2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
4 \section[DsListComp]{Desugaring list comprehensions and array comprehensions}
7 module DsListComp ( dsListComp, dsPArrComp ) where
9 #include "HsVersions.h"
11 import {-# SOURCE #-} DsExpr ( dsExpr, dsLet )
13 import BasicTypes ( Boxity(..) )
14 import TyCon ( tyConName )
15 import HsSyn ( Pat(..), HsExpr(..), Stmt(..),
16 HsMatchContext(..), HsStmtContext(..),
18 import TcHsSyn ( TypecheckedStmt, TypecheckedPat, TypecheckedHsExpr,
22 import DsMonad -- the monadery used in the desugarer
25 import CmdLineOpts ( DynFlag(..), dopt, opt_RulesOff )
26 import CoreUtils ( exprType, mkIfThenElse )
29 import Type ( mkTyVarTy, mkFunTys, mkFunTy, Type,
31 import TysPrim ( alphaTyVar )
32 import TysWiredIn ( nilDataCon, consDataCon, trueDataConId, falseDataConId,
33 unitDataConId, unitTy, mkListTy )
34 import Match ( matchSimply )
35 import PrelNames ( foldrName, buildName, replicatePName, mapPName,
36 filterPName, zipPName, crossPName, parrTyConName )
37 import PrelInfo ( pAT_ERROR_ID )
38 import SrcLoc ( noSrcLoc )
39 import Panic ( panic )
42 List comprehensions may be desugared in one of two ways: ``ordinary''
43 (as you would expect if you read SLPJ's book) and ``with foldr/build
44 turned on'' (if you read Gill {\em et al.}'s paper on the subject).
46 There will be at least one ``qualifier'' in the input.
49 dsListComp :: [TypecheckedStmt]
50 -> Type -- Type of list elements
53 dsListComp quals elt_ty
54 = getDOptsDs `thenDs` \dflags ->
55 if opt_RulesOff || dopt Opt_IgnoreInterfacePragmas dflags
56 -- Either rules are switched off, or we are ignoring what there are;
57 -- Either way foldr/build won't happen, so use the more efficient
58 -- Wadler-style desugaring
59 || isParallelComp quals
60 -- Foldr-style desugaring can't handle
61 -- parallel list comprehensions
62 then deListComp quals (mkNilExpr elt_ty)
64 else -- Foldr/build should be enabled, so desugar
65 -- into foldrs and builds
66 newTyVarsDs [alphaTyVar] `thenDs` \ [n_tyvar] ->
68 n_ty = mkTyVarTy n_tyvar
69 c_ty = mkFunTys [elt_ty, n_ty] n_ty
71 newSysLocalsDs [c_ty,n_ty] `thenDs` \ [c, n] ->
72 dfListComp c n quals `thenDs` \ result ->
73 dsLookupGlobalId buildName `thenDs` \ build_id ->
74 returnDs (Var build_id `App` Type elt_ty
75 `App` mkLams [n_tyvar, c, n] result)
77 where isParallelComp (ParStmt bndrstmtss : _) = True
78 isParallelComp _ = False
81 %************************************************************************
83 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
85 %************************************************************************
87 Just as in Phil's chapter~7 in SLPJ, using the rules for
88 optimally-compiled list comprehensions. This is what Kevin followed
89 as well, and I quite happily do the same. The TQ translation scheme
90 transforms a list of qualifiers (either boolean expressions or
91 generators) into a single expression which implements the list
92 comprehension. Because we are generating 2nd-order polymorphic
93 lambda-calculus, calls to NIL and CONS must be applied to a type
94 argument, as well as their usual value arguments.
96 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
99 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
102 TQ << [ e | b , qs ] ++ L >> =
103 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
106 TQ << [ e | p <- L1, qs ] ++ L2 >> =
112 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
117 "h", "u1", "u2", and "u3" are new variables.
120 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
121 is the TE translation scheme. Note that we carry around the @L@ list
122 already desugared. @dsListComp@ does the top TE rule mentioned above.
124 To the above, we add an additional rule to deal with parallel list
125 comprehensions. The translation goes roughly as follows:
126 [ e | p1 <- e11, let v1 = e12, p2 <- e13
127 | q1 <- e21, let v2 = e22, q2 <- e23]
129 [ e | ((x1, .., xn), (y1, ..., ym)) <-
130 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
131 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
132 where (x1, .., xn) are the variables bound in p1, v1, p2
133 (y1, .., ym) are the variables bound in q1, v2, q2
135 In the translation below, the ParStmt branch translates each parallel branch
136 into a sub-comprehension, and desugars each independently. The resulting lists
137 are fed to a zip function, we create a binding for all the variables bound in all
138 the comprehensions, and then we hand things off the the desugarer for bindings.
139 The zip function is generated here a) because it's small, and b) because then we
140 don't have to deal with arbitrary limits on the number of zip functions in the
141 prelude, nor which library the zip function came from.
142 The introduced tuples are Boxed, but only because I couldn't get it to work
143 with the Unboxed variety.
147 deListComp :: [TypecheckedStmt] -> CoreExpr -> DsM CoreExpr
149 deListComp (ParStmt stmtss_w_bndrs : quals) list
150 = mapDs do_list_comp stmtss_w_bndrs `thenDs` \ exps ->
151 mkZipBind qual_tys `thenDs` \ (zip_fn, zip_rhs) ->
153 -- Deal with [e | pat <- zip l1 .. ln] in example above
154 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
158 bndrs_s = map snd stmtss_w_bndrs
160 -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
161 pat = TuplePat pats Boxed
162 pats = map mk_hs_tuple_pat bndrs_s
164 -- Types of (x1,..,xn), (y1,..,yn) etc
165 qual_tys = map mk_bndrs_tys bndrs_s
167 do_list_comp (stmts, bndrs)
168 = dsListComp (stmts ++ [ResultStmt (mk_hs_tuple_expr bndrs) noSrcLoc])
171 mk_bndrs_tys bndrs = mkCoreTupTy (map idType bndrs)
173 -- Last: the one to return
174 deListComp [ResultStmt expr locn] list -- Figure 7.4, SLPJ, p 135, rule C above
175 = dsExpr expr `thenDs` \ core_expr ->
176 returnDs (mkConsExpr (exprType core_expr) core_expr list)
178 -- Non-last: must be a guard
179 deListComp (ExprStmt guard ty locn : quals) list -- rule B above
180 = dsExpr guard `thenDs` \ core_guard ->
181 deListComp quals list `thenDs` \ core_rest ->
182 returnDs (mkIfThenElse core_guard core_rest list)
184 -- [e | let B, qs] = let B in [e | qs]
185 deListComp (LetStmt binds : quals) list
186 = deListComp quals list `thenDs` \ core_rest ->
187 dsLet binds core_rest
189 deListComp (BindStmt pat list1 locn : quals) core_list2 -- rule A' above
190 = dsExpr list1 `thenDs` \ core_list1 ->
191 deBindComp pat core_list1 quals core_list2
196 deBindComp pat core_list1 quals core_list2
198 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
200 -- u1_ty is a [alpha] type, and u2_ty = alpha
201 u2_ty = hsPatType pat
203 res_ty = exprType core_list2
204 h_ty = u1_ty `mkFunTy` res_ty
206 newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty] `thenDs` \ [h, u1, u2, u3] ->
208 -- the "fail" value ...
210 core_fail = App (Var h) (Var u3)
211 letrec_body = App (Var h) core_list1
213 deListComp quals core_fail `thenDs` \ rest_expr ->
214 matchSimply (Var u2) (StmtCtxt ListComp) pat
215 rest_expr core_fail `thenDs` \ core_match ->
218 Case (Var u1) u1 [(DataAlt nilDataCon, [], core_list2),
219 (DataAlt consDataCon, [u2, u3], core_match)]
221 returnDs (Let (Rec [(h, rhs)]) letrec_body)
226 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
227 -- mkZipBind [t1, t2]
228 -- = (zip, \as1:[t1] as2:[t2]
231 -- (a1:as'1) -> case as2 of
233 -- (a2:as'2) -> (a2,a2) : zip as'1 as'2)]
236 = mapDs newSysLocalDs list_tys `thenDs` \ ass ->
237 mapDs newSysLocalDs elt_tys `thenDs` \ as' ->
238 mapDs newSysLocalDs list_tys `thenDs` \ as's ->
239 newSysLocalDs zip_fn_ty `thenDs` \ zip_fn ->
241 inner_rhs = mkConsExpr ret_elt_ty
242 (mkCoreTup (map Var as'))
243 (mkVarApps (Var zip_fn) as's)
244 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
246 returnDs (zip_fn, mkLams ass zip_body)
248 list_tys = map mkListTy elt_tys
249 ret_elt_ty = mkCoreTupTy elt_tys
250 zip_fn_ty = mkFunTys list_tys (mkListTy ret_elt_ty)
252 mk_case (as, a', as') rest
253 = Case (Var as) as [(DataAlt nilDataCon, [], mkNilExpr ret_elt_ty),
254 (DataAlt consDataCon, [a', as'], rest)]
256 -- Helper functions that makes an HsTuple only for non-1-sized tuples
257 mk_hs_tuple_expr :: [Id] -> TypecheckedHsExpr
258 mk_hs_tuple_expr [] = HsVar unitDataConId
259 mk_hs_tuple_expr [id] = HsVar id
260 mk_hs_tuple_expr ids = ExplicitTuple [ HsVar i | i <- ids ] Boxed
262 mk_hs_tuple_pat :: [Id] -> TypecheckedPat
263 mk_hs_tuple_pat [b] = VarPat b
264 mk_hs_tuple_pat bs = TuplePat (map VarPat bs) Boxed
268 %************************************************************************
270 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
272 %************************************************************************
274 @dfListComp@ are the rules used with foldr/build turned on:
277 TE[ e | ] c n = c e n
278 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
279 TE[ e | p <- l , q ] c n = let
280 f = \ x b -> case x of
288 dfListComp :: Id -> Id -- 'c' and 'n'
289 -> [TypecheckedStmt] -- the rest of the qual's
292 -- Last: the one to return
293 dfListComp c_id n_id [ResultStmt expr locn]
294 = dsExpr expr `thenDs` \ core_expr ->
295 returnDs (mkApps (Var c_id) [core_expr, Var n_id])
297 -- Non-last: must be a guard
298 dfListComp c_id n_id (ExprStmt guard ty locn : quals)
299 = dsExpr guard `thenDs` \ core_guard ->
300 dfListComp c_id n_id quals `thenDs` \ core_rest ->
301 returnDs (mkIfThenElse core_guard core_rest (Var n_id))
303 dfListComp c_id n_id (LetStmt binds : quals)
304 -- new in 1.3, local bindings
305 = dfListComp c_id n_id quals `thenDs` \ core_rest ->
306 dsLet binds core_rest
308 dfListComp c_id n_id (BindStmt pat list1 locn : quals)
309 -- evaluate the two lists
310 = dsExpr list1 `thenDs` \ core_list1 ->
312 -- find the required type
313 let x_ty = hsPatType pat
317 -- create some new local id's
318 newSysLocalsDs [b_ty,x_ty] `thenDs` \ [b,x] ->
320 -- build rest of the comprehesion
321 dfListComp c_id b quals `thenDs` \ core_rest ->
323 -- build the pattern match
324 matchSimply (Var x) (StmtCtxt ListComp)
325 pat core_rest (Var b) `thenDs` \ core_expr ->
327 -- now build the outermost foldr, and return
328 dsLookupGlobalId foldrName `thenDs` \ foldr_id ->
330 Var foldr_id `App` Type x_ty
332 `App` mkLams [x, b] core_expr
338 %************************************************************************
340 \subsection[DsPArrComp]{Desugaring of array comprehensions}
342 %************************************************************************
346 -- entry point for desugaring a parallel array comprehension
348 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
350 dsPArrComp :: [TypecheckedStmt]
351 -> Type -- Don't use; called with `undefined' below
354 dsLookupGlobalId replicatePName `thenDs` \repP ->
355 let unitArray = mkApps (Var repP) [Type unitTy,
359 dePArrComp qs (TuplePat [] Boxed) unitArray
363 dePArrComp :: [TypecheckedStmt]
364 -> TypecheckedPat -- the current generator pattern
365 -> CoreExpr -- the current generator expression
368 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
370 dePArrComp [ResultStmt e' _] pa cea =
371 dsLookupGlobalId mapPName `thenDs` \mapP ->
372 let ty = parrElemType cea
374 deLambda ty pa e' `thenDs` \(clam,
376 returnDs $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
378 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
380 dePArrComp (ExprStmt b _ _ : qs) pa cea =
381 dsLookupGlobalId filterPName `thenDs` \filterP ->
382 let ty = parrElemType cea
384 deLambda ty pa b `thenDs` \(clam,_) ->
385 dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])
387 -- <<[:e' | p <- e, qs:]>> pa ea =
388 -- let ef = filterP (\x -> case x of {p -> True; _ -> False}) e
390 -- <<[:e' | qs:]>> (pa, p) (crossP ea ef)
392 dePArrComp (BindStmt p e _ : qs) pa cea =
393 dsLookupGlobalId filterPName `thenDs` \filterP ->
394 dsLookupGlobalId crossPName `thenDs` \crossP ->
395 dsExpr e `thenDs` \ce ->
396 let ty'cea = parrElemType cea
397 ty'ce = parrElemType ce
398 false = Var falseDataConId
399 true = Var trueDataConId
401 newSysLocalDs ty'ce `thenDs` \v ->
402 matchSimply (Var v) (StmtCtxt PArrComp) p true false `thenDs` \pred ->
403 let cef = mkApps (Var filterP) [Type ty'ce, mkLams [v] pred, ce]
404 ty'cef = ty'ce -- filterP preserves the type
405 pa' = TuplePat [pa, p] Boxed
407 dePArrComp qs pa' (mkApps (Var crossP) [Type ty'cea, Type ty'cef, cea, cef])
409 -- <<[:e' | let ds, qs:]>> pa ea =
410 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
411 -- (mapP (\v@pa -> (v, let ds in (x_1, ..., x_n))) ea)
413 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
415 dePArrComp (LetStmt ds : qs) pa cea =
416 dsLookupGlobalId mapPName `thenDs` \mapP ->
417 let xs = collectHsBinders ds
418 ty'cea = parrElemType cea
420 newSysLocalDs ty'cea `thenDs` \v ->
421 dsLet ds (mkCoreTup (map Var xs)) `thenDs` \clet ->
422 newSysLocalDs (exprType clet) `thenDs` \let'v ->
423 let projBody = mkDsLet (NonRec let'v clet) $
424 mkCoreTup [Var v, Var let'v]
425 errTy = exprType projBody
426 errMsg = "DsListComp.dePArrComp: internal error!"
428 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
429 matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr `thenDs` \ccase ->
430 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
431 proj = mkLams [v] ccase
433 dePArrComp qs pa' (mkApps (Var mapP) [Type ty'cea, proj, cea])
435 -- <<[:e' | qs | qss:]>> pa ea =
436 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
437 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
439 -- {x_1, ..., x_n} = DV (qs)
441 dePArrComp (ParStmt [] : qss2) pa cea = dePArrComp qss2 pa cea
442 dePArrComp (ParStmt ((qs, xs):qss) : qss2) pa cea =
443 dsLookupGlobalId zipPName `thenDs` \zipP ->
444 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
445 ty'cea = parrElemType cea
446 resStmt = ResultStmt (ExplicitTuple (map HsVar xs) Boxed) noSrcLoc
448 dsPArrComp (qs ++ [resStmt]) undefined `thenDs` \cqs ->
449 let ty'cqs = parrElemType cqs
450 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
452 dePArrComp (ParStmt qss : qss2) pa' cea'
454 -- generate Core corresponding to `\p -> e'
456 deLambda :: Type -- type of the argument
457 -> TypecheckedPat -- argument pattern
458 -> TypecheckedHsExpr -- body
459 -> DsM (CoreExpr, Type)
461 newSysLocalDs ty `thenDs` \v ->
462 dsExpr e `thenDs` \ce ->
463 let errTy = exprType ce
464 errMsg = "DsListComp.deLambda: internal error!"
466 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
467 matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr `thenDs` \res ->
468 returnDs (mkLams [v] res, errTy)
470 -- obtain the element type of the parallel array produced by the given Core
473 parrElemType :: CoreExpr -> Type
475 case splitTyConApp_maybe (exprType e) of
476 Just (tycon, [ty]) | tyConName tycon == parrTyConName -> ty
478 "DsListComp.parrElemType: not a parallel array type"