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, HsType,
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 CoreUtils ( coreExprType, substCoreExpr, argToExpr,
36 mkCoreIfThenElse, unTagBinders )
37 import CostCentre ( mkUserCC )
38 import FieldLabel ( fieldLabelType, FieldLabel )
39 import Id ( idType, nullIdEnv, addOneToIdEnv,
40 dataConArgTys, dataConFieldLabels,
41 recordSelectorFieldLabel
43 import Literal ( mkMachInt, Literal(..) )
44 import Name ( Name{--O only-} )
45 import PprStyle ( PprStyle(..) )
46 import PprType ( GenType )
47 import PrelVals ( rEC_CON_ERROR_ID, rEC_UPD_ERROR_ID, voidId )
48 import Pretty ( ppShow, ppBesides, ppPStr, ppStr )
49 import TyCon ( isDataTyCon, isNewTyCon )
50 import Type ( splitSigmaTy, splitFunTy, typePrimRep,
51 getAppDataTyConExpandingDicts, getAppTyCon, applyTy,
54 import TysPrim ( voidTy )
55 import TysWiredIn ( mkTupleTy, tupleCon, nilDataCon, consDataCon,
58 import TyVar ( nullTyVarEnv, addOneToTyVarEnv, GenTyVar{-instance Eq-} )
59 import Usage ( SYN_IE(UVar) )
60 import Util ( zipEqual, pprError, panic, assertPanic )
62 mk_nil_con ty = mkCon nilDataCon [] [ty] [] -- micro utility...
65 The funny business to do with variables is that we look them up in the
66 Id-to-Id and Id-to-Id maps that the monadery is carrying
67 around; if we get hits, we use the value accordingly.
69 %************************************************************************
71 \subsection[DsExpr-vars-and-cons]{Variables and constructors}
73 %************************************************************************
76 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
78 dsExpr e@(HsVar var) = dsApp e []
81 %************************************************************************
83 \subsection[DsExpr-literals]{Literals}
85 %************************************************************************
87 We give int/float literals type Integer and Rational, respectively.
88 The typechecker will (presumably) have put \tr{from{Integer,Rational}s}
91 ToDo: put in range checks for when converting "i"
92 (or should that be in the typechecker?)
94 For numeric literals, we try to detect there use at a standard type
95 (Int, Float, etc.) are directly put in the right constructor.
96 [NB: down with the @App@ conversion.]
97 Otherwise, we punt, putting in a "NoRep" Core literal (where the
98 representation decisions are delayed)...
100 See also below where we look for @DictApps@ for \tr{plusInt}, etc.
103 dsExpr (HsLitOut (HsString s) _)
105 = returnDs (mk_nil_con charTy)
109 the_char = mkCon charDataCon [] [] [LitArg (MachChar (_HEAD_ s))]
110 the_nil = mk_nil_con charTy
112 mkConDs consDataCon [TyArg charTy, VarArg the_char, VarArg the_nil]
114 -- "_" => build (\ c n -> c 'c' n) -- LATER
116 -- "str" ==> build (\ c n -> foldr charTy T c n "str")
119 dsExpr (HsLitOut (HsString str) _)
120 = newTyVarsDs [alphaTyVar] `thenDs` \ [new_tyvar] ->
122 new_ty = mkTyVarTy new_tyvar
125 charTy `mkFunTy` (new_ty `mkFunTy` new_ty),
127 mkForallTy [alphaTyVar]
128 ((charTy `mkFunTy` (alphaTy `mkFunTy` alphaTy))
129 `mkFunTy` (alphaTy `mkFunTy` alphaTy))
130 ] `thenDs` \ [c,n,g] ->
131 returnDs (mkBuild charTy new_tyvar c n g (
133 (CoTyApp (CoTyApp (Var foldrId) charTy) new_ty) *** ensure non-prim type ***
134 [VarArg c,VarArg n,LitArg (NoRepStr str)]))
137 -- otherwise, leave it as a NoRepStr;
138 -- the Core-to-STG pass will wrap it in an application of "unpackCStringId".
140 dsExpr (HsLitOut (HsString str) _)
141 = returnDs (Lit (NoRepStr str))
143 dsExpr (HsLitOut (HsLitLit s) ty)
144 = returnDs ( mkCon data_con [] [] [LitArg (MachLitLit s kind)] )
147 = case (maybeBoxedPrimType ty) of
148 Just (boxing_data_con, prim_ty)
149 -> (boxing_data_con, typePrimRep prim_ty)
151 -> pprError "ERROR: ``literal-literal'' not a single-constructor type: "
152 (ppBesides [ppPStr s, ppStr "; type: ", ppr PprDebug ty])
154 dsExpr (HsLitOut (HsInt i) ty)
155 = returnDs (Lit (NoRepInteger i ty))
157 dsExpr (HsLitOut (HsFrac r) ty)
158 = returnDs (Lit (NoRepRational r ty))
160 -- others where we know what to do:
162 dsExpr (HsLitOut (HsIntPrim i) _)
163 = if (i >= toInteger minInt && i <= toInteger maxInt) then
164 returnDs (Lit (mkMachInt i))
166 error ("ERROR: Int constant " ++ show i ++ out_of_range_msg)
168 dsExpr (HsLitOut (HsFloatPrim f) _)
169 = returnDs (Lit (MachFloat f))
170 -- ToDo: range checking needed!
172 dsExpr (HsLitOut (HsDoublePrim d) _)
173 = returnDs (Lit (MachDouble d))
174 -- ToDo: range checking needed!
176 dsExpr (HsLitOut (HsChar c) _)
177 = returnDs ( mkCon charDataCon [] [] [LitArg (MachChar c)] )
179 dsExpr (HsLitOut (HsCharPrim c) _)
180 = returnDs (Lit (MachChar c))
182 dsExpr (HsLitOut (HsStringPrim s) _)
183 = returnDs (Lit (MachStr s))
185 -- end of literals magic. --
187 dsExpr expr@(HsLam a_Match)
188 = matchWrapper LambdaMatch [a_Match] "lambda" `thenDs` \ (binders, matching_code) ->
189 returnDs ( mkValLam binders matching_code )
191 dsExpr expr@(HsApp e1 e2) = dsApp expr []
192 dsExpr expr@(OpApp e1 op e2) = dsApp expr []
195 Operator sections. At first it looks as if we can convert
204 But no! expr might be a redex, and we can lose laziness badly this
209 for example. So we convert instead to
211 let y = expr in \x -> op y x
213 If \tr{expr} is actually just a variable, say, then the simplifier
217 dsExpr (SectionL expr op)
218 = dsExpr op `thenDs` \ core_op ->
219 dsExpr expr `thenDs` \ core_expr ->
220 dsExprToAtom (VarArg core_expr) $ \ y_atom ->
222 -- for the type of x, we need the type of op's 2nd argument
224 x_ty = case (splitSigmaTy (coreExprType core_op)) of { (_, _, tau_ty) ->
225 case (splitFunTy tau_ty) of {
226 ((_:arg2_ty:_), _) -> arg2_ty;
227 _ -> panic "dsExpr:SectionL:arg 2 ty" }}
229 newSysLocalDs x_ty `thenDs` \ x_id ->
230 returnDs (mkValLam [x_id] (core_op `App` y_atom `App` VarArg x_id))
232 -- dsExpr (SectionR op expr) -- \ x -> op x expr
233 dsExpr (SectionR op expr)
234 = dsExpr op `thenDs` \ core_op ->
235 dsExpr expr `thenDs` \ core_expr ->
236 dsExprToAtom (VarArg core_expr) $ \ y_atom ->
238 -- for the type of x, we need the type of op's 1st argument
240 x_ty = case (splitSigmaTy (coreExprType core_op)) of { (_, _, tau_ty) ->
241 case (splitFunTy tau_ty) of {
242 ((arg1_ty:_), _) -> arg1_ty;
243 _ -> panic "dsExpr:SectionR:arg 1 ty" }}
245 newSysLocalDs x_ty `thenDs` \ x_id ->
246 returnDs (mkValLam [x_id] (core_op `App` VarArg x_id `App` y_atom))
248 dsExpr (CCall label args may_gc is_asm result_ty)
249 = mapDs dsExpr args `thenDs` \ core_args ->
250 dsCCall label core_args may_gc is_asm result_ty
251 -- dsCCall does all the unboxification, etc.
253 dsExpr (HsSCC cc expr)
254 = dsExpr expr `thenDs` \ core_expr ->
255 getModuleAndGroupDs `thenDs` \ (mod_name, group_name) ->
256 returnDs ( SCC (mkUserCC cc mod_name group_name) core_expr)
258 dsExpr expr@(HsCase discrim matches src_loc)
259 = putSrcLocDs src_loc $
260 dsExpr discrim `thenDs` \ core_discrim ->
261 matchWrapper CaseMatch matches "case" `thenDs` \ ([discrim_var], matching_code) ->
262 returnDs ( mkCoLetAny (NonRec discrim_var core_discrim) matching_code )
264 dsExpr (ListComp expr quals)
265 = dsExpr expr `thenDs` \ core_expr ->
266 dsListComp core_expr quals
268 dsExpr (HsLet binds expr)
269 = dsBinds False binds `thenDs` \ core_binds ->
270 dsExpr expr `thenDs` \ core_expr ->
271 returnDs ( mkCoLetsAny core_binds core_expr )
273 dsExpr (HsDoOut stmts then_id zero_id src_loc)
274 = putSrcLocDs src_loc $
275 dsDo then_id zero_id stmts
277 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
278 = putSrcLocDs src_loc $
279 dsExpr guard_expr `thenDs` \ core_guard ->
280 dsExpr then_expr `thenDs` \ core_then ->
281 dsExpr else_expr `thenDs` \ core_else ->
282 returnDs (mkCoreIfThenElse core_guard core_then core_else)
286 Type lambda and application
287 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
289 dsExpr (TyLam tyvars expr)
290 = dsExpr expr `thenDs` \ core_expr ->
291 returnDs (mkTyLam tyvars core_expr)
293 dsExpr expr@(TyApp e tys) = dsApp expr []
297 Various data construction things
298 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
300 dsExpr (ExplicitListOut ty xs)
302 [] -> returnDs (mk_nil_con ty)
304 dsExpr y `thenDs` \ core_hd ->
305 dsExpr (ExplicitListOut ty ys) `thenDs` \ core_tl ->
306 mkConDs consDataCon [TyArg ty, VarArg core_hd, VarArg core_tl]
308 dsExpr (ExplicitTuple expr_list)
309 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
310 mkConDs (tupleCon (length expr_list))
311 (map (TyArg . coreExprType) core_exprs ++ map VarArg core_exprs)
313 -- Two cases, one for ordinary constructors and one for newtype constructors
314 dsExpr (HsCon con tys args)
315 | isDataTyCon tycon -- The usual datatype case
316 = mapDs dsExpr args `thenDs` \ args_exprs ->
317 mkConDs con (map TyArg tys ++ map VarArg args_exprs)
319 | otherwise -- The newtype case
320 = ASSERT( isNewTyCon tycon )
321 ASSERT( null rest_args )
322 dsExpr first_arg `thenDs` \ arg_expr ->
323 returnDs (Coerce (CoerceIn con) result_ty arg_expr)
326 (first_arg:rest_args) = args
327 (args_tys, result_ty) = splitFunTy (foldl applyTy (idType con) tys)
328 (tycon,_) = getAppTyCon result_ty
330 dsExpr (ArithSeqOut expr (From from))
331 = dsExpr expr `thenDs` \ expr2 ->
332 dsExpr from `thenDs` \ from2 ->
333 mkAppDs expr2 [VarArg from2]
335 dsExpr (ArithSeqOut expr (FromTo from two))
336 = dsExpr expr `thenDs` \ expr2 ->
337 dsExpr from `thenDs` \ from2 ->
338 dsExpr two `thenDs` \ two2 ->
339 mkAppDs expr2 [VarArg from2, VarArg two2]
341 dsExpr (ArithSeqOut expr (FromThen from thn))
342 = dsExpr expr `thenDs` \ expr2 ->
343 dsExpr from `thenDs` \ from2 ->
344 dsExpr thn `thenDs` \ thn2 ->
345 mkAppDs expr2 [VarArg from2, VarArg thn2]
347 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
348 = dsExpr expr `thenDs` \ expr2 ->
349 dsExpr from `thenDs` \ from2 ->
350 dsExpr thn `thenDs` \ thn2 ->
351 dsExpr two `thenDs` \ two2 ->
352 mkAppDs expr2 [VarArg from2, VarArg thn2, VarArg two2]
355 Record construction and update
356 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
357 For record construction we do this (assuming T has three arguments)
361 let err = /\a -> recConErr a
362 T (recConErr t1 "M.lhs/230/op1")
364 (recConErr t1 "M.lhs/230/op3")
366 recConErr then converts its arugment string into a proper message
367 before printing it as
369 M.lhs, line 230: missing field op1 was evaluated
373 dsExpr (RecordCon con_expr rbinds)
374 = dsExpr con_expr `thenDs` \ con_expr' ->
376 con_id = get_con con_expr'
377 (arg_tys, _) = splitFunTy (coreExprType con_expr')
380 = case [rhs | (sel_id,rhs,_) <- rbinds,
381 lbl == recordSelectorFieldLabel sel_id] of
382 (rhs:rhss) -> ASSERT( null rhss )
384 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showForErr lbl)
386 mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys (dataConFieldLabels con_id)) `thenDs` \ con_args ->
387 mkAppDs con_expr' (map VarArg con_args)
389 -- "con_expr'" is simply an application of the constructor Id
390 -- to types and (perhaps) dictionaries. This gets the constructor...
391 get_con (Var con) = con
392 get_con (App fun _) = get_con fun
395 Record update is a little harder. Suppose we have the decl:
397 data T = T1 {op1, op2, op3 :: Int}
398 | T2 {op4, op2 :: Int}
401 Then we translate as follows:
407 T1 op1 _ op3 -> T1 op1 op2 op3
408 T2 op4 _ -> T2 op4 op2
409 other -> recUpdError "M.lhs/230"
411 It's important that we use the constructor Ids for T1, T2 etc on the
412 RHSs, and do not generate a Core Con directly, because the constructor
413 might do some argument-evaluation first; and may have to throw away some
417 dsExpr (RecordUpdOut record_expr dicts rbinds)
418 = dsExpr record_expr `thenDs` \ record_expr' ->
420 -- Desugar the rbinds, and generate let-bindings if
421 -- necessary so that we don't lose sharing
422 dsRbinds rbinds $ \ rbinds' ->
424 record_ty = coreExprType record_expr'
425 (tycon, inst_tys, cons) = --trace "DsExpr.getAppDataTyConExpandingDicts" $
426 getAppDataTyConExpandingDicts record_ty
427 cons_to_upd = filter has_all_fields cons
429 -- initial_args are passed to every constructor
430 initial_args = map TyArg inst_tys ++ map VarArg dicts
432 mk_val_arg (field, arg_id)
433 = case [arg | (f, arg) <- rbinds',
434 field == recordSelectorFieldLabel f] of
435 (arg:args) -> ASSERT(null args)
440 = newSysLocalsDs (dataConArgTys con inst_tys) `thenDs` \ arg_ids ->
442 val_args = map mk_val_arg (zipEqual "dsExpr:RecordUpd" (dataConFieldLabels con) arg_ids)
444 returnDs (con, arg_ids, mkGenApp (mkGenApp (Var con) initial_args) val_args)
447 | length cons_to_upd == length cons
450 = newSysLocalDs record_ty `thenDs` \ deflt_id ->
451 mkErrorAppDs rEC_UPD_ERROR_ID record_ty "" `thenDs` \ err ->
452 returnDs (BindDefault deflt_id err)
454 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
455 mk_default `thenDs` \ deflt ->
457 returnDs (Case record_expr' (AlgAlts alts deflt))
460 has_all_fields :: Id -> Bool
461 has_all_fields con_id
464 con_fields = dataConFieldLabels con_id
465 ok (sel_id, _, _) = recordSelectorFieldLabel sel_id `elem` con_fields
468 Dictionary lambda and application
469 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
470 @DictLam@ and @DictApp@ turn into the regular old things.
471 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
472 complicated; reminiscent of fully-applied constructors.
474 dsExpr (DictLam dictvars expr)
475 = dsExpr expr `thenDs` \ core_expr ->
476 returnDs( mkValLam dictvars core_expr )
480 dsExpr expr@(DictApp e dicts) -- becomes a curried application
484 @SingleDicts@ become @Locals@; @Dicts@ turn into tuples, unless
486 @ClassDictLam dictvars methods expr@ is ``the opposite'':
488 \ x -> case x of ( dictvars-and-methods-tuple ) -> expr
491 dsExpr (SingleDict dict) -- just a local
492 = lookupEnvWithDefaultDs dict (Var dict)
494 dsExpr (Dictionary dicts methods)
495 = -- hey, these things may have been substituted away...
496 zipWithDs lookupEnvWithDefaultDs
497 dicts_and_methods dicts_and_methods_exprs
498 `thenDs` \ core_d_and_ms ->
500 (case num_of_d_and_ms of
501 0 -> returnDs (Var voidId)
503 1 -> returnDs (head core_d_and_ms) -- just a single Id
506 mkConDs (tupleCon num_of_d_and_ms)
507 (map (TyArg . coreExprType) core_d_and_ms ++ map VarArg core_d_and_ms)
510 dicts_and_methods = dicts ++ methods
511 dicts_and_methods_exprs = map Var dicts_and_methods
512 num_of_d_and_ms = length dicts_and_methods
514 dsExpr (ClassDictLam dicts methods expr)
515 = dsExpr expr `thenDs` \ core_expr ->
516 case num_of_d_and_ms of
517 0 -> newSysLocalDs voidTy `thenDs` \ new_x ->
518 returnDs (mkValLam [new_x] core_expr)
521 returnDs (mkValLam dicts_and_methods core_expr)
524 newSysLocalDs tuple_ty `thenDs` \ new_x ->
526 Lam (ValBinder new_x)
529 [(tuple_con, dicts_and_methods, core_expr)]
532 num_of_d_and_ms = length dicts + length methods
533 dicts_and_methods = dicts ++ methods
534 tuple_ty = mkTupleTy num_of_d_and_ms (map idType dicts_and_methods)
535 tuple_con = tupleCon num_of_d_and_ms
538 -- HsSyn constructs that just shouldn't be here:
539 dsExpr (HsDo _ _) = panic "dsExpr:HsDo"
540 dsExpr (ExplicitList _) = panic "dsExpr:ExplicitList"
541 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
542 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
545 out_of_range_msg -- ditto
546 = " out of range: [" ++ show minInt ++ ", " ++ show maxInt ++ "]\n"
549 %--------------------------------------------------------------------
551 @(dsApp e [t_1,..,t_n, e_1,..,e_n])@ returns something with the same
554 e t_1 ... t_n e_1 .. e_n
557 We're doing all this so we can saturate constructors (as painlessly as
561 dsApp :: TypecheckedHsExpr -- expr to desugar
562 -> [DsCoreArg] -- accumulated ty/val args: NB:
563 -> DsM CoreExpr -- final result
565 dsApp (HsApp e1 e2) args
566 = dsExpr e2 `thenDs` \ core_e2 ->
567 dsApp e1 (VarArg core_e2 : args)
569 dsApp (OpApp e1 op e2) args
570 = dsExpr e1 `thenDs` \ core_e1 ->
571 dsExpr e2 `thenDs` \ core_e2 ->
572 dsApp op (VarArg core_e1 : VarArg core_e2 : args)
574 dsApp (DictApp expr dicts) args
575 = -- now, those dicts may have been substituted away...
576 zipWithDs lookupEnvWithDefaultDs dicts (map Var dicts)
577 `thenDs` \ core_dicts ->
578 dsApp expr (map VarArg core_dicts ++ args)
580 dsApp (TyApp expr tys) args
581 = dsApp expr (map TyArg tys ++ args)
583 -- we might should look out for SectionLs, etc., here, but we don't
586 = lookupEnvDs v `thenDs` \ maybe_expr ->
587 mkAppDs (case maybe_expr of { Nothing -> Var v; Just expr -> expr }) args
589 dsApp anything_else args
590 = dsExpr anything_else `thenDs` \ core_expr ->
591 mkAppDs core_expr args
595 dsRbinds :: TypecheckedRecordBinds -- The field bindings supplied
596 -> ([(Id, CoreArg)] -> DsM CoreExpr) -- A continuation taking the field
597 -- bindings with atomic rhss
598 -> DsM CoreExpr -- The result of the continuation,
599 -- wrapped in suitable Lets
601 dsRbinds [] continue_with
604 dsRbinds ((sel_id, rhs, pun_flag) : rbinds) continue_with
605 = dsExpr rhs `thenDs` \ rhs' ->
606 dsExprToAtom (VarArg rhs') $ \ rhs_atom ->
607 dsRbinds rbinds $ \ rbinds' ->
608 continue_with ((sel_id, rhs_atom) : rbinds')
612 -- do_unfold ty_env val_env (Lam (TyBinder tyvar) body) (TyArg ty : args)
613 -- = do_unfold (addOneToTyVarEnv ty_env tyvar ty) val_env body args
615 -- do_unfold ty_env val_env (Lam (ValBinder binder) body) (arg@(VarArg expr) : args)
616 -- = dsExprToAtom arg $ \ arg_atom ->
618 -- (addOneToIdEnv val_env binder (argToExpr arg_atom))
621 -- do_unfold ty_env val_env body args
622 -- = -- Clone the remaining part of the template
623 -- uniqSMtoDsM (substCoreExpr val_env ty_env body) `thenDs` \ body' ->
625 -- -- Apply result to remaining arguments
626 -- mkAppDs body' args
629 Basically does the translation given in the Haskell~1.3 report:
631 dsDo :: Id -- id for: (>>=) m
632 -> Id -- id for: zero m
636 dsDo then_id zero_id (stmt:stmts)
638 ExprStmt expr locn -> ASSERT( null stmts ) do_expr expr locn
640 ExprStmtOut expr locn a b ->
641 do_expr expr locn `thenDs` \ expr2 ->
642 ds_rest `thenDs` \ rest ->
643 newSysLocalDs a `thenDs` \ ignored_result_id ->
644 dsApp (HsVar then_id) [TyArg a, TyArg b, VarArg expr2,
645 VarArg (mkValLam [ignored_result_id] rest)]
648 dsBinds False binds `thenDs` \ binds2 ->
649 ds_rest `thenDs` \ rest ->
650 returnDs (mkCoLetsAny binds2 rest)
652 BindStmtOut pat expr locn a b ->
653 do_expr expr locn `thenDs` \ expr2 ->
655 zero_expr = TyApp (HsVar zero_id) [b]
657 = PatMatch pat (SimpleMatch (HsDoOut stmts then_id zero_id locn))
659 = if failureFreePat pat
661 else [main_match, PatMatch (WildPat a) (SimpleMatch zero_expr)]
663 matchWrapper DoBindMatch the_matches "`do' statement"
664 `thenDs` \ (binders, matching_code) ->
665 dsApp (HsVar then_id) [TyArg a, TyArg b,
666 VarArg expr2, VarArg (mkValLam binders matching_code)]
668 ds_rest = dsDo then_id zero_id stmts
669 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
672 dsDo then_expr zero_expr [] = panic "dsDo:[]"