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"
10 #if defined(GHCI) && defined(BREAKPOINT)
11 import Foreign.StablePtr ( newStablePtr, castStablePtrToPtr )
12 import GHC.Exts ( Ptr(..), Int(..), addr2Int# )
13 import IOEnv ( ioToIOEnv )
14 import PrelNames ( breakpointJumpName, breakpointCondJumpName )
15 import TysWiredIn ( unitTy )
16 import TypeRep ( Type(..) )
17 import TyCon ( isUnLiftedTyCon )
20 import Match ( matchWrapper, matchSinglePat, matchEquations )
21 import MatchLit ( dsLit, dsOverLit )
22 import DsBinds ( dsLHsBinds, dsCoercion )
23 import DsGRHSs ( dsGuarded )
24 import DsListComp ( dsListComp, dsPArrComp )
25 import DsUtils ( mkErrorAppDs, mkStringExpr, mkConsExpr, mkNilExpr,
26 extractMatchResult, cantFailMatchResult, matchCanFail,
27 mkCoreTupTy, selectSimpleMatchVarL, lookupEvidence, selectMatchVar )
28 import DsArrows ( dsProcExpr )
32 -- Template Haskell stuff iff bootstrapped
33 import DsMeta ( dsBracket )
37 import TcHsSyn ( hsLPatType, mkVanillaTuplePat )
39 -- NB: The desugarer, which straddles the source and Core worlds, sometimes
40 -- needs to see source types (newtypes etc), and sometimes not
41 -- So WATCH OUT; check each use of split*Ty functions.
42 -- Sigh. This is a pain.
44 import TcType ( tcSplitAppTy, tcSplitFunTys, tcTyConAppTyCon,
45 tcTyConAppArgs, isUnLiftedType, Type, mkAppTy )
46 import Type ( splitFunTys, isUnboxedTupleType, mkFunTy )
48 import CoreUtils ( exprType, mkIfThenElse, bindNonRec )
50 import CostCentre ( mkUserCC )
51 import Id ( Id, idType, idName, idDataCon )
52 import PrelInfo ( rEC_CON_ERROR_ID )
53 import DataCon ( DataCon, dataConWrapId, dataConFieldLabels, dataConInstOrigArgTys )
54 import DataCon ( isVanillaDataCon )
55 import TyCon ( FieldLabel, tyConDataCons )
56 import TysWiredIn ( tupleCon )
57 import BasicTypes ( RecFlag(..), Boxity(..), ipNameName )
58 import PrelNames ( toPName,
59 returnMName, bindMName, thenMName, failMName,
61 import SrcLoc ( Located(..), unLoc, getLoc, noLoc )
62 import Util ( zipEqual, zipWithEqual )
63 import Bag ( bagToList )
69 %************************************************************************
71 dsLocalBinds, dsValBinds
73 %************************************************************************
76 dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
77 dsLocalBinds EmptyLocalBinds body = return body
78 dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
79 dsLocalBinds (HsIPBinds binds) body = dsIPBinds binds body
81 -------------------------
82 dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
83 dsValBinds (ValBindsOut binds _) body = foldrDs ds_val_bind body binds
85 -------------------------
86 dsIPBinds (IPBinds ip_binds dict_binds) body
87 = do { prs <- dsLHsBinds dict_binds
88 ; let inner = Let (Rec prs) body
89 -- The dict bindings may not be in
90 -- dependency order; hence Rec
91 ; foldrDs ds_ip_bind inner ip_binds }
93 ds_ip_bind (L _ (IPBind n e)) body
94 = dsLExpr e `thenDs` \ e' ->
95 returnDs (Let (NonRec (ipNameName n) e') body)
97 -------------------------
98 ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
99 -- Special case for bindings which bind unlifted variables
100 -- We need to do a case right away, rather than building
101 -- a tuple and doing selections.
102 -- Silently ignore INLINE and SPECIALISE pragmas...
103 ds_val_bind (NonRecursive, hsbinds) body
104 | [L _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
105 (L loc bind : null_binds) <- bagToList binds,
107 || isUnboxedTupleBind bind
108 || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
110 body_w_exports = foldr bind_export body exports
111 bind_export (tvs, g, l, _) body = ASSERT( null tvs )
112 bindNonRec g (Var l) body
114 ASSERT (null null_binds)
115 -- Non-recursive, non-overloaded bindings only come in ones
116 -- ToDo: in some bizarre case it's conceivable that there
117 -- could be dict binds in the 'binds'. (See the notes
118 -- below. Then pattern-match would fail. Urk.)
121 FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn }
122 -> matchWrapper (FunRhs (idName fun)) matches `thenDs` \ (args, rhs) ->
123 ASSERT( null args ) -- Functions aren't lifted
124 ASSERT( isIdCoercion co_fn )
125 returnDs (bindNonRec fun rhs body_w_exports)
127 PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }
128 -> -- let C x# y# = rhs in body
129 -- ==> case rhs of C x# y# -> body
131 do { rhs <- dsGuarded grhss ty
132 ; let upat = unLoc pat
133 eqn = EqnInfo { eqn_pats = [upat],
134 eqn_rhs = cantFailMatchResult body_w_exports }
135 ; var <- selectMatchVar upat
136 ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
137 ; return (scrungleMatch var rhs result) }
139 other -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
142 -- Ordinary case for bindings; none should be unlifted
143 ds_val_bind (is_rec, binds) body
144 = do { prs <- dsLHsBinds binds
145 ; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
148 other -> return (Let (Rec prs) body) }
149 -- Use a Rec regardless of is_rec.
150 -- Why? Because it allows the binds to be all
151 -- mixed up, which is what happens in one rare case
152 -- Namely, for an AbsBind with no tyvars and no dicts,
153 -- but which does have dictionary bindings.
154 -- See notes with TcSimplify.inferLoop [NO TYVARS]
155 -- It turned out that wrapping a Rec here was the easiest solution
157 -- NB The previous case dealt with unlifted bindings, so we
158 -- only have to deal with lifted ones now; so Rec is ok
160 isUnboxedTupleBind :: HsBind Id -> Bool
161 isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
162 isUnboxedTupleBind other = False
164 scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
165 -- Returns something like (let var = scrut in body)
166 -- but if var is an unboxed-tuple type, it inlines it in a fragile way
167 -- Special case to handle unboxed tuple patterns; they can't appear nested
169 -- case e of (# p1, p2 #) -> rhs
171 -- case e of (# x1, x2 #) -> ... match p1, p2 ...
173 -- let x = e in case x of ....
175 -- But there may be a big
176 -- let fail = ... in case e of ...
177 -- wrapping the whole case, which complicates matters slightly
178 -- It all seems a bit fragile. Test is dsrun013.
180 scrungleMatch var scrut body
181 | isUnboxedTupleType (idType var) = scrungle body
182 | otherwise = bindNonRec var scrut body
184 scrungle (Case (Var x) bndr ty alts)
185 | x == var = Case scrut bndr ty alts
186 scrungle (Let binds body) = Let binds (scrungle body)
187 scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
190 %************************************************************************
192 \subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
194 %************************************************************************
197 dsLExpr :: LHsExpr Id -> DsM CoreExpr
198 dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e
200 dsExpr :: HsExpr Id -> DsM CoreExpr
202 dsExpr (HsPar e) = dsLExpr e
203 dsExpr (ExprWithTySigOut e _) = dsLExpr e
204 dsExpr (HsVar var) = returnDs (Var var)
205 dsExpr (HsIPVar ip) = returnDs (Var (ipNameName ip))
206 dsExpr (HsLit lit) = dsLit lit
207 dsExpr (HsOverLit lit) = dsOverLit lit
208 dsExpr (HsCoerce co_fn e) = dsCoercion co_fn (dsExpr e)
210 dsExpr (NegApp expr neg_expr)
211 = do { core_expr <- dsLExpr expr
212 ; core_neg <- dsExpr neg_expr
213 ; return (core_neg `App` core_expr) }
215 dsExpr expr@(HsLam a_Match)
216 = matchWrapper LambdaExpr a_Match `thenDs` \ (binders, matching_code) ->
217 returnDs (mkLams binders matching_code)
219 #if defined(GHCI) && defined(BREAKPOINT)
220 dsExpr (HsApp (L _ (HsApp realFun@(L _ (HsCoerce _ fun)) (L loc arg))) _)
222 , idName funId `elem` [breakpointJumpName, breakpointCondJumpName]
223 , ids <- filter (isValidType . idType) (extractIds arg)
224 = do warnDs (text "Extracted ids:" <+> ppr ids <+> ppr (map idType ids))
225 stablePtr <- ioToIOEnv $ newStablePtr ids
226 -- Yes, I know... I'm gonna burn in hell.
227 let Ptr addr# = castStablePtrToPtr stablePtr
228 funCore <- dsLExpr realFun
229 argCore <- dsLExpr (L loc (HsLit (HsInt (fromIntegral (I# (addr2Int# addr#))))))
230 hvalCore <- dsLExpr (L loc (extractHVals ids))
231 return ((funCore `App` argCore) `App` hvalCore)
232 where extractIds :: HsExpr Id -> [Id]
233 extractIds (HsApp fn arg)
234 | HsVar argId <- unLoc arg
235 = argId:extractIds (unLoc fn)
236 | HsCoerce co_fn arg' <- unLoc arg
237 , HsVar argId <- arg' -- SLPJ: not sure what is going on here
238 = error (showSDoc (ppr co_fn)) -- argId:extractIds (unLoc fn)
240 extractHVals ids = ExplicitList unitTy (map (L loc . HsVar) ids)
241 -- checks for tyvars and unlifted kinds.
242 isValidType (TyVarTy _) = False
243 isValidType (FunTy a b) = isValidType a && isValidType b
244 isValidType (NoteTy _ t) = isValidType t
245 isValidType (AppTy a b) = isValidType a && isValidType b
246 isValidType (TyConApp con ts) = not (isUnLiftedTyCon con) && all isValidType ts
250 dsExpr expr@(HsApp fun arg)
251 = dsLExpr fun `thenDs` \ core_fun ->
252 dsLExpr arg `thenDs` \ core_arg ->
253 returnDs (core_fun `App` core_arg)
256 Operator sections. At first it looks as if we can convert
265 But no! expr might be a redex, and we can lose laziness badly this
270 for example. So we convert instead to
272 let y = expr in \x -> op y x
274 If \tr{expr} is actually just a variable, say, then the simplifier
278 dsExpr (OpApp e1 op _ e2)
279 = dsLExpr op `thenDs` \ core_op ->
280 -- for the type of y, we need the type of op's 2nd argument
281 dsLExpr e1 `thenDs` \ x_core ->
282 dsLExpr e2 `thenDs` \ y_core ->
283 returnDs (mkApps core_op [x_core, y_core])
285 dsExpr (SectionL expr op) -- Desugar (e !) to ((!) e)
286 = dsLExpr op `thenDs` \ core_op ->
287 dsLExpr expr `thenDs` \ x_core ->
288 returnDs (App core_op x_core)
290 -- dsLExpr (SectionR op expr) -- \ x -> op x expr
291 dsExpr (SectionR op expr)
292 = dsLExpr op `thenDs` \ core_op ->
293 -- for the type of x, we need the type of op's 2nd argument
295 (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
296 -- See comment with SectionL
298 dsLExpr expr `thenDs` \ y_core ->
299 newSysLocalDs x_ty `thenDs` \ x_id ->
300 newSysLocalDs y_ty `thenDs` \ y_id ->
302 returnDs (bindNonRec y_id y_core $
303 Lam x_id (mkApps core_op [Var x_id, Var y_id]))
305 dsExpr (HsSCC cc expr)
306 = dsLExpr expr `thenDs` \ core_expr ->
307 getModuleDs `thenDs` \ mod_name ->
308 returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)
311 -- hdaume: core annotation
313 dsExpr (HsCoreAnn fs expr)
314 = dsLExpr expr `thenDs` \ core_expr ->
315 returnDs (Note (CoreNote $ unpackFS fs) core_expr)
317 dsExpr (HsCase discrim matches)
318 = dsLExpr discrim `thenDs` \ core_discrim ->
319 matchWrapper CaseAlt matches `thenDs` \ ([discrim_var], matching_code) ->
320 returnDs (scrungleMatch discrim_var core_discrim matching_code)
322 dsExpr (HsLet binds body)
323 = dsLExpr body `thenDs` \ body' ->
324 dsLocalBinds binds body'
326 -- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
327 -- because the interpretation of `stmts' depends on what sort of thing it is.
329 dsExpr (HsDo ListComp stmts body result_ty)
330 = -- Special case for list comprehensions
331 dsListComp stmts body elt_ty
333 [elt_ty] = tcTyConAppArgs result_ty
335 dsExpr (HsDo DoExpr stmts body result_ty)
336 = dsDo stmts body result_ty
338 dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
339 = dsMDo tbl stmts body result_ty
341 dsExpr (HsDo PArrComp stmts body result_ty)
342 = -- Special case for array comprehensions
343 dsPArrComp (map unLoc stmts) body elt_ty
345 [elt_ty] = tcTyConAppArgs result_ty
347 dsExpr (HsIf guard_expr then_expr else_expr)
348 = dsLExpr guard_expr `thenDs` \ core_guard ->
349 dsLExpr then_expr `thenDs` \ core_then ->
350 dsLExpr else_expr `thenDs` \ core_else ->
351 returnDs (mkIfThenElse core_guard core_then core_else)
356 \underline{\bf Various data construction things}
357 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
359 dsExpr (ExplicitList ty xs)
362 go [] = returnDs (mkNilExpr ty)
363 go (x:xs) = dsLExpr x `thenDs` \ core_x ->
364 go xs `thenDs` \ core_xs ->
365 returnDs (mkConsExpr ty core_x core_xs)
367 -- we create a list from the array elements and convert them into a list using
370 -- * the main disadvantage to this scheme is that `toP' traverses the list
371 -- twice: once to determine the length and a second time to put to elements
372 -- into the array; this inefficiency could be avoided by exposing some of
373 -- the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
374 -- that we can exploit the fact that we already know the length of the array
375 -- here at compile time
377 dsExpr (ExplicitPArr ty xs)
378 = dsLookupGlobalId toPName `thenDs` \toP ->
379 dsExpr (ExplicitList ty xs) `thenDs` \coreList ->
380 returnDs (mkApps (Var toP) [Type ty, coreList])
382 dsExpr (ExplicitTuple expr_list boxity)
383 = mappM dsLExpr expr_list `thenDs` \ core_exprs ->
384 returnDs (mkConApp (tupleCon boxity (length expr_list))
385 (map (Type . exprType) core_exprs ++ core_exprs))
387 dsExpr (ArithSeq expr (From from))
388 = dsExpr expr `thenDs` \ expr2 ->
389 dsLExpr from `thenDs` \ from2 ->
390 returnDs (App expr2 from2)
392 dsExpr (ArithSeq 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 (ArithSeq expr (FromThen from thn))
399 = dsExpr expr `thenDs` \ expr2 ->
400 dsLExpr from `thenDs` \ from2 ->
401 dsLExpr thn `thenDs` \ thn2 ->
402 returnDs (mkApps expr2 [from2, thn2])
404 dsExpr (ArithSeq expr (FromThenTo from thn two))
405 = dsExpr expr `thenDs` \ expr2 ->
406 dsLExpr from `thenDs` \ from2 ->
407 dsLExpr thn `thenDs` \ thn2 ->
408 dsLExpr two `thenDs` \ two2 ->
409 returnDs (mkApps expr2 [from2, thn2, two2])
411 dsExpr (PArrSeq expr (FromTo from two))
412 = dsExpr expr `thenDs` \ expr2 ->
413 dsLExpr from `thenDs` \ from2 ->
414 dsLExpr two `thenDs` \ two2 ->
415 returnDs (mkApps expr2 [from2, two2])
417 dsExpr (PArrSeq expr (FromThenTo from thn two))
418 = dsExpr expr `thenDs` \ expr2 ->
419 dsLExpr from `thenDs` \ from2 ->
420 dsLExpr thn `thenDs` \ thn2 ->
421 dsLExpr two `thenDs` \ two2 ->
422 returnDs (mkApps expr2 [from2, thn2, two2])
424 dsExpr (PArrSeq expr _)
425 = panic "DsExpr.dsExpr: Infinite parallel array!"
426 -- the parser shouldn't have generated it and the renamer and typechecker
427 -- shouldn't have let it through
431 \underline{\bf Record construction and update}
432 % ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
433 For record construction we do this (assuming T has three arguments)
437 let err = /\a -> recConErr a
438 T (recConErr t1 "M.lhs/230/op1")
440 (recConErr t1 "M.lhs/230/op3")
442 @recConErr@ then converts its arugment string into a proper message
443 before printing it as
445 M.lhs, line 230: missing field op1 was evaluated
448 We also handle @C{}@ as valid construction syntax for an unlabelled
449 constructor @C@, setting all of @C@'s fields to bottom.
452 dsExpr (RecordCon (L _ data_con_id) con_expr rbinds)
453 = dsExpr con_expr `thenDs` \ con_expr' ->
455 (arg_tys, _) = tcSplitFunTys (exprType con_expr')
456 -- A newtype in the corner should be opaque;
457 -- hence TcType.tcSplitFunTys
459 mk_arg (arg_ty, lbl) -- Selector id has the field label as its name
460 = case [rhs | (L _ sel_id, rhs) <- rbinds, lbl == idName sel_id] of
461 (rhs:rhss) -> ASSERT( null rhss )
463 [] -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
464 unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""
466 labels = dataConFieldLabels (idDataCon data_con_id)
467 -- The data_con_id is guaranteed to be the wrapper id of the constructor
471 then mappM unlabelled_bottom arg_tys
472 else mappM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
473 `thenDs` \ con_args ->
475 returnDs (mkApps con_expr' con_args)
478 Record update is a little harder. Suppose we have the decl:
480 data T = T1 {op1, op2, op3 :: Int}
481 | T2 {op4, op2 :: Int}
484 Then we translate as follows:
490 T1 op1 _ op3 -> T1 op1 op2 op3
491 T2 op4 _ -> T2 op4 op2
492 other -> recUpdError "M.lhs/230"
494 It's important that we use the constructor Ids for @T1@, @T2@ etc on the
495 RHSs, and do not generate a Core constructor application directly, because the constructor
496 might do some argument-evaluation first; and may have to throw away some
500 dsExpr (RecordUpd record_expr [] record_in_ty record_out_ty)
501 = dsLExpr record_expr
503 dsExpr expr@(RecordUpd record_expr rbinds record_in_ty record_out_ty)
504 = dsLExpr record_expr `thenDs` \ record_expr' ->
506 -- Desugar the rbinds, and generate let-bindings if
507 -- necessary so that we don't lose sharing
510 in_inst_tys = tcTyConAppArgs record_in_ty -- Newtype opaque
511 out_inst_tys = tcTyConAppArgs record_out_ty -- Newtype opaque
512 in_out_ty = mkFunTy record_in_ty record_out_ty
514 mk_val_arg field old_arg_id
515 = case [rhs | (L _ sel_id, rhs) <- rbinds, field == idName sel_id] of
516 (rhs:rest) -> ASSERT(null rest) rhs
517 [] -> nlHsVar old_arg_id
520 = ASSERT( isVanillaDataCon con )
521 newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
522 -- This call to dataConInstOrigArgTys won't work for existentials
523 -- but existentials don't have record types anyway
525 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
526 (dataConFieldLabels con) arg_ids
527 rhs = foldl (\a b -> nlHsApp a b)
528 (nlHsTyApp (dataConWrapId con) out_inst_tys)
531 returnDs (mkSimpleMatch [mkPrefixConPat con (map nlVarPat arg_ids) record_in_ty] rhs)
533 -- Record stuff doesn't work for existentials
534 -- The type checker checks for this, but we need
535 -- worry only about the constructors that are to be updated
536 ASSERT2( all isVanillaDataCon cons_to_upd, ppr expr )
538 -- It's important to generate the match with matchWrapper,
539 -- and the right hand sides with applications of the wrapper Id
540 -- so that everything works when we are doing fancy unboxing on the
541 -- constructor aguments.
542 mappM mk_alt cons_to_upd `thenDs` \ alts ->
543 matchWrapper RecUpd (MatchGroup alts in_out_ty) `thenDs` \ ([discrim_var], matching_code) ->
545 returnDs (bindNonRec discrim_var record_expr' matching_code)
548 updated_fields :: [FieldLabel]
549 updated_fields = [ idName sel_id | (L _ sel_id,_) <- rbinds]
551 -- Get the type constructor from the record_in_ty
552 -- so that we are sure it'll have all its DataCons
553 -- (In GHCI, it's possible that some TyCons may not have all
554 -- their constructors, in a module-loop situation.)
555 tycon = tcTyConAppTyCon record_in_ty
556 data_cons = tyConDataCons tycon
557 cons_to_upd = filter has_all_fields data_cons
559 has_all_fields :: DataCon -> Bool
560 has_all_fields con_id
561 = all (`elem` con_fields) updated_fields
563 con_fields = dataConFieldLabels con_id
566 Here is where we desugar the Template Haskell brackets and escapes
569 -- Template Haskell stuff
571 #ifdef GHCI /* Only if bootstrapping */
572 dsExpr (HsBracketOut x ps) = dsBracket x ps
573 dsExpr (HsSpliceE s) = pprPanic "dsExpr:splice" (ppr s)
576 -- Arrow notation extension
577 dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
584 -- HsSyn constructs that just shouldn't be here:
585 dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
590 %--------------------------------------------------------------------
592 Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
593 handled in DsListComp). Basically does the translation given in the
599 -> Type -- Type of the whole expression
602 dsDo stmts body result_ty
603 = go (map unLoc stmts)
607 go (ExprStmt rhs then_expr _ : stmts)
608 = do { rhs2 <- dsLExpr rhs
609 ; then_expr2 <- dsExpr then_expr
611 ; returnDs (mkApps then_expr2 [rhs2, rest]) }
613 go (LetStmt binds : stmts)
614 = do { rest <- go stmts
615 ; dsLocalBinds binds rest }
617 go (BindStmt pat rhs bind_op fail_op : stmts)
618 = do { body <- go stmts
619 ; var <- selectSimpleMatchVarL pat
620 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
621 result_ty (cantFailMatchResult body)
622 ; match_code <- handle_failure pat match fail_op
623 ; rhs' <- dsLExpr rhs
624 ; bind_op' <- dsExpr bind_op
625 ; returnDs (mkApps bind_op' [rhs', Lam var match_code]) }
627 -- In a do expression, pattern-match failure just calls
628 -- the monadic 'fail' rather than throwing an exception
629 handle_failure pat match fail_op
631 = do { fail_op' <- dsExpr fail_op
632 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
633 ; extractMatchResult match (App fail_op' fail_msg) }
635 = extractMatchResult match (error "It can't fail")
637 mk_fail_msg pat = "Pattern match failure in do expression at " ++
638 showSDoc (ppr (getLoc pat))
641 Translation for RecStmt's:
642 -----------------------------
643 We turn (RecStmt [v1,..vn] stmts) into:
645 (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
652 -> Type -- Type of the whole expression
655 dsMDo tbl stmts body result_ty
656 = go (map unLoc stmts)
658 (m_ty, b_ty) = tcSplitAppTy result_ty -- result_ty must be of the form (m b)
659 mfix_id = lookupEvidence tbl mfixName
660 return_id = lookupEvidence tbl returnMName
661 bind_id = lookupEvidence tbl bindMName
662 then_id = lookupEvidence tbl thenMName
663 fail_id = lookupEvidence tbl failMName
668 go (LetStmt binds : stmts)
669 = do { rest <- go stmts
670 ; dsLocalBinds binds rest }
672 go (ExprStmt rhs _ rhs_ty : stmts)
673 = do { rhs2 <- dsLExpr rhs
675 ; returnDs (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
677 go (BindStmt pat rhs _ _ : stmts)
678 = do { body <- go stmts
679 ; var <- selectSimpleMatchVarL pat
680 ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
681 result_ty (cantFailMatchResult body)
682 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
683 ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
684 ; match_code <- extractMatchResult match fail_expr
686 ; rhs' <- dsLExpr rhs
687 ; returnDs (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty,
688 rhs', Lam var match_code]) }
690 go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
691 = ASSERT( length rec_ids > 0 )
692 ASSERT( length rec_ids == length rec_rets )
693 go (new_bind_stmt : let_stmt : stmts)
695 new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
696 let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
699 -- Remove the later_ids that appear (without fancy coercions)
700 -- in rec_rets, because there's no need to knot-tie them separately
701 -- See Note [RecStmt] in HsExpr
702 later_ids' = filter (`notElem` mono_rec_ids) later_ids
703 mono_rec_ids = [ id | HsVar id <- rec_rets ]
705 mfix_app = nlHsApp (nlHsTyApp mfix_id [tup_ty]) mfix_arg
706 mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
707 (mkFunTy tup_ty body_ty))
709 -- The rec_tup_pat must bind the rec_ids only; remember that the
710 -- trimmed_laters may share the same Names
711 -- Meanwhile, the later_pats must bind the later_vars
712 rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
713 later_pats = map nlVarPat later_ids' ++ map mk_later_pat rec_ids
714 rets = map nlHsVar later_ids' ++ map noLoc rec_rets
716 mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
717 body = noLoc $ HsDo ctxt rec_stmts return_app body_ty
718 body_ty = mkAppTy m_ty tup_ty
719 tup_ty = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
720 -- mkCoreTupTy deals with singleton case
722 return_app = nlHsApp (nlHsTyApp return_id [tup_ty])
725 mk_wild_pat :: Id -> LPat Id
726 mk_wild_pat v = noLoc $ WildPat $ idType v
728 mk_later_pat :: Id -> LPat Id
729 mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
730 | otherwise = nlVarPat v
732 mk_tup_pat :: [LPat Id] -> LPat Id
734 mk_tup_pat ps = noLoc $ mkVanillaTuplePat ps Boxed
736 mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
738 mk_ret_tup rs = noLoc $ ExplicitTuple rs Boxed