content: {-
  (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 ScopedTypeVariables, CPP, RecordWildCards, MultiWayIf #-}
  
  module Eta.Rename.RnExpr (
          rnLExpr, rnExpr, rnStmts
     ) where
  
  import Eta.Rename.RnBinds   ( rnLocalBindsAndThen, rnLocalValBindsLHS, rnLocalValBindsRHS,
                     rnMatchGroup, rnGRHS, makeMiniFixityEnv)
  import Eta.HsSyn.HsSyn
  import Eta.TypeCheck.TcRnMonad
  import Eta.BasicTypes.Module           ( getModule )
  import Eta.Rename.RnEnv
  import Eta.Rename.RnSplice         ( rnBracket, rnSpliceExpr, checkThLocalName )
  import Eta.Rename.RnTypes
  import Eta.Rename.RnPat
  import Eta.Main.DynFlags
  import Eta.BasicTypes.BasicTypes       ( FixityDirection(..) )
  import Eta.Prelude.PrelNames
  import qualified Eta.LanguageExtensions as LangExt
  import Eta.BasicTypes.Name
  import Eta.BasicTypes.NameSet
  import Eta.BasicTypes.RdrName
  import Eta.Utils.UniqSet
  import Data.List
  import Data.Array
  import Data.Ord ( comparing )
  import Eta.Utils.Util
  import Eta.Utils.ListSetOps       ( removeDups )
  import Eta.Main.ErrUtils
  import Eta.Utils.Outputable
  import qualified Eta.Utils.Outputable as Outputable
  import Eta.BasicTypes.SrcLoc
  import Eta.Utils.FastString
  import Control.Monad
  import Eta.Prelude.TysWiredIn       ( nilDataConName )
  
  #include "HsVersions.h"
  {-
  ************************************************************************
  *                                                                      *
  \subsubsection{Expressions}
  *                                                                      *
  ************************************************************************
  -}
  
  rnExprs :: [LHsExpr RdrName] -> RnM ([LHsExpr Name], 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 RdrName -> RnM (LHsExpr Name, FreeVars)
  rnLExpr = wrapLocFstM rnExpr
  
  rnExpr :: HsExpr RdrName -> RnM (HsExpr Name, FreeVars)
  
  finishHsVar :: Name -> RnM (HsExpr Name, FreeVars)
  -- Separated from rnExpr because it's also used
  -- when renaming infix expressions
  finishHsVar name
   = do { this_mod <- getModule
        ; when (nameIsLocalOrFrom this_mod name) $
          checkThLocalName name
        ; return (HsVar name, unitFV name) }
  
  rnExpr (HsVar v)
    = do { mb_name <- lookupOccRn_maybe v
         ; case mb_name of {
             Nothing -> do { if startsWithUnderscore (rdrNameOcc v)
                             then return (HsUnboundVar v, emptyFVs)
                             else do { n <- reportUnboundName v; finishHsVar n } } ;
             Just name
                | name == nilDataConName -- Treat [] as an ExplicitList, so that
                                         -- OverloadedLists works correctly
                -> rnExpr (ExplicitList placeHolderType Nothing [])
  
                | otherwise
                -> finishHsVar name }}
  
  rnExpr (HsIPVar v)
    = return (HsIPVar v, emptyFVs)
  
  rnExpr (HsOverLabel v)
    = return (HsOverLabel v, emptyFVs)
  
  rnExpr (HsLit lit@(HsString src s))
    = do { opt_OverloadedStrings <- xoptM LangExt.OverloadedStrings
         ; if opt_OverloadedStrings then
              rnExpr (HsOverLit (mkHsIsString src s placeHolderType))
           else do {
              ; rnLit lit
              ; return (HsLit lit, emptyFVs) } }
  
  rnExpr (HsLit lit)
    = do { rnLit lit
         ; return (HsLit lit, emptyFVs) }
  
  rnExpr (HsOverLit lit)
    = do { ((lit', mb_neg), fvs) <- rnOverLit lit -- See Note [Negative zero]
         ; case mb_neg of
                Nothing -> return (HsOverLit lit', fvs)
                Just neg -> return ( HsApp (noLoc neg) (noLoc (HsOverLit lit'))
                                   , fvs ) }
  
  rnExpr (HsApp fun arg)
    = do { (fun',fvFun) <- rnLExpr fun
         ; (arg',fvArg) <- rnLExpr arg
         ; return (HsApp fun' arg', fvFun `plusFV` fvArg) }
  
  rnExpr (OpApp e1 (L op_loc (HsVar op_rdr)) _ e2)
    = do  { (e1', fv_e1) <- rnLExpr e1
          ; (e2', fv_e2) <- rnLExpr e2
          ; op_name <- setSrcSpan op_loc (lookupOccRn op_rdr)
          ; (op', fv_op) <- finishHsVar op_name
                  -- NB: op' is usually just a variable, but might be
                  --     an applicatoin (assert "Foo.hs:47")
          -- 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 <- lookupFixityRn op_name
          ; final_e <- mkOpAppRn e1' (L op_loc op') fixity e2'
          ; return (final_e, fv_e1 `plusFV` fv_op `plusFV` fv_e2) }
  rnExpr (OpApp _ other_op _ _)
    = failWith (vcat [ hang (ptext (sLit "Infix application with a non-variable operator:"))
                          2 (ppr other_op)
                     , ptext (sLit "(Probably resulting from a Template Haskell splice)") ])
  
  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-ETA_REPL 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 (L loc (section@(SectionL {}))))
    = do  { (section', fvs) <- rnSection section
          ; return (HsPar (L loc section'), fvs) }
  
  rnExpr (HsPar (L loc (section@(SectionR {}))))
    = do  { (section', fvs) <- rnSection section
          ; return (HsPar (L loc section'), fvs) }
  
  rnExpr (HsPar e)
    = do  { (e', fvs_e) <- rnLExpr e
          ; return (HsPar e', fvs_e) }
  
  rnExpr expr@(SectionL {})
    = do  { addErr (sectionErr expr); rnSection expr }
  rnExpr expr@(SectionR {})
    = do  { addErr (sectionErr expr); rnSection expr }
  
  ---------------------------------------------
  rnExpr (HsCoreAnn src ann expr)
    = do { (expr', fvs_expr) <- rnLExpr expr
         ; return (HsCoreAnn src ann expr', fvs_expr) }
  
  rnExpr (HsSCC src lbl expr)
    = do { (expr', fvs_expr) <- rnLExpr expr
         ; return (HsSCC src lbl expr', fvs_expr) }
  rnExpr (HsTickPragma src info expr)
    = do { (expr', fvs_expr) <- rnLExpr expr
         ; return (HsTickPragma src info expr', fvs_expr) }
  
  rnExpr (HsLam matches)
    = do { (matches', fvMatch) <- rnMatchGroup LambdaExpr rnLExpr matches
         ; return (HsLam matches', fvMatch) }
  
  rnExpr (HsLamCase _arg matches)
    = do { (matches', fvs_ms) <- rnMatchGroup CaseAlt rnLExpr matches
         -- ; return (HsLamCase arg matches', fvs_ms) }
         ; return (HsLamCase placeHolderType matches', fvs_ms) }
  
  rnExpr (HsCase expr matches)
    = do { (new_expr, e_fvs) <- rnLExpr expr
         ; (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLExpr matches
         ; return (HsCase new_expr new_matches, e_fvs `plusFV` ms_fvs) }
  
  rnExpr (HsLet binds expr)
    = rnLocalBindsAndThen binds $ \binds' _ -> do
        { (expr',fvExpr) <- rnLExpr expr
        ; return (HsLet binds' expr', fvExpr) }
  
  rnExpr (HsDo do_or_lc stmts _)
    = do  { ((stmts', _), fvs) <-
             rnStmtsWithPostProcessing do_or_lc rnLExpr
               postProcessStmtsForApplicativeDo stmts
               (\ _ -> return ((), emptyFVs))
          ; return ( HsDo do_or_lc stmts' placeHolderType, fvs ) }
  
  rnExpr (ExplicitList _ _  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 placeHolderType (Just from_list_n_name) exps'
                       , fvs `plusFV` fvs') }
             else
              return  (ExplicitList placeHolderType Nothing exps', fvs) }
  
  rnExpr (ExplicitPArr _ exps)
    = do { (exps', fvs) <- rnExprs exps
         ; return  (ExplicitPArr placeHolderType exps', fvs) }
  
  rnExpr (ExplicitTuple tup_args boxity)
    = do { checkTupleSection tup_args
         ; checkTupSize (length tup_args)
         ; (tup_args', fvs) <- mapAndUnzipM rnTupArg tup_args
         ; return (ExplicitTuple tup_args' boxity, plusFVs fvs) }
    where
      rnTupArg (L l (Present e)) = do { (e',fvs) <- rnLExpr e
                                      ; return (L l (Present e'), fvs) }
      rnTupArg (L l (Missing _)) = return (L l (Missing placeHolderType)
                                          , emptyFVs)
  
  rnExpr (RecordCon con_id _ rbinds)
    = do  { conname <- lookupLocatedOccRn con_id
          ; (rbinds', fvRbinds) <- rnHsRecBinds (HsRecFieldCon (unLoc conname)) rbinds
          ; return (RecordCon conname noPostTcExpr rbinds',
                    fvRbinds `addOneFV` unLoc conname) }
  
  rnExpr (RecordUpd expr rbinds _ _ _)
    = do  { (expr', fvExpr) <- rnLExpr expr
          ; (rbinds', fvRbinds) <- rnHsRecBinds HsRecFieldUpd rbinds
          ; return (RecordUpd expr' rbinds' [] [] [],
                    fvExpr `plusFV` fvRbinds) }
  
  rnExpr (ExprWithTySig expr pty PlaceHolder)
    = do  { (wcs, pty') <- extractWildcards pty
          ; bindLocatedLocalsFV wcs $ \wcs_new -> do {
            (pty'', fvTy) <- rnLHsType ExprWithTySigCtx pty'
          ; (expr', fvExpr) <- bindSigTyVarsFV (hsExplicitTvs pty'') $
                               rnLExpr expr
          ; return (ExprWithTySig expr' pty'' wcs_new, fvExpr `plusFV` fvTy) } }
  
  rnExpr (HsIf _ p b1 b2)
    = do { (p', fvP) <- rnLExpr p
         ; (b1', fvB1) <- rnLExpr b1
         ; (b2', fvB2) <- rnLExpr b2
         ; (mb_ite, fvITE) <- lookupIfThenElse
         ; return (HsIf mb_ite p' b1' b2', plusFVs [fvITE, fvP, fvB1, fvB2]) }
  
  rnExpr (HsMultiIf _ty alts)
    = do { (alts', fvs) <- mapFvRn (rnGRHS IfAlt rnLExpr) alts
         -- ; return (HsMultiIf ty alts', fvs) }
         ; return (HsMultiIf placeHolderType alts', fvs) }
  
  rnExpr (HsType a)
    = do { (t, fvT) <- rnLHsType HsTypeCtx a
         ; return (HsType t, fvT) }
  
  rnExpr (ArithSeq _ _ 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 noPostTcExpr (Just from_list_name) new_seq, fvs `plusFV` fvs') }
             else
              return (ArithSeq noPostTcExpr Nothing new_seq, fvs) }
  
  rnExpr (PArrSeq _ seq)
    = do { (new_seq, fvs) <- rnArithSeq seq
         ; return (PArrSeq noPostTcExpr 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)
  rnExpr e@(EAsPat {})   = patSynErr e
  rnExpr e@(EViewPat {}) = patSynErr e
  rnExpr e@(ELazyPat {}) = patSynErr e
  
  {-
  ************************************************************************
  *                                                                      *
          Static values
  *                                                                      *
  ************************************************************************
  
  For the static form we check that the free variables are all top-level
  value bindings. This is done by checking that the name is external or
  wired-in. See the Notes about the NameSorts in Name.hs.
  -}
  
  rnExpr e@(HsStatic expr) = do
      target <- fmap hscTarget getDynFlags
      case target of
        -- SPT entries are expected to exist in object code so far, and this is
        -- not the case in interpreted mode. See bug #9878.
        HscInterpreted -> addErr $ sep
          [ text "The static form is not supported in interpreted mode."
          , text "Please use -fobject-code."
          ]
        _ -> return ()
      (expr',fvExpr) <- rnLExpr expr
      stage <- getStage
      case stage of
        Brack _ _ -> return () -- Don't check names if we are inside brackets.
                               -- We don't want to reject cases like:
                               -- \e -> [| static $(e) |]
                               -- if $(e) turns out to produce a legal expression.
        Splice _ -> addErr $ sep
               [ text "static forms cannot be used in splices:"
               , nest 2 $ ppr e
               ]
        _ -> do
         let isTopLevelName n = isExternalName n || isWiredInName n
         case nameSetElems $ filterNameSet
                               (\n -> not (isTopLevelName n || isUnboundName n))
                               fvExpr                                           of
           [] -> return ()
           fvNonGlobal -> addErr $ cat
               [ text $ "Only identifiers of top-level bindings can "
                        ++ "appear in the body of the static form:"
               , nest 2 $ ppr e
               , text "but the following identifiers were found instead:"
               , nest 2 $ vcat $ map ppr fvNonGlobal
               ]
      return (HsStatic expr', fvExpr)
  
  {-
  ************************************************************************
  *                                                                      *
          Arrow notation
  *                                                                      *
  ************************************************************************
  -}
  
  rnExpr (HsProc pat body)
    = newArrowScope $
      rnPat ProcExpr pat $ \ pat' -> do
        { (body',fvBody) <- rnCmdTop body
        ; return (HsProc 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 Name
  hsHoleExpr = HsUnboundVar (mkRdrUnqual (mkVarOcc "_"))
  
  arrowFail :: HsExpr RdrName -> RnM (HsExpr Name, FreeVars)
  arrowFail e
    = do { addErr (vcat [ ptext (sLit "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 RdrName -> RnM (HsExpr Name, FreeVars)
  rnSection section@(SectionR op expr)
    = do  { (op', fvs_op)     <- rnLExpr op
          ; (expr', fvs_expr) <- rnLExpr expr
          ; checkSectionPrec InfixR section op' expr'
          ; return (SectionR op' expr', fvs_op `plusFV` fvs_expr) }
  
  rnSection section@(SectionL expr op)
    = do  { (expr', fvs_expr) <- rnLExpr expr
          ; (op', fvs_op)     <- rnLExpr op
          ; checkSectionPrec InfixL section op' expr'
          ; return (SectionL expr' op', fvs_op `plusFV` fvs_expr) }
  
  rnSection other = pprPanic "rnSection" (ppr other)
  
  {-
  ************************************************************************
  *                                                                      *
          Records
  *                                                                      *
  ************************************************************************
  -}
  
  rnHsRecBinds :: HsRecFieldContext -> HsRecordBinds RdrName
               -> RnM (HsRecordBinds Name, FreeVars)
  rnHsRecBinds ctxt rec_binds@(HsRecFields { rec_dotdot = dd })
    = do { (flds, fvs) <- rnHsRecFields ctxt HsVar rec_binds
         ; (flds', fvss) <- mapAndUnzipM rn_field flds
         ; return (HsRecFields { rec_flds = flds', rec_dotdot = dd },
                   fvs `plusFV` plusFVs fvss) }
    where
      rn_field (L l fld) = do { (arg', fvs) <- rnLExpr (hsRecFieldArg fld)
                              ; return (L l (fld { hsRecFieldArg = arg' }), fvs) }
  
  {-
  ************************************************************************
  *                                                                      *
          Arrow commands
  *                                                                      *
  ************************************************************************
  -}
  
  rnCmdArgs :: [LHsCmdTop RdrName] -> RnM ([LHsCmdTop Name], FreeVars)
  rnCmdArgs [] = return ([], emptyFVs)
  rnCmdArgs (arg:args)
    = do { (arg',fvArg) <- rnCmdTop arg
         ; (args',fvArgs) <- rnCmdArgs args
         ; return (arg':args', fvArg `plusFV` fvArgs) }
  
  rnCmdTop :: LHsCmdTop RdrName -> RnM (LHsCmdTop Name, FreeVars)
  rnCmdTop = wrapLocFstM rnCmdTop'
   where
    rnCmdTop' (HsCmdTop cmd _ _ _)
     = do { (cmd', fvCmd) <- rnLCmd cmd
          ; let cmd_names = [arrAName, composeAName, firstAName] ++
                            nameSetElems (methodNamesCmd (unLoc cmd'))
          -- Generate the rebindable syntax for the monad
          ; (cmd_names', cmd_fvs) <- lookupSyntaxNames cmd_names
  
          ; return (HsCmdTop cmd' placeHolderType placeHolderType
                    (cmd_names `zip` cmd_names'),
                    fvCmd `plusFV` cmd_fvs) }
  
  rnLCmd :: LHsCmd RdrName -> RnM (LHsCmd Name, FreeVars)
  rnLCmd = wrapLocFstM rnCmd
  
  rnCmd :: HsCmd RdrName -> RnM (HsCmd Name, FreeVars)
  
  rnCmd (HsCmdArrApp arrow arg _ ho rtl)
    = do { (arrow',fvArrow) <- select_arrow_scope (rnLExpr arrow)
         ; (arg',fvArg) <- rnLExpr arg
         ; return (HsCmdArrApp arrow' arg' placeHolderType 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 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 op fixity cmds)
    = do { (op',fvOp) <- escapeArrowScope (rnLExpr op)
         ; (cmds',fvCmds) <- rnCmdArgs cmds
         ; return (HsCmdArrForm op' fixity cmds', fvOp `plusFV` fvCmds) }
  
  rnCmd (HsCmdApp fun arg)
    = do { (fun',fvFun) <- rnLCmd  fun
         ; (arg',fvArg) <- rnLExpr arg
         ; return (HsCmdApp fun' arg', fvFun `plusFV` fvArg) }
  
  rnCmd (HsCmdLam matches)
    = do { (matches', fvMatch) <- rnMatchGroup LambdaExpr rnLCmd matches
         ; return (HsCmdLam matches', fvMatch) }
  
  rnCmd (HsCmdPar e)
    = do  { (e', fvs_e) <- rnLCmd e
          ; return (HsCmdPar e', fvs_e) }
  
  rnCmd (HsCmdCase expr matches)
    = do { (new_expr, e_fvs) <- rnLExpr expr
         ; (new_matches, ms_fvs) <- rnMatchGroup CaseAlt rnLCmd matches
         ; return (HsCmdCase new_expr new_matches, e_fvs `plusFV` ms_fvs) }
  
  rnCmd (HsCmdIf _ p b1 b2)
    = do { (p', fvP) <- rnLExpr p
         ; (b1', fvB1) <- rnLCmd b1
         ; (b2', fvB2) <- rnLCmd b2
         ; (mb_ite, fvITE) <- lookupIfThenElse
         ; return (HsCmdIf mb_ite p' b1' b2', plusFVs [fvITE, fvP, fvB1, fvB2]) }
  
  rnCmd (HsCmdLet binds cmd)
    = rnLocalBindsAndThen binds $ \ binds' _ -> do
        { (cmd',fvExpr) <- rnLCmd cmd
        ; return (HsCmdLet binds' cmd', fvExpr) }
  
  rnCmd (HsCmdDo stmts _)
    = do  { ((stmts', _), fvs) <-
              rnStmts ArrowExpr rnLCmd stmts (\ _ -> return ((), emptyFVs))
          ; return ( HsCmdDo stmts' placeHolderType, fvs ) }
  
  rnCmd cmd@(HsCmdCast {}) = pprPanic "rnCmd" (ppr cmd)
  
  ---------------------------------------------------
  type CmdNeeds = FreeVars        -- Only inhabitants are
                                  --      appAName, choiceAName, loopAName
  
  -- find what methods the Cmd needs (loop, choice, apply)
  methodNamesLCmd :: LHsCmd Name -> CmdNeeds
  methodNamesLCmd = methodNamesCmd . unLoc
  
  methodNamesCmd :: HsCmd Name -> CmdNeeds
  
  methodNamesCmd (HsCmdArrApp _arrow _arg _ HsFirstOrderApp _rtl)
    = emptyFVs
  methodNamesCmd (HsCmdArrApp _arrow _arg _ HsHigherOrderApp _rtl)
    = unitFV appAName
  methodNamesCmd (HsCmdArrForm {}) = emptyFVs
  methodNamesCmd (HsCmdCast _ 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 stmts _) = methodNamesStmts stmts
  methodNamesCmd (HsCmdApp c _)      = methodNamesLCmd c
  methodNamesCmd (HsCmdLam match)    = methodNamesMatch match
  
  methodNamesCmd (HsCmdCase _ matches)
    = methodNamesMatch matches `addOneFV` choiceAName
  
  --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 Name (LHsCmd Name) -> FreeVars
  methodNamesMatch (MG { mg_alts = ms })
    = plusFVs (map do_one ms)
   where
      do_one (L _ (Match _ _ _ grhss)) = methodNamesGRHSs grhss
  
  -------------------------------------------------
  -- gaw 2004
  methodNamesGRHSs :: GRHSs Name (LHsCmd Name) -> FreeVars
  methodNamesGRHSs (GRHSs grhss _) = plusFVs (map methodNamesGRHS grhss)
  
  -------------------------------------------------
  
  methodNamesGRHS :: Located (GRHS Name (LHsCmd Name)) -> CmdNeeds
  methodNamesGRHS (L _ (GRHS _ rhs)) = methodNamesLCmd rhs
  
  ---------------------------------------------------
  methodNamesStmts :: [Located (StmtLR Name Name (LHsCmd Name))] -> FreeVars
  methodNamesStmts stmts = plusFVs (map methodNamesLStmt stmts)
  
  ---------------------------------------------------
  methodNamesLStmt :: Located (StmtLR Name Name (LHsCmd Name)) -> FreeVars
  methodNamesLStmt = methodNamesStmt . unLoc
  
  methodNamesStmt :: StmtLR Name Name (LHsCmd Name) -> 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
  
  {-
  ************************************************************************
  *                                                                      *
          Arithmetic sequences
  *                                                                      *
  ************************************************************************
  -}
  
  rnArithSeq :: ArithSeqInfo RdrName -> RnM (ArithSeqInfo Name, 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]) }
  
  {-
  ************************************************************************
  *                                                                      *
  *                                                                      *
  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 Name, FreeVars)]
    -> RnM ([ExprLStmt Name], 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 Name, FreeVars)
  type Cost = Int
  
  -- | Turn a sequence of statements into an ExprStmtTree using a
  -- heuristic algorithm.  /O(n^2)/
  mkStmtTreeHeuristic :: [(ExprLStmt Name, 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 Name, 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 Name]             -- ^ the "tail"
    -> FreeVars                     -- ^ free variables of the tail
    -> RnM ( [ExprLStmt Name]       -- ( 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 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 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 pat exp False, emptyFVs)
     stmtTreeArg _ctxt _tail_fvs (StmtTreeOne (L _ (BodyStmt exp _ _ _), _)) =
       return (ApplicativeArgOne 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) <- lookupStmtName ctxt returnMName
               return (HsApp (noLoc ret) tup, fvs)
       return ( ApplicativeArgMany 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 Name, FreeVars)]
    -> [[(ExprLStmt Name, 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 Name, FreeVars)] -> [[(ExprLStmt Name, 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 Name -> 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
      SigPatIn p _    -> isStrictPattern p
      SigPatOut p _   -> isStrictPattern p
      BangPat{}       -> True
      ListPat{}       -> True
      TuplePat{}      -> True
      -- SumPat{}        -> True
      PArrPat{}       -> 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 Name, FreeVars)]
    -> ( [(ExprLStmt Name, FreeVars)]
       , [(ExprLStmt Name, 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 Name (Located (body Name)), FreeVars)]
     -> Maybe ( [(LStmt Name (Located (body Name)), FreeVars)] -- LetStmts
              , [(LStmt Name (Located (body Name)), FreeVars)] -- BindStmts
              , [(LStmt Name (Located (body Name)), 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 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 binds), fvs) : rest)
      | isEmptyNameSet (bndrs `intersectNameSet` fvs)
      = go ((L loc (LetStmt 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 Name]             -- ^ The args
    -> Bool                               -- ^ True <=> need a join
    -> [ExprLStmt Name]        -- ^ The body statements
    -> RnM ([ExprLStmt Name], 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
                 (zip (fmap_op : repeat ap_op) args)
                 mb_join (panic "mkApplicativeStmt: ApplicativeStmt not handled!")
         ; 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 Name]
           -> (Bool, [ExprLStmt Name])
  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 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 Name
              -> Maybe (LHsExpr Name)
  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 :: (Name -> Bool) -> LHsExpr Name -> Bool
    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 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{@Stmt@s: in @do@ expressions}
  *                                                                      *
  ************************************************************************
  -}
  
  -- | Rename some Stmts
  rnStmts :: Outputable (body RdrName)
          => HsStmtContext Name
          -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
             -- ^ How to rename the body of each statement (e.g. rnLExpr)
          -> [LStmt RdrName (Located (body RdrName))]
             -- ^ Statements
          -> ([Name] -> RnM (thing, FreeVars))
             -- ^ if these statements scope over something, this renames it
             -- and returns the result.
          -> RnM (([LStmt Name (Located (body Name))], thing), FreeVars)
  rnStmts ctxt rnBody = rnStmtsWithPostProcessing ctxt rnBody noPostProcessStmts
  
  -- | like 'rnStmts' but applies a post-processing step to the renamed Stmts
  rnStmtsWithPostProcessing
          :: Outputable (body RdrName)
          => HsStmtContext Name
          -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
             -- ^ How to rename the body of each statement (e.g. rnLExpr)
          -> (HsStmtContext Name
                -> [(LStmt Name (Located (body Name)), FreeVars)]
                -> RnM ([LStmt Name (Located (body Name))], FreeVars))
             -- ^ postprocess the statements
          -> [LStmt RdrName (Located (body RdrName))]
             -- ^ Statements
          -> ([Name] -> RnM (thing, FreeVars))
             -- ^ if these statements scope over something, this renames it
             -- and returns the result.
          -> RnM (([LStmt Name (Located (body Name))], 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
    -> [(LStmt Name (LHsExpr Name), FreeVars)]
    -> RnM ([LStmt Name (LHsExpr Name)], 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 rearrangeForApplicativeDo ctxt stmts
              else noPostProcessStmts ctxt stmts }
  
  -- | strip the FreeVars annotations from statements
  noPostProcessStmts
    :: HsStmtContext Name
    -> [(LStmt Name (Located (body Name)), FreeVars)]
    -> RnM ([LStmt Name (Located (body Name))], FreeVars)
  noPostProcessStmts _ stmts = return (map fst stmts, emptyNameSet)
  
  
  rnStmtsWithFreeVars :: Outputable (body RdrName)
          => HsStmtContext Name
          -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
          -> [LStmt RdrName (Located (body RdrName))]
          -> ([Name] -> RnM (thing, FreeVars))
          -> RnM ( ([(LStmt Name (Located (body Name)), 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) }
  
  ----------------------
  rnStmt :: Outputable (body RdrName)
         => HsStmtContext Name
         -> (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
            -- ^ How to rename the body of the statement
         -> LStmt RdrName (Located (body RdrName))
            -- ^ The statement
         -> ([Name] -> RnM (thing, FreeVars))
            -- ^ Rename the stuff that this statement scopes over
         -> RnM ( ([(LStmt Name (Located (body Name)), 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 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/parr/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 body' then_op guard_op placeHolderType), 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
          ; (fail_op, fvs2) <- lookupStmtName ctxt failMName
          ; rnPat (StmtCtxt ctxt) pat $ \ pat' -> do
          { (thing, fvs3) <- thing_inside (collectPatBinders pat')
          ; return (([(L loc (BindStmt 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 binds)) thing_inside
    = do  { rnLocalBindsAndThen binds $ \binds' bind_fvs -> do
          { (thing, fvs) <- thing_inside (collectLocalBinders binds')
          ; return (([(L loc (LetStmt 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 = nameSetElems $ foldr (unionNameSet . (\(ds,_,_,_) -> ds))
                                              emptyNameSet segs
          ; (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)   <- lookupStmtName 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 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 varaible
                           -- 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 (noSyntaxExpr, emptyFVs)
                                  _        -> lookupStmtName 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_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"
  
  rnParallelStmts :: forall thing. HsStmtContext Name
                  -> SyntaxExpr Name
                  -> [ParStmtBlock RdrName RdrName]
                  -> ([Name] -> RnM (thing, FreeVars))
                  -> RnM (([ParStmtBlock Name Name], 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 RdrName RdrName]
              -> RnM (([ParStmtBlock Name Name], 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 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 stmts' used_bndrs return_op
             ; return ((seg':segs', thing), fvs) }
  
      cmpByOcc n1 n2 = nameOccName n1 `compare` nameOccName n2
      dupErr vs = addErr (ptext (sLit "Duplicate binding in parallel list comprehension for:")
                      <+> quotes (ppr (head vs)))
  
  lookupStmtName :: HsStmtContext Name -> Name -> RnM (HsExpr Name, FreeVars)
  -- Like lookupSyntaxName, but ListComp/PArrComp are never rebindable
  -- Neither is ArrowExpr, which has its own desugarer in DsArrows
  lookupStmtName ctxt n
    = case ctxt of
        ListComp        -> not_rebindable
        PArrComp        -> not_rebindable
        ArrowExpr       -> not_rebindable
        PatGuard {}     -> not_rebindable
  
        DoExpr          -> rebindable
        MDoExpr         -> rebindable
        MonadComp       -> rebindable
        GhciStmtCtxt    -> rebindable   -- I suppose?
  
        ParStmtCtxt   c -> lookupStmtName c n     -- Look inside to
        TransStmtCtxt c -> lookupStmtName c n     -- the parent context
    where
      rebindable     = lookupSyntaxName n
      not_rebindable = return (HsVar n, emptyFVs)
  
  {-
  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 RdrName) =>
                       (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
                    -> [LStmt RdrName (Located (body RdrName))]
                           -- assumes that the FreeVars returned includes
                           -- the FreeVars of the Segments
                    -> ([Segment (LStmt Name (Located (body Name)))] -> 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 RdrName RdrName body] -> [LFixitySig RdrName]
  collectRecStmtsFixities l =
      foldr (\ s -> \acc -> case s of
                              (L _ (LetStmt (HsValBinds (ValBindsIn _ 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 RdrName 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 Name RdrName body, FreeVars)]
  
  rn_rec_stmt_lhs _ (L loc (BodyStmt body a b c))
    = return [(L loc (BodyStmt body a b c), emptyFVs)]
  
  rn_rec_stmt_lhs _ (L loc (LastStmt body noret a))
    = return [(L loc (LastStmt 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 pat' body a b),
                 fv_pat)]
  
  rn_rec_stmt_lhs _ (L _ (LetStmt binds@(HsIPBinds _)))
    = failWith (badIpBinds (ptext (sLit "an mdo expression")) binds)
  
  rn_rec_stmt_lhs fix_env (L loc (LetStmt (HsValBinds binds)))
      = do (_bound_names, binds') <- rnLocalValBindsLHS fix_env binds
           return [(L loc (LetStmt (HsValBinds 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 EmptyLocalBinds))
    = panic "rn_rec_stmt LetStmt EmptyLocalBinds"
  
  rn_rec_stmts_lhs :: Outputable body => MiniFixityEnv
                   -> [LStmt RdrName body]
                   -> RnM [(LStmtLR Name RdrName 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 RdrName)) =>
                 (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
              -> [Name]
              -> (LStmtLR Name RdrName (Located (body RdrName)), FreeVars)
              -> RnM [Segment (LStmt Name (Located (body Name)))]
          -- 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 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 body' then_op noSyntaxExpr placeHolderType))] }
  
  rn_rec_stmt rnBody _ (L loc (BindStmt pat' body _ _), fv_pat)
    = do { (body', fv_expr) <- rnBody body
         ; (bind_op, fvs1) <- lookupSyntaxName bindMName
         ; (fail_op, fvs2) <- lookupSyntaxName failMName
         ; let bndrs = mkNameSet (collectPatBinders pat')
               fvs   = fv_expr `plusFV` fv_pat `plusFV` fvs1 `plusFV` fvs2
         ; return [(bndrs, fvs, bndrs `intersectNameSet` fvs,
                    L loc (BindStmt pat' body' bind_op fail_op))] }
  
  rn_rec_stmt _ _ (L _ (LetStmt binds@(HsIPBinds _)), _)
    = failWith (badIpBinds (ptext (sLit "an mdo expression")) binds)
  
  rn_rec_stmt _ all_bndrs (L loc (LetStmt (HsValBinds 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 (HsValBinds 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 EmptyLocalBinds), _)
    = panic "rn_rec_stmt: LetStmt EmptyLocalBinds"
  
  rn_rec_stmt _ _ stmt@(L _ (ApplicativeStmt {}), _)
    = pprPanic "rn_rec_stmt: ApplicativeStmt" (ppr stmt)
  
  rn_rec_stmts :: Outputable (body RdrName) =>
                  (Located (body RdrName) -> RnM (Located (body Name), FreeVars))
               -> [Name]
               -> [(LStmtLR Name RdrName (Located (body RdrName)), FreeVars)]
               -> RnM [Segment (LStmt Name (Located (body Name)))]
  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 Name body
                  -> [Segment (LStmt Name body)] -> FreeVars
                  -> ([LStmt Name 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 = nameSetElems (defs `intersectNameSet` fvs_later)
                        , recS_rec_ids   = nameSetElems (defs `intersectNameSet` uses) }]
      , 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 Name body)]
               -> [Segment [LStmt Name 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 Name body                   -- A RecStmt with the SyntaxOps filled in
              -> [Segment [LStmt Name body]]      -- Each Segment has a non-empty list of Stmts
              -> FreeVars                         -- Free vars used 'later'
              -> ([LStmt Name 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 = nameSetElems used_later
                                , recS_rec_ids   = nameSetElems fwds }
      non_rec    = isSingleton ss && isEmptyNameSet fwds
      used_later = defs `intersectNameSet` later_uses
                                  -- The ones needed after the RecStmt
  
  {-
  ************************************************************************
  *                                                                      *
  \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 {})   = ptext (sLit "Empty statement group in parallel comprehension")
  emptyErr (TransStmtCtxt {}) = ptext (sLit "Empty statement group preceding 'group' or 'then'")
  emptyErr ctxt               = ptext (sLit "Empty") <+> pprStmtContext ctxt
  
  ----------------------
  checkLastStmt :: Outputable (body RdrName) => HsStmtContext Name
                -> LStmt RdrName (Located (body RdrName))
                -> RnM (LStmt RdrName (Located (body RdrName)))
  checkLastStmt ctxt lstmt@(L loc stmt)
    = case ctxt of
        ListComp  -> check_comp
        MonadComp -> check_comp
        PArrComp  -> 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 = (ptext (sLit "The last statement in") <+> pprAStmtContext ctxt
                    <+> ptext (sLit "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 RdrName (Located (body RdrName))
            -> RnM ()
  checkStmt ctxt (L _ stmt)
    = do { dflags <- getDynFlags
         ; case okStmt dflags ctxt stmt of
             IsValid        -> return ()
             NotValid extra -> addErr (msg $$ extra) }
    where
     msg = sep [ ptext (sLit "Unexpected") <+> pprStmtCat stmt <+> ptext (sLit "statement")
               , ptext (sLit "in") <+> pprAStmtContext ctxt ]
  
  pprStmtCat :: Stmt a body -> SDoc
  pprStmtCat (TransStmt {})     = ptext (sLit "transform")
  pprStmtCat (LastStmt {})      = ptext (sLit "return expression")
  pprStmtCat (BodyStmt {})      = ptext (sLit "body")
  pprStmtCat (BindStmt {})      = ptext (sLit "binding")
  pprStmtCat (LetStmt {})       = ptext (sLit "let")
  pprStmtCat (RecStmt {})       = ptext (sLit "rec")
  pprStmtCat (ParStmt {})       = ptext (sLit "parallel")
  pprStmtCat (ApplicativeStmt {}) = panic "pprStmtCat: ApplicativeStmt"
  
  ------------
  emptyInvalid :: Validity  -- Payload is the empty document
  emptyInvalid = NotValid Outputable.empty
  
  okStmt, okDoStmt, okCompStmt, okParStmt, okPArrStmt
     :: DynFlags -> HsStmtContext Name
     -> Stmt RdrName (Located (body RdrName)) -> Validity
  -- Return Nothing if OK, (Just extra) if not ok
  -- The "extra" is an SDoc that is appended to an 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
        PArrComp           -> okPArrStmt dflags ctxt stmt
        TransStmtCtxt ctxt -> okStmt dflags ctxt stmt
  
  -------------
  okPatGuardStmt :: Stmt RdrName (Located (body RdrName)) -> Validity
  okPatGuardStmt stmt
    = case stmt of
        BodyStmt {} -> IsValid
        BindStmt {} -> IsValid
        LetStmt {}  -> IsValid
        _           -> emptyInvalid
  
  -------------
  okParStmt dflags ctxt stmt
    = case stmt of
        LetStmt (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 (ptext (sLit "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 (ptext (sLit "Use ParallelListComp"))
         TransStmt {}
           | LangExt.TransformListComp `xopt` dflags -> IsValid
           | otherwise -> NotValid (ptext (sLit "Use TransformListComp"))
         RecStmt {}  -> emptyInvalid
         LastStmt {} -> emptyInvalid  -- Should not happen (dealt with by checkLastStmt)
         ApplicativeStmt {} -> emptyInvalid
  
  ----------------
  okPArrStmt dflags _ stmt
    = case stmt of
         BindStmt {} -> IsValid
         LetStmt {}  -> IsValid
         BodyStmt {} -> IsValid
         ParStmt {}
           | LangExt.ParallelListComp `xopt` dflags -> IsValid
           | otherwise -> NotValid (ptext (sLit "Use ParallelListComp"))
         TransStmt {} -> emptyInvalid
         RecStmt {}   -> emptyInvalid
         LastStmt {}  -> emptyInvalid  -- Should not happen (dealt with by checkLastStmt)
         ApplicativeStmt {} -> emptyInvalid
  
  ---------
  checkTupleSection :: [LHsTupArg RdrName] -> RnM ()
  checkTupleSection args
    = do  { tuple_section <- xoptM LangExt.TupleSections
          ; checkErr (all tupArgPresent args || tuple_section) msg }
    where
      msg = ptext (sLit "Illegal tuple section: use TupleSections")
  
  ---------
  sectionErr :: HsExpr RdrName -> SDoc
  sectionErr expr
    = hang (ptext (sLit "A section must be enclosed in parentheses"))
         2 (ptext (sLit "thus:") <+> (parens (ppr expr)))
  
  patSynErr :: HsExpr RdrName -> RnM (HsExpr Name, FreeVars)
  patSynErr e = do { addErr (sep [ptext (sLit "Pattern syntax in expression context:"),
                                  nest 4 (ppr e)])
                   ; return (EWildPat, emptyFVs) }
  
  badIpBinds :: Outputable a => SDoc -> a -> SDoc
  badIpBinds what binds
    = hang (ptext (sLit "Implicit-parameter bindings illegal in") <+> what)
           2 (ppr binds)
  