2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Desugaring list comprehensions and array comprehensions
9 {-# OPTIONS -fno-warn-incomplete-patterns #-}
10 -- The above warning supression flag is a temporary kludge.
11 -- While working on this module you are encouraged to remove it and fix
12 -- any warnings in the module. See
13 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
16 module DsListComp ( dsListComp, dsPArrComp ) where
18 #include "HsVersions.h"
20 import {-# SOURCE #-} DsExpr ( dsLExpr, dsLocalBinds )
27 import DsMonad -- the monadery used in the desugarer
43 List comprehensions may be desugared in one of two ways: ``ordinary''
44 (as you would expect if you read SLPJ's book) and ``with foldr/build
45 turned on'' (if you read Gill {\em et al.}'s paper on the subject).
47 There will be at least one ``qualifier'' in the input.
50 dsListComp :: [LStmt Id]
52 -> Type -- Type of list elements
54 dsListComp lquals body elt_ty = do
56 let quals = map unLoc lquals
58 if not (dopt Opt_EnableRewriteRules dflags) || dopt Opt_IgnoreInterfacePragmas dflags
59 -- Either rules are switched off, or we are ignoring what there are;
60 -- Either way foldr/build won't happen, so use the more efficient
61 -- Wadler-style desugaring
62 || isParallelComp quals
63 -- Foldr-style desugaring can't handle parallel list comprehensions
64 then deListComp quals body (mkNilExpr elt_ty)
65 else mkBuildExpr elt_ty (\(c, _) (n, _) -> dfListComp c n quals body)
66 -- Foldr/build should be enabled, so desugar
67 -- into foldrs and builds
70 -- We must test for ParStmt anywhere, not just at the head, because an extension
71 -- to list comprehensions would be to add brackets to specify the associativity
72 -- of qualifier lists. This is really easy to do by adding extra ParStmts into the
73 -- mix of possibly a single element in length, so we do this to leave the possibility open
74 isParallelComp = any isParallelStmt
76 isParallelStmt (ParStmt _) = True
77 isParallelStmt _ = False
80 -- This function lets you desugar a inner list comprehension and a list of the binders
81 -- of that comprehension that we need in the outer comprehension into such an expression
82 -- and the type of the elements that it outputs (tuples of binders)
83 dsInnerListComp :: ([LStmt Id], [Id]) -> DsM (CoreExpr, Type)
84 dsInnerListComp (stmts, bndrs) = do
85 expr <- dsListComp stmts (mkBigLHsVarTup bndrs) bndrs_tuple_type
86 return (expr, bndrs_tuple_type)
88 bndrs_types = map idType bndrs
89 bndrs_tuple_type = mkBigCoreTupTy bndrs_types
92 -- This function factors out commonality between the desugaring strategies for TransformStmt.
93 -- Given such a statement it gives you back an expression representing how to compute the transformed
94 -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
95 dsTransformStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
96 dsTransformStmt (TransformStmt stmts binders usingExpr maybeByExpr) = do
97 (expr, binders_tuple_type) <- dsInnerListComp (stmts, binders)
98 usingExpr' <- dsLExpr usingExpr
102 Nothing -> return [expr]
104 byExpr' <- dsLExpr byExpr
106 us <- newUniqueSupply
107 [tuple_binder] <- newSysLocalsDs [binders_tuple_type]
108 let byExprWrapper = mkTupleCase us binders byExpr' tuple_binder (Var tuple_binder)
110 return [Lam tuple_binder byExprWrapper, expr]
112 let inner_list_expr = mkApps usingExpr' ((Type binders_tuple_type) : using_args)
114 let pat = mkBigLHsVarPatTup binders
115 return (inner_list_expr, pat)
117 -- This function factors out commonality between the desugaring strategies for GroupStmt.
118 -- Given such a statement it gives you back an expression representing how to compute the transformed
119 -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
120 dsGroupStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
121 dsGroupStmt (GroupStmt stmts binderMap by using) = do
122 let (fromBinders, toBinders) = unzip binderMap
124 fromBindersTypes = map idType fromBinders
125 toBindersTypes = map idType toBinders
127 toBindersTupleType = mkBigCoreTupTy toBindersTypes
129 -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
130 (expr, from_tup_ty) <- dsInnerListComp (stmts, fromBinders)
132 -- Work out what arguments should be supplied to that expression: i.e. is an extraction
133 -- function required? If so, create that desugared function and add to arguments
134 usingExpr' <- dsLExpr (either id noLoc using)
135 usingArgs <- case by of
136 Nothing -> return [expr]
137 Just by_e -> do { by_e' <- dsLExpr by_e
138 ; us <- newUniqueSupply
139 ; [from_tup_id] <- newSysLocalsDs [from_tup_ty]
140 ; let by_wrap = mkTupleCase us fromBinders by_e'
141 from_tup_id (Var from_tup_id)
142 ; return [Lam from_tup_id by_wrap, expr] }
144 -- Create an unzip function for the appropriate arity and element types and find "map"
145 (unzip_fn, unzip_rhs) <- mkUnzipBind fromBindersTypes
146 map_id <- dsLookupGlobalId mapName
148 -- Generate the expressions to build the grouped list
149 let -- First we apply the grouping function to the inner list
150 inner_list_expr = mkApps usingExpr' ((Type from_tup_ty) : usingArgs)
151 -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
152 -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
153 -- the "b" to be a tuple of "to" lists!
154 unzipped_inner_list_expr = mkApps (Var map_id)
155 [Type (mkListTy from_tup_ty), Type toBindersTupleType, Var unzip_fn, inner_list_expr]
156 -- Then finally we bind the unzip function around that expression
157 bound_unzipped_inner_list_expr = Let (Rec [(unzip_fn, unzip_rhs)]) unzipped_inner_list_expr
159 -- Build a pattern that ensures the consumer binds into the NEW binders, which hold lists rather than single values
160 let pat = mkBigLHsVarPatTup toBinders
161 return (bound_unzipped_inner_list_expr, pat)
165 %************************************************************************
167 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
169 %************************************************************************
171 Just as in Phil's chapter~7 in SLPJ, using the rules for
172 optimally-compiled list comprehensions. This is what Kevin followed
173 as well, and I quite happily do the same. The TQ translation scheme
174 transforms a list of qualifiers (either boolean expressions or
175 generators) into a single expression which implements the list
176 comprehension. Because we are generating 2nd-order polymorphic
177 lambda-calculus, calls to NIL and CONS must be applied to a type
178 argument, as well as their usual value arguments.
180 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
183 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
186 TQ << [ e | b , qs ] ++ L >> =
187 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
190 TQ << [ e | p <- L1, qs ] ++ L2 >> =
196 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
201 "h", "u1", "u2", and "u3" are new variables.
204 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
205 is the TE translation scheme. Note that we carry around the @L@ list
206 already desugared. @dsListComp@ does the top TE rule mentioned above.
208 To the above, we add an additional rule to deal with parallel list
209 comprehensions. The translation goes roughly as follows:
210 [ e | p1 <- e11, let v1 = e12, p2 <- e13
211 | q1 <- e21, let v2 = e22, q2 <- e23]
213 [ e | ((x1, .., xn), (y1, ..., ym)) <-
214 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
215 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
216 where (x1, .., xn) are the variables bound in p1, v1, p2
217 (y1, .., ym) are the variables bound in q1, v2, q2
219 In the translation below, the ParStmt branch translates each parallel branch
220 into a sub-comprehension, and desugars each independently. The resulting lists
221 are fed to a zip function, we create a binding for all the variables bound in all
222 the comprehensions, and then we hand things off the the desugarer for bindings.
223 The zip function is generated here a) because it's small, and b) because then we
224 don't have to deal with arbitrary limits on the number of zip functions in the
225 prelude, nor which library the zip function came from.
226 The introduced tuples are Boxed, but only because I couldn't get it to work
227 with the Unboxed variety.
231 deListComp :: [Stmt Id] -> LHsExpr Id -> CoreExpr -> DsM CoreExpr
233 deListComp (ParStmt stmtss_w_bndrs : quals) body list
235 exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
236 let (exps, qual_tys) = unzip exps_and_qual_tys
238 (zip_fn, zip_rhs) <- mkZipBind qual_tys
240 -- Deal with [e | pat <- zip l1 .. ln] in example above
241 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
245 bndrs_s = map snd stmtss_w_bndrs
247 -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
248 pat = mkBigLHsPatTup pats
249 pats = map mkBigLHsVarPatTup bndrs_s
251 -- Last: the one to return
252 deListComp [] body list = do -- Figure 7.4, SLPJ, p 135, rule C above
253 core_body <- dsLExpr body
254 return (mkConsExpr (exprType core_body) core_body list)
256 -- Non-last: must be a guard
257 deListComp (ExprStmt guard _ _ : quals) body list = do -- rule B above
258 core_guard <- dsLExpr guard
259 core_rest <- deListComp quals body list
260 return (mkIfThenElse core_guard core_rest list)
262 -- [e | let B, qs] = let B in [e | qs]
263 deListComp (LetStmt binds : quals) body list = do
264 core_rest <- deListComp quals body list
265 dsLocalBinds binds core_rest
267 deListComp (stmt@(TransformStmt {}) : quals) body list = do
268 (inner_list_expr, pat) <- dsTransformStmt stmt
269 deBindComp pat inner_list_expr quals body list
271 deListComp (stmt@(GroupStmt {}) : quals) body list = do
272 (inner_list_expr, pat) <- dsGroupStmt stmt
273 deBindComp pat inner_list_expr quals body list
275 deListComp (BindStmt pat list1 _ _ : quals) body core_list2 = do -- rule A' above
276 core_list1 <- dsLExpr list1
277 deBindComp pat core_list1 quals body core_list2
282 deBindComp :: OutPat Id
288 deBindComp pat core_list1 quals body core_list2 = do
290 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
292 -- u1_ty is a [alpha] type, and u2_ty = alpha
293 u2_ty = hsLPatType pat
295 res_ty = exprType core_list2
296 h_ty = u1_ty `mkFunTy` res_ty
298 [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
300 -- the "fail" value ...
302 core_fail = App (Var h) (Var u3)
303 letrec_body = App (Var h) core_list1
305 rest_expr <- deListComp quals body core_fail
306 core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail
310 Case (Var u1) u1 res_ty
311 [(DataAlt nilDataCon, [], core_list2),
312 (DataAlt consDataCon, [u2, u3], core_match)]
313 -- Increasing order of tag
315 return (Let (Rec [(h, rhs)]) letrec_body)
318 %************************************************************************
320 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
322 %************************************************************************
324 @dfListComp@ are the rules used with foldr/build turned on:
327 TE[ e | ] c n = c e n
328 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
329 TE[ e | p <- l , q ] c n = let
330 f = \ x b -> case x of
338 dfListComp :: Id -> Id -- 'c' and 'n'
339 -> [Stmt Id] -- the rest of the qual's
343 -- Last: the one to return
344 dfListComp c_id n_id [] body = do
345 core_body <- dsLExpr body
346 return (mkApps (Var c_id) [core_body, Var n_id])
348 -- Non-last: must be a guard
349 dfListComp c_id n_id (ExprStmt guard _ _ : quals) body = do
350 core_guard <- dsLExpr guard
351 core_rest <- dfListComp c_id n_id quals body
352 return (mkIfThenElse core_guard core_rest (Var n_id))
354 dfListComp c_id n_id (LetStmt binds : quals) body = do
355 -- new in 1.3, local bindings
356 core_rest <- dfListComp c_id n_id quals body
357 dsLocalBinds binds core_rest
359 dfListComp c_id n_id (stmt@(TransformStmt {}) : quals) body = do
360 (inner_list_expr, pat) <- dsTransformStmt stmt
361 -- Anyway, we bind the newly transformed list via the generic binding function
362 dfBindComp c_id n_id (pat, inner_list_expr) quals body
364 dfListComp c_id n_id (stmt@(GroupStmt {}) : quals) body = do
365 (inner_list_expr, pat) <- dsGroupStmt stmt
366 -- Anyway, we bind the newly grouped list via the generic binding function
367 dfBindComp c_id n_id (pat, inner_list_expr) quals body
369 dfListComp c_id n_id (BindStmt pat list1 _ _ : quals) body = do
370 -- evaluate the two lists
371 core_list1 <- dsLExpr list1
373 -- Do the rest of the work in the generic binding builder
374 dfBindComp c_id n_id (pat, core_list1) quals body
376 dfBindComp :: Id -> Id -- 'c' and 'n'
377 -> (LPat Id, CoreExpr)
378 -> [Stmt Id] -- the rest of the qual's
381 dfBindComp c_id n_id (pat, core_list1) quals body = do
382 -- find the required type
383 let x_ty = hsLPatType pat
386 -- create some new local id's
387 [b, x] <- newSysLocalsDs [b_ty, x_ty]
389 -- build rest of the comprehesion
390 core_rest <- dfListComp c_id b quals body
392 -- build the pattern match
393 core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
394 pat core_rest (Var b)
396 -- now build the outermost foldr, and return
397 mkFoldrExpr x_ty b_ty (mkLams [x, b] core_expr) (Var n_id) core_list1
400 %************************************************************************
402 \subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
404 %************************************************************************
408 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
409 -- mkZipBind [t1, t2]
410 -- = (zip, \as1:[t1] as2:[t2]
413 -- (a1:as'1) -> case as2 of
415 -- (a2:as'2) -> (a1, a2) : zip as'1 as'2)]
417 mkZipBind elt_tys = do
418 ass <- mapM newSysLocalDs elt_list_tys
419 as' <- mapM newSysLocalDs elt_tys
420 as's <- mapM newSysLocalDs elt_list_tys
422 zip_fn <- newSysLocalDs zip_fn_ty
424 let inner_rhs = mkConsExpr elt_tuple_ty
425 (mkBigCoreVarTup as')
426 (mkVarApps (Var zip_fn) as's)
427 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
429 return (zip_fn, mkLams ass zip_body)
431 elt_list_tys = map mkListTy elt_tys
432 elt_tuple_ty = mkBigCoreTupTy elt_tys
433 elt_tuple_list_ty = mkListTy elt_tuple_ty
435 zip_fn_ty = mkFunTys elt_list_tys elt_tuple_list_ty
437 mk_case (as, a', as') rest
438 = Case (Var as) as elt_tuple_list_ty
439 [(DataAlt nilDataCon, [], mkNilExpr elt_tuple_ty),
440 (DataAlt consDataCon, [a', as'], rest)]
441 -- Increasing order of tag
444 mkUnzipBind :: [Type] -> DsM (Id, CoreExpr)
445 -- mkUnzipBind [t1, t2]
446 -- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
448 -- (x1, x2) -> case axs of
449 -- (xs1, xs2) -> (x1 : xs1, x2 : xs2))
453 -- We use foldr here in all cases, even if rules are turned off, because we may as well!
454 mkUnzipBind elt_tys = do
455 ax <- newSysLocalDs elt_tuple_ty
456 axs <- newSysLocalDs elt_list_tuple_ty
457 ys <- newSysLocalDs elt_tuple_list_ty
458 xs <- mapM newSysLocalDs elt_tys
459 xss <- mapM newSysLocalDs elt_list_tys
461 unzip_fn <- newSysLocalDs unzip_fn_ty
463 [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]
465 let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
467 concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
468 tupled_concat_expression = mkBigCoreTup concat_expressions
470 folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
471 folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
472 folder_body = mkLams [ax, axs] folder_body_outer_case
474 unzip_body <- mkFoldrExpr elt_tuple_ty elt_list_tuple_ty folder_body nil_tuple (Var ys)
475 return (unzip_fn, mkLams [ys] unzip_body)
477 elt_tuple_ty = mkBigCoreTupTy elt_tys
478 elt_tuple_list_ty = mkListTy elt_tuple_ty
479 elt_list_tys = map mkListTy elt_tys
480 elt_list_tuple_ty = mkBigCoreTupTy elt_list_tys
482 unzip_fn_ty = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty
484 mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail
490 %************************************************************************
492 \subsection[DsPArrComp]{Desugaring of array comprehensions}
494 %************************************************************************
498 -- entry point for desugaring a parallel array comprehension
500 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
502 dsPArrComp :: [Stmt Id]
504 -> Type -- Don't use; called with `undefined' below
506 dsPArrComp [ParStmt qss] body _ = -- parallel comprehension
507 dePArrParComp qss body
509 -- Special case for simple generators:
511 -- <<[:e' | p <- e, qs:]>> = <<[: e' | qs :]>> p e
513 -- if matching again p cannot fail, or else
515 -- <<[:e' | p <- e, qs:]>> =
516 -- <<[:e' | qs:]>> p (filterP (\x -> case x of {p -> True; _ -> False}) e)
518 dsPArrComp (BindStmt p e _ _ : qs) body _ = do
519 filterP <- dsLookupGlobalId filterPName
521 let ety'ce = parrElemType ce
522 false = Var falseDataConId
523 true = Var trueDataConId
524 v <- newSysLocalDs ety'ce
525 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
526 let gen | isIrrefutableHsPat p = ce
527 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
528 dePArrComp qs body p gen
530 dsPArrComp qs body _ = do -- no ParStmt in `qs'
531 sglP <- dsLookupGlobalId singletonPName
532 let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
533 dePArrComp qs body (noLoc $ WildPat unitTy) unitArray
539 dePArrComp :: [Stmt Id]
541 -> LPat Id -- the current generator pattern
542 -> CoreExpr -- the current generator expression
545 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
547 dePArrComp [] e' pa cea = do
548 mapP <- dsLookupGlobalId mapPName
549 let ty = parrElemType cea
550 (clam, ty'e') <- deLambda ty pa e'
551 return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
553 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
555 dePArrComp (ExprStmt b _ _ : qs) body pa cea = do
556 filterP <- dsLookupGlobalId filterPName
557 let ty = parrElemType cea
558 (clam,_) <- deLambda ty pa b
559 dePArrComp qs body pa (mkApps (Var filterP) [Type ty, clam, cea])
562 -- <<[:e' | p <- e, qs:]>> pa ea =
565 -- <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
567 -- if matching again p cannot fail, or else
569 -- <<[:e' | p <- e, qs:]>> pa ea =
570 -- let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
572 -- <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
574 dePArrComp (BindStmt p e _ _ : qs) body pa cea = do
575 filterP <- dsLookupGlobalId filterPName
576 crossMapP <- dsLookupGlobalId crossMapPName
578 let ety'cea = parrElemType cea
579 ety'ce = parrElemType ce
580 false = Var falseDataConId
581 true = Var trueDataConId
582 v <- newSysLocalDs ety'ce
583 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
584 let cef | isIrrefutableHsPat p = ce
585 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
586 (clam, _) <- mkLambda ety'cea pa cef
587 let ety'cef = ety'ce -- filter doesn't change the element type
588 pa' = mkLHsPatTup [pa, p]
590 dePArrComp qs body pa' (mkApps (Var crossMapP)
591 [Type ety'cea, Type ety'cef, cea, clam])
593 -- <<[:e' | let ds, qs:]>> pa ea =
594 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
595 -- (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
597 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
599 dePArrComp (LetStmt ds : qs) body pa cea = do
600 mapP <- dsLookupGlobalId mapPName
601 let xs = collectLocalBinders ds
602 ty'cea = parrElemType cea
603 v <- newSysLocalDs ty'cea
604 clet <- dsLocalBinds ds (mkCoreTup (map Var xs))
605 let'v <- newSysLocalDs (exprType clet)
606 let projBody = mkCoreLet (NonRec let'v clet) $
607 mkCoreTup [Var v, Var let'v]
608 errTy = exprType projBody
609 errMsg = ptext (sLit "DsListComp.dePArrComp: internal error!")
610 cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
611 ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
612 let pa' = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
613 proj = mkLams [v] ccase
614 dePArrComp qs body pa' (mkApps (Var mapP)
615 [Type ty'cea, Type errTy, proj, cea])
617 -- The parser guarantees that parallel comprehensions can only appear as
618 -- singeltons qualifier lists, which we already special case in the caller.
619 -- So, encountering one here is a bug.
621 dePArrComp (ParStmt _ : _) _ _ _ =
622 panic "DsListComp.dePArrComp: malformed comprehension AST"
624 -- <<[:e' | qs | qss:]>> pa ea =
625 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
626 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
628 -- {x_1, ..., x_n} = DV (qs)
630 dePArrParComp :: [([LStmt Id], [Id])] -> LHsExpr Id -> DsM CoreExpr
631 dePArrParComp qss body = do
632 (pQss, ceQss) <- deParStmt qss
633 dePArrComp [] body pQss ceQss
636 -- empty parallel statement lists have no source representation
637 panic "DsListComp.dePArrComp: Empty parallel list comprehension"
638 deParStmt ((qs, xs):qss) = do -- first statement
639 let res_expr = mkLHsVarTuple xs
640 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
641 parStmts qss (mkLHsVarPatTup xs) cqs
643 parStmts [] pa cea = return (pa, cea)
644 parStmts ((qs, xs):qss) pa cea = do -- subsequent statements (zip'ed)
645 zipP <- dsLookupGlobalId zipPName
646 let pa' = mkLHsPatTup [pa, mkLHsVarPatTup xs]
647 ty'cea = parrElemType cea
648 res_expr = mkLHsVarTuple xs
649 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
650 let ty'cqs = parrElemType cqs
651 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
652 parStmts qss pa' cea'
654 -- generate Core corresponding to `\p -> e'
656 deLambda :: Type -- type of the argument
657 -> LPat Id -- argument pattern
658 -> LHsExpr Id -- body
659 -> DsM (CoreExpr, Type)
661 mkLambda ty p =<< dsLExpr e
663 -- generate Core for a lambda pattern match, where the body is already in Core
665 mkLambda :: Type -- type of the argument
666 -> LPat Id -- argument pattern
667 -> CoreExpr -- desugared body
668 -> DsM (CoreExpr, Type)
669 mkLambda ty p ce = do
670 v <- newSysLocalDs ty
671 let errMsg = ptext (sLit "DsListComp.deLambda: internal error!")
673 cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
674 res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
675 return (mkLams [v] res, ce'ty)
677 -- obtain the element type of the parallel array produced by the given Core
680 parrElemType :: CoreExpr -> Type
682 case splitTyConApp_maybe (exprType e) of
683 Just (tycon, [ty]) | tycon == parrTyCon -> ty
685 "DsListComp.parrElemType: not a parallel array type"