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 ( opt_FoldrBuildOn )
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 | not opt_FoldrBuildOn -- Be boring
55 || isParallelComp quals
56 = deListComp quals (mkNilExpr elt_ty)
58 | otherwise -- foldr/build lives!
59 = newTyVarsDs [alphaTyVar] `thenDs` \ [n_tyvar] ->
61 n_ty = mkTyVarTy n_tyvar
62 c_ty = mkFunTys [elt_ty, n_ty] n_ty
64 newSysLocalsDs [c_ty,n_ty] `thenDs` \ [c, n] ->
65 dfListComp c n quals `thenDs` \ result ->
66 dsLookupGlobalId buildName `thenDs` \ build_id ->
67 returnDs (Var build_id `App` Type elt_ty
68 `App` mkLams [n_tyvar, c, n] result)
70 where isParallelComp (ParStmtOut bndrstmtss : _) = True
71 isParallelComp _ = False
74 %************************************************************************
76 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
78 %************************************************************************
80 Just as in Phil's chapter~7 in SLPJ, using the rules for
81 optimally-compiled list comprehensions. This is what Kevin followed
82 as well, and I quite happily do the same. The TQ translation scheme
83 transforms a list of qualifiers (either boolean expressions or
84 generators) into a single expression which implements the list
85 comprehension. Because we are generating 2nd-order polymorphic
86 lambda-calculus, calls to NIL and CONS must be applied to a type
87 argument, as well as their usual value arguments.
89 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
92 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
95 TQ << [ e | b , qs ] ++ L >> =
96 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
99 TQ << [ e | p <- L1, qs ] ++ L2 >> =
105 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
110 "h", "u1", "u2", and "u3" are new variables.
113 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
114 is the TE translation scheme. Note that we carry around the @L@ list
115 already desugared. @dsListComp@ does the top TE rule mentioned above.
117 To the above, we add an additional rule to deal with parallel list
118 comprehensions. The translation goes roughly as follows:
119 [ e | p1 <- e11, let v1 = e12, p2 <- e13
120 | q1 <- e21, let v2 = e22, q2 <- e23]
122 [ e | ((x1, .., xn), (y1, ..., ym)) <-
123 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
124 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
125 where (x1, .., xn) are the variables bound in p1, v1, p2
126 (y1, .., ym) are the variables bound in q1, v2, q2
128 In the translation below, the ParStmtOut branch translates each parallel branch
129 into a sub-comprehension, and desugars each independently. The resulting lists
130 are fed to a zip function, we create a binding for all the variables bound in all
131 the comprehensions, and then we hand things off the the desugarer for bindings.
132 The zip function is generated here a) because it's small, and b) because then we
133 don't have to deal with arbitrary limits on the number of zip functions in the
134 prelude, nor which library the zip function came from.
135 The introduced tuples are Boxed, but only because I couldn't get it to work
136 with the Unboxed variety.
140 deListComp :: [TypecheckedStmt] -> CoreExpr -> DsM CoreExpr
142 deListComp (ParStmtOut bndrstmtss : quals) list
143 = mapDs do_list_comp bndrstmtss `thenDs` \ exps ->
144 mkZipBind qual_tys `thenDs` \ (zip_fn, zip_rhs) ->
146 -- Deal with [e | pat <- zip l1 .. ln] in example above
147 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
150 where -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
151 pat = TuplePat pats Boxed
152 pats = map (\(bs,_) -> mk_hs_tuple_pat bs) bndrstmtss
154 -- Types of (x1,..,xn), (y1,..,yn) etc
155 qual_tys = [ mk_bndrs_tys bndrs | (bndrs,_) <- bndrstmtss ]
157 do_list_comp (bndrs, stmts)
158 = dsListComp (stmts ++ [ResultStmt (mk_hs_tuple_expr bndrs) noSrcLoc])
161 mk_bndrs_tys bndrs = mkCoreTupTy (map idType bndrs)
163 -- Last: the one to return
164 deListComp [ResultStmt expr locn] list -- Figure 7.4, SLPJ, p 135, rule C above
165 = dsExpr expr `thenDs` \ core_expr ->
166 returnDs (mkConsExpr (exprType core_expr) core_expr list)
168 -- Non-last: must be a guard
169 deListComp (ExprStmt guard ty locn : quals) list -- rule B above
170 = dsExpr guard `thenDs` \ core_guard ->
171 deListComp quals list `thenDs` \ core_rest ->
172 returnDs (mkIfThenElse core_guard core_rest list)
174 -- [e | let B, qs] = let B in [e | qs]
175 deListComp (LetStmt binds : quals) list
176 = deListComp quals list `thenDs` \ core_rest ->
177 dsLet binds core_rest
179 deListComp (BindStmt pat list1 locn : quals) core_list2 -- rule A' above
180 = dsExpr list1 `thenDs` \ core_list1 ->
181 deBindComp pat core_list1 quals core_list2
186 deBindComp pat core_list1 quals core_list2
188 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
190 -- u1_ty is a [alpha] type, and u2_ty = alpha
191 u2_ty = hsPatType pat
193 res_ty = exprType core_list2
194 h_ty = u1_ty `mkFunTy` res_ty
196 newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty] `thenDs` \ [h, u1, u2, u3] ->
198 -- the "fail" value ...
200 core_fail = App (Var h) (Var u3)
201 letrec_body = App (Var h) core_list1
203 deListComp quals core_fail `thenDs` \ rest_expr ->
204 matchSimply (Var u2) (StmtCtxt ListComp) pat
205 rest_expr core_fail `thenDs` \ core_match ->
208 Case (Var u1) u1 [(DataAlt nilDataCon, [], core_list2),
209 (DataAlt consDataCon, [u2, u3], core_match)]
211 returnDs (Let (Rec [(h, rhs)]) letrec_body)
216 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
217 -- mkZipBind [t1, t2]
218 -- = (zip, \as1:[t1] as2:[t2]
221 -- (a1:as'1) -> case as2 of
223 -- (a2:as'2) -> (a2,a2) : zip as'1 as'2)]
226 = mapDs newSysLocalDs list_tys `thenDs` \ ass ->
227 mapDs newSysLocalDs elt_tys `thenDs` \ as' ->
228 mapDs newSysLocalDs list_tys `thenDs` \ as's ->
229 newSysLocalDs zip_fn_ty `thenDs` \ zip_fn ->
231 inner_rhs = mkConsExpr ret_elt_ty
232 (mkCoreTup (map Var as'))
233 (mkVarApps (Var zip_fn) as's)
234 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
236 returnDs (zip_fn, mkLams ass zip_body)
238 list_tys = map mkListTy elt_tys
239 ret_elt_ty = mkCoreTupTy elt_tys
240 zip_fn_ty = mkFunTys list_tys (mkListTy ret_elt_ty)
242 mk_case (as, a', as') rest
243 = Case (Var as) as [(DataAlt nilDataCon, [], mkNilExpr ret_elt_ty),
244 (DataAlt consDataCon, [a', as'], rest)]
246 -- Helper functions that makes an HsTuple only for non-1-sized tuples
247 mk_hs_tuple_expr :: [Id] -> TypecheckedHsExpr
248 mk_hs_tuple_expr [] = HsVar unitDataConId
249 mk_hs_tuple_expr [id] = HsVar id
250 mk_hs_tuple_expr ids = ExplicitTuple [ HsVar i | i <- ids ] Boxed
252 mk_hs_tuple_pat :: [Id] -> TypecheckedPat
253 mk_hs_tuple_pat [b] = VarPat b
254 mk_hs_tuple_pat bs = TuplePat (map VarPat bs) Boxed
258 %************************************************************************
260 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
262 %************************************************************************
264 @dfListComp@ are the rules used with foldr/build turned on:
267 TE[ e | ] c n = c e n
268 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
269 TE[ e | p <- l , q ] c n = let
270 f = \ x b -> case x of
278 dfListComp :: Id -> Id -- 'c' and 'n'
279 -> [TypecheckedStmt] -- the rest of the qual's
282 -- Last: the one to return
283 dfListComp c_id n_id [ResultStmt expr locn]
284 = dsExpr expr `thenDs` \ core_expr ->
285 returnDs (mkApps (Var c_id) [core_expr, Var n_id])
287 -- Non-last: must be a guard
288 dfListComp c_id n_id (ExprStmt guard ty locn : quals)
289 = dsExpr guard `thenDs` \ core_guard ->
290 dfListComp c_id n_id quals `thenDs` \ core_rest ->
291 returnDs (mkIfThenElse core_guard core_rest (Var n_id))
293 dfListComp c_id n_id (LetStmt binds : quals)
294 -- new in 1.3, local bindings
295 = dfListComp c_id n_id quals `thenDs` \ core_rest ->
296 dsLet binds core_rest
298 dfListComp c_id n_id (BindStmt pat list1 locn : quals)
299 -- evaluate the two lists
300 = dsExpr list1 `thenDs` \ core_list1 ->
302 -- find the required type
303 let x_ty = hsPatType pat
307 -- create some new local id's
308 newSysLocalsDs [b_ty,x_ty] `thenDs` \ [b,x] ->
310 -- build rest of the comprehesion
311 dfListComp c_id b quals `thenDs` \ core_rest ->
313 -- build the pattern match
314 matchSimply (Var x) (StmtCtxt ListComp)
315 pat core_rest (Var b) `thenDs` \ core_expr ->
317 -- now build the outermost foldr, and return
318 dsLookupGlobalId foldrName `thenDs` \ foldr_id ->
320 Var foldr_id `App` Type x_ty
322 `App` mkLams [x, b] core_expr
328 %************************************************************************
330 \subsection[DsPArrComp]{Desugaring of array comprehensions}
332 %************************************************************************
336 -- entry point for desugaring a parallel array comprehension
338 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
340 dsPArrComp :: [TypecheckedStmt]
341 -> Type -- Don't use; called with `undefined' below
344 dsLookupGlobalId replicatePName `thenDs` \repP ->
345 let unitArray = mkApps (Var repP) [Type unitTy,
349 dePArrComp qs (TuplePat [] Boxed) unitArray
353 dePArrComp :: [TypecheckedStmt]
354 -> TypecheckedPat -- the current generator pattern
355 -> CoreExpr -- the current generator expression
358 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
360 dePArrComp [ResultStmt e' _] pa cea =
361 dsLookupGlobalId mapPName `thenDs` \mapP ->
362 let ty = parrElemType cea
364 deLambda ty pa e' `thenDs` \(clam,
366 returnDs $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
368 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
370 dePArrComp (ExprStmt b _ _ : qs) pa cea =
371 dsLookupGlobalId filterPName `thenDs` \filterP ->
372 let ty = parrElemType cea
374 deLambda ty pa b `thenDs` \(clam,_) ->
375 dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])
377 -- <<[:e' | p <- e, qs:]>> pa ea =
378 -- let ef = filterP (\x -> case x of {p -> True; _ -> False}) e
380 -- <<[:e' | qs:]>> (pa, p) (crossP ea ef)
382 dePArrComp (BindStmt p e _ : qs) pa cea =
383 dsLookupGlobalId filterPName `thenDs` \filterP ->
384 dsLookupGlobalId crossPName `thenDs` \crossP ->
385 dsExpr e `thenDs` \ce ->
386 let ty'cea = parrElemType cea
387 ty'ce = parrElemType ce
388 false = Var falseDataConId
389 true = Var trueDataConId
391 newSysLocalDs ty'ce `thenDs` \v ->
392 matchSimply (Var v) (StmtCtxt PArrComp) p true false `thenDs` \pred ->
393 let cef = mkApps (Var filterP) [Type ty'ce, mkLams [v] pred, ce]
394 ty'cef = ty'ce -- filterP preserves the type
395 pa' = TuplePat [pa, p] Boxed
397 dePArrComp qs pa' (mkApps (Var crossP) [Type ty'cea, Type ty'cef, cea, cef])
399 -- <<[:e' | let ds, qs:]>> pa ea =
400 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
401 -- (mapP (\v@pa -> (v, let ds in (x_1, ..., x_n))) ea)
403 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
405 dePArrComp (LetStmt ds : qs) pa cea =
406 dsLookupGlobalId mapPName `thenDs` \mapP ->
407 let xs = collectHsBinders ds
408 ty'cea = parrElemType cea
410 newSysLocalDs ty'cea `thenDs` \v ->
411 dsLet ds (mkCoreTup (map Var xs)) `thenDs` \clet ->
412 newSysLocalDs (exprType clet) `thenDs` \let'v ->
413 let projBody = mkDsLet (NonRec let'v clet) $
414 mkCoreTup [Var v, Var let'v]
415 errTy = exprType projBody
416 errMsg = "DsListComp.dePArrComp: internal error!"
418 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
419 matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr `thenDs` \ccase ->
420 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
421 proj = mkLams [v] ccase
423 dePArrComp qs pa' (mkApps (Var mapP) [Type ty'cea, proj, cea])
425 -- <<[:e' | qs | qss:]>> pa ea =
426 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
427 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
429 -- {x_1, ..., x_n} = DV (qs)
431 dePArrComp (ParStmtOut [] : qss2) pa cea = dePArrComp qss2 pa cea
432 dePArrComp (ParStmtOut ((xs, qs):qss) : qss2) pa cea =
433 dsLookupGlobalId zipPName `thenDs` \zipP ->
434 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
435 ty'cea = parrElemType cea
436 resStmt = ResultStmt (ExplicitTuple (map HsVar xs) Boxed) noSrcLoc
438 dsPArrComp (qs ++ [resStmt]) undefined `thenDs` \cqs ->
439 let ty'cqs = parrElemType cqs
440 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
442 dePArrComp (ParStmtOut qss : qss2) pa' cea'
444 -- generate Core corresponding to `\p -> e'
446 deLambda :: Type -- type of the argument
447 -> TypecheckedPat -- argument pattern
448 -> TypecheckedHsExpr -- body
449 -> DsM (CoreExpr, Type)
451 newSysLocalDs ty `thenDs` \v ->
452 dsExpr e `thenDs` \ce ->
453 let errTy = exprType ce
454 errMsg = "DsListComp.deLambda: internal error!"
456 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
457 matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr `thenDs` \res ->
458 returnDs (mkLams [v] res, errTy)
460 -- obtain the element type of the parallel array produced by the given Core
463 parrElemType :: CoreExpr -> Type
465 case splitTyConApp_maybe (exprType e) of
466 Just (tycon, [ty]) | tyConName tycon == parrTyConName -> ty
468 "DsListComp.parrElemType: not a parallel array type"