rename/RnExpr.hs

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

    \section[RnExpr]{Renaming of expressions}

    Basically dependency analysis.

    Handles @Match@, @GRHSs@, @HsExpr@, and @Qualifier@ datatypes.  In
    general, all of these functions return a renamed thing, and a set of
    free variables.
    -}

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE MultiWayIf #-}
    {-# LANGUAGE TypeFamilies #-}

    module RnExpr (
            rnLExpr, rnExpr, rnStmts
       ) where

    #include "HsVersions.h"

    import GhcPrelude

    import RnBinds   ( rnLocalBindsAndThen, rnLocalValBindsLHS, rnLocalValBindsRHS,
                       rnMatchGroup, rnGRHS, makeMiniFixityEnv)
    import HsSyn
    import TcRnMonad
    import Module           ( getModule )
    import RnEnv
    import RnFixity
    import RnUtils          ( HsDocContext(..), bindLocalNamesFV, checkDupNames
                            , bindLocalNames
                            , mapMaybeFvRn, mapFvRn
                            , warnUnusedLocalBinds )
    import RnUnbound        ( reportUnboundName )
    import RnSplice         ( rnBracket, rnSpliceExpr, checkThLocalName )
    import RnTypes
    import RnPat
    import DynFlags
    import PrelNames

    import BasicTypes
    import Name
    import NameSet
    import RdrName
    import UniqSet
    import Data.List
    import Util
    import ListSetOps       ( removeDups )
    import ErrUtils
    import Outputable
    import SrcLoc
    import FastString
    import Control.Monad
    import TysWiredIn       ( nilDataConName )
    import qualified GHC.LanguageExtensions as LangExt

    import Data.Ord
    import Data.Array
    import qualified Data.List.NonEmpty as NE

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{Expressions}
    *                                                                      *
    ************************************************************************
    -}

    rnExprs :: [LHsExpr GhcPs] -> RnM ([LHsExpr GhcRn], FreeVars)
    rnExprs ls = rnExprs' ls emptyUniqSet
     where
      rnExprs' [] acc = return ([], acc)
      rnExprs' (expr:exprs) acc =
       do { (expr', fvExpr) <- rnLExpr expr
            -- Now we do a "seq" on the free vars because typically it's small
            -- or empty, especially in very long lists of constants
          ; let  acc' = acc `plusFV` fvExpr
          ; (exprs', fvExprs) <- acc' `seq` rnExprs' exprs acc'
          ; return (expr':exprs', fvExprs) }

    -- Variables. We look up the variable and return the resulting name.

    rnLExpr :: LHsExpr GhcPs -> RnM (LHsExpr GhcRn, FreeVars)
    rnLExpr = wrapLocFstM rnExpr

    rnExpr :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)

    finishHsVar :: Located Name -> RnM (HsExpr GhcRn, FreeVars)
    -- Separated from rnExpr because it's also used
    -- when renaming infix expressions
    finishHsVar (L l name)
     = do { this_mod <- getModule
          ; when (nameIsLocalOrFrom this_mod name) $
            checkThLocalName name
          ; return (HsVar noExt (L l name), unitFV name) }

    rnUnboundVar :: RdrName -> RnM (HsExpr GhcRn, FreeVars)
    rnUnboundVar v
     = do { if isUnqual v
            then -- Treat this as a "hole"
                 -- Do not fail right now; instead, return HsUnboundVar
                 -- and let the type checker report the error
                 do { let occ = rdrNameOcc v
                    ; uv <- if startsWithUnderscore occ
                            then return (TrueExprHole occ)
                            else OutOfScope occ <$> getGlobalRdrEnv
                    ; return (HsUnboundVar noExt uv, emptyFVs) }

            else -- Fail immediately (qualified name)
                 do { n <- reportUnboundName v
                    ; return (HsVar noExt (noLoc n), emptyFVs) } }

    rnExpr (HsVar _ (L l v))
      = do { opt_DuplicateRecordFields <- xoptM LangExt.DuplicateRecordFields
           ; mb_name <- lookupOccRn_overloaded opt_DuplicateRecordFields v
           ; case mb_name of {
               Nothing -> rnUnboundVar v ;
               Just (Left name)
                  | name == nilDataConName -- Treat [] as an ExplicitList, so that
                                           -- OverloadedLists works correctly
                  -> rnExpr (ExplicitList noExt Nothing [])

                  | otherwise
                  -> finishHsVar (L l name) ;
                Just (Right [s]) ->
                  return ( HsRecFld noExt (Unambiguous s (L l v) ), unitFV s) ;
               Just (Right fs@(_:_:_)) ->
                  return ( HsRecFld noExt (Ambiguous noExt (L l v))
                         , mkFVs fs);
               Just (Right [])         -> panic "runExpr/HsVar" } }

    rnExpr (HsIPVar x v)
      = return (HsIPVar x v, emptyFVs)

    rnExpr (HsOverLabel x _ v)
      = do { rebindable_on <- xoptM LangExt.RebindableSyntax
           ; if rebindable_on
             then do { fromLabel <- lookupOccRn (mkVarUnqual (fsLit "fromLabel"))
                     ; return (HsOverLabel x (Just fromLabel) v, unitFV fromLabel) }
             else return (HsOverLabel x Nothing v, emptyFVs) }

    rnExpr (HsLit x lit@(HsString src s))
      = do { opt_OverloadedStrings <- xoptM LangExt.OverloadedStrings
           ; if opt_OverloadedStrings then
                rnExpr (HsOverLit x (mkHsIsString src s))
             else do {
                ; rnLit lit
                ; return (HsLit x (convertLit lit), emptyFVs) } }

    rnExpr (HsLit x lit)
      = do { rnLit lit
           ; return (HsLit x(convertLit lit), emptyFVs) }

    rnExpr (HsOverLit x lit)
      = do { ((lit', mb_neg), fvs) <- rnOverLit lit -- See Note [Negative zero]
           ; case mb_neg of
                  Nothing -> return (HsOverLit x lit', fvs)
                  Just neg -> return (HsApp x (noLoc neg) (noLoc (HsOverLit x lit'))
                                     , fvs ) }

    rnExpr (HsApp x fun arg)
      = do { (fun',fvFun) <- rnLExpr fun
           ; (arg',fvArg) <- rnLExpr arg
           ; return (HsApp x fun' arg', fvFun `plusFV` fvArg) }

    rnExpr (HsAppType arg fun)
      = do { (fun',fvFun) <- rnLExpr fun
           ; (arg',fvArg) <- rnHsWcType HsTypeCtx arg
           ; return (HsAppType arg' fun', fvFun `plusFV` fvArg) }

    rnExpr (OpApp _ e1 op e2)
      = do  { (e1', fv_e1) <- rnLExpr e1
            ; (e2', fv_e2) <- rnLExpr e2
            ; (op', fv_op) <- rnLExpr op

            -- Deal with fixity
            -- When renaming code synthesised from "deriving" declarations
            -- we used to avoid fixity stuff, but we can't easily tell any
            -- more, so I've removed the test.  Adding HsPars in TcGenDeriv
            -- should prevent bad things happening.
            ; fixity <- case op' of
                  L _ (HsVar _ (L _ n)) -> lookupFixityRn n
                  L _ (HsRecFld _ f)    -> lookupFieldFixityRn f
                  _ -> return (Fixity NoSourceText minPrecedence InfixL)
                       -- c.f. lookupFixity for unbound

            ; final_e <- mkOpAppRn e1' op' fixity e2'
            ; return (final_e, fv_e1 `plusFV` fv_op `plusFV` fv_e2) }

    rnExpr (NegApp _ e _)
      = do { (e', fv_e)         <- rnLExpr e
           ; (neg_name, fv_neg) <- lookupSyntaxName negateName
           ; final_e            <- mkNegAppRn e' neg_name
           ; return (final_e, fv_e `plusFV` fv_neg) }

    ------------------------------------------
    -- Template Haskell extensions
    -- Don't ifdef-GHCI them because we want to fail gracefully
    -- (not with an rnExpr crash) in a stage-1 compiler.
    rnExpr e@(HsBracket _ br_body) = rnBracket e br_body

    rnExpr (HsSpliceE _ splice) = rnSpliceExpr splice

    ---------------------------------------------
    --      Sections
    -- See Note [Parsing sections] in Parser.y
    rnExpr (HsPar x (L loc (section@(SectionL {}))))
      = do  { (section', fvs) <- rnSection section
            ; return (HsPar x (L loc section'), fvs) }

    rnExpr (HsPar x (L loc (section@(SectionR {}))))
      = do  { (section', fvs) <- rnSection section
            ; return (HsPar x (L loc section'), fvs) }

    rnExpr (HsPar x e)
      = do  { (e', fvs_e) <- rnLExpr e
            ; return (HsPar x e', fvs_e) }

    rnExpr expr@(SectionL {})
      = do  { addErr (sectionErr expr); rnSection expr }
    rnExpr expr@(SectionR {})
      = do  { addErr (sectionErr expr); rnSection expr }

    ---------------------------------------------
    rnExpr (HsCoreAnn x src ann expr)
      = do { (expr', fvs_expr) <- rnLExpr expr
           ; return (HsCoreAnn x src ann expr', fvs_expr) }

    rnExpr (HsSCC x src lbl expr)
      = do { (expr', fvs_expr) <- rnLExpr expr
           ; return (HsSCC x src lbl expr', fvs_expr) }
    rnExpr (HsTickPragma x src info srcInfo expr)
      = do { (expr', fvs_expr) <- rnLExpr expr
           ; return (HsTickPragma x src info srcInfo expr', fvs_expr) }

    rnExpr (HsLam x matches)
      = do { (matches', fvMatch) <- rnMatchGroup LambdaExpr rnLExpr matches
           ; return (HsLam x matches', fvMatch) }

    rnExpr (HsLamCase x matches)
      = do { (matches', fvs_ms) <- rnMatchGroup CaseAlt rnLExpr matches
           ; return (HsLamCase x matches', fvs_ms) }

    rnExpr (HsCase x expr matches)
      = do { (new_expr, e_fvs) <- rnLExpr expr
           ; (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLExpr matches
           ; return (HsCase x new_expr new_matches, e_fvs `plusFV` ms_fvs) }

    rnExpr (HsLet x (L l binds) expr)
      = rnLocalBindsAndThen binds $ \binds' _ -> do
          { (expr',fvExpr) <- rnLExpr expr
          ; return (HsLet x (L l binds') expr', fvExpr) }

    rnExpr (HsDo x do_or_lc (L l stmts))
      = do  { ((stmts', _), fvs) <-
               rnStmtsWithPostProcessing do_or_lc rnLExpr
                 postProcessStmtsForApplicativeDo stmts
                 (\ _ -> return ((), emptyFVs))
            ; return ( HsDo x do_or_lc (L l stmts'), fvs ) }

    rnExpr (ExplicitList x _  exps)
      = do  { opt_OverloadedLists <- xoptM LangExt.OverloadedLists
            ; (exps', fvs) <- rnExprs exps
            ; if opt_OverloadedLists
               then do {
                ; (from_list_n_name, fvs') <- lookupSyntaxName fromListNName
                ; return (ExplicitList x (Just from_list_n_name) exps'
                         , fvs `plusFV` fvs') }
               else
                return  (ExplicitList x Nothing exps', fvs) }

    rnExpr (ExplicitTuple x tup_args boxity)
      = do { checkTupleSection tup_args
           ; checkTupSize (length tup_args)
           ; (tup_args', fvs) <- mapAndUnzipM rnTupArg tup_args
           ; return (ExplicitTuple x tup_args' boxity, plusFVs fvs) }
      where
        rnTupArg (L l (Present x e)) = do { (e',fvs) <- rnLExpr e
                                          ; return (L l (Present x e'), fvs) }
        rnTupArg (L l (Missing _)) = return (L l (Missing noExt)
                                            , emptyFVs)
        rnTupArg (L _ (XTupArg {})) = panic "rnExpr.XTupArg"

    rnExpr (ExplicitSum x alt arity expr)
      = do { (expr', fvs) <- rnLExpr expr
           ; return (ExplicitSum x alt arity expr', fvs) }

    rnExpr (RecordCon { rcon_con_name = con_id
                      , rcon_flds = rec_binds@(HsRecFields { rec_dotdot = dd }) })
      = do { con_lname@(L _ con_name) <- lookupLocatedOccRn con_id
           ; (flds, fvs)   <- rnHsRecFields (HsRecFieldCon con_name) mk_hs_var rec_binds
           ; (flds', fvss) <- mapAndUnzipM rn_field flds
           ; let rec_binds' = HsRecFields { rec_flds = flds', rec_dotdot = dd }
           ; return (RecordCon { rcon_ext = noExt
                               , rcon_con_name = con_lname, rcon_flds = rec_binds' }
                    , fvs `plusFV` plusFVs fvss `addOneFV` con_name) }
      where
        mk_hs_var l n = HsVar noExt (L l n)
        rn_field (L l fld) = do { (arg', fvs) <- rnLExpr (hsRecFieldArg fld)
                                ; return (L l (fld { hsRecFieldArg = arg' }), fvs) }

    rnExpr (RecordUpd { rupd_expr = expr, rupd_flds = rbinds })
      = do  { (expr', fvExpr) <- rnLExpr expr
            ; (rbinds', fvRbinds) <- rnHsRecUpdFields rbinds
            ; return (RecordUpd { rupd_ext = noExt, rupd_expr = expr'
                                , rupd_flds = rbinds' }
                     , fvExpr `plusFV` fvRbinds) }

    rnExpr (ExprWithTySig pty expr)
      = do  { (pty', fvTy)    <- rnHsSigWcType ExprWithTySigCtx pty
            ; (expr', fvExpr) <- bindSigTyVarsFV (hsWcScopedTvs pty') $
                                 rnLExpr expr
            ; return (ExprWithTySig pty' expr', fvExpr `plusFV` fvTy) }

    rnExpr (HsIf x _ p b1 b2)
      = do { (p', fvP) <- rnLExpr p
           ; (b1', fvB1) <- rnLExpr b1
           ; (b2', fvB2) <- rnLExpr b2
           ; (mb_ite, fvITE) <- lookupIfThenElse
           ; return (HsIf x mb_ite p' b1' b2', plusFVs [fvITE, fvP, fvB1, fvB2]) }

    rnExpr (HsMultiIf x alts)
      = do { (alts', fvs) <- mapFvRn (rnGRHS IfAlt rnLExpr) alts
           -- ; return (HsMultiIf ty alts', fvs) }
           ; return (HsMultiIf x alts', fvs) }

    rnExpr (ArithSeq x _ seq)
      = do { opt_OverloadedLists <- xoptM LangExt.OverloadedLists
           ; (new_seq, fvs) <- rnArithSeq seq
           ; if opt_OverloadedLists
               then do {
                ; (from_list_name, fvs') <- lookupSyntaxName fromListName
                ; return (ArithSeq x (Just from_list_name) new_seq
                         , fvs `plusFV` fvs') }
               else
                return (ArithSeq x Nothing new_seq, fvs) }

    {-
    These three are pattern syntax appearing in expressions.
    Since all the symbols are reservedops we can simply reject them.
    We return a (bogus) EWildPat in each case.
    -}

    rnExpr (EWildPat _)  = return (hsHoleExpr, emptyFVs)   -- "_" is just a hole
    rnExpr e@(EAsPat {})
      = do { opt_TypeApplications <- xoptM LangExt.TypeApplications
           ; let msg | opt_TypeApplications
                        = "Type application syntax requires a space before '@'"
                     | otherwise
                        = "Did you mean to enable TypeApplications?"
           ; patSynErr e (text msg)
           }
    rnExpr e@(EViewPat {}) = patSynErr e empty
    rnExpr e@(ELazyPat {}) = patSynErr e empty

    {-
    ************************************************************************
    *                                                                      *
            Static values
    *                                                                      *
    ************************************************************************

    For the static form we check that it is not used in splices.
    We also collect the free variables of the term which come from
    this module. See Note [Grand plan for static forms] in StaticPtrTable.
    -}

    rnExpr e@(HsStatic _ expr) = do
        -- Normally, you wouldn't be able to construct a static expression without
        -- first enabling -XStaticPointers in the first place, since that extension
        -- is what makes the parser treat `static` as a keyword. But this is not a
        -- sufficient safeguard, as one can construct static expressions by another
        -- mechanism: Template Haskell (see #14204). To ensure that GHC is
        -- absolutely prepared to cope with static forms, we check for
        -- -XStaticPointers here as well.
        unlessXOptM LangExt.StaticPointers $
          addErr $ hang (text "Illegal static expression:" <+> ppr e)
                      2 (text "Use StaticPointers to enable this extension")
        (expr',fvExpr) <- rnLExpr expr
        stage <- getStage
        case stage of
          Splice _ -> addErr $ sep
                 [ text "static forms cannot be used in splices:"
                 , nest 2 $ ppr e
                 ]
          _ -> return ()
        mod <- getModule
        let fvExpr' = filterNameSet (nameIsLocalOrFrom mod) fvExpr
        return (HsStatic fvExpr' expr', fvExpr)

    {-
    ************************************************************************
    *                                                                      *
            Arrow notation
    *                                                                      *
    ************************************************************************
    -}

    rnExpr (HsProc x pat body)
      = newArrowScope $
        rnPat ProcExpr pat $ \ pat' -> do
          { (body',fvBody) <- rnCmdTop body
          ; return (HsProc x pat' body', fvBody) }

    -- Ideally, these would be done in parsing, but to keep parsing simple, we do it here.
    rnExpr e@(HsArrApp {})  = arrowFail e
    rnExpr e@(HsArrForm {}) = arrowFail e

    rnExpr other = pprPanic "rnExpr: unexpected expression" (ppr other)
            -- HsWrap

    hsHoleExpr :: HsExpr (GhcPass id)
    hsHoleExpr = HsUnboundVar noExt (TrueExprHole (mkVarOcc "_"))

    arrowFail :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)
    arrowFail e
      = do { addErr (vcat [ text "Arrow command found where an expression was expected:"
                          , nest 2 (ppr e) ])
             -- Return a place-holder hole, so that we can carry on
             -- to report other errors
           ; return (hsHoleExpr, emptyFVs) }

    ----------------------
    -- See Note [Parsing sections] in Parser.y
    rnSection :: HsExpr GhcPs -> RnM (HsExpr GhcRn, FreeVars)
    rnSection section@(SectionR x op expr)
      = do  { (op', fvs_op)     <- rnLExpr op
            ; (expr', fvs_expr) <- rnLExpr expr
            ; checkSectionPrec InfixR section op' expr'
            ; return (SectionR x op' expr', fvs_op `plusFV` fvs_expr) }

    rnSection section@(SectionL x expr op)
      = do  { (expr', fvs_expr) <- rnLExpr expr
            ; (op', fvs_op)     <- rnLExpr op
            ; checkSectionPrec InfixL section op' expr'
            ; return (SectionL x expr' op', fvs_op `plusFV` fvs_expr) }

    rnSection other = pprPanic "rnSection" (ppr other)

    {-
    ************************************************************************
    *                                                                      *
            Arrow commands
    *                                                                      *
    ************************************************************************
    -}

    rnCmdArgs :: [LHsCmdTop GhcPs] -> RnM ([LHsCmdTop GhcRn], FreeVars)
    rnCmdArgs [] = return ([], emptyFVs)
    rnCmdArgs (arg:args)
      = do { (arg',fvArg) <- rnCmdTop arg
           ; (args',fvArgs) <- rnCmdArgs args
           ; return (arg':args', fvArg `plusFV` fvArgs) }

    rnCmdTop :: LHsCmdTop GhcPs -> RnM (LHsCmdTop GhcRn, FreeVars)
    rnCmdTop = wrapLocFstM rnCmdTop'
     where
      rnCmdTop' (HsCmdTop _ cmd)
       = do { (cmd', fvCmd) <- rnLCmd cmd
            ; let cmd_names = [arrAName, composeAName, firstAName] ++
                              nameSetElemsStable (methodNamesCmd (unLoc cmd'))
            -- Generate the rebindable syntax for the monad
            ; (cmd_names', cmd_fvs) <- lookupSyntaxNames cmd_names

            ; return (HsCmdTop (cmd_names `zip` cmd_names') cmd',
                      fvCmd `plusFV` cmd_fvs) }
      rnCmdTop' (XCmdTop{}) = panic "rnCmdTop"

    rnLCmd :: LHsCmd GhcPs -> RnM (LHsCmd GhcRn, FreeVars)
    rnLCmd = wrapLocFstM rnCmd

    rnCmd :: HsCmd GhcPs -> RnM (HsCmd GhcRn, FreeVars)

    rnCmd (HsCmdArrApp x arrow arg ho rtl)
      = do { (arrow',fvArrow) <- select_arrow_scope (rnLExpr arrow)
           ; (arg',fvArg) <- rnLExpr arg
           ; return (HsCmdArrApp x arrow' arg' ho rtl,
                     fvArrow `plusFV` fvArg) }
      where
        select_arrow_scope tc = case ho of
            HsHigherOrderApp -> tc
            HsFirstOrderApp  -> escapeArrowScope tc
            -- See Note [Escaping the arrow scope] in TcRnTypes
            -- Before renaming 'arrow', use the environment of the enclosing
            -- proc for the (-<) case.
            -- Local bindings, inside the enclosing proc, are not in scope
            -- inside 'arrow'.  In the higher-order case (-<<), they are.

    -- infix form
    rnCmd (HsCmdArrForm _ op _ (Just _) [arg1, arg2])
      = do { (op',fv_op) <- escapeArrowScope (rnLExpr op)
           ; let L _ (HsVar _ (L _ op_name)) = op'
           ; (arg1',fv_arg1) <- rnCmdTop arg1
           ; (arg2',fv_arg2) <- rnCmdTop arg2
            -- Deal with fixity
           ; fixity <- lookupFixityRn op_name
           ; final_e <- mkOpFormRn arg1' op' fixity arg2'
           ; return (final_e, fv_arg1 `plusFV` fv_op `plusFV` fv_arg2) }

    rnCmd (HsCmdArrForm x op f fixity cmds)
      = do { (op',fvOp) <- escapeArrowScope (rnLExpr op)
           ; (cmds',fvCmds) <- rnCmdArgs cmds
           ; return (HsCmdArrForm x op' f fixity cmds', fvOp `plusFV` fvCmds) }

    rnCmd (HsCmdApp x fun arg)
      = do { (fun',fvFun) <- rnLCmd  fun
           ; (arg',fvArg) <- rnLExpr arg
           ; return (HsCmdApp x fun' arg', fvFun `plusFV` fvArg) }

    rnCmd (HsCmdLam x matches)
      = do { (matches', fvMatch) <- rnMatchGroup LambdaExpr rnLCmd matches
           ; return (HsCmdLam x matches', fvMatch) }

    rnCmd (HsCmdPar x e)
      = do  { (e', fvs_e) <- rnLCmd e
            ; return (HsCmdPar x e', fvs_e) }

    rnCmd (HsCmdCase x expr matches)
      = do { (new_expr, e_fvs) <- rnLExpr expr
           ; (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLCmd matches
           ; return (HsCmdCase x new_expr new_matches, e_fvs `plusFV` ms_fvs) }

    rnCmd (HsCmdIf x _ p b1 b2)
      = do { (p', fvP) <- rnLExpr p
           ; (b1', fvB1) <- rnLCmd b1
           ; (b2', fvB2) <- rnLCmd b2
           ; (mb_ite, fvITE) <- lookupIfThenElse
           ; return (HsCmdIf x mb_ite p' b1' b2', plusFVs [fvITE, fvP, fvB1, fvB2])}

    rnCmd (HsCmdLet x (L l binds) cmd)
      = rnLocalBindsAndThen binds $ \ binds' _ -> do
          { (cmd',fvExpr) <- rnLCmd cmd
          ; return (HsCmdLet x (L l binds') cmd', fvExpr) }

    rnCmd (HsCmdDo x (L l stmts))
      = do  { ((stmts', _), fvs) <-
                rnStmts ArrowExpr rnLCmd stmts (\ _ -> return ((), emptyFVs))
            ; return ( HsCmdDo x (L l stmts'), fvs ) }

    rnCmd cmd@(HsCmdWrap {}) = pprPanic "rnCmd" (ppr cmd)
    rnCmd cmd@(XCmd {})      = pprPanic "rnCmd" (ppr cmd)

    ---------------------------------------------------
    type CmdNeeds = FreeVars        -- Only inhabitants are
                                    --      appAName, choiceAName, loopAName

    -- find what methods the Cmd needs (loop, choice, apply)
    methodNamesLCmd :: LHsCmd GhcRn -> CmdNeeds
    methodNamesLCmd = methodNamesCmd . unLoc

    methodNamesCmd :: HsCmd GhcRn -> CmdNeeds

    methodNamesCmd (HsCmdArrApp _ _arrow _arg HsFirstOrderApp _rtl)
      = emptyFVs
    methodNamesCmd (HsCmdArrApp _ _arrow _arg HsHigherOrderApp _rtl)
      = unitFV appAName
    methodNamesCmd (HsCmdArrForm {}) = emptyFVs
    methodNamesCmd (HsCmdWrap _ _ cmd) = methodNamesCmd cmd

    methodNamesCmd (HsCmdPar _ c) = methodNamesLCmd c

    methodNamesCmd (HsCmdIf _ _ _ c1 c2)
      = methodNamesLCmd c1 `plusFV` methodNamesLCmd c2 `addOneFV` choiceAName

    methodNamesCmd (HsCmdLet _ _ c)          = methodNamesLCmd c
    methodNamesCmd (HsCmdDo _ (L _ stmts))   = methodNamesStmts stmts
    methodNamesCmd (HsCmdApp _ c _)          = methodNamesLCmd c
    methodNamesCmd (HsCmdLam _ match)        = methodNamesMatch match

    methodNamesCmd (HsCmdCase _ _ matches)
      = methodNamesMatch matches `addOneFV` choiceAName

    methodNamesCmd (XCmd {}) = panic "methodNamesCmd"

    --methodNamesCmd _ = emptyFVs
       -- Other forms can't occur in commands, but it's not convenient
       -- to error here so we just do what's convenient.
       -- The type checker will complain later

    ---------------------------------------------------
    methodNamesMatch :: MatchGroup GhcRn (LHsCmd GhcRn) -> FreeVars
    methodNamesMatch (MG { mg_alts = L _ ms })
      = plusFVs (map do_one ms)
     where
        do_one (L _ (Match { m_grhss = grhss })) = methodNamesGRHSs grhss
        do_one (L _ (XMatch _)) = panic "methodNamesMatch.XMatch"
    methodNamesMatch (XMatchGroup _) = panic "methodNamesMatch"

    -------------------------------------------------
    -- gaw 2004
    methodNamesGRHSs :: GRHSs GhcRn (LHsCmd GhcRn) -> FreeVars
    methodNamesGRHSs (GRHSs _ grhss _) = plusFVs (map methodNamesGRHS grhss)
    methodNamesGRHSs (XGRHSs _) = panic "methodNamesGRHSs"

    -------------------------------------------------

    methodNamesGRHS :: Located (GRHS GhcRn (LHsCmd GhcRn)) -> CmdNeeds
    methodNamesGRHS (L _ (GRHS _ _ rhs)) = methodNamesLCmd rhs
    methodNamesGRHS (L _ (XGRHS _)) = panic "methodNamesGRHS"

    ---------------------------------------------------
    methodNamesStmts :: [Located (StmtLR GhcRn GhcRn (LHsCmd GhcRn))] -> FreeVars
    methodNamesStmts stmts = plusFVs (map methodNamesLStmt stmts)

    ---------------------------------------------------
    methodNamesLStmt :: Located (StmtLR GhcRn GhcRn (LHsCmd GhcRn)) -> FreeVars
    methodNamesLStmt = methodNamesStmt . unLoc

    methodNamesStmt :: StmtLR GhcRn GhcRn (LHsCmd GhcRn) -> FreeVars
    methodNamesStmt (LastStmt _ cmd _ _)           = methodNamesLCmd cmd
    methodNamesStmt (BodyStmt _ cmd _ _)           = methodNamesLCmd cmd
    methodNamesStmt (BindStmt _ _ cmd _ _)         = methodNamesLCmd cmd
    methodNamesStmt (RecStmt { recS_stmts = stmts }) =
      methodNamesStmts stmts `addOneFV` loopAName
    methodNamesStmt (LetStmt {})                   = emptyFVs
    methodNamesStmt (ParStmt {})                   = emptyFVs
    methodNamesStmt (TransStmt {})                 = emptyFVs
    methodNamesStmt ApplicativeStmt{}              = emptyFVs
       -- ParStmt and TransStmt can't occur in commands, but it's not
       -- convenient to error here so we just do what's convenient
    methodNamesStmt (XStmtLR {}) = panic "methodNamesStmt"

    {-
    ************************************************************************
    *                                                                      *
            Arithmetic sequences
    *                                                                      *
    ************************************************************************
    -}

    rnArithSeq :: ArithSeqInfo GhcPs -> RnM (ArithSeqInfo GhcRn, FreeVars)
    rnArithSeq (From expr)
     = do { (expr', fvExpr) <- rnLExpr expr
          ; return (From expr', fvExpr) }

    rnArithSeq (FromThen expr1 expr2)
     = do { (expr1', fvExpr1) <- rnLExpr expr1
          ; (expr2', fvExpr2) <- rnLExpr expr2
          ; return (FromThen expr1' expr2', fvExpr1 `plusFV` fvExpr2) }

    rnArithSeq (FromTo expr1 expr2)
     = do { (expr1', fvExpr1) <- rnLExpr expr1
          ; (expr2', fvExpr2) <- rnLExpr expr2
          ; return (FromTo expr1' expr2', fvExpr1 `plusFV` fvExpr2) }

    rnArithSeq (FromThenTo expr1 expr2 expr3)
     = do { (expr1', fvExpr1) <- rnLExpr expr1
          ; (expr2', fvExpr2) <- rnLExpr expr2
          ; (expr3', fvExpr3) <- rnLExpr expr3
          ; return (FromThenTo expr1' expr2' expr3',
                    plusFVs [fvExpr1, fvExpr2, fvExpr3]) }

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{@Stmt@s: in @do@ expressions}
    *                                                                      *
    ************************************************************************
    -}

    {-
    Note [Deterministic ApplicativeDo and RecursiveDo desugaring]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Both ApplicativeDo and RecursiveDo need to create tuples not
    present in the source text.

    For ApplicativeDo we create:

      (a,b,c) <- (\c b a -> (a,b,c)) <$>

    For RecursiveDo we create:

      mfix (\ ~(a,b,c) -> do ...; return (a',b',c'))

    The order of the components in those tuples needs to be stable
    across recompilations, otherwise they can get optimized differently
    and we end up with incompatible binaries.
    To get a stable order we use nameSetElemsStable.
    See Note [Deterministic UniqFM] to learn more about nondeterminism.
    -}

    -- | Rename some Stmts
    rnStmts :: Outputable (body GhcPs)
            => HsStmtContext Name
            -> (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
               -- ^ How to rename the body of each statement (e.g. rnLExpr)
            -> [LStmt GhcPs (Located (body GhcPs))]
               -- ^ Statements
            -> ([Name] -> RnM (thing, FreeVars))
               -- ^ if these statements scope over something, this renames it
               -- and returns the result.
            -> RnM (([LStmt GhcRn (Located (body GhcRn))], thing), FreeVars)
    rnStmts ctxt rnBody = rnStmtsWithPostProcessing ctxt rnBody noPostProcessStmts

    -- | like 'rnStmts' but applies a post-processing step to the renamed Stmts
    rnStmtsWithPostProcessing
            :: Outputable (body GhcPs)
            => HsStmtContext Name
            -> (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
               -- ^ How to rename the body of each statement (e.g. rnLExpr)
            -> (HsStmtContext Name
                  -> [(LStmt GhcRn (Located (body GhcRn)), FreeVars)]
                  -> RnM ([LStmt GhcRn (Located (body GhcRn))], FreeVars))
               -- ^ postprocess the statements
            -> [LStmt GhcPs (Located (body GhcPs))]
               -- ^ Statements
            -> ([Name] -> RnM (thing, FreeVars))
               -- ^ if these statements scope over something, this renames it
               -- and returns the result.
            -> RnM (([LStmt GhcRn (Located (body GhcRn))], thing), FreeVars)
    rnStmtsWithPostProcessing ctxt rnBody ppStmts stmts thing_inside
     = do { ((stmts', thing), fvs) <-
              rnStmtsWithFreeVars ctxt rnBody stmts thing_inside
          ; (pp_stmts, fvs') <- ppStmts ctxt stmts'
          ; return ((pp_stmts, thing), fvs `plusFV` fvs')
          }

    -- | maybe rearrange statements according to the ApplicativeDo transformation
    postProcessStmtsForApplicativeDo
      :: HsStmtContext Name
      -> [(ExprLStmt GhcRn, FreeVars)]
      -> RnM ([ExprLStmt GhcRn], FreeVars)
    postProcessStmtsForApplicativeDo ctxt stmts
      = do {
           -- rearrange the statements using ApplicativeStmt if
           -- -XApplicativeDo is on.  Also strip out the FreeVars attached
           -- to each Stmt body.
             ado_is_on <- xoptM LangExt.ApplicativeDo
           ; let is_do_expr | DoExpr <- ctxt = True
                            | otherwise = False
           ; if ado_is_on && is_do_expr
                then do { traceRn "ppsfa" (ppr stmts)
                        ; rearrangeForApplicativeDo ctxt stmts }
                else noPostProcessStmts ctxt stmts }

    -- | strip the FreeVars annotations from statements
    noPostProcessStmts
      :: HsStmtContext Name
      -> [(LStmt GhcRn (Located (body GhcRn)), FreeVars)]
      -> RnM ([LStmt GhcRn (Located (body GhcRn))], FreeVars)
    noPostProcessStmts _ stmts = return (map fst stmts, emptyNameSet)

    rnStmtsWithFreeVars :: Outputable (body GhcPs)
            => HsStmtContext Name
            -> (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
            -> [LStmt GhcPs (Located (body GhcPs))]
            -> ([Name] -> RnM (thing, FreeVars))
            -> RnM ( ([(LStmt GhcRn (Located (body GhcRn)), FreeVars)], thing)
                   , FreeVars)
    -- Each Stmt body is annotated with its FreeVars, so that
    -- we can rearrange statements for ApplicativeDo.
    --
    -- Variables bound by the Stmts, and mentioned in thing_inside,
    -- do not appear in the result FreeVars

    rnStmtsWithFreeVars ctxt _ [] thing_inside
      = do { checkEmptyStmts ctxt
           ; (thing, fvs) <- thing_inside []
           ; return (([], thing), fvs) }

    rnStmtsWithFreeVars MDoExpr rnBody stmts thing_inside    -- Deal with mdo
      = -- Behave like do { rec { ...all but last... }; last }
        do { ((stmts1, (stmts2, thing)), fvs)
               <- rnStmt MDoExpr rnBody (noLoc $ mkRecStmt all_but_last) $ \ _ ->
                  do { last_stmt' <- checkLastStmt MDoExpr last_stmt
                     ; rnStmt MDoExpr rnBody last_stmt' thing_inside }
            ; return (((stmts1 ++ stmts2), thing), fvs) }
      where
        Just (all_but_last, last_stmt) = snocView stmts

    rnStmtsWithFreeVars ctxt rnBody (lstmt@(L loc _) : lstmts) thing_inside
      | null lstmts
      = setSrcSpan loc $
        do { lstmt' <- checkLastStmt ctxt lstmt
           ; rnStmt ctxt rnBody lstmt' thing_inside }

      | otherwise
      = do { ((stmts1, (stmts2, thing)), fvs)
                <- setSrcSpan loc                         $
                   do { checkStmt ctxt lstmt
                      ; rnStmt ctxt rnBody lstmt    $ \ bndrs1 ->
                        rnStmtsWithFreeVars ctxt rnBody lstmts  $ \ bndrs2 ->
                        thing_inside (bndrs1 ++ bndrs2) }
            ; return (((stmts1 ++ stmts2), thing), fvs) }

    ----------------------

    {-
    Note [Failing pattern matches in Stmts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Many things desugar to HsStmts including monadic things like `do` and `mdo`
    statements, pattern guards, and list comprehensions (see 'HsStmtContext' for an
    exhaustive list). How we deal with pattern match failure is context-dependent.

     * In the case of list comprehensions and pattern guards we don't need any 'fail'
       function; the desugarer ignores the fail function field of 'BindStmt' entirely.
     * In the case of monadic contexts (e.g. monad comprehensions, do, and mdo
       expressions) we want pattern match failure to be desugared to the appropriate
       'fail' function (either that of Monad or MonadFail, depending on whether
       -XMonadFailDesugaring is enabled.)

    At one point we failed to make this distinction, leading to #11216.
    -}

    rnStmt :: Outputable (body GhcPs)
           => HsStmtContext Name
           -> (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
              -- ^ How to rename the body of the statement
           -> LStmt GhcPs (Located (body GhcPs))
              -- ^ The statement
           -> ([Name] -> RnM (thing, FreeVars))
              -- ^ Rename the stuff that this statement scopes over
           -> RnM ( ([(LStmt GhcRn (Located (body GhcRn)), FreeVars)], thing)
                  , FreeVars)
    -- Variables bound by the Stmt, and mentioned in thing_inside,
    -- do not appear in the result FreeVars

    rnStmt ctxt rnBody (L loc (LastStmt _ body noret _)) thing_inside
      = do  { (body', fv_expr) <- rnBody body
            ; (ret_op, fvs1)   <- lookupStmtName ctxt returnMName
            ; (thing,  fvs3)   <- thing_inside []
            ; return (([(L loc (LastStmt noExt body' noret ret_op), fv_expr)]
                      , thing), fv_expr `plusFV` fvs1 `plusFV` fvs3) }

    rnStmt ctxt rnBody (L loc (BodyStmt _ body _ _)) thing_inside
      = do  { (body', fv_expr) <- rnBody body
            ; (then_op, fvs1)  <- lookupStmtName ctxt thenMName
            ; (guard_op, fvs2) <- if isListCompExpr ctxt
                                  then lookupStmtName ctxt guardMName
                                  else return (noSyntaxExpr, emptyFVs)
                                  -- Only list/monad comprehensions use 'guard'
                                  -- Also for sub-stmts of same eg [ e | x<-xs, gd | blah ]
                                  -- Here "gd" is a guard
            ; (thing, fvs3)    <- thing_inside []
            ; return ( ([(L loc (BodyStmt noExt body' then_op guard_op), fv_expr)]
                      , thing), fv_expr `plusFV` fvs1 `plusFV` fvs2 `plusFV` fvs3) }

    rnStmt ctxt rnBody (L loc (BindStmt _ pat body _ _)) thing_inside
      = do  { (body', fv_expr) <- rnBody body
                    -- The binders do not scope over the expression
            ; (bind_op, fvs1) <- lookupStmtName ctxt bindMName

            ; xMonadFailEnabled <- fmap (xopt LangExt.MonadFailDesugaring) getDynFlags
            ; let getFailFunction
                    -- If the pattern is irrefutable (e.g.: wildcard, tuple,
                    -- ~pat, etc.) we should not need to fail.
                    | isIrrefutableHsPat pat
                                        = return (noSyntaxExpr, emptyFVs)
                    -- For non-monadic contexts (e.g. guard patterns, list
                    -- comprehensions, etc.) we should not need to fail.
                    -- See Note [Failing pattern matches in Stmts]
                    | not (isMonadFailStmtContext ctxt)
                                        = return (noSyntaxExpr, emptyFVs)
                    | xMonadFailEnabled = lookupSyntaxName failMName
                    | otherwise         = lookupSyntaxName failMName_preMFP
            ; (fail_op, fvs2) <- getFailFunction

            ; rnPat (StmtCtxt ctxt) pat $ \ pat' -> do
            { (thing, fvs3) <- thing_inside (collectPatBinders pat')
            ; return (( [( L loc (BindStmt noExt pat' body' bind_op fail_op)
                         , fv_expr )]
                      , thing),
                      fv_expr `plusFV` fvs1 `plusFV` fvs2 `plusFV` fvs3) }}
           -- fv_expr shouldn't really be filtered by the rnPatsAndThen
            -- but it does not matter because the names are unique

    rnStmt _ _ (L loc (LetStmt _ (L l binds))) thing_inside
      = do  { rnLocalBindsAndThen binds $ \binds' bind_fvs -> do
            { (thing, fvs) <- thing_inside (collectLocalBinders binds')
            ; return ( ([(L loc (LetStmt noExt (L l binds')), bind_fvs)], thing)
                     , fvs) }  }

    rnStmt ctxt rnBody (L loc (RecStmt { recS_stmts = rec_stmts })) thing_inside
      = do  { (return_op, fvs1)  <- lookupStmtName ctxt returnMName
            ; (mfix_op,   fvs2)  <- lookupStmtName ctxt mfixName
            ; (bind_op,   fvs3)  <- lookupStmtName ctxt bindMName
            ; let empty_rec_stmt = emptyRecStmtName { recS_ret_fn  = return_op
                                                    , recS_mfix_fn = mfix_op
                                                    , recS_bind_fn = bind_op }

            -- Step1: Bring all the binders of the mdo into scope
            -- (Remember that this also removes the binders from the
            -- finally-returned free-vars.)
            -- And rename each individual stmt, making a
            -- singleton segment.  At this stage the FwdRefs field
            -- isn't finished: it's empty for all except a BindStmt
            -- for which it's the fwd refs within the bind itself
            -- (This set may not be empty, because we're in a recursive
            -- context.)
            ; rnRecStmtsAndThen rnBody rec_stmts   $ \ segs -> do
            { let bndrs = nameSetElemsStable $
                            foldr (unionNameSet . (\(ds,_,_,_) -> ds))
                                  emptyNameSet
                                  segs
              -- See Note [Deterministic ApplicativeDo and RecursiveDo desugaring]
            ; (thing, fvs_later) <- thing_inside bndrs
            ; let (rec_stmts', fvs) = segmentRecStmts loc ctxt empty_rec_stmt segs fvs_later
            -- We aren't going to try to group RecStmts with
            -- ApplicativeDo, so attaching empty FVs is fine.
            ; return ( ((zip rec_stmts' (repeat emptyNameSet)), thing)
                     , fvs `plusFV` fvs1 `plusFV` fvs2 `plusFV` fvs3) } }

    rnStmt ctxt _ (L loc (ParStmt _ segs _ _)) thing_inside
      = do  { (mzip_op, fvs1)   <- lookupStmtNamePoly ctxt mzipName
            ; (bind_op, fvs2)   <- lookupStmtName ctxt bindMName
            ; (return_op, fvs3) <- lookupStmtName ctxt returnMName
            ; ((segs', thing), fvs4) <- rnParallelStmts (ParStmtCtxt ctxt) return_op segs thing_inside
            ; return (([(L loc (ParStmt noExt segs' mzip_op bind_op), fvs4)], thing)
                     , fvs1 `plusFV` fvs2 `plusFV` fvs3 `plusFV` fvs4) }

    rnStmt ctxt _ (L loc (TransStmt { trS_stmts = stmts, trS_by = by, trS_form = form
                                  , trS_using = using })) thing_inside
      = do { -- Rename the 'using' expression in the context before the transform is begun
             (using', fvs1) <- rnLExpr using

             -- Rename the stmts and the 'by' expression
             -- Keep track of the variables mentioned in the 'by' expression
           ; ((stmts', (by', used_bndrs, thing)), fvs2)
                 <- rnStmts (TransStmtCtxt ctxt) rnLExpr stmts $ \ bndrs ->
                    do { (by',   fvs_by) <- mapMaybeFvRn rnLExpr by
                       ; (thing, fvs_thing) <- thing_inside bndrs
                       ; let fvs = fvs_by `plusFV` fvs_thing
                             used_bndrs = filter (`elemNameSet` fvs) bndrs
                             -- The paper (Fig 5) has a bug here; we must treat any free variable
                             -- of the "thing inside", **or of the by-expression**, as used
                       ; return ((by', used_bndrs, thing), fvs) }

           -- Lookup `return`, `(>>=)` and `liftM` for monad comprehensions
           ; (return_op, fvs3) <- lookupStmtName ctxt returnMName
           ; (bind_op,   fvs4) <- lookupStmtName ctxt bindMName
           ; (fmap_op,   fvs5) <- case form of
                                    ThenForm -> return (noExpr, emptyFVs)
                                    _        -> lookupStmtNamePoly ctxt fmapName

           ; let all_fvs  = fvs1 `plusFV` fvs2 `plusFV` fvs3
                                 `plusFV` fvs4 `plusFV` fvs5
                 bndr_map = used_bndrs `zip` used_bndrs
                 -- See Note [TransStmt binder map] in HsExpr

           ; traceRn "rnStmt: implicitly rebound these used binders:" (ppr bndr_map)
           ; return (([(L loc (TransStmt { trS_ext = noExt
                                        , trS_stmts = stmts', trS_bndrs = bndr_map
                                        , trS_by = by', trS_using = using', trS_form = form
                                        , trS_ret = return_op, trS_bind = bind_op
                                        , trS_fmap = fmap_op }), fvs2)], thing), all_fvs) }

    rnStmt _ _ (L _ ApplicativeStmt{}) _ =
      panic "rnStmt: ApplicativeStmt"

    rnStmt _ _ (L _ XStmtLR{}) _ =
      panic "rnStmt: XStmtLR"

    rnParallelStmts :: forall thing. HsStmtContext Name
                    -> SyntaxExpr GhcRn
                    -> [ParStmtBlock GhcPs GhcPs]
                    -> ([Name] -> RnM (thing, FreeVars))
                    -> RnM (([ParStmtBlock GhcRn GhcRn], thing), FreeVars)
    -- Note [Renaming parallel Stmts]
    rnParallelStmts ctxt return_op segs thing_inside
      = do { orig_lcl_env <- getLocalRdrEnv
           ; rn_segs orig_lcl_env [] segs }
      where
        rn_segs :: LocalRdrEnv
                -> [Name] -> [ParStmtBlock GhcPs GhcPs]
                -> RnM (([ParStmtBlock GhcRn GhcRn], thing), FreeVars)
        rn_segs _ bndrs_so_far []
          = do { let (bndrs', dups) = removeDups cmpByOcc bndrs_so_far
               ; mapM_ dupErr dups
               ; (thing, fvs) <- bindLocalNames bndrs' (thing_inside bndrs')
               ; return (([], thing), fvs) }

        rn_segs env bndrs_so_far (ParStmtBlock x stmts _ _ : segs)
          = do { ((stmts', (used_bndrs, segs', thing)), fvs)
                        <- rnStmts ctxt rnLExpr stmts $ \ bndrs ->
                           setLocalRdrEnv env       $ do
                           { ((segs', thing), fvs) <- rn_segs env (bndrs ++ bndrs_so_far) segs
                           ; let used_bndrs = filter (`elemNameSet` fvs) bndrs
                           ; return ((used_bndrs, segs', thing), fvs) }

               ; let seg' = ParStmtBlock x stmts' used_bndrs return_op
               ; return ((seg':segs', thing), fvs) }
        rn_segs _ _ (XParStmtBlock{}:_) = panic "rnParallelStmts"

        cmpByOcc n1 n2 = nameOccName n1 `compare` nameOccName n2
        dupErr vs = addErr (text "Duplicate binding in parallel list comprehension for:"
                        <+> quotes (ppr (NE.head vs)))

    lookupStmtName :: HsStmtContext Name -> Name -> RnM (SyntaxExpr GhcRn, FreeVars)
    -- Like lookupSyntaxName, but respects contexts
    lookupStmtName ctxt n
      | rebindableContext ctxt
      = lookupSyntaxName n
      | otherwise
      = return (mkRnSyntaxExpr n, emptyFVs)

    lookupStmtNamePoly :: HsStmtContext Name -> Name -> RnM (HsExpr GhcRn, FreeVars)
    lookupStmtNamePoly ctxt name
      | rebindableContext ctxt
      = do { rebindable_on <- xoptM LangExt.RebindableSyntax
           ; if rebindable_on
             then do { fm <- lookupOccRn (nameRdrName name)
                     ; return (HsVar noExt (noLoc fm), unitFV fm) }
             else not_rebindable }
      | otherwise
      = not_rebindable
      where
        not_rebindable = return (HsVar noExt (noLoc name), emptyFVs)

    -- | Is this a context where we respect RebindableSyntax?
    -- but ListComp are never rebindable
    -- Neither is ArrowExpr, which has its own desugarer in DsArrows
    rebindableContext :: HsStmtContext Name -> Bool
    rebindableContext ctxt = case ctxt of
      ListComp        -> False
      ArrowExpr       -> False
      PatGuard {}     -> False

      DoExpr          -> True
      MDoExpr         -> True
      MonadComp       -> True
      GhciStmtCtxt    -> True   -- I suppose?

      ParStmtCtxt   c -> rebindableContext c     -- Look inside to
      TransStmtCtxt c -> rebindableContext c     -- the parent context

    {-
    Note [Renaming parallel Stmts]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Renaming parallel statements is painful.  Given, say
         [ a+c | a <- as, bs <- bss
               | c <- bs, a <- ds ]
    Note that
      (a) In order to report "Defined but not used" about 'bs', we must
          rename each group of Stmts with a thing_inside whose FreeVars
          include at least {a,c}

      (b) We want to report that 'a' is illegally bound in both branches

      (c) The 'bs' in the second group must obviously not be captured by
          the binding in the first group

    To satisfy (a) we nest the segements.
    To satisfy (b) we check for duplicates just before thing_inside.
    To satisfy (c) we reset the LocalRdrEnv each time.

    ************************************************************************
    *                                                                      *
    \subsubsection{mdo expressions}
    *                                                                      *
    ************************************************************************
    -}

    type FwdRefs = NameSet
    type Segment stmts = (Defs,
                          Uses,     -- May include defs
                          FwdRefs,  -- A subset of uses that are
                                    --   (a) used before they are bound in this segment, or
                                    --   (b) used here, and bound in subsequent segments
                          stmts)    -- Either Stmt or [Stmt]

    -- wrapper that does both the left- and right-hand sides
    rnRecStmtsAndThen :: Outputable (body GhcPs) =>
                         (Located (body GhcPs)
                      -> RnM (Located (body GhcRn), FreeVars))
                      -> [LStmt GhcPs (Located (body GhcPs))]
                             -- assumes that the FreeVars returned includes
                             -- the FreeVars of the Segments
                      -> ([Segment (LStmt GhcRn (Located (body GhcRn)))]
                          -> RnM (a, FreeVars))
                      -> RnM (a, FreeVars)
    rnRecStmtsAndThen rnBody s cont
      = do  { -- (A) Make the mini fixity env for all of the stmts
              fix_env <- makeMiniFixityEnv (collectRecStmtsFixities s)

              -- (B) Do the LHSes
            ; new_lhs_and_fv <- rn_rec_stmts_lhs fix_env s

              --    ...bring them and their fixities into scope
            ; let bound_names = collectLStmtsBinders (map fst new_lhs_and_fv)
                  -- Fake uses of variables introduced implicitly (warning suppression, see #4404)
                  implicit_uses = lStmtsImplicits (map fst new_lhs_and_fv)
            ; bindLocalNamesFV bound_names $
              addLocalFixities fix_env bound_names $ do

              -- (C) do the right-hand-sides and thing-inside
            { segs <- rn_rec_stmts rnBody bound_names new_lhs_and_fv
            ; (res, fvs) <- cont segs
            ; warnUnusedLocalBinds bound_names (fvs `unionNameSet` implicit_uses)
            ; return (res, fvs) }}

    -- get all the fixity decls in any Let stmt
    collectRecStmtsFixities :: [LStmtLR GhcPs GhcPs body] -> [LFixitySig GhcPs]
    collectRecStmtsFixities l =
        foldr (\ s -> \acc -> case s of
                (L _ (LetStmt _ (L _ (HsValBinds _ (ValBinds _ _ sigs))))) ->
                  foldr (\ sig -> \ acc -> case sig of
                                             (L loc (FixSig _ s)) -> (L loc s) : acc
                                             _ -> acc) acc sigs
                _ -> acc) [] l

    -- left-hand sides

    rn_rec_stmt_lhs :: Outputable body => MiniFixityEnv
                    -> LStmt GhcPs body
                       -- rename LHS, and return its FVs
                       -- Warning: we will only need the FreeVars below in the case of a BindStmt,
                       -- so we don't bother to compute it accurately in the other cases
                    -> RnM [(LStmtLR GhcRn GhcPs body, FreeVars)]

    rn_rec_stmt_lhs _ (L loc (BodyStmt _ body a b))
      = return [(L loc (BodyStmt noExt body a b), emptyFVs)]

    rn_rec_stmt_lhs _ (L loc (LastStmt _ body noret a))
      = return [(L loc (LastStmt noExt body noret a), emptyFVs)]

    rn_rec_stmt_lhs fix_env (L loc (BindStmt _ pat body a b))
      = do
          -- should the ctxt be MDo instead?
          (pat', fv_pat) <- rnBindPat (localRecNameMaker fix_env) pat
          return [(L loc (BindStmt noExt pat' body a b), fv_pat)]

    rn_rec_stmt_lhs _ (L _ (LetStmt _ (L _ binds@(HsIPBinds {}))))
      = failWith (badIpBinds (text "an mdo expression") binds)

    rn_rec_stmt_lhs fix_env (L loc (LetStmt _ (L l (HsValBinds x binds))))
        = do (_bound_names, binds') <- rnLocalValBindsLHS fix_env binds
             return [(L loc (LetStmt noExt (L l (HsValBinds x binds'))),
                     -- Warning: this is bogus; see function invariant
                     emptyFVs
                     )]

    -- XXX Do we need to do something with the return and mfix names?
    rn_rec_stmt_lhs fix_env (L _ (RecStmt { recS_stmts = stmts }))  -- Flatten Rec inside Rec
        = rn_rec_stmts_lhs fix_env stmts

    rn_rec_stmt_lhs _ stmt@(L _ (ParStmt {}))       -- Syntactically illegal in mdo
      = pprPanic "rn_rec_stmt" (ppr stmt)

    rn_rec_stmt_lhs _ stmt@(L _ (TransStmt {}))     -- Syntactically illegal in mdo
      = pprPanic "rn_rec_stmt" (ppr stmt)

    rn_rec_stmt_lhs _ stmt@(L _ (ApplicativeStmt {})) -- Shouldn't appear yet
      = pprPanic "rn_rec_stmt" (ppr stmt)

    rn_rec_stmt_lhs _ (L _ (LetStmt _ (L _ (EmptyLocalBinds _))))
      = panic "rn_rec_stmt LetStmt EmptyLocalBinds"
    rn_rec_stmt_lhs _ (L _ (LetStmt _ (L _ (XHsLocalBindsLR _))))
      = panic "rn_rec_stmt LetStmt XHsLocalBindsLR"
    rn_rec_stmt_lhs _ (L _ (XStmtLR _))
      = panic "rn_rec_stmt XStmtLR"

    rn_rec_stmts_lhs :: Outputable body => MiniFixityEnv
                     -> [LStmt GhcPs body]
                     -> RnM [(LStmtLR GhcRn GhcPs body, FreeVars)]
    rn_rec_stmts_lhs fix_env stmts
      = do { ls <- concatMapM (rn_rec_stmt_lhs fix_env) stmts
           ; let boundNames = collectLStmtsBinders (map fst ls)
                -- First do error checking: we need to check for dups here because we
                -- don't bind all of the variables from the Stmt at once
                -- with bindLocatedLocals.
           ; checkDupNames boundNames
           ; return ls }

    -- right-hand-sides

    rn_rec_stmt :: (Outputable (body GhcPs)) =>
                   (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
                -> [Name]
                -> (LStmtLR GhcRn GhcPs (Located (body GhcPs)), FreeVars)
                -> RnM [Segment (LStmt GhcRn (Located (body GhcRn)))]
            -- Rename a Stmt that is inside a RecStmt (or mdo)
            -- Assumes all binders are already in scope
            -- Turns each stmt into a singleton Stmt
    rn_rec_stmt rnBody _ (L loc (LastStmt _ body noret _), _)
      = do  { (body', fv_expr) <- rnBody body
            ; (ret_op, fvs1)   <- lookupSyntaxName returnMName
            ; return [(emptyNameSet, fv_expr `plusFV` fvs1, emptyNameSet,
                       L loc (LastStmt noExt body' noret ret_op))] }

    rn_rec_stmt rnBody _ (L loc (BodyStmt _ body _ _), _)
      = do { (body', fvs) <- rnBody body
           ; (then_op, fvs1) <- lookupSyntaxName thenMName
           ; return [(emptyNameSet, fvs `plusFV` fvs1, emptyNameSet,
                     L loc (BodyStmt noExt body' then_op noSyntaxExpr))] }

    rn_rec_stmt rnBody _ (L loc (BindStmt _ pat' body _ _), fv_pat)
      = do { (body', fv_expr) <- rnBody body
           ; (bind_op, fvs1) <- lookupSyntaxName bindMName

           ; xMonadFailEnabled <- fmap (xopt LangExt.MonadFailDesugaring) getDynFlags
           ; let failFunction | xMonadFailEnabled = failMName
                              | otherwise         = failMName_preMFP
           ; (fail_op, fvs2) <- lookupSyntaxName failFunction

           ; let bndrs = mkNameSet (collectPatBinders pat')
                 fvs   = fv_expr `plusFV` fv_pat `plusFV` fvs1 `plusFV` fvs2
           ; return [(bndrs, fvs, bndrs `intersectNameSet` fvs,
                      L loc (BindStmt noExt pat' body' bind_op fail_op))] }

    rn_rec_stmt _ _ (L _ (LetStmt _ (L _ binds@(HsIPBinds {}))), _)
      = failWith (badIpBinds (text "an mdo expression") binds)

    rn_rec_stmt _ all_bndrs (L loc (LetStmt _ (L l (HsValBinds x binds'))), _)
      = do { (binds', du_binds) <- rnLocalValBindsRHS (mkNameSet all_bndrs) binds'
               -- fixities and unused are handled above in rnRecStmtsAndThen
           ; let fvs = allUses du_binds
           ; return [(duDefs du_binds, fvs, emptyNameSet,
                     L loc (LetStmt noExt (L l (HsValBinds x binds'))))] }

    -- no RecStmt case because they get flattened above when doing the LHSes
    rn_rec_stmt _ _ stmt@(L _ (RecStmt {}), _)
      = pprPanic "rn_rec_stmt: RecStmt" (ppr stmt)

    rn_rec_stmt _ _ stmt@(L _ (ParStmt {}), _)       -- Syntactically illegal in mdo
      = pprPanic "rn_rec_stmt: ParStmt" (ppr stmt)

    rn_rec_stmt _ _ stmt@(L _ (TransStmt {}), _)     -- Syntactically illegal in mdo
      = pprPanic "rn_rec_stmt: TransStmt" (ppr stmt)

    rn_rec_stmt _ _ (L _ (LetStmt _ (L _ (XHsLocalBindsLR _))), _)
      = panic "rn_rec_stmt: LetStmt XHsLocalBindsLR"

    rn_rec_stmt _ _ (L _ (LetStmt _ (L _ (EmptyLocalBinds _))), _)
      = panic "rn_rec_stmt: LetStmt EmptyLocalBinds"

    rn_rec_stmt _ _ stmt@(L _ (ApplicativeStmt {}), _)
      = pprPanic "rn_rec_stmt: ApplicativeStmt" (ppr stmt)

    rn_rec_stmt _ _ stmt@(L _ (XStmtLR {}), _)
      = pprPanic "rn_rec_stmt: XStmtLR" (ppr stmt)

    rn_rec_stmts :: Outputable (body GhcPs) =>
                    (Located (body GhcPs) -> RnM (Located (body GhcRn), FreeVars))
                 -> [Name]
                 -> [(LStmtLR GhcRn GhcPs (Located (body GhcPs)), FreeVars)]
                 -> RnM [Segment (LStmt GhcRn (Located (body GhcRn)))]
    rn_rec_stmts rnBody bndrs stmts
      = do { segs_s <- mapM (rn_rec_stmt rnBody bndrs) stmts
           ; return (concat segs_s) }

    ---------------------------------------------
    segmentRecStmts :: SrcSpan -> HsStmtContext Name
                    -> Stmt GhcRn body
                    -> [Segment (LStmt GhcRn body)] -> FreeVars
                    -> ([LStmt GhcRn body], FreeVars)

    segmentRecStmts loc ctxt empty_rec_stmt segs fvs_later
      | null segs
      = ([], fvs_later)

      | MDoExpr <- ctxt
      = segsToStmts empty_rec_stmt grouped_segs fvs_later
                   -- Step 4: Turn the segments into Stmts
                    --         Use RecStmt when and only when there are fwd refs
                    --         Also gather up the uses from the end towards the
                    --         start, so we can tell the RecStmt which things are
                    --         used 'after' the RecStmt

      | otherwise
      = ([ L loc $
           empty_rec_stmt { recS_stmts = ss
                          , recS_later_ids = nameSetElemsStable
                                               (defs `intersectNameSet` fvs_later)
                          , recS_rec_ids   = nameSetElemsStable
                                               (defs `intersectNameSet` uses) }]
              -- See Note [Deterministic ApplicativeDo and RecursiveDo desugaring]
        , uses `plusFV` fvs_later)

      where
        (defs_s, uses_s, _, ss) = unzip4 segs
        defs = plusFVs defs_s
        uses = plusFVs uses_s

                    -- Step 2: Fill in the fwd refs.
                    --         The segments are all singletons, but their fwd-ref
                    --         field mentions all the things used by the segment
                    --         that are bound after their use
        segs_w_fwd_refs = addFwdRefs segs

                    -- Step 3: Group together the segments to make bigger segments
                    --         Invariant: in the result, no segment uses a variable
                    --                    bound in a later segment
        grouped_segs = glomSegments ctxt segs_w_fwd_refs

    ----------------------------
    addFwdRefs :: [Segment a] -> [Segment a]
    -- So far the segments only have forward refs *within* the Stmt
    --      (which happens for bind:  x <- ...x...)
    -- This function adds the cross-seg fwd ref info

    addFwdRefs segs
      = fst (foldr mk_seg ([], emptyNameSet) segs)
      where
        mk_seg (defs, uses, fwds, stmts) (segs, later_defs)
            = (new_seg : segs, all_defs)
            where
              new_seg = (defs, uses, new_fwds, stmts)
              all_defs = later_defs `unionNameSet` defs
              new_fwds = fwds `unionNameSet` (uses `intersectNameSet` later_defs)
                    -- Add the downstream fwd refs here

    {-
    Note [Segmenting mdo]
    ~~~~~~~~~~~~~~~~~~~~~
    NB. June 7 2012: We only glom segments that appear in an explicit mdo;
    and leave those found in "do rec"'s intact.  See
    http://ghc.haskell.org/trac/ghc/ticket/4148 for the discussion
    leading to this design choice.  Hence the test in segmentRecStmts.

    Note [Glomming segments]
    ~~~~~~~~~~~~~~~~~~~~~~~~
    Glomming the singleton segments of an mdo into minimal recursive groups.

    At first I thought this was just strongly connected components, but
    there's an important constraint: the order of the stmts must not change.

    Consider
         mdo { x <- ...y...
               p <- z
               y <- ...x...
               q <- x
               z <- y
               r <- x }

    Here, the first stmt mention 'y', which is bound in the third.
    But that means that the innocent second stmt (p <- z) gets caught
    up in the recursion.  And that in turn means that the binding for
    'z' has to be included... and so on.

    Start at the tail { r <- x }
    Now add the next one { z <- y ; r <- x }
    Now add one more     { q <- x ; z <- y ; r <- x }
    Now one more... but this time we have to group a bunch into rec
         { rec { y <- ...x... ; q <- x ; z <- y } ; r <- x }
    Now one more, which we can add on without a rec
         { p <- z ;
           rec { y <- ...x... ; q <- x ; z <- y } ;
           r <- x }
    Finally we add the last one; since it mentions y we have to
    glom it together with the first two groups
         { rec { x <- ...y...; p <- z ; y <- ...x... ;
                 q <- x ; z <- y } ;
           r <- x }
    -}

    glomSegments :: HsStmtContext Name
                 -> [Segment (LStmt GhcRn body)]
                 -> [Segment [LStmt GhcRn body]]
                                      -- Each segment has a non-empty list of Stmts
    -- See Note [Glomming segments]

    glomSegments _ [] = []
    glomSegments ctxt ((defs,uses,fwds,stmt) : segs)
            -- Actually stmts will always be a singleton
      = (seg_defs, seg_uses, seg_fwds, seg_stmts)  : others
      where
        segs'            = glomSegments ctxt segs
        (extras, others) = grab uses segs'
        (ds, us, fs, ss) = unzip4 extras

        seg_defs  = plusFVs ds `plusFV` defs
        seg_uses  = plusFVs us `plusFV` uses
        seg_fwds  = plusFVs fs `plusFV` fwds
        seg_stmts = stmt : concat ss

        grab :: NameSet             -- The client
             -> [Segment a]
             -> ([Segment a],       -- Needed by the 'client'
                 [Segment a])       -- Not needed by the client
            -- The result is simply a split of the input
        grab uses dus
            = (reverse yeses, reverse noes)
            where
              (noes, yeses)           = span not_needed (reverse dus)
              not_needed (defs,_,_,_) = not (intersectsNameSet defs uses)

    ----------------------------------------------------
    segsToStmts :: Stmt GhcRn body
                                      -- A RecStmt with the SyntaxOps filled in
                -> [Segment [LStmt GhcRn body]]
                                      -- Each Segment has a non-empty list of Stmts
                -> FreeVars           -- Free vars used 'later'
                -> ([LStmt GhcRn body], FreeVars)

    segsToStmts _ [] fvs_later = ([], fvs_later)
    segsToStmts empty_rec_stmt ((defs, uses, fwds, ss) : segs) fvs_later
      = ASSERT( not (null ss) )
        (new_stmt : later_stmts, later_uses `plusFV` uses)
      where
        (later_stmts, later_uses) = segsToStmts empty_rec_stmt segs fvs_later
        new_stmt | non_rec   = head ss
                 | otherwise = L (getLoc (head ss)) rec_stmt
        rec_stmt = empty_rec_stmt { recS_stmts     = ss
                                  , recS_later_ids = nameSetElemsStable used_later
                                  , recS_rec_ids   = nameSetElemsStable fwds }
              -- See Note [Deterministic ApplicativeDo and RecursiveDo desugaring]
        non_rec    = isSingleton ss && isEmptyNameSet fwds
        used_later = defs `intersectNameSet` later_uses
                                    -- The ones needed after the RecStmt

    {-
    ************************************************************************
    *                                                                      *
    ApplicativeDo
    *                                                                      *
    ************************************************************************

    Note [ApplicativeDo]

    = Example =

    For a sequence of statements

     do
         x <- A
         y <- B x
         z <- C
         return (f x y z)

    We want to transform this to

      (\(x,y) z -> f x y z) <$> (do x <- A; y <- B x; return (x,y)) <*> C

    It would be easy to notice that "y <- B x" and "z <- C" are
    independent and do something like this:

     do
         x <- A
         (y,z) <- (,) <$> B x <*> C
         return (f x y z)

    But this isn't enough! A and C were also independent, and this
    transformation loses the ability to do A and C in parallel.

    The algorithm works by first splitting the sequence of statements into
    independent "segments", and a separate "tail" (the final statement). In
    our example above, the segements would be

         [ x <- A
         , y <- B x ]

         [ z <- C ]

    and the tail is:

         return (f x y z)

    Then we take these segments and make an Applicative expression from them:

         (\(x,y) z -> return (f x y z))
           <$> do { x <- A; y <- B x; return (x,y) }
           <*> C

    Finally, we recursively apply the transformation to each segment, to
    discover any nested parallelism.

    = Syntax & spec =

      expr ::= ... | do {stmt_1; ..; stmt_n} expr | ...

      stmt ::= pat <- expr
             | (arg_1 | ... | arg_n)  -- applicative composition, n>=1
             | ...                    -- other kinds of statement (e.g. let)

      arg ::= pat <- expr
            | {stmt_1; ..; stmt_n} {var_1..var_n}

    (note that in the actual implementation,the expr in a do statement is
    represented by a LastStmt as the final stmt, this is just a
    representational issue and may change later.)

    == Transformation to introduce applicative stmts ==

    ado {} tail = tail
    ado {pat <- expr} {return expr'} = (mkArg(pat <- expr)); return expr'
    ado {one} tail = one : tail
    ado stmts tail
      | n == 1 = ado before (ado after tail)
        where (before,after) = split(stmts_1)
      | n > 1  = (mkArg(stmts_1) | ... | mkArg(stmts_n)); tail
      where
        {stmts_1 .. stmts_n} = segments(stmts)

    segments(stmts) =
      -- divide stmts into segments with no interdependencies

    mkArg({pat <- expr}) = (pat <- expr)
    mkArg({stmt_1; ...; stmt_n}) =
      {stmt_1; ...; stmt_n} {vars(stmt_1) u .. u vars(stmt_n)}

    split({stmt_1; ..; stmt_n) =
      ({stmt_1; ..; stmt_i}, {stmt_i+1; ..; stmt_n})
      -- 1 <= i <= n
      -- i is a good place to insert a bind

    == Desugaring for do ==

    dsDo {} expr = expr

    dsDo {pat <- rhs; stmts} expr =
       rhs >>= \pat -> dsDo stmts expr

    dsDo {(arg_1 | ... | arg_n)} (return expr) =
      (\argpat (arg_1) .. argpat(arg_n) -> expr)
         <$> argexpr(arg_1)
         <*> ...
         <*> argexpr(arg_n)

    dsDo {(arg_1 | ... | arg_n); stmts} expr =
      join (\argpat (arg_1) .. argpat(arg_n) -> dsDo stmts expr)
         <$> argexpr(arg_1)
         <*> ...
         <*> argexpr(arg_n)

    -}

    -- | The 'Name's of @return@ and @pure@. These may not be 'returnName' and
    -- 'pureName' due to @RebindableSyntax@.
    data MonadNames = MonadNames { return_name, pure_name :: Name }

    -- | rearrange a list of statements using ApplicativeDoStmt.  See
    -- Note [ApplicativeDo].
    rearrangeForApplicativeDo
      :: HsStmtContext Name
      -> [(ExprLStmt GhcRn, FreeVars)]
      -> RnM ([ExprLStmt GhcRn], FreeVars)

    rearrangeForApplicativeDo _ [] = return ([], emptyNameSet)
    rearrangeForApplicativeDo _ [(one,_)] = return ([one], emptyNameSet)
    rearrangeForApplicativeDo ctxt stmts0 = do
      optimal_ado <- goptM Opt_OptimalApplicativeDo
      let stmt_tree | optimal_ado = mkStmtTreeOptimal stmts
                    | otherwise = mkStmtTreeHeuristic stmts
      traceRn "rearrangeForADo" (ppr stmt_tree)
      return_name <- lookupSyntaxName' returnMName
      pure_name   <- lookupSyntaxName' pureAName
      let monad_names = MonadNames { return_name = return_name
                                   , pure_name   = pure_name }
      stmtTreeToStmts monad_names ctxt stmt_tree [last] last_fvs
      where
        (stmts,(last,last_fvs)) = findLast stmts0
        findLast [] = error "findLast"
        findLast [last] = ([],last)
        findLast (x:xs) = (x:rest,last) where (rest,last) = findLast xs

    -- | A tree of statements using a mixture of applicative and bind constructs.
    data StmtTree a
      = StmtTreeOne a
      | StmtTreeBind (StmtTree a) (StmtTree a)
      | StmtTreeApplicative [StmtTree a]

    instance Outputable a => Outputable (StmtTree a) where
      ppr (StmtTreeOne x)          = parens (text "StmtTreeOne" <+> ppr x)
      ppr (StmtTreeBind x y)       = parens (hang (text "StmtTreeBind")
                                                2 (sep [ppr x, ppr y]))
      ppr (StmtTreeApplicative xs) = parens (hang (text "StmtTreeApplicative")
                                                2 (vcat (map ppr xs)))

    flattenStmtTree :: StmtTree a -> [a]
    flattenStmtTree t = go t []
     where
      go (StmtTreeOne a) as = a : as
      go (StmtTreeBind l r) as = go l (go r as)
      go (StmtTreeApplicative ts) as = foldr go as ts

    type ExprStmtTree = StmtTree (ExprLStmt GhcRn, FreeVars)
    type Cost = Int

    -- | Turn a sequence of statements into an ExprStmtTree using a
    -- heuristic algorithm.  /O(n^2)/
    mkStmtTreeHeuristic :: [(ExprLStmt GhcRn, FreeVars)] -> ExprStmtTree
    mkStmtTreeHeuristic [one] = StmtTreeOne one
    mkStmtTreeHeuristic stmts =
      case segments stmts of
        [one] -> split one
        segs -> StmtTreeApplicative (map split segs)
     where
      split [one] = StmtTreeOne one
      split stmts =
        StmtTreeBind (mkStmtTreeHeuristic before) (mkStmtTreeHeuristic after)
        where (before, after) = splitSegment stmts

    -- | Turn a sequence of statements into an ExprStmtTree optimally,
    -- using dynamic programming.  /O(n^3)/
    mkStmtTreeOptimal :: [(ExprLStmt GhcRn, FreeVars)] -> ExprStmtTree
    mkStmtTreeOptimal stmts =
      ASSERT(not (null stmts)) -- the empty case is handled by the caller;
                               -- we don't support empty StmtTrees.
      fst (arr ! (0,n))
      where
        n = length stmts - 1
        stmt_arr = listArray (0,n) stmts

        -- lazy cache of optimal trees for subsequences of the input
        arr :: Array (Int,Int) (ExprStmtTree, Cost)
        arr = array ((0,0),(n,n))
                 [ ((lo,hi), tree lo hi)
                 | lo <- [0..n]
                 , hi <- [lo..n] ]

        -- compute the optimal tree for the sequence [lo..hi]
        tree lo hi
          | hi == lo = (StmtTreeOne (stmt_arr ! lo), 1)
          | otherwise =
             case segments [ stmt_arr ! i | i <- [lo..hi] ] of
               [] -> panic "mkStmtTree"
               [_one] -> split lo hi
               segs -> (StmtTreeApplicative trees, maximum costs)
                 where
                   bounds = scanl (\(_,hi) a -> (hi+1, hi + length a)) (0,lo-1) segs
                   (trees,costs) = unzip (map (uncurry split) (tail bounds))

        -- find the best place to split the segment [lo..hi]
        split :: Int -> Int -> (ExprStmtTree, Cost)
        split lo hi
          | hi == lo = (StmtTreeOne (stmt_arr ! lo), 1)
          | otherwise = (StmtTreeBind before after, c1+c2)
            where
             -- As per the paper, for a sequence s1...sn, we want to find
             -- the split with the minimum cost, where the cost is the
             -- sum of the cost of the left and right subsequences.
             --
             -- As an optimisation (also in the paper) if the cost of
             -- s1..s(n-1) is different from the cost of s2..sn, we know
             -- that the optimal solution is the lower of the two.  Only
             -- in the case that these two have the same cost do we need
             -- to do the exhaustive search.
             --
             ((before,c1),(after,c2))
               | hi - lo == 1
               = ((StmtTreeOne (stmt_arr ! lo), 1),
                  (StmtTreeOne (stmt_arr ! hi), 1))
               | left_cost < right_cost
               = ((left,left_cost), (StmtTreeOne (stmt_arr ! hi), 1))
               | left_cost > right_cost
               = ((StmtTreeOne (stmt_arr ! lo), 1), (right,right_cost))
               | otherwise = minimumBy (comparing cost) alternatives
               where
                 (left, left_cost) = arr ! (lo,hi-1)
                 (right, right_cost) = arr ! (lo+1,hi)
                 cost ((_,c1),(_,c2)) = c1 + c2
                 alternatives = [ (arr ! (lo,k), arr ! (k+1,hi))
                                | k <- [lo .. hi-1] ]

    -- | Turn the ExprStmtTree back into a sequence of statements, using
    -- ApplicativeStmt where necessary.
    stmtTreeToStmts
      :: MonadNames
      -> HsStmtContext Name
      -> ExprStmtTree
      -> [ExprLStmt GhcRn]             -- ^ the "tail"
      -> FreeVars                     -- ^ free variables of the tail
      -> RnM ( [ExprLStmt GhcRn]       -- ( output statements,
             , FreeVars )             -- , things we needed

    -- If we have a single bind, and we can do it without a join, transform
    -- to an ApplicativeStmt.  This corresponds to the rule
    --   dsBlock [pat <- rhs] (return expr) = expr <$> rhs
    -- In the spec, but we do it here rather than in the desugarer,
    -- because we need the typechecker to typecheck the <$> form rather than
    -- the bind form, which would give rise to a Monad constraint.
    stmtTreeToStmts monad_names ctxt (StmtTreeOne (L _ (BindStmt _ pat rhs _ _), _))
                    tail _tail_fvs
      | not (isStrictPattern pat), (False,tail') <- needJoin monad_names tail
      -- See Note [ApplicativeDo and strict patterns]
      = mkApplicativeStmt ctxt [ApplicativeArgOne noExt pat rhs False] False tail'
    stmtTreeToStmts monad_names ctxt (StmtTreeOne (L _ (BodyStmt _ rhs _ _),_))
                    tail _tail_fvs
      | (False,tail') <- needJoin monad_names tail
      = mkApplicativeStmt ctxt
          [ApplicativeArgOne noExt nlWildPatName rhs True] False tail'

    stmtTreeToStmts _monad_names _ctxt (StmtTreeOne (s,_)) tail _tail_fvs =
      return (s : tail, emptyNameSet)

    stmtTreeToStmts monad_names ctxt (StmtTreeBind before after) tail tail_fvs = do
      (stmts1, fvs1) <- stmtTreeToStmts monad_names ctxt after tail tail_fvs
      let tail1_fvs = unionNameSets (tail_fvs : map snd (flattenStmtTree after))
      (stmts2, fvs2) <- stmtTreeToStmts monad_names ctxt before stmts1 tail1_fvs
      return (stmts2, fvs1 `plusFV` fvs2)

    stmtTreeToStmts monad_names ctxt (StmtTreeApplicative trees) tail tail_fvs = do
       pairs <- mapM (stmtTreeArg ctxt tail_fvs) trees
       let (stmts', fvss) = unzip pairs
       let (need_join, tail') = needJoin monad_names tail
       (stmts, fvs) <- mkApplicativeStmt ctxt stmts' need_join tail'
       return (stmts, unionNameSets (fvs:fvss))
     where
       stmtTreeArg _ctxt _tail_fvs (StmtTreeOne (L _ (BindStmt _ pat exp _ _), _))
         = return (ApplicativeArgOne noExt pat exp False, emptyFVs)
       stmtTreeArg _ctxt _tail_fvs (StmtTreeOne (L _ (BodyStmt _ exp _ _), _)) =
         return (ApplicativeArgOne noExt nlWildPatName exp True, emptyFVs)
       stmtTreeArg ctxt tail_fvs tree = do
         let stmts = flattenStmtTree tree
             pvarset = mkNameSet (concatMap (collectStmtBinders.unLoc.fst) stmts)
                         `intersectNameSet` tail_fvs
             pvars = nameSetElemsStable pvarset
               -- See Note [Deterministic ApplicativeDo and RecursiveDo desugaring]
             pat = mkBigLHsVarPatTup pvars
             tup = mkBigLHsVarTup pvars
         (stmts',fvs2) <- stmtTreeToStmts monad_names ctxt tree [] pvarset
         (mb_ret, fvs1) <-
            if | L _ ApplicativeStmt{} <- last stmts' ->
                 return (unLoc tup, emptyNameSet)
               | otherwise -> do
                 (ret,fvs) <- lookupStmtNamePoly ctxt returnMName
                 return (HsApp noExt (noLoc ret) tup, fvs)
         return ( ApplicativeArgMany noExt stmts' mb_ret pat
                , fvs1 `plusFV` fvs2)

    -- | Divide a sequence of statements into segments, where no segment
    -- depends on any variables defined by a statement in another segment.
    segments
      :: [(ExprLStmt GhcRn, FreeVars)]
      -> [[(ExprLStmt GhcRn, FreeVars)]]
    segments stmts = map fst $ merge $ reverse $ map reverse $ walk (reverse stmts)
      where
        allvars = mkNameSet (concatMap (collectStmtBinders.unLoc.fst) stmts)

        -- We would rather not have a segment that just has LetStmts in
        -- it, so combine those with an adjacent segment where possible.
        merge [] = []
        merge (seg : segs)
           = case rest of
              [] -> [(seg,all_lets)]
              ((s,s_lets):ss) | all_lets || s_lets
                   -> (seg ++ s, all_lets && s_lets) : ss
              _otherwise -> (seg,all_lets) : rest
          where
            rest = merge segs
            all_lets = all (isLetStmt . fst) seg

        -- walk splits the statement sequence into segments, traversing
        -- the sequence from the back to the front, and keeping track of
        -- the set of free variables of the current segment.  Whenever
        -- this set of free variables is empty, we have a complete segment.
        walk :: [(ExprLStmt GhcRn, FreeVars)] -> [[(ExprLStmt GhcRn, FreeVars)]]
        walk [] = []
        walk ((stmt,fvs) : stmts) = ((stmt,fvs) : seg) : walk rest
          where (seg,rest) = chunter fvs' stmts
                (_, fvs') = stmtRefs stmt fvs

        chunter _ [] = ([], [])
        chunter vars ((stmt,fvs) : rest)
           | not (isEmptyNameSet vars)
           || isStrictPatternBind stmt
               -- See Note [ApplicativeDo and strict patterns]
           = ((stmt,fvs) : chunk, rest')
           where (chunk,rest') = chunter vars' rest
                 (pvars, evars) = stmtRefs stmt fvs
                 vars' = (vars `minusNameSet` pvars) `unionNameSet` evars
        chunter _ rest = ([], rest)

        stmtRefs stmt fvs
          | isLetStmt stmt = (pvars, fvs' `minusNameSet` pvars)
          | otherwise      = (pvars, fvs')
          where fvs' = fvs `intersectNameSet` allvars
                pvars = mkNameSet (collectStmtBinders (unLoc stmt))

        isStrictPatternBind :: ExprLStmt GhcRn -> Bool
        isStrictPatternBind (L _ (BindStmt _ pat _ _ _)) = isStrictPattern pat
        isStrictPatternBind _ = False

    {-
    Note [ApplicativeDo and strict patterns]

    A strict pattern match is really a dependency.  For example,

    do
      (x,y) <- A
      z <- B
      return C

    The pattern (_,_) must be matched strictly before we do B.  If we
    allowed this to be transformed into

      (\(x,y) -> \z -> C) <$> A <*> B

    then it could be lazier than the standard desuraging using >>=.  See #13875
    for more examples.

    Thus, whenever we have a strict pattern match, we treat it as a
    dependency between that statement and the following one.  The
    dependency prevents those two statements from being performed "in
    parallel" in an ApplicativeStmt, but doesn't otherwise affect what we
    can do with the rest of the statements in the same "do" expression.
    -}

    isStrictPattern :: LPat id -> Bool
    isStrictPattern (L _ pat) =
      case pat of
        WildPat{}       -> False
        VarPat{}        -> False
        LazyPat{}       -> False
        AsPat _ _ p     -> isStrictPattern p
        ParPat _ p      -> isStrictPattern p
        ViewPat _ _ p   -> isStrictPattern p
        SigPat _ p      -> isStrictPattern p
        BangPat{}       -> True
        ListPat{}       -> True
        TuplePat{}      -> True
        SumPat{}        -> True
        ConPatIn{}      -> True
        ConPatOut{}     -> True
        LitPat{}        -> True
        NPat{}          -> True
        NPlusKPat{}     -> True
        SplicePat{}     -> True
        _otherwise -> panic "isStrictPattern"

    isLetStmt :: LStmt a b -> Bool
    isLetStmt (L _ LetStmt{}) = True
    isLetStmt _ = False

    -- | Find a "good" place to insert a bind in an indivisible segment.
    -- This is the only place where we use heuristics.  The current
    -- heuristic is to peel off the first group of independent statements
    -- and put the bind after those.
    splitSegment
      :: [(ExprLStmt GhcRn, FreeVars)]
      -> ( [(ExprLStmt GhcRn, FreeVars)]
         , [(ExprLStmt GhcRn, FreeVars)] )
    splitSegment [one,two] = ([one],[two])
      -- there is no choice when there are only two statements; this just saves
      -- some work in a common case.
    splitSegment stmts
      | Just (lets,binds,rest) <- slurpIndependentStmts stmts
      =  if not (null lets)
           then (lets, binds++rest)
           else (lets++binds, rest)
      | otherwise
      = case stmts of
          (x:xs) -> ([x],xs)
          _other -> (stmts,[])

    slurpIndependentStmts
       :: [(LStmt GhcRn (Located (body GhcRn)), FreeVars)]
       -> Maybe ( [(LStmt GhcRn (Located (body GhcRn)), FreeVars)] -- LetStmts
                , [(LStmt GhcRn (Located (body GhcRn)), FreeVars)] -- BindStmts
                , [(LStmt GhcRn (Located (body GhcRn)), FreeVars)] )
    slurpIndependentStmts stmts = go [] [] emptyNameSet stmts
     where
      -- If we encounter a BindStmt that doesn't depend on a previous BindStmt
      -- in this group, then add it to the group. We have to be careful about
      -- strict patterns though; splitSegments expects that if we return Just
      -- then we have actually done some splitting. Otherwise it will go into
      -- an infinite loop (#14163).
      go lets indep bndrs ((L loc (BindStmt _ pat body bind_op fail_op), fvs): rest)
        | isEmptyNameSet (bndrs `intersectNameSet` fvs) && not (isStrictPattern pat)
        = go lets ((L loc (BindStmt noExt pat body bind_op fail_op), fvs) : indep)
             bndrs' rest
        where bndrs' = bndrs `unionNameSet` mkNameSet (collectPatBinders pat)
      -- If we encounter a LetStmt that doesn't depend on a BindStmt in this
      -- group, then move it to the beginning, so that it doesn't interfere with
      -- grouping more BindStmts.
      -- TODO: perhaps we shouldn't do this if there are any strict bindings,
      -- because we might be moving evaluation earlier.
      go lets indep bndrs ((L loc (LetStmt noExt binds), fvs) : rest)
        | isEmptyNameSet (bndrs `intersectNameSet` fvs)
        = go ((L loc (LetStmt noExt binds), fvs) : lets) indep bndrs rest
      go _ []  _ _ = Nothing
      go _ [_] _ _ = Nothing
      go lets indep _ stmts = Just (reverse lets, reverse indep, stmts)

    -- | Build an ApplicativeStmt, and strip the "return" from the tail
    -- if necessary.
    --
    -- For example, if we start with
    --   do x <- E1; y <- E2; return (f x y)
    -- then we get
    --   do (E1[x] | E2[y]); f x y
    --
    -- the LastStmt in this case has the return removed, but we set the
    -- flag on the LastStmt to indicate this, so that we can print out the
    -- original statement correctly in error messages.  It is easier to do
    -- it this way rather than try to ignore the return later in both the
    -- typechecker and the desugarer (I tried it that way first!).
    mkApplicativeStmt
      :: HsStmtContext Name
      -> [ApplicativeArg GhcRn]             -- ^ The args
      -> Bool                               -- ^ True <=> need a join
      -> [ExprLStmt GhcRn]        -- ^ The body statements
      -> RnM ([ExprLStmt GhcRn], FreeVars)
    mkApplicativeStmt ctxt args need_join body_stmts
      = do { (fmap_op, fvs1) <- lookupStmtName ctxt fmapName
           ; (ap_op, fvs2) <- lookupStmtName ctxt apAName
           ; (mb_join, fvs3) <-
               if need_join then
                 do { (join_op, fvs) <- lookupStmtName ctxt joinMName
                    ; return (Just join_op, fvs) }
               else
                 return (Nothing, emptyNameSet)
           ; let applicative_stmt = noLoc $ ApplicativeStmt noExt
                   (zip (fmap_op : repeat ap_op) args)
                   mb_join
           ; return ( applicative_stmt : body_stmts
                    , fvs1 `plusFV` fvs2 `plusFV` fvs3) }

    -- | Given the statements following an ApplicativeStmt, determine whether
    -- we need a @join@ or not, and remove the @return@ if necessary.
    needJoin :: MonadNames
             -> [ExprLStmt GhcRn]
             -> (Bool, [ExprLStmt GhcRn])
    needJoin _monad_names [] = (False, [])  -- we're in an ApplicativeArg
    needJoin monad_names  [L loc (LastStmt _ e _ t)]
     | Just arg <- isReturnApp monad_names e =
           (False, [L loc (LastStmt noExt arg True t)])
    needJoin _monad_names stmts = (True, stmts)

    -- | @Just e@, if the expression is @return e@ or @return $ e@,
    -- otherwise @Nothing@
    isReturnApp :: MonadNames
                -> LHsExpr GhcRn
                -> Maybe (LHsExpr GhcRn)
    isReturnApp monad_names (L _ (HsPar _ expr)) = isReturnApp monad_names expr
    isReturnApp monad_names (L _ e) = case e of
      OpApp _ l op r | is_return l, is_dollar op -> Just r
      HsApp _ f arg  | is_return f               -> Just arg
      _otherwise -> Nothing
     where
      is_var f (L _ (HsPar _ e)) = is_var f e
      is_var f (L _ (HsAppType _ e)) = is_var f e
      is_var f (L _ (HsVar _ (L _ r))) = f r
           -- TODO: I don't know how to get this right for rebindable syntax
      is_var _ _ = False

      is_return = is_var (\n -> n == return_name monad_names
                             || n == pure_name monad_names)
      is_dollar = is_var (`hasKey` dollarIdKey)

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{Errors}
    *                                                                      *
    ************************************************************************
    -}

    checkEmptyStmts :: HsStmtContext Name -> RnM ()
    -- We've seen an empty sequence of Stmts... is that ok?
    checkEmptyStmts ctxt
      = unless (okEmpty ctxt) (addErr (emptyErr ctxt))

    okEmpty :: HsStmtContext a -> Bool
    okEmpty (PatGuard {}) = True
    okEmpty _             = False

    emptyErr :: HsStmtContext Name -> SDoc
    emptyErr (ParStmtCtxt {})   = text "Empty statement group in parallel comprehension"
    emptyErr (TransStmtCtxt {}) = text "Empty statement group preceding 'group' or 'then'"
    emptyErr ctxt               = text "Empty" <+> pprStmtContext ctxt

    ----------------------
    checkLastStmt :: Outputable (body GhcPs) => HsStmtContext Name
                  -> LStmt GhcPs (Located (body GhcPs))
                  -> RnM (LStmt GhcPs (Located (body GhcPs)))
    checkLastStmt ctxt lstmt@(L loc stmt)
      = case ctxt of
          ListComp  -> check_comp
          MonadComp -> check_comp
          ArrowExpr -> check_do
          DoExpr    -> check_do
          MDoExpr   -> check_do
          _         -> check_other
      where
        check_do    -- Expect BodyStmt, and change it to LastStmt
          = case stmt of
              BodyStmt _ e _ _ -> return (L loc (mkLastStmt e))
              LastStmt {}      -> return lstmt   -- "Deriving" clauses may generate a
                                                 -- LastStmt directly (unlike the parser)
              _                -> do { addErr (hang last_error 2 (ppr stmt)); return lstmt }
        last_error = (text "The last statement in" <+> pprAStmtContext ctxt
                      <+> text "must be an expression")

        check_comp  -- Expect LastStmt; this should be enforced by the parser!
          = case stmt of
              LastStmt {} -> return lstmt
              _           -> pprPanic "checkLastStmt" (ppr lstmt)

        check_other -- Behave just as if this wasn't the last stmt
          = do { checkStmt ctxt lstmt; return lstmt }

    -- Checking when a particular Stmt is ok
    checkStmt :: HsStmtContext Name
              -> LStmt GhcPs (Located (body GhcPs))
              -> RnM ()
    checkStmt ctxt (L _ stmt)
      = do { dflags <- getDynFlags
           ; case okStmt dflags ctxt stmt of
               IsValid        -> return ()
               NotValid extra -> addErr (msg $$ extra) }
      where
       msg = sep [ text "Unexpected" <+> pprStmtCat stmt <+> ptext (sLit "statement")
                 , text "in" <+> pprAStmtContext ctxt ]

    pprStmtCat :: Stmt a body -> SDoc
    pprStmtCat (TransStmt {})     = text "transform"
    pprStmtCat (LastStmt {})      = text "return expression"
    pprStmtCat (BodyStmt {})      = text "body"
    pprStmtCat (BindStmt {})      = text "binding"
    pprStmtCat (LetStmt {})       = text "let"
    pprStmtCat (RecStmt {})       = text "rec"
    pprStmtCat (ParStmt {})       = text "parallel"
    pprStmtCat (ApplicativeStmt {}) = panic "pprStmtCat: ApplicativeStmt"
    pprStmtCat (XStmtLR {})         = panic "pprStmtCat: XStmtLR"

    ------------
    emptyInvalid :: Validity  -- Payload is the empty document
    emptyInvalid = NotValid Outputable.empty

    okStmt, okDoStmt, okCompStmt, okParStmt
       :: DynFlags -> HsStmtContext Name
       -> Stmt GhcPs (Located (body GhcPs)) -> Validity
    -- Return Nothing if OK, (Just extra) if not ok
    -- The "extra" is an SDoc that is appended to a generic error message

    okStmt dflags ctxt stmt
      = case ctxt of
          PatGuard {}        -> okPatGuardStmt stmt
          ParStmtCtxt ctxt   -> okParStmt  dflags ctxt stmt
          DoExpr             -> okDoStmt   dflags ctxt stmt
          MDoExpr            -> okDoStmt   dflags ctxt stmt
          ArrowExpr          -> okDoStmt   dflags ctxt stmt
          GhciStmtCtxt       -> okDoStmt   dflags ctxt stmt
          ListComp           -> okCompStmt dflags ctxt stmt
          MonadComp          -> okCompStmt dflags ctxt stmt
          TransStmtCtxt ctxt -> okStmt dflags ctxt stmt

    -------------
    okPatGuardStmt :: Stmt GhcPs (Located (body GhcPs)) -> Validity
    okPatGuardStmt stmt
      = case stmt of
          BodyStmt {} -> IsValid
          BindStmt {} -> IsValid
          LetStmt {}  -> IsValid
          _           -> emptyInvalid

    -------------
    okParStmt dflags ctxt stmt
      = case stmt of
          LetStmt _ (L _ (HsIPBinds {})) -> emptyInvalid
          _                              -> okStmt dflags ctxt stmt

    ----------------
    okDoStmt dflags ctxt stmt
      = case stmt of
           RecStmt {}
             | LangExt.RecursiveDo `xopt` dflags -> IsValid
             | ArrowExpr <- ctxt -> IsValid    -- Arrows allows 'rec'
             | otherwise         -> NotValid (text "Use RecursiveDo")
           BindStmt {} -> IsValid
           LetStmt {}  -> IsValid
           BodyStmt {} -> IsValid
           _           -> emptyInvalid

    ----------------
    okCompStmt dflags _ stmt
      = case stmt of
           BindStmt {} -> IsValid
           LetStmt {}  -> IsValid
           BodyStmt {} -> IsValid
           ParStmt {}
             | LangExt.ParallelListComp `xopt` dflags -> IsValid
             | otherwise -> NotValid (text "Use ParallelListComp")
           TransStmt {}
             | LangExt.TransformListComp `xopt` dflags -> IsValid
             | otherwise -> NotValid (text "Use TransformListComp")
           RecStmt {}  -> emptyInvalid
           LastStmt {} -> emptyInvalid  -- Should not happen (dealt with by checkLastStmt)
           ApplicativeStmt {} -> emptyInvalid
           XStmtLR{} -> panic "okCompStmt"

    ---------
    checkTupleSection :: [LHsTupArg GhcPs] -> RnM ()
    checkTupleSection args
      = do  { tuple_section <- xoptM LangExt.TupleSections
            ; checkErr (all tupArgPresent args || tuple_section) msg }
      where
        msg = text "Illegal tuple section: use TupleSections"

    ---------
    sectionErr :: HsExpr GhcPs -> SDoc
    sectionErr expr
      = hang (text "A section must be enclosed in parentheses")
           2 (text "thus:" <+> (parens (ppr expr)))

    patSynErr :: HsExpr GhcPs -> SDoc -> RnM (HsExpr GhcRn, FreeVars)
    patSynErr e explanation = do { addErr (sep [text "Pattern syntax in expression context:",
                                    nest 4 (ppr e)] $$
                                      explanation)
                     ; return (EWildPat noExt, emptyFVs) }

    badIpBinds :: Outputable a => SDoc -> a -> SDoc
    badIpBinds what binds
      = hang (text "Implicit-parameter bindings illegal in" <+> what)
             2 (ppr binds)