2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Desugaring list comprehensions and array comprehensions
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 import Control.Monad ( liftM2 )
46 List comprehensions may be desugared in one of two ways: ``ordinary''
47 (as you would expect if you read SLPJ's book) and ``with foldr/build
48 turned on'' (if you read Gill {\em et al.}'s paper on the subject).
50 There will be at least one ``qualifier'' in the input.
53 dsListComp :: [LStmt Id]
55 -> Type -- Type of list elements
57 dsListComp lquals body elt_ty = do
59 let quals = map unLoc lquals
61 if not (dopt Opt_RewriteRules dflags) || dopt Opt_IgnoreInterfacePragmas dflags
62 -- Either rules are switched off, or we are ignoring what there are;
63 -- Either way foldr/build won't happen, so use the more efficient
64 -- Wadler-style desugaring
65 || isParallelComp quals
66 -- Foldr-style desugaring can't handle parallel list comprehensions
67 then deListComp quals body (mkNilExpr elt_ty)
68 else do -- Foldr/build should be enabled, so desugar
69 -- into foldrs and builds
70 [n_tyvar] <- newTyVarsDs [alphaTyVar]
72 let n_ty = mkTyVarTy n_tyvar
73 c_ty = mkFunTys [elt_ty, n_ty] n_ty
74 [c, n] <- newSysLocalsDs [c_ty, n_ty]
76 result <- dfListComp c n quals body
77 build_id <- dsLookupGlobalId buildName
78 return (Var build_id `App` Type elt_ty `App` mkLams [n_tyvar, c, n] result)
81 -- We must test for ParStmt anywhere, not just at the head, because an extension
82 -- to list comprehensions would be to add brackets to specify the associativity
83 -- of qualifier lists. This is really easy to do by adding extra ParStmts into the
84 -- mix of possibly a single element in length, so we do this to leave the possibility open
85 isParallelComp = any isParallelStmt
87 isParallelStmt (ParStmt _) = True
88 isParallelStmt _ = False
91 -- This function lets you desugar a inner list comprehension and a list of the binders
92 -- of that comprehension that we need in the outer comprehension into such an expression
93 -- and the type of the elements that it outputs (tuples of binders)
94 dsInnerListComp :: ([LStmt Id], [Id]) -> DsM (CoreExpr, Type)
95 dsInnerListComp (stmts, bndrs) = do
96 expr <- dsListComp stmts (mkBigLHsVarTup bndrs) bndrs_tuple_type
97 return (expr, bndrs_tuple_type)
99 bndrs_types = map idType bndrs
100 bndrs_tuple_type = mkBigCoreTupTy bndrs_types
103 -- This function factors out commonality between the desugaring strategies for TransformStmt.
104 -- Given such a statement it gives you back an expression representing how to compute the transformed
105 -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
106 dsTransformStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
107 dsTransformStmt (TransformStmt (stmts, binders) usingExpr maybeByExpr) = do
108 (expr, binders_tuple_type) <- dsInnerListComp (stmts, binders)
109 usingExpr' <- dsLExpr usingExpr
113 Nothing -> return [expr]
115 byExpr' <- dsLExpr byExpr
117 us <- newUniqueSupply
118 [tuple_binder] <- newSysLocalsDs [binders_tuple_type]
119 let byExprWrapper = mkTupleCase us binders byExpr' tuple_binder (Var tuple_binder)
121 return [Lam tuple_binder byExprWrapper, expr]
123 let inner_list_expr = mkApps usingExpr' ((Type binders_tuple_type) : using_args)
125 let pat = mkBigLHsVarPatTup binders
126 return (inner_list_expr, pat)
128 -- This function factors out commonality between the desugaring strategies for GroupStmt.
129 -- Given such a statement it gives you back an expression representing how to compute the transformed
130 -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
131 dsGroupStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
132 dsGroupStmt (GroupStmt (stmts, binderMap) groupByClause) = do
133 let (fromBinders, toBinders) = unzip binderMap
135 fromBindersTypes = map idType fromBinders
136 toBindersTypes = map idType toBinders
138 toBindersTupleType = mkBigCoreTupTy toBindersTypes
140 -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
141 (expr, fromBindersTupleType) <- dsInnerListComp (stmts, fromBinders)
143 -- Work out what arguments should be supplied to that expression: i.e. is an extraction
144 -- function required? If so, create that desugared function and add to arguments
145 (usingExpr', usingArgs) <-
146 case groupByClause of
147 GroupByNothing usingExpr -> liftM2 (,) (dsLExpr usingExpr) (return [expr])
148 GroupBySomething usingExpr byExpr -> do
149 usingExpr' <- dsLExpr (either id noLoc usingExpr)
151 byExpr' <- dsLExpr byExpr
153 us <- newUniqueSupply
154 [fromBindersTuple] <- newSysLocalsDs [fromBindersTupleType]
155 let byExprWrapper = mkTupleCase us fromBinders byExpr' fromBindersTuple (Var fromBindersTuple)
157 return (usingExpr', [Lam fromBindersTuple byExprWrapper, expr])
159 -- Create an unzip function for the appropriate arity and element types and find "map"
160 (unzip_fn, unzip_rhs) <- mkUnzipBind fromBindersTypes
161 map_id <- dsLookupGlobalId mapName
163 -- Generate the expressions to build the grouped list
164 let -- First we apply the grouping function to the inner list
165 inner_list_expr = mkApps usingExpr' ((Type fromBindersTupleType) : usingArgs)
166 -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
167 -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
168 -- the "b" to be a tuple of "to" lists!
169 unzipped_inner_list_expr = mkApps (Var map_id)
170 [Type (mkListTy fromBindersTupleType), Type toBindersTupleType, Var unzip_fn, inner_list_expr]
171 -- Then finally we bind the unzip function around that expression
172 bound_unzipped_inner_list_expr = Let (Rec [(unzip_fn, unzip_rhs)]) unzipped_inner_list_expr
174 -- Build a pattern that ensures the consumer binds into the NEW binders, which hold lists rather than single values
175 let pat = mkBigLHsVarPatTup toBinders
176 return (bound_unzipped_inner_list_expr, pat)
180 %************************************************************************
182 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
184 %************************************************************************
186 Just as in Phil's chapter~7 in SLPJ, using the rules for
187 optimally-compiled list comprehensions. This is what Kevin followed
188 as well, and I quite happily do the same. The TQ translation scheme
189 transforms a list of qualifiers (either boolean expressions or
190 generators) into a single expression which implements the list
191 comprehension. Because we are generating 2nd-order polymorphic
192 lambda-calculus, calls to NIL and CONS must be applied to a type
193 argument, as well as their usual value arguments.
195 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
198 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
201 TQ << [ e | b , qs ] ++ L >> =
202 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
205 TQ << [ e | p <- L1, qs ] ++ L2 >> =
211 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
216 "h", "u1", "u2", and "u3" are new variables.
219 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
220 is the TE translation scheme. Note that we carry around the @L@ list
221 already desugared. @dsListComp@ does the top TE rule mentioned above.
223 To the above, we add an additional rule to deal with parallel list
224 comprehensions. The translation goes roughly as follows:
225 [ e | p1 <- e11, let v1 = e12, p2 <- e13
226 | q1 <- e21, let v2 = e22, q2 <- e23]
228 [ e | ((x1, .., xn), (y1, ..., ym)) <-
229 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
230 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
231 where (x1, .., xn) are the variables bound in p1, v1, p2
232 (y1, .., ym) are the variables bound in q1, v2, q2
234 In the translation below, the ParStmt branch translates each parallel branch
235 into a sub-comprehension, and desugars each independently. The resulting lists
236 are fed to a zip function, we create a binding for all the variables bound in all
237 the comprehensions, and then we hand things off the the desugarer for bindings.
238 The zip function is generated here a) because it's small, and b) because then we
239 don't have to deal with arbitrary limits on the number of zip functions in the
240 prelude, nor which library the zip function came from.
241 The introduced tuples are Boxed, but only because I couldn't get it to work
242 with the Unboxed variety.
246 deListComp :: [Stmt Id] -> LHsExpr Id -> CoreExpr -> DsM CoreExpr
248 deListComp (ParStmt stmtss_w_bndrs : quals) body list
250 exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
251 let (exps, qual_tys) = unzip exps_and_qual_tys
253 (zip_fn, zip_rhs) <- mkZipBind qual_tys
255 -- Deal with [e | pat <- zip l1 .. ln] in example above
256 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
260 bndrs_s = map snd stmtss_w_bndrs
262 -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
263 pat = mkBigLHsPatTup pats
264 pats = map mkBigLHsVarPatTup bndrs_s
266 -- Last: the one to return
267 deListComp [] body list = do -- Figure 7.4, SLPJ, p 135, rule C above
268 core_body <- dsLExpr body
269 return (mkConsExpr (exprType core_body) core_body list)
271 -- Non-last: must be a guard
272 deListComp (ExprStmt guard _ _ : quals) body list = do -- rule B above
273 core_guard <- dsLExpr guard
274 core_rest <- deListComp quals body list
275 return (mkIfThenElse core_guard core_rest list)
277 -- [e | let B, qs] = let B in [e | qs]
278 deListComp (LetStmt binds : quals) body list = do
279 core_rest <- deListComp quals body list
280 dsLocalBinds binds core_rest
282 deListComp (stmt@(TransformStmt _ _ _) : quals) body list = do
283 (inner_list_expr, pat) <- dsTransformStmt stmt
284 deBindComp pat inner_list_expr quals body list
286 deListComp (stmt@(GroupStmt _ _) : quals) body list = do
287 (inner_list_expr, pat) <- dsGroupStmt stmt
288 deBindComp pat inner_list_expr quals body list
290 deListComp (BindStmt pat list1 _ _ : quals) body core_list2 = do -- rule A' above
291 core_list1 <- dsLExpr list1
292 deBindComp pat core_list1 quals body core_list2
297 deBindComp pat core_list1 quals body core_list2 = do
299 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
301 -- u1_ty is a [alpha] type, and u2_ty = alpha
302 u2_ty = hsLPatType pat
304 res_ty = exprType core_list2
305 h_ty = u1_ty `mkFunTy` res_ty
307 [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
309 -- the "fail" value ...
311 core_fail = App (Var h) (Var u3)
312 letrec_body = App (Var h) core_list1
314 rest_expr <- deListComp quals body core_fail
315 core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail
319 Case (Var u1) u1 res_ty
320 [(DataAlt nilDataCon, [], core_list2),
321 (DataAlt consDataCon, [u2, u3], core_match)]
322 -- Increasing order of tag
324 return (Let (Rec [(h, rhs)]) letrec_body)
327 %************************************************************************
329 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
331 %************************************************************************
333 @dfListComp@ are the rules used with foldr/build turned on:
336 TE[ e | ] c n = c e n
337 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
338 TE[ e | p <- l , q ] c n = let
339 f = \ x b -> case x of
347 dfListComp :: Id -> Id -- 'c' and 'n'
348 -> [Stmt Id] -- the rest of the qual's
352 -- Last: the one to return
353 dfListComp c_id n_id [] body = do
354 core_body <- dsLExpr body
355 return (mkApps (Var c_id) [core_body, Var n_id])
357 -- Non-last: must be a guard
358 dfListComp c_id n_id (ExprStmt guard _ _ : quals) body = do
359 core_guard <- dsLExpr guard
360 core_rest <- dfListComp c_id n_id quals body
361 return (mkIfThenElse core_guard core_rest (Var n_id))
363 dfListComp c_id n_id (LetStmt binds : quals) body = do
364 -- new in 1.3, local bindings
365 core_rest <- dfListComp c_id n_id quals body
366 dsLocalBinds binds core_rest
368 dfListComp c_id n_id (stmt@(TransformStmt _ _ _) : quals) body = do
369 (inner_list_expr, pat) <- dsTransformStmt stmt
370 -- Anyway, we bind the newly transformed list via the generic binding function
371 dfBindComp c_id n_id (pat, inner_list_expr) quals body
373 dfListComp c_id n_id (stmt@(GroupStmt _ _) : quals) body = do
374 (inner_list_expr, pat) <- dsGroupStmt stmt
375 -- Anyway, we bind the newly grouped list via the generic binding function
376 dfBindComp c_id n_id (pat, inner_list_expr) quals body
378 dfListComp c_id n_id (BindStmt pat list1 _ _ : quals) body = do
379 -- evaluate the two lists
380 core_list1 <- dsLExpr list1
382 -- Do the rest of the work in the generic binding builder
383 dfBindComp c_id n_id (pat, core_list1) quals body
385 dfBindComp :: Id -> Id -- 'c' and 'n'
386 -> (LPat Id, CoreExpr)
387 -> [Stmt Id] -- the rest of the qual's
390 dfBindComp c_id n_id (pat, core_list1) quals body = do
391 -- find the required type
392 let x_ty = hsLPatType pat
395 -- create some new local id's
396 [b, x] <- newSysLocalsDs [b_ty, x_ty]
398 -- build rest of the comprehesion
399 core_rest <- dfListComp c_id b quals body
401 -- build the pattern match
402 core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
403 pat core_rest (Var b)
405 -- now build the outermost foldr, and return
406 foldr_id <- dsLookupGlobalId foldrName
407 return (Var foldr_id `App` Type x_ty
409 `App` mkLams [x, b] core_expr
415 %************************************************************************
417 \subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
419 %************************************************************************
423 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
424 -- mkZipBind [t1, t2]
425 -- = (zip, \as1:[t1] as2:[t2]
428 -- (a1:as'1) -> case as2 of
430 -- (a2:as'2) -> (a1, a2) : zip as'1 as'2)]
432 mkZipBind elt_tys = do
433 ass <- mapM newSysLocalDs elt_list_tys
434 as' <- mapM newSysLocalDs elt_tys
435 as's <- mapM newSysLocalDs elt_list_tys
437 zip_fn <- newSysLocalDs zip_fn_ty
439 let inner_rhs = mkConsExpr elt_tuple_ty
440 (mkBigCoreVarTup as')
441 (mkVarApps (Var zip_fn) as's)
442 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
444 return (zip_fn, mkLams ass zip_body)
446 elt_list_tys = map mkListTy elt_tys
447 elt_tuple_ty = mkBigCoreTupTy elt_tys
448 elt_tuple_list_ty = mkListTy elt_tuple_ty
450 zip_fn_ty = mkFunTys elt_list_tys elt_tuple_list_ty
452 mk_case (as, a', as') rest
453 = Case (Var as) as elt_tuple_list_ty
454 [(DataAlt nilDataCon, [], mkNilExpr elt_tuple_ty),
455 (DataAlt consDataCon, [a', as'], rest)]
456 -- Increasing order of tag
459 mkUnzipBind :: [Type] -> DsM (Id, CoreExpr)
460 -- mkUnzipBind [t1, t2]
461 -- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
463 -- (x1, x2) -> case axs of
464 -- (xs1, xs2) -> (x1 : xs1, x2 : xs2))
468 -- We use foldr here in all cases, even if rules are turned off, because we may as well!
469 mkUnzipBind elt_tys = do
470 ax <- newSysLocalDs elt_tuple_ty
471 axs <- newSysLocalDs elt_list_tuple_ty
472 ys <- newSysLocalDs elt_tuple_list_ty
473 xs <- mapM newSysLocalDs elt_tys
474 xss <- mapM newSysLocalDs elt_list_tys
476 unzip_fn <- newSysLocalDs unzip_fn_ty
478 foldr_id <- dsLookupGlobalId foldrName
479 [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]
481 let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
483 concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
484 tupled_concat_expression = mkBigCoreTup concat_expressions
486 folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
487 folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
488 folder_body = mkLams [ax, axs] folder_body_outer_case
490 unzip_body = mkApps (Var foldr_id) [Type elt_tuple_ty, Type elt_list_tuple_ty, folder_body, nil_tuple, Var ys]
491 unzip_body_saturated = mkLams [ys] unzip_body
493 return (unzip_fn, unzip_body_saturated)
495 elt_tuple_ty = mkBigCoreTupTy elt_tys
496 elt_tuple_list_ty = mkListTy elt_tuple_ty
497 elt_list_tys = map mkListTy elt_tys
498 elt_list_tuple_ty = mkBigCoreTupTy elt_list_tys
500 unzip_fn_ty = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty
502 mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail
508 %************************************************************************
510 \subsection[DsPArrComp]{Desugaring of array comprehensions}
512 %************************************************************************
516 -- entry point for desugaring a parallel array comprehension
518 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
520 dsPArrComp :: [Stmt Id]
522 -> Type -- Don't use; called with `undefined' below
524 dsPArrComp [ParStmt qss] body _ = -- parallel comprehension
525 dePArrParComp qss body
526 dsPArrComp qs body _ = do -- no ParStmt in `qs'
527 sglP <- dsLookupGlobalId singletonPName
528 let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
529 dePArrComp qs body (mkLHsPatTup []) unitArray
535 dePArrComp :: [Stmt Id]
537 -> LPat Id -- the current generator pattern
538 -> CoreExpr -- the current generator expression
541 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
543 dePArrComp [] e' pa cea = do
544 mapP <- dsLookupGlobalId mapPName
545 let ty = parrElemType cea
546 (clam, ty'e') <- deLambda ty pa e'
547 return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
549 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
551 dePArrComp (ExprStmt b _ _ : qs) body pa cea = do
552 filterP <- dsLookupGlobalId filterPName
553 let ty = parrElemType cea
554 (clam,_) <- deLambda ty pa b
555 dePArrComp qs body pa (mkApps (Var filterP) [Type ty, clam, cea])
558 -- <<[:e' | p <- e, qs:]>> pa ea =
561 -- <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
563 -- if matching again p cannot fail, or else
565 -- <<[:e' | p <- e, qs:]>> pa ea =
566 -- let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
568 -- <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
570 dePArrComp (BindStmt p e _ _ : qs) body pa cea = do
571 filterP <- dsLookupGlobalId filterPName
572 crossMapP <- dsLookupGlobalId crossMapPName
574 let ety'cea = parrElemType cea
575 ety'ce = parrElemType ce
576 false = Var falseDataConId
577 true = Var trueDataConId
578 v <- newSysLocalDs ety'ce
579 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
580 let cef | isIrrefutableHsPat p = ce
581 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
582 (clam, _) <- mkLambda ety'cea pa cef
583 let ety'cef = ety'ce -- filter doesn't change the element type
584 pa' = mkLHsPatTup [pa, p]
586 dePArrComp qs body pa' (mkApps (Var crossMapP)
587 [Type ety'cea, Type ety'cef, cea, clam])
589 -- <<[:e' | let ds, qs:]>> pa ea =
590 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
591 -- (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
593 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
595 dePArrComp (LetStmt ds : qs) body pa cea = do
596 mapP <- dsLookupGlobalId mapPName
597 let xs = map unLoc (collectLocalBinders ds)
598 ty'cea = parrElemType cea
599 v <- newSysLocalDs ty'cea
600 clet <- dsLocalBinds ds (mkCoreTup (map Var xs))
601 let'v <- newSysLocalDs (exprType clet)
602 let projBody = mkDsLet (NonRec let'v clet) $
603 mkCoreTup [Var v, Var let'v]
604 errTy = exprType projBody
605 errMsg = "DsListComp.dePArrComp: internal error!"
606 cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
607 ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
608 let pa' = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
609 proj = mkLams [v] ccase
610 dePArrComp qs body pa' (mkApps (Var mapP)
611 [Type ty'cea, Type errTy, proj, cea])
613 -- The parser guarantees that parallel comprehensions can only appear as
614 -- singeltons qualifier lists, which we already special case in the caller.
615 -- So, encountering one here is a bug.
617 dePArrComp (ParStmt _ : _) _ _ _ =
618 panic "DsListComp.dePArrComp: malformed comprehension AST"
620 -- <<[:e' | qs | qss:]>> pa ea =
621 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
622 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
624 -- {x_1, ..., x_n} = DV (qs)
626 dePArrParComp qss body = do
627 (pQss, ceQss) <- deParStmt qss
628 dePArrComp [] body pQss ceQss
631 -- empty parallel statement lists have no source representation
632 panic "DsListComp.dePArrComp: Empty parallel list comprehension"
633 deParStmt ((qs, xs):qss) = do -- first statement
634 let res_expr = mkLHsVarTup xs
635 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
636 parStmts qss (mkLHsVarPatTup xs) cqs
638 parStmts [] pa cea = return (pa, cea)
639 parStmts ((qs, xs):qss) pa cea = do -- subsequent statements (zip'ed)
640 zipP <- dsLookupGlobalId zipPName
641 let pa' = mkLHsPatTup [pa, mkLHsVarPatTup xs]
642 ty'cea = parrElemType cea
643 res_expr = mkLHsVarTup xs
644 cqs <- dsPArrComp (map unLoc qs) res_expr undefined
645 let ty'cqs = parrElemType cqs
646 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
647 parStmts qss pa' cea'
649 -- generate Core corresponding to `\p -> e'
651 deLambda :: Type -- type of the argument
652 -> LPat Id -- argument pattern
653 -> LHsExpr Id -- body
654 -> DsM (CoreExpr, Type)
656 mkLambda ty p =<< dsLExpr e
658 -- generate Core for a lambda pattern match, where the body is already in Core
660 mkLambda :: Type -- type of the argument
661 -> LPat Id -- argument pattern
662 -> CoreExpr -- desugared body
663 -> DsM (CoreExpr, Type)
664 mkLambda ty p ce = do
665 v <- newSysLocalDs ty
666 let errMsg = do "DsListComp.deLambda: internal error!"
668 cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
669 res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
670 return (mkLams [v] res, ce'ty)
672 -- obtain the element type of the parallel array produced by the given Core
675 parrElemType :: CoreExpr -> Type
677 case splitTyConApp_maybe (exprType e) of
678 Just (tycon, [ty]) | tycon == parrTyCon -> ty
680 "DsListComp.parrElemType: not a parallel array type"