Problem decoding json from input args with Aeson - haskell

Question

I'm trying to write a Haskell program that expects Json string provided as input. Expected Json is a list of Person objects. I want to handle a case when user did not provide input by defaulting to Json of empty list.
It seems like I'm running into type conversion issue between ByteString and String. I do have OverloadedStrings turned on but it doesn't seem to help here. Here's the simplified code.
{-# LANGUAGE OverloadedStrings #-}
import GHC.Generics
import System.Environment
import Data.Aeson
import Data.Maybe (fromMaybe, listToMaybe)
import Data.Text (Text)
data Person = Person { pName :: Text, pAge :: Int } deriving (Show,
Generic)
instance ToJSON Person
instance FromJSON Person
main :: IO (Maybe [Person])
main = do
args <- getArgs
arg1 <- pure $ (fromMaybe "[]" (listToMaybe args))
-- let arg1 = "[{\"pName\": \"James\", \"pAge\": 30}]"
return $ decode arg1 :: IO (Maybe [Person])
Error I'm getting is:
• Couldn't match type ‘[Char]’
with ‘Data.ByteString.Lazy.Internal.ByteString’
Expected type: Data.ByteString.Lazy.Internal.ByteString
Actual type: String
• In the first argument of ‘decode’, namely ‘arg1’
In the second argument of ‘($)’, namely ‘decode arg1’
In a stmt of a 'do' block:
return $ decode arg1 :: IO (Maybe [Person])
If I uncomment let arg1 to simulate what arg1 should be, then code compiles.

Answer 1

The problem is that getArgs returns a [String], not a [ByteString]. decode really wants a ByteString as input.
OverloadedStrings doesn't help with that; it only affects string literals in your code, not external inputs. That's why the version with the hardcoded argument (let arg1 = "[{\"pName\": \"James\", \"pAge\": 30}]") works: arg1 automatically becomes a ByteString to make decode arg1 work, but getArgs has an incompatible type.
One possible fix would be to encode the command line strings to bytes somehow, but there seems to be an easier alternative:
import System.Posix.Env.ByteString (getArgs)
which gives you a
getArgs :: IO [ByteString]
(I haven't actually tested this code; you may also have to use decodeStrict instead of decode.)