content: {-# LANGUAGE LambdaCase #-}
  {-# 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
  
  import GHC.Prelude
  
  import {-# SOURCE #-} GHC.HsToCore.Expr ( dsExpr, dsLExpr, dsLocalBinds, dsSyntaxExpr )
  
  import GHC.Hs
  import GHC.Hs.Syn.Type
  import GHC.Core
  import GHC.Core.Make
  
  import GHC.HsToCore.Monad          -- the monadery used in the desugarer
  import GHC.HsToCore.Utils
  
  import GHC.Driver.DynFlags
  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 Data.Foldable ( toList )
  
  {-
  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' ]
  
      -- 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 <- dsLExpr 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 (toList qual_tys)
  
          -- Deal with [e | pat <- zip l1 .. ln] in example above
         ; deBindComp pat (Let (Rec [(zip_fn, zip_rhs)]) (mkApps (Var zip_fn) (toList exps)))
                      quals list }
    where
          bndrs_s = [bs | ParStmtBlock _ _ bs _ <- toList 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 (XStmtLR 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 representation 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 (HsDoStmt ListComp)) ManyTy 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 <- dsLExpr 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 _ _ (XStmtLR 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 <- newSysLocalMDs b_ty
      x <- newSysLocalMDs x_ty
  
      -- build rest of the comprehension
      core_rest <- dfListComp c_id b quals
  
      -- build the pattern match
      core_expr <- matchSimply (Var x) (StmtCtxt (HsDoStmt ListComp)) ManyTy
                  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  <- newSysLocalsMDs elt_list_tys
      as'  <- newSysLocalsMDs elt_tys
      as's <- newSysLocalsMDs elt_list_tys
  
      zip_fn <- newSysLocalMDs 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  <- newSysLocalMDs elt_tuple_ty
         ; axs <- newSysLocalMDs elt_list_tuple_ty
         ; ys  <- newSysLocalMDs elt_tuple_list_ty
         ; xs  <- newSysLocalsMDs elt_tys
         ; xss <- newSysLocalsMDs elt_list_tys
  
         ; unzip_fn <- newSysLocalMDs unzip_fn_ty
  
         ; 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 <- mkBigTupleCase xss tupled_concat_expression (Var axs)
         ; folder_body_outer_case <- mkBigTupleCase xs folder_body_inner_case (Var ax)
         ; let 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
         ; body        <- dsMcStmts stmts_rest
         ; n_tup_var'  <- newSysLocalMDs n_tup_ty'
         ; tup_n_expr' <- mkMcUnzipM form fmap_op n_tup_var' from_bndr_tys
         ; let rhs'  = mkApps usingExpr' usingArgs'
         ; body'       <- mkBigTupleCase to_bndrs body 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 parStmtBlockIds :: ParStmtBlock GhcTc GhcTc -> [Id]
               parStmtBlockIds = \ case
                   ParStmtBlock _ _ bs _ -> bs
               -- The pattern variables
               pats = fmap (mkBigLHsVarPatTupId . parStmtBlockIds) 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@(XStmtLR ApplicativeStmt {}) _ = pprPanic "dsMcStmt: unexpected stmt" (ppr stmt)
  dsMcStmt stmt@(RecStmt {}) _ = 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 { tup_id <- newSysLocalMDs (mkBigCoreVarTupTy ids)
         ; tup_case <- mkBigTupleCase ids body (Var tup_id)
         ; return (Lam tup_id tup_case) }
  
  -- 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  { var   <- selectSimpleMatchVarL ManyTy pat
          ; match <- matchSinglePatVar var Nothing (StmtCtxt (HsDoStmt (DoExpr Nothing))) pat
                        res1_ty (MR_Infallible $ dsMcStmts stmts)
              -- NB: dsMcStmts needs to happen inside matchSinglePatVar, and not
              -- before it, so that long-distance information is properly threaded.
              -- See Note [Long-distance information in do notation] in GHC.HsToCore.Expr.
          ; match_code <- dsHandleMonadicFailure MonadComp pat res1_ty 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 representation-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       <- newSysLocalsMDs elt_tys
         ; let tup_ty = mkBigCoreTupTy elt_tys
         ; tup_xs   <- newSysLocalMDs 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 $
                          mkBigTupleSelector xs (getNth xs n) tup_xs (Var tup_xs)
  
         ; return (mkBigCoreTup (map mk_elt [0..length elt_tys - 1])) }
  