deSugar/DsArrows.hs

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

    Desugaring arrow commands
    -}

    {-# LANGUAGE CPP #-}

    module DsArrows ( dsProcExpr ) where

    #include "HsVersions.h"

    import Match
    import DsUtils
    import DsMonad

    import HsSyn    hiding (collectPatBinders, collectPatsBinders, collectLStmtsBinders, collectLStmtBinders, collectStmtBinders )
    import TcHsSyn
    import qualified HsUtils

    -- NB: The desugarer, which straddles the source and Core worlds, sometimes
    --     needs to see source types (newtypes etc), and sometimes not
    --     So WATCH OUT; check each use of split*Ty functions.
    -- Sigh.  This is a pain.

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

    import TcType
    import TcEvidence
    import CoreSyn
    import CoreFVs
    import CoreUtils
    import MkCore
    import DsBinds (dsHsWrapper)

    import Name
    import Var
    import Id
    import DataCon
    import TysWiredIn
    import BasicTypes
    import PrelNames
    import Outputable
    import Bag
    import VarSet
    import SrcLoc
    import ListSetOps( assocDefault )
    import Data.List

    data DsCmdEnv = DsCmdEnv {
            arr_id, compose_id, first_id, app_id, choice_id, loop_id :: CoreExpr
        }

    mkCmdEnv :: CmdSyntaxTable Id -> DsM ([CoreBind], DsCmdEnv)
    -- See Note [CmdSyntaxTable] in HsExpr
    mkCmdEnv tc_meths
      = do { (meth_binds, prs) <- mapAndUnzipM mk_bind tc_meths
           ; return (meth_binds, DsCmdEnv {
                   arr_id     = Var (find_meth prs arrAName),
                   compose_id = Var (find_meth prs composeAName),
                   first_id   = Var (find_meth prs firstAName),
                   app_id     = Var (find_meth prs appAName),
                   choice_id  = Var (find_meth prs choiceAName),
                   loop_id    = Var (find_meth prs loopAName)
                 }) }
      where
        mk_bind (std_name, expr)
          = do { rhs <- dsExpr expr
               ; id <- newSysLocalDs (exprType rhs)
               ; return (NonRec id rhs, (std_name, id)) }

        find_meth prs std_name
          = assocDefault (mk_panic std_name) prs std_name
        mk_panic std_name = pprPanic "mkCmdEnv" (text "Not found:" <+> ppr std_name)

    -- arr :: forall b c. (b -> c) -> a b c
    do_arr :: DsCmdEnv -> Type -> Type -> CoreExpr -> CoreExpr
    do_arr ids b_ty c_ty f = mkApps (arr_id ids) [Type b_ty, Type c_ty, f]

    -- (>>>) :: forall b c d. a b c -> a c d -> a b d
    do_compose :: DsCmdEnv -> Type -> Type -> Type ->
                    CoreExpr -> CoreExpr -> CoreExpr
    do_compose ids b_ty c_ty d_ty f g
      = mkApps (compose_id ids) [Type b_ty, Type c_ty, Type d_ty, f, g]

    -- first :: forall b c d. a b c -> a (b,d) (c,d)
    do_first :: DsCmdEnv -> Type -> Type -> Type -> CoreExpr -> CoreExpr
    do_first ids b_ty c_ty d_ty f
      = mkApps (first_id ids) [Type b_ty, Type c_ty, Type d_ty, f]

    -- app :: forall b c. a (a b c, b) c
    do_app :: DsCmdEnv -> Type -> Type -> CoreExpr
    do_app ids b_ty c_ty = mkApps (app_id ids) [Type b_ty, Type c_ty]

    -- (|||) :: forall b d c. a b d -> a c d -> a (Either b c) d
    -- note the swapping of d and c
    do_choice :: DsCmdEnv -> Type -> Type -> Type ->
                    CoreExpr -> CoreExpr -> CoreExpr
    do_choice ids b_ty c_ty d_ty f g
      = mkApps (choice_id ids) [Type b_ty, Type d_ty, Type c_ty, f, g]

    -- loop :: forall b d c. a (b,d) (c,d) -> a b c
    -- note the swapping of d and c
    do_loop :: DsCmdEnv -> Type -> Type -> Type -> CoreExpr -> CoreExpr
    do_loop ids b_ty c_ty d_ty f
      = mkApps (loop_id ids) [Type b_ty, Type d_ty, Type c_ty, f]

    -- premap :: forall b c d. (b -> c) -> a c d -> a b d
    -- premap f g = arr f >>> g
    do_premap :: DsCmdEnv -> Type -> Type -> Type ->
                    CoreExpr -> CoreExpr -> CoreExpr
    do_premap ids b_ty c_ty d_ty f g
       = do_compose ids b_ty c_ty d_ty (do_arr ids b_ty c_ty f) g

    mkFailExpr :: HsMatchContext Id -> Type -> DsM CoreExpr
    mkFailExpr ctxt ty
      = mkErrorAppDs pAT_ERROR_ID ty (matchContextErrString ctxt)

    -- construct CoreExpr for \ (a :: a_ty, b :: b_ty) -> a
    mkFstExpr :: Type -> Type -> DsM CoreExpr
    mkFstExpr a_ty b_ty = do
        a_var <- newSysLocalDs a_ty
        b_var <- newSysLocalDs b_ty
        pair_var <- newSysLocalDs (mkCorePairTy a_ty b_ty)
        return (Lam pair_var
                   (coreCasePair pair_var a_var b_var (Var a_var)))

    -- construct CoreExpr for \ (a :: a_ty, b :: b_ty) -> b
    mkSndExpr :: Type -> Type -> DsM CoreExpr
    mkSndExpr a_ty b_ty = do
        a_var <- newSysLocalDs a_ty
        b_var <- newSysLocalDs b_ty
        pair_var <- newSysLocalDs (mkCorePairTy a_ty b_ty)
        return (Lam pair_var
                   (coreCasePair pair_var a_var b_var (Var b_var)))

    {-
    Build case analysis of a tuple.  This cannot be done in the DsM monad,
    because the list of variables is typically not yet defined.
    -}

    -- coreCaseTuple [u1..] v [x1..xn] body
    --      = case v of v { (x1, .., xn) -> body }
    -- But the matching may be nested if the tuple is very big

    coreCaseTuple :: UniqSupply -> Id -> [Id] -> CoreExpr -> CoreExpr
    coreCaseTuple uniqs scrut_var vars body
      = mkTupleCase uniqs vars body scrut_var (Var scrut_var)

    coreCasePair :: Id -> Id -> Id -> CoreExpr -> CoreExpr
    coreCasePair scrut_var var1 var2 body
      = Case (Var scrut_var) scrut_var (exprType body)
             [(DataAlt (tupleDataCon Boxed 2), [var1, var2], body)]

    mkCorePairTy :: Type -> Type -> Type
    mkCorePairTy t1 t2 = mkBoxedTupleTy [t1, t2]

    mkCorePairExpr :: CoreExpr -> CoreExpr -> CoreExpr
    mkCorePairExpr e1 e2 = mkCoreTup [e1, e2]

    mkCoreUnitExpr :: CoreExpr
    mkCoreUnitExpr = mkCoreTup []

    {-
    The input is divided into a local environment, which is a flat tuple
    (unless it's too big), and a stack, which is a right-nested pair.
    In general, the input has the form

            ((x1,...,xn), (s1,...(sk,())...))

    where xi are the environment values, and si the ones on the stack,
    with s1 being the "top", the first one to be matched with a lambda.
    -}

    envStackType :: [Id] -> Type -> Type
    envStackType ids stack_ty = mkCorePairTy (mkBigCoreVarTupTy ids) stack_ty

    -- splitTypeAt n (t1,... (tn,t)...) = ([t1, ..., tn], t)
    splitTypeAt :: Int -> Type -> ([Type], Type)
    splitTypeAt n ty
      | n == 0 = ([], ty)
      | otherwise = case tcTyConAppArgs ty of
          [t, ty'] -> let (ts, ty_r) = splitTypeAt (n-1) ty' in (t:ts, ty_r)
          _ -> pprPanic "splitTypeAt" (ppr ty)

    ----------------------------------------------
    --              buildEnvStack
    --
    --      ((x1,...,xn),stk)

    buildEnvStack :: [Id] -> Id -> CoreExpr
    buildEnvStack env_ids stack_id
      = mkCorePairExpr (mkBigCoreVarTup env_ids) (Var stack_id)

    ----------------------------------------------
    --              matchEnvStack
    --
    --      \ ((x1,...,xn),stk) -> body
    --      =>
    --      \ pair ->
    --      case pair of (tup,stk) ->
    --      case tup of (x1,...,xn) ->
    --      body

    matchEnvStack   :: [Id]         -- x1..xn
                    -> Id           -- stk
                    -> CoreExpr     -- e
                    -> DsM CoreExpr
    matchEnvStack env_ids stack_id body = do
        uniqs <- newUniqueSupply
        tup_var <- newSysLocalDs (mkBigCoreVarTupTy env_ids)
        let match_env = coreCaseTuple uniqs tup_var env_ids body
        pair_id <- newSysLocalDs (mkCorePairTy (idType tup_var) (idType stack_id))
        return (Lam pair_id (coreCasePair pair_id tup_var stack_id match_env))

    ----------------------------------------------
    --              matchEnv
    --
    --      \ (x1,...,xn) -> body
    --      =>
    --      \ tup ->
    --      case tup of (x1,...,xn) ->
    --      body

    matchEnv :: [Id]        -- x1..xn
             -> CoreExpr    -- e
             -> DsM CoreExpr
    matchEnv env_ids body = do
        uniqs <- newUniqueSupply
        tup_id <- newSysLocalDs (mkBigCoreVarTupTy env_ids)
        return (Lam tup_id (coreCaseTuple uniqs tup_id env_ids body))

    ----------------------------------------------
    --              matchVarStack
    --
    --      case (x1, ...(xn, s)...) -> e
    --      =>
    --      case z0 of (x1,z1) ->
    --      case zn-1 of (xn,s) ->
    --      e
    matchVarStack :: [Id] -> Id -> CoreExpr -> DsM (Id, CoreExpr)
    matchVarStack [] stack_id body = return (stack_id, body)
    matchVarStack (param_id:param_ids) stack_id body = do
        (tail_id, tail_code) <- matchVarStack param_ids stack_id body
        pair_id <- newSysLocalDs (mkCorePairTy (idType param_id) (idType tail_id))
        return (pair_id, coreCasePair pair_id param_id tail_id tail_code)

    mkHsEnvStackExpr :: [Id] -> Id -> LHsExpr Id
    mkHsEnvStackExpr env_ids stack_id
      = mkLHsTupleExpr [mkLHsVarTuple env_ids, nlHsVar stack_id]

    -- Translation of arrow abstraction

    -- D; xs |-a c : () --> t'      ---> c'
    -- --------------------------
    -- D |- proc p -> c :: a t t'   ---> premap (\ p -> ((xs),())) c'
    --
    --              where (xs) is the tuple of variables bound by p

    dsProcExpr
            :: LPat Id
            -> LHsCmdTop Id
            -> DsM CoreExpr
    dsProcExpr pat (L _ (HsCmdTop cmd _unitTy cmd_ty ids)) = do
        (meth_binds, meth_ids) <- mkCmdEnv ids
        let locals = mkVarSet (collectPatBinders pat)
        (core_cmd, _free_vars, env_ids) <- dsfixCmd meth_ids locals unitTy cmd_ty cmd
        let env_ty = mkBigCoreVarTupTy env_ids
        let env_stk_ty = mkCorePairTy env_ty unitTy
        let env_stk_expr = mkCorePairExpr (mkBigCoreVarTup env_ids) mkCoreUnitExpr
        fail_expr <- mkFailExpr ProcExpr env_stk_ty
        var <- selectSimpleMatchVarL pat
        match_code <- matchSimply (Var var) ProcExpr pat env_stk_expr fail_expr
        let pat_ty = hsLPatType pat
        let proc_code = do_premap meth_ids pat_ty env_stk_ty cmd_ty
                        (Lam var match_code)
                        core_cmd
        return (mkLets meth_binds proc_code)

    {-
    Translation of a command judgement of the form

            D; xs |-a c : stk --> t

    to an expression e such that

            D |- e :: a (xs, stk) t
    -}

    dsLCmd :: DsCmdEnv -> IdSet -> Type -> Type -> LHsCmd Id -> [Id]
           -> DsM (CoreExpr, IdSet)
    dsLCmd ids local_vars stk_ty res_ty cmd env_ids
      = dsCmd ids local_vars stk_ty res_ty (unLoc cmd) env_ids

    dsCmd   :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this command
            -> Type                 -- type of the stack (right-nested tuple)
            -> Type                 -- return type of the command
            -> HsCmd Id             -- command to desugar
            -> [Id]                 -- list of vars in the input to this command
                                    -- This is typically fed back,
                                    -- so don't pull on it too early
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet)          -- subset of local vars that occur free

    -- D |- fun :: a t1 t2
    -- D, xs |- arg :: t1
    -- -----------------------------
    -- D; xs |-a fun -< arg : stk --> t2
    --
    --              ---> premap (\ ((xs), _stk) -> arg) fun

    dsCmd ids local_vars stack_ty res_ty
            (HsCmdArrApp arrow arg arrow_ty HsFirstOrderApp _)
            env_ids = do
        let
            (a_arg_ty, _res_ty') = tcSplitAppTy arrow_ty
            (_a_ty, arg_ty) = tcSplitAppTy a_arg_ty
        core_arrow <- dsLExpr arrow
        core_arg   <- dsLExpr arg
        stack_id   <- newSysLocalDs stack_ty
        core_make_arg <- matchEnvStack env_ids stack_id core_arg
        return (do_premap ids
                  (envStackType env_ids stack_ty)
                  arg_ty
                  res_ty
                  core_make_arg
                  core_arrow,
                exprFreeIds core_arg `intersectVarSet` local_vars)

    -- D, xs |- fun :: a t1 t2
    -- D, xs |- arg :: t1
    -- ------------------------------
    -- D; xs |-a fun -<< arg : stk --> t2
    --
    --              ---> premap (\ ((xs), _stk) -> (fun, arg)) app

    dsCmd ids local_vars stack_ty res_ty
            (HsCmdArrApp arrow arg arrow_ty HsHigherOrderApp _)
            env_ids = do
        let
            (a_arg_ty, _res_ty') = tcSplitAppTy arrow_ty
            (_a_ty, arg_ty) = tcSplitAppTy a_arg_ty

        core_arrow <- dsLExpr arrow
        core_arg   <- dsLExpr arg
        stack_id   <- newSysLocalDs stack_ty
        core_make_pair <- matchEnvStack env_ids stack_id
              (mkCorePairExpr core_arrow core_arg)

        return (do_premap ids
                  (envStackType env_ids stack_ty)
                  (mkCorePairTy arrow_ty arg_ty)
                  res_ty
                  core_make_pair
                  (do_app ids arg_ty res_ty),
                (exprFreeIds core_arrow `unionVarSet` exprFreeIds core_arg)
                  `intersectVarSet` local_vars)

    -- D; ys |-a cmd : (t,stk) --> t'
    -- D, xs |-  exp :: t
    -- ------------------------
    -- D; xs |-a cmd exp : stk --> t'
    --
    --              ---> premap (\ ((xs),stk) -> ((ys),(e,stk))) cmd

    dsCmd ids local_vars stack_ty res_ty (HsCmdApp cmd arg) env_ids = do
        core_arg <- dsLExpr arg
        let
            arg_ty = exprType core_arg
            stack_ty' = mkCorePairTy arg_ty stack_ty
        (core_cmd, free_vars, env_ids')
                 <- dsfixCmd ids local_vars stack_ty' res_ty cmd
        stack_id <- newSysLocalDs stack_ty
        arg_id <- newSysLocalDs arg_ty
        -- push the argument expression onto the stack
        let
            stack' = mkCorePairExpr (Var arg_id) (Var stack_id)
            core_body = bindNonRec arg_id core_arg
                            (mkCorePairExpr (mkBigCoreVarTup env_ids') stack')

        -- match the environment and stack against the input
        core_map <- matchEnvStack env_ids stack_id core_body
        return (do_premap ids
                          (envStackType env_ids stack_ty)
                          (envStackType env_ids' stack_ty')
                          res_ty
                          core_map
                          core_cmd,
                free_vars `unionVarSet`
                  (exprFreeIds core_arg `intersectVarSet` local_vars))

    -- D; ys |-a cmd : stk t'
    -- -----------------------------------------------
    -- D; xs |-a \ p1 ... pk -> cmd : (t1,...(tk,stk)...) t'
    --
    --              ---> premap (\ ((xs), (p1, ... (pk,stk)...)) -> ((ys),stk)) cmd

    dsCmd ids local_vars stack_ty res_ty
            (HsCmdLam (MG { mg_alts = L _ [L _ (Match _ pats _
                                               (GRHSs [L _ (GRHS [] body)] _ ))] }))
            env_ids = do
        let pat_vars = mkVarSet (collectPatsBinders pats)
        let
            local_vars' = pat_vars `unionVarSet` local_vars
            (pat_tys, stack_ty') = splitTypeAt (length pats) stack_ty
        (core_body, free_vars, env_ids') <- dsfixCmd ids local_vars' stack_ty' res_ty body
        param_ids <- mapM newSysLocalDs pat_tys
        stack_id' <- newSysLocalDs stack_ty'

        -- the expression is built from the inside out, so the actions
        -- are presented in reverse order

        let
            -- build a new environment, plus what's left of the stack
            core_expr = buildEnvStack env_ids' stack_id'
            in_ty = envStackType env_ids stack_ty
            in_ty' = envStackType env_ids' stack_ty'

        fail_expr <- mkFailExpr LambdaExpr in_ty'
        -- match the patterns against the parameters
        match_code <- matchSimplys (map Var param_ids) LambdaExpr pats core_expr fail_expr
        -- match the parameters against the top of the old stack
        (stack_id, param_code) <- matchVarStack param_ids stack_id' match_code
        -- match the old environment and stack against the input
        select_code <- matchEnvStack env_ids stack_id param_code
        return (do_premap ids in_ty in_ty' res_ty select_code core_body,
                free_vars `minusVarSet` pat_vars)

    dsCmd ids local_vars stack_ty res_ty (HsCmdPar cmd) env_ids
      = dsLCmd ids local_vars stack_ty res_ty cmd env_ids

    -- D, xs |- e :: Bool
    -- D; xs1 |-a c1 : stk --> t
    -- D; xs2 |-a c2 : stk --> t
    -- ----------------------------------------
    -- D; xs |-a if e then c1 else c2 : stk --> t
    --
    --              ---> premap (\ ((xs),stk) ->
    --                       if e then Left ((xs1),stk) else Right ((xs2),stk))
    --                     (c1 ||| c2)

    dsCmd ids local_vars stack_ty res_ty (HsCmdIf mb_fun cond then_cmd else_cmd)
            env_ids = do
        core_cond <- dsLExpr cond
        (core_then, fvs_then, then_ids) <- dsfixCmd ids local_vars stack_ty res_ty then_cmd
        (core_else, fvs_else, else_ids) <- dsfixCmd ids local_vars stack_ty res_ty else_cmd
        stack_id   <- newSysLocalDs stack_ty
        either_con <- dsLookupTyCon eitherTyConName
        left_con   <- dsLookupDataCon leftDataConName
        right_con  <- dsLookupDataCon rightDataConName

        let mk_left_expr ty1 ty2 e = mkCoreConApps left_con   [Type ty1, Type ty2, e]
            mk_right_expr ty1 ty2 e = mkCoreConApps right_con [Type ty1, Type ty2, e]

            in_ty = envStackType env_ids stack_ty
            then_ty = envStackType then_ids stack_ty
            else_ty = envStackType else_ids stack_ty
            sum_ty = mkTyConApp either_con [then_ty, else_ty]
            fvs_cond = exprFreeIds core_cond `intersectVarSet` local_vars

            core_left  = mk_left_expr  then_ty else_ty (buildEnvStack then_ids stack_id)
            core_right = mk_right_expr then_ty else_ty (buildEnvStack else_ids stack_id)

        core_if <- case mb_fun of
           Just fun -> do { fun_apps <- dsSyntaxExpr fun [core_cond, core_left, core_right]
                          ; matchEnvStack env_ids stack_id fun_apps }
           Nothing  -> matchEnvStack env_ids stack_id $
                       mkIfThenElse core_cond core_left core_right

        return (do_premap ids in_ty sum_ty res_ty
                    core_if
                    (do_choice ids then_ty else_ty res_ty core_then core_else),
            fvs_cond `unionVarSet` fvs_then `unionVarSet` fvs_else)

    {-
    Case commands are treated in much the same way as if commands
    (see above) except that there are more alternatives.  For example

            case e of { p1 -> c1; p2 -> c2; p3 -> c3 }

    is translated to

            premap (\ ((xs)*ts) -> case e of
                    p1 -> (Left (Left (xs1)*ts))
                    p2 -> Left ((Right (xs2)*ts))
                    p3 -> Right ((xs3)*ts))
            ((c1 ||| c2) ||| c3)

    The idea is to extract the commands from the case, build a balanced tree
    of choices, and replace the commands with expressions that build tagged
    tuples, obtaining a case expression that can be desugared normally.
    To build all this, we use triples describing segments of the list of
    case bodies, containing the following fields:
     * a list of expressions of the form (Left|Right)* ((xs)*ts), to be put
       into the case replacing the commands
     * a sum type that is the common type of these expressions, and also the
       input type of the arrow
     * a CoreExpr for an arrow built by combining the translated command
       bodies with |||.
    -}

    dsCmd ids local_vars stack_ty res_ty
          (HsCmdCase exp (MG { mg_alts = L l matches, mg_arg_tys = arg_tys
                             , mg_origin = origin }))
          env_ids = do
        stack_id <- newSysLocalDs stack_ty

        -- Extract and desugar the leaf commands in the case, building tuple
        -- expressions that will (after tagging) replace these leaves

        let
            leaves = concatMap leavesMatch matches
            make_branch (leaf, bound_vars) = do
                (core_leaf, _fvs, leaf_ids) <-
                      dsfixCmd ids (bound_vars `unionVarSet` local_vars) stack_ty res_ty leaf
                return ([mkHsEnvStackExpr leaf_ids stack_id],
                        envStackType leaf_ids stack_ty,
                        core_leaf)

        branches <- mapM make_branch leaves
        either_con <- dsLookupTyCon eitherTyConName
        left_con <- dsLookupDataCon leftDataConName
        right_con <- dsLookupDataCon rightDataConName
        let
            left_id  = HsVar (noLoc (dataConWrapId left_con))
            right_id = HsVar (noLoc (dataConWrapId right_con))
            left_expr  ty1 ty2 e = noLoc $ HsApp (noLoc $ HsWrap (mkWpTyApps [ty1, ty2]) left_id ) e
            right_expr ty1 ty2 e = noLoc $ HsApp (noLoc $ HsWrap (mkWpTyApps [ty1, ty2]) right_id) e

            -- Prefix each tuple with a distinct series of Left's and Right's,
            -- in a balanced way, keeping track of the types.

            merge_branches (builds1, in_ty1, core_exp1)
                           (builds2, in_ty2, core_exp2)
              = (map (left_expr in_ty1 in_ty2) builds1 ++
                    map (right_expr in_ty1 in_ty2) builds2,
                 mkTyConApp either_con [in_ty1, in_ty2],
                 do_choice ids in_ty1 in_ty2 res_ty core_exp1 core_exp2)
            (leaves', sum_ty, core_choices) = foldb merge_branches branches

            -- Replace the commands in the case with these tagged tuples,
            -- yielding a HsExpr Id we can feed to dsExpr.

            (_, matches') = mapAccumL (replaceLeavesMatch res_ty) leaves' matches
            in_ty = envStackType env_ids stack_ty

        core_body <- dsExpr (HsCase exp (MG { mg_alts = L l matches'
                                            , mg_arg_tys = arg_tys
                                            , mg_res_ty = sum_ty, mg_origin = origin }))
            -- Note that we replace the HsCase result type by sum_ty,
            -- which is the type of matches'

        core_matches <- matchEnvStack env_ids stack_id core_body
        return (do_premap ids in_ty sum_ty res_ty core_matches core_choices,
                exprFreeIds core_body  `intersectVarSet` local_vars)

    -- D; ys |-a cmd : stk --> t
    -- ----------------------------------
    -- D; xs |-a let binds in cmd : stk --> t
    --
    --              ---> premap (\ ((xs),stk) -> let binds in ((ys),stk)) c

    dsCmd ids local_vars stack_ty res_ty (HsCmdLet (L _ binds) body) env_ids = do
        let
            defined_vars = mkVarSet (collectLocalBinders binds)
            local_vars' = defined_vars `unionVarSet` local_vars

        (core_body, _free_vars, env_ids') <- dsfixCmd ids local_vars' stack_ty res_ty body
        stack_id <- newSysLocalDs stack_ty
        -- build a new environment, plus the stack, using the let bindings
        core_binds <- dsLocalBinds binds (buildEnvStack env_ids' stack_id)
        -- match the old environment and stack against the input
        core_map <- matchEnvStack env_ids stack_id core_binds
        return (do_premap ids
                            (envStackType env_ids stack_ty)
                            (envStackType env_ids' stack_ty)
                            res_ty
                            core_map
                            core_body,
            exprFreeIds core_binds `intersectVarSet` local_vars)

    -- D; xs |-a ss : t
    -- ----------------------------------
    -- D; xs |-a do { ss } : () --> t
    --
    --              ---> premap (\ (env,stk) -> env) c

    dsCmd ids local_vars stack_ty res_ty (HsCmdDo (L _ stmts) _) env_ids = do
        (core_stmts, env_ids') <- dsCmdDo ids local_vars res_ty stmts env_ids
        let env_ty = mkBigCoreVarTupTy env_ids
        core_fst <- mkFstExpr env_ty stack_ty
        return (do_premap ids
                    (mkCorePairTy env_ty stack_ty)
                    env_ty
                    res_ty
                    core_fst
                    core_stmts,
            env_ids')

    -- D |- e :: forall e. a1 (e,stk1) t1 -> ... an (e,stkn) tn -> a (e,stk) t
    -- D; xs |-a ci :: stki --> ti
    -- -----------------------------------
    -- D; xs |-a (|e c1 ... cn|) :: stk --> t       ---> e [t_xs] c1 ... cn

    dsCmd _ids local_vars _stack_ty _res_ty (HsCmdArrForm op _ args) env_ids = do
        let env_ty = mkBigCoreVarTupTy env_ids
        core_op <- dsLExpr op
        (core_args, fv_sets) <- mapAndUnzipM (dsTrimCmdArg local_vars env_ids) args
        return (mkApps (App core_op (Type env_ty)) core_args,
                unionVarSets fv_sets)

    dsCmd ids local_vars stack_ty res_ty (HsCmdWrap wrap cmd) env_ids = do
        (core_cmd, env_ids') <- dsCmd ids local_vars stack_ty res_ty cmd env_ids
        wrapped_cmd <- dsHsWrapper wrap core_cmd
        return (wrapped_cmd, env_ids')

    dsCmd _ _ _ _ _ c = pprPanic "dsCmd" (ppr c)

    -- D; ys |-a c : stk --> t      (ys <= xs)
    -- ---------------------
    -- D; xs |-a c : stk --> t      ---> premap (\ ((xs),stk) -> ((ys),stk)) c

    dsTrimCmdArg
            :: IdSet                -- set of local vars available to this command
            -> [Id]                 -- list of vars in the input to this command
            -> LHsCmdTop Id         -- command argument to desugar
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet)          -- subset of local vars that occur free
    dsTrimCmdArg local_vars env_ids (L _ (HsCmdTop cmd stack_ty cmd_ty ids)) = do
        (meth_binds, meth_ids) <- mkCmdEnv ids
        (core_cmd, free_vars, env_ids') <- dsfixCmd meth_ids local_vars stack_ty cmd_ty cmd
        stack_id <- newSysLocalDs stack_ty
        trim_code <- matchEnvStack env_ids stack_id (buildEnvStack env_ids' stack_id)
        let
            in_ty = envStackType env_ids stack_ty
            in_ty' = envStackType env_ids' stack_ty
            arg_code = if env_ids' == env_ids then core_cmd else
                    do_premap meth_ids in_ty in_ty' cmd_ty trim_code core_cmd
        return (mkLets meth_binds arg_code, free_vars)

    -- Given D; xs |-a c : stk --> t, builds c with xs fed back.
    -- Typically needs to be prefixed with arr (\(p, stk) -> ((xs),stk))

    dsfixCmd
            :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this command
            -> Type                 -- type of the stack (right-nested tuple)
            -> Type                 -- return type of the command
            -> LHsCmd Id            -- command to desugar
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet,          -- subset of local vars that occur free
                    [Id])           -- the same local vars as a list, fed back
    dsfixCmd ids local_vars stk_ty cmd_ty cmd
      = trimInput (dsLCmd ids local_vars stk_ty cmd_ty cmd)

    -- Feed back the list of local variables actually used a command,
    -- for use as the input tuple of the generated arrow.

    trimInput
            :: ([Id] -> DsM (CoreExpr, IdSet))
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet,          -- subset of local vars that occur free
                    [Id])           -- same local vars as a list, fed back to
                                    -- the inner function to form the tuple of
                                    -- inputs to the arrow.
    trimInput build_arrow
      = fixDs (\ ~(_,_,env_ids) -> do
            (core_cmd, free_vars) <- build_arrow env_ids
            return (core_cmd, free_vars, varSetElems free_vars))

    {-
    Translation of command judgements of the form

            D |-a do { ss } : t
    -}

    dsCmdDo :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this statement
            -> Type                 -- return type of the statement
            -> [CmdLStmt Id]        -- statements to desugar
            -> [Id]                 -- list of vars in the input to this statement
                                    -- This is typically fed back,
                                    -- so don't pull on it too early
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet)          -- subset of local vars that occur free

    dsCmdDo _ _ _ [] _ = panic "dsCmdDo"

    -- D; xs |-a c : () --> t
    -- --------------------------
    -- D; xs |-a do { c } : t
    --
    --              ---> premap (\ (xs) -> ((xs), ())) c

    dsCmdDo ids local_vars res_ty [L _ (LastStmt body _ _)] env_ids = do
        (core_body, env_ids') <- dsLCmd ids local_vars unitTy res_ty body env_ids
        let env_ty = mkBigCoreVarTupTy env_ids
        env_var <- newSysLocalDs env_ty
        let core_map = Lam env_var (mkCorePairExpr (Var env_var) mkCoreUnitExpr)
        return (do_premap ids
                            env_ty
                            (mkCorePairTy env_ty unitTy)
                            res_ty
                            core_map
                            core_body,
            env_ids')

    dsCmdDo ids local_vars res_ty (stmt:stmts) env_ids = do
        let bound_vars  = mkVarSet (collectLStmtBinders stmt)
        let local_vars' = bound_vars `unionVarSet` local_vars
        (core_stmts, _, env_ids') <- trimInput (dsCmdDo ids local_vars' res_ty stmts)
        (core_stmt, fv_stmt) <- dsCmdLStmt ids local_vars env_ids' stmt env_ids
        return (do_compose ids
                    (mkBigCoreVarTupTy env_ids)
                    (mkBigCoreVarTupTy env_ids')
                    res_ty
                    core_stmt
                    core_stmts,
                  fv_stmt)

    {-
    A statement maps one local environment to another, and is represented
    as an arrow from one tuple type to another.  A statement sequence is
    translated to a composition of such arrows.
    -}

    dsCmdLStmt :: DsCmdEnv -> IdSet -> [Id] -> CmdLStmt Id -> [Id]
               -> DsM (CoreExpr, IdSet)
    dsCmdLStmt ids local_vars out_ids cmd env_ids
      = dsCmdStmt ids local_vars out_ids (unLoc cmd) env_ids

    dsCmdStmt
            :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this statement
            -> [Id]                 -- list of vars in the output of this statement
            -> CmdStmt Id           -- statement to desugar
            -> [Id]                 -- list of vars in the input to this statement
                                    -- This is typically fed back,
                                    -- so don't pull on it too early
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet)          -- subset of local vars that occur free

    -- D; xs1 |-a c : () --> t
    -- D; xs' |-a do { ss } : t'
    -- ------------------------------
    -- D; xs  |-a do { c; ss } : t'
    --
    --              ---> premap (\ ((xs)) -> (((xs1),()),(xs')))
    --                      (first c >>> arr snd) >>> ss

    dsCmdStmt ids local_vars out_ids (BodyStmt cmd _ _ c_ty) env_ids = do
        (core_cmd, fv_cmd, env_ids1) <- dsfixCmd ids local_vars unitTy c_ty cmd
        core_mux <- matchEnv env_ids
            (mkCorePairExpr
                (mkCorePairExpr (mkBigCoreVarTup env_ids1) mkCoreUnitExpr)
                (mkBigCoreVarTup out_ids))
        let
            in_ty = mkBigCoreVarTupTy env_ids
            in_ty1 = mkCorePairTy (mkBigCoreVarTupTy env_ids1) unitTy
            out_ty = mkBigCoreVarTupTy out_ids
            before_c_ty = mkCorePairTy in_ty1 out_ty
            after_c_ty = mkCorePairTy c_ty out_ty
        snd_fn <- mkSndExpr c_ty out_ty
        return (do_premap ids in_ty before_c_ty out_ty core_mux $
                    do_compose ids before_c_ty after_c_ty out_ty
                            (do_first ids in_ty1 c_ty out_ty core_cmd) $
                    do_arr ids after_c_ty out_ty snd_fn,
                  extendVarSetList fv_cmd out_ids)

    -- D; xs1 |-a c : () --> t
    -- D; xs' |-a do { ss } : t'            xs2 = xs' - defs(p)
    -- -----------------------------------
    -- D; xs  |-a do { p <- c; ss } : t'
    --
    --              ---> premap (\ (xs) -> (((xs1),()),(xs2)))
    --                      (first c >>> arr (\ (p, (xs2)) -> (xs'))) >>> ss
    --
    -- It would be simpler and more consistent to do this using second,
    -- but that's likely to be defined in terms of first.

    dsCmdStmt ids local_vars out_ids (BindStmt pat cmd _ _ _) env_ids = do
        let pat_ty = hsLPatType pat
        (core_cmd, fv_cmd, env_ids1) <- dsfixCmd ids local_vars unitTy pat_ty cmd
        let pat_vars = mkVarSet (collectPatBinders pat)
        let
            env_ids2 = varSetElems (mkVarSet out_ids `minusVarSet` pat_vars)
            env_ty2 = mkBigCoreVarTupTy env_ids2

        -- multiplexing function
        --          \ (xs) -> (((xs1),()),(xs2))

        core_mux <- matchEnv env_ids
            (mkCorePairExpr
                (mkCorePairExpr (mkBigCoreVarTup env_ids1) mkCoreUnitExpr)
                (mkBigCoreVarTup env_ids2))

        -- projection function
        --          \ (p, (xs2)) -> (zs)

        env_id <- newSysLocalDs env_ty2
        uniqs <- newUniqueSupply
        let
            after_c_ty = mkCorePairTy pat_ty env_ty2
            out_ty = mkBigCoreVarTupTy out_ids
            body_expr = coreCaseTuple uniqs env_id env_ids2 (mkBigCoreVarTup out_ids)

        fail_expr <- mkFailExpr (StmtCtxt DoExpr) out_ty
        pat_id    <- selectSimpleMatchVarL pat
        match_code <- matchSimply (Var pat_id) (StmtCtxt DoExpr) pat body_expr fail_expr
        pair_id   <- newSysLocalDs after_c_ty
        let
            proj_expr = Lam pair_id (coreCasePair pair_id pat_id env_id match_code)

        -- put it all together
        let
            in_ty = mkBigCoreVarTupTy env_ids
            in_ty1 = mkCorePairTy (mkBigCoreVarTupTy env_ids1) unitTy
            in_ty2 = mkBigCoreVarTupTy env_ids2
            before_c_ty = mkCorePairTy in_ty1 in_ty2
        return (do_premap ids in_ty before_c_ty out_ty core_mux $
                    do_compose ids before_c_ty after_c_ty out_ty
                            (do_first ids in_ty1 pat_ty in_ty2 core_cmd) $
                    do_arr ids after_c_ty out_ty proj_expr,
                  fv_cmd `unionVarSet` (mkVarSet out_ids `minusVarSet` pat_vars))

    -- D; xs' |-a do { ss } : t
    -- --------------------------------------
    -- D; xs  |-a do { let binds; ss } : t
    --
    --              ---> arr (\ (xs) -> let binds in (xs')) >>> ss

    dsCmdStmt ids local_vars out_ids (LetStmt (L _ binds)) env_ids = do
        -- build a new environment using the let bindings
        core_binds <- dsLocalBinds binds (mkBigCoreVarTup out_ids)
        -- match the old environment against the input
        core_map <- matchEnv env_ids core_binds
        return (do_arr ids
                            (mkBigCoreVarTupTy env_ids)
                            (mkBigCoreVarTupTy out_ids)
                            core_map,
                exprFreeIds core_binds `intersectVarSet` local_vars)

    -- D; ys  |-a do { ss; returnA -< ((xs1), (ys2)) } : ...
    -- D; xs' |-a do { ss' } : t
    -- ------------------------------------
    -- D; xs  |-a do { rec ss; ss' } : t
    --
    --                      xs1 = xs' /\ defs(ss)
    --                      xs2 = xs' - defs(ss)
    --                      ys1 = ys - defs(ss)
    --                      ys2 = ys /\ defs(ss)
    --
    --              ---> arr (\(xs) -> ((ys1),(xs2))) >>>
    --                      first (loop (arr (\((ys1),~(ys2)) -> (ys)) >>> ss)) >>>
    --                      arr (\((xs1),(xs2)) -> (xs')) >>> ss'

    dsCmdStmt ids local_vars out_ids
            (RecStmt { recS_stmts = stmts
                     , recS_later_ids = later_ids, recS_rec_ids = rec_ids
                     , recS_later_rets = later_rets, recS_rec_rets = rec_rets })
            env_ids = do
        let
            env2_id_set = mkVarSet out_ids `minusVarSet` mkVarSet later_ids
            env2_ids = varSetElems env2_id_set
            env2_ty = mkBigCoreVarTupTy env2_ids

        -- post_loop_fn = \((later_ids),(env2_ids)) -> (out_ids)

        uniqs <- newUniqueSupply
        env2_id <- newSysLocalDs env2_ty
        let
            later_ty = mkBigCoreVarTupTy later_ids
            post_pair_ty = mkCorePairTy later_ty env2_ty
            post_loop_body = coreCaseTuple uniqs env2_id env2_ids (mkBigCoreVarTup out_ids)

        post_loop_fn <- matchEnvStack later_ids env2_id post_loop_body

        --- loop (...)

        (core_loop, env1_id_set, env1_ids)
                   <- dsRecCmd ids local_vars stmts later_ids later_rets rec_ids rec_rets

        -- pre_loop_fn = \(env_ids) -> ((env1_ids),(env2_ids))

        let
            env1_ty = mkBigCoreVarTupTy env1_ids
            pre_pair_ty = mkCorePairTy env1_ty env2_ty
            pre_loop_body = mkCorePairExpr (mkBigCoreVarTup env1_ids)
                                            (mkBigCoreVarTup env2_ids)

        pre_loop_fn <- matchEnv env_ids pre_loop_body

        -- arr pre_loop_fn >>> first (loop (...)) >>> arr post_loop_fn

        let
            env_ty = mkBigCoreVarTupTy env_ids
            out_ty = mkBigCoreVarTupTy out_ids
            core_body = do_premap ids env_ty pre_pair_ty out_ty
                    pre_loop_fn
                    (do_compose ids pre_pair_ty post_pair_ty out_ty
                            (do_first ids env1_ty later_ty env2_ty
                                    core_loop)
                            (do_arr ids post_pair_ty out_ty
                                    post_loop_fn))

        return (core_body, env1_id_set `unionVarSet` env2_id_set)

    dsCmdStmt _ _ _ _ s = pprPanic "dsCmdStmt" (ppr s)

    --      loop (premap (\ ((env1_ids), ~(rec_ids)) -> (env_ids))
    --            (ss >>> arr (\ (out_ids) -> ((later_rets),(rec_rets))))) >>>

    dsRecCmd
            :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this statement
            -> [CmdLStmt Id]        -- list of statements inside the RecCmd
            -> [Id]                 -- list of vars defined here and used later
            -> [HsExpr Id]          -- expressions corresponding to later_ids
            -> [Id]                 -- list of vars fed back through the loop
            -> [HsExpr Id]          -- expressions corresponding to rec_ids
            -> DsM (CoreExpr,       -- desugared statement
                    IdSet,          -- subset of local vars that occur free
                    [Id])           -- same local vars as a list

    dsRecCmd ids local_vars stmts later_ids later_rets rec_ids rec_rets = do
        let
            later_id_set = mkVarSet later_ids
            rec_id_set = mkVarSet rec_ids
            local_vars' = rec_id_set `unionVarSet` later_id_set `unionVarSet` local_vars

        -- mk_pair_fn = \ (out_ids) -> ((later_rets),(rec_rets))

        core_later_rets <- mapM dsExpr later_rets
        core_rec_rets <- mapM dsExpr rec_rets
        let
            -- possibly polymorphic version of vars of later_ids and rec_ids
            out_ids = varSetElems (unionVarSets (map exprFreeIds (core_later_rets ++ core_rec_rets)))
            out_ty = mkBigCoreVarTupTy out_ids

            later_tuple = mkBigCoreTup core_later_rets
            later_ty = mkBigCoreVarTupTy later_ids

            rec_tuple = mkBigCoreTup core_rec_rets
            rec_ty = mkBigCoreVarTupTy rec_ids

            out_pair = mkCorePairExpr later_tuple rec_tuple
            out_pair_ty = mkCorePairTy later_ty rec_ty

        mk_pair_fn <- matchEnv out_ids out_pair

        -- ss

        (core_stmts, fv_stmts, env_ids) <- dsfixCmdStmts ids local_vars' out_ids stmts

        -- squash_pair_fn = \ ((env1_ids), ~(rec_ids)) -> (env_ids)

        rec_id <- newSysLocalDs rec_ty
        let
            env1_id_set = fv_stmts `minusVarSet` rec_id_set
            env1_ids = varSetElems env1_id_set
            env1_ty = mkBigCoreVarTupTy env1_ids
            in_pair_ty = mkCorePairTy env1_ty rec_ty
            core_body = mkBigCoreTup (map selectVar env_ids)
              where
                selectVar v
                    | v `elemVarSet` rec_id_set
                      = mkTupleSelector rec_ids v rec_id (Var rec_id)
                    | otherwise = Var v

        squash_pair_fn <- matchEnvStack env1_ids rec_id core_body

        -- loop (premap squash_pair_fn (ss >>> arr mk_pair_fn))

        let
            env_ty = mkBigCoreVarTupTy env_ids
            core_loop = do_loop ids env1_ty later_ty rec_ty
                    (do_premap ids in_pair_ty env_ty out_pair_ty
                            squash_pair_fn
                            (do_compose ids env_ty out_ty out_pair_ty
                                    core_stmts
                                    (do_arr ids out_ty out_pair_ty mk_pair_fn)))

        return (core_loop, env1_id_set, env1_ids)

    {-
    A sequence of statements (as in a rec) is desugared to an arrow between
    two environments (no stack)
    -}

    dsfixCmdStmts
            :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this statement
            -> [Id]                 -- output vars of these statements
            -> [CmdLStmt Id]        -- statements to desugar
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet,          -- subset of local vars that occur free
                    [Id])           -- same local vars as a list

    dsfixCmdStmts ids local_vars out_ids stmts
      = trimInput (dsCmdStmts ids local_vars out_ids stmts)

    dsCmdStmts
            :: DsCmdEnv             -- arrow combinators
            -> IdSet                -- set of local vars available to this statement
            -> [Id]                 -- output vars of these statements
            -> [CmdLStmt Id]        -- statements to desugar
            -> [Id]                 -- list of vars in the input to these statements
            -> DsM (CoreExpr,       -- desugared expression
                    IdSet)          -- subset of local vars that occur free

    dsCmdStmts ids local_vars out_ids [stmt] env_ids
      = dsCmdLStmt ids local_vars out_ids stmt env_ids

    dsCmdStmts ids local_vars out_ids (stmt:stmts) env_ids = do
        let bound_vars  = mkVarSet (collectLStmtBinders stmt)
        let local_vars' = bound_vars `unionVarSet` local_vars
        (core_stmts, _fv_stmts, env_ids') <- dsfixCmdStmts ids local_vars' out_ids stmts
        (core_stmt, fv_stmt) <- dsCmdLStmt ids local_vars env_ids' stmt env_ids
        return (do_compose ids
                    (mkBigCoreVarTupTy env_ids)
                    (mkBigCoreVarTupTy env_ids')
                    (mkBigCoreVarTupTy out_ids)
                    core_stmt
                    core_stmts,
                  fv_stmt)

    dsCmdStmts _ _ _ [] _ = panic "dsCmdStmts []"

    -- Match a list of expressions against a list of patterns, left-to-right.

    matchSimplys :: [CoreExpr]              -- Scrutinees
                 -> HsMatchContext Name     -- Match kind
                 -> [LPat Id]               -- Patterns they should match
                 -> CoreExpr                -- Return this if they all match
                 -> CoreExpr                -- Return this if they don't
                 -> DsM CoreExpr
    matchSimplys [] _ctxt [] result_expr _fail_expr = return result_expr
    matchSimplys (exp:exps) ctxt (pat:pats) result_expr fail_expr = do
        match_code <- matchSimplys exps ctxt pats result_expr fail_expr
        matchSimply exp ctxt pat match_code fail_expr
    matchSimplys _ _ _ _ _ = panic "matchSimplys"

    -- List of leaf expressions, with set of variables bound in each

    leavesMatch :: LMatch Id (Located (body Id)) -> [(Located (body Id), IdSet)]
    leavesMatch (L _ (Match _ pats _ (GRHSs grhss (L _ binds))))
      = let
            defined_vars = mkVarSet (collectPatsBinders pats)
                            `unionVarSet`
                           mkVarSet (collectLocalBinders binds)
        in
        [(body,
          mkVarSet (collectLStmtsBinders stmts)
            `unionVarSet` defined_vars)
        | L _ (GRHS stmts body) <- grhss]

    -- Replace the leaf commands in a match

    replaceLeavesMatch
            :: Type                                 -- new result type
            -> [Located (body' Id)]                 -- replacement leaf expressions of that type
            -> LMatch Id (Located (body Id))        -- the matches of a case command
            -> ([Located (body' Id)],               -- remaining leaf expressions
                LMatch Id (Located (body' Id)))     -- updated match
    replaceLeavesMatch _res_ty leaves (L loc (Match mf pat mt (GRHSs grhss binds)))
      = let
            (leaves', grhss') = mapAccumL replaceLeavesGRHS leaves grhss
        in
        (leaves', L loc (Match mf pat mt (GRHSs grhss' binds)))

    replaceLeavesGRHS
            :: [Located (body' Id)]                 -- replacement leaf expressions of that type
            -> LGRHS Id (Located (body Id))         -- rhss of a case command
            -> ([Located (body' Id)],               -- remaining leaf expressions
                LGRHS Id (Located (body' Id)))      -- updated GRHS
    replaceLeavesGRHS (leaf:leaves) (L loc (GRHS stmts _))
      = (leaves, L loc (GRHS stmts leaf))
    replaceLeavesGRHS [] _ = panic "replaceLeavesGRHS []"

    -- Balanced fold of a non-empty list.

    foldb :: (a -> a -> a) -> [a] -> a
    foldb _ [] = error "foldb of empty list"
    foldb _ [x] = x
    foldb f xs = foldb f (fold_pairs xs)
      where
        fold_pairs [] = []
        fold_pairs [x] = [x]
        fold_pairs (x1:x2:xs) = f x1 x2:fold_pairs xs

    {-
    Note [Dictionary binders in ConPatOut] See also same Note in HsUtils
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The following functions to collect value variables from patterns are
    copied from HsUtils, with one change: we also collect the dictionary
    bindings (pat_binds) from ConPatOut.  We need them for cases like

    h :: Arrow a => Int -> a (Int,Int) Int
    h x = proc (y,z) -> case compare x y of
                    GT -> returnA -< z+x

    The type checker turns the case into

                    case compare x y of
                      GT { p77 = plusInt } -> returnA -< p77 z x

    Here p77 is a local binding for the (+) operation.

    See comments in HsUtils for why the other version does not include
    these bindings.
    -}

    collectPatBinders :: LPat Id -> [Id]
    collectPatBinders pat = collectl pat []

    collectPatsBinders :: [LPat Id] -> [Id]
    collectPatsBinders pats = foldr collectl [] pats

    ---------------------
    collectl :: LPat Id -> [Id] -> [Id]
    -- See Note [Dictionary binders in ConPatOut]
    collectl (L _ pat) bndrs
      = go pat
      where
        go (VarPat (L _ var))         = var : bndrs
        go (WildPat _)                = bndrs
        go (LazyPat pat)              = collectl pat bndrs
        go (BangPat pat)              = collectl pat bndrs
        go (AsPat (L _ a) pat)        = a : collectl pat bndrs
        go (ParPat  pat)              = collectl pat bndrs

        go (ListPat pats _ _)         = foldr collectl bndrs pats
        go (PArrPat pats _)           = foldr collectl bndrs pats
        go (TuplePat pats _ _)        = foldr collectl bndrs pats

        go (ConPatIn _ ps)            = foldr collectl bndrs (hsConPatArgs ps)
        go (ConPatOut {pat_args=ps, pat_binds=ds}) =
                                        collectEvBinders ds
                                        ++ foldr collectl bndrs (hsConPatArgs ps)
        go (LitPat _)                 = bndrs
        go (NPat {})                  = bndrs
        go (NPlusKPat (L _ n) _ _ _ _ _) = n : bndrs

        go (SigPatIn pat _)           = collectl pat bndrs
        go (SigPatOut pat _)          = collectl pat bndrs
        go (CoPat _ pat _)            = collectl (noLoc pat) bndrs
        go (ViewPat _ pat _)          = collectl pat bndrs
        go p@(SplicePat {})           = pprPanic "collectl/go" (ppr p)

    collectEvBinders :: TcEvBinds -> [Id]
    collectEvBinders (EvBinds bs)   = foldrBag add_ev_bndr [] bs
    collectEvBinders (TcEvBinds {}) = panic "ToDo: collectEvBinders"

    add_ev_bndr :: EvBind -> [Id] -> [Id]
    add_ev_bndr (EvBind { eb_lhs = b }) bs | isId b    = b:bs
                                           | otherwise = bs
      -- A worry: what about coercion variable binders??

    collectLStmtsBinders :: [LStmt Id body] -> [Id]
    collectLStmtsBinders = concatMap collectLStmtBinders

    collectLStmtBinders :: LStmt Id body -> [Id]
    collectLStmtBinders = collectStmtBinders . unLoc

    collectStmtBinders :: Stmt Id body -> [Id]
    collectStmtBinders (RecStmt { recS_later_ids = later_ids }) = later_ids
    collectStmtBinders stmt = HsUtils.collectStmtBinders stmt