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, ScopedTypeVariables #-}

    module RnExpr (
            rnLExpr, rnExpr, rnStmts
       ) where

    #include "HsVersions.h"

    import {-# SOURCE #-} TcSplice( runQuasiQuoteExpr )

    import RnBinds   ( rnLocalBindsAndThen, rnLocalValBindsLHS, rnLocalValBindsRHS,
                       rnMatchGroup, rnGRHS, makeMiniFixityEnv)
    import HsSyn
    import TcRnMonad
    import Module           ( getModule )
    import RnEnv
    import RnSplice         ( rnBracket, rnSpliceExpr, checkThLocalName )
    import RnTypes
    import RnPat
    import DynFlags
    import BasicTypes       ( FixityDirection(..) )
    import PrelNames

    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 )

    {-
    ************************************************************************
    *                                                                      *
    \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 (HsLit lit@(HsString src s))
      = do { opt_OverloadedStrings <- xoptM Opt_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', fvs) <- rnOverLit lit
           ; return (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-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 is_typed splice) = rnSpliceExpr is_typed splice

    rnExpr (HsQuasiQuoteE qq)
      = do { lexpr' <- runQuasiQuoteExpr qq
             -- Wrap the result of the quasi-quoter in parens so that we don't
             -- lose the outermost location set by runQuasiQuote (#7918)
           ; rnExpr (HsPar lexpr') }

    ---------------------------------------------
    --      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) <- rnStmts do_or_lc rnLExpr stmts (\ _ -> return ((), emptyFVs))
            ; return ( HsDo do_or_lc stmts' placeHolderType, fvs ) }

    rnExpr (ExplicitList _ _  exps)
      = do  { opt_OverloadedLists <- xoptM Opt_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 Opt_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
       -- 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]) }

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

    rnStmts :: 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))], thing), FreeVars)
    -- Variables bound by the Stmts, and mentioned in thing_inside,
    -- do not appear in the result FreeVars

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

    rnStmts 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

    rnStmts 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 ->
                        rnStmts 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))
           -> LStmt RdrName (Located (body RdrName))
           -> ([Name] -> RnM (thing, FreeVars))
           -> RnM (([LStmt Name (Located (body Name))], 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 _)) thing_inside
      = do  { (body', fv_expr) <- rnBody body
            ; (ret_op, fvs1)   <- lookupStmtName ctxt returnMName
            ; (thing,  fvs3)   <- thing_inside []
            ; return (([L loc (LastStmt body' ret_op)], 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)], 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)], 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' -> do
            { (thing, fvs) <- thing_inside (collectLocalBinders binds')
            ; return (([L loc (LetStmt binds')], 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
            ; return ((rec_stmts', 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)], 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 (text "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 })], thing), all_fvs) }

    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 by 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 a))
      = return [(L loc (LastStmt body 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 _ (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 _), _)
      = do  { (body', fv_expr) <- rnBody body
            ; (ret_op, fvs1)   <- lookupSyntaxName returnMName
            ; return [(emptyNameSet, fv_expr `plusFV` fvs1, emptyNameSet,
                       L loc (LastStmt body' 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
           ; return [(duDefs du_binds, allUses du_binds,
                      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_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")

    ------------
    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 {}
             | Opt_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 {}
             | Opt_ParallelListComp `xopt` dflags -> IsValid
             | otherwise -> NotValid (ptext (sLit "Use ParallelListComp"))
           TransStmt {}
             | Opt_TransformListComp `xopt` dflags -> IsValid
             | otherwise -> NotValid (ptext (sLit "Use TransformListComp"))
           RecStmt {}  -> emptyInvalid
           LastStmt {} -> emptyInvalid  -- Should not happen (dealt with by checkLastStmt)

    ----------------
    okPArrStmt dflags _ stmt
      = case stmt of
           BindStmt {} -> IsValid
           LetStmt {}  -> IsValid
           BodyStmt {} -> IsValid
           ParStmt {}
             | Opt_ParallelListComp `xopt` dflags -> IsValid
             | otherwise -> NotValid (ptext (sLit "Use ParallelListComp"))
           TransStmt {} -> emptyInvalid
           RecStmt {}   -> emptyInvalid
           LastStmt {}  -> emptyInvalid  -- Should not happen (dealt with by checkLastStmt)

    ---------
    checkTupleSection :: [LHsTupArg RdrName] -> RnM ()
    checkTupleSection args
      = do  { tuple_section <- xoptM Opt_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)