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 #if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ <= 201
13 IMPORT_DELOOPER(DsLoop) -- partly to get dsBinds, partly to chk dsExpr
15 import {-# SOURCE #-} DsBinds (dsBinds )
18 import HsSyn ( failureFreePat,
19 HsExpr(..), OutPat(..), HsLit(..), ArithSeqInfo(..),
20 Stmt(..), DoOrListComp(..), Match(..), HsBinds, HsType, Fixity,
23 import TcHsSyn ( SYN_IE(TypecheckedHsExpr), SYN_IE(TypecheckedHsBinds),
24 SYN_IE(TypecheckedRecordBinds), SYN_IE(TypecheckedPat),
25 SYN_IE(TypecheckedStmt)
30 import DsCCall ( dsCCall )
31 import DsHsSyn ( outPatType )
32 import DsListComp ( dsListComp )
33 import DsUtils ( mkAppDs, mkConDs, mkPrimDs, dsExprToAtomGivenTy, mkTupleExpr,
34 mkErrorAppDs, showForErr, EquationInfo,
35 MatchResult, SYN_IE(DsCoreArg)
37 import Match ( matchWrapper )
39 import CoreUtils ( coreExprType, substCoreExpr, argToExpr,
40 mkCoreIfThenElse, unTagBinders )
41 import CostCentre ( mkUserCC )
42 import FieldLabel ( fieldLabelType, FieldLabel )
43 import Id ( idType, nullIdEnv, addOneToIdEnv,
44 dataConArgTys, dataConFieldLabels,
45 recordSelectorFieldLabel, SYN_IE(Id)
47 import Literal ( mkMachInt, Literal(..) )
48 import Name ( Name{--O only-} )
49 import Outputable ( PprStyle(..), Outputable(..) )
50 import PprType ( GenType )
51 import PrelVals ( rEC_CON_ERROR_ID, rEC_UPD_ERROR_ID, voidId )
52 import Pretty ( Doc, hcat, ptext, text )
53 import Type ( splitSigmaTy, splitFunTy, typePrimRep,
54 getAppDataTyConExpandingDicts, maybeAppTyCon, getAppTyCon, applyTy,
55 maybeBoxedPrimType, splitAppTy, SYN_IE(Type)
57 import TysPrim ( voidTy )
58 import TysWiredIn ( mkTupleTy, tupleCon, nilDataCon, consDataCon, listTyCon, mkListTy,
61 import TyVar ( nullTyVarEnv, addOneToTyVarEnv, GenTyVar{-instance Eq-} )
62 import Usage ( SYN_IE(UVar) )
63 import Maybes ( maybeToBool )
64 import Util ( zipEqual, pprError, panic, assertPanic )
66 mk_nil_con ty = mkCon nilDataCon [] [ty] [] -- micro utility...
69 The funny business to do with variables is that we look them up in the
70 Id-to-Id and Id-to-Id maps that the monadery is carrying
71 around; if we get hits, we use the value accordingly.
73 %************************************************************************
75 \subsection[DsExpr-vars-and-cons]{Variables and constructors}
77 %************************************************************************
80 dsExpr :: TypecheckedHsExpr -> DsM CoreExpr
82 dsExpr e@(HsVar var) = dsId var
85 %************************************************************************
87 \subsection[DsExpr-literals]{Literals}
89 %************************************************************************
91 We give int/float literals type Integer and Rational, respectively.
92 The typechecker will (presumably) have put \tr{from{Integer,Rational}s}
95 ToDo: put in range checks for when converting "i"
96 (or should that be in the typechecker?)
98 For numeric literals, we try to detect there use at a standard type
99 (Int, Float, etc.) are directly put in the right constructor.
100 [NB: down with the @App@ conversion.]
101 Otherwise, we punt, putting in a "NoRep" Core literal (where the
102 representation decisions are delayed)...
104 See also below where we look for @DictApps@ for \tr{plusInt}, etc.
107 dsExpr (HsLitOut (HsString s) _)
109 = returnDs (mk_nil_con charTy)
113 the_char = mkCon charDataCon [] [] [LitArg (MachChar (_HEAD_ s))]
114 the_nil = mk_nil_con charTy
116 mkConDs consDataCon [TyArg charTy, VarArg the_char, VarArg the_nil]
118 -- "_" => build (\ c n -> c 'c' n) -- LATER
120 -- "str" ==> build (\ c n -> foldr charTy T c n "str")
123 dsExpr (HsLitOut (HsString str) _)
124 = newTyVarsDs [alphaTyVar] `thenDs` \ [new_tyvar] ->
126 new_ty = mkTyVarTy new_tyvar
129 charTy `mkFunTy` (new_ty `mkFunTy` new_ty),
131 mkForallTy [alphaTyVar]
132 ((charTy `mkFunTy` (alphaTy `mkFunTy` alphaTy))
133 `mkFunTy` (alphaTy `mkFunTy` alphaTy))
134 ] `thenDs` \ [c,n,g] ->
135 returnDs (mkBuild charTy new_tyvar c n g (
137 (CoTyApp (CoTyApp (Var foldrId) charTy) new_ty) *** ensure non-prim type ***
138 [VarArg c,VarArg n,LitArg (NoRepStr str)]))
141 -- otherwise, leave it as a NoRepStr;
142 -- the Core-to-STG pass will wrap it in an application of "unpackCStringId".
144 dsExpr (HsLitOut (HsString str) _)
145 = returnDs (Lit (NoRepStr str))
147 dsExpr (HsLitOut (HsLitLit s) ty)
148 = returnDs ( mkCon data_con [] [] [LitArg (MachLitLit s kind)] )
151 = case (maybeBoxedPrimType ty) of
152 Just (boxing_data_con, prim_ty)
153 -> (boxing_data_con, typePrimRep prim_ty)
155 -> pprError "ERROR: ``literal-literal'' not a single-constructor type: "
156 (hcat [ptext s, text "; type: ", ppr PprDebug ty])
158 dsExpr (HsLitOut (HsInt i) ty)
159 = returnDs (Lit (NoRepInteger i ty))
161 dsExpr (HsLitOut (HsFrac r) ty)
162 = returnDs (Lit (NoRepRational r ty))
164 -- others where we know what to do:
166 dsExpr (HsLitOut (HsIntPrim i) _)
167 = if (i >= toInteger minInt && i <= toInteger maxInt) then
168 returnDs (Lit (mkMachInt i))
170 error ("ERROR: Int constant " ++ show i ++ out_of_range_msg)
172 dsExpr (HsLitOut (HsFloatPrim f) _)
173 = returnDs (Lit (MachFloat f))
174 -- ToDo: range checking needed!
176 dsExpr (HsLitOut (HsDoublePrim d) _)
177 = returnDs (Lit (MachDouble d))
178 -- ToDo: range checking needed!
180 dsExpr (HsLitOut (HsChar c) _)
181 = returnDs ( mkCon charDataCon [] [] [LitArg (MachChar c)] )
183 dsExpr (HsLitOut (HsCharPrim c) _)
184 = returnDs (Lit (MachChar c))
186 dsExpr (HsLitOut (HsStringPrim s) _)
187 = returnDs (Lit (MachStr s))
189 -- end of literals magic. --
191 dsExpr expr@(HsLam a_Match)
192 = matchWrapper LambdaMatch [a_Match] "lambda" `thenDs` \ (binders, matching_code) ->
193 returnDs ( mkValLam binders matching_code )
195 dsExpr expr@(HsApp fun arg)
196 = dsExpr fun `thenDs` \ core_fun ->
197 dsExpr arg `thenDs` \ core_arg ->
198 dsExprToAtomGivenTy core_arg (coreExprType core_arg) $ \ atom_arg ->
199 returnDs (core_fun `App` atom_arg)
203 Operator sections. At first it looks as if we can convert
212 But no! expr might be a redex, and we can lose laziness badly this
217 for example. So we convert instead to
219 let y = expr in \x -> op y x
221 If \tr{expr} is actually just a variable, say, then the simplifier
225 dsExpr (OpApp e1 op _ e2)
226 = dsExpr op `thenDs` \ core_op ->
227 -- for the type of y, we need the type of op's 2nd argument
229 (x_ty:y_ty:_, _) = splitFunTy (coreExprType core_op)
231 dsExpr e1 `thenDs` \ x_core ->
232 dsExpr e2 `thenDs` \ y_core ->
233 dsExprToAtomGivenTy x_core x_ty $ \ x_atom ->
234 dsExprToAtomGivenTy y_core y_ty $ \ y_atom ->
235 returnDs (core_op `App` x_atom `App` y_atom)
237 dsExpr (SectionL expr op)
238 = dsExpr op `thenDs` \ core_op ->
239 -- for the type of y, we need the type of op's 2nd argument
241 (x_ty:y_ty:_, _) = splitFunTy (coreExprType core_op)
243 dsExpr expr `thenDs` \ x_core ->
244 dsExprToAtomGivenTy x_core x_ty $ \ x_atom ->
246 newSysLocalDs y_ty `thenDs` \ y_id ->
247 returnDs (mkValLam [y_id] (core_op `App` x_atom `App` VarArg y_id))
249 -- dsExpr (SectionR op expr) -- \ x -> op x expr
250 dsExpr (SectionR op expr)
251 = dsExpr op `thenDs` \ core_op ->
252 -- for the type of x, we need the type of op's 2nd argument
254 (x_ty:y_ty:_, _) = splitFunTy (coreExprType core_op)
256 dsExpr expr `thenDs` \ y_expr ->
257 dsExprToAtomGivenTy y_expr y_ty $ \ y_atom ->
259 newSysLocalDs x_ty `thenDs` \ x_id ->
260 returnDs (mkValLam [x_id] (core_op `App` VarArg x_id `App` y_atom))
262 dsExpr (CCall label args may_gc is_asm result_ty)
263 = mapDs dsExpr args `thenDs` \ core_args ->
264 dsCCall label core_args may_gc is_asm result_ty
265 -- dsCCall does all the unboxification, etc.
267 dsExpr (HsSCC cc expr)
268 = dsExpr expr `thenDs` \ core_expr ->
269 getModuleAndGroupDs `thenDs` \ (mod_name, group_name) ->
270 returnDs ( SCC (mkUserCC cc mod_name group_name) core_expr)
272 dsExpr expr@(HsCase discrim matches src_loc)
273 = putSrcLocDs src_loc $
274 dsExpr discrim `thenDs` \ core_discrim ->
275 matchWrapper CaseMatch matches "case" `thenDs` \ ([discrim_var], matching_code) ->
276 returnDs ( mkCoLetAny (NonRec discrim_var core_discrim) matching_code )
278 dsExpr (HsLet binds expr)
279 = dsBinds False binds `thenDs` \ core_binds ->
280 dsExpr expr `thenDs` \ core_expr ->
281 returnDs ( mkCoLetsAny core_binds core_expr )
283 dsExpr (HsDoOut do_or_lc stmts return_id then_id zero_id result_ty src_loc)
284 | maybeToBool maybe_list_comp
285 = -- Special case for list comprehensions
286 putSrcLocDs src_loc $
287 dsListComp stmts elt_ty
290 = putSrcLocDs src_loc $
291 dsDo do_or_lc stmts return_id then_id zero_id result_ty
294 = case (do_or_lc, maybeAppTyCon result_ty) of
295 (ListComp, Just (tycon, [elt_ty]))
299 -- We need the ListComp form to use deListComp (rather than the "do" form)
300 -- because the "return" in a do block is a call to "PrelBase.return", and
301 -- not a ReturnStmt. Only the ListComp form has ReturnStmts
303 Just elt_ty = maybe_list_comp
305 dsExpr (HsIf guard_expr then_expr else_expr src_loc)
306 = putSrcLocDs src_loc $
307 dsExpr guard_expr `thenDs` \ core_guard ->
308 dsExpr then_expr `thenDs` \ core_then ->
309 dsExpr else_expr `thenDs` \ core_else ->
310 returnDs (mkCoreIfThenElse core_guard core_then core_else)
314 Type lambda and application
315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 dsExpr (TyLam tyvars expr)
318 = dsExpr expr `thenDs` \ core_expr ->
319 returnDs (mkTyLam tyvars core_expr)
321 dsExpr (TyApp expr tys)
322 = dsExpr expr `thenDs` \ core_expr ->
323 returnDs (mkTyApp core_expr tys)
327 Various data construction things
328 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
330 dsExpr (ExplicitListOut ty xs)
333 list_ty = mkListTy ty
335 -- xs can ocasaionlly be huge, so don't try to take
336 -- coreExprType of core_xs, as dsArgToAtom does
337 -- (that gives a quadratic algorithm)
338 go [] = returnDs (mk_nil_con ty)
339 go (x:xs) = dsExpr x `thenDs` \ core_x ->
340 dsExprToAtomGivenTy core_x ty $ \ arg_x ->
341 go xs `thenDs` \ core_xs ->
342 dsExprToAtomGivenTy core_xs list_ty $ \ arg_xs ->
343 returnDs (Con consDataCon [TyArg ty, arg_x, arg_xs])
345 dsExpr (ExplicitTuple expr_list)
346 = mapDs dsExpr expr_list `thenDs` \ core_exprs ->
347 mkConDs (tupleCon (length expr_list))
348 (map (TyArg . coreExprType) core_exprs ++ map VarArg core_exprs)
350 dsExpr (ArithSeqOut expr (From from))
351 = dsExpr expr `thenDs` \ expr2 ->
352 dsExpr from `thenDs` \ from2 ->
353 mkAppDs expr2 [VarArg from2]
355 dsExpr (ArithSeqOut expr (FromTo from two))
356 = dsExpr expr `thenDs` \ expr2 ->
357 dsExpr from `thenDs` \ from2 ->
358 dsExpr two `thenDs` \ two2 ->
359 mkAppDs expr2 [VarArg from2, VarArg two2]
361 dsExpr (ArithSeqOut expr (FromThen from thn))
362 = dsExpr expr `thenDs` \ expr2 ->
363 dsExpr from `thenDs` \ from2 ->
364 dsExpr thn `thenDs` \ thn2 ->
365 mkAppDs expr2 [VarArg from2, VarArg thn2]
367 dsExpr (ArithSeqOut expr (FromThenTo from thn two))
368 = dsExpr expr `thenDs` \ expr2 ->
369 dsExpr from `thenDs` \ from2 ->
370 dsExpr thn `thenDs` \ thn2 ->
371 dsExpr two `thenDs` \ two2 ->
372 mkAppDs expr2 [VarArg from2, VarArg thn2, VarArg two2]
375 Record construction and update
376 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
377 For record construction we do this (assuming T has three arguments)
381 let err = /\a -> recConErr a
382 T (recConErr t1 "M.lhs/230/op1")
384 (recConErr t1 "M.lhs/230/op3")
386 recConErr then converts its arugment string into a proper message
387 before printing it as
389 M.lhs, line 230: missing field op1 was evaluated
393 dsExpr (RecordConOut con_id con_expr rbinds)
394 = dsExpr con_expr `thenDs` \ con_expr' ->
396 (arg_tys, _) = splitFunTy (coreExprType con_expr')
399 = case [rhs | (sel_id,rhs,_) <- rbinds,
400 lbl == recordSelectorFieldLabel sel_id] of
401 (rhs:rhss) -> ASSERT( null rhss )
403 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showForErr lbl)
405 mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys (dataConFieldLabels con_id)) `thenDs` \ con_args ->
406 mkAppDs con_expr' (map VarArg con_args)
409 Record update is a little harder. Suppose we have the decl:
411 data T = T1 {op1, op2, op3 :: Int}
412 | T2 {op4, op2 :: Int}
415 Then we translate as follows:
421 T1 op1 _ op3 -> T1 op1 op2 op3
422 T2 op4 _ -> T2 op4 op2
423 other -> recUpdError "M.lhs/230"
425 It's important that we use the constructor Ids for T1, T2 etc on the
426 RHSs, and do not generate a Core Con directly, because the constructor
427 might do some argument-evaluation first; and may have to throw away some
431 dsExpr (RecordUpdOut record_expr record_out_ty dicts rbinds)
432 = dsExpr record_expr `thenDs` \ record_expr' ->
434 -- Desugar the rbinds, and generate let-bindings if
435 -- necessary so that we don't lose sharing
436 dsRbinds rbinds $ \ rbinds' ->
438 record_in_ty = coreExprType record_expr'
439 (tycon, in_inst_tys, cons) = getAppDataTyConExpandingDicts record_in_ty
440 (_, out_inst_tys, _) = getAppDataTyConExpandingDicts record_out_ty
441 cons_to_upd = filter has_all_fields cons
443 -- initial_args are passed to every constructor
444 initial_args = map TyArg out_inst_tys ++ map VarArg dicts
446 mk_val_arg (field, arg_id)
447 = case [arg | (f, arg) <- rbinds',
448 field == recordSelectorFieldLabel f] of
449 (arg:args) -> ASSERT(null args)
454 = newSysLocalsDs (dataConArgTys con in_inst_tys) `thenDs` \ arg_ids ->
456 val_args = map mk_val_arg (zipEqual "dsExpr:RecordUpd" (dataConFieldLabels con) arg_ids)
458 returnDs (con, arg_ids, mkGenApp (mkGenApp (Var con) initial_args) val_args)
461 | length cons_to_upd == length cons
464 = newSysLocalDs record_in_ty `thenDs` \ deflt_id ->
465 mkErrorAppDs rEC_UPD_ERROR_ID record_out_ty "" `thenDs` \ err ->
466 returnDs (BindDefault deflt_id err)
468 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
469 mk_default `thenDs` \ deflt ->
471 returnDs (Case record_expr' (AlgAlts alts deflt))
474 has_all_fields :: Id -> Bool
475 has_all_fields con_id
478 con_fields = dataConFieldLabels con_id
479 ok (sel_id, _, _) = recordSelectorFieldLabel sel_id `elem` con_fields
482 Dictionary lambda and application
483 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
484 @DictLam@ and @DictApp@ turn into the regular old things.
485 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
486 complicated; reminiscent of fully-applied constructors.
488 dsExpr (DictLam dictvars expr)
489 = dsExpr expr `thenDs` \ core_expr ->
490 returnDs (mkValLam dictvars core_expr)
494 dsExpr (DictApp expr dicts) -- becomes a curried application
495 = mapDs lookupEnvDs dicts `thenDs` \ core_dicts ->
496 dsExpr expr `thenDs` \ core_expr ->
497 returnDs (foldl (\f d -> f `App` (VarArg d)) core_expr core_dicts)
500 @SingleDicts@ become @Locals@; @Dicts@ turn into tuples, unless
502 @ClassDictLam dictvars methods expr@ is ``the opposite'':
504 \ x -> case x of ( dictvars-and-methods-tuple ) -> expr
507 dsExpr (SingleDict dict) -- just a local
508 = lookupEnvDs dict `thenDs` \ dict' ->
511 dsExpr (Dictionary [] []) -- Empty dictionary represented by void,
512 = returnDs (Var voidId) -- (not, as would happen if we took the next case, by ())
514 dsExpr (Dictionary dicts methods)
515 = mapDs lookupEnvDs (dicts ++ methods) `thenDs` \ d_and_ms' ->
516 returnDs (mkTupleExpr d_and_ms')
518 dsExpr (ClassDictLam dicts methods expr)
519 = dsExpr expr `thenDs` \ core_expr ->
520 case num_of_d_and_ms of
521 0 -> newSysLocalDs voidTy `thenDs` \ new_x ->
522 returnDs (mkValLam [new_x] core_expr)
525 returnDs (mkValLam dicts_and_methods core_expr)
528 newSysLocalDs tuple_ty `thenDs` \ new_x ->
530 Lam (ValBinder new_x)
533 [(tuple_con, dicts_and_methods, core_expr)]
536 num_of_d_and_ms = length dicts + length methods
537 dicts_and_methods = dicts ++ methods
538 tuple_ty = mkTupleTy num_of_d_and_ms (map idType dicts_and_methods)
539 tuple_con = tupleCon num_of_d_and_ms
542 -- HsSyn constructs that just shouldn't be here:
543 dsExpr (HsDo _ _ _) = panic "dsExpr:HsDo"
544 dsExpr (ExplicitList _) = panic "dsExpr:ExplicitList"
545 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
546 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
549 out_of_range_msg -- ditto
550 = " out of range: [" ++ show minInt ++ ", " ++ show maxInt ++ "]\n"
554 %--------------------------------------------------------------------
558 = lookupEnvDs v `thenDs` \ v' ->
563 dsRbinds :: TypecheckedRecordBinds -- The field bindings supplied
564 -> ([(Id, CoreArg)] -> DsM CoreExpr) -- A continuation taking the field
565 -- bindings with atomic rhss
566 -> DsM CoreExpr -- The result of the continuation,
567 -- wrapped in suitable Lets
569 dsRbinds [] continue_with
572 dsRbinds ((sel_id, rhs, pun_flag) : rbinds) continue_with
573 = dsExpr rhs `thenDs` \ rhs' ->
574 dsExprToAtomGivenTy rhs' (coreExprType rhs') $ \ rhs_atom ->
575 dsRbinds rbinds $ \ rbinds' ->
576 continue_with ((sel_id, rhs_atom) : rbinds')
580 -- do_unfold ty_env val_env (Lam (TyBinder tyvar) body) (TyArg ty : args)
581 -- = do_unfold (addOneToTyVarEnv ty_env tyvar ty) val_env body args
583 -- do_unfold ty_env val_env (Lam (ValBinder binder) body) (arg@(VarArg expr) : args)
584 -- = dsExprToAtom arg $ \ arg_atom ->
586 -- (addOneToIdEnv val_env binder (argToExpr arg_atom))
589 -- do_unfold ty_env val_env body args
590 -- = -- Clone the remaining part of the template
591 -- uniqSMtoDsM (substCoreExpr val_env ty_env body) `thenDs` \ body' ->
593 -- -- Apply result to remaining arguments
594 -- mkAppDs body' args
597 Basically does the translation given in the Haskell~1.3 report:
601 -> Id -- id for: return m
602 -> Id -- id for: (>>=) m
603 -> Id -- id for: zero m
604 -> Type -- Element type; the whole expression has type (m t)
607 dsDo do_or_lc stmts return_id then_id zero_id result_ty
608 = dsId return_id `thenDs` \ return_ds ->
609 dsId then_id `thenDs` \ then_ds ->
610 dsId zero_id `thenDs` \ zero_ds ->
612 (_, b_ty) = splitAppTy result_ty -- result_ty must be of the form (m b)
615 = dsExpr expr `thenDs` \ expr2 ->
616 mkAppDs return_ds [TyArg b_ty, VarArg expr2]
618 go (GuardStmt expr locn : stmts)
619 = do_expr expr locn `thenDs` \ expr2 ->
620 go stmts `thenDs` \ rest ->
621 mkAppDs zero_ds [TyArg b_ty] `thenDs` \ zero_expr ->
622 returnDs (mkCoreIfThenElse expr2 rest zero_expr)
624 go (ExprStmt expr locn : stmts)
625 = do_expr expr locn `thenDs` \ expr2 ->
627 (_, a_ty) = splitAppTy (coreExprType expr2) -- Must be of form (m a)
632 go stmts `thenDs` \ rest ->
633 newSysLocalDs a_ty `thenDs` \ ignored_result_id ->
634 mkAppDs then_ds [TyArg a_ty, TyArg b_ty, VarArg expr2,
635 VarArg (mkValLam [ignored_result_id] rest)]
637 go (LetStmt binds : stmts )
638 = dsBinds False binds `thenDs` \ binds2 ->
639 go stmts `thenDs` \ rest ->
640 returnDs (mkCoLetsAny binds2 rest)
642 go (BindStmt pat expr locn : stmts)
644 dsExpr expr `thenDs` \ expr2 ->
646 (_, a_ty) = splitAppTy (coreExprType expr2) -- Must be of form (m a)
647 zero_expr = TyApp (HsVar zero_id) [b_ty]
648 main_match = PatMatch pat (SimpleMatch (
649 HsDoOut do_or_lc stmts return_id then_id zero_id result_ty locn))
651 = if failureFreePat pat
653 else [main_match, PatMatch (WildPat a_ty) (SimpleMatch zero_expr)]
655 matchWrapper DoBindMatch the_matches match_msg
656 `thenDs` \ (binders, matching_code) ->
657 mkAppDs then_ds [TyArg a_ty, TyArg b_ty,
658 VarArg expr2, VarArg (mkValLam binders matching_code)]
663 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
665 match_msg = case do_or_lc of
666 DoStmt -> "`do' statement"
667 ListComp -> "comprehension"