Happy Birthday, bos!

{-# LANGUAGE OverloadedStrings #-}

import qualified Facebook as FB

import Control.Applicative
import Control.Monad (mzero)
import Control.Monad.IO.Class (liftIO)
import qualified Data.Aeson as A
import Data.Time
import Data.Text
import Formatting
import Network.HTTP.Conduit (withManager)

data UserWithBirthday = UserWithBirthday
  { birthday :: Text
  } deriving Show

instance A.FromJSON UserWithBirthday where
  parseJSON (A.Object v) = UserWithBirthday <$>
                         v A..: "birthday"
  parseJSON _ = mzero

-- Copy paste a valid Access Token (make sure to request the 'user_birthday' permission) from:
-- https://developers.facebook.com/tools/explorer/
fbTokenData :: Text
fbTokenData = "paste-your-token-here"

userAccessToken :: FB.UserAccessToken
userAccessToken = FB.UserAccessToken (FB.Id "0") fbTokenData farInTheFuture
  where
    Just farInTheFuture = parseTime (error "farInTheFuture") "%Y" "3000"

parseFbBirthday :: Text -> UTCTime
parseFbBirthday formatted = case parseTime (error "yearsOld") "%m/%d/%Y" (unpack formatted) of
  Just t -> t
  Nothing -> undefined

yearsOld :: UTCTime -> UTCTime -> Text
yearsOld time curTime = sformat ("Happy " % ords % " birthday!") (round $ (diffUTCTime curTime time) / 31536000)

main :: IO ()
main = do
  curTime <- getCurrentTime
  withManager $ \manager -> do
    FB.runNoAuthFacebookT manager $ do
      userWithBirthday <- FB.getObject "/v2.2/me" [ ("fields", "birthday") ] (Just userAccessToken)
      liftIO $ print $ yearsOld (parseFbBirthday (birthday userWithBirthday)) curTime