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 (RecordCon con_expr rbinds)
394 = dsExpr con_expr `thenDs` \ con_expr' ->
396 con_id = get_con con_expr'
397 (arg_tys, _) = splitFunTy (coreExprType con_expr')
400 = case [rhs | (sel_id,rhs,_) <- rbinds,
401 lbl == recordSelectorFieldLabel sel_id] of
402 (rhs:rhss) -> ASSERT( null rhss )
404 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showForErr lbl)
406 mapDs mk_arg (zipEqual "dsExpr:RecordCon" arg_tys (dataConFieldLabels con_id)) `thenDs` \ con_args ->
407 mkAppDs con_expr' (map VarArg con_args)
409 -- "con_expr'" is simply an application of the constructor Id
410 -- to types and (perhaps) dictionaries. This gets the constructor...
411 get_con (Var con) = con
412 get_con (App fun _) = get_con fun
415 Record update is a little harder. Suppose we have the decl:
417 data T = T1 {op1, op2, op3 :: Int}
418 | T2 {op4, op2 :: Int}
421 Then we translate as follows:
427 T1 op1 _ op3 -> T1 op1 op2 op3
428 T2 op4 _ -> T2 op4 op2
429 other -> recUpdError "M.lhs/230"
431 It's important that we use the constructor Ids for T1, T2 etc on the
432 RHSs, and do not generate a Core Con directly, because the constructor
433 might do some argument-evaluation first; and may have to throw away some
437 dsExpr (RecordUpdOut record_expr record_out_ty dicts rbinds)
438 = dsExpr record_expr `thenDs` \ record_expr' ->
440 -- Desugar the rbinds, and generate let-bindings if
441 -- necessary so that we don't lose sharing
442 dsRbinds rbinds $ \ rbinds' ->
444 record_in_ty = coreExprType record_expr'
445 (tycon, in_inst_tys, cons) = getAppDataTyConExpandingDicts record_in_ty
446 (_, out_inst_tys, _) = getAppDataTyConExpandingDicts record_out_ty
447 cons_to_upd = filter has_all_fields cons
449 -- initial_args are passed to every constructor
450 initial_args = map TyArg out_inst_tys ++ map VarArg dicts
452 mk_val_arg (field, arg_id)
453 = case [arg | (f, arg) <- rbinds',
454 field == recordSelectorFieldLabel f] of
455 (arg:args) -> ASSERT(null args)
460 = newSysLocalsDs (dataConArgTys con in_inst_tys) `thenDs` \ arg_ids ->
462 val_args = map mk_val_arg (zipEqual "dsExpr:RecordUpd" (dataConFieldLabels con) arg_ids)
464 returnDs (con, arg_ids, mkGenApp (mkGenApp (Var con) initial_args) val_args)
467 | length cons_to_upd == length cons
470 = newSysLocalDs record_in_ty `thenDs` \ deflt_id ->
471 mkErrorAppDs rEC_UPD_ERROR_ID record_out_ty "" `thenDs` \ err ->
472 returnDs (BindDefault deflt_id err)
474 mapDs mk_alt cons_to_upd `thenDs` \ alts ->
475 mk_default `thenDs` \ deflt ->
477 returnDs (Case record_expr' (AlgAlts alts deflt))
480 has_all_fields :: Id -> Bool
481 has_all_fields con_id
484 con_fields = dataConFieldLabels con_id
485 ok (sel_id, _, _) = recordSelectorFieldLabel sel_id `elem` con_fields
488 Dictionary lambda and application
489 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
490 @DictLam@ and @DictApp@ turn into the regular old things.
491 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
492 complicated; reminiscent of fully-applied constructors.
494 dsExpr (DictLam dictvars expr)
495 = dsExpr expr `thenDs` \ core_expr ->
496 returnDs (mkValLam dictvars core_expr)
500 dsExpr (DictApp expr dicts) -- becomes a curried application
501 = mapDs lookupEnvDs dicts `thenDs` \ core_dicts ->
502 dsExpr expr `thenDs` \ core_expr ->
503 returnDs (foldl (\f d -> f `App` (VarArg d)) core_expr core_dicts)
506 @SingleDicts@ become @Locals@; @Dicts@ turn into tuples, unless
508 @ClassDictLam dictvars methods expr@ is ``the opposite'':
510 \ x -> case x of ( dictvars-and-methods-tuple ) -> expr
513 dsExpr (SingleDict dict) -- just a local
514 = lookupEnvDs dict `thenDs` \ dict' ->
517 dsExpr (Dictionary [] []) -- Empty dictionary represented by void,
518 = returnDs (Var voidId) -- (not, as would happen if we took the next case, by ())
520 dsExpr (Dictionary dicts methods)
521 = mapDs lookupEnvDs (dicts ++ methods) `thenDs` \ d_and_ms' ->
522 returnDs (mkTupleExpr d_and_ms')
524 dsExpr (ClassDictLam dicts methods expr)
525 = dsExpr expr `thenDs` \ core_expr ->
526 case num_of_d_and_ms of
527 0 -> newSysLocalDs voidTy `thenDs` \ new_x ->
528 returnDs (mkValLam [new_x] core_expr)
531 returnDs (mkValLam dicts_and_methods core_expr)
534 newSysLocalDs tuple_ty `thenDs` \ new_x ->
536 Lam (ValBinder new_x)
539 [(tuple_con, dicts_and_methods, core_expr)]
542 num_of_d_and_ms = length dicts + length methods
543 dicts_and_methods = dicts ++ methods
544 tuple_ty = mkTupleTy num_of_d_and_ms (map idType dicts_and_methods)
545 tuple_con = tupleCon num_of_d_and_ms
548 -- HsSyn constructs that just shouldn't be here:
549 dsExpr (HsDo _ _ _) = panic "dsExpr:HsDo"
550 dsExpr (ExplicitList _) = panic "dsExpr:ExplicitList"
551 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
552 dsExpr (ArithSeqIn _) = panic "dsExpr:ArithSeqIn"
555 out_of_range_msg -- ditto
556 = " out of range: [" ++ show minInt ++ ", " ++ show maxInt ++ "]\n"
560 %--------------------------------------------------------------------
564 = lookupEnvDs v `thenDs` \ v' ->
569 dsRbinds :: TypecheckedRecordBinds -- The field bindings supplied
570 -> ([(Id, CoreArg)] -> DsM CoreExpr) -- A continuation taking the field
571 -- bindings with atomic rhss
572 -> DsM CoreExpr -- The result of the continuation,
573 -- wrapped in suitable Lets
575 dsRbinds [] continue_with
578 dsRbinds ((sel_id, rhs, pun_flag) : rbinds) continue_with
579 = dsExpr rhs `thenDs` \ rhs' ->
580 dsExprToAtomGivenTy rhs' (coreExprType rhs') $ \ rhs_atom ->
581 dsRbinds rbinds $ \ rbinds' ->
582 continue_with ((sel_id, rhs_atom) : rbinds')
586 -- do_unfold ty_env val_env (Lam (TyBinder tyvar) body) (TyArg ty : args)
587 -- = do_unfold (addOneToTyVarEnv ty_env tyvar ty) val_env body args
589 -- do_unfold ty_env val_env (Lam (ValBinder binder) body) (arg@(VarArg expr) : args)
590 -- = dsExprToAtom arg $ \ arg_atom ->
592 -- (addOneToIdEnv val_env binder (argToExpr arg_atom))
595 -- do_unfold ty_env val_env body args
596 -- = -- Clone the remaining part of the template
597 -- uniqSMtoDsM (substCoreExpr val_env ty_env body) `thenDs` \ body' ->
599 -- -- Apply result to remaining arguments
600 -- mkAppDs body' args
603 Basically does the translation given in the Haskell~1.3 report:
607 -> Id -- id for: return m
608 -> Id -- id for: (>>=) m
609 -> Id -- id for: zero m
610 -> Type -- Element type; the whole expression has type (m t)
613 dsDo do_or_lc stmts return_id then_id zero_id result_ty
614 = dsId return_id `thenDs` \ return_ds ->
615 dsId then_id `thenDs` \ then_ds ->
616 dsId zero_id `thenDs` \ zero_ds ->
618 (_, b_ty) = splitAppTy result_ty -- result_ty must be of the form (m b)
621 = dsExpr expr `thenDs` \ expr2 ->
622 mkAppDs return_ds [TyArg b_ty, VarArg expr2]
624 go (GuardStmt expr locn : stmts)
625 = do_expr expr locn `thenDs` \ expr2 ->
626 go stmts `thenDs` \ rest ->
627 mkAppDs zero_ds [TyArg b_ty] `thenDs` \ zero_expr ->
628 returnDs (mkCoreIfThenElse expr2 rest zero_expr)
630 go (ExprStmt expr locn : stmts)
631 = do_expr expr locn `thenDs` \ expr2 ->
633 (_, a_ty) = splitAppTy (coreExprType expr2) -- Must be of form (m a)
638 go stmts `thenDs` \ rest ->
639 newSysLocalDs a_ty `thenDs` \ ignored_result_id ->
640 mkAppDs then_ds [TyArg a_ty, TyArg b_ty, VarArg expr2,
641 VarArg (mkValLam [ignored_result_id] rest)]
643 go (LetStmt binds : stmts )
644 = dsBinds False binds `thenDs` \ binds2 ->
645 go stmts `thenDs` \ rest ->
646 returnDs (mkCoLetsAny binds2 rest)
648 go (BindStmt pat expr locn : stmts)
650 dsExpr expr `thenDs` \ expr2 ->
652 (_, a_ty) = splitAppTy (coreExprType expr2) -- Must be of form (m a)
653 zero_expr = TyApp (HsVar zero_id) [b_ty]
654 main_match = PatMatch pat (SimpleMatch (
655 HsDoOut do_or_lc stmts return_id then_id zero_id result_ty locn))
657 = if failureFreePat pat
659 else [main_match, PatMatch (WildPat a_ty) (SimpleMatch zero_expr)]
661 matchWrapper DoBindMatch the_matches match_msg
662 `thenDs` \ (binders, matching_code) ->
663 mkAppDs then_ds [TyArg a_ty, TyArg b_ty,
664 VarArg expr2, VarArg (mkValLam binders matching_code)]
669 do_expr expr locn = putSrcLocDs locn (dsExpr expr)
671 match_msg = case do_or_lc of
672 DoStmt -> "`do' statement"
673 ListComp -> "comprehension"