Parse date in free format from JSON with Aeson

Already is night, so post will be short :) This is an example how to parse JSON data (date stamp) in free format:

{-# LANGUAGE OverloadedStrings #-}
module Main where

import           Control.Monad        (mzero)
import           Data.Aeson
import qualified Data.ByteString.Lazy as B
import           Data.Text
import           Data.Time
import           Data.Maybe (fromJust)

main :: IO ()
main = getJSON >>= print

data Person =
  Person {  name  :: !Text
          , age   :: Int
          , birth :: UTCTime
            } deriving Show

prsTime :: String -> UTCTime
prsTime = fromJust . parseTimeM True defaultTimeLocale "%0Y,%m"

instance FromJSON Person where
  parseJSON (Object v) =
      Person <$> v .: "name"
             <*> v .: "age"
             <*> (prsTime <$> v .: "birth")
  parseJSON _ = mzero


jsonFile :: FilePath
jsonFile = "js.json"

getJSON :: IO (Maybe Person)
getJSON = decode <$> B.readFile jsonFile

To build I change .cabal file to:

...
build-depends:       base
                     , js
                     , text
                     , aeson
                     , bytestring >= 0.10
                     , time
...

Our testing JSON file D:\prj\js\js.json will be:

{
    "name": "alex",
    "age": 20,
    "birth": "2017,10"
}

so, as you can see our date has format "YYYY,mm". Build and run as usual:

D:\prj\js> stack build
D:\prj\js> stack exec js-exe
Just (Person {name = "alex", age = 20, birth = 2017-10-01 00:00:00 UTC})