    {-# LANGUAGE BangPatterns #-}
    {-# LANGUAGE CPP #-}
    {-# LANGUAGE FlexibleInstances #-}
    {-# LANGUAGE NamedFieldPuns #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    {-# LANGUAGE UndecidableInstances #-}
    #if __GLASGOW_HASKELL__ >= 800
    -- a) THQ works on cross-compilers and unregisterised GHCs
    -- b) may make compilation faster as no dynamic loading is ever needed (not sure about this)
    -- c) removes one hindrance to have code inferred as SafeHaskell safe
    {-# LANGUAGE TemplateHaskellQuotes #-}
    #else
    {-# LANGUAGE TemplateHaskell #-}
    #endif

    #include "incoherent-compat.h"
    #include "overlapping-compat.h"

    {-|
    Module:      Data.Aeson.TH
    Copyright:   (c) 2011-2016 Bryan O'Sullivan
                 (c) 2011 MailRank, Inc.
    License:     BSD3
    Stability:   experimental
    Portability: portable

    Functions to mechanically derive 'ToJSON' and 'FromJSON' instances. Note that
    you need to enable the @TemplateHaskell@ language extension in order to use this
    module.

    An example shows how instances are generated for arbitrary data types. First we
    define a data type:

    @
    data D a = Nullary
             | Unary Int
             | Product String Char a
             | Record { testOne   :: Double
                      , testTwo   :: Bool
                      , testThree :: D a
                      } deriving Eq
    @

    Next we derive the necessary instances. Note that we make use of the
    feature to change record field names. In this case we drop the first 4
    characters of every field name. We also modify constructor names by
    lower-casing them:

    @
    $('deriveJSON' 'defaultOptions'{'fieldLabelModifier' = 'drop' 4, 'constructorTagModifier' = map toLower} ''D)
    @

    Now we can use the newly created instances.

    @
    d :: D 'Int'
    d = Record { testOne = 3.14159
               , testTwo = 'True'
               , testThree = Product \"test\" \'A\' 123
               }
    @

    >>> fromJSON (toJSON d) == Success d
    > True

    This also works for data family instances, but instead of passing in the data
    family name (with double quotes), we pass in a data family instance
    constructor (with a single quote):

    @
    data family DF a
    data instance DF Int = DF1 Int
                         | DF2 Int Int
                         deriving Eq

    $('deriveJSON' 'defaultOptions' 'DF1)
    -- Alternatively, one could pass 'DF2 instead
    @

    Please note that you can derive instances for tuples using the following syntax:

    @
    -- FromJSON and ToJSON instances for 4-tuples.
    $('deriveJSON' 'defaultOptions' ''(,,,))
    @

    -}
    module Data.Aeson.TH
        (
          -- * Encoding configuration
          Options(..)
        , SumEncoding(..)
        , defaultOptions
        , defaultTaggedObject

         -- * FromJSON and ToJSON derivation
        , deriveJSON
        , deriveJSON1
        , deriveJSON2

        , deriveToJSON
        , deriveToJSON1
        , deriveToJSON2
        , deriveFromJSON
        , deriveFromJSON1
        , deriveFromJSON2

        , mkToJSON
        , mkLiftToJSON
        , mkLiftToJSON2
        , mkToEncoding
        , mkLiftToEncoding
        , mkLiftToEncoding2
        , mkParseJSON
        , mkLiftParseJSON
        , mkLiftParseJSON2
        ) where

    import Prelude.Compat

    import Control.Applicative ((<|>))
    import Data.Aeson (Object, (.:), FromJSON(..), FromJSON1(..), FromJSON2(..), ToJSON(..), ToJSON1(..), ToJSON2(..))
    import Data.Aeson.Types (Options(..), Parser, SumEncoding(..), Value(..), defaultOptions, defaultTaggedObject)
    import Data.Aeson.Types.Internal ((<?>), JSONPathElement(Key))
    import Data.Aeson.Types.FromJSON (parseOptionalFieldWith)
    import Data.Aeson.Types.ToJSON (fromPairs, pair)
    import Control.Monad (liftM2, unless, when)
    import Data.Foldable (foldr')
    #if MIN_VERSION_template_haskell(2,8,0) && !MIN_VERSION_template_haskell(2,10,0)
    import Data.List (nub)
    #endif
    import Data.List (foldl', genericLength, intercalate, partition, union)
    import Data.List.NonEmpty ((<|), NonEmpty((:|)))
    import Data.Map (Map)
    import Data.Maybe (catMaybes, fromMaybe, mapMaybe)
    import qualified Data.Monoid as Monoid
    import Data.Set (Set)
    #if MIN_VERSION_template_haskell(2,8,0)
    import Language.Haskell.TH hiding (Arity)
    #else
    import Language.Haskell.TH
    #endif
    import Language.Haskell.TH.Datatype
    #if MIN_VERSION_template_haskell(2,7,0) && !(MIN_VERSION_template_haskell(2,8,0))
    import Language.Haskell.TH.Lib (starK)
    #endif
    #if MIN_VERSION_template_haskell(2,8,0) && !(MIN_VERSION_template_haskell(2,10,0))
    import Language.Haskell.TH.Syntax (mkNameG_tc)
    #endif
    import Text.Printf (printf)
    import qualified Data.Aeson.Encoding.Internal as E
    import qualified Data.Foldable as F (all)
    import qualified Data.HashMap.Strict as H (lookup, toList)
    import qualified Data.List.NonEmpty as NE (length, reverse)
    import qualified Data.Map as M (fromList, keys, lookup , singleton, size)
    import qualified Data.Semigroup as Semigroup (Option(..))
    import qualified Data.Set as Set (empty, insert, member)
    import qualified Data.Text as T (Text, pack, unpack)
    import qualified Data.Vector as V (unsafeIndex, null, length, create, empty)
    import qualified Data.Vector.Mutable as VM (unsafeNew, unsafeWrite)

    {-# ANN module "Hlint: ignore Reduce duplication" #-}

    --------------------------------------------------------------------------------
    -- Convenience
    --------------------------------------------------------------------------------

    -- | Generates both 'ToJSON' and 'FromJSON' instance declarations for the given
    -- data type or data family instance constructor.
    --
    -- This is a convienience function which is equivalent to calling both
    -- 'deriveToJSON' and 'deriveFromJSON'.
    deriveJSON :: Options
               -- ^ Encoding options.
               -> Name
               -- ^ Name of the type for which to generate 'ToJSON' and 'FromJSON'
               -- instances.
               -> Q [Dec]
    deriveJSON = deriveJSONBoth deriveToJSON deriveFromJSON

    -- | Generates both 'ToJSON1' and 'FromJSON1' instance declarations for the given
    -- data type or data family instance constructor.
    --
    -- This is a convienience function which is equivalent to calling both
    -- 'deriveToJSON1' and 'deriveFromJSON1'.
    deriveJSON1 :: Options
                -- ^ Encoding options.
                -> Name
                -- ^ Name of the type for which to generate 'ToJSON1' and 'FromJSON1'
                -- instances.
                -> Q [Dec]
    deriveJSON1 = deriveJSONBoth deriveToJSON1 deriveFromJSON1

    -- | Generates both 'ToJSON2' and 'FromJSON2' instance declarations for the given
    -- data type or data family instance constructor.
    --
    -- This is a convienience function which is equivalent to calling both
    -- 'deriveToJSON2' and 'deriveFromJSON2'.
    deriveJSON2 :: Options
                -- ^ Encoding options.
                -> Name
                -- ^ Name of the type for which to generate 'ToJSON2' and 'FromJSON2'
                -- instances.
                -> Q [Dec]
    deriveJSON2 = deriveJSONBoth deriveToJSON2 deriveFromJSON2

    --------------------------------------------------------------------------------
    -- ToJSON
    --------------------------------------------------------------------------------

    {-
    TODO: Don't constrain phantom type variables.

    data Foo a = Foo Int
    instance (ToJSON a) ⇒ ToJSON Foo where ...

    The above (ToJSON a) constraint is not necessary and perhaps undesirable.
    -}

    -- | Generates a 'ToJSON' instance declaration for the given data type or
    -- data family instance constructor.
    deriveToJSON :: Options
                 -- ^ Encoding options.
                 -> Name
                 -- ^ Name of the type for which to generate a 'ToJSON' instance
                 -- declaration.
                 -> Q [Dec]
    deriveToJSON = deriveToJSONCommon toJSONClass

    -- | Generates a 'ToJSON1' instance declaration for the given data type or
    -- data family instance constructor.
    deriveToJSON1 :: Options
                  -- ^ Encoding options.
                  -> Name
                  -- ^ Name of the type for which to generate a 'ToJSON1' instance
                  -- declaration.
                  -> Q [Dec]
    deriveToJSON1 = deriveToJSONCommon toJSON1Class

    -- | Generates a 'ToJSON2' instance declaration for the given data type or
    -- data family instance constructor.
    deriveToJSON2 :: Options
                  -- ^ Encoding options.
                  -> Name
                  -- ^ Name of the type for which to generate a 'ToJSON2' instance
                  -- declaration.
                  -> Q [Dec]
    deriveToJSON2 = deriveToJSONCommon toJSON2Class

    deriveToJSONCommon :: JSONClass
                       -- ^ The ToJSON variant being derived.
                       -> Options
                       -- ^ Encoding options.
                       -> Name
                       -- ^ Name of the type for which to generate an instance.
                       -> Q [Dec]
    deriveToJSONCommon = deriveJSONClass [ (ToJSON,     \jc _ -> consToValue Value jc)
                                         , (ToEncoding, \jc _ -> consToValue Encoding jc)
                                         ]

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a 'Value'.
    mkToJSON :: Options -- ^ Encoding options.
             -> Name -- ^ Name of the type to encode.
             -> Q Exp
    mkToJSON = mkToJSONCommon toJSONClass

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a 'Value' by using the given encoding
    -- function on occurrences of the last type parameter.
    mkLiftToJSON :: Options -- ^ Encoding options.
                 -> Name -- ^ Name of the type to encode.
                 -> Q Exp
    mkLiftToJSON = mkToJSONCommon toJSON1Class

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a 'Value' by using the given encoding
    -- functions on occurrences of the last two type parameters.
    mkLiftToJSON2 :: Options -- ^ Encoding options.
                  -> Name -- ^ Name of the type to encode.
                  -> Q Exp
    mkLiftToJSON2 = mkToJSONCommon toJSON2Class

    mkToJSONCommon :: JSONClass -- ^ Which class's method is being derived.
                   -> Options -- ^ Encoding options.
                   -> Name -- ^ Name of the encoded type.
                   -> Q Exp
    mkToJSONCommon = mkFunCommon (\jc _ -> consToValue Value jc)

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a JSON string.
    mkToEncoding :: Options -- ^ Encoding options.
                 -> Name -- ^ Name of the type to encode.
                 -> Q Exp
    mkToEncoding = mkToEncodingCommon toJSONClass

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a JSON string by using the given encoding
    -- function on occurrences of the last type parameter.
    mkLiftToEncoding :: Options -- ^ Encoding options.
                     -> Name -- ^ Name of the type to encode.
                     -> Q Exp
    mkLiftToEncoding = mkToEncodingCommon toJSON1Class

    -- | Generates a lambda expression which encodes the given data type or
    -- data family instance constructor as a JSON string by using the given encoding
    -- functions on occurrences of the last two type parameters.
    mkLiftToEncoding2 :: Options -- ^ Encoding options.
                      -> Name -- ^ Name of the type to encode.
                      -> Q Exp
    mkLiftToEncoding2 = mkToEncodingCommon toJSON2Class

    mkToEncodingCommon :: JSONClass -- ^ Which class's method is being derived.
                       -> Options -- ^ Encoding options.
                       -> Name -- ^ Name of the encoded type.
                       -> Q Exp
    mkToEncodingCommon = mkFunCommon (\jc _ -> consToValue Encoding jc)

    -- | Helper function used by both 'deriveToJSON' and 'mkToJSON'. Generates
    -- code to generate a 'Value' or 'Encoding' of a number of constructors. All
    -- constructors must be from the same type.
    consToValue :: ToJSONFun
                -- ^ The method ('toJSON' or 'toEncoding') being derived.
                -> JSONClass
                -- ^ The ToJSON variant being derived.
                -> Options
                -- ^ Encoding options.
                -> [Type]
                -- ^ The types from the data type/data family instance declaration
                -> [ConstructorInfo]
                -- ^ Constructors for which to generate JSON generating code.
                -> Q Exp

    consToValue _ _ _ _ [] = error $ "Data.Aeson.TH.consToValue: "
                                 ++ "Not a single constructor given!"

    consToValue target jc opts vars cons = do
        value <- newName "value"
        tjs   <- newNameList "_tj"  $ arityInt jc
        tjls  <- newNameList "_tjl" $ arityInt jc
        let zippedTJs      = zip tjs tjls
            interleavedTJs = interleave tjs tjls
            lastTyVars     = map varTToName $ drop (length vars - arityInt jc) vars
            tvMap          = M.fromList $ zip lastTyVars zippedTJs
        lamE (map varP $ interleavedTJs ++ [value]) $
            caseE (varE value) (matches tvMap)
      where
        matches tvMap = case cons of
          -- A single constructor is directly encoded. The constructor itself may be
          -- forgotten.
          [con] | not (tagSingleConstructors opts) -> [argsToValue target jc tvMap opts False con]
          _ | allNullaryToStringTag opts && all isNullary cons ->
                  [ match (conP conName []) (normalB $ conStr target opts conName) []
                  | con <- cons
                  , let conName = constructorName con
                  ]
            | otherwise -> [argsToValue target jc tvMap opts True con | con <- cons]

    -- | Name of the constructor as a quoted 'Value' or 'Encoding'.
    conStr :: ToJSONFun -> Options -> Name -> Q Exp
    conStr Value opts = appE [|String|] . conTxt opts
    conStr Encoding opts = appE [|E.text|] . conTxt opts

    -- | Name of the constructor as a quoted 'Text'.
    conTxt :: Options -> Name -> Q Exp
    conTxt opts = appE [|T.pack|] . stringE . conString opts

    -- | Name of the constructor.
    conString :: Options -> Name -> String
    conString opts = constructorTagModifier opts . nameBase

    -- | If constructor is nullary.
    isNullary :: ConstructorInfo -> Bool
    isNullary ConstructorInfo { constructorVariant = NormalConstructor
                              , constructorFields  = tys } = null tys
    isNullary _ = False

    -- | Wrap fields of a non-record constructor. See 'sumToValue'.
    opaqueSumToValue :: ToJSONFun -> Options -> Bool -> Bool -> Name -> ExpQ -> ExpQ
    opaqueSumToValue target opts multiCons nullary conName value =
      sumToValue target opts multiCons nullary conName
        value
        pairs
      where
        pairs contentsFieldName = pairE contentsFieldName value

    -- | Wrap fields of a record constructor. See 'sumToValue'.
    recordSumToValue :: ToJSONFun -> Options -> Bool -> Bool -> Name -> ExpQ -> ExpQ
    recordSumToValue target opts multiCons nullary conName pairs =
      sumToValue target opts multiCons nullary conName
        (fromPairsE pairs)
        (const pairs)

    -- | Wrap fields of a constructor.
    sumToValue
      :: ToJSONFun
      -- ^ The method being derived.
      -> Options
      -- ^ Deriving options.
      -> Bool
      -- ^ Does this type have multiple constructors.
      -> Bool
      -- ^ Is this constructor nullary.
      -> Name
      -- ^ Constructor name.
      -> ExpQ
      -- ^ Fields of the constructor as a 'Value' or 'Encoding'.
      -> (String -> ExpQ)
      -- ^ Representation of an 'Object' fragment used for the 'TaggedObject'
      -- variant; of type @[(Text,Value)]@ or @[Encoding]@, depending on the method
      -- being derived.
      --
      -- - For non-records, produces a pair @"contentsFieldName":value@,
      --   given a @contentsFieldName@ as an argument. See 'opaqueSumToValue'.
      -- - For records, produces the list of pairs corresponding to fields of the
      --   encoded value (ignores the argument). See 'recordSumToValue'.
      -> ExpQ
    sumToValue target opts multiCons nullary conName value pairs
        | multiCons =
            case sumEncoding opts of
              TwoElemArray ->
                array target [conStr target opts conName, value]
              TaggedObject{tagFieldName, contentsFieldName} ->
                -- TODO: Maybe throw an error in case
                -- tagFieldName overwrites a field in pairs.
                let tag = pairE tagFieldName (conStr target opts conName)
                    content = pairs contentsFieldName
                in fromPairsE $
                  if nullary then tag else infixApp tag [|(Monoid.<>)|] content
              ObjectWithSingleField ->
                objectE [(conString opts conName, value)]
              UntaggedValue | nullary -> conStr target opts conName
              UntaggedValue -> value
        | otherwise = value

    -- | Generates code to generate the JSON encoding of a single constructor.
    argsToValue :: ToJSONFun -> JSONClass -> TyVarMap -> Options -> Bool -> ConstructorInfo -> Q Match

    -- Polyadic constructors with special case for unary constructors.
    argsToValue target jc tvMap opts multiCons
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = NormalConstructor
                      , constructorFields  = argTys } = do
        argTys' <- mapM resolveTypeSynonyms argTys
        let len = length argTys'
        args <- newNameList "arg" len
        let js = case [ dispatchToJSON target jc conName tvMap argTy
                          `appE` varE arg
                      | (arg, argTy) <- zip args argTys'
                      ] of
                   -- Single argument is directly converted.
                   [e] -> e
                   -- Zero and multiple arguments are converted to a JSON array.
                   es -> array target es

        match (conP conName $ map varP args)
              (normalB $ opaqueSumToValue target opts multiCons (null argTys') conName js)
              []

    -- Records.
    argsToValue target jc tvMap opts multiCons
      info@ConstructorInfo { constructorName    = conName
                           , constructorVariant = RecordConstructor fields
                           , constructorFields  = argTys } =
        case (unwrapUnaryRecords opts, not multiCons, argTys) of
          (True,True,[_]) -> argsToValue target jc tvMap opts multiCons
                                         (info{constructorVariant = NormalConstructor})
          _ -> do
            argTys' <- mapM resolveTypeSynonyms argTys
            args <- newNameList "arg" $ length argTys'
            let pairs | omitNothingFields opts = infixApp maybeFields
                                                          [|(Monoid.<>)|]
                                                          restFields
                      | otherwise = mconcatE (map pureToPair argCons)

                argCons = zip3 (map varE args) argTys' fields

                maybeFields = mconcatE (map maybeToPair maybes)

                restFields = mconcatE (map pureToPair rest)

                (maybes0, rest0) = partition isMaybe argCons
                (options, rest) = partition isOption rest0
                maybes = maybes0 ++ map optionToMaybe options

                maybeToPair = toPairLifted True
                pureToPair = toPairLifted False

                toPairLifted lifted (arg, argTy, field) =
                  let toValue = dispatchToJSON target jc conName tvMap argTy
                      fieldName = fieldLabel opts field
                      e arg' = pairE fieldName (toValue `appE` arg')
                  in if lifted
                    then do
                      x <- newName "x"
                      [|maybe mempty|] `appE` lam1E (varP x) (e (varE x)) `appE` arg
                    else e arg

            match (conP conName $ map varP args)
                  (normalB $ recordSumToValue target opts multiCons (null argTys) conName pairs)
                  []

    -- Infix constructors.
    argsToValue target jc tvMap opts multiCons
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = InfixConstructor
                      , constructorFields  = argTys } = do
        [alTy, arTy] <- mapM resolveTypeSynonyms argTys
        al <- newName "argL"
        ar <- newName "argR"
        match (infixP (varP al) conName (varP ar))
              ( normalB
              $ opaqueSumToValue target opts multiCons False conName
              $ array target
                  [ dispatchToJSON target jc conName tvMap aTy
                      `appE` varE a
                  | (a, aTy) <- [(al,alTy), (ar,arTy)]
                  ]
              )
              []

    isMaybe :: (a, Type, b) -> Bool
    isMaybe (_, AppT (ConT t) _, _) = t == ''Maybe
    isMaybe _                       = False

    isOption :: (a, Type, b) -> Bool
    isOption (_, AppT (ConT t) _, _) = t == ''Semigroup.Option
    isOption _                       = False

    optionToMaybe :: (ExpQ, b, c) -> (ExpQ, b, c)
    optionToMaybe (a, b, c) = ([|Semigroup.getOption|] `appE` a, b, c)

    (<^>) :: ExpQ -> ExpQ -> ExpQ
    (<^>) a b = infixApp a [|(E.><)|] b
    infixr 6 <^>

    (<%>) :: ExpQ -> ExpQ -> ExpQ
    (<%>) a b = a <^> [|E.comma|] <^> b
    infixr 4 <%>

    -- | Wrap a list of quoted 'Value's in a quoted 'Array' (of type 'Value').
    array :: ToJSONFun -> [ExpQ] -> ExpQ
    array Encoding [] = [|E.emptyArray_|]
    array Value [] = [|Array V.empty|]
    array Encoding es = [|E.wrapArray|] `appE` foldr1 (<%>) es
    array Value es = do
      mv <- newName "mv"
      let newMV = bindS (varP mv)
                        ([|VM.unsafeNew|] `appE`
                          litE (integerL $ fromIntegral (length es)))
          stmts = [ noBindS $
                      [|VM.unsafeWrite|] `appE`
                        varE mv `appE`
                          litE (integerL ix) `appE`
                            e
                  | (ix, e) <- zip [(0::Integer)..] es
                  ]
          ret = noBindS $ [|return|] `appE` varE mv
      [|Array|] `appE`
                 (varE 'V.create `appE`
                   doE (newMV:stmts++[ret]))

    -- | Wrap an associative list of keys and quoted values in a quoted 'Object'.
    objectE :: [(String, ExpQ)] -> ExpQ
    objectE = fromPairsE . mconcatE . fmap (uncurry pairE)

    -- | 'mconcat' a list of fixed length.
    --
    -- > mconcatE [ [|x|], [|y|], [|z|] ] = [| x <> (y <> z) |]
    mconcatE :: [ExpQ] -> ExpQ
    mconcatE [] = [|Monoid.mempty|]
    mconcatE [x] = x
    mconcatE (x : xs) = infixApp x [|(Monoid.<>)|] (mconcatE xs)

    fromPairsE :: ExpQ -> ExpQ
    fromPairsE = ([|fromPairs|] `appE`)

    -- | Create (an encoding of) a key-value pair.
    --
    -- > pairE "k" [|v|] = [|pair "k" v|]
    pairE :: String -> ExpQ -> ExpQ
    pairE k v = [|pair k|] `appE` v

    --------------------------------------------------------------------------------
    -- FromJSON
    --------------------------------------------------------------------------------

    -- | Generates a 'FromJSON' instance declaration for the given data type or
    -- data family instance constructor.
    deriveFromJSON :: Options
                   -- ^ Encoding options.
                   -> Name
                   -- ^ Name of the type for which to generate a 'FromJSON' instance
                   -- declaration.
                   -> Q [Dec]
    deriveFromJSON = deriveFromJSONCommon fromJSONClass

    -- | Generates a 'FromJSON1' instance declaration for the given data type or
    -- data family instance constructor.
    deriveFromJSON1 :: Options
                    -- ^ Encoding options.
                    -> Name
                    -- ^ Name of the type for which to generate a 'FromJSON1' instance
                    -- declaration.
                    -> Q [Dec]
    deriveFromJSON1 = deriveFromJSONCommon fromJSON1Class

    -- | Generates a 'FromJSON2' instance declaration for the given data type or
    -- data family instance constructor.
    deriveFromJSON2 :: Options
                    -- ^ Encoding options.
                    -> Name
                    -- ^ Name of the type for which to generate a 'FromJSON3' instance
                    -- declaration.
                    -> Q [Dec]
    deriveFromJSON2 = deriveFromJSONCommon fromJSON2Class

    deriveFromJSONCommon :: JSONClass
                         -- ^ The FromJSON variant being derived.
                         -> Options
                         -- ^ Encoding options.
                         -> Name
                         -- ^ Name of the type for which to generate an instance.
                         -- declaration.
                         -> Q [Dec]
    deriveFromJSONCommon = deriveJSONClass [(ParseJSON, consFromJSON)]

    -- | Generates a lambda expression which parses the JSON encoding of the given
    -- data type or data family instance constructor.
    mkParseJSON :: Options -- ^ Encoding options.
                -> Name -- ^ Name of the encoded type.
                -> Q Exp
    mkParseJSON = mkParseJSONCommon fromJSONClass

    -- | Generates a lambda expression which parses the JSON encoding of the given
    -- data type or data family instance constructor by using the given parsing
    -- function on occurrences of the last type parameter.
    mkLiftParseJSON :: Options -- ^ Encoding options.
                    -> Name -- ^ Name of the encoded type.
                    -> Q Exp
    mkLiftParseJSON = mkParseJSONCommon fromJSON1Class

    -- | Generates a lambda expression which parses the JSON encoding of the given
    -- data type or data family instance constructor by using the given parsing
    -- functions on occurrences of the last two type parameters.
    mkLiftParseJSON2 :: Options -- ^ Encoding options.
                     -> Name -- ^ Name of the encoded type.
                     -> Q Exp
    mkLiftParseJSON2 = mkParseJSONCommon fromJSON2Class

    mkParseJSONCommon :: JSONClass -- ^ Which class's method is being derived.
                      -> Options -- ^ Encoding options.
                      -> Name -- ^ Name of the encoded type.
                      -> Q Exp
    mkParseJSONCommon = mkFunCommon consFromJSON

    -- | Helper function used by both 'deriveFromJSON' and 'mkParseJSON'. Generates
    -- code to parse the JSON encoding of a number of constructors. All constructors
    -- must be from the same type.
    consFromJSON :: JSONClass
                 -- ^ The FromJSON variant being derived.
                 -> Name
                 -- ^ Name of the type to which the constructors belong.
                 -> Options
                 -- ^ Encoding options
                 -> [Type]
                 -- ^ The types from the data type/data family instance declaration
                 -> [ConstructorInfo]
                 -- ^ Constructors for which to generate JSON parsing code.
                 -> Q Exp

    consFromJSON _ _ _ _ [] = error $ "Data.Aeson.TH.consFromJSON: "
                                    ++ "Not a single constructor given!"

    consFromJSON jc tName opts vars cons = do
      value <- newName "value"
      pjs   <- newNameList "_pj"  $ arityInt jc
      pjls  <- newNameList "_pjl" $ arityInt jc
      let zippedPJs      = zip pjs pjls
          interleavedPJs = interleave pjs pjls
          lastTyVars     = map varTToName $ drop (length vars - arityInt jc) vars
          tvMap          = M.fromList $ zip lastTyVars zippedPJs
      lamE (map varP $ interleavedPJs ++ [value]) $ lamExpr value tvMap

      where
        checkExi tvMap con = checkExistentialContext jc tvMap
                                                     (constructorContext con)
                                                     (constructorName con)

        lamExpr value tvMap = case cons of
          [con]
            | not (tagSingleConstructors opts)
                -> checkExi tvMap con $ parseArgs jc tvMap tName opts con (Right value)
          _ | sumEncoding opts == UntaggedValue
                -> parseUntaggedValue tvMap cons value
            | otherwise
                -> caseE (varE value) $
                       if allNullaryToStringTag opts && all isNullary cons
                       then allNullaryMatches
                       else mixedMatches tvMap

        allNullaryMatches =
          [ do txt <- newName "txt"
               match (conP 'String [varP txt])
                     (guardedB $
                      [ liftM2 (,) (normalG $
                                      infixApp (varE txt)
                                               [|(==)|]
                                               (conTxt opts conName)
                                   )
                                   ([|pure|] `appE` conE conName)
                      | con <- cons
                      , let conName = constructorName con
                      ]
                      ++
                      [ liftM2 (,)
                          (normalG [|otherwise|])
                          ( [|noMatchFail|]
                            `appE` litE (stringL $ show tName)
                            `appE` ([|T.unpack|] `appE` varE txt)
                          )
                      ]
                     )
                     []
          , do other <- newName "other"
               match (varP other)
                     (normalB $ [|noStringFail|]
                        `appE` litE (stringL $ show tName)
                        `appE` ([|valueConName|] `appE` varE other)
                     )
                     []
          ]

        mixedMatches tvMap =
            case sumEncoding opts of
              TaggedObject {tagFieldName, contentsFieldName} ->
                parseObject $ parseTaggedObject tvMap tagFieldName contentsFieldName
              UntaggedValue -> error "UntaggedValue: Should be handled already"
              ObjectWithSingleField ->
                parseObject $ parseObjectWithSingleField tvMap
              TwoElemArray ->
                [ do arr <- newName "array"
                     match (conP 'Array [varP arr])
                           (guardedB
                            [ liftM2 (,) (normalG $ infixApp ([|V.length|] `appE` varE arr)
                                                             [|(==)|]
                                                             (litE $ integerL 2))
                                         (parse2ElemArray tvMap arr)
                            , liftM2 (,) (normalG [|otherwise|])
                                         ([|not2ElemArray|]
                                           `appE` litE (stringL $ show tName)
                                           `appE` ([|V.length|] `appE` varE arr))
                            ]
                           )
                           []
                , do other <- newName "other"
                     match (varP other)
                           ( normalB
                             $ [|noArrayFail|]
                                 `appE` litE (stringL $ show tName)
                                 `appE` ([|valueConName|] `appE` varE other)
                           )
                           []
                ]

        parseObject f =
            [ do obj <- newName "obj"
                 match (conP 'Object [varP obj]) (normalB $ f obj) []
            , do other <- newName "other"
                 match (varP other)
                       ( normalB
                         $ [|noObjectFail|]
                             `appE` litE (stringL $ show tName)
                             `appE` ([|valueConName|] `appE` varE other)
                       )
                       []
            ]

        parseTaggedObject tvMap typFieldName valFieldName obj = do
          conKey <- newName "conKey"
          doE [ bindS (varP conKey)
                      (infixApp (varE obj)
                                [|(.:)|]
                                ([|T.pack|] `appE` stringE typFieldName))
              , noBindS $ parseContents tvMap conKey (Left (valFieldName, obj)) 'conNotFoundFailTaggedObject
              ]

        parseUntaggedValue tvMap cons' conVal =
            foldr1 (\e e' -> infixApp e [|(<|>)|] e')
                   (map (\x -> parseValue tvMap x conVal) cons')

        parseValue _tvMap
            ConstructorInfo { constructorName    = conName
                            , constructorVariant = NormalConstructor
                            , constructorFields  = [] }
            conVal = do
          str <- newName "str"
          caseE (varE conVal)
            [ match (conP 'String [varP str])
                    (guardedB
                      [ liftM2 (,) (normalG $ infixApp (varE str) [|(==)|] (conTxt opts conName)
                                   )
                                   ([|pure|] `appE` conE conName)
                      ]
                    )
                    []
            , matchFailed tName conName "String"
            ]
        parseValue tvMap con conVal =
          checkExi tvMap con $ parseArgs jc tvMap tName opts con (Right conVal)

        parse2ElemArray tvMap arr = do
          conKey <- newName "conKey"
          conVal <- newName "conVal"
          let letIx n ix =
                  valD (varP n)
                       (normalB ([|V.unsafeIndex|] `appE`
                                   varE arr `appE`
                                   litE (integerL ix)))
                       []
          letE [ letIx conKey 0
               , letIx conVal 1
               ]
               (caseE (varE conKey)
                      [ do txt <- newName "txt"
                           match (conP 'String [varP txt])
                                 (normalB $ parseContents tvMap
                                                          txt
                                                          (Right conVal)
                                                          'conNotFoundFail2ElemArray
                                 )
                                 []
                      , do other <- newName "other"
                           match (varP other)
                                 ( normalB
                                   $ [|firstElemNoStringFail|]
                                         `appE` litE (stringL $ show tName)
                                         `appE` ([|valueConName|] `appE` varE other)
                                 )
                                 []
                      ]
               )

        parseObjectWithSingleField tvMap obj = do
          conKey <- newName "conKey"
          conVal <- newName "conVal"
          caseE ([e|H.toList|] `appE` varE obj)
                [ match (listP [tupP [varP conKey, varP conVal]])
                        (normalB $ parseContents tvMap conKey (Right conVal) 'conNotFoundFailObjectSingleField)
                        []
                , do other <- newName "other"
                     match (varP other)
                           (normalB $ [|wrongPairCountFail|]
                                      `appE` litE (stringL $ show tName)
                                      `appE` ([|show . length|] `appE` varE other)
                           )
                           []
                ]

        parseContents tvMap conKey contents errorFun =
            caseE (varE conKey)
                  [ match wildP
                          ( guardedB $
                            [ do g <- normalG $ infixApp (varE conKey)
                                                         [|(==)|]
                                                         ([|T.pack|] `appE`
                                                            conNameExp opts con)
                                 e <- checkExi tvMap con $
                                      parseArgs jc tvMap tName opts con contents
                                 return (g, e)
                            | con <- cons
                            ]
                            ++
                            [ liftM2 (,)
                                     (normalG [e|otherwise|])
                                     ( varE errorFun
                                       `appE` litE (stringL $ show tName)
                                       `appE` listE (map ( litE
                                                         . stringL
                                                         . constructorTagModifier opts
                                                         . nameBase
                                                         . constructorName
                                                         ) cons
                                                    )
                                       `appE` ([|T.unpack|] `appE` varE conKey)
                                     )
                            ]
                          )
                          []
                  ]

    parseNullaryMatches :: Name -> Name -> [Q Match]
    parseNullaryMatches tName conName =
        [ do arr <- newName "arr"
             match (conP 'Array [varP arr])
                   (guardedB
                    [ liftM2 (,) (normalG $ [|V.null|] `appE` varE arr)
                                 ([|pure|] `appE` conE conName)
                    , liftM2 (,) (normalG [|otherwise|])
                                 (parseTypeMismatch tName conName
                                    (litE $ stringL "an empty Array")
                                    (infixApp (litE $ stringL "Array of length ")
                                              [|(++)|]
                                              ([|show . V.length|] `appE` varE arr)
                                    )
                                 )
                    ]
                   )
                   []
        , matchFailed tName conName "Array"
        ]

    parseUnaryMatches :: JSONClass -> TyVarMap -> Type -> Name -> [Q Match]
    parseUnaryMatches jc tvMap argTy conName =
        [ do arg <- newName "arg"
             match (varP arg)
                   ( normalB $ infixApp (conE conName)
                                        [|(<$>)|]
                                        (dispatchParseJSON jc conName tvMap argTy
                                          `appE` varE arg)
                   )
                   []
        ]

    parseRecord :: JSONClass
                -> TyVarMap
                -> [Type]
                -> Options
                -> Name
                -> Name
                -> [Name]
                -> Name
                -> ExpQ
    parseRecord jc tvMap argTys opts tName conName fields obj =
        foldl' (\a b -> infixApp a [|(<*>)|] b)
               (infixApp (conE conName) [|(<$>)|] x)
               xs
        where
          x:xs = [ [|lookupField|]
                   `appE` dispatchParseJSON jc conName tvMap argTy
                   `appE` litE (stringL $ show tName)
                   `appE` litE (stringL $ constructorTagModifier opts $ nameBase conName)
                   `appE` varE obj
                   `appE` ( [|T.pack|] `appE` stringE (fieldLabel opts field)
                          )
                 | (field, argTy) <- zip fields argTys
                 ]

    getValField :: Name -> String -> [MatchQ] -> Q Exp
    getValField obj valFieldName matches = do
      val <- newName "val"
      doE [ bindS (varP val) $ infixApp (varE obj)
                                        [|(.:)|]
                                        ([|T.pack|] `appE`
                                           litE (stringL valFieldName))
          , noBindS $ caseE (varE val) matches
          ]

    matchCases :: Either (String, Name) Name -> [MatchQ] -> Q Exp
    matchCases (Left (valFieldName, obj)) = getValField obj valFieldName
    matchCases (Right valName)            = caseE (varE valName)

    -- | Generates code to parse the JSON encoding of a single constructor.
    parseArgs :: JSONClass -- ^ The FromJSON variant being derived.
              -> TyVarMap -- ^ Maps the last type variables to their decoding
                          --   function arguments.
              -> Name -- ^ Name of the type to which the constructor belongs.
              -> Options -- ^ Encoding options.
              -> ConstructorInfo -- ^ Constructor for which to generate JSON parsing code.
              -> Either (String, Name) Name -- ^ Left (valFieldName, objName) or
                                            --   Right valName
              -> Q Exp
    -- Nullary constructors.
    parseArgs _ _ _ _
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = NormalConstructor
                      , constructorFields  = [] }
      (Left _) =
        [|pure|] `appE` conE conName
    parseArgs _ _ tName _
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = NormalConstructor
                      , constructorFields  = [] }
      (Right valName) =
        caseE (varE valName) $ parseNullaryMatches tName conName

    -- Unary constructors.
    parseArgs jc tvMap _ _
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = NormalConstructor
                      , constructorFields  = [argTy] }
      contents = do
        argTy' <- resolveTypeSynonyms argTy
        matchCases contents $ parseUnaryMatches jc tvMap argTy' conName

    -- Polyadic constructors.
    parseArgs jc tvMap tName _
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = NormalConstructor
                      , constructorFields  = argTys }
      contents = do
        argTys' <- mapM resolveTypeSynonyms argTys
        let len = genericLength argTys'
        matchCases contents $ parseProduct jc tvMap argTys' tName conName len

    -- Records.
    parseArgs jc tvMap tName opts
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = RecordConstructor fields
                      , constructorFields  = argTys }
      (Left (_, obj)) = do
        argTys' <- mapM resolveTypeSynonyms argTys
        parseRecord jc tvMap argTys' opts tName conName fields obj
    parseArgs jc tvMap tName opts
      info@ConstructorInfo { constructorName    = conName
                           , constructorVariant = RecordConstructor fields
                           , constructorFields  = argTys }
      (Right valName) =
        case (unwrapUnaryRecords opts,argTys) of
          (True,[_])-> parseArgs jc tvMap tName opts
                                 (info{constructorVariant = NormalConstructor})
                                 (Right valName)
          _ -> do
            obj <- newName "recObj"
            argTys' <- mapM resolveTypeSynonyms argTys
            caseE (varE valName)
              [ match (conP 'Object [varP obj]) (normalB $
                  parseRecord jc tvMap argTys' opts tName conName fields obj) []
              , matchFailed tName conName "Object"
              ]

    -- Infix constructors. Apart from syntax these are the same as
    -- polyadic constructors.
    parseArgs jc tvMap tName _
      ConstructorInfo { constructorName    = conName
                      , constructorVariant = InfixConstructor
                      , constructorFields  = argTys }
      contents = do
        argTys' <- mapM resolveTypeSynonyms argTys
        matchCases contents $ parseProduct jc tvMap argTys' tName conName 2

    -- | Generates code to parse the JSON encoding of an n-ary
    -- constructor.
    parseProduct :: JSONClass -- ^ The FromJSON variant being derived.
                 -> TyVarMap -- ^ Maps the last type variables to their decoding
                             --   function arguments.
                 -> [Type] -- ^ The argument types of the constructor.
                 -> Name -- ^ Name of the type to which the constructor belongs.
                 -> Name -- ^ 'Con'structor name.
                 -> Integer -- ^ 'Con'structor arity.
                 -> [Q Match]
    parseProduct jc tvMap argTys tName conName numArgs =
        [ do arr <- newName "arr"
             -- List of: "parseJSON (arr `V.unsafeIndex` <IX>)"
             let x:xs = [ dispatchParseJSON jc conName tvMap argTy
                          `appE`
                          infixApp (varE arr)
                                   [|V.unsafeIndex|]
                                   (litE $ integerL ix)
                        | (argTy, ix) <- zip argTys [0 .. numArgs - 1]
                        ]
             match (conP 'Array [varP arr])
                   (normalB $ condE ( infixApp ([|V.length|] `appE` varE arr)
                                               [|(==)|]
                                               (litE $ integerL numArgs)
                                    )
                                    ( foldl' (\a b -> infixApp a [|(<*>)|] b)
                                             (infixApp (conE conName) [|(<$>)|] x)
                                             xs
                                    )
                                    ( parseTypeMismatch tName conName
                                        (litE $ stringL $ "Array of length " ++ show numArgs)
                                        ( infixApp (litE $ stringL "Array of length ")
                                                   [|(++)|]
                                                   ([|show . V.length|] `appE` varE arr)
                                        )
                                    )
                   )
                   []
        , matchFailed tName conName "Array"
        ]

    --------------------------------------------------------------------------------
    -- Parsing errors
    --------------------------------------------------------------------------------

    matchFailed :: Name -> Name -> String -> MatchQ
    matchFailed tName conName expected = do
      other <- newName "other"
      match (varP other)
            ( normalB $ parseTypeMismatch tName conName
                          (litE $ stringL expected)
                          ([|valueConName|] `appE` varE other)
            )
            []

    parseTypeMismatch :: Name -> Name -> ExpQ -> ExpQ -> ExpQ
    parseTypeMismatch tName conName expected actual =
        foldl appE
              [|parseTypeMismatch'|]
              [ litE $ stringL $ nameBase conName
              , litE $ stringL $ show tName
              , expected
              , actual
              ]

    class LookupField a where
        lookupField :: (Value -> Parser a) -> String -> String
                    -> Object -> T.Text -> Parser a

    instance OVERLAPPABLE_ LookupField a where
        lookupField = lookupFieldWith

    instance INCOHERENT_ LookupField (Maybe a) where
        lookupField pj _ _ = parseOptionalFieldWith pj

    instance INCOHERENT_ LookupField (Semigroup.Option a) where
        lookupField pj tName rec obj key =
            fmap Semigroup.Option
                 (lookupField (fmap Semigroup.getOption . pj) tName rec obj key)

    lookupFieldWith :: (Value -> Parser a) -> String -> String
                    -> Object -> T.Text -> Parser a
    lookupFieldWith pj tName rec obj key =
        case H.lookup key obj of
          Nothing -> unknownFieldFail tName rec (T.unpack key)
          Just v  -> pj v <?> Key key

    unknownFieldFail :: String -> String -> String -> Parser fail
    unknownFieldFail tName rec key =
        fail $ printf "When parsing the record %s of type %s the key %s was not present."
                      rec tName key

    noArrayFail :: String -> String -> Parser fail
    noArrayFail t o = fail $ printf "When parsing %s expected Array but got %s." t o

    noObjectFail :: String -> String -> Parser fail
    noObjectFail t o = fail $ printf "When parsing %s expected Object but got %s." t o

    firstElemNoStringFail :: String -> String -> Parser fail
    firstElemNoStringFail t o = fail $ printf "When parsing %s expected an Array of 2 elements where the first element is a String but got %s at the first element." t o

    wrongPairCountFail :: String -> String -> Parser fail
    wrongPairCountFail t n =
        fail $ printf "When parsing %s expected an Object with a single tag/contents pair but got %s pairs."
                      t n

    noStringFail :: String -> String -> Parser fail
    noStringFail t o = fail $ printf "When parsing %s expected String but got %s." t o

    noMatchFail :: String -> String -> Parser fail
    noMatchFail t o =
        fail $ printf "When parsing %s expected a String with the tag of a constructor but got %s." t o

    not2ElemArray :: String -> Int -> Parser fail
    not2ElemArray t i = fail $ printf "When parsing %s expected an Array of 2 elements but got %i elements" t i

    conNotFoundFail2ElemArray :: String -> [String] -> String -> Parser fail
    conNotFoundFail2ElemArray t cs o =
        fail $ printf "When parsing %s expected a 2-element Array with a tag and contents element where the tag is one of [%s], but got %s."
                      t (intercalate ", " cs) o

    conNotFoundFailObjectSingleField :: String -> [String] -> String -> Parser fail
    conNotFoundFailObjectSingleField t cs o =
        fail $ printf "When parsing %s expected an Object with a single tag/contents pair where the tag is one of [%s], but got %s."
                      t (intercalate ", " cs) o

    conNotFoundFailTaggedObject :: String -> [String] -> String -> Parser fail
    conNotFoundFailTaggedObject t cs o =
        fail $ printf "When parsing %s expected an Object with a tag field where the value is one of [%s], but got %s."
                      t (intercalate ", " cs) o

    parseTypeMismatch' :: String -> String -> String -> String -> Parser fail
    parseTypeMismatch' conName tName expected actual =
        fail $ printf "When parsing the constructor %s of type %s expected %s but got %s."
                      conName tName expected actual

    --------------------------------------------------------------------------------
    -- Shared ToJSON and FromJSON code
    --------------------------------------------------------------------------------

    -- | Functionality common to 'deriveJSON', 'deriveJSON1', and 'deriveJSON2'.
    deriveJSONBoth :: (Options -> Name -> Q [Dec])
                   -- ^ Function which derives a flavor of 'ToJSON'.
                   -> (Options -> Name -> Q [Dec])
                   -- ^ Function which derives a flavor of 'FromJSON'.
                   -> Options
                   -- ^ Encoding options.
                   -> Name
                   -- ^ Name of the type for which to generate 'ToJSON' and 'FromJSON'
                   -- instances.
                   -> Q [Dec]
    deriveJSONBoth dtj dfj opts name =
        liftM2 (++) (dtj opts name) (dfj opts name)

    -- | Functionality common to @deriveToJSON(1)(2)@ and @deriveFromJSON(1)(2)@.
    deriveJSONClass :: [(JSONFun, JSONClass -> Name -> Options -> [Type]
                                            -> [ConstructorInfo] -> Q Exp)]
                    -- ^ The class methods and the functions which derive them.
                    -> JSONClass
                    -- ^ The class for which to generate an instance.
                    -> Options
                    -- ^ Encoding options.
                    -> Name
                    -- ^ Name of the type for which to generate a class instance
                    -- declaration.
                    -> Q [Dec]
    deriveJSONClass consFuns jc opts name = do
      info <- reifyDatatype name
      case info of
        DatatypeInfo { datatypeContext = ctxt
                     , datatypeName    = parentName
                     , datatypeVars    = vars
                     , datatypeVariant = variant
                     , datatypeCons    = cons
                     } -> do
          (instanceCxt, instanceType)
            <- buildTypeInstance parentName jc ctxt vars variant
          (:[]) <$> instanceD (return instanceCxt)
                              (return instanceType)
                              (methodDecs parentName vars cons)
      where
        methodDecs :: Name -> [Type] -> [ConstructorInfo] -> [Q Dec]
        methodDecs parentName vars cons = flip map consFuns $ \(jf, jfMaker) ->
          funD (jsonFunValName jf (arity jc))
               [ clause []
                        (normalB $ jfMaker jc parentName opts vars cons)
                        []
               ]

    mkFunCommon :: (JSONClass -> Name -> Options -> [Type] -> [ConstructorInfo] -> Q Exp)
                -- ^ The function which derives the expression.
                -> JSONClass
                -- ^ Which class's method is being derived.
                -> Options
                -- ^ Encoding options.
                -> Name
                -- ^ Name of the encoded type.
                -> Q Exp
    mkFunCommon consFun jc opts name = do
      info <- reifyDatatype name
      case info of
        DatatypeInfo { datatypeContext = ctxt
                     , datatypeName    = parentName
                     , datatypeVars    = vars
                     , datatypeVariant = variant
                     , datatypeCons    = cons
                     } -> do
          -- We force buildTypeInstance here since it performs some checks for whether
          -- or not the provided datatype's kind matches the derived method's
          -- typeclass, and produces errors if it can't.
          !_ <- buildTypeInstance parentName jc ctxt vars variant
          consFun jc parentName opts vars cons

    dispatchFunByType :: JSONClass
                      -> JSONFun
                      -> Name
                      -> TyVarMap
                      -> Bool -- True if we are using the function argument that works
                              -- on lists (e.g., [a] -> Value). False is we are using
                              -- the function argument that works on single values
                              -- (e.g., a -> Value).
                      -> Type
                      -> Q Exp
    dispatchFunByType _ jf _ tvMap list (VarT tyName) =
        varE $ case M.lookup tyName tvMap of
                    Just (tfjExp, tfjlExp) -> if list then tfjlExp else tfjExp
                    Nothing                -> jsonFunValOrListName list jf Arity0
    dispatchFunByType jc jf conName tvMap list (SigT ty _) =
        dispatchFunByType jc jf conName tvMap list ty
    dispatchFunByType jc jf conName tvMap list (ForallT _ _ ty) =
        dispatchFunByType jc jf conName tvMap list ty
    dispatchFunByType jc jf conName tvMap list ty = do
        let tyCon :: Type
            tyArgs :: [Type]
            tyCon :| tyArgs = unapplyTy ty

            numLastArgs :: Int
            numLastArgs = min (arityInt jc) (length tyArgs)

            lhsArgs, rhsArgs :: [Type]
            (lhsArgs, rhsArgs) = splitAt (length tyArgs - numLastArgs) tyArgs

            tyVarNames :: [Name]
            tyVarNames = M.keys tvMap

        itf <- isTyFamily tyCon
        if any (`mentionsName` tyVarNames) lhsArgs
              || itf && any (`mentionsName` tyVarNames) tyArgs
           then outOfPlaceTyVarError jc conName
           else if any (`mentionsName` tyVarNames) rhsArgs
                then appsE $ varE (jsonFunValOrListName list jf $ toEnum numLastArgs)
                             : zipWith (dispatchFunByType jc jf conName tvMap)
                                       (cycle [False,True])
                                       (interleave rhsArgs rhsArgs)
                else varE $ jsonFunValOrListName list jf Arity0

    dispatchToJSON
      :: ToJSONFun -> JSONClass -> Name -> TyVarMap -> Type -> Q Exp
    dispatchToJSON target jc n tvMap =
        dispatchFunByType jc (targetToJSONFun target) n tvMap False

    dispatchParseJSON
      :: JSONClass -> Name -> TyVarMap -> Type -> Q Exp
    dispatchParseJSON  jc n tvMap = dispatchFunByType jc ParseJSON  n tvMap False

    --------------------------------------------------------------------------------
    -- Utility functions
    --------------------------------------------------------------------------------

    -- For the given Types, generate an instance context and head.
    buildTypeInstance :: Name
                      -- ^ The type constructor or data family name
                      -> JSONClass
                      -- ^ The typeclass to derive
                      -> Cxt
                      -- ^ The datatype context
                      -> [Type]
                      -- ^ The types to instantiate the instance with
                      -> DatatypeVariant
                      -- ^ Are we dealing with a data family instance or not
                      -> Q (Cxt, Type)
    buildTypeInstance tyConName jc dataCxt varTysOrig variant = do
        -- Make sure to expand through type/kind synonyms! Otherwise, the
        -- eta-reduction check might get tripped up over type variables in a
        -- synonym that are actually dropped.
        -- (See GHC Trac #11416 for a scenario where this actually happened.)
        varTysExp <- mapM resolveTypeSynonyms varTysOrig

        let remainingLength :: Int
            remainingLength = length varTysOrig - arityInt jc

            droppedTysExp :: [Type]
            droppedTysExp = drop remainingLength varTysExp

            droppedStarKindStati :: [StarKindStatus]
            droppedStarKindStati = map canRealizeKindStar droppedTysExp

        -- Check there are enough types to drop and that all of them are either of
        -- kind * or kind k (for some kind variable k). If not, throw an error.
        when (remainingLength < 0 || elem NotKindStar droppedStarKindStati) $
          derivingKindError jc tyConName

        let droppedKindVarNames :: [Name]
            droppedKindVarNames = catKindVarNames droppedStarKindStati

            -- Substitute kind * for any dropped kind variables
            varTysExpSubst :: [Type]
            varTysExpSubst = map (substNamesWithKindStar droppedKindVarNames) varTysExp

            remainingTysExpSubst, droppedTysExpSubst :: [Type]
            (remainingTysExpSubst, droppedTysExpSubst) =
              splitAt remainingLength varTysExpSubst

            -- All of the type variables mentioned in the dropped types
            -- (post-synonym expansion)
            droppedTyVarNames :: [Name]
            droppedTyVarNames = freeVariables droppedTysExpSubst

        -- If any of the dropped types were polykinded, ensure that they are of kind *
        -- after substituting * for the dropped kind variables. If not, throw an error.
        unless (all hasKindStar droppedTysExpSubst) $
          derivingKindError jc tyConName

        let preds    :: [Maybe Pred]
            kvNames  :: [[Name]]
            kvNames' :: [Name]
            -- Derive instance constraints (and any kind variables which are specialized
            -- to * in those constraints)
            (preds, kvNames) = unzip $ map (deriveConstraint jc) remainingTysExpSubst
            kvNames' = concat kvNames

            -- Substitute the kind variables specialized in the constraints with *
            remainingTysExpSubst' :: [Type]
            remainingTysExpSubst' =
              map (substNamesWithKindStar kvNames') remainingTysExpSubst

            -- We now substitute all of the specialized-to-* kind variable names with
            -- *, but in the original types, not the synonym-expanded types. The reason
            -- we do this is a superficial one: we want the derived instance to resemble
            -- the datatype written in source code as closely as possible. For example,
            -- for the following data family instance:
            --
            --   data family Fam a
            --   newtype instance Fam String = Fam String
            --
            -- We'd want to generate the instance:
            --
            --   instance C (Fam String)
            --
            -- Not:
            --
            --   instance C (Fam [Char])
            remainingTysOrigSubst :: [Type]
            remainingTysOrigSubst =
              map (substNamesWithKindStar (droppedKindVarNames `union` kvNames'))
                $ take remainingLength varTysOrig

            isDataFamily :: Bool
            isDataFamily = case variant of
                             Datatype        -> False
                             Newtype         -> False
                             DataInstance    -> True
                             NewtypeInstance -> True

            remainingTysOrigSubst' :: [Type]
            -- See Note [Kind signatures in derived instances] for an explanation
            -- of the isDataFamily check.
            remainingTysOrigSubst' =
              if isDataFamily
                 then remainingTysOrigSubst
                 else map unSigT remainingTysOrigSubst

            instanceCxt :: Cxt
            instanceCxt = catMaybes preds

            instanceType :: Type
            instanceType = AppT (ConT $ jsonClassName jc)
                         $ applyTyCon tyConName remainingTysOrigSubst'

        -- If the datatype context mentions any of the dropped type variables,
        -- we can't derive an instance, so throw an error.
        when (any (`predMentionsName` droppedTyVarNames) dataCxt) $
          datatypeContextError tyConName instanceType
        -- Also ensure the dropped types can be safely eta-reduced. Otherwise,
        -- throw an error.
        unless (canEtaReduce remainingTysExpSubst' droppedTysExpSubst) $
          etaReductionError instanceType
        return (instanceCxt, instanceType)

    -- | Attempt to derive a constraint on a Type. If successful, return
    -- Just the constraint and any kind variable names constrained to *.
    -- Otherwise, return Nothing and the empty list.
    --
    -- See Note [Type inference in derived instances] for the heuristics used to
    -- come up with constraints.
    deriveConstraint :: JSONClass -> Type -> (Maybe Pred, [Name])
    deriveConstraint jc t
      | not (isTyVar t) = (Nothing, [])
      | hasKindStar t   = (Just (applyCon (jcConstraint Arity0) tName), [])
      | otherwise = case hasKindVarChain 1 t of
          Just ns | jcArity >= Arity1
                  -> (Just (applyCon (jcConstraint Arity1) tName), ns)
          _ -> case hasKindVarChain 2 t of
               Just ns | jcArity == Arity2
                       -> (Just (applyCon (jcConstraint Arity2) tName), ns)
               _ -> (Nothing, [])
      where
        tName :: Name
        tName = varTToName t

        jcArity :: Arity
        jcArity = arity jc

        jcConstraint :: Arity -> Name
        jcConstraint = jsonClassName . JSONClass (direction jc)

    {-
    Note [Kind signatures in derived instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    It is possible to put explicit kind signatures into the derived instances, e.g.,

      instance C a => C (Data (f :: * -> *)) where ...

    But it is preferable to avoid this if possible. If we come up with an incorrect
    kind signature (which is entirely possible, since Template Haskell doesn't always
    have the best track record with reifying kind signatures), then GHC will flat-out
    reject the instance, which is quite unfortunate.

    Plain old datatypes have the advantage that you can avoid using any kind signatures
    at all in their instances. This is because a datatype declaration uses all type
    variables, so the types that we use in a derived instance uniquely determine their
    kinds. As long as we plug in the right types, the kind inferencer can do the rest
    of the work. For this reason, we use unSigT to remove all kind signatures before
    splicing in the instance context and head.

    Data family instances are trickier, since a data family can have two instances that
    are distinguished by kind alone, e.g.,

      data family Fam (a :: k)
      data instance Fam (a :: * -> *)
      data instance Fam (a :: *)

    If we dropped the kind signatures for C (Fam a), then GHC will have no way of
    knowing which instance we are talking about. To avoid this scenario, we always
    include explicit kind signatures in data family instances. There is a chance that
    the inferred kind signatures will be incorrect, but if so, we can always fall back
    on the mk- functions.

    Note [Type inference in derived instances]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Type inference is can be tricky to get right, and we want to avoid recreating the
    entirety of GHC's type inferencer in Template Haskell. For this reason, we will
    probably never come up with derived instance contexts that are as accurate as
    GHC's. But that doesn't mean we can't do anything! There are a couple of simple
    things we can do to make instance contexts that work for 80% of use cases:

    1. If one of the last type parameters is polykinded, then its kind will be
       specialized to * in the derived instance. We note what kind variable the type
       parameter had and substitute it with * in the other types as well. For example,
       imagine you had

         data Data (a :: k) (b :: k)

       Then you'd want to derived instance to be:

         instance C (Data (a :: *))

       Not:

         instance C (Data (a :: k))

    2. We naïvely come up with instance constraints using the following criteria:

       (i)   If there's a type parameter n of kind *, generate a ToJSON n/FromJSON n
             constraint.
       (ii)  If there's a type parameter n of kind k1 -> k2 (where k1/k2 are * or kind
             variables), then generate a ToJSON1 n/FromJSON1 n constraint, and if
             k1/k2 are kind variables, then substitute k1/k2 with * elsewhere in the
             types. We must consider the case where they are kind variables because
             you might have a scenario like this:

               newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1)
                 = Compose (f (g a))

             Which would have a derived ToJSON1 instance of:

               instance (ToJSON1 f, ToJSON1 g) => ToJSON1 (Compose f g) where ...
       (iii) If there's a type parameter n of kind k1 -> k2 -> k3 (where k1/k2/k3 are
             * or kind variables), then generate a ToJSON2 n/FromJSON2 n constraint
             and perform kind substitution as in the other cases.
    -}

    checkExistentialContext :: JSONClass -> TyVarMap -> Cxt -> Name
                            -> Q a -> Q a
    checkExistentialContext jc tvMap ctxt conName q =
      if (any (`predMentionsName` M.keys tvMap) ctxt
           || M.size tvMap < arityInt jc)
           && not (allowExQuant jc)
         then existentialContextError conName
         else q

    {-
    Note [Matching functions with GADT type variables]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    When deriving ToJSON2, there is a tricky corner case to consider:

      data Both a b where
        BothCon :: x -> x -> Both x x

    Which encoding functions should be applied to which arguments of BothCon?
    We have a choice, since both the function of type (a -> Value) and of type
    (b -> Value) can be applied to either argument. In such a scenario, the
    second encoding function takes precedence over the first encoding function, so the
    derived ToJSON2 instance would be something like:

      instance ToJSON2 Both where
        liftToJSON2 tj1 tj2 p (BothCon x1 x2) = Array $ create $ do
          mv <- unsafeNew 2
          unsafeWrite mv 0 (tj1 x1)
          unsafeWrite mv 1 (tj2 x2)
          return mv

    This is not an arbitrary choice, as this definition ensures that
    liftToJSON2 toJSON = liftToJSON for a derived ToJSON1 instance for
    Both.
    -}

    -- A mapping of type variable Names to their encoding/decoding function Names.
    -- For example, in a ToJSON2 declaration, a TyVarMap might look like
    --
    -- { a ~> (tj1, tjl1)
    -- , b ~> (tj2, tjl2) }
    --
    -- where a and b are the last two type variables of the datatype, tj1 and tjl1 are
    -- the function arguments of types (a -> Value) and ([a] -> Value), and tj2 and tjl2
    -- are the function arguments of types (b -> Value) and ([b] -> Value).
    type TyVarMap = Map Name (Name, Name)

    -- | Returns True if a Type has kind *.
    hasKindStar :: Type -> Bool
    hasKindStar VarT{}         = True
    #if MIN_VERSION_template_haskell(2,8,0)
    hasKindStar (SigT _ StarT) = True
    #else
    hasKindStar (SigT _ StarK) = True
    #endif
    hasKindStar _              = False

    -- Returns True is a kind is equal to *, or if it is a kind variable.
    isStarOrVar :: Kind -> Bool
    #if MIN_VERSION_template_haskell(2,8,0)
    isStarOrVar StarT  = True
    isStarOrVar VarT{} = True
    #else
    isStarOrVar StarK  = True
    #endif
    isStarOrVar _      = False

    -- Generate a list of fresh names with a common prefix, and numbered suffixes.
    newNameList :: String -> Int -> Q [Name]
    newNameList prefix len = mapM newName [prefix ++ show n | n <- [1..len]]

    -- | @hasKindVarChain n kind@ Checks if @kind@ is of the form
    -- k_0 -> k_1 -> ... -> k_(n-1), where k0, k1, ..., and k_(n-1) can be * or
    -- kind variables.
    hasKindVarChain :: Int -> Type -> Maybe [Name]
    hasKindVarChain kindArrows t =
      let uk = uncurryKind (tyKind t)
      in if (NE.length uk - 1 == kindArrows) && F.all isStarOrVar uk
            then Just (concatMap freeVariables uk)
            else Nothing

    -- | If a Type is a SigT, returns its kind signature. Otherwise, return *.
    tyKind :: Type -> Kind
    tyKind (SigT _ k) = k
    tyKind _          = starK

    -- | Extract Just the Name from a type variable. If the argument Type is not a
    -- type variable, return Nothing.
    varTToNameMaybe :: Type -> Maybe Name
    varTToNameMaybe (VarT n)   = Just n
    varTToNameMaybe (SigT t _) = varTToNameMaybe t
    varTToNameMaybe _          = Nothing

    -- | Extract the Name from a type variable. If the argument Type is not a
    -- type variable, throw an error.
    varTToName :: Type -> Name
    varTToName = fromMaybe (error "Not a type variable!") . varTToNameMaybe

    interleave :: [a] -> [a] -> [a]
    interleave (a1:a1s) (a2:a2s) = a1:a2:interleave a1s a2s
    interleave _        _        = []

    -- | Fully applies a type constructor to its type variables.
    applyTyCon :: Name -> [Type] -> Type
    applyTyCon = foldl' AppT . ConT

    -- | Is the given type a variable?
    isTyVar :: Type -> Bool
    isTyVar (VarT _)   = True
    isTyVar (SigT t _) = isTyVar t
    isTyVar _          = False

    -- | Is the given type a type family constructor (and not a data family constructor)?
    isTyFamily :: Type -> Q Bool
    isTyFamily (ConT n) = do
        info <- reify n
        return $ case info of
    #if MIN_VERSION_template_haskell(2,11,0)
             FamilyI OpenTypeFamilyD{} _       -> True
    #else
             FamilyI (FamilyD TypeFam _ _ _) _ -> True
    #endif
    #if MIN_VERSION_template_haskell(2,9,0)
             FamilyI ClosedTypeFamilyD{} _     -> True
    #endif
             _ -> False
    isTyFamily _ = return False

    -- | Peel off a kind signature from a Type (if it has one).
    unSigT :: Type -> Type
    unSigT (SigT t _) = t
    unSigT t          = t

    -- | Are all of the items in a list (which have an ordering) distinct?
    --
    -- This uses Set (as opposed to nub) for better asymptotic time complexity.
    allDistinct :: Ord a => [a] -> Bool
    allDistinct = allDistinct' Set.empty
      where
        allDistinct' :: Ord a => Set a -> [a] -> Bool
        allDistinct' uniqs (x:xs)
            | x `Set.member` uniqs = False
            | otherwise            = allDistinct' (Set.insert x uniqs) xs
        allDistinct' _ _           = True

    -- | Does the given type mention any of the Names in the list?
    mentionsName :: Type -> [Name] -> Bool
    mentionsName = go
      where
        go :: Type -> [Name] -> Bool
        go (AppT t1 t2) names = go t1 names || go t2 names
        go (SigT t _k)  names = go t names
    #if MIN_VERSION_template_haskell(2,8,0)
                                  || go _k names
    #endif
        go (VarT n)     names = n `elem` names
        go _            _     = False

    -- | Does an instance predicate mention any of the Names in the list?
    predMentionsName :: Pred -> [Name] -> Bool
    #if MIN_VERSION_template_haskell(2,10,0)
    predMentionsName = mentionsName
    #else
    predMentionsName (ClassP n tys) names = n `elem` names || any (`mentionsName` names) tys
    predMentionsName (EqualP t1 t2) names = mentionsName t1 names || mentionsName t2 names
    #endif

    -- | Split an applied type into its individual components. For example, this:
    --
    -- @
    -- Either Int Char
    -- @
    --
    -- would split to this:
    --
    -- @
    -- [Either, Int, Char]
    -- @
    unapplyTy :: Type -> NonEmpty Type
    unapplyTy = NE.reverse . go
      where
        go :: Type -> NonEmpty Type
        go (AppT t1 t2)    = t2 <| go t1
        go (SigT t _)      = go t
        go (ForallT _ _ t) = go t
        go t               = t :| []

    -- | Split a type signature by the arrows on its spine. For example, this:
    --
    -- @
    -- forall a b. (a ~ b) => (a -> b) -> Char -> ()
    -- @
    --
    -- would split to this:
    --
    -- @
    -- (a ~ b, [a -> b, Char, ()])
    -- @
    uncurryTy :: Type -> (Cxt, NonEmpty Type)
    uncurryTy (AppT (AppT ArrowT t1) t2) =
      let (ctxt, tys) = uncurryTy t2
      in (ctxt, t1 <| tys)
    uncurryTy (SigT t _) = uncurryTy t
    uncurryTy (ForallT _ ctxt t) =
      let (ctxt', tys) = uncurryTy t
      in (ctxt ++ ctxt', tys)
    uncurryTy t = ([], t :| [])

    -- | Like uncurryType, except on a kind level.
    uncurryKind :: Kind -> NonEmpty Kind
    #if MIN_VERSION_template_haskell(2,8,0)
    uncurryKind = snd . uncurryTy
    #else
    uncurryKind (ArrowK k1 k2) = k1 <| uncurryKind k2
    uncurryKind k              = k :| []
    #endif

    createKindChain :: Int -> Kind
    createKindChain = go starK
      where
        go :: Kind -> Int -> Kind
        go k 0 = k
    #if MIN_VERSION_template_haskell(2,8,0)
        go k !n = go (AppT (AppT ArrowT StarT) k) (n - 1)
    #else
        go k !n = go (ArrowK StarK k) (n - 1)
    #endif

    -- | Makes a string literal expression from a constructor's name.
    conNameExp :: Options -> ConstructorInfo -> Q Exp
    conNameExp opts = litE
                    . stringL
                    . constructorTagModifier opts
                    . nameBase
                    . constructorName

    -- | Extracts a record field label.
    fieldLabel :: Options -- ^ Encoding options
               -> Name
               -> String
    fieldLabel opts = fieldLabelModifier opts . nameBase

    -- | The name of the outermost 'Value' constructor.
    valueConName :: Value -> String
    valueConName (Object _) = "Object"
    valueConName (Array  _) = "Array"
    valueConName (String _) = "String"
    valueConName (Number _) = "Number"
    valueConName (Bool   _) = "Boolean"
    valueConName Null       = "Null"

    applyCon :: Name -> Name -> Pred
    applyCon con t =
    #if MIN_VERSION_template_haskell(2,10,0)
              AppT (ConT con) (VarT t)
    #else
              ClassP con [VarT t]
    #endif

    -- | Checks to see if the last types in a data family instance can be safely eta-
    -- reduced (i.e., dropped), given the other types. This checks for three conditions:
    --
    -- (1) All of the dropped types are type variables
    -- (2) All of the dropped types are distinct
    -- (3) None of the remaining types mention any of the dropped types
    canEtaReduce :: [Type] -> [Type] -> Bool
    canEtaReduce remaining dropped =
           all isTyVar dropped
        && allDistinct droppedNames -- Make sure not to pass something of type [Type], since Type
                                    -- didn't have an Ord instance until template-haskell-2.10.0.0
        && not (any (`mentionsName` droppedNames) remaining)
      where
        droppedNames :: [Name]
        droppedNames = map varTToName dropped

    -------------------------------------------------------------------------------
    -- Expanding type synonyms
    -------------------------------------------------------------------------------

    applySubstitutionKind :: Map Name Kind -> Type -> Type
    #if MIN_VERSION_template_haskell(2,8,0)
    applySubstitutionKind = applySubstitution
    #else
    applySubstitutionKind _ t = t
    #endif

    substNameWithKind :: Name -> Kind -> Type -> Type
    substNameWithKind n k = applySubstitutionKind (M.singleton n k)

    substNamesWithKindStar :: [Name] -> Type -> Type
    substNamesWithKindStar ns t = foldr' (`substNameWithKind` starK) t ns

    -------------------------------------------------------------------------------
    -- Error messages
    -------------------------------------------------------------------------------

    -- | Either the given data type doesn't have enough type variables, or one of
    -- the type variables to be eta-reduced cannot realize kind *.
    derivingKindError :: JSONClass -> Name -> Q a
    derivingKindError jc tyConName = fail
      . showString "Cannot derive well-kinded instance of form ‘"
      . showString className
      . showChar ' '
      . showParen True
        ( showString (nameBase tyConName)
        . showString " ..."
        )
      . showString "‘\n\tClass "
      . showString className
      . showString " expects an argument of kind "
      . showString (pprint . createKindChain $ arityInt jc)
      $ ""
      where
        className :: String
        className = nameBase $ jsonClassName jc

    -- | One of the last type variables cannot be eta-reduced (see the canEtaReduce
    -- function for the criteria it would have to meet).
    etaReductionError :: Type -> Q a
    etaReductionError instanceType = fail $
        "Cannot eta-reduce to an instance of form \n\tinstance (...) => "
        ++ pprint instanceType

    -- | The data type has a DatatypeContext which mentions one of the eta-reduced
    -- type variables.
    datatypeContextError :: Name -> Type -> Q a
    datatypeContextError dataName instanceType = fail
        . showString "Can't make a derived instance of ‘"
        . showString (pprint instanceType)
        . showString "‘:\n\tData type ‘"
        . showString (nameBase dataName)
        . showString "‘ must not have a class context involving the last type argument(s)"
        $ ""

    -- | The data type mentions one of the n eta-reduced type variables in a place other
    -- than the last nth positions of a data type in a constructor's field.
    outOfPlaceTyVarError :: JSONClass -> Name -> a
    outOfPlaceTyVarError jc conName = error
        . showString "Constructor ‘"
        . showString (nameBase conName)
        . showString "‘ must only use its last "
        . shows n
        . showString " type variable(s) within the last "
        . shows n
        . showString " argument(s) of a data type"
        $ ""
      where
        n :: Int
        n = arityInt jc

    -- | The data type has an existential constraint which mentions one of the
    -- eta-reduced type variables.
    existentialContextError :: Name -> a
    existentialContextError conName = error
      . showString "Constructor ‘"
      . showString (nameBase conName)
      . showString "‘ must be truly polymorphic in the last argument(s) of the data type"
      $ ""

    -------------------------------------------------------------------------------
    -- Class-specific constants
    -------------------------------------------------------------------------------

    -- | A representation of the arity of the ToJSON/FromJSON typeclass being derived.
    data Arity = Arity0 | Arity1 | Arity2
      deriving (Enum, Eq, Ord)

    -- | Whether ToJSON(1)(2) or FromJSON(1)(2) is being derived.
    data Direction = To | From

    -- | A representation of which typeclass method is being spliced in.
    data JSONFun = ToJSON | ToEncoding | ParseJSON

    -- | A refinement of JSONFun to [ToJSON, ToEncoding].
    data ToJSONFun = Value | Encoding

    targetToJSONFun :: ToJSONFun -> JSONFun
    targetToJSONFun Value = ToJSON
    targetToJSONFun Encoding = ToEncoding

    -- | A representation of which typeclass is being derived.
    data JSONClass = JSONClass { direction :: Direction, arity :: Arity }

    toJSONClass, toJSON1Class, toJSON2Class,
        fromJSONClass, fromJSON1Class, fromJSON2Class :: JSONClass
    toJSONClass    = JSONClass To   Arity0
    toJSON1Class   = JSONClass To   Arity1
    toJSON2Class   = JSONClass To   Arity2
    fromJSONClass  = JSONClass From Arity0
    fromJSON1Class = JSONClass From Arity1
    fromJSON2Class = JSONClass From Arity2

    jsonClassName :: JSONClass -> Name
    jsonClassName (JSONClass To   Arity0) = ''ToJSON
    jsonClassName (JSONClass To   Arity1) = ''ToJSON1
    jsonClassName (JSONClass To   Arity2) = ''ToJSON2
    jsonClassName (JSONClass From Arity0) = ''FromJSON
    jsonClassName (JSONClass From Arity1) = ''FromJSON1
    jsonClassName (JSONClass From Arity2) = ''FromJSON2

    jsonFunValName :: JSONFun -> Arity -> Name
    jsonFunValName ToJSON     Arity0 = 'toJSON
    jsonFunValName ToJSON     Arity1 = 'liftToJSON
    jsonFunValName ToJSON     Arity2 = 'liftToJSON2
    jsonFunValName ToEncoding Arity0 = 'toEncoding
    jsonFunValName ToEncoding Arity1 = 'liftToEncoding
    jsonFunValName ToEncoding Arity2 = 'liftToEncoding2
    jsonFunValName ParseJSON  Arity0 = 'parseJSON
    jsonFunValName ParseJSON  Arity1 = 'liftParseJSON
    jsonFunValName ParseJSON  Arity2 = 'liftParseJSON2

    jsonFunListName :: JSONFun -> Arity -> Name
    jsonFunListName ToJSON     Arity0 = 'toJSONList
    jsonFunListName ToJSON     Arity1 = 'liftToJSONList
    jsonFunListName ToJSON     Arity2 = 'liftToJSONList2
    jsonFunListName ToEncoding Arity0 = 'toEncodingList
    jsonFunListName ToEncoding Arity1 = 'liftToEncodingList
    jsonFunListName ToEncoding Arity2 = 'liftToEncodingList2
    jsonFunListName ParseJSON  Arity0 = 'parseJSONList
    jsonFunListName ParseJSON  Arity1 = 'liftParseJSONList
    jsonFunListName ParseJSON  Arity2 = 'liftParseJSONList2

    jsonFunValOrListName :: Bool -- e.g., toJSONList if True, toJSON if False
                         -> JSONFun -> Arity -> Name
    jsonFunValOrListName False = jsonFunValName
    jsonFunValOrListName True  = jsonFunListName

    arityInt :: JSONClass -> Int
    arityInt = fromEnum . arity

    allowExQuant :: JSONClass -> Bool
    allowExQuant (JSONClass To _) = True
    allowExQuant _                = False

    -------------------------------------------------------------------------------
    -- StarKindStatus
    -------------------------------------------------------------------------------

    -- | Whether a type is not of kind *, is of kind *, or is a kind variable.
    data StarKindStatus = NotKindStar
                        | KindStar
                        | IsKindVar Name
      deriving Eq

    -- | Does a Type have kind * or k (for some kind variable k)?
    canRealizeKindStar :: Type -> StarKindStatus
    canRealizeKindStar t = case t of
        _ | hasKindStar t -> KindStar
    #if MIN_VERSION_template_haskell(2,8,0)
        SigT _ (VarT k) -> IsKindVar k
    #endif
        _ -> NotKindStar

    -- | Returns 'Just' the kind variable 'Name' of a 'StarKindStatus' if it exists.
    -- Otherwise, returns 'Nothing'.
    starKindStatusToName :: StarKindStatus -> Maybe Name
    starKindStatusToName (IsKindVar n) = Just n
    starKindStatusToName _             = Nothing

    -- | Concat together all of the StarKindStatuses that are IsKindVar and extract
    -- the kind variables' Names out.
    catKindVarNames :: [StarKindStatus] -> [Name]
    catKindVarNames = mapMaybe starKindStatusToName