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,
35 import Match ( matchSimply )
36 import PrelNames ( foldrName, buildName, replicatePName, mapPName,
37 filterPName, zipPName, crossPName, parrTyConName )
38 import PrelInfo ( pAT_ERROR_ID )
39 import SrcLoc ( noSrcLoc )
40 import Panic ( panic )
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 :: [TypecheckedStmt]
51 -> Type -- Type of list elements
54 dsListComp quals elt_ty
55 | not opt_FoldrBuildOn -- Be boring
56 || isParallelComp quals
57 = deListComp quals (mkNilExpr elt_ty)
59 | otherwise -- foldr/build lives!
60 = newTyVarsDs [alphaTyVar] `thenDs` \ [n_tyvar] ->
62 n_ty = mkTyVarTy n_tyvar
63 c_ty = mkFunTys [elt_ty, n_ty] n_ty
65 newSysLocalsDs [c_ty,n_ty] `thenDs` \ [c, n] ->
66 dfListComp c n quals `thenDs` \ result ->
67 dsLookupGlobalId buildName `thenDs` \ build_id ->
68 returnDs (Var build_id `App` Type elt_ty
69 `App` mkLams [n_tyvar, c, n] result)
71 where isParallelComp (ParStmtOut bndrstmtss : _) = True
72 isParallelComp _ = False
75 %************************************************************************
77 \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
79 %************************************************************************
81 Just as in Phil's chapter~7 in SLPJ, using the rules for
82 optimally-compiled list comprehensions. This is what Kevin followed
83 as well, and I quite happily do the same. The TQ translation scheme
84 transforms a list of qualifiers (either boolean expressions or
85 generators) into a single expression which implements the list
86 comprehension. Because we are generating 2nd-order polymorphic
87 lambda-calculus, calls to NIL and CONS must be applied to a type
88 argument, as well as their usual value arguments.
90 TE << [ e | qs ] >> = TQ << [ e | qs ] ++ Nil (typeOf e) >>
93 TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>
96 TQ << [ e | b , qs ] ++ L >> =
97 if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>
100 TQ << [ e | p <- L1, qs ] ++ L2 >> =
106 (( \ TE << p >> -> ( TQ << [e | qs] ++ (h u3) >> )) u2)
111 "h", "u1", "u2", and "u3" are new variables.
114 @deListComp@ is the TQ translation scheme. Roughly speaking, @dsExpr@
115 is the TE translation scheme. Note that we carry around the @L@ list
116 already desugared. @dsListComp@ does the top TE rule mentioned above.
118 To the above, we add an additional rule to deal with parallel list
119 comprehensions. The translation goes roughly as follows:
120 [ e | p1 <- e11, let v1 = e12, p2 <- e13
121 | q1 <- e21, let v2 = e22, q2 <- e23]
123 [ e | ((x1, .., xn), (y1, ..., ym)) <-
124 zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
125 [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
126 where (x1, .., xn) are the variables bound in p1, v1, p2
127 (y1, .., ym) are the variables bound in q1, v2, q2
129 In the translation below, the ParStmtOut branch translates each parallel branch
130 into a sub-comprehension, and desugars each independently. The resulting lists
131 are fed to a zip function, we create a binding for all the variables bound in all
132 the comprehensions, and then we hand things off the the desugarer for bindings.
133 The zip function is generated here a) because it's small, and b) because then we
134 don't have to deal with arbitrary limits on the number of zip functions in the
135 prelude, nor which library the zip function came from.
136 The introduced tuples are Boxed, but only because I couldn't get it to work
137 with the Unboxed variety.
141 deListComp :: [TypecheckedStmt] -> CoreExpr -> DsM CoreExpr
143 deListComp (ParStmtOut bndrstmtss : quals) list
144 = mapDs do_list_comp bndrstmtss `thenDs` \ exps ->
145 mkZipBind qual_tys `thenDs` \ (zip_fn, zip_rhs) ->
147 -- Deal with [e | pat <- zip l1 .. ln] in example above
148 deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
151 where -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
152 pat = TuplePat pats Boxed
153 pats = map (\(bs,_) -> mk_hs_tuple_pat bs) bndrstmtss
155 -- Types of (x1,..,xn), (y1,..,yn) etc
156 qual_tys = [ mk_bndrs_tys bndrs | (bndrs,_) <- bndrstmtss ]
158 do_list_comp (bndrs, stmts)
159 = dsListComp (stmts ++ [ResultStmt (mk_hs_tuple_expr bndrs) noSrcLoc])
162 mk_bndrs_tys bndrs = mk_tuple_ty (map idType bndrs)
164 -- Last: the one to return
165 deListComp [ResultStmt expr locn] list -- Figure 7.4, SLPJ, p 135, rule C above
166 = dsExpr expr `thenDs` \ core_expr ->
167 returnDs (mkConsExpr (exprType core_expr) core_expr list)
169 -- Non-last: must be a guard
170 deListComp (ExprStmt guard ty locn : quals) list -- rule B above
171 = dsExpr guard `thenDs` \ core_guard ->
172 deListComp quals list `thenDs` \ core_rest ->
173 returnDs (mkIfThenElse core_guard core_rest list)
175 -- [e | let B, qs] = let B in [e | qs]
176 deListComp (LetStmt binds : quals) list
177 = deListComp quals list `thenDs` \ core_rest ->
178 dsLet binds core_rest
180 deListComp (BindStmt pat list1 locn : quals) core_list2 -- rule A' above
181 = dsExpr list1 `thenDs` \ core_list1 ->
182 deBindComp pat core_list1 quals core_list2
187 deBindComp pat core_list1 quals core_list2
189 u3_ty@u1_ty = exprType core_list1 -- two names, same thing
191 -- u1_ty is a [alpha] type, and u2_ty = alpha
192 u2_ty = hsPatType pat
194 res_ty = exprType core_list2
195 h_ty = u1_ty `mkFunTy` res_ty
197 newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty] `thenDs` \ [h, u1, u2, u3] ->
199 -- the "fail" value ...
201 core_fail = App (Var h) (Var u3)
202 letrec_body = App (Var h) core_list1
204 deListComp quals core_fail `thenDs` \ rest_expr ->
205 matchSimply (Var u2) (StmtCtxt ListComp) pat
206 rest_expr core_fail `thenDs` \ core_match ->
209 Case (Var u1) u1 [(DataAlt nilDataCon, [], core_list2),
210 (DataAlt consDataCon, [u2, u3], core_match)]
212 returnDs (Let (Rec [(h, rhs)]) letrec_body)
217 mkZipBind :: [Type] -> DsM (Id, CoreExpr)
218 -- mkZipBind [t1, t2]
219 -- = (zip, \as1:[t1] as2:[t2]
222 -- (a1:as'1) -> case as2 of
224 -- (a2:as'2) -> (a2,a2) : zip as'1 as'2)]
227 = mapDs newSysLocalDs list_tys `thenDs` \ ass ->
228 mapDs newSysLocalDs elt_tys `thenDs` \ as' ->
229 mapDs newSysLocalDs list_tys `thenDs` \ as's ->
230 newSysLocalDs zip_fn_ty `thenDs` \ zip_fn ->
232 inner_rhs = mkConsExpr ret_elt_ty
233 (mkCoreTup (map Var as'))
234 (mkVarApps (Var zip_fn) as's)
235 zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
237 returnDs (zip_fn, mkLams ass zip_body)
239 list_tys = map mkListTy elt_tys
240 ret_elt_ty = mk_tuple_ty elt_tys
241 zip_fn_ty = mkFunTys list_tys (mkListTy ret_elt_ty)
243 mk_case (as, a', as') rest
244 = Case (Var as) as [(DataAlt nilDataCon, [], mkNilExpr ret_elt_ty),
245 (DataAlt consDataCon, [a', as'], rest)]
248 mk_tuple_ty :: [Type] -> Type
249 mk_tuple_ty [ty] = ty
250 mk_tuple_ty tys = mkTupleTy Boxed (length tys) tys
252 -- Helper functions that makes an HsTuple only for non-1-sized tuples
253 mk_hs_tuple_expr :: [Id] -> TypecheckedHsExpr
254 mk_hs_tuple_expr [] = HsVar unitDataConId
255 mk_hs_tuple_expr [id] = HsVar id
256 mk_hs_tuple_expr ids = ExplicitTuple [ HsVar i | i <- ids ] Boxed
258 mk_hs_tuple_pat :: [Id] -> TypecheckedPat
259 mk_hs_tuple_pat [b] = VarPat b
260 mk_hs_tuple_pat bs = TuplePat (map VarPat bs) Boxed
264 %************************************************************************
266 \subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
268 %************************************************************************
270 @dfListComp@ are the rules used with foldr/build turned on:
273 TE[ e | ] c n = c e n
274 TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
275 TE[ e | p <- l , q ] c n = let
276 f = \ x b -> case x of
284 dfListComp :: Id -> Id -- 'c' and 'n'
285 -> [TypecheckedStmt] -- the rest of the qual's
288 -- Last: the one to return
289 dfListComp c_id n_id [ResultStmt expr locn]
290 = dsExpr expr `thenDs` \ core_expr ->
291 returnDs (mkApps (Var c_id) [core_expr, Var n_id])
293 -- Non-last: must be a guard
294 dfListComp c_id n_id (ExprStmt guard ty locn : quals)
295 = dsExpr guard `thenDs` \ core_guard ->
296 dfListComp c_id n_id quals `thenDs` \ core_rest ->
297 returnDs (mkIfThenElse core_guard core_rest (Var n_id))
299 dfListComp c_id n_id (LetStmt binds : quals)
300 -- new in 1.3, local bindings
301 = dfListComp c_id n_id quals `thenDs` \ core_rest ->
302 dsLet binds core_rest
304 dfListComp c_id n_id (BindStmt pat list1 locn : quals)
305 -- evaluate the two lists
306 = dsExpr list1 `thenDs` \ core_list1 ->
308 -- find the required type
309 let x_ty = hsPatType pat
313 -- create some new local id's
314 newSysLocalsDs [b_ty,x_ty] `thenDs` \ [b,x] ->
316 -- build rest of the comprehesion
317 dfListComp c_id b quals `thenDs` \ core_rest ->
319 -- build the pattern match
320 matchSimply (Var x) (StmtCtxt ListComp)
321 pat core_rest (Var b) `thenDs` \ core_expr ->
323 -- now build the outermost foldr, and return
324 dsLookupGlobalId foldrName `thenDs` \ foldr_id ->
326 Var foldr_id `App` Type x_ty
328 `App` mkLams [x, b] core_expr
334 %************************************************************************
336 \subsection[DsPArrComp]{Desugaring of array comprehensions}
338 %************************************************************************
342 -- entry point for desugaring a parallel array comprehension
344 -- [:e | qss:] = <<[:e | qss:]>> () [:():]
346 dsPArrComp :: [TypecheckedStmt]
347 -> Type -- Don't use; called with `undefined' below
350 dsLookupGlobalId replicatePName `thenDs` \repP ->
351 let unitArray = mkApps (Var repP) [Type unitTy,
355 dePArrComp qs (TuplePat [] Boxed) unitArray
359 dePArrComp :: [TypecheckedStmt]
360 -> TypecheckedPat -- the current generator pattern
361 -> CoreExpr -- the current generator expression
364 -- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
366 dePArrComp [ResultStmt e' _] pa cea =
367 dsLookupGlobalId mapPName `thenDs` \mapP ->
368 let ty = parrElemType cea
370 deLambda ty pa e' `thenDs` \(clam,
372 returnDs $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
374 -- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
376 dePArrComp (ExprStmt b _ _ : qs) pa cea =
377 dsLookupGlobalId filterPName `thenDs` \filterP ->
378 let ty = parrElemType cea
380 deLambda ty pa b `thenDs` \(clam,_) ->
381 dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])
383 -- <<[:e' | p <- e, qs:]>> pa ea =
384 -- let ef = filterP (\x -> case x of {p -> True; _ -> False}) e
386 -- <<[:e' | qs:]>> (pa, p) (crossP ea ef)
388 dePArrComp (BindStmt p e _ : qs) pa cea =
389 dsLookupGlobalId filterPName `thenDs` \filterP ->
390 dsLookupGlobalId crossPName `thenDs` \crossP ->
391 dsExpr e `thenDs` \ce ->
392 let ty'cea = parrElemType cea
393 ty'ce = parrElemType ce
394 false = Var falseDataConId
395 true = Var trueDataConId
397 newSysLocalDs ty'ce `thenDs` \v ->
398 matchSimply (Var v) (StmtCtxt PArrComp) p true false `thenDs` \pred ->
399 let cef = mkApps (Var filterP) [Type ty'ce, mkLams [v] pred, ce]
400 ty'cef = ty'ce -- filterP preserves the type
401 pa' = TuplePat [pa, p] Boxed
403 dePArrComp qs pa' (mkApps (Var crossP) [Type ty'cea, Type ty'cef, cea, cef])
405 -- <<[:e' | let ds, qs:]>> pa ea =
406 -- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
407 -- (mapP (\v@pa -> (v, let ds in (x_1, ..., x_n))) ea)
409 -- {x_1, ..., x_n} = DV (ds) -- Defined Variables
411 dePArrComp (LetStmt ds : qs) pa cea =
412 dsLookupGlobalId mapPName `thenDs` \mapP ->
413 let xs = collectHsBinders ds
414 ty'cea = parrElemType cea
416 newSysLocalDs ty'cea `thenDs` \v ->
417 dsLet ds (mkCoreTup (map Var xs)) `thenDs` \clet ->
418 newSysLocalDs (exprType clet) `thenDs` \let'v ->
419 let projBody = mkDsLet (NonRec let'v clet) $
420 mkCoreTup [Var v, Var let'v]
421 errTy = exprType projBody
422 errMsg = "DsListComp.dePArrComp: internal error!"
424 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
425 matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr `thenDs` \ccase ->
426 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
427 proj = mkLams [v] ccase
429 dePArrComp qs pa' (mkApps (Var mapP) [Type ty'cea, proj, cea])
431 -- <<[:e' | qs | qss:]>> pa ea =
432 -- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
433 -- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
435 -- {x_1, ..., x_n} = DV (qs)
437 dePArrComp (ParStmtOut [] : qss2) pa cea = dePArrComp qss2 pa cea
438 dePArrComp (ParStmtOut ((xs, qs):qss) : qss2) pa cea =
439 dsLookupGlobalId zipPName `thenDs` \zipP ->
440 let pa' = TuplePat [pa, TuplePat (map VarPat xs) Boxed] Boxed
441 ty'cea = parrElemType cea
442 resStmt = ResultStmt (ExplicitTuple (map HsVar xs) Boxed) noSrcLoc
444 dsPArrComp (qs ++ [resStmt]) undefined `thenDs` \cqs ->
445 let ty'cqs = parrElemType cqs
446 cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
448 dePArrComp (ParStmtOut qss : qss2) pa' cea'
450 -- generate Core corresponding to `\p -> e'
452 deLambda :: Type -- type of the argument
453 -> TypecheckedPat -- argument pattern
454 -> TypecheckedHsExpr -- body
455 -> DsM (CoreExpr, Type)
457 newSysLocalDs ty `thenDs` \v ->
458 dsExpr e `thenDs` \ce ->
459 let errTy = exprType ce
460 errMsg = "DsListComp.deLambda: internal error!"
462 mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
463 matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr `thenDs` \res ->
464 returnDs (mkLams [v] res, errTy)
466 -- obtain the element type of the parallel array produced by the given Core
469 parrElemType :: CoreExpr -> Type
471 case splitTyConApp_maybe (exprType e) of
472 Just (tycon, [ty]) | tyConName tycon == parrTyConName -> ty
474 "DsListComp.parrElemType: not a parallel array type"