deSugar/DsListComp.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Desugaring list comprehensions, monad comprehensions and array comprehensions
    -}

    {-# LANGUAGE CPP, NamedFieldPuns #-}

    module DsListComp ( dsListComp, dsPArrComp, dsMonadComp ) where

    #include "HsVersions.h"

    import {-# SOURCE #-} DsExpr ( dsExpr, dsLExpr, dsLExprNoLP, dsLocalBinds, dsSyntaxExpr )

    import HsSyn
    import TcHsSyn
    import CoreSyn
    import MkCore

    import DsMonad          -- the monadery used in the desugarer
    import DsUtils

    import DynFlags
    import CoreUtils
    import Id
    import Type
    import TysWiredIn
    import Match
    import PrelNames
    import SrcLoc
    import Outputable
    import TcType
    import ListSetOps( getNth )
    import Util

    {-
    List comprehensions may be desugared in one of two ways: ``ordinary''
    (as you would expect if you read SLPJ's book) and ``with foldr/build
    turned on'' (if you read Gill {\em et al.}'s paper on the subject).

    There will be at least one ``qualifier'' in the input.
    -}

    dsListComp :: [ExprLStmt Id]
               -> Type              -- Type of entire list
               -> DsM CoreExpr
    dsListComp lquals res_ty = do
        dflags <- getDynFlags
        let quals = map unLoc lquals
            elt_ty = case tcTyConAppArgs res_ty of
                       [elt_ty] -> elt_ty
                       _ -> pprPanic "dsListComp" (ppr res_ty $$ ppr lquals)

        if not (gopt Opt_EnableRewriteRules dflags) || gopt 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 mkBuildExpr elt_ty (\(c, _) (n, _) -> dfListComp c n quals)
                 -- Foldr/build should be enabled, so desugar
                 -- into foldrs and builds

      where
        -- We must test for ParStmt anywhere, not just at the head, because an extension
        -- to list comprehensions would be to add brackets to specify the associativity
        -- of qualifier lists. This is really easy to do by adding extra ParStmts into the
        -- mix of possibly a single element in length, so we do this to leave the possibility open
        isParallelComp = any isParallelStmt

        isParallelStmt (ParStmt {}) = True
        isParallelStmt _            = False

    -- This function lets you desugar a inner list comprehension and a list of the binders
    -- of that comprehension that we need in the outer comprehension into such an expression
    -- and the type of the elements that it outputs (tuples of binders)
    dsInnerListComp :: (ParStmtBlock Id Id) -> DsM (CoreExpr, Type)
    dsInnerListComp (ParStmtBlock stmts bndrs _)
      = do { let bndrs_tuple_type = mkBigCoreVarTupTy bndrs
                 list_ty          = mkListTy bndrs_tuple_type

                 -- really use original bndrs below!
           ; expr <- dsListComp (stmts ++ [noLoc $ mkLastStmt (mkBigLHsVarTupId bndrs)]) list_ty

           ; return (expr, bndrs_tuple_type) }

    -- This function factors out commonality between the desugaring strategies for GroupStmt.
    -- Given such a statement it gives you back an expression representing how to compute the transformed
    -- list and the tuple that you need to bind from that list in order to proceed with your desugaring
    dsTransStmt :: ExprStmt Id -> DsM (CoreExpr, LPat Id)
    dsTransStmt (TransStmt { trS_form = form, trS_stmts = stmts, trS_bndrs = binderMap
                           , trS_by = by, trS_using = using }) = do
        let (from_bndrs, to_bndrs) = unzip binderMap

        let from_bndrs_tys  = map idType from_bndrs
            to_bndrs_tys    = map idType to_bndrs

            to_bndrs_tup_ty = mkBigCoreTupTy to_bndrs_tys

        -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
        (expr', from_tup_ty) <- dsInnerListComp (ParStmtBlock stmts from_bndrs noSyntaxExpr)

        -- Work out what arguments should be supplied to that expression: i.e. is an extraction
        -- function required? If so, create that desugared function and add to arguments
        usingExpr' <- dsLExpr using
        usingArgs' <- case by of
                        Nothing   -> return [expr']
                        Just by_e -> do { by_e' <- dsLExpr by_e
                                        ; lam' <- matchTuple from_bndrs by_e'
                                        ; return [lam', expr'] }

        -- Create an unzip function for the appropriate arity and element types and find "map"
        unzip_stuff' <- mkUnzipBind form from_bndrs_tys
        map_id <- dsLookupGlobalId mapName

        -- Generate the expressions to build the grouped list
        let -- First we apply the grouping function to the inner list
            inner_list_expr' = mkApps usingExpr' usingArgs'
            -- Then we map our "unzip" across it to turn the lists of tuples into tuples of lists
            -- We make sure we instantiate the type variable "a" to be a list of "from" tuples and
            -- the "b" to be a tuple of "to" lists!
            -- Then finally we bind the unzip function around that expression
            bound_unzipped_inner_list_expr'
              = case unzip_stuff' of
                  Nothing -> inner_list_expr'
                  Just (unzip_fn', unzip_rhs') ->
                    Let (Rec [(unzip_fn', unzip_rhs')]) $
                    mkApps (Var map_id) $
                    [ Type (mkListTy from_tup_ty)
                    , Type to_bndrs_tup_ty
                    , Var unzip_fn'
                    , inner_list_expr' ]

        dsNoLevPoly (tcFunResultTyN (length usingArgs') (exprType usingExpr'))
          (text "In the result of a" <+> quotes (text "using") <+> text "function:" <+> ppr using)

        -- Build a pattern that ensures the consumer binds into the NEW binders,
        -- which hold lists rather than single values
        let pat = mkBigLHsVarPatTupId to_bndrs  -- NB: no '!
        return (bound_unzipped_inner_list_expr', pat)

    dsTransStmt _ = panic "dsTransStmt: Not given a TransStmt"

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DsListComp-ordinary]{Ordinary desugaring of list comprehensions}
    *                                                                      *
    ************************************************************************

    Just as in Phil's chapter~7 in SLPJ, using the rules for
    optimally-compiled list comprehensions.  This is what Kevin followed
    as well, and I quite happily do the same.  The TQ translation scheme
    transforms a list of qualifiers (either boolean expressions or
    generators) into a single expression which implements the list
    comprehension.  Because we are generating 2nd-order polymorphic
    lambda-calculus, calls to NIL and CONS must be applied to a type
    argument, as well as their usual value arguments.
    \begin{verbatim}
    TE << [ e | qs ] >>  =  TQ << [ e | qs ] ++ Nil (typeOf e) >>

    (Rule C)
    TQ << [ e | ] ++ L >> = Cons (typeOf e) TE <<e>> TE <<L>>

    (Rule B)
    TQ << [ e | b , qs ] ++ L >> =
        if TE << b >> then TQ << [ e | qs ] ++ L >> else TE << L >>

    (Rule A')
    TQ << [ e | p <- L1, qs ]  ++  L2 >> =
      letrec
        h = \ u1 ->
              case u1 of
                []        ->  TE << L2 >>
                (u2 : u3) ->
                      (( \ TE << p >> -> ( TQ << [e | qs]  ++  (h u3) >> )) u2)
                        [] (h u3)
      in
        h ( TE << L1 >> )

    "h", "u1", "u2", and "u3" are new variables.
    \end{verbatim}

    @deListComp@ is the TQ translation scheme.  Roughly speaking, @dsExpr@
    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.
    -}

    deListComp :: [ExprStmt Id] -> CoreExpr -> DsM CoreExpr

    deListComp [] _ = panic "deListComp"

    deListComp (LastStmt body _ _ : quals) list
      =     -- Figure 7.4, SLPJ, p 135, rule C above
        ASSERT( null quals )
        do { core_body <- dsLExpr body
           ; return (mkConsExpr (exprType core_body) core_body list) }

            -- Non-last: must be a guard
    deListComp (BodyStmt guard _ _ _ : quals) list = do  -- rule B above
        core_guard <- dsLExpr guard
        core_rest <- deListComp quals list
        return (mkIfThenElse core_guard core_rest list)

    -- [e | let B, qs] = let B in [e | qs]
    deListComp (LetStmt binds : quals) list = do
        core_rest <- deListComp quals list
        dsLocalBinds binds core_rest

    deListComp (stmt@(TransStmt {}) : quals) list = do
        (inner_list_expr, pat) <- dsTransStmt stmt
        deBindComp pat inner_list_expr quals list

    deListComp (BindStmt pat list1 _ _ _ : quals) core_list2 = do -- rule A' above
        core_list1 <- dsLExprNoLP list1
        deBindComp pat core_list1 quals core_list2

    deListComp (ParStmt stmtss_w_bndrs _ _ _ : quals) list
      = do { exps_and_qual_tys <- mapM dsInnerListComp stmtss_w_bndrs
           ; let (exps, qual_tys) = unzip exps_and_qual_tys

           ; (zip_fn, zip_rhs) <- mkZipBind qual_tys

            -- 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 }
      where
            bndrs_s = [bs | ParStmtBlock _ bs _ <- stmtss_w_bndrs]

            -- pat is the pattern ((x1,..,xn), (y1,..,ym)) in the example above
            pat  = mkBigLHsPatTupId pats
            pats = map mkBigLHsVarPatTupId bndrs_s

    deListComp (RecStmt {} : _) _ = panic "deListComp RecStmt"

    deListComp (ApplicativeStmt {} : _) _ =
      panic "deListComp ApplicativeStmt"

    deBindComp :: OutPat Id
               -> CoreExpr
               -> [ExprStmt Id]
               -> CoreExpr
               -> DsM (Expr Id)
    deBindComp pat core_list1 quals core_list2 = do
        let u3_ty@u1_ty = exprType core_list1       -- two names, same thing

            -- u1_ty is a [alpha] type, and u2_ty = alpha
        let u2_ty = hsLPatType pat

        let res_ty = exprType core_list2
            h_ty   = u1_ty `mkFunTy` res_ty

           -- no levity polymorphism here, as list comprehensions don't work
           -- with RebindableSyntax. NB: These are *not* monad comps.
        [h, u1, u2, u3] <- newSysLocalsDs [h_ty, u1_ty, u2_ty, u3_ty]

        -- the "fail" value ...
        let
            core_fail   = App (Var h) (Var u3)
            letrec_body = App (Var h) core_list1

        rest_expr <- deListComp quals core_fail
        core_match <- matchSimply (Var u2) (StmtCtxt ListComp) pat rest_expr core_fail

        let
            rhs = Lam u1 $
                  Case (Var u1) u1 res_ty
                       [(DataAlt nilDataCon,  [],       core_list2),
                        (DataAlt consDataCon, [u2, u3], core_match)]
                            -- Increasing order of tag

        return (Let (Rec [(h, rhs)]) letrec_body)

    {-
    ************************************************************************
    *                                                                      *
    \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 = let
                                    f = \ x b -> case x of
                                                      p -> TE[ e | q ] c b
                                                      _ -> b
                               in
                               foldr f n l
    \end{verbatim}
    -}

    dfListComp :: Id -> Id         -- 'c' and 'n'
               -> [ExprStmt Id]    -- the rest of the qual's
               -> DsM CoreExpr

    dfListComp _ _ [] = panic "dfListComp"

    dfListComp c_id n_id (LastStmt body _ _ : quals)
      = ASSERT( null quals )
        do { core_body <- dsLExprNoLP body
           ; return (mkApps (Var c_id) [core_body, Var n_id]) }

            -- Non-last: must be a guard
    dfListComp c_id n_id (BodyStmt guard _ _ _  : quals) = do
        core_guard <- dsLExpr guard
        core_rest <- dfListComp c_id n_id quals
        return (mkIfThenElse core_guard core_rest (Var n_id))

    dfListComp c_id n_id (LetStmt binds : quals) = do
        -- new in 1.3, local bindings
        core_rest <- dfListComp c_id n_id quals
        dsLocalBinds binds core_rest

    dfListComp c_id n_id (stmt@(TransStmt {}) : quals) = do
        (inner_list_expr, pat) <- dsTransStmt stmt
        -- Anyway, we bind the newly grouped list via the generic binding function
        dfBindComp c_id n_id (pat, inner_list_expr) quals

    dfListComp c_id n_id (BindStmt pat list1 _ _ _ : quals) = do
        -- evaluate the two lists
        core_list1 <- dsLExpr list1

        -- Do the rest of the work in the generic binding builder
        dfBindComp c_id n_id (pat, core_list1) quals

    dfListComp _ _ (ParStmt {} : _) = panic "dfListComp ParStmt"
    dfListComp _ _ (RecStmt {} : _) = panic "dfListComp RecStmt"
    dfListComp _ _ (ApplicativeStmt {} : _) =
      panic "dfListComp ApplicativeStmt"

    dfBindComp :: Id -> Id          -- 'c' and 'n'
               -> (LPat Id, CoreExpr)
               -> [ExprStmt Id]     -- the rest of the qual's
               -> DsM CoreExpr
    dfBindComp c_id n_id (pat, core_list1) quals = do
        -- find the required type
        let x_ty   = hsLPatType pat
        let b_ty   = idType n_id

        -- create some new local id's
        b <- newSysLocalDs b_ty
        x <- newSysLocalDs x_ty

        -- build rest of the comprehesion
        core_rest <- dfListComp c_id b quals

        -- build the pattern match
        core_expr <- matchSimply (Var x) (StmtCtxt ListComp)
                    pat core_rest (Var b)

        -- now build the outermost foldr, and return
        mkFoldrExpr x_ty b_ty (mkLams [x, b] core_expr) (Var n_id) core_list1

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DsFunGeneration]{Generation of zip/unzip functions for use in desugaring}
    *                                                                      *
    ************************************************************************
    -}

    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) -> (a1, a2) : zip as'1 as'2)]

    mkZipBind elt_tys = do
        ass  <- mapM newSysLocalDs  elt_list_tys
        as'  <- mapM newSysLocalDs  elt_tys
        as's <- mapM newSysLocalDs  elt_list_tys

        zip_fn <- newSysLocalDs zip_fn_ty

        let inner_rhs = mkConsExpr elt_tuple_ty
                            (mkBigCoreVarTup as')
                            (mkVarApps (Var zip_fn) as's)
            zip_body  = foldr mk_case inner_rhs (zip3 ass as' as's)

        return (zip_fn, mkLams ass zip_body)
      where
        elt_list_tys      = map mkListTy elt_tys
        elt_tuple_ty      = mkBigCoreTupTy elt_tys
        elt_tuple_list_ty = mkListTy elt_tuple_ty

        zip_fn_ty         = mkFunTys elt_list_tys elt_tuple_list_ty

        mk_case (as, a', as') rest
              = Case (Var as) as elt_tuple_list_ty
                      [(DataAlt nilDataCon,  [],        mkNilExpr elt_tuple_ty),
                       (DataAlt consDataCon, [a', as'], rest)]
                            -- Increasing order of tag

    mkUnzipBind :: TransForm -> [Type] -> DsM (Maybe (Id, CoreExpr))
    -- mkUnzipBind [t1, t2]
    -- = (unzip, \ys :: [(t1, t2)] -> foldr (\ax :: (t1, t2) axs :: ([t1], [t2])
    --     -> case ax of
    --      (x1, x2) -> case axs of
    --                (xs1, xs2) -> (x1 : xs1, x2 : xs2))
    --      ([], [])
    --      ys)
    --
    -- We use foldr here in all cases, even if rules are turned off, because we may as well!
    mkUnzipBind ThenForm _
     = return Nothing    -- No unzipping for ThenForm
    mkUnzipBind _ elt_tys
      = do { ax  <- newSysLocalDs elt_tuple_ty
           ; axs <- newSysLocalDs elt_list_tuple_ty
           ; ys  <- newSysLocalDs elt_tuple_list_ty
           ; xs  <- mapM newSysLocalDs elt_tys
           ; xss <- mapM newSysLocalDs elt_list_tys

           ; unzip_fn <- newSysLocalDs unzip_fn_ty

           ; [us1, us2] <- sequence [newUniqueSupply, newUniqueSupply]

           ; let nil_tuple = mkBigCoreTup (map mkNilExpr elt_tys)
                 concat_expressions = map mkConcatExpression (zip3 elt_tys (map Var xs) (map Var xss))
                 tupled_concat_expression = mkBigCoreTup concat_expressions

                 folder_body_inner_case = mkTupleCase us1 xss tupled_concat_expression axs (Var axs)
                 folder_body_outer_case = mkTupleCase us2 xs folder_body_inner_case ax (Var ax)
                 folder_body = mkLams [ax, axs] folder_body_outer_case

           ; unzip_body <- mkFoldrExpr elt_tuple_ty elt_list_tuple_ty folder_body nil_tuple (Var ys)
           ; return (Just (unzip_fn, mkLams [ys] unzip_body)) }
      where
        elt_tuple_ty       = mkBigCoreTupTy elt_tys
        elt_tuple_list_ty  = mkListTy elt_tuple_ty
        elt_list_tys       = map mkListTy elt_tys
        elt_list_tuple_ty  = mkBigCoreTupTy elt_list_tys

        unzip_fn_ty        = elt_tuple_list_ty `mkFunTy` elt_list_tuple_ty

        mkConcatExpression (list_element_ty, head, tail) = mkConsExpr list_element_ty head tail

    {-
    ************************************************************************
    *                                                                      *
    \subsection[DsPArrComp]{Desugaring of array comprehensions}
    *                                                                      *
    ************************************************************************
    -}

    -- entry point for desugaring a parallel array comprehension
    --
    --   [:e | qss:] = <<[:e | qss:]>> () [:():]
    --
    dsPArrComp :: [ExprStmt Id]
                -> DsM CoreExpr

    -- Special case for parallel comprehension
    dsPArrComp (ParStmt qss _ _ _ : quals) = dePArrParComp qss quals

    -- Special case for simple generators:
    --
    --  <<[:e' | p <- e, qs:]>> = <<[: e' | qs :]>> p e
    --
    -- if matching again p cannot fail, or else
    --
    --  <<[:e' | p <- e, qs:]>> =
    --    <<[:e' | qs:]>> p (filterP (\x -> case x of {p -> True; _ -> False}) e)
    --
    dsPArrComp (BindStmt p e _ _ _ : qs) = do
        filterP <- dsDPHBuiltin filterPVar
        ce <- dsLExprNoLP e
        let ety'ce  = parrElemType ce
            false   = Var falseDataConId
            true    = Var trueDataConId
        v <- newSysLocalDs ety'ce
        pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
        let gen | isIrrefutableHsPat p = ce
                | otherwise            = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
        dePArrComp qs p gen

    dsPArrComp qs = do -- no ParStmt in `qs'
        sglP <- dsDPHBuiltin singletonPVar
        let unitArray = mkApps (Var sglP) [Type unitTy, mkCoreTup []]
        dePArrComp qs (noLoc $ WildPat unitTy) unitArray

    -- the work horse
    --
    dePArrComp :: [ExprStmt Id]
               -> LPat Id           -- the current generator pattern
               -> CoreExpr          -- the current generator expression
               -> DsM CoreExpr

    dePArrComp [] _ _ = panic "dePArrComp"

    --
    --  <<[:e' | :]>> pa ea = mapP (\pa -> e') ea
    --
    dePArrComp (LastStmt e' _ _ : quals) pa cea
      = ASSERT( null quals )
        do { mapP <- dsDPHBuiltin mapPVar
           ; let ty = parrElemType cea
           ; (clam, ty'e') <- deLambda ty pa e'
           ; return $ mkApps (Var mapP) [Type ty, Type ty'e', clam, cea] }
    --
    --  <<[:e' | b, qs:]>> pa ea = <<[:e' | qs:]>> pa (filterP (\pa -> b) ea)
    --
    dePArrComp (BodyStmt b _ _ _ : qs) pa cea = do
        filterP <- dsDPHBuiltin filterPVar
        let ty = parrElemType cea
        (clam,_) <- deLambda ty pa b
        dePArrComp qs pa (mkApps (Var filterP) [Type ty, clam, cea])

    --
    --  <<[:e' | p <- e, qs:]>> pa ea =
    --    let ef = \pa -> e
    --    in
    --    <<[:e' | qs:]>> (pa, p) (crossMap ea ef)
    --
    -- if matching again p cannot fail, or else
    --
    --  <<[:e' | p <- e, qs:]>> pa ea =
    --    let ef = \pa -> filterP (\x -> case x of {p -> True; _ -> False}) e
    --    in
    --    <<[:e' | qs:]>> (pa, p) (crossMapP ea ef)
    --
    dePArrComp (BindStmt p e _ _ _ : qs) pa cea = do
        filterP <- dsDPHBuiltin filterPVar
        crossMapP <- dsDPHBuiltin crossMapPVar
        ce <- dsLExpr e
        let ety'cea = parrElemType cea
            ety'ce  = parrElemType ce
            false   = Var falseDataConId
            true    = Var trueDataConId
        v <- newSysLocalDs ety'ce
        pred <- matchSimply (Var v) (StmtCtxt PArrComp) p true false
        let cef | isIrrefutableHsPat p = ce
                | otherwise            = mkApps (Var filterP) [Type ety'ce, mkLams [v] pred, ce]
        (clam, _) <- mkLambda ety'cea pa cef
        let ety'cef = ety'ce                    -- filter doesn't change the element type
            pa'     = mkLHsPatTup [pa, p]

        dePArrComp qs pa' (mkApps (Var crossMapP)
                                     [Type ety'cea, Type ety'cef, cea, clam])
    --
    --  <<[:e' | let ds, qs:]>> pa ea =
    --    <<[:e' | qs:]>> (pa, (x_1, ..., x_n))
    --                    (mapP (\v@pa -> let ds in (v, (x_1, ..., x_n))) ea)
    --  where
    --    {x_1, ..., x_n} = DV (ds)         -- Defined Variables
    --
    dePArrComp (LetStmt lds@(L _ ds) : qs) pa cea = do
        mapP <- dsDPHBuiltin mapPVar
        let xs = collectLocalBinders ds
            ty'cea = parrElemType cea
        v <- newSysLocalDs ty'cea
        clet <- dsLocalBinds lds (mkCoreTup (map Var xs))
        let'v <- newSysLocalDs (exprType clet)
        let projBody = mkCoreLet (NonRec let'v clet) $
                       mkCoreTup [Var v, Var let'v]
            errTy    = exprType projBody
            errMsg   = text "DsListComp.dePArrComp: internal error!"
        cerr <- mkErrorAppDs pAT_ERROR_ID errTy errMsg
        ccase <- matchSimply (Var v) (StmtCtxt PArrComp) pa projBody cerr
        let pa'    = mkLHsPatTup [pa, mkLHsPatTup (map nlVarPat xs)]
            proj   = mkLams [v] ccase
        dePArrComp qs pa' (mkApps (Var mapP)
                                       [Type ty'cea, Type errTy, proj, cea])
    --
    -- The parser guarantees that parallel comprehensions can only appear as
    -- singleton qualifier lists, which we already special case in the caller.
    -- So, encountering one here is a bug.
    --
    dePArrComp (ParStmt {} : _) _ _ =
      panic "DsListComp.dePArrComp: malformed comprehension AST: ParStmt"
    dePArrComp (TransStmt {} : _) _ _ = panic "DsListComp.dePArrComp: TransStmt"
    dePArrComp (RecStmt   {} : _) _ _ = panic "DsListComp.dePArrComp: RecStmt"
    dePArrComp (ApplicativeStmt   {} : _) _ _ =
      panic "DsListComp.dePArrComp: ApplicativeStmt"

    --  <<[: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)
    --
    dePArrParComp :: [ParStmtBlock Id Id] -> [ExprStmt Id] -> DsM CoreExpr
    dePArrParComp qss quals = do
        (pQss, ceQss) <- deParStmt qss
        dePArrComp quals pQss ceQss
      where
        deParStmt []             =
          -- empty parallel statement lists have no source representation
          panic "DsListComp.dePArrComp: Empty parallel list comprehension"
        deParStmt (ParStmtBlock qs xs _:qss) = do        -- first statement
          let res_expr = mkLHsVarTuple xs
          cqs <- dsPArrComp (map unLoc qs ++ [mkLastStmt res_expr])
          parStmts qss (mkLHsVarPatTup xs) cqs
        ---
        parStmts []             pa cea = return (pa, cea)
        parStmts (ParStmtBlock qs xs _:qss) pa cea = do  -- subsequent statements (zip'ed)
          zipP <- dsDPHBuiltin zipPVar
          let pa'      = mkLHsPatTup [pa, mkLHsVarPatTup xs]
              ty'cea   = parrElemType cea
              res_expr = mkLHsVarTuple xs
          cqs <- dsPArrComp (map unLoc qs ++ [mkLastStmt res_expr])
          let ty'cqs = parrElemType cqs
              cea'   = mkApps (Var zipP) [Type ty'cea, Type ty'cqs, cea, cqs]
          parStmts qss pa' cea'

    -- generate Core corresponding to `\p -> e'
    --
    deLambda :: Type                       -- type of the argument (not levity-polymorphic)
             -> LPat Id                    -- argument pattern
             -> LHsExpr Id                 -- body
             -> DsM (CoreExpr, Type)
    deLambda ty p e =
        mkLambda ty p =<< dsLExpr e

    -- generate Core for a lambda pattern match, where the body is already in Core
    --
    mkLambda :: Type                        -- type of the argument (not levity-polymorphic)
             -> LPat Id                     -- argument pattern
             -> CoreExpr                    -- desugared body
             -> DsM (CoreExpr, Type)
    mkLambda ty p ce = do
        v <- newSysLocalDs ty
        let errMsg = text "DsListComp.deLambda: internal error!"
            ce'ty  = exprType ce
        cerr <- mkErrorAppDs pAT_ERROR_ID ce'ty errMsg
        res <- matchSimply (Var v) (StmtCtxt PArrComp) p ce cerr
        return (mkLams [v] res, ce'ty)

    -- 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"

    -- Translation for monad comprehensions

    -- Entry point for monad comprehension desugaring
    dsMonadComp :: [ExprLStmt Id] -> DsM CoreExpr
    dsMonadComp stmts = dsMcStmts stmts

    dsMcStmts :: [ExprLStmt Id] -> DsM CoreExpr
    dsMcStmts []                    = panic "dsMcStmts"
    dsMcStmts (L loc stmt : lstmts) = putSrcSpanDs loc (dsMcStmt stmt lstmts)

    ---------------
    dsMcStmt :: ExprStmt Id -> [ExprLStmt Id] -> DsM CoreExpr

    dsMcStmt (LastStmt body _ ret_op) stmts
      = ASSERT( null stmts )
        do { body' <- dsLExpr body
           ; dsSyntaxExpr ret_op [body'] }

    --   [ .. | let binds, stmts ]
    dsMcStmt (LetStmt binds) stmts
      = do { rest <- dsMcStmts stmts
           ; dsLocalBinds binds rest }

    --   [ .. | a <- m, stmts ]
    dsMcStmt (BindStmt pat rhs bind_op fail_op bind_ty) stmts
      = do { rhs' <- dsLExpr rhs
           ; dsMcBindStmt pat rhs' bind_op fail_op bind_ty stmts }

    -- Apply `guard` to the `exp` expression
    --
    --   [ .. | exp, stmts ]
    --
    dsMcStmt (BodyStmt exp then_exp guard_exp _) stmts
      = do { exp'       <- dsLExpr exp
           ; rest       <- dsMcStmts stmts
           ; guard_exp' <- dsSyntaxExpr guard_exp [exp']
           ; dsSyntaxExpr then_exp [guard_exp', rest] }

    -- Group statements desugar like this:
    --
    --   [| (q, then group by e using f); rest |]
    --   --->  f {qt} (\qv -> e) [| q; return qv |] >>= \ n_tup ->
    --         case unzip n_tup of qv' -> [| rest |]
    --
    -- where   variables (v1:t1, ..., vk:tk) are bound by q
    --         qv = (v1, ..., vk)
    --         qt = (t1, ..., tk)
    --         (>>=) :: m2 a -> (a -> m3 b) -> m3 b
    --         f :: forall a. (a -> t) -> m1 a -> m2 (n a)
    --         n_tup :: n qt
    --         unzip :: n qt -> (n t1, ..., n tk)    (needs Functor n)

    dsMcStmt (TransStmt { trS_stmts = stmts, trS_bndrs = bndrs
                        , trS_by = by, trS_using = using
                        , trS_ret = return_op, trS_bind = bind_op
                        , trS_bind_arg_ty = n_tup_ty'  -- n (a,b,c)
                        , trS_fmap = fmap_op, trS_form = form }) stmts_rest
      = do { let (from_bndrs, to_bndrs) = unzip bndrs

           ; let from_bndr_tys = map idType from_bndrs     -- Types ty

           -- Desugar an inner comprehension which outputs a list of tuples of the "from" binders
           ; expr' <- dsInnerMonadComp stmts from_bndrs return_op

           -- Work out what arguments should be supplied to that expression: i.e. is an extraction
           -- function required? If so, create that desugared function and add to arguments
           ; usingExpr' <- dsLExpr using
           ; usingArgs' <- case by of
                             Nothing   -> return [expr']
                             Just by_e -> do { by_e' <- dsLExpr by_e
                                             ; lam' <- matchTuple from_bndrs by_e'
                                             ; return [lam', expr'] }

           -- Generate the expressions to build the grouped list
           -- Build a pattern that ensures the consumer binds into the NEW binders,
           -- which hold monads rather than single values
           ; let tup_n_ty' = mkBigCoreVarTupTy to_bndrs

           ; body        <- dsMcStmts stmts_rest
           ; n_tup_var'  <- newSysLocalDsNoLP n_tup_ty'
           ; tup_n_var'  <- newSysLocalDs tup_n_ty'
           ; tup_n_expr' <- mkMcUnzipM form fmap_op n_tup_var' from_bndr_tys
           ; us          <- newUniqueSupply
           ; let rhs'  = mkApps usingExpr' usingArgs'
                 body' = mkTupleCase us to_bndrs body tup_n_var' tup_n_expr'

           ; dsSyntaxExpr bind_op [rhs', Lam n_tup_var' body'] }

    -- Parallel statements. Use `Control.Monad.Zip.mzip` to zip parallel
    -- statements, for example:
    --
    --   [ body | qs1 | qs2 | qs3 ]
    --     ->  [ body | (bndrs1, (bndrs2, bndrs3))
    --                     <- [bndrs1 | qs1] `mzip` ([bndrs2 | qs2] `mzip` [bndrs3 | qs3]) ]
    --
    -- where `mzip` has type
    --   mzip :: forall a b. m a -> m b -> m (a,b)
    -- NB: we need a polymorphic mzip because we call it several times

    dsMcStmt (ParStmt blocks mzip_op bind_op bind_ty) stmts_rest
     = do  { exps_w_tys  <- mapM ds_inner blocks   -- Pairs (exp :: m ty, ty)
           ; mzip_op'    <- dsExpr mzip_op

           ; let -- The pattern variables
                 pats = [ mkBigLHsVarPatTupId bs | ParStmtBlock _ bs _ <- blocks]
                 -- Pattern with tuples of variables
                 -- [v1,v2,v3]  =>  (v1, (v2, v3))
                 pat = foldr1 (\p1 p2 -> mkLHsPatTup [p1, p2]) pats
                 (rhs, _) = foldr1 (\(e1,t1) (e2,t2) ->
                                     (mkApps mzip_op' [Type t1, Type t2, e1, e2],
                                      mkBoxedTupleTy [t1,t2]))
                                   exps_w_tys

           ; dsMcBindStmt pat rhs bind_op noSyntaxExpr bind_ty stmts_rest }
      where
        ds_inner (ParStmtBlock stmts bndrs return_op)
           = do { exp <- dsInnerMonadComp stmts bndrs return_op
                ; return (exp, mkBigCoreVarTupTy bndrs) }

    dsMcStmt stmt _ = pprPanic "dsMcStmt: unexpected stmt" (ppr stmt)

    matchTuple :: [Id] -> CoreExpr -> DsM CoreExpr
    -- (matchTuple [a,b,c] body)
    --       returns the Core term
    --  \x. case x of (a,b,c) -> body
    matchTuple ids body
      = do { us <- newUniqueSupply
           ; tup_id <- newSysLocalDs (mkBigCoreVarTupTy ids)
           ; return (Lam tup_id $ mkTupleCase us ids body tup_id (Var tup_id)) }

    -- general `rhs' >>= \pat -> stmts` desugaring where `rhs'` is already a
    -- desugared `CoreExpr`
    dsMcBindStmt :: LPat Id
                 -> CoreExpr        -- ^ the desugared rhs of the bind statement
                 -> SyntaxExpr Id
                 -> SyntaxExpr Id
                 -> Type            -- ^ S in (>>=) :: Q -> (R -> S) -> T
                 -> [ExprLStmt Id]
                 -> DsM CoreExpr
    dsMcBindStmt pat rhs' bind_op fail_op res1_ty stmts
      = do  { body     <- dsMcStmts stmts
            ; var      <- selectSimpleMatchVarL pat
            ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
                                      res1_ty (cantFailMatchResult body)
            ; match_code <- handle_failure pat match fail_op
            ; dsSyntaxExpr bind_op [rhs', Lam var match_code] }

      where
        -- In a monad comprehension expression, pattern-match failure just calls
        -- the monadic `fail` rather than throwing an exception
        handle_failure pat match fail_op
          | matchCanFail match
            = do { dflags <- getDynFlags
                 ; fail_msg <- mkStringExpr (mk_fail_msg dflags pat)
                 ; fail_expr <- dsSyntaxExpr fail_op [fail_msg]
                 ; extractMatchResult match fail_expr }
          | otherwise
            = extractMatchResult match (error "It can't fail")

        mk_fail_msg :: DynFlags -> Located e -> String
        mk_fail_msg dflags pat
            = "Pattern match failure in monad comprehension at " ++
              showPpr dflags (getLoc pat)

    -- Desugar nested monad comprehensions, for example in `then..` constructs
    --    dsInnerMonadComp quals [a,b,c] ret_op
    -- returns the desugaring of
    --       [ (a,b,c) | quals ]

    dsInnerMonadComp :: [ExprLStmt Id]
                     -> [Id]            -- Return a tuple of these variables
                     -> SyntaxExpr Id   -- The monomorphic "return" operator
                     -> DsM CoreExpr
    dsInnerMonadComp stmts bndrs ret_op
      = dsMcStmts (stmts ++ [noLoc (LastStmt (mkBigLHsVarTupId bndrs) False ret_op)])

    -- The `unzip` function for `GroupStmt` in a monad comprehensions
    --
    --   unzip :: m (a,b,..) -> (m a,m b,..)
    --   unzip m_tuple = ( liftM selN1 m_tuple
    --                   , liftM selN2 m_tuple
    --                   , .. )
    --
    --   mkMcUnzipM fmap ys [t1, t2]
    --     = ( fmap (selN1 :: (t1, t2) -> t1) ys
    --       , fmap (selN2 :: (t1, t2) -> t2) ys )

    mkMcUnzipM :: TransForm
               -> HsExpr TcId       -- fmap
               -> Id                -- Of type n (a,b,c)
               -> [Type]            -- [a,b,c]   (not levity-polymorphic)
               -> DsM CoreExpr      -- Of type (n a, n b, n c)
    mkMcUnzipM ThenForm _ ys _
      = return (Var ys) -- No unzipping to do

    mkMcUnzipM _ fmap_op ys elt_tys
      = do { fmap_op' <- dsExpr fmap_op
           ; xs       <- mapM newSysLocalDs elt_tys
           ; let tup_ty = mkBigCoreTupTy elt_tys
           ; tup_xs   <- newSysLocalDs tup_ty

           ; let mk_elt i = mkApps fmap_op'  -- fmap :: forall a b. (a -> b) -> n a -> n b
                               [ Type tup_ty, Type (getNth elt_tys i)
                               , mk_sel i, Var ys]

                 mk_sel n = Lam tup_xs $
                            mkTupleSelector xs (getNth xs n) tup_xs (Var tup_xs)

           ; return (mkBigCoreTup (map mk_elt [0..length elt_tys - 1])) }