In another thread, I wrote:
main = (>> return ()) $ runListT $ do
arg <- ListT getArgs
n <- ListT $ return [1..3]
liftIO $ putStrLn ((show n) ++ ") " ++ arg)
The frequent occurence of "ListT $ return" in my code when I use the ListT
monad transformer has made me wonder why there isn't a standard typeclass
`MonadList', like those for the other monad transformers, encapsulating
the essence of being a "list-like" monad -- in this case, the ability to
select from a list of things. I quickly wrote one for myself:
class MonadList m where
option :: [a] -> m a
instance MonadList [] where
option = id
instance (Monad m) => MonadList (ListT m) where
option = ListT . return
Has anyone else thought about or done something like this?
Mike
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