GHC/HsToCore/ListComp.hs

    {-# LANGUAGE CPP            #-}
    {-# LANGUAGE TypeFamilies   #-}

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

    Desugaring list comprehensions, monad comprehensions and array comprehensions
    -}

    module GHC.HsToCore.ListComp ( dsListComp, dsMonadComp ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import {-# SOURCE #-} GHC.HsToCore.Expr ( dsExpr, dsLExpr, dsLExprNoLP, dsLocalBinds, dsSyntaxExpr )

    import GHC.Hs
    import GHC.Tc.Utils.Zonk
    import GHC.Core
    import GHC.Core.Make

    import GHC.HsToCore.Monad          -- the monadery used in the desugarer
    import GHC.HsToCore.Utils

    import GHC.Driver.Session
    import GHC.Core.Utils
    import GHC.Types.Id
    import GHC.Core.Type
    import GHC.Builtin.Types
    import GHC.HsToCore.Match
    import GHC.Builtin.Names
    import GHC.Types.SrcLoc
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Tc.Utils.TcType
    import GHC.Data.List.SetOps( getNth )
    import GHC.Utils.Misc

    {-
    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 GhcTc]
               -> 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 GhcTc GhcTc) -> 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 ++ [noLocA $ 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 GhcTc -> DsM (CoreExpr, LPat GhcTc)
    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 noExtField 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"

    {-
    ************************************************************************
    *                                                                      *
    *           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 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 GhcTc] -> 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 :: LPat GhcTc
               -> CoreExpr
               -> [ExprStmt GhcTc]
               -> 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 `mkVisFunTyMany` 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 $ map unrestricted [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
                       [Alt (DataAlt nilDataCon)  []       core_list2
                       ,Alt (DataAlt consDataCon) [u2, u3] core_match]
                            -- Increasing order of tag

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

    {-
    ************************************************************************
    *                                                                      *
    *           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 GhcTc]    -- 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 GhcTc, CoreExpr)
               -> [ExprStmt GhcTc]     -- 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 Many b_ty
        x <- newSysLocalDs Many x_ty

        -- build rest of the comprehension
        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 Many)  elt_list_tys
        as'  <- mapM (newSysLocalDs Many)  elt_tys
        as's <- mapM (newSysLocalDs Many)  elt_list_tys

        zip_fn <- newSysLocalDs Many 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         = mkVisFunTysMany elt_list_tys elt_tuple_list_ty

        mk_case (as, a', as') rest
              = Case (Var as) as elt_tuple_list_ty
                      [ Alt (DataAlt nilDataCon)  []        (mkNilExpr elt_tuple_ty)
                      , Alt (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 Many elt_tuple_ty
           ; axs <- newSysLocalDs Many elt_list_tuple_ty
           ; ys  <- newSysLocalDs Many elt_tuple_list_ty
           ; xs  <- mapM (newSysLocalDs Many) elt_tys
           ; xss <- mapM (newSysLocalDs Many) elt_list_tys

           ; unzip_fn <- newSysLocalDs Many 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 `mkVisFunTyMany` elt_list_tuple_ty

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

    -- Translation for monad comprehensions

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

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

    ---------------
    dsMcStmt :: ExprStmt GhcTc -> [ExprLStmt GhcTc] -> 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 xbs pat rhs) stmts
      = do { rhs' <- dsLExpr rhs
           ; dsMcBindStmt pat rhs' (xbstc_bindOp xbs) (xbstc_failOp xbs) (xbstc_boundResultType xbs) 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_ext = 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 Many n_tup_ty'
           ; tup_n_var'  <- newSysLocalDs Many 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 bind_ty blocks mzip_op bind_op) 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 Nothing bind_ty stmts_rest }
      where
        ds_inner :: ParStmtBlock GhcTc GhcTc -> DsM (CoreExpr, Type)
        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 Many (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 GhcTc
                 -> CoreExpr        -- ^ the desugared rhs of the bind statement
                 -> SyntaxExpr GhcTc
                 -> Maybe (SyntaxExpr GhcTc)
                 -> Type            -- ^ S in (>>=) :: Q -> (R -> S) -> T
                 -> [ExprLStmt GhcTc]
                 -> DsM CoreExpr
    dsMcBindStmt pat rhs' bind_op fail_op res1_ty stmts
      = do  { body     <- dsMcStmts stmts
            ; var      <- selectSimpleMatchVarL Many pat
            ; match <- matchSinglePatVar var Nothing (StmtCtxt (DoExpr Nothing)) pat
                                      res1_ty (cantFailMatchResult body)
            ; match_code <- dsHandleMonadicFailure (MonadComp :: HsStmtContext GhcRn) pat match fail_op
            ; dsSyntaxExpr bind_op [rhs', Lam var match_code] }

    -- 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 GhcTc]
                     -> [Id]               -- Return a tuple of these variables
                     -> SyntaxExpr GhcTc   -- The monomorphic "return" operator
                     -> DsM CoreExpr
    dsInnerMonadComp stmts bndrs ret_op
      = dsMcStmts (stmts ++
                     [noLocA (LastStmt noExtField (mkBigLHsVarTupId bndrs) Nothing 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 GhcTc      -- 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 Many) elt_tys
           ; let tup_ty = mkBigCoreTupTy elt_tys
           ; tup_xs   <- newSysLocalDs Many 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])) }