2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 Desugaring list comprehensions, monad comprehensions and array comprehensions
9 {-# LANGUAGE NamedFieldPuns #-}
10 {-# OPTIONS -fno-warn-incomplete-patterns #-}
11 -- The above warning supression flag is a temporary kludge.
12 -- While working on this module you are encouraged to remove it and fix
13 -- any warnings in the module. See
14 -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
17 module DsListComp ( dsListComp, dsPArrComp, dsMonadComp ) where
19 #include "HsVersions.h"
21 import {-# SOURCE #-} DsExpr ( dsExpr, dsLExpr, dsLocalBinds )
28 import DsMonad -- the monadery used in the desugarer
44 List comprehensions may be desugared in one of two ways: ``ordinary''
45 (as you would expect if you read SLPJ's book) and ``with foldr/build
46 turned on'' (if you read Gill {\em et al.}'s paper on the subject).
48 There will be at least one ``qualifier'' in the input.
51 dsListComp :: [LStmt Id]
52 -> Type -- Type of entire list
54 dsListComp lquals res_ty = do
56 let quals = map unLoc lquals
57 elt_ty = case tcTyConAppArgs res_ty of
59 _ -> pprPanic "dsListComp" (ppr res_ty $$ ppr lquals)
61 if not (dopt Opt_EnableRewriteRules 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 (mkNilExpr elt_ty)
68 else mkBuildExpr elt_ty (\(c, _) (n, _) -> dfListComp c n quals)
69 -- Foldr/build should be enabled, so desugar
70 -- into foldrs and builds
73 -- We must test for ParStmt anywhere, not just at the head, because an extension
74 -- to list comprehensions would be to add brackets to specify the associativity
75 -- of qualifier lists. This is really easy to do by adding extra ParStmts into the
76 -- mix of possibly a single element in length, so we do this to leave the possibility open
77 isParallelComp = any isParallelStmt
79 isParallelStmt (ParStmt _ _ _ _) = True
80 isParallelStmt _ = False
83 -- This function lets you desugar a inner list comprehension and a list of the binders
84 -- of that comprehension that we need in the outer comprehension into such an expression
85 -- and the type of the elements that it outputs (tuples of binders)
86 dsInnerListComp :: ([LStmt Id], [Id]) -> DsM (CoreExpr, Type)
87 dsInnerListComp (stmts, bndrs)
88 = do { expr <- dsListComp (stmts ++ [noLoc $ mkLastStmt (mkBigLHsVarTup bndrs)])
89 (mkListTy bndrs_tuple_type)
90 ; return (expr, bndrs_tuple_type) }
92 bndrs_tuple_type = mkBigCoreVarTupTy bndrs
94 -- This function factors out commonality between the desugaring strategies for GroupStmt.
95 -- Given such a statement it gives you back an expression representing how to compute the transformed
96 -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
97 dsTransStmt :: Stmt Id -> DsM (CoreExpr, LPat Id)
98 dsTransStmt (TransStmt { trS_form = form, trS_stmts = stmts, trS_bndrs = binderMap
99 , trS_by = by, trS_using = using }) = do
100 let (from_bndrs, to_bndrs) = unzip binderMap
101 from_bndrs_tys = map idType from_bndrs
102 to_bndrs_tys = map idType to_bndrs
103 to_bndrs_tup_ty = mkBigCoreTupTy to_bndrs_tys
105 -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
106 (expr, from_tup_ty) <- dsInnerListComp (stmts, from_bndrs)
108 -- Work out what arguments should be supplied to that expression: i.e. is an extraction
109 -- function required? If so, create that desugared function and add to arguments
110 usingExpr' <- dsLExpr using
111 usingArgs <- case by of
112 Nothing -> return [expr]
113 Just by_e -> do { by_e' <- dsLExpr by_e
114 ; lam <- matchTuple from_bndrs by_e'
115 ; return [lam, expr] }
117 -- Create an unzip function for the appropriate arity and element types and find "map"
118 unzip_stuff <- mkUnzipBind form from_bndrs_tys
119 map_id <- dsLookupGlobalId mapName
121 -- Generate the expressions to build the grouped list
122 let -- First we apply the grouping function to the inner list
123 inner_list_expr = mkApps usingExpr' usingArgs
124 -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
125 -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
126 -- the "b" to be a tuple of "to" lists!
127 -- Then finally we bind the unzip function around that expression
128 bound_unzipped_inner_list_expr
129 = case unzip_stuff of
130 Nothing -> inner_list_expr
131 Just (unzip_fn, unzip_rhs) -> Let (Rec [(unzip_fn, unzip_rhs)]) $
132 mkApps (Var map_id) $
133 [ Type (mkListTy from_tup_ty)
134 , Type to_bndrs_tup_ty
138 -- Build a pattern that ensures the consumer binds into the NEW binders,
139 -- which hold lists rather than single values
140 let pat = mkBigLHsVarPatTup to_bndrs
141 return (bound_unzipped_inner_list_expr, pat)
144 %************************************************************************
146 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
148 %************************************************************************
150 Just as in Phil's chapter~7 in SLPJ, using the rules for
151 optimally-compiled list comprehensions. This is what Kevin followed
152 as well, and I quite happily do the same. The TQ translation scheme
153 transforms a list of qualifiers (either boolean expressions or
154 generators) into a single expression which implements the list
155 comprehension. Because we are generating 2nd-order polymorphic
156 lambda-calculus, calls to NIL and CONS must be applied to a type
157 argument, as well as their usual value arguments.
159 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
162 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
165 TQ << [ e | b , qs ] ++ L >> =
166 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
169 TQ << [ e | p <- L1, qs ] ++ L2 >> =
175 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
180 "h", "u1", "u2", and "u3" are new variables.
183 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
184 is the TE translation scheme. Note that we carry around the @L@ list
185 already desugared. @dsListComp@ does the top TE rule mentioned above.
187 To the above, we add an additional rule to deal with parallel list
188 comprehensions. The translation goes roughly as follows:
189 [ e | p1 <- e11, let v1 = e12, p2 <- e13
190 | q1 <- e21, let v2 = e22, q2 <- e23]
192 [ e | ((x1, .., xn), (y1, ..., ym)) <-
193 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
194 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
195 where (x1, .., xn) are the variables bound in p1, v1, p2
196 (y1, .., ym) are the variables bound in q1, v2, q2
198 In the translation below, the ParStmt branch translates each parallel branch
199 into a sub-comprehension, and desugars each independently. The resulting lists
200 are fed to a zip function, we create a binding for all the variables bound in all
201 the comprehensions, and then we hand things off the the desugarer for bindings.
202 The zip function is generated here a) because it's small, and b) because then we
203 don't have to deal with arbitrary limits on the number of zip functions in the
204 prelude, nor which library the zip function came from.
205 The introduced tuples are Boxed, but only because I couldn't get it to work
206 with the Unboxed variety.
210 deListComp :: [Stmt Id] -> CoreExpr -> DsM CoreExpr
212 deListComp [] _ = panic "deListComp"
214 deListComp (LastStmt body _ : quals) list
215 = -- Figure 7.4, SLPJ, p 135, rule C above
217 do { core_body <- dsLExpr body
218 ; return (mkConsExpr (exprType core_body) core_body list) }
220 -- Non-last: must be a guard
221 deListComp (ExprStmt guard _ _ _ : quals) list = do -- rule B above
222 core_guard <- dsLExpr guard
223 core_rest <- deListComp quals list
224 return (mkIfThenElse core_guard core_rest list)
226 -- [e | let B, qs] = let B in [e | qs]
227 deListComp (LetStmt binds : quals) list = do
228 core_rest <- deListComp quals list
229 dsLocalBinds binds core_rest
231 deListComp (stmt@(TransStmt {}) : quals) list = do
232 (inner_list_expr, pat) <- dsTransStmt stmt
233 deBindComp pat inner_list_expr quals list
235 deListComp (BindStmt pat list1 _ _ : quals) core_list2 = do -- rule A' above
236 core_list1 <- dsLExpr list1
237 deBindComp pat core_list1 quals core_list2
239 deListComp (ParStmt stmtss_w_bndrs _ _ _ : quals) list
240 = do { exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
241 ; let (exps, qual_tys) = unzip exps_and_qual_tys
243 ; (zip_fn, zip_rhs) <- mkZipBind qual_tys
245 -- Deal with [e | pat <- zip l1 .. ln] in example above
246 ; 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
258 deBindComp :: OutPat Id
263 deBindComp pat core_list1 quals core_list2 = do
265 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
267 -- u1_ty is a [alpha] type, and u2_ty = alpha
268 u2_ty = hsLPatType pat
270 res_ty = exprType core_list2
271 h_ty = u1_ty `mkFunTy` res_ty
273 [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
275 -- the "fail" value ...
277 core_fail = App (Var h) (Var u3)
278 letrec_body = App (Var h) core_list1
280 rest_expr <- deListComp quals core_fail
281 core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail
285 Case (Var u1) u1 res_ty
286 [(DataAlt nilDataCon, [], core_list2),
287 (DataAlt consDataCon, [u2, u3], core_match)]
288 -- Increasing order of tag
290 return (Let (Rec [(h, rhs)]) letrec_body)
293 %************************************************************************
295 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
297 %************************************************************************
299 @dfListComp@ are the rules used with foldr/build turned on:
302 TE[ e | ] c n = c e n
303 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
304 TE[ e | p <- l , q ] c n = let
305 f = \ x b -> case x of
313 dfListComp :: Id -> Id -- 'c' and 'n'
314 -> [Stmt Id] -- the rest of the qual's
317 dfListComp _ _ [] = panic "dfListComp"
319 dfListComp c_id n_id (LastStmt body _ : quals)
320 = ASSERT( null quals )
321 do { core_body <- dsLExpr body
322 ; return (mkApps (Var c_id) [core_body, Var n_id]) }
324 -- Non-last: must be a guard
325 dfListComp c_id n_id (ExprStmt guard _ _ _ : quals) = do
326 core_guard <- dsLExpr guard
327 core_rest <- dfListComp c_id n_id quals
328 return (mkIfThenElse core_guard core_rest (Var n_id))
330 dfListComp c_id n_id (LetStmt binds : quals) = do
331 -- new in 1.3, local bindings
332 core_rest <- dfListComp c_id n_id quals
333 dsLocalBinds binds core_rest
335 dfListComp c_id n_id (stmt@(TransStmt {}) : quals) = do
336 (inner_list_expr, pat) <- dsTransStmt stmt
337 -- Anyway, we bind the newly grouped list via the generic binding function
338 dfBindComp c_id n_id (pat, inner_list_expr) quals
340 dfListComp c_id n_id (BindStmt pat list1 _ _ : quals) = do
341 -- evaluate the two lists
342 core_list1 <- dsLExpr list1
344 -- Do the rest of the work in the generic binding builder
345 dfBindComp c_id n_id (pat, core_list1) quals
347 dfBindComp :: Id -> Id -- 'c' and 'n'
348 -> (LPat Id, CoreExpr)
349 -> [Stmt Id] -- the rest of the qual's
351 dfBindComp c_id n_id (pat, core_list1) quals = do
352 -- find the required type
353 let x_ty = hsLPatType pat
356 -- create some new local id's
357 [b, x] <- newSysLocalsDs [b_ty, x_ty]
359 -- build rest of the comprehesion
360 core_rest <- dfListComp c_id b quals
362 -- build the pattern match
363 core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
364 pat core_rest (Var b)
366 -- now build the outermost foldr, and return
367 mkFoldrExpr x_ty b_ty (mkLams [x, b] core_expr) (Var n_id) core_list1
370 %************************************************************************
372 \subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
374 %************************************************************************
378 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
379 -- mkZipBind [t1, t2]
380 -- = (zip, \as1:[t1] as2:[t2]
383 -- (a1:as'1) -> case as2 of
385 -- (a2:as'2) -> (a1, a2) : zip as'1 as'2)]
387 mkZipBind elt_tys = do
388 ass <- mapM newSysLocalDs elt_list_tys
389 as' <- mapM newSysLocalDs elt_tys
390 as's <- mapM newSysLocalDs elt_list_tys
392 zip_fn <- newSysLocalDs zip_fn_ty
394 let inner_rhs = mkConsExpr elt_tuple_ty
395 (mkBigCoreVarTup as')
396 (mkVarApps (Var zip_fn) as's)
397 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
399 return (zip_fn, mkLams ass zip_body)
401 elt_list_tys = map mkListTy elt_tys
402 elt_tuple_ty = mkBigCoreTupTy elt_tys
403 elt_tuple_list_ty = mkListTy elt_tuple_ty
405 zip_fn_ty = mkFunTys elt_list_tys elt_tuple_list_ty
407 mk_case (as, a', as') rest
408 = Case (Var as) as elt_tuple_list_ty
409 [(DataAlt nilDataCon, [], mkNilExpr elt_tuple_ty),
410 (DataAlt consDataCon, [a', as'], rest)]
411 -- Increasing order of tag
414 mkUnzipBind :: TransForm -> [Type] -> DsM (Maybe (Id, CoreExpr))
415 -- mkUnzipBind [t1, t2]
416 -- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
418 -- (x1, x2) -> case axs of
419 -- (xs1, xs2) -> (x1 : xs1, x2 : xs2))
423 -- We use foldr here in all cases, even if rules are turned off, because we may as well!
424 mkUnzipBind ThenForm _
425 = return Nothing -- No unzipping for ThenForm
426 mkUnzipBind _ elt_tys
427 = do { ax <- newSysLocalDs elt_tuple_ty
428 ; axs <- newSysLocalDs elt_list_tuple_ty
429 ; ys <- newSysLocalDs elt_tuple_list_ty
430 ; xs <- mapM newSysLocalDs elt_tys
431 ; xss <- mapM newSysLocalDs elt_list_tys
433 ; unzip_fn <- newSysLocalDs unzip_fn_ty
435 ; [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]
437 ; let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
438 concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
439 tupled_concat_expression = mkBigCoreTup concat_expressions
441 folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
442 folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
443 folder_body = mkLams [ax, axs] folder_body_outer_case
445 ; unzip_body <- mkFoldrExpr elt_tuple_ty elt_list_tuple_ty folder_body nil_tuple (Var ys)
446 ; return (Just (unzip_fn, mkLams [ys] unzip_body)) }
448 elt_tuple_ty = mkBigCoreTupTy elt_tys
449 elt_tuple_list_ty = mkListTy elt_tuple_ty
450 elt_list_tys = map mkListTy elt_tys
451 elt_list_tuple_ty = mkBigCoreTupTy elt_list_tys
453 unzip_fn_ty = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty
455 mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail
458 %************************************************************************
460 \subsection[DsPArrComp]{Desugaring of array comprehensions}
462 %************************************************************************
466 -- entry point for desugaring a parallel array comprehension
468 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
470 dsPArrComp :: [Stmt Id]
473 -- Special case for parallel comprehension
474 dsPArrComp (ParStmt qss _ _ _ : quals) = dePArrParComp qss quals
476 -- Special case for simple generators:
478 -- <<[:e' | p <- e, qs:]>> = <<[: e' | qs :]>> p e
480 -- if matching again p cannot fail, or else
482 -- <<[:e' | p <- e, qs:]>> =
483 -- <<[:e' | qs:]>> p (filterP (\x -> case x of {p -> True; _ -> False}) e)
485 dsPArrComp (BindStmt p e _ _ : qs) = do
486 filterP <- dsLookupDPHId filterPName
488 let ety'ce = parrElemType ce
489 false = Var falseDataConId
490 true = Var trueDataConId
491 v <- newSysLocalDs ety'ce
492 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
493 let gen | isIrrefutableHsPat p = ce
494 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
497 dsPArrComp qs = do -- no ParStmt in `qs'
498 sglP <- dsLookupDPHId singletonPName
499 let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
500 dePArrComp qs (noLoc $ WildPat unitTy) unitArray
506 dePArrComp :: [Stmt Id]
507 -> LPat Id -- the current generator pattern
508 -> CoreExpr -- the current generator expression
511 dePArrComp [] _ _ = panic "dePArrComp"
514 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
516 dePArrComp (LastStmt e' _ : quals) pa cea
517 = ASSERT( null quals )
518 do { mapP <- dsLookupDPHId mapPName
519 ; let ty = parrElemType cea
520 ; (clam, ty'e') <- deLambda ty pa e'
521 ; return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea] }
523 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
525 dePArrComp (ExprStmt b _ _ _ : qs) pa cea = do
526 filterP <- dsLookupDPHId filterPName
527 let ty = parrElemType cea
528 (clam,_) <- deLambda ty pa b
529 dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])
532 -- <<[:e' | p <- e, qs:]>> pa ea =
535 -- <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
537 -- if matching again p cannot fail, or else
539 -- <<[:e' | p <- e, qs:]>> pa ea =
540 -- let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
542 -- <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
544 dePArrComp (BindStmt p e _ _ : qs) pa cea = do
545 filterP <- dsLookupDPHId filterPName
546 crossMapP <- dsLookupDPHId crossMapPName
548 let ety'cea = parrElemType cea
549 ety'ce = parrElemType ce
550 false = Var falseDataConId
551 true = Var trueDataConId
552 v <- newSysLocalDs ety'ce
553 pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
554 let cef | isIrrefutableHsPat p = ce
555 | otherwise = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
556 (clam, _) <- mkLambda ety'cea pa cef
557 let ety'cef = ety'ce -- filter doesn't change the element type
558 pa' = mkLHsPatTup [pa, p]
560 dePArrComp qs pa' (mkApps (Var crossMapP)
561 [Type ety'cea, Type ety'cef, cea, clam])
563 -- <<[:e' | let ds, qs:]>> pa ea =
564 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
565 -- (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
567 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
569 dePArrComp (LetStmt ds : qs) pa cea = do
570 mapP <- dsLookupDPHId mapPName
571 let xs = collectLocalBinders ds
572 ty'cea = parrElemType cea
573 v <- newSysLocalDs ty'cea
574 clet <- dsLocalBinds ds (mkCoreTup (map Var xs))
575 let'v <- newSysLocalDs (exprType clet)
576 let projBody = mkCoreLet (NonRec let'v clet) $
577 mkCoreTup [Var v, Var let'v]
578 errTy = exprType projBody
579 errMsg = ptext (sLit "DsListComp.dePArrComp: internal error!")
580 cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
581 ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
582 let pa' = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
583 proj = mkLams [v] ccase
584 dePArrComp qs pa' (mkApps (Var mapP)
585 [Type ty'cea, Type errTy, proj, cea])
587 -- The parser guarantees that parallel comprehensions can only appear as
588 -- singeltons qualifier lists, which we already special case in the caller.
589 -- So, encountering one here is a bug.
591 dePArrComp (ParStmt _ _ _ _ : _) _ _ =
592 panic "DsListComp.dePArrComp: malformed comprehension AST"
594 -- <<[:e' | qs | qss:]>> pa ea =
595 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
596 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
598 -- {x_1, ..., x_n} = DV (qs)
600 dePArrParComp :: [([LStmt Id], [Id])] -> [Stmt Id] -> DsM CoreExpr
601 dePArrParComp qss quals = do
602 (pQss, ceQss) <- deParStmt qss
603 dePArrComp quals pQss ceQss
606 -- empty parallel statement lists have no source representation
607 panic "DsListComp.dePArrComp: Empty parallel list comprehension"
608 deParStmt ((qs, xs):qss) = do -- first statement
609 let res_expr = mkLHsVarTuple xs
610 cqs <- dsPArrComp (map unLoc qs ++ [mkLastStmt res_expr])
611 parStmts qss (mkLHsVarPatTup xs) cqs
613 parStmts [] pa cea = return (pa, cea)
614 parStmts ((qs, xs):qss) pa cea = do -- subsequent statements (zip'ed)
615 zipP <- dsLookupDPHId zipPName
616 let pa' = mkLHsPatTup [pa, mkLHsVarPatTup xs]
617 ty'cea = parrElemType cea
618 res_expr = mkLHsVarTuple xs
619 cqs <- dsPArrComp (map unLoc qs ++ [mkLastStmt res_expr])
620 let ty'cqs = parrElemType cqs
621 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
622 parStmts qss pa' cea'
624 -- generate Core corresponding to `\p -> e'
626 deLambda :: Type -- type of the argument
627 -> LPat Id -- argument pattern
628 -> LHsExpr Id -- body
629 -> DsM (CoreExpr, Type)
631 mkLambda ty p =<< dsLExpr e
633 -- generate Core for a lambda pattern match, where the body is already in Core
635 mkLambda :: Type -- type of the argument
636 -> LPat Id -- argument pattern
637 -> CoreExpr -- desugared body
638 -> DsM (CoreExpr, Type)
639 mkLambda ty p ce = do
640 v <- newSysLocalDs ty
641 let errMsg = ptext (sLit "DsListComp.deLambda: internal error!")
643 cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
644 res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
645 return (mkLams [v] res, ce'ty)
647 -- obtain the element type of the parallel array produced by the given Core
650 parrElemType :: CoreExpr -> Type
652 case splitTyConApp_maybe (exprType e) of
653 Just (tycon, [ty]) | tycon == parrTyCon -> ty
655 "DsListComp.parrElemType: not a parallel array type"
658 Translation for monad comprehensions
661 -- Entry point for monad comprehension desugaring
662 dsMonadComp :: [LStmt Id] -> DsM CoreExpr
663 dsMonadComp stmts = dsMcStmts stmts
665 dsMcStmts :: [LStmt Id] -> DsM CoreExpr
666 dsMcStmts [] = panic "dsMcStmts"
667 dsMcStmts (L loc stmt : lstmts) = putSrcSpanDs loc (dsMcStmt stmt lstmts)
670 dsMcStmt :: Stmt Id -> [LStmt Id] -> DsM CoreExpr
672 dsMcStmt (LastStmt body ret_op) stmts
673 = ASSERT( null stmts )
674 do { body' <- dsLExpr body
675 ; ret_op' <- dsExpr ret_op
676 ; return (App ret_op' body') }
678 -- [ .. | let binds, stmts ]
679 dsMcStmt (LetStmt binds) stmts
680 = do { rest <- dsMcStmts stmts
681 ; dsLocalBinds binds rest }
683 -- [ .. | a <- m, stmts ]
684 dsMcStmt (BindStmt pat rhs bind_op fail_op) stmts
685 = do { rhs' <- dsLExpr rhs
686 ; dsMcBindStmt pat rhs' bind_op fail_op stmts }
688 -- Apply `guard` to the `exp` expression
690 -- [ .. | exp, stmts ]
692 dsMcStmt (ExprStmt exp then_exp guard_exp _) stmts
693 = do { exp' <- dsLExpr exp
694 ; guard_exp' <- dsExpr guard_exp
695 ; then_exp' <- dsExpr then_exp
696 ; rest <- dsMcStmts stmts
697 ; return $ mkApps then_exp' [ mkApps guard_exp' [exp']
700 -- Group statements desugar like this:
702 -- [| (q, then group by e using f); rest |]
703 -- ---> f {qt} (\qv -> e) [| q; return qv |] >>= \ n_tup ->
704 -- case unzip n_tup of qv' -> [| rest |]
706 -- where variables (v1:t1, ..., vk:tk) are bound by q
707 -- qv = (v1, ..., vk)
708 -- qt = (t1, ..., tk)
709 -- (>>=) :: m2 a -> (a -> m3 b) -> m3 b
710 -- f :: forall a. (a -> t) -> m1 a -> m2 (n a)
712 -- unzip :: n qt -> (n t1, ..., n tk) (needs Functor n)
714 dsMcStmt (TransStmt { trS_stmts = stmts, trS_bndrs = bndrs
715 , trS_by = by, trS_using = using
716 , trS_ret = return_op, trS_bind = bind_op
717 , trS_fmap = fmap_op, trS_form = form }) stmts_rest
718 = do { let (from_bndrs, to_bndrs) = unzip bndrs
719 from_bndr_tys = map idType from_bndrs -- Types ty
721 -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
722 ; expr <- dsInnerMonadComp stmts from_bndrs return_op
724 -- Work out what arguments should be supplied to that expression: i.e. is an extraction
725 -- function required? If so, create that desugared function and add to arguments
726 ; usingExpr' <- dsLExpr using
727 ; usingArgs <- case by of
728 Nothing -> return [expr]
729 Just by_e -> do { by_e' <- dsLExpr by_e
730 ; lam <- matchTuple from_bndrs by_e'
731 ; return [lam, expr] }
733 -- Generate the expressions to build the grouped list
734 -- Build a pattern that ensures the consumer binds into the NEW binders,
735 -- which hold monads rather than single values
736 ; bind_op' <- dsExpr bind_op
737 ; let bind_ty = exprType bind_op' -- m2 (n (a,b,c)) -> (n (a,b,c) -> r1) -> r2
738 n_tup_ty = funArgTy $ funArgTy $ funResultTy bind_ty -- n (a,b,c)
739 tup_n_ty = mkBigCoreVarTupTy to_bndrs
741 ; body <- dsMcStmts stmts_rest
742 ; n_tup_var <- newSysLocalDs n_tup_ty
743 ; tup_n_var <- newSysLocalDs tup_n_ty
744 ; tup_n_expr <- mkMcUnzipM form fmap_op n_tup_var from_bndr_tys
745 ; us <- newUniqueSupply
746 ; let rhs' = mkApps usingExpr' usingArgs
747 body' = mkTupleCase us to_bndrs body tup_n_var tup_n_expr
749 ; return (mkApps bind_op' [rhs', Lam n_tup_var body']) }
751 -- Parallel statements. Use `Control.Monad.Zip.mzip` to zip parallel
752 -- statements, for example:
754 -- [ body | qs1 | qs2 | qs3 ]
755 -- -> [ body | (bndrs1, (bndrs2, bndrs3))
756 -- <- [bndrs1 | qs1] `mzip` ([bndrs2 | qs2] `mzip` [bndrs3 | qs3]) ]
758 -- where `mzip` has type
759 -- mzip :: forall a b. m a -> m b -> m (a,b)
760 -- NB: we need a polymorphic mzip because we call it several times
762 dsMcStmt (ParStmt pairs mzip_op bind_op return_op) stmts_rest
763 = do { exps_w_tys <- mapM ds_inner pairs -- Pairs (exp :: m ty, ty)
764 ; mzip_op' <- dsExpr mzip_op
766 ; let -- The pattern variables
767 pats = map (mkBigLHsVarPatTup . snd) pairs
768 -- Pattern with tuples of variables
769 -- [v1,v2,v3] => (v1, (v2, v3))
770 pat = foldr1 (\p1 p2 -> mkLHsPatTup [p1, p2]) pats
771 (rhs, _) = foldr1 (\(e1,t1) (e2,t2) ->
772 (mkApps mzip_op' [Type t1, Type t2, e1, e2],
773 mkBoxedTupleTy [t1,t2]))
776 ; dsMcBindStmt pat rhs bind_op noSyntaxExpr stmts_rest }
778 ds_inner (stmts, bndrs) = do { exp <- dsInnerMonadComp stmts bndrs mono_ret_op
779 ; return (exp, tup_ty) }
781 mono_ret_op = HsWrap (WpTyApp tup_ty) return_op
782 tup_ty = mkBigCoreVarTupTy bndrs
784 dsMcStmt stmt _ = pprPanic "dsMcStmt: unexpected stmt" (ppr stmt)
787 matchTuple :: [Id] -> CoreExpr -> DsM CoreExpr
788 -- (matchTuple [a,b,c] body)
789 -- returns the Core term
790 -- \x. case x of (a,b,c) -> body
792 = do { us <- newUniqueSupply
793 ; tup_id <- newSysLocalDs (mkBigCoreVarTupTy ids)
794 ; return (Lam tup_id $ mkTupleCase us ids body tup_id (Var tup_id)) }
796 -- general `rhs' >>= \pat -> stmts` desugaring where `rhs'` is already a
797 -- desugared `CoreExpr`
798 dsMcBindStmt :: LPat Id
799 -> CoreExpr -- ^ the desugared rhs of the bind statement
804 dsMcBindStmt pat rhs' bind_op fail_op stmts
805 = do { body <- dsMcStmts stmts
806 ; bind_op' <- dsExpr bind_op
807 ; var <- selectSimpleMatchVarL pat
808 ; let bind_ty = exprType bind_op' -- rhs -> (pat -> res1) -> res2
809 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
810 ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
811 res1_ty (cantFailMatchResult body)
812 ; match_code <- handle_failure pat match fail_op
813 ; return (mkApps bind_op' [rhs', Lam var match_code]) }
816 -- In a monad comprehension expression, pattern-match failure just calls
817 -- the monadic `fail` rather than throwing an exception
818 handle_failure pat match fail_op
820 = do { fail_op' <- dsExpr fail_op
821 ; fail_msg <- mkStringExpr (mk_fail_msg pat)
822 ; extractMatchResult match (App fail_op' fail_msg) }
824 = extractMatchResult match (error "It can't fail")
826 mk_fail_msg :: Located e -> String
827 mk_fail_msg pat = "Pattern match failure in monad comprehension at " ++
828 showSDoc (ppr (getLoc pat))
830 -- Desugar nested monad comprehensions, for example in `then..` constructs
831 -- dsInnerMonadComp quals [a,b,c] ret_op
832 -- returns the desugaring of
833 -- [ (a,b,c) | quals ]
835 dsInnerMonadComp :: [LStmt Id]
836 -> [Id] -- Return a tuple of these variables
837 -> HsExpr Id -- The monomorphic "return" operator
839 dsInnerMonadComp stmts bndrs ret_op
840 = dsMcStmts (stmts ++ [noLoc (LastStmt (mkBigLHsVarTup bndrs) ret_op)])
842 -- The `unzip` function for `GroupStmt` in a monad comprehensions
844 -- unzip :: m (a,b,..) -> (m a,m b,..)
845 -- unzip m_tuple = ( liftM selN1 m_tuple
846 -- , liftM selN2 m_tuple
849 -- mkMcUnzipM fmap ys [t1, t2]
850 -- = ( fmap (selN1 :: (t1, t2) -> t1) ys
851 -- , fmap (selN2 :: (t1, t2) -> t2) ys )
853 mkMcUnzipM :: TransForm
854 -> SyntaxExpr TcId -- fmap
855 -> Id -- Of type n (a,b,c)
857 -> DsM CoreExpr -- Of type (n a, n b, n c)
858 mkMcUnzipM ThenForm _ ys _
859 = return (Var ys) -- No unzipping to do
861 mkMcUnzipM _ fmap_op ys elt_tys
862 = do { fmap_op' <- dsExpr fmap_op
863 ; xs <- mapM newSysLocalDs elt_tys
864 ; let tup_ty = mkBigCoreTupTy elt_tys
865 ; tup_xs <- newSysLocalDs tup_ty
867 ; let mk_elt i = mkApps fmap_op' -- fmap :: forall a b. (a -> b) -> n a -> n b
868 [ Type tup_ty, Type (elt_tys !! i)
871 mk_sel n = Lam tup_xs $
872 mkTupleSelector xs (xs !! n) tup_xs (Var tup_xs)
874 ; return (mkBigCoreTup (map mk_elt [0..length elt_tys - 1])) }