Allow using unit as an environment variable about envy HOT 4 CLOSED

Ah, this is super interesting. Yea I don't see why not. Previously I've always just initialized the manager inside of FromEnv since there is a MonadIO instance. But your way seems to be a nice clever use of types.

instance Var () where
  toVar = const "()"
  fromVar = const $ Just ()

from envy.

Ah, I didn't know about the MonadIO instance because the Parser data type isn't exported. Regardless, if you do that you'll have to use undefined (or error or unsafePerformIO) in defConfig.

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Right, if you use liftIO in FromEnv, the assumption is that you're no longer using generics. But I can definitely see a case where you'd like to parse the environment w/o initializing connection pools, and the Var () approach seems pretty nice. Before, I'd have to do something like this.

instance FromEnv Config where
  fromEnv = do
    Config <$> liftIO createConPool 
           <*> envMaybe "PG_PORT" .!= 5432

But again, I can see a case for not wanting to do this, and initializing the connection pool at a later stage.

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@tfausak, and yea, Parser (minus constructors), should probably be exported.

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