typecheck/TcGenGenerics.hs

    {-
    (c) The University of Glasgow 2011

    The deriving code for the Generic class
    (equivalent to the code in TcGenDeriv, for other classes)
    -}

    {-# LANGUAGE CPP, ScopedTypeVariables, TupleSections #-}
    {-# LANGUAGE FlexibleContexts #-}
    {-# LANGUAGE TypeFamilies #-}

    module TcGenGenerics (canDoGenerics, canDoGenerics1,
                          GenericKind(..),
                          gen_Generic_binds, get_gen1_constrained_tys) where

    import GhcPrelude

    import HsSyn
    import Type
    import TcType
    import TcGenDeriv
    import TcGenFunctor
    import DataCon
    import TyCon
    import FamInstEnv       ( FamInst, FamFlavor(..), mkSingleCoAxiom )
    import FamInst
    import Module           ( moduleName, moduleNameFS
                            , moduleUnitId, unitIdFS, getModule )
    import IfaceEnv         ( newGlobalBinder )
    import Name      hiding ( varName )
    import RdrName
    import BasicTypes
    import TysPrim
    import TysWiredIn
    import PrelNames
    import TcEnv
    import TcRnMonad
    import HscTypes
    import ErrUtils( Validity(..), andValid )
    import SrcLoc
    import Bag
    import VarEnv
    import VarSet (elemVarSet)
    import Outputable
    import FastString
    import Util

    import Control.Monad (mplus)
    import Data.List (zip4, partition)
    import Data.Maybe (isJust)

    #include "HsVersions.h"

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Bindings for the new generic deriving mechanism}
    *                                                                      *
    ************************************************************************

    For the generic representation we need to generate:
    \begin{itemize}
    \item A Generic instance
    \item A Rep type instance
    \item Many auxiliary datatypes and instances for them (for the meta-information)
    \end{itemize}
    -}

    gen_Generic_binds :: GenericKind -> TyCon -> [Type]
                     -> TcM (LHsBinds GhcPs, FamInst)
    gen_Generic_binds gk tc inst_tys = do
      repTyInsts <- tc_mkRepFamInsts gk tc inst_tys
      return (mkBindsRep gk tc, repTyInsts)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Generating representation types}
    *                                                                      *
    ************************************************************************
    -}

    get_gen1_constrained_tys :: TyVar -> Type -> [Type]
    -- called by TcDeriv.inferConstraints; generates a list of types, each of which
    -- must be a Functor in order for the Generic1 instance to work.
    get_gen1_constrained_tys argVar
      = argTyFold argVar $ ArgTyAlg { ata_rec0 = const []
                                    , ata_par1 = [], ata_rec1 = const []
                                    , ata_comp = (:) }

    {-

    Note [Requirements for deriving Generic and Rep]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    In the following, T, Tfun, and Targ are "meta-variables" ranging over type
    expressions.

    (Generic T) and (Rep T) are derivable for some type expression T if the
    following constraints are satisfied.

      (a) D is a type constructor *value*. In other words, D is either a type
          constructor or it is equivalent to the head of a data family instance (up to
          alpha-renaming).

      (b) D cannot have a "stupid context".

      (c) The right-hand side of D cannot include existential types, universally
          quantified types, or "exotic" unlifted types. An exotic unlifted type
          is one which is not listed in the definition of allowedUnliftedTy
          (i.e., one for which we have no representation type).
          See Note [Generics and unlifted types]

      (d) T :: *.

    (Generic1 T) and (Rep1 T) are derivable for some type expression T if the
    following constraints are satisfied.

      (a),(b),(c) As above.

      (d) T must expect arguments, and its last parameter must have kind *.

          We use `a' to denote the parameter of D that corresponds to the last
          parameter of T.

      (e) For any type-level application (Tfun Targ) in the right-hand side of D
          where the head of Tfun is not a tuple constructor:

          (b1) `a' must not occur in Tfun.

          (b2) If `a' occurs in Targ, then Tfun :: * -> *.

    -}

    canDoGenerics :: TyCon -> Validity
    -- canDoGenerics determines if Generic/Rep can be derived.
    --
    -- Check (a) from Note [Requirements for deriving Generic and Rep] is taken
    -- care of because canDoGenerics is applied to rep tycons.
    --
    -- It returns IsValid if deriving is possible. It returns (NotValid reason)
    -- if not.
    canDoGenerics tc
      = mergeErrors (
              -- Check (b) from Note [Requirements for deriving Generic and Rep].
                  (if (not (null (tyConStupidTheta tc)))
                    then (NotValid (tc_name <+> text "must not have a datatype context"))
                    else IsValid)
              -- See comment below
                : (map bad_con (tyConDataCons tc)))
      where
        -- The tc can be a representation tycon. When we want to display it to the
        -- user (in an error message) we should print its parent
        tc_name = ppr $ case tyConFamInst_maybe tc of
            Just (ptc, _) -> ptc
            _             -> tc

            -- Check (c) from Note [Requirements for deriving Generic and Rep].
            --
            -- If any of the constructors has an exotic unlifted type as argument,
            -- then we can't build the embedding-projection pair, because
            -- it relies on instantiating *polymorphic* sum and product types
            -- at the argument types of the constructors
        bad_con dc = if (any bad_arg_type (dataConOrigArgTys dc))
                      then (NotValid (ppr dc <+> text
                        "must not have exotic unlifted or polymorphic arguments"))
                      else (if (not (isVanillaDataCon dc))
                              then (NotValid (ppr dc <+> text "must be a vanilla data constructor"))
                              else IsValid)

            -- Nor can we do the job if it's an existential data constructor,
            -- Nor if the args are polymorphic types (I don't think)
        bad_arg_type ty = (isUnliftedType ty && not (allowedUnliftedTy ty))
                          || not (isTauTy ty)

    -- Returns True the Type argument is an unlifted type which has a
    -- corresponding generic representation type. For example,
    -- (allowedUnliftedTy Int#) would return True since there is the UInt
    -- representation type.
    allowedUnliftedTy :: Type -> Bool
    allowedUnliftedTy = isJust . unboxedRepRDRs

    mergeErrors :: [Validity] -> Validity
    mergeErrors []             = IsValid
    mergeErrors (NotValid s:t) = case mergeErrors t of
      IsValid     -> NotValid s
      NotValid s' -> NotValid (s <> text ", and" $$ s')
    mergeErrors (IsValid : t) = mergeErrors t

    -- A datatype used only inside of canDoGenerics1. It's the result of analysing
    -- a type term.
    data Check_for_CanDoGenerics1 = CCDG1
      { _ccdg1_hasParam :: Bool       -- does the parameter of interest occurs in
                                      -- this type?
      , _ccdg1_errors   :: Validity   -- errors generated by this type
      }

    {-

    Note [degenerate use of FFoldType]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    We use foldDataConArgs here only for its ability to treat tuples
    specially. foldDataConArgs also tracks covariance (though it assumes all
    higher-order type parameters are covariant) and has hooks for special handling
    of functions and polytypes, but we do *not* use those.

    The key issue is that Generic1 deriving currently offers no sophisticated
    support for functions. For example, we cannot handle

      data F a = F ((a -> Int) -> Int)

    even though a is occurring covariantly.

    In fact, our rule is harsh: a is simply not allowed to occur within the first
    argument of (->). We treat (->) the same as any other non-tuple tycon.

    Unfortunately, this means we have to track "the parameter occurs in this type"
    explicitly, even though foldDataConArgs is also doing this internally.

    -}

    -- canDoGenerics1 determines if a Generic1/Rep1 can be derived.
    --
    -- Checks (a) through (c) from Note [Requirements for deriving Generic and Rep]
    -- are taken care of by the call to canDoGenerics.
    --
    -- It returns IsValid if deriving is possible. It returns (NotValid reason)
    -- if not.
    canDoGenerics1 :: TyCon -> Validity
    canDoGenerics1 rep_tc =
      canDoGenerics rep_tc `andValid` additionalChecks
      where
        additionalChecks
            -- check (d) from Note [Requirements for deriving Generic and Rep]
          | null (tyConTyVars rep_tc) = NotValid $
              text "Data type" <+> quotes (ppr rep_tc)
          <+> text "must have some type parameters"

          | otherwise = mergeErrors $ concatMap check_con data_cons

        data_cons = tyConDataCons rep_tc
        check_con con = case check_vanilla con of
          j@(NotValid {}) -> [j]
          IsValid -> _ccdg1_errors `map` foldDataConArgs (ft_check con) con

        bad :: DataCon -> SDoc -> SDoc
        bad con msg = text "Constructor" <+> quotes (ppr con) <+> msg

        check_vanilla :: DataCon -> Validity
        check_vanilla con | isVanillaDataCon con = IsValid
                          | otherwise            = NotValid (bad con existential)

        bmzero      = CCDG1 False IsValid
        bmbad con s = CCDG1 True $ NotValid $ bad con s
        bmplus (CCDG1 b1 m1) (CCDG1 b2 m2) = CCDG1 (b1 || b2) (m1 `andValid` m2)

        -- check (e) from Note [Requirements for deriving Generic and Rep]
        -- See also Note [degenerate use of FFoldType]
        ft_check :: DataCon -> FFoldType Check_for_CanDoGenerics1
        ft_check con = FT
          { ft_triv = bmzero

          , ft_var = caseVar, ft_co_var = caseVar

          -- (component_0,component_1,...,component_n)
          , ft_tup = \_ components -> if any _ccdg1_hasParam (init components)
                                      then bmbad con wrong_arg
                                      else foldr bmplus bmzero components

          -- (dom -> rng), where the head of ty is not a tuple tycon
          , ft_fun = \dom rng -> -- cf #8516
              if _ccdg1_hasParam dom
              then bmbad con wrong_arg
              else bmplus dom rng

          -- (ty arg), where head of ty is neither (->) nor a tuple constructor and
          -- the parameter of interest does not occur in ty
          , ft_ty_app = \_ arg -> arg

          , ft_bad_app = bmbad con wrong_arg
          , ft_forall  = \_ body -> body -- polytypes are handled elsewhere
          }
          where
            caseVar = CCDG1 True IsValid

        existential = text "must not have existential arguments"
        wrong_arg   = text "applies a type to an argument involving the last parameter"
                   $$ text "but the applied type is not of kind * -> *"

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Generating the RHS of a generic default method}
    *                                                                      *
    ************************************************************************
    -}

    type US = Int   -- Local unique supply, just a plain Int
    type Alt = (LPat GhcPs, LHsExpr GhcPs)

    -- GenericKind serves to mark if a datatype derives Generic (Gen0) or
    -- Generic1 (Gen1).
    data GenericKind = Gen0 | Gen1

    -- as above, but with a payload of the TyCon's name for "the" parameter
    data GenericKind_ = Gen0_ | Gen1_ TyVar

    -- as above, but using a single datacon's name for "the" parameter
    data GenericKind_DC = Gen0_DC | Gen1_DC TyVar

    forgetArgVar :: GenericKind_DC -> GenericKind
    forgetArgVar Gen0_DC   = Gen0
    forgetArgVar Gen1_DC{} = Gen1

    -- When working only within a single datacon, "the" parameter's name should
    -- match that datacon's name for it.
    gk2gkDC :: GenericKind_ -> DataCon -> GenericKind_DC
    gk2gkDC Gen0_   _ = Gen0_DC
    gk2gkDC Gen1_{} d = Gen1_DC $ last $ dataConUnivTyVars d

    -- Bindings for the Generic instance
    mkBindsRep :: GenericKind -> TyCon -> LHsBinds GhcPs
    mkBindsRep gk tycon =
        unitBag (mkRdrFunBind (L loc from01_RDR) [from_eqn])
      `unionBags`
        unitBag (mkRdrFunBind (L loc to01_RDR) [to_eqn])
          where
            -- The topmost M1 (the datatype metadata) has the exact same type
            -- across all cases of a from/to definition, and can be factored out
            -- to save some allocations during typechecking.
            -- See Note [Generics compilation speed tricks]
            from_eqn = mkHsCaseAlt x_Pat $ mkM1_E
                                           $ nlHsPar $ nlHsCase x_Expr from_matches
            to_eqn   = mkHsCaseAlt (mkM1_P x_Pat) $ nlHsCase x_Expr to_matches

            from_matches  = [mkHsCaseAlt pat rhs | (pat,rhs) <- from_alts]
            to_matches    = [mkHsCaseAlt pat rhs | (pat,rhs) <- to_alts  ]
            loc           = srcLocSpan (getSrcLoc tycon)
            datacons      = tyConDataCons tycon

            (from01_RDR, to01_RDR) = case gk of
                                       Gen0 -> (from_RDR,  to_RDR)
                                       Gen1 -> (from1_RDR, to1_RDR)

            -- Recurse over the sum first
            from_alts, to_alts :: [Alt]
            (from_alts, to_alts) = mkSum gk_ (1 :: US) datacons
              where gk_ = case gk of
                      Gen0 -> Gen0_
                      Gen1 -> ASSERT(tyvars `lengthAtLeast` 1)
                              Gen1_ (last tyvars)
                        where tyvars = tyConTyVars tycon

    --------------------------------------------------------------------------------
    -- The type synonym instance and synonym
    --       type instance Rep (D a b) = Rep_D a b
    --       type Rep_D a b = ...representation type for D ...
    --------------------------------------------------------------------------------

    tc_mkRepFamInsts :: GenericKind   -- Gen0 or Gen1
                     -> TyCon         -- The type to generate representation for
                     -> [Type]        -- The type(s) to which Generic(1) is applied
                                      -- in the generated instance
                     -> TcM FamInst   -- Generated representation0 coercion
    tc_mkRepFamInsts gk tycon inst_tys =
           -- Consider the example input tycon `D`, where data D a b = D_ a
           -- Also consider `R:DInt`, where { data family D x y :: * -> *
           --                               ; data instance D Int a b = D_ a }
      do { -- `rep` = GHC.Generics.Rep or GHC.Generics.Rep1 (type family)
           fam_tc <- case gk of
             Gen0 -> tcLookupTyCon repTyConName
             Gen1 -> tcLookupTyCon rep1TyConName

         ; fam_envs <- tcGetFamInstEnvs

         ; let -- If the derived instance is
               --   instance Generic (Foo x)
               -- then:
               --   `arg_ki` = *, `inst_ty` = Foo x :: *
               --
               -- If the derived instance is
               --   instance Generic1 (Bar x :: k -> *)
               -- then:
               --   `arg_k` = k, `inst_ty` = Bar x :: k -> *
               (arg_ki, inst_ty) = case (gk, inst_tys) of
                 (Gen0, [inst_t])        -> (liftedTypeKind, inst_t)
                 (Gen1, [arg_k, inst_t]) -> (arg_k,          inst_t)
                 _ -> pprPanic "tc_mkRepFamInsts" (ppr inst_tys)

         ; let mbFamInst         = tyConFamInst_maybe tycon
               -- If we're examining a data family instance, we grab the parent
               -- TyCon (ptc) and use it to determine the type arguments
               -- (inst_args) for the data family *instance*'s type variables.
               ptc               = maybe tycon fst mbFamInst
               (_, inst_args, _) = tcLookupDataFamInst fam_envs ptc $ snd
                                     $ tcSplitTyConApp inst_ty

         ; let -- `tyvars` = [a,b]
               (tyvars, gk_) = case gk of
                 Gen0 -> (all_tyvars, Gen0_)
                 Gen1 -> ASSERT(not $ null all_tyvars)
                         (init all_tyvars, Gen1_ $ last all_tyvars)
                 where all_tyvars = tyConTyVars tycon

           -- `repTy` = D1 ... (C1 ... (S1 ... (Rec0 a))) :: * -> *
         ; repTy <- tc_mkRepTy gk_ tycon arg_ki

           -- `rep_name` is a name we generate for the synonym
         ; mod <- getModule
         ; loc <- getSrcSpanM
         ; let tc_occ  = nameOccName (tyConName tycon)
               rep_occ = case gk of Gen0 -> mkGenR tc_occ; Gen1 -> mkGen1R tc_occ
         ; rep_name <- newGlobalBinder mod rep_occ loc

           -- We make sure to substitute the tyvars with their user-supplied
           -- type arguments before generating the Rep/Rep1 instance, since some
           -- of the tyvars might have been instantiated when deriving.
           -- See Note [Generating a correctly typed Rep instance].
         ; let (env_tyvars, env_inst_args)
                 = case gk_ of
                     Gen0_ -> (tyvars, inst_args)
                     Gen1_ last_tv
                              -- See the "wrinkle" in
                              -- Note [Generating a correctly typed Rep instance]
                           -> ( last_tv : tyvars
                              , anyTypeOfKind (tyVarKind last_tv) : inst_args )
               env        = zipTyEnv env_tyvars env_inst_args
               in_scope   = mkInScopeSet (tyCoVarsOfTypes inst_tys)
               subst      = mkTvSubst in_scope env
               repTy'     = substTy  subst repTy
               tcv'       = tyCoVarsOfTypeList inst_ty
               (tv', cv') = partition isTyVar tcv'
               tvs'       = scopedSort tv'
               cvs'       = scopedSort cv'
               axiom      = mkSingleCoAxiom Nominal rep_name tvs' [] cvs'
                                            fam_tc inst_tys repTy'

         ; newFamInst SynFamilyInst axiom  }

    --------------------------------------------------------------------------------
    -- Type representation
    --------------------------------------------------------------------------------

    -- | See documentation of 'argTyFold'; that function uses the fields of this
    -- type to interpret the structure of a type when that type is considered as an
    -- argument to a constructor that is being represented with 'Rep1'.
    data ArgTyAlg a = ArgTyAlg
      { ata_rec0 :: (Type -> a)
      , ata_par1 :: a, ata_rec1 :: (Type -> a)
      , ata_comp :: (Type -> a -> a)
      }

    -- | @argTyFold@ implements a generalised and safer variant of the @arg@
    -- function from Figure 3 in <http://dreixel.net/research/pdf/gdmh.pdf>. @arg@
    -- is conceptually equivalent to:
    --
    -- > arg t = case t of
    -- >   _ | isTyVar t         -> if (t == argVar) then Par1 else Par0 t
    -- >   App f [t'] |
    -- >     representable1 f &&
    -- >     t' == argVar        -> Rec1 f
    -- >   App f [t'] |
    -- >     representable1 f &&
    -- >     t' has tyvars       -> f :.: (arg t')
    -- >   _                     -> Rec0 t
    --
    -- where @argVar@ is the last type variable in the data type declaration we are
    -- finding the representation for.
    --
    -- @argTyFold@ is more general than @arg@ because it uses 'ArgTyAlg' to
    -- abstract out the concrete invocations of @Par0@, @Rec0@, @Par1@, @Rec1@, and
    -- @:.:@.
    --
    -- @argTyFold@ is safer than @arg@ because @arg@ would lead to a GHC panic for
    -- some data types. The problematic case is when @t@ is an application of a
    -- non-representable type @f@ to @argVar@: @App f [argVar]@ is caught by the
    -- @_@ pattern, and ends up represented as @Rec0 t@. This type occurs /free/ in
    -- the RHS of the eventual @Rep1@ instance, which is therefore ill-formed. Some
    -- representable1 checks have been relaxed, and others were moved to
    -- @canDoGenerics1@.
    argTyFold :: forall a. TyVar -> ArgTyAlg a -> Type -> a
    argTyFold argVar (ArgTyAlg {ata_rec0 = mkRec0,
                                ata_par1 = mkPar1, ata_rec1 = mkRec1,
                                ata_comp = mkComp}) =
      -- mkRec0 is the default; use it if there is no interesting structure
      -- (e.g. occurrences of parameters or recursive occurrences)
      \t -> maybe (mkRec0 t) id $ go t where
      go :: Type -> -- type to fold through
            Maybe a -- the result (e.g. representation type), unless it's trivial
      go t = isParam `mplus` isApp where

        isParam = do -- handles parameters
          t' <- getTyVar_maybe t
          Just $ if t' == argVar then mkPar1 -- moreover, it is "the" parameter
                 else mkRec0 t -- NB mkRec0 instead of the conventional mkPar0

        isApp = do -- handles applications
          (phi, beta) <- tcSplitAppTy_maybe t

          let interesting = argVar `elemVarSet` exactTyCoVarsOfType beta

          -- Does it have no interesting structure to represent?
          if not interesting then Nothing
            else -- Is the argument the parameter? Special case for mkRec1.
              if Just argVar == getTyVar_maybe beta then Just $ mkRec1 phi
                else mkComp phi `fmap` go beta -- It must be a composition.

    tc_mkRepTy ::  -- Gen0_ or Gen1_, for Rep or Rep1
                   GenericKind_
                  -- The type to generate representation for
                -> TyCon
                  -- The kind of the representation type's argument
                  -- See Note [Handling kinds in a Rep instance]
                -> Kind
                   -- Generated representation0 type
                -> TcM Type
    tc_mkRepTy gk_ tycon k =
      do
        d1      <- tcLookupTyCon d1TyConName
        c1      <- tcLookupTyCon c1TyConName
        s1      <- tcLookupTyCon s1TyConName
        rec0    <- tcLookupTyCon rec0TyConName
        rec1    <- tcLookupTyCon rec1TyConName
        par1    <- tcLookupTyCon par1TyConName
        u1      <- tcLookupTyCon u1TyConName
        v1      <- tcLookupTyCon v1TyConName
        plus    <- tcLookupTyCon sumTyConName
        times   <- tcLookupTyCon prodTyConName
        comp    <- tcLookupTyCon compTyConName
        uAddr   <- tcLookupTyCon uAddrTyConName
        uChar   <- tcLookupTyCon uCharTyConName
        uDouble <- tcLookupTyCon uDoubleTyConName
        uFloat  <- tcLookupTyCon uFloatTyConName
        uInt    <- tcLookupTyCon uIntTyConName
        uWord   <- tcLookupTyCon uWordTyConName

        let tcLookupPromDataCon = fmap promoteDataCon . tcLookupDataCon

        md         <- tcLookupPromDataCon metaDataDataConName
        mc         <- tcLookupPromDataCon metaConsDataConName
        ms         <- tcLookupPromDataCon metaSelDataConName
        pPrefix    <- tcLookupPromDataCon prefixIDataConName
        pInfix     <- tcLookupPromDataCon infixIDataConName
        pLA        <- tcLookupPromDataCon leftAssociativeDataConName
        pRA        <- tcLookupPromDataCon rightAssociativeDataConName
        pNA        <- tcLookupPromDataCon notAssociativeDataConName
        pSUpk      <- tcLookupPromDataCon sourceUnpackDataConName
        pSNUpk     <- tcLookupPromDataCon sourceNoUnpackDataConName
        pNSUpkness <- tcLookupPromDataCon noSourceUnpackednessDataConName
        pSLzy      <- tcLookupPromDataCon sourceLazyDataConName
        pSStr      <- tcLookupPromDataCon sourceStrictDataConName
        pNSStrness <- tcLookupPromDataCon noSourceStrictnessDataConName
        pDLzy      <- tcLookupPromDataCon decidedLazyDataConName
        pDStr      <- tcLookupPromDataCon decidedStrictDataConName
        pDUpk      <- tcLookupPromDataCon decidedUnpackDataConName

        fix_env <- getFixityEnv

        let mkSum' a b = mkTyConApp plus  [k,a,b]
            mkProd a b = mkTyConApp times [k,a,b]
            mkRec0 a   = mkBoxTy uAddr uChar uDouble uFloat uInt uWord rec0 k a
            mkRec1 a   = mkTyConApp rec1  [k,a]
            mkPar1     = mkTyConTy  par1
            mkD    a   = mkTyConApp d1 [ k, metaDataTy, sumP (tyConDataCons a) ]
            mkC      a = mkTyConApp c1 [ k
                                       , metaConsTy a
                                       , prod (dataConInstOrigArgTys a
                                                . mkTyVarTys . tyConTyVars $ tycon)
                                              (dataConSrcBangs    a)
                                              (dataConImplBangs   a)
                                              (dataConFieldLabels a)]
            mkS mlbl su ss ib a = mkTyConApp s1 [k, metaSelTy mlbl su ss ib, a]

            -- Sums and products are done in the same way for both Rep and Rep1
            sumP [] = mkTyConApp v1 [k]
            sumP l  = foldBal mkSum' . map mkC  $ l
            -- The Bool is True if this constructor has labelled fields
            prod :: [Type] -> [HsSrcBang] -> [HsImplBang] -> [FieldLabel] -> Type
            prod [] _  _  _  = mkTyConApp u1 [k]
            prod l  sb ib fl = foldBal mkProd
                                       [ ASSERT(null fl || lengthExceeds fl j)
                                         arg t sb' ib' (if null fl
                                                           then Nothing
                                                           else Just (fl !! j))
                                       | (t,sb',ib',j) <- zip4 l sb ib [0..] ]

            arg :: Type -> HsSrcBang -> HsImplBang -> Maybe FieldLabel -> Type
            arg t (HsSrcBang _ su ss) ib fl = mkS fl su ss ib $ case gk_ of
                -- Here we previously used Par0 if t was a type variable, but we
                -- realized that we can't always guarantee that we are wrapping-up
                -- all type variables in Par0. So we decided to stop using Par0
                -- altogether, and use Rec0 all the time.
                          Gen0_        -> mkRec0 t
                          Gen1_ argVar -> argPar argVar t
              where
                -- Builds argument representation for Rep1 (more complicated due to
                -- the presence of composition).
                argPar argVar = argTyFold argVar $ ArgTyAlg
                  {ata_rec0 = mkRec0, ata_par1 = mkPar1,
                   ata_rec1 = mkRec1, ata_comp = mkComp comp k}

            tyConName_user = case tyConFamInst_maybe tycon of
                               Just (ptycon, _) -> tyConName ptycon
                               Nothing          -> tyConName tycon

            dtName  = mkStrLitTy . occNameFS . nameOccName $ tyConName_user
            mdName  = mkStrLitTy . moduleNameFS . moduleName
                    . nameModule . tyConName $ tycon
            pkgName = mkStrLitTy . unitIdFS . moduleUnitId
                    . nameModule . tyConName $ tycon
            isNT    = mkTyConTy $ if isNewTyCon tycon
                                  then promotedTrueDataCon
                                  else promotedFalseDataCon

            ctName = mkStrLitTy . occNameFS . nameOccName . dataConName
            ctFix c
                | dataConIsInfix c
                = case lookupFixity fix_env (dataConName c) of
                       Fixity _ n InfixL -> buildFix n pLA
                       Fixity _ n InfixR -> buildFix n pRA
                       Fixity _ n InfixN -> buildFix n pNA
                | otherwise = mkTyConTy pPrefix
            buildFix n assoc = mkTyConApp pInfix [ mkTyConTy assoc
                                                 , mkNumLitTy (fromIntegral n)]

            isRec c = mkTyConTy $ if dataConFieldLabels c `lengthExceeds` 0
                                  then promotedTrueDataCon
                                  else promotedFalseDataCon

            selName = mkStrLitTy . flLabel

            mbSel Nothing  = mkTyConApp promotedNothingDataCon [typeSymbolKind]
            mbSel (Just s) = mkTyConApp promotedJustDataCon
                                        [typeSymbolKind, selName s]

            metaDataTy   = mkTyConApp md [dtName, mdName, pkgName, isNT]
            metaConsTy c = mkTyConApp mc [ctName c, ctFix c, isRec c]
            metaSelTy mlbl su ss ib =
                mkTyConApp ms [mbSel mlbl, pSUpkness, pSStrness, pDStrness]
              where
                pSUpkness = mkTyConTy $ case su of
                                             SrcUnpack   -> pSUpk
                                             SrcNoUnpack -> pSNUpk
                                             NoSrcUnpack -> pNSUpkness

                pSStrness = mkTyConTy $ case ss of
                                             SrcLazy     -> pSLzy
                                             SrcStrict   -> pSStr
                                             NoSrcStrict -> pNSStrness

                pDStrness = mkTyConTy $ case ib of
                                             HsLazy      -> pDLzy
                                             HsStrict    -> pDStr
                                             HsUnpack{}  -> pDUpk

        return (mkD tycon)

    mkComp :: TyCon -> Kind -> Type -> Type -> Type
    mkComp comp k f g
      | k1_first  = mkTyConApp comp  [k,liftedTypeKind,f,g]
      | otherwise = mkTyConApp comp  [liftedTypeKind,k,f,g]
      where
        -- Which of these is the case?
        --     newtype (:.:) {k1} {k2} (f :: k2->*) (g :: k1->k2) (p :: k1) = ...
        -- or  newtype (:.:) {k2} {k1} (f :: k2->*) (g :: k1->k2) (p :: k1) = ...
        -- We want to instantiate with k1=k, and k2=*
        --    Reason for k2=*: see Note [Handling kinds in a Rep instance]
        -- But we need to know which way round!
        k1_first = k_first == p_kind_var
        [k_first,_,_,_,p] = tyConTyVars comp
        Just p_kind_var = getTyVar_maybe (tyVarKind p)

    -- Given the TyCons for each URec-related type synonym, check to see if the
    -- given type is an unlifted type that generics understands. If so, return
    -- its representation type. Otherwise, return Rec0.
    -- See Note [Generics and unlifted types]
    mkBoxTy :: TyCon -- UAddr
            -> TyCon -- UChar
            -> TyCon -- UDouble
            -> TyCon -- UFloat
            -> TyCon -- UInt
            -> TyCon -- UWord
            -> TyCon -- Rec0
            -> Kind  -- What to instantiate Rec0's kind variable with
            -> Type
            -> Type
    mkBoxTy uAddr uChar uDouble uFloat uInt uWord rec0 k ty
      | ty `eqType` addrPrimTy   = mkTyConApp uAddr   [k]
      | ty `eqType` charPrimTy   = mkTyConApp uChar   [k]
      | ty `eqType` doublePrimTy = mkTyConApp uDouble [k]
      | ty `eqType` floatPrimTy  = mkTyConApp uFloat  [k]
      | ty `eqType` intPrimTy    = mkTyConApp uInt    [k]
      | ty `eqType` wordPrimTy   = mkTyConApp uWord   [k]
      | otherwise                = mkTyConApp rec0    [k,ty]

    --------------------------------------------------------------------------------
    -- Dealing with sums
    --------------------------------------------------------------------------------

    mkSum :: GenericKind_ -- Generic or Generic1?
          -> US          -- Base for generating unique names
          -> [DataCon]   -- The data constructors
          -> ([Alt],     -- Alternatives for the T->Trep "from" function
              [Alt])     -- Alternatives for the Trep->T "to" function

    -- Datatype without any constructors
    mkSum _ _ [] = ([from_alt], [to_alt])
      where
        from_alt = (x_Pat, nlHsCase x_Expr [])
        to_alt   = (x_Pat, nlHsCase x_Expr [])
                   -- These M1s are meta-information for the datatype

    -- Datatype with at least one constructor
    mkSum gk_ us datacons =
      -- switch the payload of gk_ to be datacon-centric instead of tycon-centric
     unzip [ mk1Sum (gk2gkDC gk_ d) us i (length datacons) d
               | (d,i) <- zip datacons [1..] ]

    -- Build the sum for a particular constructor
    mk1Sum :: GenericKind_DC -- Generic or Generic1?
           -> US        -- Base for generating unique names
           -> Int       -- The index of this constructor
           -> Int       -- Total number of constructors
           -> DataCon   -- The data constructor
           -> (Alt,     -- Alternative for the T->Trep "from" function
               Alt)     -- Alternative for the Trep->T "to" function
    mk1Sum gk_ us i n datacon = (from_alt, to_alt)
      where
        gk = forgetArgVar gk_

        -- Existentials already excluded
        argTys = dataConOrigArgTys datacon
        n_args = dataConSourceArity datacon

        datacon_varTys = zip (map mkGenericLocal [us .. us+n_args-1]) argTys
        datacon_vars = map fst datacon_varTys
        us'          = us + n_args

        datacon_rdr  = getRdrName datacon

        from_alt     = (nlConVarPat datacon_rdr datacon_vars, from_alt_rhs)
        from_alt_rhs = genLR_E i n (mkProd_E gk_ us' datacon_varTys)

        to_alt     = ( genLR_P i n (mkProd_P gk us' datacon_varTys)
                     , to_alt_rhs
                     ) -- These M1s are meta-information for the datatype
        to_alt_rhs = case gk_ of
          Gen0_DC        -> nlHsVarApps datacon_rdr datacon_vars
          Gen1_DC argVar -> nlHsApps datacon_rdr $ map argTo datacon_varTys
            where
              argTo (var, ty) = converter ty `nlHsApp` nlHsVar var where
                converter = argTyFold argVar $ ArgTyAlg
                  {ata_rec0 = nlHsVar . unboxRepRDR,
                   ata_par1 = nlHsVar unPar1_RDR,
                   ata_rec1 = const $ nlHsVar unRec1_RDR,
                   ata_comp = \_ cnv -> (nlHsVar fmap_RDR `nlHsApp` cnv)
                                        `nlHsCompose` nlHsVar unComp1_RDR}

    -- Generates the L1/R1 sum pattern
    genLR_P :: Int -> Int -> LPat GhcPs -> LPat GhcPs
    genLR_P i n p
      | n == 0       = error "impossible"
      | n == 1       = p
      | i <= div n 2 = nlParPat $ nlConPat l1DataCon_RDR [genLR_P i     (div n 2) p]
      | otherwise    = nlParPat $ nlConPat r1DataCon_RDR [genLR_P (i-m) (n-m)     p]
                         where m = div n 2

    -- Generates the L1/R1 sum expression
    genLR_E :: Int -> Int -> LHsExpr GhcPs -> LHsExpr GhcPs
    genLR_E i n e
      | n == 0       = error "impossible"
      | n == 1       = e
      | i <= div n 2 = nlHsVar l1DataCon_RDR `nlHsApp`
                                                nlHsPar (genLR_E i     (div n 2) e)
      | otherwise    = nlHsVar r1DataCon_RDR `nlHsApp`
                                                nlHsPar (genLR_E (i-m) (n-m)     e)
                         where m = div n 2

    --------------------------------------------------------------------------------
    -- Dealing with products
    --------------------------------------------------------------------------------

    -- Build a product expression
    mkProd_E :: GenericKind_DC    -- Generic or Generic1?
             -> US                -- Base for unique names
             -> [(RdrName, Type)]
                           -- List of variables matched on the lhs and their types
             -> LHsExpr GhcPs   -- Resulting product expression
    mkProd_E _   _ []     = mkM1_E (nlHsVar u1DataCon_RDR)
    mkProd_E gk_ _ varTys = mkM1_E (foldBal prod appVars)
                         -- These M1s are meta-information for the constructor
      where
        appVars = map (wrapArg_E gk_) varTys
        prod a b = prodDataCon_RDR `nlHsApps` [a,b]

    wrapArg_E :: GenericKind_DC -> (RdrName, Type) -> LHsExpr GhcPs
    wrapArg_E Gen0_DC          (var, ty) = mkM1_E $
                                boxRepRDR ty `nlHsVarApps` [var]
                             -- This M1 is meta-information for the selector
    wrapArg_E (Gen1_DC argVar) (var, ty) = mkM1_E $
                                converter ty `nlHsApp` nlHsVar var
                             -- This M1 is meta-information for the selector
      where converter = argTyFold argVar $ ArgTyAlg
              {ata_rec0 = nlHsVar . boxRepRDR,
               ata_par1 = nlHsVar par1DataCon_RDR,
               ata_rec1 = const $ nlHsVar rec1DataCon_RDR,
               ata_comp = \_ cnv -> nlHsVar comp1DataCon_RDR `nlHsCompose`
                                      (nlHsVar fmap_RDR `nlHsApp` cnv)}

    boxRepRDR :: Type -> RdrName
    boxRepRDR = maybe k1DataCon_RDR fst . unboxedRepRDRs

    unboxRepRDR :: Type -> RdrName
    unboxRepRDR = maybe unK1_RDR snd . unboxedRepRDRs

    -- Retrieve the RDRs associated with each URec data family instance
    -- constructor. See Note [Generics and unlifted types]
    unboxedRepRDRs :: Type -> Maybe (RdrName, RdrName)
    unboxedRepRDRs ty
      | ty `eqType` addrPrimTy   = Just (uAddrDataCon_RDR,   uAddrHash_RDR)
      | ty `eqType` charPrimTy   = Just (uCharDataCon_RDR,   uCharHash_RDR)
      | ty `eqType` doublePrimTy = Just (uDoubleDataCon_RDR, uDoubleHash_RDR)
      | ty `eqType` floatPrimTy  = Just (uFloatDataCon_RDR,  uFloatHash_RDR)
      | ty `eqType` intPrimTy    = Just (uIntDataCon_RDR,    uIntHash_RDR)
      | ty `eqType` wordPrimTy   = Just (uWordDataCon_RDR,   uWordHash_RDR)
      | otherwise          = Nothing

    -- Build a product pattern
    mkProd_P :: GenericKind       -- Gen0 or Gen1
             -> US                -- Base for unique names
             -> [(RdrName, Type)] -- List of variables to match,
                                  --   along with their types
             -> LPat GhcPs      -- Resulting product pattern
    mkProd_P _  _ []     = mkM1_P (nlNullaryConPat u1DataCon_RDR)
    mkProd_P gk _ varTys = mkM1_P (foldBal prod appVars)
                         -- These M1s are meta-information for the constructor
      where
        appVars = unzipWith (wrapArg_P gk) varTys
        prod a b = nlParPat $ prodDataCon_RDR `nlConPat` [a,b]

    wrapArg_P :: GenericKind -> RdrName -> Type -> LPat GhcPs
    wrapArg_P Gen0 v ty = mkM1_P (nlParPat $ boxRepRDR ty `nlConVarPat` [v])
                       -- This M1 is meta-information for the selector
    wrapArg_P Gen1 v _  = nlParPat $ m1DataCon_RDR `nlConVarPat` [v]

    mkGenericLocal :: US -> RdrName
    mkGenericLocal u = mkVarUnqual (mkFastString ("g" ++ show u))

    x_RDR :: RdrName
    x_RDR = mkVarUnqual (fsLit "x")

    x_Expr :: LHsExpr GhcPs
    x_Expr = nlHsVar x_RDR

    x_Pat :: LPat GhcPs
    x_Pat = nlVarPat x_RDR

    mkM1_E :: LHsExpr GhcPs -> LHsExpr GhcPs
    mkM1_E e = nlHsVar m1DataCon_RDR `nlHsApp` e

    mkM1_P :: LPat GhcPs -> LPat GhcPs
    mkM1_P p = nlParPat $ m1DataCon_RDR `nlConPat` [p]

    nlHsCompose :: LHsExpr GhcPs -> LHsExpr GhcPs -> LHsExpr GhcPs
    nlHsCompose x y = compose_RDR `nlHsApps` [x, y]

    -- | Variant of foldr1 for producing balanced lists
    foldBal :: (a -> a -> a) -> [a] -> a
    foldBal op = foldBal' op (error "foldBal: empty list")

    foldBal' :: (a -> a -> a) -> a -> [a] -> a
    foldBal' _  x []  = x
    foldBal' _  _ [y] = y
    foldBal' op x l   = let (a,b) = splitAt (length l `div` 2) l
                        in foldBal' op x a `op` foldBal' op x b

    {-
    Note [Generics and unlifted types]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Normally, all constants are marked with K1/Rec0. The exception to this rule is
    when a data constructor has an unlifted argument (e.g., Int#, Char#, etc.). In
    that case, we must use a data family instance of URec (from GHC.Generics) to
    mark it. As a result, before we can generate K1 or unK1, we must first check
    to see if the type is actually one of the unlifted types for which URec has a
    data family instance; if so, we generate that instead.

    See wiki:Commentary/Compiler/GenericDeriving#Handlingunliftedtypes for more
    details on why URec is implemented the way it is.

    Note [Generating a correctly typed Rep instance]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    tc_mkRepTy derives the RHS of the Rep(1) type family instance when deriving
    Generic(1). That is, it derives the ellipsis in the following:

        instance Generic Foo where
          type Rep Foo = ...

    However, tc_mkRepTy only has knowledge of the *TyCon* of the type for which
    a Generic(1) instance is being derived, not the fully instantiated type. As a
    result, tc_mkRepTy builds the most generalized Rep(1) instance possible using
    the type variables it learns from the TyCon (i.e., it uses tyConTyVars). This
    can cause problems when the instance has instantiated type variables
    (see Trac #11732). As an example:

        data T a = MkT a
        deriving instance Generic (T Int)
        ==>
        instance Generic (T Int) where
          type Rep (T Int) = (... (Rec0 a)) -- wrong!

    -XStandaloneDeriving is one way for the type variables to become instantiated.
    Another way is when Generic1 is being derived for a datatype with a visible
    kind binder, e.g.,

       data P k (a :: k) = MkP k deriving Generic1
       ==>
       instance Generic1 (P *) where
         type Rep1 (P *) = (... (Rec0 k)) -- wrong!

    See Note [Unify kinds in deriving] in TcDeriv.

    In any such scenario, we must prevent a discrepancy between the LHS and RHS of
    a Rep(1) instance. To do so, we create a type variable substitution that maps
    the tyConTyVars of the TyCon to their counterparts in the fully instantiated
    type. (For example, using T above as example, you'd map a :-> Int.) We then
    apply the substitution to the RHS before generating the instance.

    A wrinkle in all of this: when forming the type variable substitution for
    Generic1 instances, we map the last type variable of the tycon to Any. Why?
    It's because of wily data types like this one (#15012):

       data T a = MkT (FakeOut a)
       type FakeOut a = Int

    If we ignore a, then we'll produce the following Rep1 instance:

       instance Generic1 T where
         type Rep1 T = ... (Rec0 (FakeOut a))
         ...

    Oh no! Now we have `a` on the RHS, but it's completely unbound. Instead, we
    ensure that `a` is mapped to Any:

       instance Generic1 T where
         type Rep1 T = ... (Rec0 (FakeOut Any))
         ...

    And now all is good.

    Alternatively, we could have avoided this problem by expanding all type
    synonyms on the RHSes of Rep1 instances. But we might blow up the size of
    these types even further by doing this, so we choose not to do so.

    Note [Handling kinds in a Rep instance]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Because Generic1 is poly-kinded, the representation types were generalized to
    be kind-polymorphic as well. As a result, tc_mkRepTy must explicitly apply
    the kind of the instance being derived to all the representation type
    constructors. For instance, if you have

        data Empty (a :: k) = Empty deriving Generic1

    Then the generated code is now approximately (with -fprint-explicit-kinds
    syntax):

        instance Generic1 k (Empty k) where
          type Rep1 k (Empty k) = U1 k

    Most representation types have only one kind variable, making them easy to deal
    with. The only non-trivial case is (:.:), which is only used in Generic1
    instances:

        newtype (:.:) (f :: k2 -> *) (g :: k1 -> k2) (p :: k1) =
            Comp1 { unComp1 :: f (g p) }

    Here, we do something a bit counter-intuitive: we make k1 be the kind of the
    instance being derived, and we always make k2 be *. Why *? It's because
    the code that GHC generates using (:.:) is always of the form x :.: Rec1 y
    for some types x and y. In other words, the second type to which (:.:) is
    applied always has kind k -> *, for some kind k, so k2 cannot possibly be
    anything other than * in a generated Generic1 instance.

    Note [Generics compilation speed tricks]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Deriving Generic(1) is known to have a large constant factor during
    compilation, which contributes to noticeable compilation slowdowns when
    deriving Generic(1) for large datatypes (see Trac #5642).

    To ease the pain, there is a trick one can play when generating definitions for
    to(1) and from(1). If you have a datatype like:

      data Letter = A | B | C | D

    then a naïve Generic instance for Letter would be:

      instance Generic Letter where
        type Rep Letter = D1 ('MetaData ...) ...

        to (M1 (L1 (L1 (M1 U1)))) = A
        to (M1 (L1 (R1 (M1 U1)))) = B
        to (M1 (R1 (L1 (M1 U1)))) = C
        to (M1 (R1 (R1 (M1 U1)))) = D

        from A = M1 (L1 (L1 (M1 U1)))
        from B = M1 (L1 (R1 (M1 U1)))
        from C = M1 (R1 (L1 (M1 U1)))
        from D = M1 (R1 (R1 (M1 U1)))

    Notice that in every LHS pattern-match of the 'to' definition, and in every RHS
    expression in the 'from' definition, the topmost constructor is M1. This
    corresponds to the datatype-specific metadata (the D1 in the Rep Letter
    instance). But this is wasteful from a typechecking perspective, since this
    definition requires GHC to typecheck an application of M1 in every single case,
    leading to an O(n) increase in the number of coercions the typechecker has to
    solve, which in turn increases allocations and degrades compilation speed.

    Luckily, since the topmost M1 has the exact same type across every case, we can
    factor it out reduce the typechecker's burden:

      instance Generic Letter where
        type Rep Letter = D1 ('MetaData ...) ...

        to (M1 x) = case x of
          L1 (L1 (M1 U1)) -> A
          L1 (R1 (M1 U1)) -> B
          R1 (L1 (M1 U1)) -> C
          R1 (R1 (M1 U1)) -> D

        from x = M1 (case x of
          A -> L1 (L1 (M1 U1))
          B -> L1 (R1 (M1 U1))
          C -> R1 (L1 (M1 U1))
          D -> R1 (R1 (M1 U1)))

    A simple change, but one that pays off, since it goes turns an O(n) amount of
    coercions to an O(1) amount.
    -}