UUID.hs

-- | Description : UUID generator and data structure
--
-- Generate, parse, and pretty print UUIDs for use with IPython.
module IHaskell.IPython.Message.UUID (UUID, random, randoms) where

import           Control.Monad (mzero, replicateM)
import           Control.Applicative ((<$>))
import           Data.Text (pack)
import           Data.Aeson
import           Data.UUID.V4 (nextRandom)

-- | A UUID (universally unique identifier).
data UUID =
     -- We use an internal string representation because for the purposes of IPython, it
     -- matters whether the letters are uppercase or lowercase and whether the dashes are
     -- present in the correct locations. For the purposes of new UUIDs, it does not matter,
     -- but IPython expects UUIDs passed to kernels to be returned unchanged, so we cannot
     -- actually parse them.
      UUID String
  deriving (Show, Read, Eq, Ord)

-- | Generate a list of random UUIDs.
randoms :: Int      -- ^ Number of UUIDs to generate.
        -> IO [UUID]
randoms n = replicateM n random

-- | Generate a single random UUID.
random :: IO UUID
random = UUID <$> show <$> nextRandom

-- Allows reading and writing UUIDs as Strings in JSON. 
instance FromJSON UUID where
  parseJSON val@(String _) = UUID <$> parseJSON val

  -- UUIDs must be Strings.
  parseJSON _ = mzero

instance ToJSON UUID where
  -- Extract the string from the UUID.
  toJSON (UUID str) = String $ pack str