Re: [Haskell-cafe] Lifting IO actions into Applicatives

On Tue, Oct 01, 2013 at 09:29:00AM +0200, Niklas Haas wrote:

On Tue, 1 Oct 2013 02:21:13 -0500, John Lato wrote:

...

It's not a solution per se, but it seems to me that there's no need for the Monad superclass constraint on MonadIO. If that were removed, we could just have

class LiftIO t where liftIO :: IO a -> t a

and it would Just Work.

One concern with this is that it's not exactly clear what the semantics are on LiftIO (is liftIO a >> liftIO b equal to liftIO (a >> b) or not?) and the interaction between LiftIO and Applicative/Monad would have to be some sort of ugly ad-hoc law like we have with Bounded/Enum etc.

What is this 'ad-hoc law' you are talking about ?

Furthermore, you might end up specifying a lot of ‘redundant’ constraints like (Monad m, LiftIO m) that way, just a thing to keep in mind. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370