hsSyn/Convert.hs

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

    This module converts Template Haskell syntax into HsSyn
    -}

    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE ScopedTypeVariables #-}
    {-# LANGUAGE TypeFamilies #-}
    {-# LANGUAGE ViewPatterns #-}

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

    import GhcPrelude

    import HsSyn as Hs
    import PrelNames
    import RdrName
    import qualified Name
    import Module
    import RdrHsSyn
    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 Lexeme
    import Util
    import FastString
    import Outputable
    import MonadUtils ( foldrM )

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

    import Data.Maybe( catMaybes, isNothing )
    import Language.Haskell.TH as TH hiding (sigP)
    import Language.Haskell.TH.Syntax as TH

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

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

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

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

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

    -------------------------------------------------------------------
    newtype CvtM a = CvtM { unCvtM :: SrcSpan -> Either MsgDoc (SrcSpan, 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 Functor CvtM where
        fmap = liftM

    instance Applicative CvtM where
        pure x = CvtM $ \loc -> Right (loc,x)
        (<*>) = ap

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

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

    setL :: SrcSpan -> CvtM ()
    setL loc = CvtM (\_ -> Right (loc, ()))

    returnL :: HasSrcSpan a => SrcSpanLess a -> CvtM a
    returnL x = CvtM (\loc -> Right (loc, cL loc x))

    returnJustL :: HasSrcSpan a => SrcSpanLess a -> CvtM (Maybe a)
    returnJustL = fmap Just . returnL

    wrapParL :: HasSrcSpan a =>
                (a -> SrcSpanLess a) -> SrcSpanLess a -> CvtM (SrcSpanLess  a)
    wrapParL add_par x = CvtM (\loc -> Right (loc, add_par (cL 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 (text "When splicing a TH" <+> text what <> colon)
                     2 (if debugStyle sty
                        then text (show item)
                        else text (pprint item))

    wrapL :: HasSrcSpan a => CvtM (SrcSpanLess a) -> CvtM a
    wrapL (CvtM m) = CvtM (\loc -> case m loc of
                                   Left err -> Left err
                                   Right (loc',v) -> Right (loc',cL loc v))

    -------------------------------------------------------------------
    cvtDecs :: [TH.Dec] -> CvtM [LHsDecl GhcPs]
    cvtDecs = fmap catMaybes . mapM cvtDec

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

      | otherwise
      = do  { pat' <- cvtPat pat
            ; body' <- cvtGuard body
            ; ds' <- cvtLocalDecs (text "a where clause") ds
            ; returnJustL $ Hs.ValD noExt $
              PatBind { pat_lhs = pat'
                      , pat_rhs = GRHSs noExt body' (noLoc ds')
                      , pat_ext = noExt
                      , pat_ticks = ([],[]) } }

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

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

    cvtDec (TH.InfixD fx nm)
      -- Fixity signatures are allowed for variables, constructors, and types
      -- the renamer automatically looks for types during renaming, even when
      -- the RdrName says it's a variable or a constructor. So, just assume
      -- it's a variable or constructor and proceed.
      = do { nm' <- vcNameL nm
           ; returnJustL (Hs.SigD noExt (FixSig noExt
                                          (FixitySig noExt [nm'] (cvtFixity fx)))) }

    cvtDec (PragmaD prag)
      = cvtPragmaD prag

    cvtDec (TySynD tc tvs rhs)
      = do  { (_, tc', tvs') <- cvt_tycl_hdr [] tc tvs
            ; rhs' <- cvtType rhs
            ; returnJustL $ TyClD noExt $
              SynDecl { tcdSExt = noExt, tcdLName = tc', tcdTyVars = tvs'
                      , tcdFixity = Prefix
                      , tcdRhs = rhs' } }

    cvtDec (DataD ctxt tc tvs ksig constrs derivs)
      = do  { let isGadtCon (GadtC    _ _ _) = True
                  isGadtCon (RecGadtC _ _ _) = True
                  isGadtCon (ForallC  _ _ c) = isGadtCon c
                  isGadtCon _                = False
                  isGadtDecl  = all isGadtCon constrs
                  isH98Decl   = all (not . isGadtCon) constrs
            ; unless (isGadtDecl || isH98Decl)
                     (failWith (text "Cannot mix GADT constructors with Haskell 98"
                            <+> text "constructors"))
            ; unless (isNothing ksig || isGadtDecl)
                     (failWith (text "Kind signatures are only allowed on GADTs"))
            ; (ctxt', tc', tvs') <- cvt_tycl_hdr ctxt tc tvs
            ; ksig' <- cvtKind `traverse` ksig
            ; cons' <- mapM cvtConstr constrs
            ; derivs' <- cvtDerivs derivs
            ; let defn = HsDataDefn { dd_ext = noExt
                                    , dd_ND = DataType, dd_cType = Nothing
                                    , dd_ctxt = ctxt'
                                    , dd_kindSig = ksig'
                                    , dd_cons = cons', dd_derivs = derivs' }
            ; returnJustL $ TyClD noExt (DataDecl
                                            { tcdDExt = noExt
                                            , tcdLName = tc', tcdTyVars = tvs'
                                            , tcdFixity = Prefix
                                            , tcdDataDefn = defn }) }

    cvtDec (NewtypeD ctxt tc tvs ksig constr derivs)
      = do  { (ctxt', tc', tvs') <- cvt_tycl_hdr ctxt tc tvs
            ; ksig' <- cvtKind `traverse` ksig
            ; con' <- cvtConstr constr
            ; derivs' <- cvtDerivs derivs
            ; let defn = HsDataDefn { dd_ext = noExt
                                    , dd_ND = NewType, dd_cType = Nothing
                                    , dd_ctxt = ctxt'
                                    , dd_kindSig = ksig'
                                    , dd_cons = [con']
                                    , dd_derivs = derivs' }
            ; returnJustL $ TyClD noExt (DataDecl
                                        { tcdDExt = noExt
                                        , tcdLName = tc', tcdTyVars = tvs'
                                        , tcdFixity = Prefix
                                        , tcdDataDefn = defn }) }

    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 (text "a class declaration") decs
            ; unless (null adts')
                (failWith $ (text "Default data instance declarations"
                         <+> text "are not allowed:")
                       $$ (Outputable.ppr adts'))
            ; at_defs <- mapM cvt_at_def ats'
            ; returnJustL $ TyClD noExt $
              ClassDecl { tcdCExt = noExt
                        , tcdCtxt = cxt', tcdLName = tc', tcdTyVars = tvs'
                        , tcdFixity = Prefix
                        , tcdFDs = fds', tcdSigs = Hs.mkClassOpSigs sigs'
                        , tcdMeths = binds'
                        , tcdATs = fams', tcdATDefs = at_defs, tcdDocs = [] }
                                  -- no docs in TH ^^
            }
      where
        cvt_at_def :: LTyFamInstDecl GhcPs -> CvtM (LTyFamDefltEqn GhcPs)
        -- Very similar to what happens in RdrHsSyn.mkClassDecl
        cvt_at_def decl = case RdrHsSyn.mkATDefault decl of
                            Right (def, _) -> return def
                            Left (_, msg) -> failWith msg

    cvtDec (InstanceD o ctxt ty decs)
      = do  { let doc = text "an instance declaration"
            ; (binds', sigs', fams', ats', adts') <- cvt_ci_decs doc decs
            ; unless (null fams') (failWith (mkBadDecMsg doc fams'))
            ; ctxt' <- cvtContext ctxt
            ; (dL->L loc ty') <- cvtType ty
            ; let inst_ty' = mkHsQualTy ctxt loc ctxt' $ cL loc ty'
            ; returnJustL $ InstD noExt $ ClsInstD noExt $
              ClsInstDecl { cid_ext = noExt, cid_poly_ty = mkLHsSigType inst_ty'
                          , cid_binds = binds'
                          , cid_sigs = Hs.mkClassOpSigs sigs'
                          , cid_tyfam_insts = ats', cid_datafam_insts = adts'
                          , cid_overlap_mode = fmap (cL loc . overlap) o } }
      where
      overlap pragma =
        case pragma of
          TH.Overlaps      -> Hs.Overlaps     (SourceText "OVERLAPS")
          TH.Overlappable  -> Hs.Overlappable (SourceText "OVERLAPPABLE")
          TH.Overlapping   -> Hs.Overlapping  (SourceText "OVERLAPPING")
          TH.Incoherent    -> Hs.Incoherent   (SourceText "INCOHERENT")

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

    cvtDec (DataFamilyD tc tvs kind)
      = do { (_, tc', tvs') <- cvt_tycl_hdr [] tc tvs
           ; result <- cvtMaybeKindToFamilyResultSig kind
           ; returnJustL $ TyClD noExt $ FamDecl noExt $
             FamilyDecl noExt DataFamily tc' tvs' Prefix result Nothing }

    cvtDec (DataInstD ctxt bndrs tys ksig constrs derivs)
      = do { (ctxt', tc', bndrs', typats') <- cvt_datainst_hdr ctxt bndrs tys
           ; ksig' <- cvtKind `traverse` ksig
           ; cons' <- mapM cvtConstr constrs
           ; derivs' <- cvtDerivs derivs
           ; let defn = HsDataDefn { dd_ext = noExt
                                   , dd_ND = DataType, dd_cType = Nothing
                                   , dd_ctxt = ctxt'
                                   , dd_kindSig = ksig'
                                   , dd_cons = cons', dd_derivs = derivs' }

           ; returnJustL $ InstD noExt $ DataFamInstD
               { dfid_ext = noExt
               , dfid_inst = DataFamInstDecl { dfid_eqn = mkHsImplicitBndrs $
                               FamEqn { feqn_ext = noExt
                                      , feqn_tycon = tc'
                                      , feqn_bndrs = bndrs'
                                      , feqn_pats = typats'
                                      , feqn_rhs = defn
                                      , feqn_fixity = Prefix } }}}

    cvtDec (NewtypeInstD ctxt bndrs tys ksig constr derivs)
      = do { (ctxt', tc', bndrs', typats') <- cvt_datainst_hdr ctxt bndrs tys
           ; ksig' <- cvtKind `traverse` ksig
           ; con' <- cvtConstr constr
           ; derivs' <- cvtDerivs derivs
           ; let defn = HsDataDefn { dd_ext = noExt
                                   , dd_ND = NewType, dd_cType = Nothing
                                   , dd_ctxt = ctxt'
                                   , dd_kindSig = ksig'
                                   , dd_cons = [con'], dd_derivs = derivs' }
           ; returnJustL $ InstD noExt $ DataFamInstD
               { dfid_ext = noExt
               , dfid_inst = DataFamInstDecl { dfid_eqn = mkHsImplicitBndrs $
                               FamEqn { feqn_ext = noExt
                                      , feqn_tycon = tc'
                                      , feqn_bndrs = bndrs'
                                      , feqn_pats = typats'
                                      , feqn_rhs = defn
                                      , feqn_fixity = Prefix } }}}

    cvtDec (TySynInstD eqn)
      = do  { (dL->L _ eqn') <- cvtTySynEqn eqn
            ; returnJustL $ InstD noExt $ TyFamInstD
                { tfid_ext = noExt
                , tfid_inst = TyFamInstDecl { tfid_eqn = eqn' } } }

    cvtDec (OpenTypeFamilyD head)
      = do { (tc', tyvars', result', injectivity') <- cvt_tyfam_head head
           ; returnJustL $ TyClD noExt $ FamDecl noExt $
             FamilyDecl noExt OpenTypeFamily tc' tyvars' Prefix result' injectivity'
           }

    cvtDec (ClosedTypeFamilyD head eqns)
      = do { (tc', tyvars', result', injectivity') <- cvt_tyfam_head head
           ; eqns' <- mapM cvtTySynEqn eqns
           ; returnJustL $ TyClD noExt $ FamDecl noExt $
             FamilyDecl noExt (ClosedTypeFamily (Just eqns')) tc' tyvars' Prefix
                               result' injectivity' }

    cvtDec (TH.RoleAnnotD tc roles)
      = do { tc' <- tconNameL tc
           ; let roles' = map (noLoc . cvtRole) roles
           ; returnJustL $ Hs.RoleAnnotD noExt (RoleAnnotDecl noExt tc' roles') }

    cvtDec (TH.StandaloneDerivD ds cxt ty)
      = do { cxt' <- cvtContext cxt
           ; ds'  <- traverse cvtDerivStrategy ds
           ; (dL->L loc ty') <- cvtType ty
           ; let inst_ty' = mkHsQualTy cxt loc cxt' $ cL loc ty'
           ; returnJustL $ DerivD noExt $
             DerivDecl { deriv_ext =noExt
                       , deriv_strategy = ds'
                       , deriv_type = mkLHsSigWcType inst_ty'
                       , deriv_overlap_mode = Nothing } }

    cvtDec (TH.DefaultSigD nm typ)
      = do { nm' <- vNameL nm
           ; ty' <- cvtType typ
           ; returnJustL $ Hs.SigD noExt
                         $ ClassOpSig noExt True [nm'] (mkLHsSigType ty')}

    cvtDec (TH.PatSynD nm args dir pat)
      = do { nm'   <- cNameL nm
           ; args' <- cvtArgs args
           ; dir'  <- cvtDir nm' dir
           ; pat'  <- cvtPat pat
           ; returnJustL $ Hs.ValD noExt $ PatSynBind noExt $
               PSB noExt nm' args' pat' dir' }
      where
        cvtArgs (TH.PrefixPatSyn args) = Hs.PrefixCon <$> mapM vNameL args
        cvtArgs (TH.InfixPatSyn a1 a2) = Hs.InfixCon <$> vNameL a1 <*> vNameL a2
        cvtArgs (TH.RecordPatSyn sels)
          = do { sels' <- mapM vNameL sels
               ; vars' <- mapM (vNameL . mkNameS . nameBase) sels
               ; return $ Hs.RecCon $ zipWith RecordPatSynField sels' vars' }

        cvtDir _ Unidir          = return Unidirectional
        cvtDir _ ImplBidir       = return ImplicitBidirectional
        cvtDir n (ExplBidir cls) =
          do { ms <- mapM (cvtClause (mkPrefixFunRhs n)) cls
             ; return $ ExplicitBidirectional $ mkMatchGroup FromSource ms }

    cvtDec (TH.PatSynSigD nm ty)
      = do { nm' <- cNameL nm
           ; ty' <- cvtPatSynSigTy ty
           ; returnJustL $ Hs.SigD noExt $ PatSynSig noExt [nm'] (mkLHsSigType ty')}

    -- Implicit parameter bindings are handled in cvtLocalDecs and
    -- cvtImplicitParamBind. They are not allowed in any other scope, so
    -- reaching this case indicates an error.
    cvtDec (TH.ImplicitParamBindD _ _)
      = failWith (text "Implicit parameter binding only allowed in let or where")

    ----------------
    cvtTySynEqn :: TySynEqn -> CvtM (LTyFamInstEqn GhcPs)
    cvtTySynEqn (TySynEqn mb_bndrs lhs rhs)
      = do { mb_bndrs' <- traverse (mapM cvt_tv) mb_bndrs
           ; (head_ty, args) <- split_ty_app lhs
           ; case head_ty of
               ConT nm -> do { nm' <- tconNameL nm
                             ; rhs' <- cvtType rhs
                             ; let args' = map wrap_tyarg args
                             ; returnL $ mkHsImplicitBndrs
                                $ FamEqn { feqn_ext    = noExt
                                         , feqn_tycon  = nm'
                                         , feqn_bndrs  = mb_bndrs'
                                         , feqn_pats   = args'
                                         , feqn_fixity = Prefix
                                         , feqn_rhs    = rhs' } }
               InfixT t1 nm t2 -> do { nm' <- tconNameL nm
                                     ; args' <- mapM cvtType [t1,t2]
                                     ; rhs' <- cvtType rhs
                                     ; returnL $ mkHsImplicitBndrs
                                          $ FamEqn { feqn_ext    = noExt
                                                   , feqn_tycon  = nm'
                                                   , feqn_bndrs  = mb_bndrs'
                                                   , feqn_pats   =
                                                    (map HsValArg args') ++ args
                                                   , feqn_fixity = Hs.Infix
                                                   , feqn_rhs    = rhs' } }
               _ -> failWith $ text "Invalid type family instance LHS:"
                              <+> text (show lhs)
            }

    ----------------
    cvt_ci_decs :: MsgDoc -> [TH.Dec]
                -> CvtM (LHsBinds GhcPs,
                         [LSig GhcPs],
                         [LFamilyDecl GhcPs],
                         [LTyFamInstDecl GhcPs],
                         [LDataFamInstDecl GhcPs])
    -- Convert the declarations inside a class or instance decl
    -- ie signatures, bindings, and associated types
    cvt_ci_decs doc decs
      = do  { decs' <- cvtDecs 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))
              --We use FromSource as the origin of the bind
              -- because the TH declaration is user-written
            ; return (listToBag binds', sigs', fams', ats', adts') }

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

    cvt_datainst_hdr :: TH.Cxt -> Maybe [TH.TyVarBndr] -> TH.Type
                   -> CvtM ( LHsContext GhcPs
                           , Located RdrName
                           , Maybe [LHsTyVarBndr GhcPs]
                           , HsTyPats GhcPs)
    cvt_datainst_hdr cxt bndrs tys
      = do { cxt' <- cvtContext cxt
           ; bndrs' <- traverse (mapM cvt_tv) bndrs
           ; (head_ty, args) <- split_ty_app tys
           ; case head_ty of
              ConT nm -> do { nm' <- tconNameL nm
                            ; let args' = map wrap_tyarg args
                            ; return (cxt', nm', bndrs', args') }
              InfixT t1 nm t2 -> do { nm' <- tconNameL nm
                                    ; args' <- mapM cvtType [t1,t2]
                                    ; return (cxt', nm', bndrs',
                                             ((map HsValArg args') ++ args)) }
              _ -> failWith $ text "Invalid type instance header:"
                              <+> text (show tys) }

    ----------------
    cvt_tyfam_head :: TypeFamilyHead
                   -> CvtM ( Located RdrName
                           , LHsQTyVars GhcPs
                           , Hs.LFamilyResultSig GhcPs
                           , Maybe (Hs.LInjectivityAnn GhcPs))

    cvt_tyfam_head (TypeFamilyHead tc tyvars result injectivity)
      = do {(_, tc', tyvars') <- cvt_tycl_hdr [] tc tyvars
           ; result' <- cvtFamilyResultSig result
           ; injectivity' <- traverse cvtInjectivityAnnotation injectivity
           ; return (tc', tyvars', result', injectivity') }

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

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

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

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

    is_sig :: LHsDecl GhcPs -> Either (LSig GhcPs) (LHsDecl GhcPs)
    is_sig (dL->L loc (Hs.SigD _ sig)) = Left (cL loc sig)
    is_sig decl                        = Right decl

    is_bind :: LHsDecl GhcPs -> Either (LHsBind GhcPs) (LHsDecl GhcPs)
    is_bind (dL->L loc (Hs.ValD _ bind)) = Left (cL loc bind)
    is_bind decl                         = Right decl

    is_ip_bind :: TH.Dec -> Either (String, TH.Exp) TH.Dec
    is_ip_bind (TH.ImplicitParamBindD n e) = Left (n, e)
    is_ip_bind decl             = Right decl

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

    ---------------------------------------------------
    --      Data types
    ---------------------------------------------------

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

    cvtConstr (NormalC c strtys)
      = do  { c'   <- cNameL c
            ; tys' <- mapM cvt_arg strtys
            ; returnL $ mkConDeclH98 c' Nothing Nothing (PrefixCon tys') }

    cvtConstr (RecC c varstrtys)
      = do  { c'    <- cNameL c
            ; args' <- mapM cvt_id_arg varstrtys
            ; returnL $ mkConDeclH98 c' Nothing Nothing
                                       (RecCon (noLoc args')) }

    cvtConstr (InfixC st1 c st2)
      = do  { c'   <- cNameL c
            ; st1' <- cvt_arg st1
            ; st2' <- cvt_arg st2
            ; returnL $ mkConDeclH98 c' Nothing Nothing (InfixCon st1' st2') }

    cvtConstr (ForallC tvs ctxt con)
      = do  { tvs'      <- cvtTvs tvs
            ; ctxt'     <- cvtContext ctxt
            ; (dL->L _ con')  <- cvtConstr con
            ; returnL $ add_forall tvs' ctxt' con' }
      where
        add_cxt lcxt         Nothing           = Just lcxt
        add_cxt (dL->L loc cxt1) (Just (dL->L _ cxt2))
          = Just (cL loc (cxt1 ++ cxt2))

        add_forall tvs' cxt' con@(ConDeclGADT { con_qvars = qvars, con_mb_cxt = cxt })
          = con { con_forall = noLoc $ not (null all_tvs)
                , con_qvars  = mkHsQTvs all_tvs
                , con_mb_cxt = add_cxt cxt' cxt }
          where
            all_tvs = hsQTvExplicit tvs' ++ hsQTvExplicit qvars

        add_forall tvs' cxt' con@(ConDeclH98 { con_ex_tvs = ex_tvs, con_mb_cxt = cxt })
          = con { con_forall = noLoc $ not (null all_tvs)
                , con_ex_tvs = all_tvs
                , con_mb_cxt = add_cxt cxt' cxt }
          where
            all_tvs = hsQTvExplicit tvs' ++ ex_tvs

        add_forall _ _ (XConDecl _) = panic "cvtConstr"

    cvtConstr (GadtC c strtys ty)
      = do  { c'      <- mapM cNameL c
            ; args    <- mapM cvt_arg strtys
            ; (dL->L _ ty') <- cvtType ty
            ; c_ty    <- mk_arr_apps args ty'
            ; returnL $ fst $ mkGadtDecl c' c_ty}

    cvtConstr (RecGadtC c varstrtys ty)
      = do  { c'       <- mapM cNameL c
            ; ty'      <- cvtType ty
            ; rec_flds <- mapM cvt_id_arg varstrtys
            ; let rec_ty = noLoc (HsFunTy noExt
                                               (noLoc $ HsRecTy noExt rec_flds) ty')
            ; returnL $ fst $ mkGadtDecl c' rec_ty }

    cvtSrcUnpackedness :: TH.SourceUnpackedness -> SrcUnpackedness
    cvtSrcUnpackedness NoSourceUnpackedness = NoSrcUnpack
    cvtSrcUnpackedness SourceNoUnpack       = SrcNoUnpack
    cvtSrcUnpackedness SourceUnpack         = SrcUnpack

    cvtSrcStrictness :: TH.SourceStrictness -> SrcStrictness
    cvtSrcStrictness NoSourceStrictness = NoSrcStrict
    cvtSrcStrictness SourceLazy         = SrcLazy
    cvtSrcStrictness SourceStrict       = SrcStrict

    cvt_arg :: (TH.Bang, TH.Type) -> CvtM (LHsType GhcPs)
    cvt_arg (Bang su ss, ty)
      = do { ty'' <- cvtType ty
           ; let ty' = parenthesizeHsType appPrec ty''
                 su' = cvtSrcUnpackedness su
                 ss' = cvtSrcStrictness ss
           ; returnL $ HsBangTy noExt (HsSrcBang NoSourceText su' ss') ty' }

    cvt_id_arg :: (TH.Name, TH.Bang, TH.Type) -> CvtM (LConDeclField GhcPs)
    cvt_id_arg (i, str, ty)
      = do  { (dL->L li i') <- vNameL i
            ; ty' <- cvt_arg (str,ty)
            ; return $ noLoc (ConDeclField
                              { cd_fld_ext = noExt
                              , cd_fld_names
                                  = [cL li $ FieldOcc noExt (cL li i')]
                              , cd_fld_type =  ty'
                              , cd_fld_doc = Nothing}) }

    cvtDerivs :: [TH.DerivClause] -> CvtM (HsDeriving GhcPs)
    cvtDerivs cs = do { cs' <- mapM cvtDerivClause cs
                      ; returnL cs' }

    cvt_fundep :: FunDep -> CvtM (LHsFunDep GhcPs)
    cvt_fundep (FunDep xs ys) = do { xs' <- mapM tNameL xs
                                   ; ys' <- mapM tNameL ys
                                   ; returnL (xs', ys') }

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

    cvtForD :: Foreign -> CvtM (ForeignDecl GhcPs)
    cvtForD (ImportF callconv safety from nm ty)
      -- the prim and javascript calling conventions do not support headers
      -- and are inserted verbatim, analogous to mkImport in RdrHsSyn
      | callconv == TH.Prim || callconv == TH.JavaScript
      = mk_imp (CImport (noLoc (cvt_conv callconv)) (noLoc safety') Nothing
                        (CFunction (StaticTarget (SourceText from)
                                                 (mkFastString from) Nothing
                                                 True))
                        (noLoc $ quotedSourceText from))
      | Just impspec <- parseCImport (noLoc (cvt_conv callconv)) (noLoc safety')
                                     (mkFastString (TH.nameBase nm))
                                     from (noLoc $ quotedSourceText from)
      = mk_imp impspec
      | otherwise
      = failWith $ text (show from) <+> text "is not a valid ccall impent"
      where
        mk_imp impspec
          = do { nm' <- vNameL nm
               ; ty' <- cvtType ty
               ; return (ForeignImport { fd_i_ext = noExt
                                       , fd_name = nm'
                                       , fd_sig_ty = mkLHsSigType ty'
                                       , fd_fi = impspec })
               }
        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 (noLoc (CExportStatic (SourceText as)
                                                    (mkFastString as)
                                                    (cvt_conv callconv)))
                                                    (noLoc (SourceText as))
            ; return $ ForeignExport { fd_e_ext = noExt
                                     , fd_name = nm'
                                     , fd_sig_ty = mkLHsSigType ty'
                                     , fd_fe = e } }

    cvt_conv :: TH.Callconv -> CCallConv
    cvt_conv TH.CCall      = CCallConv
    cvt_conv TH.StdCall    = StdCallConv
    cvt_conv TH.CApi       = CApiConv
    cvt_conv TH.Prim       = PrimCallConv
    cvt_conv TH.JavaScript = JavaScriptCallConv

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

    cvtPragmaD :: Pragma -> CvtM (Maybe (LHsDecl GhcPs))
    cvtPragmaD (InlineP nm inline rm phases)
      = do { nm' <- vNameL nm
           ; let dflt = dfltActivation inline
           ; let src TH.NoInline  = "{-# NOINLINE"
                 src TH.Inline    = "{-# INLINE"
                 src TH.Inlinable = "{-# INLINABLE"
           ; let ip   = InlinePragma { inl_src    = SourceText $ src inline
                                     , inl_inline = cvtInline inline
                                     , inl_rule   = cvtRuleMatch rm
                                     , inl_act    = cvtPhases phases dflt
                                     , inl_sat    = Nothing }
           ; returnJustL $ Hs.SigD noExt $ InlineSig noExt nm' ip }

    cvtPragmaD (SpecialiseP nm ty inline phases)
      = do { nm' <- vNameL nm
           ; ty' <- cvtType ty
           ; let src TH.NoInline  = "{-# SPECIALISE NOINLINE"
                 src TH.Inline    = "{-# SPECIALISE INLINE"
                 src TH.Inlinable = "{-# SPECIALISE INLINE"
           ; let (inline', dflt,srcText) = case inline of
                   Just inline1 -> (cvtInline inline1, dfltActivation inline1,
                                    src inline1)
                   Nothing      -> (NoUserInline,   AlwaysActive,
                                    "{-# SPECIALISE")
           ; let ip = InlinePragma { inl_src    = SourceText srcText
                                   , inl_inline = inline'
                                   , inl_rule   = Hs.FunLike
                                   , inl_act    = cvtPhases phases dflt
                                   , inl_sat    = Nothing }
           ; returnJustL $ Hs.SigD noExt $ SpecSig noExt nm' [mkLHsSigType ty'] ip }

    cvtPragmaD (SpecialiseInstP ty)
      = do { ty' <- cvtType ty
           ; returnJustL $ Hs.SigD noExt $
             SpecInstSig noExt (SourceText "{-# SPECIALISE") (mkLHsSigType ty') }

    cvtPragmaD (RuleP nm ty_bndrs tm_bndrs lhs rhs phases)
      = do { let nm' = mkFastString nm
           ; let act = cvtPhases phases AlwaysActive
           ; ty_bndrs' <- traverse (mapM cvt_tv) ty_bndrs
           ; tm_bndrs' <- mapM cvtRuleBndr tm_bndrs
           ; lhs'   <- cvtl lhs
           ; rhs'   <- cvtl rhs
           ; returnJustL $ Hs.RuleD noExt
                $ HsRules { rds_ext = noExt
                          , rds_src = SourceText "{-# RULES"
                          , rds_rules = [noLoc $
                              HsRule { rd_ext  = noExt
                                     , rd_name = (noLoc (quotedSourceText nm,nm'))
                                     , rd_act  = act
                                     , rd_tyvs = ty_bndrs'
                                     , rd_tmvs = tm_bndrs'
                                     , rd_lhs  = lhs'
                                     , rd_rhs  = rhs' }] }

              }

    cvtPragmaD (AnnP target exp)
      = do { exp' <- cvtl exp
           ; target' <- case target of
             ModuleAnnotation  -> return ModuleAnnProvenance
             TypeAnnotation n  -> do
               n' <- tconName n
               return (TypeAnnProvenance  (noLoc n'))
             ValueAnnotation n -> do
               n' <- vcName n
               return (ValueAnnProvenance (noLoc n'))
           ; returnJustL $ Hs.AnnD noExt
                         $ HsAnnotation noExt (SourceText "{-# ANN") target' exp'
           }

    cvtPragmaD (LineP line file)
      = do { setL (srcLocSpan (mkSrcLoc (fsLit file) line 1))
           ; return Nothing
           }
    cvtPragmaD (CompleteP cls mty)
      = do { cls' <- noLoc <$> mapM cNameL cls
           ; mty'  <- traverse tconNameL mty
           ; returnJustL $ Hs.SigD noExt
                       $ CompleteMatchSig noExt NoSourceText cls' mty' }

    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 NoSourceText i
    cvtPhases (BeforePhase i) _    = ActiveBefore NoSourceText i

    cvtRuleBndr :: TH.RuleBndr -> CvtM (Hs.LRuleBndr GhcPs)
    cvtRuleBndr (RuleVar n)
      = do { n' <- vNameL n
           ; return $ noLoc $ Hs.RuleBndr noExt n' }
    cvtRuleBndr (TypedRuleVar n ty)
      = do { n'  <- vNameL n
           ; ty' <- cvtType ty
           ; return $ noLoc $ Hs.RuleBndrSig noExt n' $ mkLHsSigWcType ty' }

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

    cvtLocalDecs :: MsgDoc -> [TH.Dec] -> CvtM (HsLocalBinds GhcPs)
    cvtLocalDecs doc ds
      = case partitionWith is_ip_bind ds of
          ([], []) -> return (EmptyLocalBinds noExt)
          ([], _) -> do
            ds' <- cvtDecs 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 noExt (ValBinds noExt (listToBag binds) sigs))
          (ip_binds, []) -> do
            binds <- mapM (uncurry cvtImplicitParamBind) ip_binds
            return (HsIPBinds noExt (IPBinds noExt binds))
          ((_:_), (_:_)) ->
            failWith (text "Implicit parameters mixed with other bindings")

    cvtClause :: HsMatchContext RdrName
              -> TH.Clause -> CvtM (Hs.LMatch GhcPs (LHsExpr GhcPs))
    cvtClause ctxt (Clause ps body wheres)
      = do  { ps' <- cvtPats ps
            ; let pps = map (parenthesizePat appPrec) ps'
            ; g'  <- cvtGuard body
            ; ds' <- cvtLocalDecs (text "a where clause") wheres
            ; returnL $ Hs.Match noExt ctxt pps (GRHSs noExt g' (noLoc ds')) }

    cvtImplicitParamBind :: String -> TH.Exp -> CvtM (LIPBind GhcPs)
    cvtImplicitParamBind n e = do
        n' <- wrapL (ipName n)
        e' <- cvtl e
        returnL (IPBind noExt (Left n') e')

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

    cvtl :: TH.Exp -> CvtM (LHsExpr GhcPs)
    cvtl e = wrapL (cvt e)
      where
        cvt (VarE s)        = do { s' <- vName s; return $ HsVar noExt (noLoc s') }
        cvt (ConE s)        = do { s' <- cName s; return $ HsVar noExt (noLoc s') }
        cvt (LitE l)
          | overloadedLit l = go cvtOverLit (HsOverLit noExt)
                                 (hsOverLitNeedsParens appPrec)
          | otherwise       = go cvtLit (HsLit noExt)
                                 (hsLitNeedsParens appPrec)
          where
            go :: (Lit -> CvtM (l GhcPs))
               -> (l GhcPs -> HsExpr GhcPs)
               -> (l GhcPs -> Bool)
               -> CvtM (HsExpr GhcPs)
            go cvt_lit mk_expr is_compound_lit = do
              l' <- cvt_lit l
              let e' = mk_expr l'
              return $ if is_compound_lit l' then HsPar noExt (noLoc e') else e'
        cvt (AppE x@(LamE _ _) y) = do { x' <- cvtl x; y' <- cvtl y
                                       ; return $ HsApp noExt (mkLHsPar x')
                                                              (mkLHsPar y')}
        cvt (AppE x y)            = do { x' <- cvtl x; y' <- cvtl y
                                       ; return $ HsApp noExt (mkLHsPar x')
                                                              (mkLHsPar y')}
        cvt (AppTypeE e t) = do { e' <- cvtl e
                                ; t' <- cvtType t
                                ; let tp = parenthesizeHsType appPrec t'
                                ; return $ HsAppType noExt e'
                                         $ mkHsWildCardBndrs tp }
        cvt (LamE [] e)    = cvt e -- Degenerate case. We convert the body as its
                                   -- own expression to avoid pretty-printing
                                   -- oddities that can result from zero-argument
                                   -- lambda expressions. See #13856.
        cvt (LamE ps e)    = do { ps' <- cvtPats ps; e' <- cvtl e
                                ; let pats = map (parenthesizePat appPrec) ps'
                                ; return $ HsLam noExt (mkMatchGroup FromSource
                                                 [mkSimpleMatch LambdaExpr
                                                 pats e'])}
        cvt (LamCaseE ms)  = do { ms' <- mapM (cvtMatch CaseAlt) ms
                                ; return $ HsLamCase noExt
                                                       (mkMatchGroup FromSource ms')
                                }
        cvt (TupE [e])     = do { e' <- cvtl e; return $ HsPar noExt e' }
                                     -- Note [Dropping constructors]
                                     -- Singleton tuples treated like nothing (just parens)
        cvt (TupE es)      = do { es' <- mapM cvtl es
                                ; return $ ExplicitTuple noExt
                                                 (map (noLoc . (Present noExt)) es')
                                                                             Boxed }
        cvt (UnboxedTupE es)      = do { es' <- mapM cvtl es
                                       ; return $ ExplicitTuple noExt
                                               (map (noLoc . (Present noExt)) es')
                                                                           Unboxed }
        cvt (UnboxedSumE e alt arity) = do { e' <- cvtl e
                                           ; unboxedSumChecks alt arity
                                           ; return $ ExplicitSum noExt
                                                                       alt arity e'}
        cvt (CondE x y z)  = do { x' <- cvtl x; y' <- cvtl y; z' <- cvtl z;
                                ; return $ HsIf noExt (Just noSyntaxExpr) x' y' z' }
        cvt (MultiIfE alts)
          | null alts      = failWith (text "Multi-way if-expression with no alternatives")
          | otherwise      = do { alts' <- mapM cvtpair alts
                                ; return $ HsMultiIf noExt alts' }
        cvt (LetE ds e)    = do { ds' <- cvtLocalDecs (text "a let expression") ds
                                ; e' <- cvtl e; return $ HsLet noExt (noLoc ds') e'}
        cvt (CaseE e ms)   = do { e' <- cvtl e; ms' <- mapM (cvtMatch CaseAlt) ms
                                ; return $ HsCase noExt e'
                                                     (mkMatchGroup FromSource ms') }
        cvt (DoE ss)       = cvtHsDo DoExpr ss
        cvt (MDoE ss)      = cvtHsDo MDoExpr ss
        cvt (CompE ss)     = cvtHsDo ListComp ss
        cvt (ArithSeqE dd) = do { dd' <- cvtDD dd
                                ; return $ ArithSeq noExt Nothing dd' }
        cvt (ListE xs)
          | Just s <- allCharLs xs       = do { l' <- cvtLit (StringL s)
                                              ; return (HsLit noExt l') }
                 -- Note [Converting strings]
          | otherwise       = do { xs' <- mapM cvtl xs
                                 ; return $ ExplicitList noExt Nothing xs'
                                 }

        -- Infix expressions
        cvt (InfixE (Just x) s (Just y)) =
          do { x' <- cvtl x
             ; s' <- cvtl s
             ; y' <- cvtl y
             ; let px = parenthesizeHsExpr opPrec x'
                   py = parenthesizeHsExpr opPrec y'
             ; wrapParL (HsPar noExt)
               $ OpApp noExt px s' py }
               -- 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 noExt) $
                                                              SectionR noExt s' y' }
                                                -- See Note [Sections in HsSyn] in HsExpr
        cvt (InfixE (Just x) s Nothing ) = do { x' <- cvtl x; s' <- cvtl s
                                              ; wrapParL (HsPar noExt) $
                                                              SectionL noExt x' s' }

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

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

        cvt (ParensE e)      = do { e' <- cvtl e; return $ HsPar noExt e' }
        cvt (SigE e t)       = do { e' <- cvtl e; t' <- cvtType t
                                  ; let pe = parenthesizeHsExpr sigPrec e'
                                  ; return $ ExprWithTySig noExt pe (mkLHsSigWcType t') }
        cvt (RecConE c flds) = do { c' <- cNameL c
                                  ; flds' <- mapM (cvtFld (mkFieldOcc . noLoc)) flds
                                  ; return $ mkRdrRecordCon c' (HsRecFields flds' Nothing) }
        cvt (RecUpdE e flds) = do { e' <- cvtl e
                                  ; flds'
                                      <- mapM (cvtFld (mkAmbiguousFieldOcc . noLoc))
                                               flds
                                  ; return $ mkRdrRecordUpd e' flds' }
        cvt (StaticE e)      = fmap (HsStatic noExt) $ cvtl e
        cvt (UnboundVarE s)  = do -- Use of 'vcName' here instead of 'vName' is
                                  -- important, because UnboundVarE may contain
                                  -- constructor names - see #14627.
                                  { s' <- vcName s
                                  ; return $ HsVar noExt (noLoc s') }
        cvt (LabelE s)       = do { return $ HsOverLabel noExt Nothing (fsLit s) }
        cvt (ImplicitParamVarE n) = do { n' <- ipName n; return $ HsIPVar noExt n' }

    {- 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 :: (RdrName -> t) -> (TH.Name, TH.Exp)
           -> CvtM (LHsRecField' t (LHsExpr GhcPs))
    cvtFld f (v,e)
      = do  { v' <- vNameL v; e' <- cvtl e
            ; return (noLoc $ HsRecField { hsRecFieldLbl = fmap f v'
                                         , hsRecFieldArg = e'
                                         , hsRecPun      = False}) }

    cvtDD :: Range -> CvtM (ArithSeqInfo GhcPs)
    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@, @UInfixP@, and @UInfixT@ 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@, @mkConOppPatRn@, and
    @mkHsOpTyRn@ in RnTypes), which expects that the input will be completely
    right-biased for types and left-biased for everything else. So we left-bias the
    trees of @UInfixP@ and @UInfixE@ and right-bias the trees of @UInfixT@.

    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@, @cvtOpAppP@, and @cvtOpAppT@ are responsible for this
    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 GhcPs -> TH.Exp -> TH.Exp -> CvtM (HsExpr GhcPs)
    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 noExt x op' y') }

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

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

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

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

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

    cvtStmt :: TH.Stmt -> CvtM (Hs.LStmt GhcPs (LHsExpr GhcPs))
    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 (text "a let binding") ds
                                ; returnL $ LetStmt noExt (noLoc ds') }
    cvtStmt (TH.ParS dss)  = do { dss' <- mapM cvt_one dss
                                ; returnL $ ParStmt noExt dss' noExpr noSyntaxExpr }
      where
        cvt_one ds = do { ds' <- cvtStmts ds
                        ; return (ParStmtBlock noExt ds' undefined noSyntaxExpr) }
    cvtStmt (TH.RecS ss) = do { ss' <- mapM cvtStmt ss; returnL (mkRecStmt ss') }

    cvtMatch :: HsMatchContext RdrName
             -> TH.Match -> CvtM (Hs.LMatch GhcPs (LHsExpr GhcPs))
    cvtMatch ctxt (TH.Match p body decs)
      = do  { p' <- cvtPat p
            ; let lp = case p' of
                         (dL->L loc SigPat{}) -> cL loc (ParPat NoExt p') -- #14875
                         _                    -> p'
            ; g' <- cvtGuard body
            ; decs' <- cvtLocalDecs (text "a where clause") decs
            ; returnL $ Hs.Match noExt ctxt [lp] (GRHSs noExt g' (noLoc decs')) }

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

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

    cvtOverLit :: Lit -> CvtM (HsOverLit GhcPs)
    cvtOverLit (IntegerL i)
      = do { force i; return $ mkHsIntegral   (mkIntegralLit i) }
    cvtOverLit (RationalL r)
      = do { force r; return $ mkHsFractional (mkFractionalLit r) }
    cvtOverLit (StringL s)
      = do { let { s' = mkFastString s }
           ; force s'
           ; return $ mkHsIsString (quotedSourceText s) s'
           }
    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 GhcPs)
    cvtLit (IntPrimL i)    = do { force i; return $ HsIntPrim NoSourceText i }
    cvtLit (WordPrimL w)   = do { force w; return $ HsWordPrim NoSourceText w }
    cvtLit (FloatPrimL f)
      = do { force f; return $ HsFloatPrim noExt (mkFractionalLit f) }
    cvtLit (DoublePrimL f)
      = do { force f; return $ HsDoublePrim noExt (mkFractionalLit f) }
    cvtLit (CharL c)       = do { force c; return $ HsChar NoSourceText c }
    cvtLit (CharPrimL c)   = do { force c; return $ HsCharPrim NoSourceText c }
    cvtLit (StringL s)     = do { let { s' = mkFastString s }
                                ; force s'
                                ; return $ HsString (quotedSourceText s) s' }
    cvtLit (StringPrimL s) = do { let { s' = BS.pack s }
                                ; force s'
                                ; return $ HsStringPrim NoSourceText s' }
    cvtLit _ = panic "Convert.cvtLit: Unexpected literal"
            -- cvtLit should not be called on IntegerL, RationalL
            -- That precondition is established right here in
            -- Convert.hs, hence panic

    quotedSourceText :: String -> SourceText
    quotedSourceText s = SourceText $ "\"" ++ s ++ "\""

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

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

    cvtp :: TH.Pat -> CvtM (Hs.Pat GhcPs)
    cvtp (TH.LitP l)
      | overloadedLit l    = do { l' <- cvtOverLit l
                                ; return (mkNPat (noLoc l') Nothing) }
                                      -- Not right for negative patterns;
                                      -- need to think about that!
      | otherwise          = do { l' <- cvtLit l; return $ Hs.LitPat noExt l' }
    cvtp (TH.VarP s)       = do { s' <- vName s
                                ; return $ Hs.VarPat noExt (noLoc s') }
    cvtp (TupP [p])        = do { p' <- cvtPat p; return $ ParPat noExt p' }
                                             -- Note [Dropping constructors]
    cvtp (TupP ps)         = do { ps' <- cvtPats ps
                                ; return $ TuplePat noExt ps' Boxed }
    cvtp (UnboxedTupP ps)  = do { ps' <- cvtPats ps
                                ; return $ TuplePat noExt ps' Unboxed }
    cvtp (UnboxedSumP p alt arity)
                           = do { p' <- cvtPat p
                                ; unboxedSumChecks alt arity
                                ; return $ SumPat noExt p' alt arity }
    cvtp (ConP s ps)       = do { s' <- cNameL s; ps' <- cvtPats ps
                                ; let pps = map (parenthesizePat appPrec) ps'
                                ; return $ ConPatIn s' (PrefixCon pps) }
    cvtp (InfixP p1 s p2)  = do { s' <- cNameL s; p1' <- cvtPat p1; p2' <- cvtPat p2
                                ; wrapParL (ParPat noExt) $
                                  ConPatIn s' $
                                  InfixCon (parenthesizePat opPrec p1')
                                           (parenthesizePat opPrec 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;
                                ; case unLoc p' of  -- may be wrapped ConPatIn
                                    ParPat {} -> return $ unLoc p'
                                    _         -> return $ ParPat noExt p' }
    cvtp (TildeP p)        = do { p' <- cvtPat p; return $ LazyPat noExt p' }
    cvtp (BangP p)         = do { p' <- cvtPat p; return $ BangPat noExt p' }
    cvtp (TH.AsP s p)      = do { s' <- vNameL s; p' <- cvtPat p
                                ; return $ AsPat noExt s' p' }
    cvtp TH.WildP          = return $ WildPat noExt
    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 noExt ps'}
    cvtp (SigP p t)        = do { p' <- cvtPat p; t' <- cvtType t
                                ; return $ SigPat noExt p' (mkLHsSigWcType t') }
    cvtp (ViewP e p)       = do { e' <- cvtl e; p' <- cvtPat p
                                ; return $ ViewPat noExt e' p'}

    cvtPatFld :: (TH.Name, TH.Pat) -> CvtM (LHsRecField GhcPs (LPat GhcPs))
    cvtPatFld (s,p)
      = do  { (dL->L ls s') <- vNameL s
            ; p' <- cvtPat p
            ; return (noLoc $ HsRecField { hsRecFieldLbl
                                             = cL ls $ mkFieldOcc (cL ls 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 GhcPs -> TH.Name -> TH.Pat -> CvtM (Hs.Pat GhcPs)
    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 (LHsQTyVars GhcPs)
    cvtTvs tvs = do { tvs' <- mapM cvt_tv tvs; return (mkHsQTvs tvs') }

    cvt_tv :: TH.TyVarBndr -> CvtM (LHsTyVarBndr GhcPs)
    cvt_tv (TH.PlainTV nm)
      = do { nm' <- tNameL nm
           ; returnL $ UserTyVar noExt nm' }
    cvt_tv (TH.KindedTV nm ki)
      = do { nm' <- tNameL nm
           ; ki' <- cvtKind ki
           ; returnL $ KindedTyVar noExt nm' ki' }

    cvtRole :: TH.Role -> Maybe Coercion.Role
    cvtRole TH.NominalR          = Just Coercion.Nominal
    cvtRole TH.RepresentationalR = Just Coercion.Representational
    cvtRole TH.PhantomR          = Just Coercion.Phantom
    cvtRole TH.InferR            = Nothing

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

    cvtPred :: TH.Pred -> CvtM (LHsType GhcPs)
    cvtPred = cvtType

    cvtDerivClause :: TH.DerivClause
                   -> CvtM (LHsDerivingClause GhcPs)
    cvtDerivClause (TH.DerivClause ds ctxt)
      = do { ctxt' <- fmap (map mkLHsSigType) <$> cvtContext ctxt
           ; ds'   <- traverse cvtDerivStrategy ds
           ; returnL $ HsDerivingClause noExt ds' ctxt' }

    cvtDerivStrategy :: TH.DerivStrategy -> CvtM (Hs.LDerivStrategy GhcPs)
    cvtDerivStrategy TH.StockStrategy    = returnL Hs.StockStrategy
    cvtDerivStrategy TH.AnyclassStrategy = returnL Hs.AnyclassStrategy
    cvtDerivStrategy TH.NewtypeStrategy  = returnL Hs.NewtypeStrategy
    cvtDerivStrategy (TH.ViaStrategy ty) = do
      ty' <- cvtType ty
      returnL $ Hs.ViaStrategy (mkLHsSigType ty')

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

    cvtTypeKind :: String -> TH.Type -> CvtM (LHsType GhcPs)
    cvtTypeKind ty_str ty
      = do { (head_ty, tys') <- split_ty_app ty
           ; let m_normals = mapM extract_normal tys'
                                    where extract_normal (HsValArg ty) = Just ty
                                          extract_normal _ = Nothing

           ; case head_ty of
               TupleT n
                | Just normals <- m_normals
                , normals `lengthIs` n         -- Saturated
                   -> if n==1 then return (head normals) -- Singleton tuples treated
                                                         -- like nothing (ie just parens)
                              else returnL (HsTupleTy noExt
                                            HsBoxedOrConstraintTuple normals)
                | n == 1
                   -> failWith (ptext (sLit ("Illegal 1-tuple " ++ ty_str ++ " constructor")))
                | otherwise
                -> mk_apps
                   (HsTyVar noExt NotPromoted (noLoc (getRdrName (tupleTyCon Boxed n))))
                   tys'
               UnboxedTupleT n
                 | Just normals <- m_normals
                 , normals `lengthIs` n               -- Saturated
                 -> returnL (HsTupleTy noExt HsUnboxedTuple normals)
                 | otherwise
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName (tupleTyCon Unboxed n))))
                    tys'
               UnboxedSumT n
                 | n < 2
                -> failWith $
                       vcat [ text "Illegal sum arity:" <+> text (show n)
                            , nest 2 $
                                text "Sums must have an arity of at least 2" ]
                 | Just normals <- m_normals
                 , normals `lengthIs` n -- Saturated
                 -> returnL (HsSumTy noExt normals)
                 | otherwise
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName (sumTyCon n))))
                    tys'
               ArrowT
                 | Just normals <- m_normals
                 , [x',y'] <- normals -> do
                     x'' <- case unLoc x' of
                              HsFunTy{}    -> returnL (HsParTy noExt x')
                              HsForAllTy{} -> returnL (HsParTy noExt x') -- #14646
                              HsQualTy{}   -> returnL (HsParTy noExt x') -- #15324
                              _            -> return $
                                              parenthesizeHsType sigPrec x'
                     let y'' = parenthesizeHsType sigPrec y'
                     returnL (HsFunTy noExt x'' y'')
                 | otherwise
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName funTyCon)))
                    tys'
               ListT
                 | Just normals <- m_normals
                 , [x'] <- normals -> do
                    returnL (HsListTy noExt x')
                 | otherwise
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName listTyCon)))
                    tys'

               VarT nm -> do { nm' <- tNameL nm
                             ; mk_apps (HsTyVar noExt NotPromoted nm') tys' }
               ConT nm -> do { nm' <- tconName nm
                             ; -- ConT can contain both data constructor (i.e.,
                               -- promoted) names and other (i.e, unpromoted)
                               -- names, as opposed to PromotedT, which can only
                               -- contain data constructor names. See #15572.
                               let prom = if isRdrDataCon nm'
                                          then IsPromoted
                                          else NotPromoted
                             ; mk_apps (HsTyVar noExt prom (noLoc nm')) tys'}

               ForallT tvs cxt ty
                 | null tys'
                 -> do { tvs' <- cvtTvs tvs
                       ; cxt' <- cvtContext cxt
                       ; let pcxt = parenthesizeHsContext funPrec cxt'
                       ; ty'  <- cvtType ty
                       ; loc <- getL
                       ; let hs_ty  = mkHsForAllTy tvs loc tvs' rho_ty
                             rho_ty = mkHsQualTy cxt loc pcxt ty'

                       ; return hs_ty }

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

               LitT lit
                 -> mk_apps (HsTyLit noExt (cvtTyLit lit)) tys'

               WildCardT
                 -> mk_apps mkAnonWildCardTy tys'

               InfixT t1 s t2
                 -> do { s'  <- tconName s
                       ; t1' <- cvtType t1
                       ; t2' <- cvtType t2
                       ; mk_apps
                          (HsTyVar noExt NotPromoted (noLoc s'))
                          ([HsValArg t1', HsValArg t2'] ++ tys')
                       }

               UInfixT t1 s t2
                 -> do { t2' <- cvtType t2
                       ; t <- cvtOpAppT t1 s t2'
                       ; mk_apps (unLoc t) tys'
                       } -- Note [Converting UInfix]

               ParensT t
                 -> do { t' <- cvtType t
                       ; mk_apps (HsParTy noExt t') tys'
                       }

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

               PromotedTupleT n
                  | n == 1
                  -> failWith (ptext (sLit ("Illegal promoted 1-tuple " ++ ty_str)))
                  | Just normals <- m_normals
                  , normals `lengthIs` n   -- Saturated
                  -> returnL (HsExplicitTupleTy noExt normals)
                  | otherwise
                  -> mk_apps
                     (HsTyVar noExt IsPromoted (noLoc (getRdrName (tupleDataCon Boxed n))))
                     tys'

               PromotedNilT
                 -> mk_apps (HsExplicitListTy noExt IsPromoted []) tys'

               PromotedConsT  -- See Note [Representing concrete syntax in types]
                              -- in Language.Haskell.TH.Syntax
                  | Just normals <- m_normals
                  , [ty1, dL->L _ (HsExplicitListTy _ ip tys2)] <- normals
                  -> do
                      returnL (HsExplicitListTy noExt ip (ty1:tys2))
                  | otherwise
                  -> mk_apps
                     (HsTyVar noExt IsPromoted (noLoc (getRdrName consDataCon)))
                     tys'

               StarT
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName liftedTypeKindTyCon)))
                    tys'

               ConstraintT
                 -> mk_apps
                    (HsTyVar noExt NotPromoted (noLoc (getRdrName constraintKindTyCon)))
                    tys'

               EqualityT
                 | Just normals <- m_normals
                 , [x',y'] <- normals ->
                       let px = parenthesizeHsType opPrec x'
                           py = parenthesizeHsType opPrec y'
                       in returnL (HsOpTy noExt px (noLoc eqTyCon_RDR) py)
                   -- The long-term goal is to remove the above case entirely and
                   -- subsume it under the case for InfixT. See #15815, comment:6,
                   -- for more details.

                 | otherwise ->
                       mk_apps (HsTyVar noExt NotPromoted
                                (noLoc eqTyCon_RDR)) tys'
               ImplicitParamT n t
                 -> do { n' <- wrapL $ ipName n
                       ; t' <- cvtType t
                       ; returnL (HsIParamTy noExt n' t')
                       }

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

    -- | Constructs an application of a type to arguments passed in a list.
    mk_apps :: HsType GhcPs -> [LHsTypeArg GhcPs] -> CvtM (LHsType GhcPs)
    mk_apps head_ty type_args = do
      head_ty' <- returnL head_ty
      -- We must parenthesize the function type in case of an explicit
      -- signature. For instance, in `(Maybe :: Type -> Type) Int`, there
      -- _must_ be parentheses around `Maybe :: Type -> Type`.
      let phead_ty :: LHsType GhcPs
          phead_ty = parenthesizeHsType sigPrec head_ty'

          go :: [LHsTypeArg GhcPs] -> CvtM (LHsType GhcPs)
          go [] = pure head_ty'
          go (arg:args) =
            case arg of
              HsValArg ty  -> do p_ty <- add_parens ty
                                 mk_apps (HsAppTy noExt phead_ty p_ty) args
              HsTypeArg l ki -> do p_ki <- add_parens ki
                                   mk_apps (HsAppKindTy l phead_ty p_ki) args
              HsArgPar _   -> mk_apps (HsParTy noExt phead_ty) args

      go type_args
       where
        -- See Note [Adding parens for splices]
        add_parens lt@(dL->L _ t)
          | hsTypeNeedsParens appPrec t = returnL (HsParTy noExt lt)
          | otherwise                   = return lt

    wrap_tyarg :: LHsTypeArg GhcPs -> LHsTypeArg GhcPs
    wrap_tyarg (HsValArg ty)    = HsValArg  $ parenthesizeHsType appPrec ty
    wrap_tyarg (HsTypeArg l ki) = HsTypeArg l $ parenthesizeHsType appPrec ki
    wrap_tyarg ta@(HsArgPar {}) = ta -- Already parenthesized

    -- ---------------------------------------------------------------------
    -- Note [Adding parens for splices]
    {-
    The hsSyn representation of parsed source explicitly contains all the original
    parens, as written in the source.

    When a Template Haskell (TH) splice is evaluated, the original splice is first
    renamed and type checked and then finally converted to core in DsMeta. This core
    is then run in the TH engine, and the result comes back as a TH AST.

    In the process, all parens are stripped out, as they are not needed.

    This Convert module then converts the TH AST back to hsSyn AST.

    In order to pretty-print this hsSyn AST, parens need to be adde back at certain
    points so that the code is readable with its original meaning.

    So scattered through Convert.hs are various points where parens are added.

    See (among other closed issued) https://ghc.haskell.org/trac/ghc/ticket/14289
    -}
    -- ---------------------------------------------------------------------

    -- | Constructs an arrow type with a specified return type
    mk_arr_apps :: [LHsType GhcPs] -> HsType GhcPs -> CvtM (LHsType GhcPs)
    mk_arr_apps tys return_ty = foldrM go return_ty tys >>= returnL
        where go :: LHsType GhcPs -> HsType GhcPs -> CvtM (HsType GhcPs)
              go arg ret_ty = do { ret_ty_l <- returnL ret_ty
                                 ; return (HsFunTy noExt arg ret_ty_l) }

    split_ty_app :: TH.Type -> CvtM (TH.Type, [LHsTypeArg GhcPs])
    split_ty_app ty = go ty []
      where
        go (AppT f a) as' = do { a' <- cvtType a; go f (HsValArg a':as') }
        go (AppKindT ty ki) as' = do { ki' <- cvtKind ki
                                     ; go ty (HsTypeArg noSrcSpan ki':as') }
        go (ParensT t) as' = do { loc <- getL; go t (HsArgPar loc: as') }
        go f as           = return (f,as)

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

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

    See the @cvtOpApp@ documentation for how this function works.
    -}
    cvtOpAppT :: TH.Type -> TH.Name -> LHsType GhcPs -> CvtM (LHsType GhcPs)
    cvtOpAppT (UInfixT x op2 y) op1 z
      = do { l <- cvtOpAppT y op1 z
           ; cvtOpAppT x op2 l }
    cvtOpAppT x op y
      = do { op' <- tconNameL op
           ; x' <- cvtType x
           ; returnL (mkHsOpTy x' op' y) }

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

    -- | Convert Maybe Kind to a type family result signature. Used with data
    -- families where naming of the result is not possible (thus only kind or no
    -- signature is possible).
    cvtMaybeKindToFamilyResultSig :: Maybe TH.Kind
                                  -> CvtM (LFamilyResultSig GhcPs)
    cvtMaybeKindToFamilyResultSig Nothing   = returnL (Hs.NoSig noExt)
    cvtMaybeKindToFamilyResultSig (Just ki) = do { ki' <- cvtKind ki
                                                 ; returnL (Hs.KindSig noExt ki') }

    -- | Convert type family result signature. Used with both open and closed type
    -- families.
    cvtFamilyResultSig :: TH.FamilyResultSig -> CvtM (Hs.LFamilyResultSig GhcPs)
    cvtFamilyResultSig TH.NoSig           = returnL (Hs.NoSig noExt)
    cvtFamilyResultSig (TH.KindSig ki)    = do { ki' <- cvtKind ki
                                               ; returnL (Hs.KindSig noExt  ki') }
    cvtFamilyResultSig (TH.TyVarSig bndr) = do { tv <- cvt_tv bndr
                                               ; returnL (Hs.TyVarSig noExt tv) }

    -- | Convert injectivity annotation of a type family.
    cvtInjectivityAnnotation :: TH.InjectivityAnn
                             -> CvtM (Hs.LInjectivityAnn GhcPs)
    cvtInjectivityAnnotation (TH.InjectivityAnn annLHS annRHS)
      = do { annLHS' <- tNameL annLHS
           ; annRHS' <- mapM tNameL annRHS
           ; returnL (Hs.InjectivityAnn annLHS' annRHS') }

    cvtPatSynSigTy :: TH.Type -> CvtM (LHsType GhcPs)
    -- pattern synonym types are of peculiar shapes, which is why we treat
    -- them separately from regular types;
    -- see Note [Pattern synonym type signatures and Template Haskell]
    cvtPatSynSigTy (ForallT univs reqs (ForallT exis provs ty))
      | null exis, null provs = cvtType (ForallT univs reqs ty)
      | null univs, null reqs = do { l   <- getL
                                   ; ty' <- cvtType (ForallT exis provs ty)
                                   ; return $ cL l (HsQualTy { hst_ctxt = cL l []
                                                             , hst_xqual = noExt
                                                             , hst_body = ty' }) }
      | null reqs             = do { l      <- getL
                                   ; univs' <- hsQTvExplicit <$> cvtTvs univs
                                   ; ty'    <- cvtType (ForallT exis provs ty)
                                   ; let forTy = HsForAllTy
                                                  { hst_bndrs = univs'
                                                  , hst_xforall = noExt
                                                  , hst_body = cL l cxtTy }
                                         cxtTy = HsQualTy { hst_ctxt = cL l []
                                                          , hst_xqual = noExt
                                                          , hst_body = ty' }
                                   ; return $ cL l forTy }
      | otherwise             = cvtType (ForallT univs reqs (ForallT exis provs ty))
    cvtPatSynSigTy ty         = cvtType ty

    -----------------------------------------------------------
    cvtFixity :: TH.Fixity -> Hs.Fixity
    cvtFixity (TH.Fixity prec dir) = Hs.Fixity NoSourceText 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

    -- Checks that are performed when converting unboxed sum expressions and
    -- patterns alike.
    unboxedSumChecks :: TH.SumAlt -> TH.SumArity -> CvtM ()
    unboxedSumChecks alt arity
        | alt > arity
        = failWith $ text "Sum alternative"    <+> text (show alt)
                 <+> text "exceeds its arity," <+> text (show arity)
        | alt <= 0
        = failWith $ vcat [ text "Illegal sum alternative:" <+> text (show alt)
                          , nest 2 $ text "Sum alternatives must start from 1" ]
        | arity < 2
        = failWith $ vcat [ text "Illegal sum arity:" <+> text (show arity)
                          , nest 2 $ text "Sums must have an arity of at least 2" ]
        | otherwise
        = return ()

    -- | If passed an empty list of 'TH.TyVarBndr's, this simply returns the
    -- third argument (an 'LHsType'). Otherwise, return an 'HsForAllTy'
    -- using the provided 'LHsQTyVars' and 'LHsType'.
    mkHsForAllTy :: [TH.TyVarBndr]
                 -- ^ The original Template Haskell type variable binders
                 -> SrcSpan
                 -- ^ The location of the returned 'LHsType' if it needs an
                 --   explicit forall
                 -> LHsQTyVars GhcPs
                 -- ^ The converted type variable binders
                 -> LHsType GhcPs
                 -- ^ The converted rho type
                 -> LHsType GhcPs
                 -- ^ The complete type, quantified with a forall if necessary
    mkHsForAllTy tvs loc tvs' rho_ty
      | null tvs  = rho_ty
      | otherwise = cL loc $ HsForAllTy { hst_bndrs = hsQTvExplicit tvs'
                                        , hst_xforall = noExt
                                        , hst_body = rho_ty }

    -- | If passed an empty 'TH.Cxt', this simply returns the third argument
    -- (an 'LHsType'). Otherwise, return an 'HsQualTy' using the provided
    -- 'LHsContext' and 'LHsType'.

    -- It's important that we don't build an HsQualTy if the context is empty,
    -- as the pretty-printer for HsType _always_ prints contexts, even if
    -- they're empty. See Trac #13183.
    mkHsQualTy :: TH.Cxt
               -- ^ The original Template Haskell context
               -> SrcSpan
               -- ^ The location of the returned 'LHsType' if it needs an
               --   explicit context
               -> LHsContext GhcPs
               -- ^ The converted context
               -> LHsType GhcPs
               -- ^ The converted tau type
               -> LHsType GhcPs
               -- ^ The complete type, qualified with a context if necessary
    mkHsQualTy ctxt loc ctxt' ty
      | null ctxt = ty
      | otherwise = cL loc $ HsQualTy { hst_xqual = noExt
                                      , hst_ctxt  = ctxt'
                                      , hst_body  = ty }

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

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

    -- Variable names
    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

    -- Variable *or* constructor names; check by looking at the first char
    vcNameL n = wrapL (vcName n)
    vcName n = if isVarName n then vName n else cName n

    -- Type variable names
    tNameL n = wrapL (tName n)
    tName n = cvtName OccName.tvName n

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

    ipName :: String -> CvtM HsIPName
    ipName n
      = do { unless (okVarOcc n) (failWith (badOcc OccName.varName n))
           ; return (HsIPName (fsLit 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 ns str
      | OccName.isVarNameSpace ns     = okVarOcc str
      | OccName.isDataConNameSpace ns = okConOcc str
      | otherwise                     = okTcOcc  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
      = text "Illegal" <+> pprNameSpace ctxt_ns
            <+> text "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_maybe occ -> nameRdrName $! name
                  | otherwise                           -> mkRdrUnqual $! occ
                  -- We check for built-in syntax here, because the TH
                  -- user might have written a (NameS "(,,)"), for example
      where
        occ :: OccName.OccName
        occ = mk_occ ctxt_ns th_occ

    -- Return an unqualified exact RdrName if we're dealing with built-in syntax.
    -- See Trac #13776.
    thOrigRdrName :: String -> TH.NameSpace -> PkgName -> ModName -> RdrName
    thOrigRdrName occ th_ns pkg mod =
      let occ' = mk_occ (mk_ghc_ns th_ns) occ
      in case isBuiltInOcc_maybe occ' of
           Just name -> nameRdrName name
           Nothing   -> (mkOrig $! (mkModule (mk_pkg pkg) (mk_mod mod))) $! 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

    -- 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 -> UnitId
    mk_pkg pkg = stringToUnitId (TH.pkgString pkg)

    mk_uniq :: Int -> Unique
    mk_uniq u = mkUniqueGrimily u

    {-
    Note [Binders in Template Haskell]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider this TH term construction:
      do { x1 <- TH.newName "x"   -- newName :: String -> Q TH.Name
         ; x2 <- TH.newName "x"   -- Builds a NameU
         ; x3 <- TH.newName "x"

         ; let x = mkName "x"     -- mkName :: String -> TH.Name
                                  -- Builds a NameS

         ; return (LamE (..pattern [x1,x2]..) $
                   LamE (VarPat x3) $
                   ..tuple (x1,x2,x3,x)) }

    It represents the term   \[x1,x2]. \x3. (x1,x2,x3,x)

    a) We don't want to complain about "x" being bound twice in
       the pattern [x1,x2]
    b) We don't want x3 to shadow the x1,x2
    c) We *do* want 'x' (dynamically bound with mkName) to bind
       to the innermost binding of "x", namely x3.
    d) When pretty printing, we want to print a unique with x1,x2
       etc, else they'll all print as "x" which isn't very helpful

    When we convert all this to HsSyn, the TH.Names are converted with
    thRdrName.  To achieve (b) we want the binders to be Exact RdrNames.
    Achieving (a) is a bit awkward, because
       - We must check for duplicate and shadowed names on Names,
         not RdrNames, *after* renaming.
         See Note [Collect binders only after renaming] in HsUtils

       - But to achieve (a) we must distinguish between the Exact
         RdrNames arising from TH and the Unqual RdrNames that would
         come from a user writing \[x,x] -> blah

    So in Convert.thRdrName we translate
       TH Name                          RdrName
       --------------------------------------------------------
       NameU (arising from newName) --> Exact (Name{ System })
       NameS (arising from mkName)  --> Unqual

    Notice that the NameUs generate *System* Names.  Then, when
    figuring out shadowing and duplicates, we can filter out
    System Names.

    This use of System Names fits with other uses of System Names, eg for
    temporary variables "a". Since there are lots of things called "a" we
    usually want to print the name with the unique, and that is indeed
    the way System Names are printed.

    There's a small complication of course; see Note [Looking up Exact
    RdrNames] in RnEnv.
    -}

    {-
    Note [Pattern synonym type signatures and Template Haskell]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    In general, the type signature of a pattern synonym

      pattern P x1 x2 .. xn = <some-pattern>

    is of the form

       forall univs. reqs => forall exis. provs => t1 -> t2 -> ... -> tn -> t

    with the following parts:

       1) the (possibly empty lists of) universally quantified type
          variables `univs` and required constraints `reqs` on them.
       2) the (possibly empty lists of) existentially quantified type
          variables `exis` and the provided constraints `provs` on them.
       3) the types `t1`, `t2`, .., `tn` of the pattern synonym's arguments x1,
          x2, .., xn, respectively
       4) the type `t` of <some-pattern>, mentioning only universals from `univs`.

    Due to the two forall quantifiers and constraint contexts (either of
    which might be empty), pattern synonym type signatures are treated
    specially in `deSugar/DsMeta.hs`, `hsSyn/Convert.hs`, and
    `typecheck/TcSplice.hs`:

       (a) When desugaring a pattern synonym from HsSyn to TH.Dec in
           `deSugar/DsMeta.hs`, we represent its *full* type signature in TH, i.e.:

               ForallT univs reqs (ForallT exis provs ty)
                  (where ty is the AST representation of t1 -> t2 -> ... -> tn -> t)

       (b) When converting pattern synonyms from TH.Dec to HsSyn in
           `hsSyn/Convert.hs`, we convert their TH type signatures back to an
           appropriate Haskell pattern synonym type of the form

             forall univs. reqs => forall exis. provs => t1 -> t2 -> ... -> tn -> t

           where initial empty `univs` type variables or an empty `reqs`
           constraint context are represented *explicitly* as `() =>`.

       (c) When reifying a pattern synonym in `typecheck/TcSplice.hs`, we always
           return its *full* type, i.e.:

               ForallT univs reqs (ForallT exis provs ty)
                  (where ty is the AST representation of t1 -> t2 -> ... -> tn -> t)

    The key point is to always represent a pattern synonym's *full* type
    in cases (a) and (c) to make it clear which of the two forall
    quantifiers and/or constraint contexts are specified, and which are
    not. See GHC's user's guide on pattern synonyms for more information
    about pattern synonym type signatures.

    -}