs` \ core_then ->
283 dsExpr else_expr `thenDs` \ core_else ->
284 returnDs (mkCoreIfThenElse core_guard core_then core_else)
288 Type lambda and application
289 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
291 dsExpr (TyLam tyvars expr)
292 = dsExpr expr `thenDs` \ core_expr ->
293 returnDs (mkTyLam tyvars core_expr)
295 dsExpr expr@(TyApp e tys) = dsApp expr []
299 Various data construction things
300 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
302 dsExpr (ExplicitListOut ty xs)
304 [] -> returnDs (mk_nil_con ty)
306 dsExpr y `thenDs` \ core_hd ->
307 dsExpr (ExplicitListOut ty ys) `thenDs` \ core_tl ->
308 mkConDs consDataCon [TyArg ty, VarArg core_hd, VarArg core_tl]
310 dsExpr (ExplicitTuple expr_list)
311 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
312 mkConDs (mkTupleCon (length expr_list))
313 (map (TyArg . coreExprType) core_exprs ++ map VarArg core_exprs)
315 -- Two cases, one for ordinary constructors and one for newtype constructors
316 dsExpr (HsCon con tys args)
317 | isDataTyCon tycon -- The usual datatype case
318 = mapDs dsExpr args `thenDs` \ args_exprs ->
319 mkConDs con (map TyArg tys ++ map VarArg args_exprs)
321 | otherwise -- The newtype case
322 = ASSERT( isNewTyCon tycon )
323 ASSERT( null rest_args )
324 dsExpr first_arg `thenDs` \ arg_expr ->
325 returnDs (Coerce (CoerceIn con) result_ty arg_expr)
328 (first_arg:rest_args) = args
329 (args_tys, result_ty) = splitFunTy (foldl applyTy (idType con) tys)
330 (tycon,_) = getAppTyCon result_ty
332 dsExpr (ArithSeqOut expr (From from))
333 = dsExpr expr `thenDs` \ expr2 ->
334 dsExpr from `thenDs` \ from2 ->
335 mkAppDs expr2 [VarArg from2]
337 dsExpr (ArithSeqOut expr (FromTo from two))
338 = dsExpr expr `thenDs` \ expr2 ->
339 dsExpr from `thenDs` \ from2 ->
340 dsExpr two `thenDs` \ two2 ->
341 mkAppDs expr2 [VarArg from2, VarArg two2]
343 dsExpr (ArithSeqOut expr (FromThen from thn))
344 = dsExpr expr `thenDs` \ expr2 ->
345 dsExpr from `thenDs` \ from2 ->
346 dsExpr thn `thenDs` \ thn2 ->
347 mkAppDs expr2 [VarArg from2, VarArg thn2]
349 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
350 = dsExpr expr `thenDs` \ expr2 ->
351 dsExpr from `thenDs` \ from2 ->
352 dsExpr thn `thenDs` \ thn2 ->
353 dsExpr two `thenDs` \ two2 ->
354 mkAppDs expr2 [VarArg from2, VarArg thn2, VarArg two2]
357 Record construction and update
358 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
359 For record construction we do this (assuming T has three arguments)
363 let err = /\a -> recConErr a
364 T (recConErr t1 "M.lhs/230/op1")
366 (recConErr t1 "M.lhs/230/op3")
368 recConErr then converts its arugment string into a proper message
369 before printing it as
371 M.lhs, line 230: missing field op1 was evaluated
375 dsExpr (RecordCon con_expr rbinds)
376 = dsExpr con_expr `thenDs` \ con_expr' ->
378 con_id = get_con con_expr'
379 (arg_tys, _) = splitFunTy (coreExprType con_expr')
382 = case [rhs | (sel_id,rhs,_) <- rbinds,
383 lbl == recordSelectorFieldLabel sel_id] of
384 (rhs:rhss) -> ASSERT( null rhss )
386 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showForErr lbl)
388 mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys (dataConFieldLabels con_id)) `thenDs` \ con_args ->
389 mkAppDs con_expr' (map VarArg con_args)
391 -- "con_expr'" is simply an application of the constructor Id
392 -- to types and (perhaps) dictionaries. This gets the constructor...
393 get_con (Var con) = con
394 get_con (App fun _) = get_con fun
397 Record update is a little harder. Suppose we have the decl:
399 data T = T1 {op1, op2, op3 :: Int}
400 | T2 {op4, op2 :: Int}
403 Then we translate as follows:
409 T1 op1 _ op3 -> T1 op1 op2 op3
410 T2 op4 _ -> T2 op4 op2
411 other -> recUpdError "M.lhs/230"
413 It's important that we use the constructor Ids for T1, T2 etc on the
414 RHSs, and do not generate a Core Con directly, because the constructor
415 might do some argument-evaluation first; and may have to throw away some
419 dsExpr (RecordUpdOut record_expr dicts rbinds)
420 = dsExpr record_expr `thenDs` \ record_expr' ->
422 -- Desugar the rbinds, and generate let-bindings if
423 -- necessary so that we don't lose sharing
424 dsRbinds rbinds $ \ rbinds' ->
426 record_ty = coreExprType record_expr'
427 (tycon, inst_tys, cons) = --trace "DsExpr.getAppDataTyConExpandingDicts" $
428 getAppDataTyConExpandingDicts record_ty
429 cons_to_upd = filter has_all_fields cons
431 -- initial_args are passed to every constructor
432 initial_args = map TyArg inst_tys ++ map VarArg dicts
434 mk_val_arg (field, arg_id)
435 = case [arg | (f, arg) <- rbinds',
436 field == recordSelectorFieldLabel f] of
437 (arg:args) -> ASSERT(null args)
442 = newSysLocalsDs (dataConArgTys con inst_tys) `thenDs` \ arg_ids ->
444 val_args = map mk_val_arg (zipEqual "dsExpr:RecordUpd" (dataConFieldLabels con) arg_ids)
446 returnDs (con, arg_ids, mkGenApp (mkGenApp (Var con) initial_args) val_args)
449 | length cons_to_upd == length cons
452 = newSysLocalDs record_ty `thenDs` \ deflt_id ->
453 mkErrorAppDs rEC_UPD_ERROR_ID record_ty "" `thenDs` \ err ->
454 returnDs (BindDefault deflt_id err)
456 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
457 mk_default `thenDs` \ deflt ->
459 returnDs (Case record_expr' (AlgAlts alts deflt))
462 has_all_fields :: Id -> Bool
463 has_all_fields con_id
466 con_fields = dataConFieldLabels con_id
467 ok (sel_id, _, _) = recordSelectorFieldLabel sel_id `elem` con_fields
470 Dictionary lambda and application
471 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
472 @DictLam@ and @DictApp@ turn into the regular old things.
473 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
474 complicated; reminiscent of fully-applied constructors.
476 dsExpr (DictLam dictvars expr)
477 = dsExpr expr `thenDs` \ core_expr ->
478 returnDs( mkValLam dictvars core_expr )
482 dsExpr expr@(DictApp e dicts) -- becomes a curried application
486 @SingleDicts@ become @Locals@; @Dicts@ turn into tuples, unless
488 @ClassDictLam dictvars methods expr@ is ``the opposite'':
490 \ x -> case x of ( dictvars-and-methods-tuple ) -> expr
493 dsExpr (SingleDict dict) -- just a local
494 = lookupEnvWithDefaultDs dict (Var dict)
496 dsExpr (Dictionary dicts methods)
497 = -- hey, these things may have been substituted away...
498 zipWithDs lookupEnvWithDefaultDs
499 dicts_and_methods dicts_and_methods_exprs
500 `thenDs` \ core_d_and_ms ->
502 (case num_of_d_and_ms of
503 0 -> returnDs (Var voidId)
505 1 -> returnDs (head core_d_and_ms) -- just a single Id
508 mkConDs (mkTupleCon num_of_d_and_ms)
509 (map (TyArg . coreExprType) core_d_and_ms ++ map VarArg core_d_and_ms)
512 dicts_and_methods = dicts ++ methods
513 dicts_and_methods_exprs = map Var dicts_and_methods
514 num_of_d_and_ms = length dicts_and_methods
516 dsExpr (ClassDictLam dicts methods expr)
517 = dsExpr expr `thenDs` \ core_expr ->
518 case num_of_d_and_ms of
519 0 -> newSysLocalDs voidTy `thenDs` \ new_x ->
520 returnDs (mkValLam [new_x] core_expr)
523 returnDs (mkValLam dicts_and_methods core_expr)
526 newSysLocalDs tuple_ty `thenDs` \ new_x ->
528 Lam (ValBinder new_x)
531 [(tuple_con, dicts_and_methods, core_expr)]
534 num_of_d_and_ms = length dicts + length methods
535 dicts_and_methods = dicts ++ methods
536 tuple_ty = mkTupleTy num_of_d_and_ms (map idType dicts_and_methods)
537 tuple_con = mkTupleCon num_of_d_and_ms
540 -- HsSyn constructs that just shouldn't be here:
541 dsExpr (HsDo _ _) = panic "dsExpr:HsDo"
542 dsExpr (ExplicitList _) = panic "dsExpr:ExplicitList"
543 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
544 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
547 out_of_range_msg -- ditto
548 = " out of range: [" ++ show minInt ++ ", " ++ show maxInt ++ "]\n"
551 %--------------------------------------------------------------------
553 @(dsApp e [t_1,..,t_n, e_1,..,e_n])@ returns something with the same
556 e t_1 ... t_n e_1 .. e_n
559 We're doing all this so we can saturate constructors (as painlessly as
563 dsApp :: TypecheckedHsExpr -- expr to desugar
564 -> [DsCoreArg] -- accumulated ty/val args: NB:
565 -> DsM CoreExpr -- final result
567 dsApp (HsApp e1 e2) args
568 = dsExpr e2 `thenDs` \ core_e2 ->
569 dsApp e1 (VarArg core_e2 : args)
571 dsApp (OpApp e1 op e2) args
572 = dsExpr e1 `thenDs` \ core_e1 ->
573 dsExpr e2 `thenDs` \ core_e2 ->
574 dsApp op (VarArg core_e1 : VarArg core_e2 : args)
576 dsApp (DictApp expr dicts) args
577 = -- now, those dicts may have been substituted away...
578 zipWithDs lookupEnvWithDefaultDs dicts (map Var dicts)
579 `thenDs` \ core_dicts ->
580 dsApp expr (map VarArg core_dicts ++ args)
582 dsApp (TyApp expr tys) args
583 = dsApp expr (map TyArg tys ++ args)
585 -- we might should look out for SectionLs, etc., here, but we don't
588 = lookupEnvDs v `thenDs` \ maybe_expr ->
589 mkAppDs (case maybe_expr of { Nothing -> Var v; Just expr -> expr }) args
591 dsApp anything_else args
592 = dsExpr anything_else `thenDs` \ core_expr ->
593 mkAppDs core_expr args
597 dsRbinds :: TypecheckedRecordBinds -- The field bindings supplied
598 -> ([(Id, CoreArg)] -> DsM CoreExpr) -- A continuation taking the field
599 -- bindings with atomic rhss
600 -> DsM CoreExpr -- The result of the continuation,
601 -- wrapped in suitable Lets
603 dsRbinds [] continue_with
606 dsRbinds ((sel_id, rhs, pun_flag) : rbinds) continue_with
607 = dsExpr rhs `thenDs` \ rhs' ->
608 dsExprToAtom (VarArg rhs') $ \ rhs_atom ->
609 dsRbinds rbinds $ \ rbinds' ->
610 continue_with ((sel_id, rhs_atom) : rbinds')
614 -- do_unfold ty_env val_env (Lam (TyBinder tyvar) body) (TyArg ty : args)
615 -- = do_unfold (addOneToTyVarEnv ty_env tyvar ty) val_env body args
617 -- do_unfold ty_env val_env (Lam (ValBinder binder) body) (arg@(VarArg expr) : args)
618 -- = dsExprToAtom arg $ \ arg_atom ->
620 -- (addOneToIdEnv val_env binder (argToExpr arg_atom))
623 -- do_unfold ty_env val_env body args
624 -- = -- Clone the remaining part of the template
625 -- uniqSMtoDsM (substCoreExpr val_env ty_env body) `thenDs` \ body' ->
627 -- -- Apply result to remaining arguments
628 -- mkAppDs body' args
631 Basically does the translation given in the Haskell~1.3 report:
633 dsDo :: Id -- id for: (>>=) m
634 -> Id -- id for: zero m
638 dsDo then_id zero_id (stmt:stmts)
640 ExprStmt expr locn -> ASSERT( null stmts ) do_expr expr locn
642 ExprStmtOut expr locn a b ->
643 do_expr expr locn `thenDs` \ expr2 ->
644 ds_rest `thenDs` \ rest ->
645 newSysLocalDs a `thenDs` \ ignored_result_id ->
646 dsApp (HsVar then_id) [TyArg a, TyArg b, VarArg expr2,
647 VarArg (mkValLam [ignored_result_id] rest)]
650 dsBinds False binds `thenDs` \ binds2 ->
651 ds_rest `thenDs` \ rest ->
652 returnDs (mkCoLetsAny binds2 rest)
654 BindStmtOut pat expr locn a b ->
655 do_expr expr locn `thenDs` \ expr2 ->
657 zero_expr = TyApp (HsVar zero_id) [b]
659 = PatMatch pat (SimpleMatch (HsDoOut stmts then_id zero_id locn))
661 = if failureFreePat pat
663 else [main_match, PatMatch (WildPat a) (SimpleMatch zero_expr)]
665 matchWrapper DoBindMatch the_matches "`do' statement"
666 `thenDs` \ (binders, matching_code) ->
667 dsApp (HsVar then_id) [TyArg a, TyArg b,
668 VarArg expr2, VarArg (mkValLam binders matching_code)]
670 ds_rest = dsDo then_id zero_id stmts
671 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
674 dsDo then_expr zero_expr [] = panic "dsDo:[]"