I don't think some of the things he writes, like "MonadIO m => m () -> IO ()" are generally possible, for example. There are specific instances of MonadIO (ErrorT, etc.) that have a function that does that, but MonadIO itself only has the liftIO function, which is "MonadIO m => IO a -> m a".
Good call. I agree in disagreeing with his type for 'keep'. I think he was aiming at the idea of "any computation which is liftable to an IO computation" instead of "computation which can be evaluated within an IO context".
In my opinion, since 'keep' takes a function which is, in general, fully capable of any effectful action, it necessarily is of the type
keep :: IO a -> ...
but since it's roughly ignoring any potential return 'a', yet still evaluating the function repeatedly, the best type is something like
I think he actually meant "MonadIO m => m a -> m a".
Anyway, if I were going to use Haskell-style type signatures in an article about shell scripts written in Perl, I would probably skip the whole typeclass and monad concept, and just say something like:
keep :: Action -> Action
This doesn't say much about the infinite nature of the process, unfortunately, but modeling effects as though they are pure functions just gets you into trouble anyway. His textual description is much more natural and easier to understand.
Anyway, I like articles that combine both Perl and Haskell :) They make me feel less insane.
