Haskell, Aeson - no instance for (ToJSON ByteString)

Question

So happy making it this far, encountered a new hurdle: Got this code made to be encoded to JSON. However no matter when type I use as an instance, the compiler complains. Now I am obviously doing something wrong, but it is exactly what is in the documentation (when using DeriveGeneric obviously).

{-# LANGUAGE OverloadedStrings, DeriveGeneric #-}

import Data.Aeson
import Data.Text as T
import Data.ByteString.Lazy as B
import Data.ByteString.Lazy.Char8 as BC
import GHC.Generics

-- decode :: FromJSON a => B.ByteString -> Maybe a
-- decode' :: FromJSON a => B.ByteString -> Either String a
-- encode :: ToJSON => a -> B.ByteString

data System = System  { system :: BC.ByteString
                  , make :: BC.ByteString
                  , code :: Int
                  } deriving (Generic, Show)
instance ToJSON System
-- instance FromJSON System

platform = System { system = "FPGA"
                  , make = "Xilinx"
                  , code = 10165
                  }
encodePlatform :: BC.ByteString
encodePlatform = encode platform

Compiler output:

    • No instance for (ToJSON ByteString)
        arising from a use of ‘aeson-1.4.1.0:Data.Aeson.Types.ToJSON.$dmtoJSON’
    • In the expression:
        aeson-1.4.1.0:Data.Aeson.Types.ToJSON.$dmtoJSON @(System)
      In an equation for ‘toJSON’:
          toJSON = aeson-1.4.1.0:Data.Aeson.Types.ToJSON.$dmtoJSON @(System)
      In the instance declaration for ‘ToJSON System’
   |
17 | instance ToJSON System

If ByteString is really the correct type here, then you should probably not be converting it to JSON. Use binary serialisation instead. If those fields actually contain text rather than binary data, then make the type Text. If they do contain binary data but you need to use JSON, use base64 or something like that. — leftaroundabout

Sibi Sibi · Accepted Answer · 2018-11-14T08:38:43

That's because there is no ByteString instance for ToJSON typeclass. Historically, it used to be present but it was removed because JSON strings should be valid unicode.

You can find more details here:

For fixing it, I would convert it to Text type and then encode into JSON.

Haskell, Aeson - no instance for (ToJSON ByteString)

1 Answers