%
-% (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
+% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
-\section[DsListComp]{Desugaring list comprehensions}
+\section[DsListComp]{Desugaring list comprehensions and array comprehensions}
\begin{code}
-#include "HsVersions.h"
+module DsListComp ( dsListComp, dsPArrComp ) where
-module DsListComp ( dsListComp ) where
+#include "HsVersions.h"
-IMP_Ubiq()
-IMPORT_DELOOPER(DsLoop) -- break dsExpr-ish loop
+import {-# SOURCE #-} DsExpr ( dsLExpr, dsLet )
-import HsSyn ( Qualifier(..), HsExpr, HsBinds )
-import TcHsSyn ( TypecheckedQual(..), TypecheckedHsExpr(..) , TypecheckedHsBinds(..) )
-import DsHsSyn ( outPatType )
+import BasicTypes ( Boxity(..) )
+import HsSyn
+import TcHsSyn ( hsPatType )
import CoreSyn
import DsMonad -- the monadery used in the desugarer
import DsUtils
-import CmdLineOpts ( opt_FoldrBuildOn )
-import CoreUtils ( coreExprType, mkCoreIfThenElse )
-import PrelVals ( mkBuild, foldrId )
-import Type ( mkTyVarTy, mkForAllTy, mkFunTys )
-import TysPrim ( alphaTy )
-import TysWiredIn ( nilDataCon, consDataCon, listTyCon )
-import TyVar ( alphaTyVar )
+import CmdLineOpts ( DynFlag(..), dopt, opt_RulesOff )
+import CoreUtils ( exprType, mkIfThenElse )
+import Id ( idType )
+import Var ( Id )
+import Type ( mkTyVarTy, mkFunTys, mkFunTy, Type,
+ splitTyConApp_maybe )
+import TysPrim ( alphaTyVar )
+import TysWiredIn ( nilDataCon, consDataCon, trueDataConId, falseDataConId,
+ unitDataConId, unitTy, mkListTy, parrTyCon )
import Match ( matchSimply )
-import Util ( panic )
+import PrelNames ( foldrName, buildName, replicatePName, mapPName,
+ filterPName, zipPName, crossPName )
+import PrelInfo ( pAT_ERROR_ID )
+import SrcLoc ( noLoc, unLoc )
+import Panic ( panic )
\end{code}
List comprehensions may be desugared in one of two ways: ``ordinary''
There will be at least one ``qualifier'' in the input.
\begin{code}
-dsListComp :: CoreExpr -> [TypecheckedQual] -> DsM CoreExpr
-
-dsListComp expr quals
- = let
- expr_ty = coreExprType expr
+dsListComp :: [LStmt Id]
+ -> Type -- Type of list elements
+ -> DsM CoreExpr
+dsListComp lquals elt_ty
+ = getDOptsDs `thenDs` \dflags ->
+ let
+ quals = map unLoc lquals
in
- if not opt_FoldrBuildOn then -- be boring
- deListComp expr quals (nIL_EXPR expr_ty)
-
- else -- foldr/build lives!
- new_alpha_tyvar `thenDs` \ (n_tyvar, n_ty) ->
- let
- alpha_to_alpha = mkFunTys [alphaTy] alphaTy
-
- c_ty = mkFunTys [expr_ty, n_ty] n_ty
- g_ty = mkForAllTy alphaTyVar (
- (mkFunTys [expr_ty, alpha_to_alpha] alpha_to_alpha))
- in
- newSysLocalsDs [c_ty,n_ty,g_ty] `thenDs` \ [c, n, g] ->
-
- dfListComp expr expr_ty
- c_ty c
- n_ty n
- quals `thenDs` \ result ->
-
- returnDs (mkBuild expr_ty n_tyvar c n g result)
- where
- nIL_EXPR ty = mkCon nilDataCon [] [ty] []
-
- new_alpha_tyvar :: DsM (TyVar, Type)
- new_alpha_tyvar
- = newTyVarsDs [alphaTyVar] `thenDs` \ [new_ty] ->
- returnDs (new_ty, mkTyVarTy new_ty)
+ if opt_RulesOff || dopt Opt_IgnoreInterfacePragmas dflags
+ -- Either rules are switched off, or we are ignoring what there are;
+ -- Either way foldr/build won't happen, so use the more efficient
+ -- Wadler-style desugaring
+ || isParallelComp quals
+ -- Foldr-style desugaring can't handle
+ -- parallel list comprehensions
+ then deListComp quals (mkNilExpr elt_ty)
+
+ else -- Foldr/build should be enabled, so desugar
+ -- into foldrs and builds
+ newTyVarsDs [alphaTyVar] `thenDs` \ [n_tyvar] ->
+ let
+ n_ty = mkTyVarTy n_tyvar
+ c_ty = mkFunTys [elt_ty, n_ty] n_ty
+ in
+ newSysLocalsDs [c_ty,n_ty] `thenDs` \ [c, n] ->
+ dfListComp c n quals `thenDs` \ result ->
+ dsLookupGlobalId buildName `thenDs` \ build_id ->
+ returnDs (Var build_id `App` Type elt_ty
+ `App` mkLams [n_tyvar, c, n] result)
+
+ where isParallelComp (ParStmt bndrstmtss : _) = True
+ isParallelComp _ = False
\end{code}
%************************************************************************
is the TE translation scheme. Note that we carry around the @L@ list
already desugared. @dsListComp@ does the top TE rule mentioned above.
+To the above, we add an additional rule to deal with parallel list
+comprehensions. The translation goes roughly as follows:
+ [ e | p1 <- e11, let v1 = e12, p2 <- e13
+ | q1 <- e21, let v2 = e22, q2 <- e23]
+ =>
+ [ e | ((x1, .., xn), (y1, ..., ym)) <-
+ zip [(x1,..,xn) | p1 <- e11, let v1 = e12, p2 <- e13]
+ [(y1,..,ym) | q1 <- e21, let v2 = e22, q2 <- e23]]
+where (x1, .., xn) are the variables bound in p1, v1, p2
+ (y1, .., ym) are the variables bound in q1, v2, q2
+
+In the translation below, the ParStmt branch translates each parallel branch
+into a sub-comprehension, and desugars each independently. The resulting lists
+are fed to a zip function, we create a binding for all the variables bound in all
+the comprehensions, and then we hand things off the the desugarer for bindings.
+The zip function is generated here a) because it's small, and b) because then we
+don't have to deal with arbitrary limits on the number of zip functions in the
+prelude, nor which library the zip function came from.
+The introduced tuples are Boxed, but only because I couldn't get it to work
+with the Unboxed variety.
+
\begin{code}
-deListComp :: CoreExpr -> [TypecheckedQual] -> CoreExpr -> DsM CoreExpr
+deListComp :: [Stmt Id] -> CoreExpr -> DsM CoreExpr
-deListComp expr [] list -- Figure 7.4, SLPJ, p 135, rule C above
- = mkConDs consDataCon [TyArg (coreExprType expr), VarArg expr, VarArg list]
+deListComp (ParStmt stmtss_w_bndrs : quals) list
+ = mappM do_list_comp stmtss_w_bndrs `thenDs` \ exps ->
+ mkZipBind qual_tys `thenDs` \ (zip_fn, zip_rhs) ->
-deListComp expr (FilterQual filt : quals) list -- rule B above
- = dsExpr filt `thenDs` \ core_filt ->
- deListComp expr quals list `thenDs` \ core_rest ->
- returnDs ( mkCoreIfThenElse core_filt core_rest list )
+ -- Deal with [e | pat <- zip l1 .. ln] in example above
+ deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) exps))
+ quals list
-deListComp expr (LetQual binds : quals) list
- = panic "deListComp:LetQual"
+ where
+ bndrs_s = map snd stmtss_w_bndrs
-deListComp expr ((GeneratorQual pat list1):quals) core_list2 -- rule A' above
- = dsExpr list1 `thenDs` \ core_list1 ->
- let
- u3_ty@u1_ty = coreExprType core_list1 -- two names, same thing
+ -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
+ pat = noLoc (TuplePat pats Boxed)
+ pats = map mk_hs_tuple_pat bndrs_s
+
+ -- Types of (x1,..,xn), (y1,..,yn) etc
+ qual_tys = map mk_bndrs_tys bndrs_s
+
+ do_list_comp (stmts, bndrs)
+ = dsListComp (stmts ++ [noLoc $ ResultStmt (mk_hs_tuple_expr bndrs)])
+ (mk_bndrs_tys bndrs)
+
+ mk_bndrs_tys bndrs = mkCoreTupTy (map idType bndrs)
+
+ -- Last: the one to return
+deListComp [ResultStmt expr] list -- Figure 7.4, SLPJ, p 135, rule C above
+ = dsLExpr expr `thenDs` \ core_expr ->
+ returnDs (mkConsExpr (exprType core_expr) core_expr list)
+
+ -- Non-last: must be a guard
+deListComp (ExprStmt guard ty : quals) list -- rule B above
+ = dsLExpr guard `thenDs` \ core_guard ->
+ deListComp quals list `thenDs` \ core_rest ->
+ returnDs (mkIfThenElse core_guard core_rest list)
+
+-- [e | let B, qs] = let B in [e | qs]
+deListComp (LetStmt binds : quals) list
+ = deListComp quals list `thenDs` \ core_rest ->
+ dsLet binds core_rest
+
+deListComp (BindStmt pat list1 : quals) core_list2 -- rule A' above
+ = dsLExpr list1 `thenDs` \ core_list1 ->
+ deBindComp pat core_list1 quals core_list2
+\end{code}
+
+
+\begin{code}
+deBindComp pat core_list1 quals core_list2
+ = let
+ u3_ty@u1_ty = exprType core_list1 -- two names, same thing
-- u1_ty is a [alpha] type, and u2_ty = alpha
- u2_ty = outPatType pat
+ u2_ty = hsPatType pat
+
+ res_ty = exprType core_list2
+ h_ty = u1_ty `mkFunTy` res_ty
+ in
+ newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty] `thenDs` \ [h, u1, u2, u3] ->
- res_ty = coreExprType core_list2
- h_ty = mkFunTys [u1_ty] res_ty
+ -- the "fail" value ...
+ let
+ core_fail = App (Var h) (Var u3)
+ letrec_body = App (Var h) core_list1
in
- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]
- `thenDs` \ [h', u1, u2, u3] ->
- {-
- Make the function h unfoldable by the deforester.
- Since it only occurs once in the body, we can't get
- an increase in code size by unfolding it.
- -}
+ deListComp quals core_fail `thenDs` \ rest_expr ->
+ matchSimply (Var u2) (StmtCtxt ListComp) pat
+ rest_expr core_fail `thenDs` \ core_match ->
let
- h = if False -- LATER: sw_chkr DoDeforest???
- then panic "deListComp:deforest"
- -- replaceIdInfo h' (addInfo (getIdInfo h') DoDeforest)
- else h'
+ rhs = Lam u1 $
+ Case (Var u1) u1 [(DataAlt nilDataCon, [], core_list2),
+ (DataAlt consDataCon, [u2, u3], core_match)]
in
- -- the "fail" value ...
- mkAppDs (Var h) [VarArg (Var u3)] `thenDs` \ core_fail ->
-
- deListComp expr quals core_fail `thenDs` \ rest_expr ->
-
- matchSimply (Var u2) pat res_ty rest_expr core_fail `thenDs` \ core_match ->
+ returnDs (Let (Rec [(h, rhs)]) letrec_body)
+\end{code}
- mkAppDs (Var h) [VarArg core_list1] `thenDs` \ letrec_body ->
- returnDs (
- mkCoLetrecAny [
- ( h,
- (Lam (ValBinder u1)
- (Case (Var u1)
- (AlgAlts
- [(nilDataCon, [], core_list2),
- (consDataCon, [u2, u3], core_match)]
- NoDefault)))
- )] letrec_body
- )
+\begin{code}
+mkZipBind :: [Type] -> DsM (Id, CoreExpr)
+-- mkZipBind [t1, t2]
+-- = (zip, \as1:[t1] as2:[t2]
+-- -> case as1 of
+-- [] -> []
+-- (a1:as'1) -> case as2 of
+-- [] -> []
+-- (a2:as'2) -> (a2,a2) : zip as'1 as'2)]
+
+mkZipBind elt_tys
+ = mappM newSysLocalDs list_tys `thenDs` \ ass ->
+ mappM newSysLocalDs elt_tys `thenDs` \ as' ->
+ mappM newSysLocalDs list_tys `thenDs` \ as's ->
+ newSysLocalDs zip_fn_ty `thenDs` \ zip_fn ->
+ let
+ inner_rhs = mkConsExpr ret_elt_ty
+ (mkCoreTup (map Var as'))
+ (mkVarApps (Var zip_fn) as's)
+ zip_body = foldr mk_case inner_rhs (zip3 ass as' as's)
+ in
+ returnDs (zip_fn, mkLams ass zip_body)
+ where
+ list_tys = map mkListTy elt_tys
+ ret_elt_ty = mkCoreTupTy elt_tys
+ zip_fn_ty = mkFunTys list_tys (mkListTy ret_elt_ty)
+
+ mk_case (as, a', as') rest
+ = Case (Var as) as [(DataAlt nilDataCon, [], mkNilExpr ret_elt_ty),
+ (DataAlt consDataCon, [a', as'], rest)]
+
+-- Helper functions that makes an HsTuple only for non-1-sized tuples
+mk_hs_tuple_expr :: [Id] -> LHsExpr Id
+mk_hs_tuple_expr [] = nlHsVar unitDataConId
+mk_hs_tuple_expr [id] = nlHsVar id
+mk_hs_tuple_expr ids = noLoc $ ExplicitTuple [ nlHsVar i | i <- ids ] Boxed
+
+mk_hs_tuple_pat :: [Id] -> LPat Id
+mk_hs_tuple_pat [b] = nlVarPat b
+mk_hs_tuple_pat bs = noLoc $ TuplePat (map nlVarPat bs) Boxed
\end{code}
+
%************************************************************************
%* *
\subsection[DsListComp-foldr-build]{Foldr/Build desugaring of list comprehensions}
%************************************************************************
@dfListComp@ are the rules used with foldr/build turned on:
+
\begin{verbatim}
-TE < [ e | ] >> c n = c e n
-TE << [ e | b , q ] >> c n = if b then TE << [ e | q ] >> c n else n
-TE << [ e | p <- l , q ] c n = foldr
- (\ TE << p >> b -> TE << [ e | q ] >> c b
- _ b -> b) n l
+TE[ e | ] c n = c e n
+TE[ e | b , q ] c n = if b then TE[ e | q ] c n else n
+TE[ e | p <- l , q ] c n = let
+ f = \ x b -> case x of
+ p -> TE[ e | q ] c b
+ _ -> b
+ in
+ foldr f n l
\end{verbatim}
+
\begin{code}
-dfListComp :: CoreExpr -- the inside of the comp
- -> Type -- the type of the inside
- -> Type -> Id -- 'c'; its type and id
- -> Type -> Id -- 'n'; its type and id
- -> [TypecheckedQual] -- the rest of the qual's
+dfListComp :: Id -> Id -- 'c' and 'n'
+ -> [Stmt Id] -- the rest of the qual's
-> DsM CoreExpr
-dfListComp expr expr_ty c_ty c_id n_ty n_id []
- = mkAppDs (Var c_id) [VarArg expr, VarArg (Var n_id)]
+ -- Last: the one to return
+dfListComp c_id n_id [ResultStmt expr]
+ = dsLExpr expr `thenDs` \ core_expr ->
+ returnDs (mkApps (Var c_id) [core_expr, Var n_id])
-dfListComp expr expr_ty c_ty c_id n_ty n_id (FilterQual filt : quals)
- = dsExpr filt `thenDs` \ core_filt ->
- dfListComp expr expr_ty c_ty c_id n_ty n_id quals
- `thenDs` \ core_rest ->
- returnDs (mkCoreIfThenElse core_filt core_rest (Var n_id))
+ -- Non-last: must be a guard
+dfListComp c_id n_id (ExprStmt guard ty : quals)
+ = dsLExpr guard `thenDs` \ core_guard ->
+ dfListComp c_id n_id quals `thenDs` \ core_rest ->
+ returnDs (mkIfThenElse core_guard core_rest (Var n_id))
-dfListComp expr expr_ty c_ty c_id n_ty n_id (LetQual binds : quals)
- = panic "dfListComp:LetQual"
+dfListComp c_id n_id (LetStmt binds : quals)
+ -- new in 1.3, local bindings
+ = dfListComp c_id n_id quals `thenDs` \ core_rest ->
+ dsLet binds core_rest
-dfListComp expr expr_ty c_ty c_id n_ty n_id (GeneratorQual pat list1 : quals)
+dfListComp c_id n_id (BindStmt pat list1 : quals)
-- evaluate the two lists
- = dsExpr list1 `thenDs` \ core_list1 ->
+ = dsLExpr list1 `thenDs` \ core_list1 ->
-- find the required type
-
- let p_ty = outPatType pat
- b_ty = n_ty -- alias b_ty to n_ty
- fn_ty = mkFunTys [p_ty, b_ty] b_ty
- lst_ty = coreExprType core_list1
+ let x_ty = hsPatType pat
+ b_ty = idType n_id
in
-- create some new local id's
-
- newSysLocalsDs [b_ty,p_ty,fn_ty,lst_ty] `thenDs` \ [b,p,fn,lst] ->
+ newSysLocalsDs [b_ty,x_ty] `thenDs` \ [b,x] ->
-- build rest of the comprehesion
+ dfListComp c_id b quals `thenDs` \ core_rest ->
- dfListComp expr expr_ty c_ty c_id b_ty b quals `thenDs` \ core_rest ->
-- build the pattern match
-
- matchSimply (Var p) pat b_ty core_rest (Var b) `thenDs` \ core_expr ->
+ matchSimply (Var x) (StmtCtxt ListComp)
+ pat core_rest (Var b) `thenDs` \ core_expr ->
-- now build the outermost foldr, and return
-
+ dsLookupGlobalId foldrName `thenDs` \ foldr_id ->
returnDs (
- mkCoLetsAny
- [NonRec fn (mkValLam [p, b] core_expr),
- NonRec lst core_list1]
- (mkFoldr p_ty n_ty fn n_id lst)
+ Var foldr_id `App` Type x_ty
+ `App` Type b_ty
+ `App` mkLams [x, b] core_expr
+ `App` Var n_id
+ `App` core_list1
)
+\end{code}
+
+%************************************************************************
+%* *
+\subsection[DsPArrComp]{Desugaring of array comprehensions}
+%* *
+%************************************************************************
+
+\begin{code}
+
+-- entry point for desugaring a parallel array comprehension
+--
+-- [:e | qss:] = <<[:e | qss:]>> () [:():]
+--
+dsPArrComp :: [Stmt Id]
+ -> Type -- Don't use; called with `undefined' below
+ -> DsM CoreExpr
+dsPArrComp qs _ =
+ dsLookupGlobalId replicatePName `thenDs` \repP ->
+ let unitArray = mkApps (Var repP) [Type unitTy,
+ mkIntExpr 1,
+ mkCoreTup []]
+ in
+ dePArrComp qs (noLoc (TuplePat [] Boxed)) unitArray
-mkFoldr a b f z xs
- = mkValApp (mkTyApp (Var foldrId) [a,b]) [VarArg f, VarArg z, VarArg xs]
+-- the work horse
+--
+dePArrComp :: [Stmt Id]
+ -> LPat Id -- the current generator pattern
+ -> CoreExpr -- the current generator expression
+ -> DsM CoreExpr
+--
+-- <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
+--
+dePArrComp [ResultStmt e'] pa cea =
+ dsLookupGlobalId mapPName `thenDs` \mapP ->
+ let ty = parrElemType cea
+ in
+ deLambda ty pa e' `thenDs` \(clam,
+ ty'e') ->
+ returnDs $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea]
+--
+-- <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
+--
+dePArrComp (ExprStmt b _ : qs) pa cea =
+ dsLookupGlobalId filterPName `thenDs` \filterP ->
+ let ty = parrElemType cea
+ in
+ deLambda ty pa b `thenDs` \(clam,_) ->
+ dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])
+--
+-- <<[:e' | p <- e, qs:]>> pa ea =
+-- let ef = filterP (\x -> case x of {p -> True; _ -> False}) e
+-- in
+-- <<[:e' | qs:]>> (pa, p) (crossP ea ef)
+--
+dePArrComp (BindStmt p e : qs) pa cea =
+ dsLookupGlobalId filterPName `thenDs` \filterP ->
+ dsLookupGlobalId crossPName `thenDs` \crossP ->
+ dsLExpr e `thenDs` \ce ->
+ let ty'cea = parrElemType cea
+ ty'ce = parrElemType ce
+ false = Var falseDataConId
+ true = Var trueDataConId
+ in
+ newSysLocalDs ty'ce `thenDs` \v ->
+ matchSimply (Var v) (StmtCtxt PArrComp) p true false `thenDs` \pred ->
+ let cef = mkApps (Var filterP) [Type ty'ce, mkLams [v] pred, ce]
+ ty'cef = ty'ce -- filterP preserves the type
+ pa' = noLoc (TuplePat [pa, p] Boxed)
+ in
+ dePArrComp qs pa' (mkApps (Var crossP) [Type ty'cea, Type ty'cef, cea, cef])
+--
+-- <<[:e' | let ds, qs:]>> pa ea =
+-- <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
+-- (mapP (\v@pa -> (v, let ds in (x_1, ..., x_n))) ea)
+-- where
+-- {x_1, ..., x_n} = DV (ds) -- Defined Variables
+--
+dePArrComp (LetStmt ds : qs) pa cea =
+ dsLookupGlobalId mapPName `thenDs` \mapP ->
+ let xs = map unLoc (collectGroupBinders ds)
+ ty'cea = parrElemType cea
+ in
+ newSysLocalDs ty'cea `thenDs` \v ->
+ dsLet ds (mkCoreTup (map Var xs)) `thenDs` \clet ->
+ newSysLocalDs (exprType clet) `thenDs` \let'v ->
+ let projBody = mkDsLet (NonRec let'v clet) $
+ mkCoreTup [Var v, Var let'v]
+ errTy = exprType projBody
+ errMsg = "DsListComp.dePArrComp: internal error!"
+ in
+ mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
+ matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr `thenDs` \ccase ->
+ let pa' = noLoc $ TuplePat [pa, noLoc (TuplePat (map nlVarPat xs) Boxed)] Boxed
+ proj = mkLams [v] ccase
+ in
+ dePArrComp qs pa' (mkApps (Var mapP) [Type ty'cea, proj, cea])
+--
+-- <<[:e' | qs | qss:]>> pa ea =
+-- <<[:e' | qss:]>> (pa, (x_1, ..., x_n))
+-- (zipP ea <<[:(x_1, ..., x_n) | qs:]>>)
+-- where
+-- {x_1, ..., x_n} = DV (qs)
+--
+dePArrComp (ParStmt [] : qss2) pa cea = dePArrComp qss2 pa cea
+dePArrComp (ParStmt ((qs, xs):qss) : qss2) pa cea =
+ dsLookupGlobalId zipPName `thenDs` \zipP ->
+ let pa' = noLoc $ TuplePat [pa, noLoc (TuplePat (map nlVarPat xs) Boxed)] Boxed
+ ty'cea = parrElemType cea
+ resStmt = ResultStmt (noLoc $ ExplicitTuple (map nlHsVar xs) Boxed)
+ in
+ dsPArrComp (map unLoc qs ++ [resStmt]) undefined `thenDs` \cqs ->
+ let ty'cqs = parrElemType cqs
+ cea' = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
+ in
+ dePArrComp (ParStmt qss : qss2) pa' cea'
+
+-- generate Core corresponding to `\p -> e'
+--
+deLambda :: Type -- type of the argument
+ -> LPat Id -- argument pattern
+ -> LHsExpr Id -- body
+ -> DsM (CoreExpr, Type)
+deLambda ty p e =
+ newSysLocalDs ty `thenDs` \v ->
+ dsLExpr e `thenDs` \ce ->
+ let errTy = exprType ce
+ errMsg = "DsListComp.deLambda: internal error!"
+ in
+ mkErrorAppDs pAT_ERROR_ID errTy errMsg `thenDs` \cerr ->
+ matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr `thenDs` \res ->
+ returnDs (mkLams [v] res, errTy)
+
+-- obtain the element type of the parallel array produced by the given Core
+-- expression
+--
+parrElemType :: CoreExpr -> Type
+parrElemType e =
+ case splitTyConApp_maybe (exprType e) of
+ Just (tycon, [ty]) | tycon == parrTyCon -> ty
+ _ -> panic
+ "DsListComp.parrElemType: not a parallel array type"
\end{code}
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