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, dsCoercion )
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 { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn })]
125 -> putSrcSpanDs loc $
126 matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
127 ASSERT( null args ) -- Functions aren't lifted
128 ASSERT( isIdCoercion co_fn )
129 returnDs (bindNonRec fun rhs body_w_exports)
131 [L loc (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty })]
132 -> putSrcSpanDs loc $
133 dsGuarded grhss ty `thenDs` \ rhs ->
134 mk_error_app pat `thenDs` \ error_expr ->
135 matchSimply rhs PatBindRhs pat body_w_exports error_expr
137 other -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
139 -- Ordinary case for bindings
140 ds_val_bind (is_rec, binds) body
141 = dsLHsBinds binds `thenDs` \ prs ->
142 returnDs (Let (Rec prs) body)
143 -- Use a Rec regardless of is_rec.
144 -- Why? Because it allows the binds to be all
145 -- mixed up, which is what happens in one rare case
146 -- Namely, for an AbsBind with no tyvars and no dicts,
147 -- but which does have dictionary bindings.
148 -- See notes with TcSimplify.inferLoop [NO TYVARS]
149 -- It turned out that wrapping a Rec here was the easiest solution
151 -- NB The previous case dealt with unlifted bindings, so we
152 -- only have to deal with lifted ones now; so Rec is ok
155 %************************************************************************
157 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
159 %************************************************************************
162 dsLExpr :: LHsExpr Id -> DsM CoreExpr
163 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
165 dsExpr :: HsExpr Id -> DsM CoreExpr
167 dsExpr (HsPar e) = dsLExpr e
168 dsExpr (ExprWithTySigOut e _) = dsLExpr e
169 dsExpr (HsVar var) = returnDs (Var var)
170 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
171 dsExpr (HsLit lit) = dsLit lit
172 dsExpr (HsOverLit lit) = dsOverLit lit
174 dsExpr (NegApp expr neg_expr)
175 = do { core_expr <- dsLExpr expr
176 ; core_neg <- dsExpr neg_expr
177 ; return (core_neg `App` core_expr) }
179 dsExpr expr@(HsLam a_Match)
180 = matchWrapper LambdaExpr a_Match `thenDs` \ (binders, matching_code) ->
181 returnDs (mkLams binders matching_code)
183 dsExpr expr@(HsApp fun arg)
184 = dsLExpr fun `thenDs` \ core_fun ->
185 dsLExpr arg `thenDs` \ core_arg ->
186 returnDs (core_fun `App` core_arg)
189 Operator sections. At first it looks as if we can convert
198 But no! expr might be a redex, and we can lose laziness badly this
203 for example. So we convert instead to
205 let y = expr in \x -> op y x
207 If \tr{expr} is actually just a variable, say, then the simplifier
211 dsExpr (OpApp e1 op _ e2)
212 = dsLExpr op `thenDs` \ core_op ->
213 -- for the type of y, we need the type of op's 2nd argument
214 dsLExpr e1 `thenDs` \ x_core ->
215 dsLExpr e2 `thenDs` \ y_core ->
216 returnDs (mkApps core_op [x_core, y_core])
218 dsExpr (SectionL expr op)
219 = dsLExpr op `thenDs` \ core_op ->
220 -- for the type of y, we need the type of op's 2nd argument
222 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
223 -- Must look through an implicit-parameter type;
224 -- newtype impossible; hence Type.splitFunTys
226 dsLExpr expr `thenDs` \ x_core ->
227 newSysLocalDs x_ty `thenDs` \ x_id ->
228 newSysLocalDs y_ty `thenDs` \ y_id ->
230 returnDs (bindNonRec x_id x_core $
231 Lam y_id (mkApps core_op [Var x_id, Var y_id]))
233 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
234 dsExpr (SectionR op expr)
235 = dsLExpr op `thenDs` \ core_op ->
236 -- for the type of x, we need the type of op's 2nd argument
238 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
239 -- See comment with SectionL
241 dsLExpr expr `thenDs` \ y_core ->
242 newSysLocalDs x_ty `thenDs` \ x_id ->
243 newSysLocalDs y_ty `thenDs` \ y_id ->
245 returnDs (bindNonRec y_id y_core $
246 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
248 dsExpr (HsSCC cc expr)
249 = dsLExpr expr `thenDs` \ core_expr ->
250 getModuleDs `thenDs` \ mod_name ->
251 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
254 -- hdaume: core annotation
256 dsExpr (HsCoreAnn fs expr)
257 = dsLExpr expr `thenDs` \ core_expr ->
258 returnDs (Note (CoreNote $ unpackFS fs) core_expr)
260 -- Special case to handle unboxed tuple patterns; they can't appear nested
262 -- case e of (# p1, p2 #) -> rhs
264 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
266 -- let x = e in case x of ....
268 -- But there may be a big
269 -- let fail = ... in case e of ...
270 -- wrapping the whole case, which complicates matters slightly
271 -- It all seems a bit fragile. Test is dsrun013.
273 dsExpr (HsCase discrim matches@(MatchGroup _ ty))
274 | isUnboxedTupleType (funArgTy ty)
275 = dsLExpr discrim `thenDs` \ core_discrim ->
276 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
278 scrungle (Case (Var x) bndr ty alts)
279 | x == discrim_var = Case core_discrim bndr ty alts
280 scrungle (Let binds body) = Let binds (scrungle body)
281 scrungle other = panic ("dsLExpr: tuple pattern:\n" ++ showSDoc (ppr other))
283 returnDs (scrungle matching_code)
285 dsExpr (HsCase discrim matches)
286 = dsLExpr discrim `thenDs` \ core_discrim ->
287 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
288 returnDs (bindNonRec discrim_var core_discrim matching_code)
290 dsExpr (HsLet binds body)
291 = dsLExpr body `thenDs` \ body' ->
292 dsLocalBinds binds body'
294 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
295 -- because the interpretation of `stmts' depends on what sort of thing it is.
297 dsExpr (HsDo ListComp stmts body result_ty)
298 = -- Special case for list comprehensions
299 dsListComp stmts body elt_ty
301 [elt_ty] = tcTyConAppArgs result_ty
303 dsExpr (HsDo DoExpr stmts body result_ty)
304 = dsDo stmts body result_ty
306 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
307 = dsMDo tbl stmts body result_ty
309 dsExpr (HsDo PArrComp stmts body result_ty)
310 = -- Special case for array comprehensions
311 dsPArrComp (map unLoc stmts) body elt_ty
313 [elt_ty] = tcTyConAppArgs result_ty
315 dsExpr (HsIf guard_expr then_expr else_expr)
316 = dsLExpr guard_expr `thenDs` \ core_guard ->
317 dsLExpr then_expr `thenDs` \ core_then ->
318 dsLExpr else_expr `thenDs` \ core_else ->
319 returnDs (mkIfThenElse core_guard core_then core_else)
324 \underline{\bf Type lambda and application}
325 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~
327 dsExpr (TyLam tyvars expr)
328 = dsLExpr expr `thenDs` \ core_expr ->
329 returnDs (mkLams tyvars core_expr)
331 dsExpr (TyApp expr tys)
332 = dsLExpr expr `thenDs` \ core_expr ->
333 returnDs (mkTyApps core_expr tys)
338 \underline{\bf Various data construction things}
339 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
341 dsExpr (ExplicitList ty xs)
344 go [] = returnDs (mkNilExpr ty)
345 go (x:xs) = dsLExpr x `thenDs` \ core_x ->
346 go xs `thenDs` \ core_xs ->
347 returnDs (mkConsExpr ty core_x core_xs)
349 -- we create a list from the array elements and convert them into a list using
352 -- * the main disadvantage to this scheme is that `toP' traverses the list
353 -- twice: once to determine the length and a second time to put to elements
354 -- into the array; this inefficiency could be avoided by exposing some of
355 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
356 -- that we can exploit the fact that we already know the length of the array
357 -- here at compile time
359 dsExpr (ExplicitPArr ty xs)
360 = dsLookupGlobalId toPName `thenDs` \toP ->
361 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
362 returnDs (mkApps (Var toP) [Type ty, coreList])
364 dsExpr (ExplicitTuple expr_list boxity)
365 = mappM dsLExpr expr_list `thenDs` \ core_exprs ->
366 returnDs (mkConApp (tupleCon boxity (length expr_list))
367 (map (Type . exprType) core_exprs ++ core_exprs))
369 dsExpr (ArithSeq expr (From from))
370 = dsExpr expr `thenDs` \ expr2 ->
371 dsLExpr from `thenDs` \ from2 ->
372 returnDs (App expr2 from2)
374 dsExpr (ArithSeq expr (FromTo from two))
375 = dsExpr expr `thenDs` \ expr2 ->
376 dsLExpr from `thenDs` \ from2 ->
377 dsLExpr two `thenDs` \ two2 ->
378 returnDs (mkApps expr2 [from2, two2])
380 dsExpr (ArithSeq expr (FromThen from thn))
381 = dsExpr expr `thenDs` \ expr2 ->
382 dsLExpr from `thenDs` \ from2 ->
383 dsLExpr thn `thenDs` \ thn2 ->
384 returnDs (mkApps expr2 [from2, thn2])
386 dsExpr (ArithSeq expr (FromThenTo from thn two))
387 = dsExpr expr `thenDs` \ expr2 ->
388 dsLExpr from `thenDs` \ from2 ->
389 dsLExpr thn `thenDs` \ thn2 ->
390 dsLExpr two `thenDs` \ two2 ->
391 returnDs (mkApps expr2 [from2, thn2, two2])
393 dsExpr (PArrSeq expr (FromTo from two))
394 = dsExpr expr `thenDs` \ expr2 ->
395 dsLExpr from `thenDs` \ from2 ->
396 dsLExpr two `thenDs` \ two2 ->
397 returnDs (mkApps expr2 [from2, two2])
399 dsExpr (PArrSeq expr (FromThenTo from thn two))
400 = dsExpr expr `thenDs` \ expr2 ->
401 dsLExpr from `thenDs` \ from2 ->
402 dsLExpr thn `thenDs` \ thn2 ->
403 dsLExpr two `thenDs` \ two2 ->
404 returnDs (mkApps expr2 [from2, thn2, two2])
406 dsExpr (PArrSeq expr _)
407 = panic "DsExpr.dsExpr: Infinite parallel array!"
408 -- the parser shouldn't have generated it and the renamer and typechecker
409 -- shouldn't have let it through
413 \underline{\bf Record construction and update}
414 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
415 For record construction we do this (assuming T has three arguments)
419 let err = /\a -> recConErr a
420 T (recConErr t1 "M.lhs/230/op1")
422 (recConErr t1 "M.lhs/230/op3")
424 @recConErr@ then converts its arugment string into a proper message
425 before printing it as
427 M.lhs, line 230: missing field op1 was evaluated
430 We also handle @C{}@ as valid construction syntax for an unlabelled
431 constructor @C@, setting all of @C@'s fields to bottom.
434 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds)
435 = dsExpr con_expr `thenDs` \ con_expr' ->
437 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
438 -- A newtype in the corner should be opaque;
439 -- hence TcType.tcSplitFunTys
441 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
442 = case [rhs | (L _ sel_id, rhs) <- rbinds, lbl == idName sel_id] of
443 (rhs:rhss) -> ASSERT( null rhss )
445 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
446 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
448 labels = dataConFieldLabels (idDataCon data_con_id)
449 -- The data_con_id is guaranteed to be the wrapper id of the constructor
453 then mappM unlabelled_bottom arg_tys
454 else mappM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
455 `thenDs` \ con_args ->
457 returnDs (mkApps con_expr' con_args)
460 Record update is a little harder. Suppose we have the decl:
462 data T = T1 {op1, op2, op3 :: Int}
463 | T2 {op4, op2 :: Int}
466 Then we translate as follows:
472 T1 op1 _ op3 -> T1 op1 op2 op3
473 T2 op4 _ -> T2 op4 op2
474 other -> recUpdError "M.lhs/230"
476 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
477 RHSs, and do not generate a Core constructor application directly, because the constructor
478 might do some argument-evaluation first; and may have to throw away some
482 dsExpr (RecordUpd record_expr [] record_in_ty record_out_ty)
483 = dsLExpr record_expr
485 dsExpr expr@(RecordUpd record_expr rbinds record_in_ty record_out_ty)
486 = dsLExpr record_expr `thenDs` \ record_expr' ->
488 -- Desugar the rbinds, and generate let-bindings if
489 -- necessary so that we don't lose sharing
492 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
493 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
494 in_out_ty = mkFunTy record_in_ty record_out_ty
496 mk_val_arg field old_arg_id
497 = case [rhs | (L _ sel_id, rhs) <- rbinds, field == idName sel_id] of
498 (rhs:rest) -> ASSERT(null rest) rhs
499 [] -> nlHsVar old_arg_id
502 = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
503 -- This call to dataConInstOrigArgTys won't work for existentials
504 -- but existentials don't have record types anyway
506 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
507 (dataConFieldLabels con) arg_ids
508 rhs = foldl (\a b -> nlHsApp a b)
509 (noLoc $ TyApp (nlHsVar (dataConWrapId con))
513 returnDs (mkSimpleMatch [noLoc $ ConPatOut (noLoc con) [] [] emptyLHsBinds
514 (PrefixCon (map nlVarPat arg_ids)) record_in_ty]
517 -- Record stuff doesn't work for existentials
518 -- The type checker checks for this, but we need
519 -- worry only about the constructors that are to be updated
520 ASSERT2( all isVanillaDataCon cons_to_upd, ppr expr )
522 -- It's important to generate the match with matchWrapper,
523 -- and the right hand sides with applications of the wrapper Id
524 -- so that everything works when we are doing fancy unboxing on the
525 -- constructor aguments.
526 mappM mk_alt cons_to_upd `thenDs` \ alts ->
527 matchWrapper RecUpd (MatchGroup alts in_out_ty) `thenDs` \ ([discrim_var], matching_code) ->
529 returnDs (bindNonRec discrim_var record_expr' matching_code)
532 updated_fields :: [FieldLabel]
533 updated_fields = [ idName sel_id | (L _ sel_id,_) <- rbinds]
535 -- Get the type constructor from the record_in_ty
536 -- so that we are sure it'll have all its DataCons
537 -- (In GHCI, it's possible that some TyCons may not have all
538 -- their constructors, in a module-loop situation.)
539 tycon = tcTyConAppTyCon record_in_ty
540 data_cons = tyConDataCons tycon
541 cons_to_upd = filter has_all_fields data_cons
543 has_all_fields :: DataCon -> Bool
544 has_all_fields con_id
545 = all (`elem` con_fields) updated_fields
547 con_fields = dataConFieldLabels con_id
552 \underline{\bf Dictionary lambda and application}
553 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
554 @DictLam@ and @DictApp@ turn into the regular old things.
555 (OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
556 complicated; reminiscent of fully-applied constructors.
558 dsExpr (DictLam dictvars expr)
559 = dsLExpr expr `thenDs` \ core_expr ->
560 returnDs (mkLams dictvars core_expr)
564 dsExpr (DictApp expr dicts) -- becomes a curried application
565 = dsLExpr expr `thenDs` \ core_expr ->
566 returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)
568 dsExpr (HsCoerce co_fn e) = dsCoercion co_fn (dsExpr e)
571 Here is where we desugar the Template Haskell brackets and escapes
574 -- Template Haskell stuff
576 #ifdef GHCI /* Only if bootstrapping */
577 dsExpr (HsBracketOut x ps) = dsBracket x ps
578 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
581 -- Arrow notation extension
582 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
589 -- HsSyn constructs that just shouldn't be here:
590 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
595 %--------------------------------------------------------------------
597 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
598 handled in DsListComp). Basically does the translation given in the
604 -> Type -- Type of the whole expression
607 dsDo stmts body result_ty
608 = go (map unLoc stmts)
612 go (ExprStmt rhs then_expr _ : stmts)
613 = do { rhs2 <- dsLExpr rhs
614 ; then_expr2 <- dsExpr then_expr
616 ; returnDs (mkApps then_expr2 [rhs2, rest]) }
618 go (LetStmt binds : stmts)
619 = do { rest <- go stmts
620 ; dsLocalBinds binds rest }
622 go (BindStmt pat rhs bind_op fail_op : stmts)
623 = do { body <- go stmts
624 ; var <- selectSimpleMatchVarL pat
625 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
626 result_ty (cantFailMatchResult body)
627 ; match_code <- handle_failure pat match fail_op
628 ; rhs' <- dsLExpr rhs
629 ; bind_op' <- dsExpr bind_op
630 ; returnDs (mkApps bind_op' [rhs', Lam var match_code]) }
632 -- In a do expression, pattern-match failure just calls
633 -- the monadic 'fail' rather than throwing an exception
634 handle_failure pat match fail_op
636 = do { fail_op' <- dsExpr fail_op
637 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
638 ; extractMatchResult match (App fail_op' fail_msg) }
640 = extractMatchResult match (error "It can't fail")
642 mk_fail_msg pat = "Pattern match failure in do expression at " ++
643 showSDoc (ppr (getLoc pat))
646 Translation for RecStmt's:
647 -----------------------------
648 We turn (RecStmt [v1,..vn] stmts) into:
650 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
657 -> Type -- Type of the whole expression
660 dsMDo tbl stmts body result_ty
661 = go (map unLoc stmts)
663 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
664 mfix_id = lookupEvidence tbl mfixName
665 return_id = lookupEvidence tbl returnMName
666 bind_id = lookupEvidence tbl bindMName
667 then_id = lookupEvidence tbl thenMName
668 fail_id = lookupEvidence tbl failMName
673 go (LetStmt binds : stmts)
674 = do { rest <- go stmts
675 ; dsLocalBinds binds rest }
677 go (ExprStmt rhs _ rhs_ty : stmts)
678 = do { rhs2 <- dsLExpr rhs
680 ; returnDs (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
682 go (BindStmt pat rhs _ _ : stmts)
683 = do { body <- go stmts
684 ; var <- selectSimpleMatchVarL pat
685 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
686 result_ty (cantFailMatchResult body)
687 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
688 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
689 ; match_code <- extractMatchResult match fail_expr
691 ; rhs' <- dsLExpr rhs
692 ; returnDs (mkApps (Var bind_id) [Type (hsPatType pat), Type b_ty,
693 rhs', Lam var match_code]) }
695 go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
696 = ASSERT( length rec_ids > 0 )
697 ASSERT( length rec_ids == length rec_rets )
698 go (new_bind_stmt : let_stmt : stmts)
700 new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
701 let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
704 -- Remove the later_ids that appear (without fancy coercions)
705 -- in rec_rets, because there's no need to knot-tie them separately
706 -- See Note [RecStmt] in HsExpr
707 later_ids' = filter (`notElem` mono_rec_ids) later_ids
708 mono_rec_ids = [ id | HsVar id <- rec_rets ]
710 mfix_app = nlHsApp (noLoc $ TyApp (nlHsVar mfix_id) [tup_ty]) mfix_arg
711 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
712 (mkFunTy tup_ty body_ty))
714 -- The rec_tup_pat must bind the rec_ids only; remember that the
715 -- trimmed_laters may share the same Names
716 -- Meanwhile, the later_pats must bind the later_vars
717 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
718 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
719 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
721 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
722 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
723 body_ty = mkAppTy m_ty tup_ty
724 tup_ty = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
725 -- mkCoreTupTy deals with singleton case
727 return_app = nlHsApp (noLoc $ TyApp (nlHsVar return_id) [tup_ty])
730 mk_wild_pat :: Id -> LPat Id
731 mk_wild_pat v = noLoc $ WildPat $ idType v
733 mk_later_pat :: Id -> LPat Id
734 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
735 | otherwise = nlVarPat v
737 mk_tup_pat :: [LPat Id] -> LPat Id
739 mk_tup_pat ps = noLoc $ TuplePat ps Boxed
741 mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
743 mk_ret_tup rs = noLoc $ ExplicitTuple rs Boxed