Data/Aeson/TH.hs

{-# LANGUAGE CPP, NoImplicitPrelude, TemplateHaskell #-}

{-|
Module: Data.Aeson.TH
Copyright: (c) 2011 Bryan O'Sullivan
 (c) 2011 MailRank, Inc.
License: Apache
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.

@
$('deriveJSON' ('drop' 4) ''D)
@

This will result in the following (simplified) code to be spliced in your program:

@
import Control.Applicative
import Control.Monad
import Data.Aeson
import Data.Aeson.TH
import qualified Data.HashMap.Strict as H
import qualified Data.Text as T
import qualified Data.Vector as V

instance 'ToJSON' a => 'ToJSON' (D a) where
 'toJSON' =
 \value ->
 case value of
 Nullary ->
 'object' [T.pack \"Nullary\" .= 'toJSON' ([] :: [()])]
 Unary arg1 ->
 'object' [T.pack \"Unary\" .= 'toJSON' arg1]
 Product arg1 arg2 arg3 ->
 'object' [ T.pack \"Product\"
 .= ('Array' $ 'V.create' $ do
 mv <- 'VM.unsafeNew' 3
 'VM.unsafeWrite' mv 0 ('toJSON' arg1)
 'VM.unsafeWrite' mv 1 ('toJSON' arg2)
 'VM.unsafeWrite' mv 2 ('toJSON' arg3)
 return mv)
 ]
 Record arg1 arg2 arg3 ->
 'object' [ T.pack \"Record\"
 .= 'object' [ T.pack \"One\" '.=' arg1
 , T.pack \"Two\" '.=' arg2
 , T.pack \"Three\" '.=' arg3
 ]
 ]
@

@
instance 'FromJSON' a => 'FromJSON' (D a) where
 'parseJSON' =
 \value ->
 case value of
 'Object' obj ->
 case H.toList obj of
 [(conKey, conVal)] ->
 case conKey of
 _ | conKey == T.pack \"Nullary\" ->
 case conVal of
 'Array' arr ->
 if V.null arr
 then pure Nullary
 else fail \"\<error message\>\"
 _ -> fail \"\<error message\>\"
 | conKey == T.pack \"Unary\" ->
 case conVal of
 arg -> Unary \<$\> parseJSON arg
 | conKey == T.pack \"Product\" ->
 case conVal of
 'Array' arr ->
 if V.length arr == 3
 then Product \<$\> 'parseJSON' (arr `V.unsafeIndex` 0)
 \<*\> 'parseJSON' (arr `V.unsafeIndex` 1)
 \<*\> 'parseJSON' (arr `V.unsafeIndex` 2)
 else fail \"\<error message\>\"
 _ -> fail \"\<error message\>\"
 | conKey == T.pack \"Record\" ->
 case conVal of
 'Object' recObj ->
 if H.size recObj == 3
 then Record \<$\> recObj '.:' T.pack \"One\"
 \<*\> recObj '.:' T.pack \"Two\"
 \<*\> recObj '.:' T.pack \"Three\"
 else fail \"\<error message\>\"
 _ -> fail \"\<error message\>\"
 | otherwise -> fail \"\<error message\>\"
 _ -> fail \"\<error message\>\"
 _ -> fail \"\<error message\>\"
@

Note that every \"\<error message\>\" is in fact a descriptive message which
provides as much information as is reasonable about the failed parse.

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

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

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

-}

module Data.Aeson.TH
 ( deriveJSON

 , deriveToJSON
 , deriveFromJSON

 , mkToJSON
 , mkParseJSON
 ) where

--------------------------------------------------------------------------------
-- Imports
--------------------------------------------------------------------------------

-- from aeson:
import Data.Aeson ( toJSON, Object, object, (.=)
 , ToJSON, toJSON
 , FromJSON, parseJSON
 )
import Data.Aeson.Types ( Value(..), Parser )
-- from base:
import Control.Applicative ( pure, (<$>), (<*>) )
import Control.Monad ( return, mapM, liftM2, fail )
import Data.Bool ( otherwise )
import Data.Eq ( (==) )
import Data.Function ( ($), (.), id )
import Data.Functor ( fmap )
import Data.List ( (++), foldl, foldl', intercalate
 , length, map, zip, genericLength
 )
import Data.Maybe ( Maybe(Nothing, Just) )
import Prelude ( String, (-), Integer, fromIntegral, error )
import Text.Printf ( printf )
import Text.Show ( show )
#if __GLASGOW_HASKELL__ < 700
import Control.Monad ( (>>=) )
import Prelude ( fromInteger )
#endif
-- from unordered-containers:
import qualified Data.HashMap.Strict as H ( lookup, toList, size )
-- from template-haskell:
import Language.Haskell.TH
-- from text:
import qualified Data.Text as T ( Text, pack, unpack )
-- from vector:
import qualified Data.Vector as V ( unsafeIndex, null, length, create )
import qualified Data.Vector.Mutable as VM ( unsafeNew, unsafeWrite )


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

-- | Generates both 'ToJSON' and 'FromJSON' instance declarations for the given
-- data type.
--
-- This is a convienience function which is equivalent to calling both
-- 'deriveToJSON' and 'deriveFromJSON'.
deriveJSON :: (String -> String)
 -- ^ Function to change field names.
 -> Name
 -- ^ Name of the type for which to generate 'ToJSON' and 'FromJSON'
 -- instances.
 -> Q [Dec]
deriveJSON withField name =
 liftM2 (++)
 (deriveToJSON withField name)
 (deriveFromJSON withField name)


--------------------------------------------------------------------------------
-- 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.
--
-- Example:
--
-- @
-- data Foo = Foo 'Char' 'Int'
-- $('deriveToJSON' 'id' ''Foo)
-- @
--
-- This will splice in the following code:
--
-- @
-- instance 'ToJSON' Foo where
-- 'toJSON' =
-- \value -> case value of
-- Foo arg1 arg2 -> 'Array' $ 'V.create' $ do
-- mv <- 'VM.unsafeNew' 2
-- 'VM.unsafeWrite' mv 0 ('toJSON' arg1)
-- 'VM.unsafeWrite' mv 1 ('toJSON' arg2)
-- return mv
-- @
deriveToJSON :: (String -> String)
 -- ^ Function to change field names.
 -> Name
 -- ^ Name of the type for which to generate a 'ToJSON' instance
 -- declaration.
 -> Q [Dec]
deriveToJSON withField name =
 withType name $ \tvbs cons -> fmap (:[]) $ fromCons tvbs cons
 where
 fromCons :: [TyVarBndr] -> [Con] -> Q Dec
 fromCons tvbs cons =
 instanceD (return $ map (\t -> ClassP ''ToJSON [VarT t]) typeNames)
 (classType `appT` instanceType)
 [ funD 'toJSON
 [ clause []
 (normalB $ consToJSON withField cons)
 []
 ]
 ]
 where
 classType = conT ''ToJSON
 typeNames = map tvbName tvbs
 instanceType = foldl' appT (conT name) $ map varT typeNames

-- | Generates a lambda expression which encodes the given data type as JSON.
--
-- Example:
--
-- @
-- data Foo = Foo Int
-- @
--
-- @
-- encodeFoo :: Foo -> 'Value'
-- encodeFoo = $('mkToJSON' id ''Foo)
-- @
--
-- This will splice in the following code:
--
-- @
-- \value -> case value of Foo arg1 -> 'toJSON' arg1
-- @
mkToJSON :: (String -> String) -- ^ Function to change field names.
 -> Name -- ^ Name of the type to encode.
 -> Q Exp
mkToJSON withField name = withType name (\_ cons -> consToJSON withField cons)

-- | Helper function used by both 'deriveToJSON' and 'mkToJSON'. Generates code
-- to generate the JSON encoding of a number of constructors. All constructors
-- must be from the same type.
consToJSON :: (String -> String)
 -- ^ Function to change field names.
 -> [Con]
 -- ^ Constructors for which to generate JSON generating code.
 -> Q Exp
consToJSON _ [] = error $ "Data.Aeson.TH.consToJSON: "
 ++ "Not a single constructor given!"
-- A single constructor is directly encoded. The constructor itself may be
-- forgotten.
consToJSON withField [con] = do
 value <- newName "value"
 lam1E (varP value)
 $ caseE (varE value)
 [encodeArgs id withField con]
-- With multiple constructors we need to remember which constructor is
-- encoded. This is done by generating a JSON object which maps to constructor's
-- name to the JSON encoding of its contents.
consToJSON withField cons = do
 value <- newName "value"
 lam1E (varP value)
 $ caseE (varE value)
 [ encodeArgs (wrap $ getConName con) withField con
 | con <- cons
 ]
 where
 wrap :: Name -> Q Exp -> Q Exp
 wrap name exp =
 let fieldName = [e|T.pack|] `appE` litE (stringL $ nameBase name)
 in [e|object|] `appE` listE [ infixApp fieldName
 [e|(.=)|]
 exp
 ]

-- | Generates code to generate the JSON encoding of a single constructor.
encodeArgs :: (Q Exp -> Q Exp) -> (String -> String) -> Con -> Q Match
-- Nullary constructors. Generates code that explicitly matches against the
-- constructor even though it doesn't contain data. This is useful to prevent
-- type errors.
encodeArgs withExp _ (NormalC conName []) =
 match (conP conName [])
 (normalB $ withExp [e|toJSON ([] :: [()])|])
 []
-- Polyadic constructors with special case for unary constructors.
encodeArgs withExp _ (NormalC conName ts) = do
 let len = length ts
 args <- mapM newName ["arg" ++ show n | n <- [1..len]]
 js <- case [[e|toJSON|] `appE` varE arg | arg <- args] of
 -- Single argument is directly converted.
 [e] -> return e
 -- Multiple arguments are converted to a JSON array.
 es -> do
 mv <- newName "mv"
 let newMV = bindS (varP mv)
 ([e|VM.unsafeNew|] `appE`
 litE (integerL $ fromIntegral len))
 stmts = [ noBindS $
 [e|VM.unsafeWrite|] `appE`
 (varE mv) `appE`
 litE (integerL ix) `appE`
 e
 | (ix, e) <- zip [(0::Integer)..] es
 ]
 ret = noBindS $ [e|return|] `appE` varE mv
 return $ [e|Array|] `appE`
 (varE 'V.create `appE`
 doE (newMV:stmts++[ret]))
 match (conP conName $ map varP args)
 (normalB $ withExp js)
 []
-- Records.
encodeArgs withExp withField (RecC conName ts) = do
 args <- mapM newName ["arg" ++ show n | (_, n) <- zip ts [1 :: Integer ..]]
 let js = [ infixApp ([e|T.pack|] `appE` fieldNameExp withField field)
 [e|(.=)|]
 (varE arg)
 | (arg, (field, _, _)) <- zip args ts
 ]
 match (conP conName $ map varP args)
 (normalB $ withExp $ [e|object|] `appE` listE js)
 []
-- Infix constructors.
encodeArgs withExp _ (InfixC _ conName _) = do
 al <- newName "argL"
 ar <- newName "argR"
 match (infixP (varP al) conName (varP ar))
 ( normalB
 $ withExp
 $ [e|toJSON|] `appE` listE [ [e|toJSON|] `appE` varE a
 | a <- [al,ar]
 ]
 )
 []
-- Existentially quantified constructors.
encodeArgs withExp withField (ForallC _ _ con) =
 encodeArgs withExp withField con


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

-- | Generates a 'FromJSON' instance declaration for the given data type.
--
-- Example:
--
-- @
-- data Foo = Foo Char Int
-- $('deriveFromJSON' id ''Foo)
-- @
--
-- This will splice in the following code:
--
-- @
-- instance 'FromJSON' Foo where
-- 'parseJSON' =
-- \value -> case value of
-- 'Array' arr ->
-- if (V.length arr == 2)
-- then Foo \<$\> 'parseJSON' (arr `V.unsafeIndex` 0)
-- \<*\> 'parseJSON' (arr `V.unsafeIndex` 1)
-- else fail \"\<error message\>\"
-- other -> fail \"\<error message\>\"
-- @
deriveFromJSON :: (String -> String)
 -- ^ Function to change field names.
 -> Name
 -- ^ Name of the type for which to generate a 'FromJSON' instance
 -- declaration.
 -> Q [Dec]
deriveFromJSON withField name =
 withType name $ \tvbs cons -> fmap (:[]) $ fromCons tvbs cons
 where
 fromCons :: [TyVarBndr] -> [Con] -> Q Dec
 fromCons tvbs cons =
 instanceD (return $ map (\t -> ClassP ''FromJSON [VarT t]) typeNames)
 (classType `appT` instanceType)
 [ funD 'parseJSON
 [ clause []
 (normalB $ consFromJSON name withField cons)
 []
 ]
 ]
 where
 classType = conT ''FromJSON
 typeNames = map tvbName tvbs
 instanceType = foldl' appT (conT name) $ map varT typeNames

-- | Generates a lambda expression which parses the JSON encoding of the given
-- data type.
--
-- Example:
--
-- @
-- data Foo = Foo 'Int'
-- @
--
-- @
-- parseFoo :: 'Value' -> 'Parser' Foo
-- parseFoo = $('mkParseJSON' id ''Foo)
-- @
--
-- This will splice in the following code:
--
-- @
-- \\value -> case value of arg -> Foo \<$\> 'parseJSON' arg
-- @
mkParseJSON :: (String -> String) -- ^ Function to change field names.
 -> Name -- ^ Name of the encoded type.
 -> Q Exp
mkParseJSON withField name =
 withType name (\_ cons -> consFromJSON name withField cons)

-- | 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 :: Name
 -- ^ Name of the type to which the constructors belong.
 -> (String -> String)
 -- ^ Function to change field names.
 -> [Con]
 -- ^ Constructors for which to generate JSON parsing code.
 -> Q Exp
consFromJSON _ _ [] = error $ "Data.Aeson.TH.consFromJSON: "
 ++ "Not a single constructor given!"
consFromJSON tName withField [con] = do
 value <- newName "value"
 lam1E (varP value)
 $ caseE (varE value)
 (parseArgs tName withField con)
consFromJSON tName withField cons = do
 value <- newName "value"
 obj <- newName "obj"
 conKey <- newName "conKey"
 conVal <- newName "conVal"

 let -- Convert the Data.Map inside the Object to a list and pattern match
 -- against it. It must contain a single element otherwise the parse will
 -- fail.
 caseLst = caseE ([e|H.toList|] `appE` varE obj)
 [ match (listP [tupP [varP conKey, varP conVal]])
 (normalB caseKey)
 []
 , do other <- newName "other"
 match (varP other)
 (normalB $ [|wrongPairCountFail|]
 `appE` (litE $ stringL $ show tName)
 `appE` ([|show . length|] `appE` varE other)
 )
 []
 ]

 caseKey = caseE (varE conKey)
 [match wildP (guardedB guards) []]
 guards = [ do g <- normalG $ infixApp (varE conKey)
 [|(==)|]
 ( [|T.pack|]
 `appE` conNameExp con
 )
 e <- caseE (varE conVal)
 (parseArgs tName withField con)
 return (g, e)
 | con <- cons
 ]
 ++
 [ liftM2 (,)
 (normalG [e|otherwise|])
 ( [|conNotFoundFail|]
 `appE` (litE $ stringL $ show tName)
 `appE` listE (map (litE . stringL . nameBase . getConName) cons)
 `appE` ([|T.unpack|] `appE` varE conKey)
 )
 ]

 lam1E (varP value)
 $ caseE (varE value)
 [ match (conP 'Object [varP obj])
 (normalB caseLst)
 []
 , do other <- newName "other"
 match (varP other)
 ( normalB
 $ [|noObjectFail|]
 `appE` (litE $ stringL $ show tName)
 `appE` ([|valueConName|] `appE` varE other)
 )
 []
 ]

-- | Generates code to parse the JSON encoding of a single constructor.
parseArgs :: Name -- ^ Name of the type to which the constructor belongs.
 -> (String -> String) -- ^ Function to change field names.
 -> Con -- ^ Constructor for which to generate JSON parsing code.
 -> [Q Match]
-- Nullary constructors.
parseArgs tName _ (NormalC conName []) =
 [ do arr <- newName "arr"
 match (conP 'Array [varP arr])
 ( normalB $ condE ([|V.null|] `appE` varE arr)
 ([e|pure|] `appE` conE conName)
 ( parseTypeMismatch tName conName
 (litE $ stringL "an empty Array")
 ( infixApp (litE $ stringL $ "Array of length ")
 [|(++)|]
 ([|show . V.length|] `appE` varE arr)
 )
 )
 )
 []
 , matchFailed tName conName "Array"
 ]
-- Unary constructors.
parseArgs _ _ (NormalC conName [_]) =
 [ do arg <- newName "arg"
 match (varP arg)
 ( normalB $ infixApp (conE conName)
 [e|(<$>)|]
 ([e|parseJSON|] `appE` varE arg)
 )
 []
 ]
-- Polyadic constructors.
parseArgs tName _ (NormalC conName ts) = parseProduct tName conName $ genericLength ts
-- Records.
parseArgs tName withField (RecC conName ts) =
 [ do obj <- newName "recObj"
 let x:xs = [ [|lookupField|]
 `appE` (litE $ stringL $ show tName)
 `appE` (litE $ stringL $ nameBase conName)
 `appE` (varE obj)
 `appE` ( [e|T.pack|]
 `appE`
 fieldNameExp withField field
 )
 | (field, _, _) <- ts
 ]
 match (conP 'Object [varP obj])
 ( normalB $ condE ( infixApp ([|H.size|] `appE` varE obj)
 [|(==)|]
 (litE $ integerL $ genericLength ts)
 )
 ( foldl' (\a b -> infixApp a [|(<*>)|] b)
 (infixApp (conE conName) [|(<$>)|] x)
 xs
 )
 ( parseTypeMismatch tName conName
 ( litE $ stringL $ "Object with "
 ++ show (length ts)
 ++ " name/value pairs"
 )
 ( infixApp ([|show . H.size|] `appE` varE obj)
 [|(++)|]
 (litE $ stringL $ " name/value pairs")
 )
 )
 )
 []
 , matchFailed tName conName "Object"
 ]
-- Infix constructors. Apart from syntax these are the same as
-- polyadic constructors.
parseArgs tName _ (InfixC _ conName _) = parseProduct tName conName 2
-- Existentially quantified constructors. We ignore the quantifiers
-- and proceed with the contained constructor.
parseArgs tName withField (ForallC _ _ con) = parseArgs tName withField con

-- | Generates code to parse the JSON encoding of an n-ary
-- constructor.
parseProduct :: Name -- ^ Name of the type to which the constructor belongs.
 -> Name -- ^ 'Con'structor name.
 -> Integer -- ^ 'Con'structor arity.
 -> [Q Match]
parseProduct tName conName numArgs =
 [ do arr <- newName "arr"
 -- List of: "parseJSON (arr `V.unsafeIndex` <IX>)"
 let x:xs = [ [|parseJSON|]
 `appE`
 infixApp (varE arr)
 [|V.unsafeIndex|]
 (litE $ integerL ix)
 | ix <- [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
 ]

lookupField :: (FromJSON a) => String -> String -> Object -> T.Text -> Parser a
lookupField tName rec obj key =
 case H.lookup key obj of
 Nothing -> unknownFieldFail tName rec (T.unpack key)
 Just v -> parseJSON v

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

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

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

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

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


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

-- | Boilerplate for top level splices.
--
-- The given 'Name' must be from a type constructor. Furthermore, the
-- type constructor must be either a data type or a newtype. Any other
-- value will result in an exception.
withType :: Name
 -> ([TyVarBndr] -> [Con] -> Q a)
 -- ^ Function that generates the actual code. Will be applied
 -- to the type variable binders and constructors extracted
 -- from the given 'Name'.
 -> Q a
 -- ^ Resulting value in the 'Q'uasi monad.
withType name f = do
 info <- reify name
 case info of
 TyConI dec ->
 case dec of
 DataD _ _ tvbs cons _ -> f tvbs cons
 NewtypeD _ _ tvbs con _ -> f tvbs [con]
 other -> error $ "Data.Aeson.TH.withType: Unsupported type: "
 ++ show other
 _ -> error "Data.Aeson.TH.withType: I need the name of a type."

-- | Extracts the name from a constructor.
getConName :: Con -> Name
getConName (NormalC name _) = name
getConName (RecC name _) = name
getConName (InfixC _ name _) = name
getConName (ForallC _ _ con) = getConName con

-- | Extracts the name from a type variable binder.
tvbName :: TyVarBndr -> Name
tvbName (PlainTV name ) = name
tvbName (KindedTV name _) = name

-- | Makes a string literal expression from a constructor's name.
conNameExp :: Con -> Q Exp
conNameExp = litE . stringL . nameBase . getConName

-- | Creates a string literal expression from a record field name.
fieldNameExp :: (String -> String) -- ^ Function to change the field name.
 -> Name
 -> Q Exp
fieldNameExp f = litE . stringL . f . 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"