Encode/Decode Sum Types manually using Aeson (Haskell) - No Generic

JSONSumTypeHaskell.hs

{-# LANGUAGE OverloadedStrings #-}

module Stuff where

import Data.Aeson
import Data.ByteString.Lazy.Char8 as BC

data FooBar = Foo | Bar deriving (Eq, Show)

instance ToJSON FooBar where
  toJSON Foo = String "Fu"
  toJSON Bar = String "Baz"

instance FromJSON FooBar where
  parseJSON (String "Fu") = pure Foo
  parseJSON (String "Baz") = pure Bar
  parseJSON invalid = fail "Unable to parse"

-- Returns "\"Fu\""
fooJSON :: BC.ByteString
fooJSON = encode Foo

-- Returns "\"Baz\""
barJSON :: BC.ByteString
barJSON = encode Bar

-- Returns Foo
fooFromJSON :: Maybe FooBar
fooFromJSON = decode "\"Fu\""

-- Returns Bar
barFromJSON :: Maybe FooBar
barFromJSON = decode "\"Baz\""

-- Returns Nothing
invalidFromWrongJSON :: Maybe FooBar
invalidFromWrongJSON = decode "Boom"