module Convert( convertToHsExpr, convertToPat, convertToHsDecls,
                convertToHsType,
                thRdrNameGuesses ) where

import HsSyn as Hs
import qualified Class
import RdrName
import qualified Name
import Module
import RdrHsSyn
import qualified OccName
import OccName
import SrcLoc
import Type
import qualified Coercion ( Role(..) )
import TysWiredIn
import BasicTypes as Hs
import ForeignCall
import Unique
import ErrUtils
import Bag
import Util
import FastString
import Outputable

import qualified Data.ByteString as BS
import Control.Monad( unless )

import Language.Haskell.TH as TH hiding (sigP)
import Language.Haskell.TH.Syntax as TH
import GHC.Exts

-------------------------------------------------------------------
--              The external interface

convertToHsDecls :: SrcSpan -> [TH.Dec] -> Either MsgDoc [LHsDecl RdrName]
convertToHsDecls loc ds = initCvt loc (mapM cvt_dec ds)
  where
    cvt_dec d = wrapMsg "declaration" d (cvtDec d)

convertToHsExpr :: SrcSpan -> TH.Exp -> Either MsgDoc (LHsExpr RdrName)
convertToHsExpr loc e
  = initCvt loc $ wrapMsg "expression" e $ cvtl e

convertToPat :: SrcSpan -> TH.Pat -> Either MsgDoc (LPat RdrName)
convertToPat loc p
  = initCvt loc $ wrapMsg "pattern" p $ cvtPat p

convertToHsType :: SrcSpan -> TH.Type -> Either MsgDoc (LHsType RdrName)
convertToHsType loc t
  = initCvt loc $ wrapMsg "type" t $ cvtType t

-------------------------------------------------------------------
newtype CvtM a = CvtM { unCvtM :: SrcSpan -> Either MsgDoc a }
        -- Push down the source location;
        -- Can fail, with a single error message

-- NB: If the conversion succeeds with (Right x), there should
--     be no exception values hiding in x
-- Reason: so a (head []) in TH code doesn't subsequently
--         make GHC crash when it tries to walk the generated tree

-- Use the loc everywhere, for lack of anything better
-- In particular, we want it on binding locations, so that variables bound in
-- the spliced-in declarations get a location that at least relates to the splice point

instance Monad CvtM where
  return x       = CvtM $ \_   -> Right x
  (CvtM m) >>= k = CvtM $ \loc -> case m loc of
                                  Left err -> Left err
                                  Right v  -> unCvtM (k v) loc

initCvt :: SrcSpan -> CvtM a -> Either MsgDoc a
initCvt loc (CvtM m) = m loc

force :: a -> CvtM ()
force a = a `seq` return ()

failWith :: MsgDoc -> CvtM a
failWith m = CvtM (\_ -> Left m)

getL :: CvtM SrcSpan
getL = CvtM (\loc -> Right loc)

returnL :: a -> CvtM (Located a)
returnL x = CvtM (\loc -> Right (L loc x))

wrapParL :: (Located a -> a) -> a -> CvtM a
wrapParL add_par x = CvtM (\loc -> Right (add_par (L loc x)))

wrapMsg :: (Show a, TH.Ppr a) => String -> a -> CvtM b -> CvtM b
-- E.g  wrapMsg "declaration" dec thing
wrapMsg what item (CvtM m)
  = CvtM (\loc -> case m loc of
                     Left err -> Left (err $$ getPprStyle msg)
                     Right v  -> Right v)
  where
        -- Show the item in pretty syntax normally,
        -- but with all its constructors if you say -dppr-debug
    msg sty = hang (ptext (sLit "When splicing a TH") <+> text what <> colon)
                 2 (if debugStyle sty
                    then text (show item)
                    else text (pprint item))

wrapL :: CvtM a -> CvtM (Located a)
wrapL (CvtM m) = CvtM (\loc -> case m loc of
                               Left err -> Left err
                               Right v  -> Right (L loc v))

-------------------------------------------------------------------
cvtDec :: TH.Dec -> CvtM (LHsDecl RdrName)
cvtDec (TH.ValD pat body ds)
  | TH.VarP s <- pat
  = do  { s' <- vNameL s
        ; cl' <- cvtClause (Clause [] body ds)
        ; returnL $ Hs.ValD $ mkFunBind s' [cl'] }

  | otherwise
  = do  { pat' <- cvtPat pat
        ; body' <- cvtGuard body
        ; ds' <- cvtLocalDecs (ptext (sLit "a where clause")) ds
        ; returnL $ Hs.ValD $
          PatBind { pat_lhs = pat', pat_rhs = GRHSs body' ds'
                  , pat_rhs_ty = void, bind_fvs = placeHolderNames
                  , pat_ticks = (Nothing,[]) } }

cvtDec (TH.FunD nm cls)
  | null cls
  = failWith (ptext (sLit "Function binding for")
                 <+> quotes (text (TH.pprint nm))
                 <+> ptext (sLit "has no equations"))
  | otherwise
  = do  { nm' <- vNameL nm
        ; cls' <- mapM cvtClause cls
        ; returnL $ Hs.ValD $ mkFunBind nm' cls' }

cvtDec (TH.SigD nm typ)
  = do  { nm' <- vNameL nm
        ; ty' <- cvtType typ
        ; returnL $ Hs.SigD (TypeSig [nm'] ty') }

cvtDec (TH.InfixD fx nm)
  = do { nm' <- vNameL nm
       ; returnL (Hs.SigD (FixSig (FixitySig nm' (cvtFixity fx)))) }

cvtDec (PragmaD prag)
  = cvtPragmaD prag

cvtDec (TySynD tc tvs rhs)
  = do  { (_, tc', tvs') <- cvt_tycl_hdr [] tc tvs
        ; rhs' <- cvtType rhs
        ; returnL $ TyClD (SynDecl { tcdLName = tc'
                                  , tcdTyVars = tvs', tcdFVs = placeHolderNames
                                  , tcdRhs = rhs' }) }

cvtDec (DataD ctxt tc tvs constrs derivs)
  = do  { (ctxt', tc', tvs') <- cvt_tycl_hdr ctxt tc tvs
        ; cons' <- mapM cvtConstr constrs
        ; derivs' <- cvtDerivs derivs
        ; let defn = HsDataDefn { dd_ND = DataType, dd_cType = Nothing
                                , dd_ctxt = ctxt'
                                , dd_kindSig = Nothing
                                , dd_cons = cons', dd_derivs = derivs' }
        ; returnL $ TyClD (DataDecl { tcdLName = tc', tcdTyVars = tvs'
                                    , tcdDataDefn = defn, tcdFVs = placeHolderNames }) }

cvtDec (NewtypeD ctxt tc tvs constr derivs)
  = do  { (ctxt', tc', tvs') <- cvt_tycl_hdr ctxt tc tvs
        ; con' <- cvtConstr constr
        ; derivs' <- cvtDerivs derivs
        ; let defn = HsDataDefn { dd_ND = NewType, dd_cType = Nothing
                                , dd_ctxt = ctxt'
                                , dd_kindSig = Nothing
                                , dd_cons = [con'], dd_derivs = derivs' }
        ; returnL $ TyClD (DataDecl { tcdLName = tc', tcdTyVars = tvs'
                                    , tcdDataDefn = defn, tcdFVs = placeHolderNames }) }

cvtDec (ClassD ctxt cl tvs fds decs)
  = do  { (cxt', tc', tvs') <- cvt_tycl_hdr ctxt cl tvs
        ; fds'  <- mapM cvt_fundep fds
        ; (binds', sigs', fams', ats', adts') <- cvt_ci_decs (ptext (sLit "a class declaration")) decs
        ; unless (null adts')
            (failWith $ (ptext (sLit "Default data instance declarations are not allowed:"))
                   $$ (Outputable.ppr adts'))
        ; returnL $ TyClD $
          ClassDecl { tcdCtxt = cxt', tcdLName = tc', tcdTyVars = tvs'
                    , tcdFDs = fds', tcdSigs = sigs', tcdMeths = binds'
                    , tcdATs = fams', tcdATDefs = ats', tcdDocs = []
                    , tcdFVs = placeHolderNames }
                              -- no docs in TH ^^
        }

cvtDec (InstanceD ctxt ty decs)
  = do  { let doc = ptext (sLit "an instance declaration")
        ; (binds', sigs', fams', ats', adts') <- cvt_ci_decs doc decs
        ; unless (null fams') (failWith (mkBadDecMsg doc fams'))
        ; ctxt' <- cvtContext ctxt
        ; L loc ty' <- cvtType ty
        ; let inst_ty' = L loc $ mkImplicitHsForAllTy ctxt' $ L loc ty'
        ; returnL $ InstD (ClsInstD (ClsInstDecl inst_ty' binds' sigs' ats' adts')) }

cvtDec (ForeignD ford)
  = do { ford' <- cvtForD ford
       ; returnL $ ForD ford' }

cvtDec (FamilyD flav tc tvs kind)
  = do { (_, tc', tvs') <- cvt_tycl_hdr [] tc tvs
       ; kind' <- cvtMaybeKind kind
       ; returnL $ TyClD (FamDecl (FamilyDecl (cvtFamFlavour flav) tc' tvs' kind')) }
  where
    cvtFamFlavour TypeFam = OpenTypeFamily
    cvtFamFlavour DataFam = DataFamily

cvtDec (DataInstD ctxt tc tys constrs derivs)
  = do { (ctxt', tc', typats') <- cvt_tyinst_hdr ctxt tc tys
       ; cons' <- mapM cvtConstr constrs
       ; derivs' <- cvtDerivs derivs
       ; let defn = HsDataDefn { dd_ND = DataType, dd_cType = Nothing
                               , dd_ctxt = ctxt'
                               , dd_kindSig = Nothing
                               , dd_cons = cons', dd_derivs = derivs' }

       ; returnL $ InstD $ DataFamInstD
           { dfid_inst = DataFamInstDecl { dfid_tycon = tc', dfid_pats = typats'
                                         , dfid_defn = defn, dfid_fvs = placeHolderNames } }}

cvtDec (NewtypeInstD ctxt tc tys constr derivs)
  = do { (ctxt', tc', typats') <- cvt_tyinst_hdr ctxt tc tys
       ; con' <- cvtConstr constr
       ; derivs' <- cvtDerivs derivs
       ; let defn = HsDataDefn { dd_ND = NewType, dd_cType = Nothing
                               , dd_ctxt = ctxt'
                               , dd_kindSig = Nothing
                               , dd_cons = [con'], dd_derivs = derivs' }
       ; returnL $ InstD $ DataFamInstD
           { dfid_inst = DataFamInstDecl { dfid_tycon = tc', dfid_pats = typats'
                                         , dfid_defn = defn, dfid_fvs = placeHolderNames } }}

cvtDec (TySynInstD tc eqn)
  = do  { tc' <- tconNameL tc
        ; eqn' <- cvtTySynEqn tc' eqn
        ; returnL $ InstD $ TyFamInstD
            { tfid_inst = TyFamInstDecl { tfid_eqn = eqn'
                                        , tfid_fvs = placeHolderNames } } }

cvtDec (ClosedTypeFamilyD tc tyvars mkind eqns)
  | not $ null eqns
  = do { (_, tc', tvs') <- cvt_tycl_hdr [] tc tyvars
       ; mkind' <- cvtMaybeKind mkind
       ; eqns' <- mapM (cvtTySynEqn tc') eqns
       ; returnL $ TyClD (FamDecl (FamilyDecl (ClosedTypeFamily eqns') tc' tvs' mkind')) }
  | otherwise
  = failWith (ptext (sLit "Illegal empty closed type family"))
----------------
cvtTySynEqn :: Located RdrName -> TySynEqn -> CvtM (LTyFamInstEqn RdrName)
cvtTySynEqn tc (TySynEqn lhs rhs)
  = do  { lhs' <- mapM cvtType lhs
        ; rhs' <- cvtType rhs
        ; returnL $ TyFamInstEqn { tfie_tycon = tc
                                 , tfie_pats = mkHsWithBndrs lhs'
                                 , tfie_rhs = rhs' } }

----------------
cvt_ci_decs :: MsgDoc -> [TH.Dec]
            -> CvtM (LHsBinds RdrName,
                     [LSig RdrName],
                     [LFamilyDecl RdrName],
                     [LTyFamInstDecl RdrName],
                     [LDataFamInstDecl RdrName])
-- Convert the declarations inside a class or instance decl
-- ie signatures, bindings, and associated types
cvt_ci_decs doc decs
  = do  { decs' <- mapM cvtDec decs
        ; let (ats', bind_sig_decs') = partitionWith is_tyfam_inst decs'
        ; let (adts', no_ats')       = partitionWith is_datafam_inst bind_sig_decs'
        ; let (sigs', prob_binds')   = partitionWith is_sig no_ats'
        ; let (binds', prob_fams')   = partitionWith is_bind prob_binds'
        ; let (fams', bads)          = partitionWith is_fam_decl prob_fams'
        ; unless (null bads) (failWith (mkBadDecMsg doc bads))
        ; return (listToBag binds', sigs', fams', ats', adts') }

----------------
cvt_tycl_hdr :: TH.Cxt -> TH.Name -> [TH.TyVarBndr]
             -> CvtM ( LHsContext RdrName
                     , Located RdrName
                     , LHsTyVarBndrs RdrName)
cvt_tycl_hdr cxt tc tvs
  = do { cxt' <- cvtContext cxt
       ; tc'  <- tconNameL tc
       ; tvs' <- cvtTvs tvs
       ; return (cxt', tc', tvs')
       }

cvt_tyinst_hdr :: TH.Cxt -> TH.Name -> [TH.Type]
               -> CvtM ( LHsContext RdrName
                       , Located RdrName
                       , HsWithBndrs [LHsType RdrName])
cvt_tyinst_hdr cxt tc tys
  = do { cxt' <- cvtContext cxt
       ; tc'  <- tconNameL tc
       ; tys' <- mapM cvtType tys
       ; return (cxt', tc', mkHsWithBndrs tys') }

-------------------------------------------------------------------
--              Partitioning declarations
-------------------------------------------------------------------

is_fam_decl :: LHsDecl RdrName -> Either (LFamilyDecl RdrName) (LHsDecl RdrName)
is_fam_decl (L loc (TyClD (FamDecl { tcdFam = d }))) = Left (L loc d)
is_fam_decl decl = Right decl

is_tyfam_inst :: LHsDecl RdrName -> Either (LTyFamInstDecl RdrName) (LHsDecl RdrName)
is_tyfam_inst (L loc (Hs.InstD (TyFamInstD { tfid_inst = d }))) = Left (L loc d)
is_tyfam_inst decl                                              = Right decl

is_datafam_inst :: LHsDecl RdrName -> Either (LDataFamInstDecl RdrName) (LHsDecl RdrName)
is_datafam_inst (L loc (Hs.InstD (DataFamInstD { dfid_inst = d }))) = Left (L loc d)
is_datafam_inst decl                                                = Right decl

is_sig :: LHsDecl RdrName -> Either (LSig RdrName) (LHsDecl RdrName)
is_sig (L loc (Hs.SigD sig)) = Left (L loc sig)
is_sig decl                  = Right decl

is_bind :: LHsDecl RdrName -> Either (LHsBind RdrName) (LHsDecl RdrName)
is_bind (L loc (Hs.ValD bind)) = Left (L loc bind)
is_bind decl                   = Right decl

mkBadDecMsg :: Outputable a => MsgDoc -> [a] -> MsgDoc
mkBadDecMsg doc bads
  = sep [ ptext (sLit "Illegal declaration(s) in") <+> doc <> colon
        , nest 2 (vcat (map Outputable.ppr bads)) ]

---------------------------------------------------
--      Data types
-- Can't handle GADTs yet
---------------------------------------------------

cvtConstr :: TH.Con -> CvtM (LConDecl RdrName)

cvtConstr (NormalC c strtys)
  = do  { c'   <- cNameL c
        ; cxt' <- returnL []
        ; tys' <- mapM cvt_arg strtys
        ; returnL $ mkSimpleConDecl c' noExistentials cxt' (PrefixCon tys') }

cvtConstr (RecC c varstrtys)
  = do  { c'    <- cNameL c
        ; cxt'  <- returnL []
        ; args' <- mapM cvt_id_arg varstrtys
        ; returnL $ mkSimpleConDecl c' noExistentials cxt' (RecCon args') }

cvtConstr (InfixC st1 c st2)
  = do  { c' <- cNameL c
        ; cxt' <- returnL []
        ; st1' <- cvt_arg st1
        ; st2' <- cvt_arg st2
        ; returnL $ mkSimpleConDecl c' noExistentials cxt' (InfixCon st1' st2') }

cvtConstr (ForallC tvs ctxt con)
  = do  { tvs'  <- cvtTvs tvs
        ; L loc ctxt' <- cvtContext ctxt
        ; L _ con' <- cvtConstr con
        ; returnL $ con' { con_qvars = mkHsQTvs (hsQTvBndrs tvs' ++ hsQTvBndrs (con_qvars con'))
                         , con_cxt = L loc (ctxt' ++ (unLoc $ con_cxt con')) } }

cvt_arg :: (TH.Strict, TH.Type) -> CvtM (LHsType RdrName)
cvt_arg (NotStrict, ty) = cvtType ty
cvt_arg (IsStrict,  ty) = do { ty' <- cvtType ty; returnL $ HsBangTy (HsUserBang Nothing     True) ty' }
cvt_arg (Unpacked,  ty) = do { ty' <- cvtType ty; returnL $ HsBangTy (HsUserBang (Just True) True) ty' }

cvt_id_arg :: (TH.Name, TH.Strict, TH.Type) -> CvtM (ConDeclField RdrName)
cvt_id_arg (i, str, ty)
  = do  { i' <- vNameL i
        ; ty' <- cvt_arg (str,ty)
        ; return (ConDeclField { cd_fld_name = i', cd_fld_type =  ty', cd_fld_doc = Nothing}) }

cvtDerivs :: [TH.Name] -> CvtM (Maybe [LHsType RdrName])
cvtDerivs [] = return Nothing
cvtDerivs cs = do { cs' <- mapM cvt_one cs
                  ; return (Just cs') }
        where
          cvt_one c = do { c' <- tconName c
                         ; returnL $ HsTyVar c' }

cvt_fundep :: FunDep -> CvtM (Located (Class.FunDep RdrName))
cvt_fundep (FunDep xs ys) = do { xs' <- mapM tName xs; ys' <- mapM tName ys; returnL (xs', ys') }

noExistentials :: [LHsTyVarBndr RdrName]
noExistentials = []

------------------------------------------
--      Foreign declarations
------------------------------------------

cvtForD :: Foreign -> CvtM (ForeignDecl RdrName)
cvtForD (ImportF callconv safety from nm ty)
  | Just impspec <- parseCImport (cvt_conv callconv) safety'
                                 (mkFastString (TH.nameBase nm)) from
  = do { nm' <- vNameL nm
       ; ty' <- cvtType ty
       ; return (ForeignImport nm' ty' noForeignImportCoercionYet impspec)
       }
  | otherwise
  = failWith $ text (show from) <+> ptext (sLit "is not a valid ccall impent")
  where
    safety' = case safety of
                     Unsafe     -> PlayRisky
                     Safe       -> PlaySafe
                     Interruptible -> PlayInterruptible

cvtForD (ExportF callconv as nm ty)
  = do  { nm' <- vNameL nm
        ; ty' <- cvtType ty
        ; let e = CExport (CExportStatic (mkFastString as) (cvt_conv callconv))
        ; return $ ForeignExport nm' ty' noForeignExportCoercionYet e }

cvt_conv :: TH.Callconv -> CCallConv
cvt_conv TH.CCall   = CCallConv
cvt_conv TH.StdCall = StdCallConv

------------------------------------------
--              Pragmas
------------------------------------------

cvtPragmaD :: Pragma -> CvtM (LHsDecl RdrName)
cvtPragmaD (InlineP nm inline rm phases)
  = do { nm' <- vNameL nm
       ; let dflt = dfltActivation inline
       ; let ip   = InlinePragma { inl_inline = cvtInline inline
                                 , inl_rule   = cvtRuleMatch rm
                                 , inl_act    = cvtPhases phases dflt
                                 , inl_sat    = Nothing }
       ; returnL $ Hs.SigD $ InlineSig nm' ip }

cvtPragmaD (SpecialiseP nm ty inline phases)
  = do { nm' <- vNameL nm
       ; ty' <- cvtType ty
       ; let (inline', dflt) = case inline of
               Just inline1 -> (cvtInline inline1, dfltActivation inline1)
               Nothing      -> (EmptyInlineSpec,   AlwaysActive)
       ; let ip = InlinePragma { inl_inline = inline'
                               , inl_rule   = Hs.FunLike
                               , inl_act    = cvtPhases phases dflt
                               , inl_sat    = Nothing }
       ; returnL $ Hs.SigD $ SpecSig nm' ty' ip }

cvtPragmaD (SpecialiseInstP ty)
  = do { ty' <- cvtType ty
       ; returnL $ Hs.SigD $ SpecInstSig ty' }

cvtPragmaD (RuleP nm bndrs lhs rhs phases)
  = do { let nm' = mkFastString nm
       ; let act = cvtPhases phases AlwaysActive
       ; bndrs' <- mapM cvtRuleBndr bndrs
       ; lhs'   <- cvtl lhs
       ; rhs'   <- cvtl rhs
       ; returnL $ Hs.RuleD $ HsRule nm' act bndrs'
                                     lhs' placeHolderNames
                                     rhs' placeHolderNames
       }

dfltActivation :: TH.Inline -> Activation
dfltActivation TH.NoInline = NeverActive
dfltActivation _           = AlwaysActive

cvtInline :: TH.Inline -> Hs.InlineSpec
cvtInline TH.NoInline  = Hs.NoInline
cvtInline TH.Inline    = Hs.Inline
cvtInline TH.Inlinable = Hs.Inlinable

cvtRuleMatch :: TH.RuleMatch -> RuleMatchInfo
cvtRuleMatch TH.ConLike = Hs.ConLike
cvtRuleMatch TH.FunLike = Hs.FunLike

cvtPhases :: TH.Phases -> Activation -> Activation
cvtPhases AllPhases       dflt = dflt
cvtPhases (FromPhase i)   _    = ActiveAfter i
cvtPhases (BeforePhase i) _    = ActiveBefore i

cvtRuleBndr :: TH.RuleBndr -> CvtM (Hs.RuleBndr RdrName)
cvtRuleBndr (RuleVar n)
  = do { n' <- vNameL n
       ; return $ Hs.RuleBndr n' }
cvtRuleBndr (TypedRuleVar n ty)
  = do { n'  <- vNameL n
       ; ty' <- cvtType ty
       ; return $ Hs.RuleBndrSig n' $ mkHsWithBndrs ty' }

---------------------------------------------------
--              Declarations
---------------------------------------------------

cvtLocalDecs :: MsgDoc -> [TH.Dec] -> CvtM (HsLocalBinds RdrName)
cvtLocalDecs doc ds
  | null ds
  = return EmptyLocalBinds
  | otherwise
  = do { ds' <- mapM cvtDec ds
       ; let (binds, prob_sigs) = partitionWith is_bind ds'
       ; let (sigs, bads) = partitionWith is_sig prob_sigs
       ; unless (null bads) (failWith (mkBadDecMsg doc bads))
       ; return (HsValBinds (ValBindsIn (listToBag binds) sigs)) }

cvtClause :: TH.Clause -> CvtM (Hs.LMatch RdrName (LHsExpr RdrName))
cvtClause (Clause ps body wheres)
  = do  { ps' <- cvtPats ps
        ; g'  <- cvtGuard body
        ; ds' <- cvtLocalDecs (ptext (sLit "a where clause")) wheres
        ; returnL $ Hs.Match ps' Nothing (GRHSs g' ds') }


-------------------------------------------------------------------
--              Expressions
-------------------------------------------------------------------

cvtl :: TH.Exp -> CvtM (LHsExpr RdrName)
cvtl e = wrapL (cvt e)
  where
    cvt (VarE s)        = do { s' <- vName s; return $ HsVar s' }
    cvt (ConE s)        = do { s' <- cName s; return $ HsVar s' }
    cvt (LitE l)
      | overloadedLit l = do { l' <- cvtOverLit l; return $ HsOverLit l' }
      | otherwise       = do { l' <- cvtLit l;     return $ HsLit l' }

    cvt (AppE x y)     = do { x' <- cvtl x; y' <- cvtl y; return $ HsApp x' y' }
    cvt (LamE ps e)    = do { ps' <- cvtPats ps; e' <- cvtl e
                            ; return $ HsLam (mkMatchGroup [mkSimpleMatch ps' e']) }
    cvt (LamCaseE ms)  = do { ms' <- mapM cvtMatch ms
                            ; return $ HsLamCase placeHolderType
                                                 (mkMatchGroup ms')
                            }
    cvt (TupE [e])     = do { e' <- cvtl e; return $ HsPar e' }
                                 -- Note [Dropping constructors]
                                 -- Singleton tuples treated like nothing (just parens)
    cvt (TupE es)      = do { es' <- mapM cvtl es; return $ ExplicitTuple (map Present es') Boxed }
    cvt (UnboxedTupE es)      = do { es' <- mapM cvtl es; return $ ExplicitTuple (map Present es') Unboxed }
    cvt (CondE x y z)  = do { x' <- cvtl x; y' <- cvtl y; z' <- cvtl z;
                            ; return $ HsIf (Just noSyntaxExpr) x' y' z' }
    cvt (MultiIfE alts)
      | null alts      = failWith (ptext (sLit "Multi-way if-expression with no alternatives"))
      | otherwise      = do { alts' <- mapM cvtpair alts
                            ; return $ HsMultiIf placeHolderType alts' }
    cvt (LetE ds e)    = do { ds' <- cvtLocalDecs (ptext (sLit "a let expression")) ds
                            ; e' <- cvtl e; return $ HsLet ds' e' }
    cvt (CaseE e ms)   = do { e' <- cvtl e; ms' <- mapM cvtMatch ms
                            ; return $ HsCase e' (mkMatchGroup ms') }
    cvt (DoE ss)       = cvtHsDo DoExpr ss
    cvt (CompE ss)     = cvtHsDo ListComp ss
    cvt (ArithSeqE dd) = do { dd' <- cvtDD dd; return $ ArithSeq noPostTcExpr Nothing dd' }
    cvt (ListE xs)
      | Just s <- allCharLs xs       = do { l' <- cvtLit (StringL s); return (HsLit l') }
             -- Note [Converting strings]
      | otherwise                    = do { xs' <- mapM cvtl xs; return $ ExplicitList void Nothing xs' }

    -- Infix expressions
    cvt (InfixE (Just x) s (Just y)) = do { x' <- cvtl x; s' <- cvtl s; y' <- cvtl y
                                          ; wrapParL HsPar $
                                            OpApp (mkLHsPar x') s' undefined (mkLHsPar y') }
                                            -- Parenthesise both arguments and result,
                                            -- to ensure this operator application does
                                            -- does not get re-associated
                            -- See Note [Operator association]
    cvt (InfixE Nothing  s (Just y)) = do { s' <- cvtl s; y' <- cvtl y
                                          ; wrapParL HsPar $ SectionR s' y' }
                                            -- See Note [Sections in HsSyn] in HsExpr
    cvt (InfixE (Just x) s Nothing ) = do { x' <- cvtl x; s' <- cvtl s
                                          ; wrapParL HsPar $ SectionL x' s' }

    cvt (InfixE Nothing  s Nothing ) = do { s' <- cvtl s; return $ HsPar s' }
                                       -- Can I indicate this is an infix thing?
                                       -- Note [Dropping constructors]

    cvt (UInfixE x s y)  = do { x' <- cvtl x
                              ; let x'' = case x' of
                                            L _ (OpApp {}) -> x'
                                            _ -> mkLHsPar x'
                              ; cvtOpApp x'' s y } --  Note [Converting UInfix]

    cvt (ParensE e)      = do { e' <- cvtl e; return $ HsPar e' }
    cvt (SigE e t)       = do { e' <- cvtl e; t' <- cvtType t
                              ; return $ ExprWithTySig e' t' }
    cvt (RecConE c flds) = do { c' <- cNameL c
                              ; flds' <- mapM cvtFld flds
                              ; return $ RecordCon c' noPostTcExpr (HsRecFields flds' Nothing)}
    cvt (RecUpdE e flds) = do { e' <- cvtl e
                              ; flds' <- mapM cvtFld flds
                              ; return $ RecordUpd e' (HsRecFields flds' Nothing) [] [] [] }

{- Note [Dropping constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we drop constructors from the input (for instance, when we encounter @TupE [e]@)
we must insert parentheses around the argument. Otherwise, @UInfix@ constructors in @e@
could meet @UInfix@ constructors containing the @TupE [e]@. For example:

  UInfixE x * (TupE [UInfixE y + z])

If we drop the singleton tuple but don't insert parentheses, the @UInfixE@s would meet
and the above expression would be reassociated to

  OpApp (OpApp x * y) + z

which we don't want.
-}

cvtFld :: (TH.Name, TH.Exp) -> CvtM (HsRecField RdrName (LHsExpr RdrName))
cvtFld (v,e)
  = do  { v' <- vNameL v; e' <- cvtl e
        ; return (HsRecField { hsRecFieldId = v', hsRecFieldArg = e', hsRecPun = False}) }

cvtDD :: Range -> CvtM (ArithSeqInfo RdrName)
cvtDD (FromR x)           = do { x' <- cvtl x; return $ From x' }
cvtDD (FromThenR x y)     = do { x' <- cvtl x; y' <- cvtl y; return $ FromThen x' y' }
cvtDD (FromToR x y)       = do { x' <- cvtl x; y' <- cvtl y; return $ FromTo x' y' }
cvtDD (FromThenToR x y z) = do { x' <- cvtl x; y' <- cvtl y; z' <- cvtl z; return $ FromThenTo x' y' z' }

{- Note [Operator assocation]
We must be quite careful about adding parens:
  * Infix (UInfix ...) op arg      Needs parens round the first arg
  * Infix (Infix ...) op arg       Needs parens round the first arg
  * UInfix (UInfix ...) op arg     No parens for first arg
  * UInfix (Infix ...) op arg      Needs parens round first arg


Note [Converting UInfix]
~~~~~~~~~~~~~~~~~~~~~~~~
When converting @UInfixE@ and @UInfixP@ values, we want to readjust
the trees to reflect the fixities of the underlying operators:

  UInfixE x * (UInfixE y + z) ---> (x * y) + z

This is done by the renamer (see @mkOppAppRn@ and @mkConOppPatRn@ in
RnTypes), which expects that the input will be completely left-biased.
So we left-bias the trees  of @UInfixP@ and @UInfixE@ that we come across.

Sample input:

  UInfixE
   (UInfixE x op1 y)
   op2
   (UInfixE z op3 w)

Sample output:

  OpApp
    (OpApp
      (OpApp x op1 y)
      op2
      z)
    op3
    w

The functions @cvtOpApp@ and @cvtOpAppP@ are responsible for this
left-biasing.
-}

{- | @cvtOpApp x op y@ converts @op@ and @y@ and produces the operator application @x `op` y@.
The produced tree of infix expressions will be left-biased, provided @x@ is.

We can see that @cvtOpApp@ is correct as follows. The inductive hypothesis
is that @cvtOpApp x op y@ is left-biased, provided @x@ is. It is clear that
this holds for both branches (of @cvtOpApp@), provided we assume it holds for
the recursive calls to @cvtOpApp@.

When we call @cvtOpApp@ from @cvtl@, the first argument will always be left-biased
since we have already run @cvtl@ on it.
-}
cvtOpApp :: LHsExpr RdrName -> TH.Exp -> TH.Exp -> CvtM (HsExpr RdrName)
cvtOpApp x op1 (UInfixE y op2 z)
  = do { l <- wrapL $ cvtOpApp x op1 y
       ; cvtOpApp l op2 z }
cvtOpApp x op y
  = do { op' <- cvtl op
       ; y' <- cvtl y
       ; return (OpApp x op' undefined y') }

-------------------------------------
--      Do notation and statements
-------------------------------------

cvtHsDo :: HsStmtContext Name.Name -> [TH.Stmt] -> CvtM (HsExpr RdrName)
cvtHsDo do_or_lc stmts
  | null stmts = failWith (ptext (sLit "Empty stmt list in do-block"))
  | otherwise
  = do  { stmts' <- cvtStmts stmts
        ; let Just (stmts'', last') = snocView stmts'

        ; last'' <- case last' of
                    L loc (BodyStmt body _ _ _) -> return (L loc (mkLastStmt body))
                    _ -> failWith (bad_last last')

        ; return $ HsDo do_or_lc (stmts'' ++ [last'']) void }
  where
    bad_last stmt = vcat [ ptext (sLit "Illegal last statement of") <+> pprAStmtContext do_or_lc <> colon
                         , nest 2 $ Outputable.ppr stmt
                         , ptext (sLit "(It should be an expression.)") ]

cvtStmts :: [TH.Stmt] -> CvtM [Hs.LStmt RdrName (LHsExpr RdrName)]
cvtStmts = mapM cvtStmt

cvtStmt :: TH.Stmt -> CvtM (Hs.LStmt RdrName (LHsExpr RdrName))
cvtStmt (NoBindS e)    = do { e' <- cvtl e; returnL $ mkBodyStmt e' }
cvtStmt (TH.BindS p e) = do { p' <- cvtPat p; e' <- cvtl e; returnL $ mkBindStmt p' e' }
cvtStmt (TH.LetS ds)   = do { ds' <- cvtLocalDecs (ptext (sLit "a let binding")) ds
                            ; returnL $ LetStmt ds' }
cvtStmt (TH.ParS dss)  = do { dss' <- mapM cvt_one dss; returnL $ ParStmt dss' noSyntaxExpr noSyntaxExpr }
                       where
                         cvt_one ds = do { ds' <- cvtStmts ds; return (ParStmtBlock ds' undefined noSyntaxExpr) }

cvtMatch :: TH.Match -> CvtM (Hs.LMatch RdrName (LHsExpr RdrName))
cvtMatch (TH.Match p body decs)
  = do  { p' <- cvtPat p
        ; g' <- cvtGuard body
        ; decs' <- cvtLocalDecs (ptext (sLit "a where clause")) decs
        ; returnL $ Hs.Match [p'] Nothing (GRHSs g' decs') }

cvtGuard :: TH.Body -> CvtM [LGRHS RdrName (LHsExpr RdrName)]
cvtGuard (GuardedB pairs) = mapM cvtpair pairs
cvtGuard (NormalB e)      = do { e' <- cvtl e; g' <- returnL $ GRHS [] e'; return [g'] }

cvtpair :: (TH.Guard, TH.Exp) -> CvtM (LGRHS RdrName (LHsExpr RdrName))
cvtpair (NormalG ge,rhs) = do { ge' <- cvtl ge; rhs' <- cvtl rhs
                              ; g' <- returnL $ mkBodyStmt ge'
                              ; returnL $ GRHS [g'] rhs' }
cvtpair (PatG gs,rhs)    = do { gs' <- cvtStmts gs; rhs' <- cvtl rhs
                              ; returnL $ GRHS gs' rhs' }

cvtOverLit :: Lit -> CvtM (HsOverLit RdrName)
cvtOverLit (IntegerL i)
  = do { force i; return $ mkHsIntegral i placeHolderType}
cvtOverLit (RationalL r)
  = do { force r; return $ mkHsFractional (cvtFractionalLit r) placeHolderType}
cvtOverLit (StringL s)
  = do { let { s' = mkFastString s }
       ; force s'
       ; return $ mkHsIsString s' placeHolderType
       }
cvtOverLit _ = panic "Convert.cvtOverLit: Unexpected overloaded literal"
-- An Integer is like an (overloaded) '3' in a Haskell source program
-- Similarly 3.5 for fractionals

{- Note [Converting strings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If we get (ListE [CharL 'x', CharL 'y']) we'd like to convert to
a string literal for "xy".  Of course, we might hope to get
(LitE (StringL "xy")), but not always, and allCharLs fails quickly
if it isn't a literal string
-}

allCharLs :: [TH.Exp] -> Maybe String
-- Note [Converting strings]
-- NB: only fire up this setup for a non-empty list, else
--     there's a danger of returning "" for [] :: [Int]!
allCharLs xs
  = case xs of
      LitE (CharL c) : ys -> go [c] ys
      _                   -> Nothing
  where
    go cs []                    = Just (reverse cs)
    go cs (LitE (CharL c) : ys) = go (c:cs) ys
    go _  _                     = Nothing

cvtLit :: Lit -> CvtM HsLit
cvtLit (IntPrimL i)    = do { force i; return $ HsIntPrim i }
cvtLit (WordPrimL w)   = do { force w; return $ HsWordPrim w }
cvtLit (FloatPrimL f)  = do { force f; return $ HsFloatPrim (cvtFractionalLit f) }
cvtLit (DoublePrimL f) = do { force f; return $ HsDoublePrim (cvtFractionalLit f) }
cvtLit (CharL c)       = do { force c; return $ HsChar c }
cvtLit (StringL s)     = do { let { s' = mkFastString s }
                            ; force s'
                            ; return $ HsString s' }
cvtLit (StringPrimL s) = do { let { s' = BS.pack s }
                            ; force s'
                            ; return $ HsStringPrim s' }
cvtLit _ = panic "Convert.cvtLit: Unexpected literal"
        -- cvtLit should not be called on IntegerL, RationalL
        -- That precondition is established right here in
        -- Convert.lhs, hence panic

cvtPats :: [TH.Pat] -> CvtM [Hs.LPat RdrName]
cvtPats pats = mapM cvtPat pats

cvtPat :: TH.Pat -> CvtM (Hs.LPat RdrName)
cvtPat pat = wrapL (cvtp pat)

cvtp :: TH.Pat -> CvtM (Hs.Pat RdrName)
cvtp (TH.LitP l)
  | overloadedLit l    = do { l' <- cvtOverLit l
                            ; return (mkNPat l' Nothing) }
                                  -- Not right for negative patterns;
                                  -- need to think about that!
  | otherwise          = do { l' <- cvtLit l; return $ Hs.LitPat l' }
cvtp (TH.VarP s)       = do { s' <- vName s; return $ Hs.VarPat s' }
cvtp (TupP [p])        = do { p' <- cvtPat p; return $ ParPat p' } -- Note [Dropping constructors]
cvtp (TupP ps)         = do { ps' <- cvtPats ps; return $ TuplePat ps' Boxed void }
cvtp (UnboxedTupP ps)  = do { ps' <- cvtPats ps; return $ TuplePat ps' Unboxed void }
cvtp (ConP s ps)       = do { s' <- cNameL s; ps' <- cvtPats ps
                            ; return $ ConPatIn s' (PrefixCon ps') }
cvtp (InfixP p1 s p2)  = do { s' <- cNameL s; p1' <- cvtPat p1; p2' <- cvtPat p2
                            ; wrapParL ParPat $
                              ConPatIn s' (InfixCon (mkParPat p1') (mkParPat p2')) }
                            -- See Note [Operator association]
cvtp (UInfixP p1 s p2) = do { p1' <- cvtPat p1; cvtOpAppP p1' s p2 } -- Note [Converting UInfix]
cvtp (ParensP p)       = do { p' <- cvtPat p; return $ ParPat p' }
cvtp (TildeP p)        = do { p' <- cvtPat p; return $ LazyPat p' }
cvtp (BangP p)         = do { p' <- cvtPat p; return $ BangPat p' }
cvtp (TH.AsP s p)      = do { s' <- vNameL s; p' <- cvtPat p; return $ AsPat s' p' }
cvtp TH.WildP          = return $ WildPat void
cvtp (RecP c fs)       = do { c' <- cNameL c; fs' <- mapM cvtPatFld fs
                            ; return $ ConPatIn c' $ Hs.RecCon (HsRecFields fs' Nothing) }
cvtp (ListP ps)        = do { ps' <- cvtPats ps; return $ ListPat ps' void Nothing }
cvtp (SigP p t)        = do { p' <- cvtPat p; t' <- cvtType t
                            ; return $ SigPatIn p' (mkHsWithBndrs t') }
cvtp (ViewP e p)       = do { e' <- cvtl e; p' <- cvtPat p; return $ ViewPat e' p' void }

cvtPatFld :: (TH.Name, TH.Pat) -> CvtM (HsRecField RdrName (LPat RdrName))
cvtPatFld (s,p)
  = do  { s' <- vNameL s; p' <- cvtPat p
        ; return (HsRecField { hsRecFieldId = s', hsRecFieldArg = p', hsRecPun = False}) }

{- | @cvtOpAppP x op y@ converts @op@ and @y@ and produces the operator application @x `op` y@.
The produced tree of infix patterns will be left-biased, provided @x@ is.

See the @cvtOpApp@ documentation for how this function works.
-}
cvtOpAppP :: Hs.LPat RdrName -> TH.Name -> TH.Pat -> CvtM (Hs.Pat RdrName)
cvtOpAppP x op1 (UInfixP y op2 z)
  = do { l <- wrapL $ cvtOpAppP x op1 y
       ; cvtOpAppP l op2 z }
cvtOpAppP x op y
  = do { op' <- cNameL op
       ; y' <- cvtPat y
       ; return (ConPatIn op' (InfixCon x y')) }

-----------------------------------------------------------
--      Types and type variables

cvtTvs :: [TH.TyVarBndr] -> CvtM (LHsTyVarBndrs RdrName)
cvtTvs tvs = do { tvs' <- mapM cvt_tv tvs; return (mkHsQTvs tvs') }

cvt_tv :: TH.TyVarBndr -> CvtM (LHsTyVarBndr RdrName)
cvt_tv (TH.PlainTV nm)
  = do { nm' <- tName nm
       ; returnL $ HsTyVarBndr nm' Nothing Nothing }
cvt_tv (TH.KindedTV nm ki)
  = do { nm' <- tName nm
       ; ki' <- cvtKind ki
       ; returnL $ HsTyVarBndr nm' (Just ki') Nothing }
cvt_tv (TH.RoledTV nm r)
  = do { nm' <- tName nm
       ; r'  <- cvtRole r
       ; returnL $ HsTyVarBndr nm' Nothing (Just r') }
cvt_tv (TH.KindedRoledTV nm k r)
  = do { nm' <- tName nm
       ; k'  <- cvtKind k
       ; r'  <- cvtRole r
       ; returnL $ HsTyVarBndr nm' (Just k') (Just r') }

cvtRole :: TH.Role -> CvtM Coercion.Role
cvtRole TH.Nominal          = return Coercion.Nominal
cvtRole TH.Representational = return Coercion.Representational
cvtRole TH.Phantom          = return Coercion.Phantom

cvtContext :: TH.Cxt -> CvtM (LHsContext RdrName)
cvtContext tys = do { preds' <- mapM cvtPred tys; returnL preds' }

cvtPred :: TH.Pred -> CvtM (LHsType RdrName)
cvtPred (TH.ClassP cla tys)
  = do { cla' <- if isVarName cla then tName cla else tconName cla
       ; tys' <- mapM cvtType tys
       ; mk_apps (HsTyVar cla') tys'
       }
cvtPred (TH.EqualP ty1 ty2)
  = do { ty1' <- cvtType ty1
       ; ty2' <- cvtType ty2
       ; returnL $ HsEqTy ty1' ty2'
       }

cvtType :: TH.Type -> CvtM (LHsType RdrName)
cvtType = cvtTypeKind "type"

cvtTypeKind :: String -> TH.Type -> CvtM (LHsType RdrName)
cvtTypeKind ty_str ty
  = do { (head_ty, tys') <- split_ty_app ty
       ; case head_ty of
           TupleT n
             | length tys' == n         -- Saturated
             -> if n==1 then return (head tys') -- Singleton tuples treated
                                                -- like nothing (ie just parens)
                        else returnL (HsTupleTy HsBoxedTuple tys')
             | n == 1
             -> failWith (ptext (sLit ("Illegal 1-tuple " ++ ty_str ++ " constructor")))
             | otherwise
             -> mk_apps (HsTyVar (getRdrName (tupleTyCon BoxedTuple n))) tys'
           UnboxedTupleT n
             | length tys' == n         -- Saturated
             -> if n==1 then return (head tys') -- Singleton tuples treated
                                                -- like nothing (ie just parens)
                        else returnL (HsTupleTy HsUnboxedTuple tys')
             | otherwise
             -> mk_apps (HsTyVar (getRdrName (tupleTyCon UnboxedTuple n))) tys'
           ArrowT
             | [x',y'] <- tys' -> returnL (HsFunTy x' y')
             | otherwise       -> mk_apps (HsTyVar (getRdrName funTyCon)) tys'
           ListT
             | [x']    <- tys' -> returnL (HsListTy x')
             | otherwise       -> mk_apps (HsTyVar (getRdrName listTyCon)) tys'
           VarT nm -> do { nm' <- tName nm;    mk_apps (HsTyVar nm') tys' }
           ConT nm -> do { nm' <- tconName nm; mk_apps (HsTyVar nm') tys' }

           ForallT tvs cxt ty
             | null tys'
             -> do { tvs' <- cvtTvs tvs
                   ; cxt' <- cvtContext cxt
                   ; ty'  <- cvtType ty
                   ; returnL $ mkExplicitHsForAllTy (hsQTvBndrs tvs') cxt' ty'
                   }

           SigT ty ki
             -> do { ty' <- cvtType ty
                   ; ki' <- cvtKind ki
                   ; mk_apps (HsKindSig ty' ki') tys'
                   }

           LitT lit
             -> returnL (HsTyLit (cvtTyLit lit))

           PromotedT nm -> do { nm' <- cName nm; mk_apps (HsTyVar nm') tys' }
                 -- Promoted data constructor; hence cName

           PromotedTupleT n
             | n == 1
             -> failWith (ptext (sLit ("Illegal promoted 1-tuple " ++ ty_str)))
             | m == n   -- Saturated
             -> do  { let kis = replicate m placeHolderKind
                    ; returnL (HsExplicitTupleTy kis tys')
                    }
             where
               m = length tys'

           PromotedNilT
             -> returnL (HsExplicitListTy placeHolderKind [])

           PromotedConsT  -- See Note [Representing concrete syntax in types]
                          -- in Language.Haskell.TH.Syntax
             | [ty1, L _ (HsExplicitListTy _ tys2)] <- tys'
             -> returnL (HsExplicitListTy placeHolderKind (ty1:tys2))
             | otherwise
             -> mk_apps (HsTyVar (getRdrName consDataCon)) tys'

           StarT
             -> returnL (HsTyVar (getRdrName liftedTypeKindTyCon))

           ConstraintT
             -> returnL (HsTyVar (getRdrName constraintKindTyCon))

           _ -> failWith (ptext (sLit ("Malformed " ++ ty_str)) <+> text (show ty))
    }

mk_apps :: HsType RdrName -> [LHsType RdrName] -> CvtM (LHsType RdrName)
mk_apps head_ty []       = returnL head_ty
mk_apps head_ty (ty:tys) = do { head_ty' <- returnL head_ty
                              ; mk_apps (HsAppTy head_ty' ty) tys }

split_ty_app :: TH.Type -> CvtM (TH.Type, [LHsType RdrName])
split_ty_app ty = go ty []
  where
    go (AppT f a) as' = do { a' <- cvtType a; go f (a':as') }
    go f as           = return (f,as)

cvtTyLit :: TH.TyLit -> HsTyLit
cvtTyLit (NumTyLit i) = HsNumTy i
cvtTyLit (StrTyLit s) = HsStrTy (fsLit s)

cvtKind :: TH.Kind -> CvtM (LHsKind RdrName)
cvtKind = cvtTypeKind "kind"

cvtMaybeKind :: Maybe TH.Kind -> CvtM (Maybe (LHsKind RdrName))
cvtMaybeKind Nothing = return Nothing
cvtMaybeKind (Just ki) = do { ki' <- cvtKind ki
                            ; return (Just ki') }

-----------------------------------------------------------
cvtFixity :: TH.Fixity -> Hs.Fixity
cvtFixity (TH.Fixity prec dir) = Hs.Fixity prec (cvt_dir dir)
   where
     cvt_dir TH.InfixL = Hs.InfixL
     cvt_dir TH.InfixR = Hs.InfixR
     cvt_dir TH.InfixN = Hs.InfixN

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


-----------------------------------------------------------
-- some useful things

overloadedLit :: Lit -> Bool
-- True for literals that Haskell treats as overloaded
overloadedLit (IntegerL  _) = True
overloadedLit (RationalL _) = True
overloadedLit _             = False

void :: Type.Type
void = placeHolderType

cvtFractionalLit :: Rational -> FractionalLit
cvtFractionalLit r = FL { fl_text = show (fromRational r :: Double), fl_value = r }

--------------------------------------------------------------------
--      Turning Name back into RdrName
--------------------------------------------------------------------

-- variable names
vNameL, cNameL, tconNameL :: TH.Name -> CvtM (Located RdrName)
vName,  cName,  tName,  tconName  :: TH.Name -> CvtM RdrName

vNameL n = wrapL (vName n)
vName n = cvtName OccName.varName n

-- Constructor function names; this is Haskell source, hence srcDataName
cNameL n = wrapL (cName n)
cName n = cvtName OccName.dataName n

-- Type variable names
tName n = cvtName OccName.tvName n

-- Type Constructor names
tconNameL n = wrapL (tconName n)
tconName n = cvtName OccName.tcClsName n

cvtName :: OccName.NameSpace -> TH.Name -> CvtM RdrName
cvtName ctxt_ns (TH.Name occ flavour)
  | not (okOcc ctxt_ns occ_str) = failWith (badOcc ctxt_ns occ_str)
  | otherwise
  = do { loc <- getL
       ; let rdr_name = thRdrName loc ctxt_ns occ_str flavour
       ; force rdr_name
       ; return rdr_name }
  where
    occ_str = TH.occString occ

okOcc :: OccName.NameSpace -> String -> Bool
okOcc _  []      = False
okOcc ns str@(c:_)
  | OccName.isVarNameSpace ns = startsVarId c || startsVarSym c
  | otherwise                 = startsConId c || startsConSym c || str == "[]"

-- Determine the name space of a name in a type
--
isVarName :: TH.Name -> Bool
isVarName (TH.Name occ _)
  = case TH.occString occ of
      ""    -> False
      (c:_) -> startsVarId c || startsVarSym c

badOcc :: OccName.NameSpace -> String -> SDoc
badOcc ctxt_ns occ
  = ptext (sLit "Illegal") <+> pprNameSpace ctxt_ns
        <+> ptext (sLit "name:") <+> quotes (text occ)

thRdrName :: SrcSpan -> OccName.NameSpace -> String -> TH.NameFlavour -> RdrName
-- This turns a TH Name into a RdrName; used for both binders and occurrences
-- See Note [Binders in Template Haskell]
-- The passed-in name space tells what the context is expecting;
--      use it unless the TH name knows what name-space it comes
--      from, in which case use the latter
--
-- We pass in a SrcSpan (gotten from the monad) because this function
-- is used for *binders* and if we make an Exact Name we want it
-- to have a binding site inside it.  (cf Trac #5434)
--
-- ToDo: we may generate silly RdrNames, by passing a name space
--       that doesn't match the string, like VarName ":+",
--       which will give confusing error messages later
--
-- The strict applications ensure that any buried exceptions get forced
thRdrName loc ctxt_ns th_occ th_name
  = case th_name of
     TH.NameG th_ns pkg mod -> thOrigRdrName th_occ th_ns pkg mod
     TH.NameQ mod  -> (mkRdrQual  $! mk_mod mod) $! occ
     TH.NameL uniq -> nameRdrName $! (((Name.mkInternalName $! mk_uniq uniq) $! occ) loc)
     TH.NameU uniq -> nameRdrName $! (((Name.mkSystemNameAt $! mk_uniq uniq) $! occ) loc)
     TH.NameS | Just name <- isBuiltInOcc ctxt_ns th_occ -> nameRdrName $! name
              | otherwise                                -> mkRdrUnqual $! occ
  where
    occ :: OccName.OccName
    occ = mk_occ ctxt_ns th_occ

thOrigRdrName :: String -> TH.NameSpace -> PkgName -> ModName -> RdrName
thOrigRdrName occ th_ns pkg mod = (mkOrig $! (mkModule (mk_pkg pkg) (mk_mod mod))) $! (mk_occ (mk_ghc_ns th_ns) occ)

thRdrNameGuesses :: TH.Name -> [RdrName]
thRdrNameGuesses (TH.Name occ flavour)
  -- This special case for NameG ensures that we don't generate duplicates in the output list
  | TH.NameG th_ns pkg mod <- flavour = [ thOrigRdrName occ_str th_ns pkg mod]
  | otherwise                         = [ thRdrName noSrcSpan gns occ_str flavour
                                        | gns <- guessed_nss]
  where
    -- guessed_ns are the name spaces guessed from looking at the TH name
    guessed_nss | isLexCon (mkFastString occ_str) = [OccName.tcName,  OccName.dataName]
                | otherwise                       = [OccName.varName, OccName.tvName]
    occ_str = TH.occString occ

isBuiltInOcc :: OccName.NameSpace -> String -> Maybe Name.Name
-- Built in syntax isn't "in scope" so an Unqual RdrName won't do
-- We must generate an Exact name, just as the parser does
isBuiltInOcc ctxt_ns occ
  = case occ of
        ":"              -> Just (Name.getName consDataCon)
        "[]"             -> Just (Name.getName nilDataCon)
        "()"             -> Just (tup_name 0)
        '(' : ',' : rest -> go_tuple 2 rest
        _                -> Nothing
  where
    go_tuple n ")"          = Just (tup_name n)
    go_tuple n (',' : rest) = go_tuple (n+1) rest
    go_tuple _ _            = Nothing

    tup_name n
        | OccName.isTcClsNameSpace ctxt_ns = Name.getName (tupleTyCon BoxedTuple n)
        | otherwise                        = Name.getName (tupleCon BoxedTuple n)

-- The packing and unpacking is rather turgid :-(
mk_occ :: OccName.NameSpace -> String -> OccName.OccName
mk_occ ns occ = OccName.mkOccName ns occ

mk_ghc_ns :: TH.NameSpace -> OccName.NameSpace
mk_ghc_ns TH.DataName  = OccName.dataName
mk_ghc_ns TH.TcClsName = OccName.tcClsName
mk_ghc_ns TH.VarName   = OccName.varName

mk_mod :: TH.ModName -> ModuleName
mk_mod mod = mkModuleName (TH.modString mod)

mk_pkg :: TH.PkgName -> PackageId
mk_pkg pkg = stringToPackageId (TH.pkgString pkg)

mk_uniq :: Int# -> Unique
mk_uniq u = mkUniqueGrimily (I# u)