a sample of reading a yaml file by Haskell

haskell_read_yaml_sample.hs

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

import GHC.Generics

import Data.Yaml
import Control.Applicative

data Config    = Config    { app  :: AppConfig, db  :: DbConfig                    } deriving (Show, Generic)
data AppConfig = AppConfig { port :: Int                                           } deriving (Show)
data DbConfig  = DbConfig  { user :: String, password :: String, database :: String} deriving (Show, Generic)

-- instance FromJSON Config where
--   parseJSON (Object o) = Config <$> 
--      o .: "app" <*> 
--      o .: "db"
--   parseJSON x          = fail $ show x

instance FromJSON Config
instance FromJSON AppConfig where
  parseJSON (Object m) = AppConfig <$> m .:? "port" .!= 8081 -- default
instance FromJSON DbConfig

readConfig :: IO Config
readConfig =
    either (error . show) id <$>
        decodeFileEither "./conf/conf.yaml"

main :: IO ()
main = do
  conf <- readConfig
  putStrLn $ show conf
  return ()

-- 
-- app :
--   port :
-- db  :
--   user     : "admin"
--   password : "pass"
--   database : "mydb"