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 )
26 import DsMonad -- the monadery used in the desugarer
40 import Control.Monad ( liftM2 )
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_RewriteRules 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) groupByClause) = 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, fromBindersTupleType) <- 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', usingArgs) <-
135 case groupByClause of
136 GroupByNothing usingExpr -> liftM2 (,) (dsLExpr usingExpr) (return [expr])
137 GroupBySomething usingExpr byExpr -> do
138 usingExpr' <- dsLExpr (either id noLoc usingExpr)
140 byExpr' <- dsLExpr byExpr
142 us <- newUniqueSupply
143 [fromBindersTuple] <- newSysLocalsDs [fromBindersTupleType]
144 let byExprWrapper = mkTupleCase us fromBinders byExpr' fromBindersTuple (Var fromBindersTuple)
146 return (usingExpr', [Lam fromBindersTuple byExprWrapper, expr])
148 -- Create an unzip function for the appropriate arity and element types and find "map"
149 (unzip_fn, unzip_rhs) <- mkUnzipBind fromBindersTypes
150 map_id <- dsLookupGlobalId mapName
152 -- Generate the expressions to build the grouped list
153 let -- First we apply the grouping function to the inner list
154 inner_list_expr = mkApps usingExpr' ((Type fromBindersTupleType) : usingArgs)
155 -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
156 -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
157 -- the "b" to be a tuple of "to" lists!
158 unzipped_inner_list_expr = mkApps (Var map_id)
159 [Type (mkListTy fromBindersTupleType), Type toBindersTupleType, Var unzip_fn, inner_list_expr]
160 -- Then finally we bind the unzip function around that expression
161 bound_unzipped_inner_list_expr = Let (Rec [(unzip_fn, unzip_rhs)]) unzipped_inner_list_expr
163 -- Build a pattern that ensures the consumer binds into the NEW binders, which hold lists rather than single values
164 let pat = mkBigLHsVarPatTup toBinders
165 return (bound_unzipped_inner_list_expr, pat)
169 %************************************************************************
171 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
173 %************************************************************************
175 Just as in Phil's chapter~7 in SLPJ, using the rules for
176 optimally-compiled list comprehensions. This is what Kevin followed
177 as well, and I quite happily do the same. The TQ translation scheme
178 transforms a list of qualifiers (either boolean expressions or
179 generators) into a single expression which implements the list
180 comprehension. Because we are generating 2nd-order polymorphic
181 lambda-calculus, calls to NIL and CONS must be applied to a type
182 argument, as well as their usual value arguments.
184 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
187 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
190 TQ << [ e | b , qs ] ++ L >> =
191 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
194 TQ << [ e | p <- L1, qs ] ++ L2 >> =
200 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
205 "h", "u1", "u2", and "u3" are new variables.
208 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
209 is the TE translation scheme. Note that we carry around the @L@ list
210 already desugared. @dsListComp@ does the top TE rule mentioned above.
212 To the above, we add an additional rule to deal with parallel list
213 comprehensions. The translation goes roughly as follows:
214 [ e | p1 <- e11, let v1 = e12, p2 <- e13
215 | q1 <- e21, let v2 = e22, q2 <- e23]
217 [ e | ((x1, .., xn), (y1, ..., ym)) <-
218 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
219 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
220 where (x1, .., xn) are the variables bound in p1, v1, p2
221 (y1, .., ym) are the variables bound in q1, v2, q2
223 In the translation below, the ParStmt branch translates each parallel branch
224 into a sub-comprehension, and desugars each independently. The resulting lists
225 are fed to a zip function, we create a binding for all the variables bound in all
226 the comprehensions, and then we hand things off the the desugarer for bindings.
227 The zip function is generated here a) because it's small, and b) because then we
228 don't have to deal with arbitrary limits on the number of zip functions in the
229 prelude, nor which library the zip function came from.
230 The introduced tuples are Boxed, but only because I couldn't get it to work
231 with the Unboxed variety.
235 deListComp :: [Stmt Id] -> LHsExpr Id -> CoreExpr -> DsM CoreExpr
237 deListComp (ParStmt stmtss_w_bndrs : quals) body list
239 exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
240 let (exps, qual_tys) = unzip exps_and_qual_tys
242 (zip_fn, zip_rhs) <- mkZipBind qual_tys
244 -- Deal with [e | pat <- zip l1 .. ln] in example above
245 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
249 bndrs_s = map snd stmtss_w_bndrs
251 -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
252 pat = mkBigLHsPatTup pats
253 pats = map mkBigLHsVarPatTup bndrs_s
255 -- Last: the one to return
256 deListComp [] body list = do -- Figure 7.4, SLPJ, p 135, rule C above
257 core_body <- dsLExpr body
258 return (mkConsExpr (exprType core_body) core_body list)
260 -- Non-last: must be a guard
261 deListComp (ExprStmt guard _ _ : quals) body list = do -- rule B above
262 core_guard <- dsLExpr guard
263 core_rest <- deListComp quals body list
264 return (mkIfThenElse core_guard core_rest list)
266 -- [e | let B, qs] = let B in [e | qs]
267 deListComp (LetStmt binds : quals) body list = do
268 core_rest <- deListComp quals body list
269 dsLocalBinds binds core_rest
271 deListComp (stmt@(TransformStmt _ _ _) : quals) body list = do
272 (inner_list_expr, pat) <- dsTransformStmt stmt
273 deBindComp pat inner_list_expr quals body list
275 deListComp (stmt@(GroupStmt _ _) : quals) body list = do
276 (inner_list_expr, pat) <- dsGroupStmt stmt
277 deBindComp pat inner_list_expr quals body list
279 deListComp (BindStmt pat list1 _ _ : quals) body core_list2 = do -- rule A' above
280 core_list1 <- dsLExpr list1
281 deBindComp pat core_list1 quals body core_list2
286 deBindComp :: OutPat Id
292 deBindComp pat core_list1 quals body core_list2 = do
294 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
296 -- u1_ty is a [alpha] type, and u2_ty = alpha
297 u2_ty = hsLPatType pat
299 res_ty = exprType core_list2
300 h_ty = u1_ty `mkFunTy` res_ty
302 [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
304 -- the "fail" value ...
306 core_fail = App (Var h) (Var u3)
307 letrec_body = App (Var h) core_list1
309 rest_expr <- deListComp quals body core_fail
310 core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail
314 Case (Var u1) u1 res_ty
315 [(DataAlt nilDataCon, [], core_list2),
316 (DataAlt consDataCon, [u2, u3], core_match)]
317 -- Increasing order of tag
319 return (Let (Rec [(h, rhs)]) letrec_body)
322 %************************************************************************
324 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
326 %************************************************************************
328 @dfListComp@ are the rules used with foldr/build turned on:
331 TE[ e | ] c n = c e n
332 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
333 TE[ e | p <- l , q ] c n = let
334 f = \ x b -> case x of
342 dfListComp :: Id -> Id -- 'c' and 'n'
343 -> [Stmt Id] -- the rest of the qual's
347 -- Last: the one to return
348 dfListComp c_id n_id [] body = do
349 core_body <- dsLExpr body
350 return (mkApps (Var c_id) [core_body, Var n_id])
352 -- Non-last: must be a guard
353 dfListComp c_id n_id (ExprStmt guard _ _ : quals) body = do
354 core_guard <- dsLExpr guard
355 core_rest <- dfListComp c_id n_id quals body
356 return (mkIfThenElse core_guard core_rest (Var n_id))
358 dfListComp c_id n_id (LetStmt binds : quals) body = do
359 -- new in 1.3, local bindings
360 core_rest <- dfListComp c_id n_id quals body
361 dsLocalBinds binds core_rest
363 dfListComp c_id n_id (stmt@(TransformStmt _ _ _) : quals) body = do
364 (inner_list_expr, pat) <- dsTransformStmt stmt
365 -- Anyway, we bind the newly transformed list via the generic binding function
366 dfBindComp c_id n_id (pat, inner_list_expr) quals body
368 dfListComp c_id n_id (stmt@(GroupStmt _ _) : quals) body = do
369 (inner_list_expr, pat) <- dsGroupStmt stmt
370 -- Anyway, we bind the newly grouped list via the generic binding function
371 dfBindComp c_id n_id (pat, inner_list_expr) quals body
373 dfListComp c_id n_id (BindStmt pat list1 _ _ : quals) body = do
374 -- evaluate the two lists
375 core_list1 <- dsLExpr list1
377 -- Do the rest of the work in the generic binding builder
378 dfBindComp c_id n_id (pat, core_list1) quals body
380 dfBindComp :: Id -> Id -- 'c' and 'n'
381 -> (LPat Id, CoreExpr)
382 -> [Stmt Id] -- the rest of the qual's
385 dfBindComp c_id n_id (pat, core_list1) quals body = do
386 -- find the required type
387 let x_ty = hsLPatType pat
390 -- create some new local id's
391 [b, x] <- newSysLocalsDs [b_ty, x_ty]
393 -- build rest of the comprehesion
394 core_rest <- dfListComp c_id b quals body
396 -- build the pattern match
397 core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
398 pat core_rest (Var b)
400 -- now build the outermost foldr, and return
401 mkFoldrExpr x_ty b_ty (mkLams [x, b] core_expr) (Var n_id) core_list1
404 %************************************************************************
406 \subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
408 %************************************************************************
412 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
413 -- mkZipBind [t1, t2]
414 -- = (zip, \as1:[t1] as2:[t2]
417 -- (a1:as'1) -> case as2 of
419 -- (a2:as'2) -> (a1, a2) : zip as'1 as'2)]
421 mkZipBind elt_tys = do
422 ass <- mapM newSysLocalDs elt_list_tys
423 as' <- mapM newSysLocalDs elt_tys
424 as's <- mapM newSysLocalDs elt_list_tys
426 zip_fn <- newSysLocalDs zip_fn_ty
428 let inner_rhs = mkConsExpr elt_tuple_ty
429 (mkBigCoreVarTup as')
430 (mkVarApps (Var zip_fn) as's)
431 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
433 return (zip_fn, mkLams ass zip_body)
435 elt_list_tys = map mkListTy elt_tys
436 elt_tuple_ty = mkBigCoreTupTy elt_tys
437 elt_tuple_list_ty = mkListTy elt_tuple_ty
439 zip_fn_ty = mkFunTys elt_list_tys elt_tuple_list_ty
441 mk_case (as, a', as') rest
442 = Case (Var as) as elt_tuple_list_ty
443 [(DataAlt nilDataCon, [], mkNilExpr elt_tuple_ty),
444 (DataAlt consDataCon, [a', as'], rest)]
445 -- Increasing order of tag
448 mkUnzipBind :: [Type] -> DsM (Id, CoreExpr)
449 -- mkUnzipBind [t1, t2]
450 -- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
452 -- (x1, x2) -> case axs of
453 -- (xs1, xs2) -> (x1 : xs1, x2 : xs2))
457 -- We use foldr here in all cases, even if rules are turned off, because we may as well!
458 mkUnzipBind elt_tys = do
459 ax <- newSysLocalDs elt_tuple_ty
460 axs <- newSysLocalDs elt_list_tuple_ty
461 ys <- newSysLocalDs elt_tuple_list_ty
462 xs <- mapM newSysLocalDs elt_tys
463 xss <- mapM newSysLocalDs elt_list_tys
465 unzip_fn <- newSysLocalDs unzip_fn_ty
467 [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]
469 let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
471 concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
472 tupled_concat_expression = mkBigCoreTup concat_expressions
474 folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
475 folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
476 folder_body = mkLams [ax, axs] folder_body_outer_case
478 unzip_body <- mkFoldrExpr elt_tuple_ty elt_list_tuple_ty folder_body nil_tuple (Var ys)
479 return (unzip_fn, mkLams [ys] unzip_body)
481 elt_tuple_ty = mkBigCoreTupTy elt_tys
482 elt_tuple_list_ty = mkListTy elt_tuple_ty
483 elt_list_tys = map mkListTy elt_tys
484 elt_list_tuple_ty = mkBigCoreTupTy elt_list_tys
486 unzip_fn_ty = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty
488 mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail
494 %************************************************************************
496 \subsection[DsPArrComp]{Desugaring of array comprehensions}
498 %************************************************************************
502 -- entry point for desugaring a parallel array comprehension
504 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
506 dsPArrComp :: [Stmt Id]
508 -> Type -- Don't use; called with `undefined' below
510 dsPArrComp [ParStmt qss] body _ = -- parallel comprehension
511 dePArrParComp qss body
512 dsPArrComp qs body _ = do -- no ParStmt in `qs'
513 sglP <- dsLookupGlobalId singletonPName
514 let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
515 dePArrComp qs body (mkLHsPatTup []) unitArray
521 dePArrComp :: [Stmt Id]
523 -> LPat Id -- the current generator pattern
524 -> CoreExpr -- the current generator expression
527 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
529 dePArrComp [] e' pa cea = do
530 mapP <- dsLookupGlobalId mapPName
531 let ty = parrElemType cea
532 (clam, ty'e') <- deLambda ty pa e'
533 return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
535 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
537 dePArrComp (ExprStmt b _ _ : qs) body pa cea = do
538 filterP <- dsLookupGlobalId filterPName
539 let ty = parrElemType cea
540 (clam,_) <- deLambda ty pa b
541 dePArrComp qs body pa (mkApps (Var filterP) [Type ty, clam, cea])
544 -- <<[:e' | p <- e, qs:]>> pa ea =
547 -- <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
549 -- if matching again p cannot fail, or else
551 -- <<[:e' | p <- e, qs:]>> pa ea =
552 -- let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
554 -- <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
556 dePArrComp (BindStmt p e _ _ : qs) body pa cea = do
557 filterP <- dsLookupGlobalId filterPName
558 crossMapP <- dsLookupGlobalId crossMapPName
560 let ety'cea = parrElemType cea
561 ety'ce = parrElemType ce
562 false = Var falseDataConId
563 true = Var trueDataConId
564 v <- newSysLocalDs ety'ce
565 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
566 let cef | isIrrefutableHsPat p = ce
567 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
568 (clam, _) <- mkLambda ety'cea pa cef
569 let ety'cef = ety'ce -- filter doesn't change the element type
570 pa' = mkLHsPatTup [pa, p]
572 dePArrComp qs body pa' (mkApps (Var crossMapP)
573 [Type ety'cea, Type ety'cef, cea, clam])
575 -- <<[:e' | let ds, qs:]>> pa ea =
576 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
577 -- (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
579 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
581 dePArrComp (LetStmt ds : qs) body pa cea = do
582 mapP <- dsLookupGlobalId mapPName
583 let xs = map unLoc (collectLocalBinders ds)
584 ty'cea = parrElemType cea
585 v <- newSysLocalDs ty'cea
586 clet <- dsLocalBinds ds (mkCoreTup (map Var xs))
587 let'v <- newSysLocalDs (exprType clet)
588 let projBody = mkDsLet (NonRec let'v clet) $
589 mkCoreTup [Var v, Var let'v]
590 errTy = exprType projBody
591 errMsg = "DsListComp.dePArrComp: internal error!"
592 cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
593 ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
594 let pa' = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
595 proj = mkLams [v] ccase
596 dePArrComp qs body pa' (mkApps (Var mapP)
597 [Type ty'cea, Type errTy, proj, cea])
599 -- The parser guarantees that parallel comprehensions can only appear as
600 -- singeltons qualifier lists, which we already special case in the caller.
601 -- So, encountering one here is a bug.
603 dePArrComp (ParStmt _ : _) _ _ _ =
604 panic "DsListComp.dePArrComp: malformed comprehension AST"
606 -- <<[:e' | qs | qss:]>> pa ea =
607 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
608 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
610 -- {x_1, ..., x_n} = DV (qs)
612 dePArrParComp :: [([LStmt Id], [Id])] -> LHsExpr Id -> DsM CoreExpr
613 dePArrParComp qss body = do
614 (pQss, ceQss) <- deParStmt qss
615 dePArrComp [] body pQss ceQss
618 -- empty parallel statement lists have no source representation
619 panic "DsListComp.dePArrComp: Empty parallel list comprehension"
620 deParStmt ((qs, xs):qss) = do -- first statement
621 let res_expr = mkLHsVarTup xs
622 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
623 parStmts qss (mkLHsVarPatTup xs) cqs
625 parStmts [] pa cea = return (pa, cea)
626 parStmts ((qs, xs):qss) pa cea = do -- subsequent statements (zip'ed)
627 zipP <- dsLookupGlobalId zipPName
628 let pa' = mkLHsPatTup [pa, mkLHsVarPatTup xs]
629 ty'cea = parrElemType cea
630 res_expr = mkLHsVarTup xs
631 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
632 let ty'cqs = parrElemType cqs
633 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
634 parStmts qss pa' cea'
636 -- generate Core corresponding to `\p -> e'
638 deLambda :: Type -- type of the argument
639 -> LPat Id -- argument pattern
640 -> LHsExpr Id -- body
641 -> DsM (CoreExpr, Type)
643 mkLambda ty p =<< dsLExpr e
645 -- generate Core for a lambda pattern match, where the body is already in Core
647 mkLambda :: Type -- type of the argument
648 -> LPat Id -- argument pattern
649 -> CoreExpr -- desugared body
650 -> DsM (CoreExpr, Type)
651 mkLambda ty p ce = do
652 v <- newSysLocalDs ty
653 let errMsg = do "DsListComp.deLambda: internal error!"
655 cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
656 res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
657 return (mkLams [v] res, ce'ty)
659 -- obtain the element type of the parallel array produced by the given Core
662 parrElemType :: CoreExpr -> Type
664 case splitTyConApp_maybe (exprType e) of
665 Just (tycon, [ty]) | tycon == parrTyCon -> ty
667 "DsListComp.parrElemType: not a parallel array type"