Parsing config files in Haskell with Yaml

app:
  port: 9000
  limit: 100
db:
  host: "my.database.com"
  port: 5439
  user: "admin"
  database: "mydb"
s3:
  bucket: "my-s3-bucket"
  access: "ABASDSAMPLEACESSKEY" 
  secret: "312312YASDASAMPLESECRETKEY"

import Control.Applicative
import Data.Yaml -- you need to install yaml beforehand with cabal

data MyConfig = MyConfig {
    app :: AppConfig,
    db :: DbConfig,
    s3 :: S3Config
  } deriving Show
data AppConfig = AppConfig {
    port :: Int,
    limit :: Int
  } deriving Show
data DbConfig = DbConfig {
    host :: String,
    port :: Int,
    user :: String,
    database :: String
  } deriving Show
data S3Config = S3Config {
    bucket :: String,
    access :: String,
    secret :: String
  } deriving Show

instance FromJSON MyConfig where
    parseJSON (Object m) = MyConfig <$>
        m .: "app" <*>
        m .: "db" <*>
        m .: "s3"
    parseJSON x = fail ("not an object: " ++ show x)       
instance FromJSON AppConfig where
    parseJSON (Object m) = AppConfig <$>
        m .: "port" <*>
        m .: "limit"
    parseJSON x = fail ("not an object: " ++ show x)
instance FromJSON DbConfig where
    parseJSON (Object m) = DbConfig <$>
        m .: "host" <*>
        m .: "port" <*>
        m .: "user" <*>
        m .: "database"
    parseJSON x = fail ("not an object: " ++ show x)
instance FromJSON S3Config where
    parseJSON (Object m) = S3Config <$>
        m .: "bucket" <*>
        m .: "access" <*>
        m .: "secret"
    parseJSON x = fail ("not an object: " ++ show x)