solution.hs

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric #-}

module Main where

import           Data.Aeson
import           Data.Aeson.Encode.Pretty
import           Data.Aeson.Types
import           Data.ByteString as B
import qualified Data.ByteString.Lazy.Char8 as LC
import           GHC.Generics
import           Data.Text

data Root = Root
    { coord :: Coord
    } deriving (Generic, Show)

data Coord = Coord
    { lon   :: Double
    , lat   :: Double
    } deriving (Generic, Show)

instance FromJSON Root
instance FromJSON Coord

main :: IO ()
main = do
  jsonString <- B.readFile "/home/denis/test.json"
  let result = decodeStrict $ jsonString :: Maybe Root
  case result of
    Nothing -> print "error"
    Just r  -> print r

{-
{
   "coord": {
      "lon":44.51,
      "lat":40.18
   }
}
-}