2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[DsExpr]{Matching expressions (Exprs)}
7 module DsExpr ( dsExpr, dsLExpr, dsLocalBinds, dsValBinds, dsLit ) where
9 #include "HsVersions.h"
12 import Match ( matchWrapper, matchSimply, matchSinglePat )
13 import MatchLit ( dsLit, dsOverLit )
14 import DsBinds ( dsLHsBinds )
15 import DsGRHSs ( dsGuarded )
16 import DsListComp ( dsListComp, dsPArrComp )
17 import DsUtils ( mkErrorAppDs, mkStringExpr, mkConsExpr, mkNilExpr,
18 extractMatchResult, cantFailMatchResult, matchCanFail,
19 mkCoreTupTy, selectSimpleMatchVarL, lookupEvidence )
20 import DsArrows ( dsProcExpr )
24 -- Template Haskell stuff iff bootstrapped
25 import DsMeta ( dsBracket )
29 import TcHsSyn ( hsPatType )
31 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
32 -- needs to see source types (newtypes etc), and sometimes not
33 -- So WATCH OUT; check each use of split*Ty functions.
34 -- Sigh. This is a pain.
36 import TcType ( tcSplitAppTy, tcSplitFunTys, tcTyConAppTyCon,
37 tcTyConAppArgs, isUnLiftedType, Type, mkAppTy )
38 import Type ( funArgTy, splitFunTys, isUnboxedTupleType, mkFunTy )
40 import CoreUtils ( exprType, mkIfThenElse, bindNonRec )
42 import CostCentre ( mkUserCC )
43 import Id ( Id, idType, idName, idDataCon )
44 import PrelInfo ( rEC_CON_ERROR_ID, iRREFUT_PAT_ERROR_ID )
45 import DataCon ( DataCon, dataConWrapId, dataConFieldLabels, dataConInstOrigArgTys )
46 import DataCon ( isVanillaDataCon )
47 import TyCon ( FieldLabel, tyConDataCons )
48 import TysWiredIn ( tupleCon )
49 import BasicTypes ( RecFlag(..), Boxity(..), ipNameName )
50 import PrelNames ( toPName,
51 returnMName, bindMName, thenMName, failMName,
53 import SrcLoc ( Located(..), unLoc, getLoc, noLoc )
54 import Util ( zipEqual, zipWithEqual )
55 import Bag ( bagToList )
61 %************************************************************************
65 %************************************************************************
67 @dsLet@ is a match-result transformer, taking the @MatchResult@ for the body
68 and transforming it into one for the let-bindings enclosing the body.
70 This may seem a bit odd, but (source) let bindings can contain unboxed
75 This must be transformed to a case expression and, if the type has
76 more than one constructor, may fail.
79 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
80 dsLocalBinds EmptyLocalBinds body = return body
81 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
82 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
84 -------------------------
85 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
86 dsValBinds (ValBindsOut binds _) body = foldrDs ds_val_bind body binds
88 -------------------------
89 dsIPBinds (IPBinds ip_binds dict_binds) body
90 = do { prs <- dsLHsBinds dict_binds
91 ; let inner = foldr (\(x,r) e -> Let (NonRec x r) e) body prs
92 ; foldrDs ds_ip_bind inner ip_binds }
94 ds_ip_bind (L _ (IPBind n e)) body
95 = dsLExpr e `thenDs` \ e' ->
96 returnDs (Let (NonRec (ipNameName n) e') body)
98 -------------------------
99 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
100 -- Special case for bindings which bind unlifted variables
101 -- We need to do a case right away, rather than building
102 -- a tuple and doing selections.
103 -- Silently ignore INLINE and SPECIALISE pragmas...
104 ds_val_bind (is_rec, hsbinds) body
105 | [L _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
106 or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
107 = ASSERT (case is_rec of {NonRecursive -> True; other -> False})
108 -- Unlifted bindings are always non-recursive
109 -- and are always a Fun or Pat monobind
111 -- ToDo: in some bizarre case it's conceivable that there
112 -- could be dict binds in the 'binds'. (See the notes
113 -- below. Then pattern-match would fail. Urk.)
115 body_w_exports = foldr bind_export body exports
116 bind_export (tvs, g, l, _) body = ASSERT( null tvs )
117 bindNonRec g (Var l) body
119 mk_error_app pat = mkErrorAppDs iRREFUT_PAT_ERROR_ID
123 case bagToList binds of
124 [L loc (FunBind (L _ fun) _ matches _)]
125 -> putSrcSpanDs loc $
126 matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
127 ASSERT( null args ) -- Functions aren't lifted
128 returnDs (bindNonRec fun rhs body_w_exports)
130 [L loc (PatBind pat grhss ty _)]
131 -> putSrcSpanDs loc $
132 dsGuarded grhss ty `thenDs` \ rhs ->
133 mk_error_app pat `thenDs` \ error_expr ->
134 matchSimply rhs PatBindRhs pat body_w_exports error_expr
136 other -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
138 -- Ordinary case for bindings
139 ds_val_bind (is_rec, binds) body
140 = dsLHsBinds binds `thenDs` \ prs ->
141 returnDs (Let (Rec prs) body)
142 -- Use a Rec regardless of is_rec.
143 -- Why? Because it allows the binds to be all
144 -- mixed up, which is what happens in one rare case
145 -- Namely, for an AbsBind with no tyvars and no dicts,
146 -- but which does have dictionary bindings.
147 -- See notes with TcSimplify.inferLoop [NO TYVARS]
148 -- It turned out that wrapping a Rec here was the easiest solution
150 -- NB The previous case dealt with unlifted bindings, so we
151 -- only have to deal with lifted ones now; so Rec is ok
154 %************************************************************************
156 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
158 %************************************************************************
161 dsLExpr :: LHsExpr Id -> DsM CoreExpr
162 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
164 dsExpr :: HsExpr Id -> DsM CoreExpr
166 dsExpr (HsPar e) = dsLExpr e
167 dsExpr (ExprWithTySigOut e _) = dsLExpr e
168 dsExpr (HsVar var) = returnDs (Var var)
169 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
170 dsExpr (HsLit lit) = dsLit lit
171 dsExpr (HsOverLit lit) = dsOverLit lit
173 dsExpr (NegApp expr neg_expr)
174 = do { core_expr <- dsLExpr expr
175 ; core_neg <- dsExpr neg_expr
176 ; return (core_neg `App` core_expr) }
178 dsExpr expr@(HsLam a_Match)
179 = matchWrapper LambdaExpr a_Match `thenDs` \ (binders, matching_code) ->
180 returnDs (mkLams binders matching_code)
182 dsExpr expr@(HsApp fun arg)
183 = dsLExpr fun `thenDs` \ core_fun ->
184 dsLExpr arg `thenDs` \ core_arg ->
185 returnDs (core_fun `App` core_arg)
188 Operator sections. At first it looks as if we can convert
197 But no! expr might be a redex, and we can lose laziness badly this
202 for example. So we convert instead to
204 let y = expr in \x -> op y x
206 If \tr{expr} is actually just a variable, say, then the simplifier
210 dsExpr (OpApp e1 op _ e2)
211 = dsLExpr op `thenDs` \ core_op ->
212 -- for the type of y, we need the type of op's 2nd argument
213 dsLExpr e1 `thenDs` \ x_core ->
214 dsLExpr e2 `thenDs` \ y_core ->
215 returnDs (mkApps core_op [x_core, y_core])
217 dsExpr (SectionL expr op)
218 = dsLExpr op `thenDs` \ core_op ->
219 -- for the type of y, we need the type of op's 2nd argument
221 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
222 -- Must look through an implicit-parameter type;
223 -- newtype impossible; hence Type.splitFunTys
225 dsLExpr expr `thenDs` \ x_core ->
226 newSysLocalDs x_ty `thenDs` \ x_id ->
227 newSysLocalDs y_ty `thenDs` \ y_id ->
229 returnDs (bindNonRec x_id x_core $
230 Lam y_id (mkApps core_op [Var x_id, Var y_id]))
232 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
233 dsExpr (SectionR op expr)
234 = dsLExpr op `thenDs` \ core_op ->
235 -- for the type of x, we need the type of op's 2nd argument
237 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
238 -- See comment with SectionL
240 dsLExpr expr `thenDs` \ y_core ->
241 newSysLocalDs x_ty `thenDs` \ x_id ->
242 newSysLocalDs y_ty `thenDs` \ y_id ->
244 returnDs (bindNonRec y_id y_core $
245 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
247 dsExpr (HsSCC cc expr)
248 = dsLExpr expr `thenDs` \ core_expr ->
249 getModuleDs `thenDs` \ mod_name ->
250 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
253 -- hdaume: core annotation
255 dsExpr (HsCoreAnn fs expr)
256 = dsLExpr expr `thenDs` \ core_expr ->
257 returnDs (Note (CoreNote $ unpackFS fs) core_expr)
259 -- Special case to handle unboxed tuple patterns; they can't appear nested
261 -- case e of (# p1, p2 #) -> rhs
263 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
265 -- let x = e in case x of ....
267 -- But there may be a big
268 -- let fail = ... in case e of ...
269 -- wrapping the whole case, which complicates matters slightly
270 -- It all seems a bit fragile. Test is dsrun013.
272 dsExpr (HsCase discrim matches@(MatchGroup _ ty))
273 | isUnboxedTupleType (funArgTy ty)
274 = dsLExpr discrim `thenDs` \ core_discrim ->
275 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
277 scrungle (Case (Var x) bndr ty alts)
278 | x == discrim_var = Case core_discrim bndr ty alts
279 scrungle (Let binds body) = Let binds (scrungle body)
280 scrungle other = panic ("dsLExpr: tuple pattern:\n" ++ showSDoc (ppr other))
282 returnDs (scrungle matching_code)
284 dsExpr (HsCase discrim matches)
285 = dsLExpr discrim `thenDs` \ core_discrim ->
286 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
287 returnDs (bindNonRec discrim_var core_discrim matching_code)
289 dsExpr (HsLet binds body)
290 = dsLExpr body `thenDs` \ body' ->
291 dsLocalBinds binds body'
293 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
294 -- because the interpretation of `stmts' depends on what sort of thing it is.
296 dsExpr (HsDo ListComp stmts body result_ty)
297 = -- Special case for list comprehensions
298 dsListComp stmts body elt_ty
300 [elt_ty] = tcTyConAppArgs result_ty
302 dsExpr (HsDo DoExpr stmts body result_ty)
303 = dsDo stmts body result_ty
305 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
306 = dsMDo tbl stmts body result_ty
308 dsExpr (HsDo PArrComp stmts body result_ty)
309 = -- Special case for array comprehensions
310 dsPArrComp (map unLoc stmts) body elt_ty
312 [elt_ty] = tcTyConAppArgs result_ty
314 dsExpr (HsIf guard_expr then_expr else_expr)
315 = dsLExpr guard_expr `thenDs` \ core_guard ->
316 dsLExpr then_expr `thenDs` \ core_then ->
317 dsLExpr else_expr `thenDs` \ core_else ->
318 returnDs (mkIfThenElse core_guard core_then core_else)
323 \underline{\bf Type lambda and application}
324 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
326 dsExpr (TyLam tyvars expr)
327 = dsLExpr expr `thenDs` \ core_expr ->
328 returnDs (mkLams tyvars core_expr)
330 dsExpr (TyApp expr tys)
331 = dsLExpr expr `thenDs` \ core_expr ->
332 returnDs (mkTyApps core_expr tys)
337 \underline{\bf Various data construction things}
338 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
340 dsExpr (ExplicitList ty xs)
343 go [] = returnDs (mkNilExpr ty)
344 go (x:xs) = dsLExpr x `thenDs` \ core_x ->
345 go xs `thenDs` \ core_xs ->
346 returnDs (mkConsExpr ty core_x core_xs)
348 -- we create a list from the array elements and convert them into a list using
351 -- * the main disadvantage to this scheme is that `toP' traverses the list
352 -- twice: once to determine the length and a second time to put to elements
353 -- into the array; this inefficiency could be avoided by exposing some of
354 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
355 -- that we can exploit the fact that we already know the length of the array
356 -- here at compile time
358 dsExpr (ExplicitPArr ty xs)
359 = dsLookupGlobalId toPName `thenDs` \toP ->
360 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
361 returnDs (mkApps (Var toP) [Type ty, coreList])
363 dsExpr (ExplicitTuple expr_list boxity)
364 = mappM dsLExpr expr_list `thenDs` \ core_exprs ->
365 returnDs (mkConApp (tupleCon boxity (length expr_list))
366 (map (Type . exprType) core_exprs ++ core_exprs))
368 dsExpr (ArithSeq expr (From from))
369 = dsExpr expr `thenDs` \ expr2 ->
370 dsLExpr from `thenDs` \ from2 ->
371 returnDs (App expr2 from2)
373 dsExpr (ArithSeq expr (FromTo from two))
374 = dsExpr expr `thenDs` \ expr2 ->
375 dsLExpr from `thenDs` \ from2 ->
376 dsLExpr two `thenDs` \ two2 ->
377 returnDs (mkApps expr2 [from2, two2])
379 dsExpr (ArithSeq expr (FromThen from thn))
380 = dsExpr expr `thenDs` \ expr2 ->
381 dsLExpr from `thenDs` \ from2 ->
382 dsLExpr thn `thenDs` \ thn2 ->
383 returnDs (mkApps expr2 [from2, thn2])
385 dsExpr (ArithSeq expr (FromThenTo from thn two))
386 = dsExpr expr `thenDs` \ expr2 ->
387 dsLExpr from `thenDs` \ from2 ->
388 dsLExpr thn `thenDs` \ thn2 ->
389 dsLExpr two `thenDs` \ two2 ->
390 returnDs (mkApps expr2 [from2, thn2, two2])
392 dsExpr (PArrSeq expr (FromTo from two))
393 = dsExpr expr `thenDs` \ expr2 ->
394 dsLExpr from `thenDs` \ from2 ->
395 dsLExpr two `thenDs` \ two2 ->
396 returnDs (mkApps expr2 [from2, two2])
398 dsExpr (PArrSeq expr (FromThenTo from thn two))
399 = dsExpr expr `thenDs` \ expr2 ->
400 dsLExpr from `thenDs` \ from2 ->
401 dsLExpr thn `thenDs` \ thn2 ->
402 dsLExpr two `thenDs` \ two2 ->
403 returnDs (mkApps expr2 [from2, thn2, two2])
405 dsExpr (PArrSeq expr _)
406 = panic "DsExpr.dsExpr: Infinite parallel array!"
407 -- the parser shouldn't have generated it and the renamer and typechecker
408 -- shouldn't have let it through
412 \underline{\bf Record construction and update}
413 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
414 For record construction we do this (assuming T has three arguments)
418 let err = /\a -> recConErr a
419 T (recConErr t1 "M.lhs/230/op1")
421 (recConErr t1 "M.lhs/230/op3")
423 @recConErr@ then converts its arugment string into a proper message
424 before printing it as
426 M.lhs, line 230: missing field op1 was evaluated
429 We also handle @C{}@ as valid construction syntax for an unlabelled
430 constructor @C@, setting all of @C@'s fields to bottom.
433 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds)
434 = dsExpr con_expr `thenDs` \ con_expr' ->
436 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
437 -- A newtype in the corner should be opaque;
438 -- hence TcType.tcSplitFunTys
440 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
441 = case [rhs | (L _ sel_id, rhs) <- rbinds, lbl == idName sel_id] of
442 (rhs:rhss) -> ASSERT( null rhss )
444 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
445 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
447 labels = dataConFieldLabels (idDataCon data_con_id)
448 -- The data_con_id is guaranteed to be the wrapper id of the constructor
452 then mappM unlabelled_bottom arg_tys
453 else mappM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
454 `thenDs` \ con_args ->
456 returnDs (mkApps con_expr' con_args)
459 Record update is a little harder. Suppose we have the decl:
461 data T = T1 {op1, op2, op3 :: Int}
462 | T2 {op4, op2 :: Int}
465 Then we translate as follows:
471 T1 op1 _ op3 -> T1 op1 op2 op3
472 T2 op4 _ -> T2 op4 op2
473 other -> recUpdError "M.lhs/230"
475 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
476 RHSs, and do not generate a Core constructor application directly, because the constructor
477 might do some argument-evaluation first; and may have to throw away some
481 dsExpr (RecordUpd record_expr [] record_in_ty record_out_ty)
482 = dsLExpr record_expr
484 dsExpr expr@(RecordUpd record_expr rbinds record_in_ty record_out_ty)
485 = dsLExpr record_expr `thenDs` \ record_expr' ->
487 -- Desugar the rbinds, and generate let-bindings if
488 -- necessary so that we don't lose sharing
491 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
492 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
493 in_out_ty = mkFunTy record_in_ty record_out_ty
495 mk_val_arg field old_arg_id
496 = case [rhs | (L _ sel_id, rhs) <- rbinds, field == idName sel_id] of
497 (rhs:rest) -> ASSERT(null rest) rhs
498 [] -> nlHsVar old_arg_id
501 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
502 -- This call to dataConInstOrigArgTys won't work for existentials
503 -- but existentials don't have record types anyway
505 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
506 (dataConFieldLabels con) arg_ids
507 rhs = foldl (\a b -> nlHsApp a b)
508 (noLoc $ TyApp (nlHsVar (dataConWrapId con))
512 returnDs (mkSimpleMatch [noLoc $ ConPatOut (noLoc con) [] [] emptyLHsBinds
513 (PrefixCon (map nlVarPat arg_ids)) record_in_ty]
516 -- Record stuff doesn't work for existentials
517 -- The type checker checks for this, but we need
518 -- worry only about the constructors that are to be updated
519 ASSERT2( all isVanillaDataCon cons_to_upd, ppr expr )
521 -- It's important to generate the match with matchWrapper,
522 -- and the right hand sides with applications of the wrapper Id
523 -- so that everything works when we are doing fancy unboxing on the
524 -- constructor aguments.
525 mappM mk_alt cons_to_upd `thenDs` \ alts ->
526 matchWrapper RecUpd (MatchGroup alts in_out_ty) `thenDs` \ ([discrim_var], matching_code) ->
528 returnDs (bindNonRec discrim_var record_expr' matching_code)
531 updated_fields :: [FieldLabel]
532 updated_fields = [ idName sel_id | (L _ sel_id,_) <- rbinds]
534 -- Get the type constructor from the record_in_ty
535 -- so that we are sure it'll have all its DataCons
536 -- (In GHCI, it's possible that some TyCons may not have all
537 -- their constructors, in a module-loop situation.)
538 tycon = tcTyConAppTyCon record_in_ty
539 data_cons = tyConDataCons tycon
540 cons_to_upd = filter has_all_fields data_cons
542 has_all_fields :: DataCon -> Bool
543 has_all_fields con_id
544 = all (`elem` con_fields) updated_fields
546 con_fields = dataConFieldLabels con_id
551 \underline{\bf Dictionary lambda and application}
552 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
553 @DictLam@ and @DictApp@ turn into the regular old things.
554 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
555 complicated; reminiscent of fully-applied constructors.
557 dsExpr (DictLam dictvars expr)
558 = dsLExpr expr `thenDs` \ core_expr ->
559 returnDs (mkLams dictvars core_expr)
563 dsExpr (DictApp expr dicts) -- becomes a curried application
564 = dsLExpr expr `thenDs` \ core_expr ->
565 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
568 Here is where we desugar the Template Haskell brackets and escapes
571 -- Template Haskell stuff
573 #ifdef GHCI /* Only if bootstrapping */
574 dsExpr (HsBracketOut x ps) = dsBracket x ps
575 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
578 -- Arrow notation extension
579 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
586 -- HsSyn constructs that just shouldn't be here:
587 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
592 %--------------------------------------------------------------------
594 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
595 handled in DsListComp). Basically does the translation given in the
601 -> Type -- Type of the whole expression
604 dsDo stmts body result_ty
605 = go (map unLoc stmts)
609 go (ExprStmt rhs then_expr _ : stmts)
610 = do { rhs2 <- dsLExpr rhs
611 ; then_expr2 <- dsExpr then_expr
613 ; returnDs (mkApps then_expr2 [rhs2, rest]) }
615 go (LetStmt binds : stmts)
616 = do { rest <- go stmts
617 ; dsLocalBinds binds rest }
619 go (BindStmt pat rhs bind_op fail_op : stmts)
620 = do { body <- go stmts
621 ; var <- selectSimpleMatchVarL pat
622 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
623 result_ty (cantFailMatchResult body)
624 ; match_code <- handle_failure pat match fail_op
625 ; rhs' <- dsLExpr rhs
626 ; bind_op' <- dsExpr bind_op
627 ; returnDs (mkApps bind_op' [rhs', Lam var match_code]) }
629 -- In a do expression, pattern-match failure just calls
630 -- the monadic 'fail' rather than throwing an exception
631 handle_failure pat match fail_op
633 = do { fail_op' <- dsExpr fail_op
634 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
635 ; extractMatchResult match (App fail_op' fail_msg) }
637 = extractMatchResult match (error "It can't fail")
639 mk_fail_msg pat = "Pattern match failure in do expression at " ++
640 showSDoc (ppr (getLoc pat))
643 Translation for RecStmt's:
644 -----------------------------
645 We turn (RecStmt [v1,..vn] stmts) into:
647 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
654 -> Type -- Type of the whole expression
657 dsMDo tbl stmts body result_ty
658 = go (map unLoc stmts)
660 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
661 mfix_id = lookupEvidence tbl mfixName
662 return_id = lookupEvidence tbl returnMName
663 bind_id = lookupEvidence tbl bindMName
664 then_id = lookupEvidence tbl thenMName
665 fail_id = lookupEvidence tbl failMName
670 go (LetStmt binds : stmts)
671 = do { rest <- go stmts
672 ; dsLocalBinds binds rest }
674 go (ExprStmt rhs _ rhs_ty : stmts)
675 = do { rhs2 <- dsLExpr rhs
677 ; returnDs (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
679 go (BindStmt pat rhs _ _ : stmts)
680 = do { body <- go stmts
681 ; var <- selectSimpleMatchVarL pat
682 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
683 result_ty (cantFailMatchResult body)
684 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
685 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
686 ; match_code <- extractMatchResult match fail_expr
688 ; rhs' <- dsLExpr rhs
689 ; returnDs (mkApps (Var bind_id) [Type (hsPatType pat), Type b_ty,
690 rhs', Lam var match_code]) }
692 go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
693 = ASSERT( length rec_ids > 0 )
694 ASSERT( length rec_ids == length rec_rets )
695 go (new_bind_stmt : let_stmt : stmts)
697 new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
698 let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
701 -- Remove the later_ids that appear (without fancy coercions)
702 -- in rec_rets, because there's no need to knot-tie them separately
703 -- See Note [RecStmt] in HsExpr
704 later_ids' = filter (`notElem` mono_rec_ids) later_ids
705 mono_rec_ids = [ id | HsVar id <- rec_rets ]
707 mfix_app = nlHsApp (noLoc $ TyApp (nlHsVar mfix_id) [tup_ty]) mfix_arg
708 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
709 (mkFunTy tup_ty body_ty))
711 -- The rec_tup_pat must bind the rec_ids only; remember that the
712 -- trimmed_laters may share the same Names
713 -- Meanwhile, the later_pats must bind the later_vars
714 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
715 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
716 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
718 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
719 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
720 body_ty = mkAppTy m_ty tup_ty
721 tup_ty = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
722 -- mkCoreTupTy deals with singleton case
724 return_app = nlHsApp (noLoc $ TyApp (nlHsVar return_id) [tup_ty])
727 mk_wild_pat :: Id -> LPat Id
728 mk_wild_pat v = noLoc $ WildPat $ idType v
730 mk_later_pat :: Id -> LPat Id
731 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
732 | otherwise = nlVarPat v
734 mk_tup_pat :: [LPat Id] -> LPat Id
736 mk_tup_pat ps = noLoc $ TuplePat ps Boxed
738 mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
740 mk_ret_tup rs = noLoc $ ExplicitTuple rs Boxed