[Haskell-cafe] Lifting IO actions into Applicatives

[Daniil Frumin]

Isn't it the case that there could be more than one natural transformation
between functors?


On Tue, Oct 1, 2013 at 10:00 PM, John Wiegley <johnw at fpcomplete.com> wrote:

> >>>>> Yitzchak Gale <gale at sefer.org> writes:
>
> > In fact, it even makes sense to define it as FunctorIO, with the only
> laws
> > being that liftIO commutes with fmap and preserves id, i.e., that it is a
> > natural transformation. (Those laws are also needed for ApplicativeIO and
> > MonadIO.)
>
> Given that we are moving toward Applicative (and thus Functor) as a
> superclass
> of Monad, why not just solve the MonadIO problem and similar type classes
> with
> natural transformations?  It requires 3 extensions, but these are
> extensions I
> believe should become part of Haskell anyway:
>
>     {-# LANGUAGE FlexibleInstances #-}
>     {-# LANGUAGE MultiParamTypeClasses #-}
>     {-# LANGUAGE RankNTypes #-}
>
>     module NatTrans where
>
>     import Control.Monad.IO.Class
>     import Control.Monad.Trans.Maybe
>
>     class (Functor s, Functor t) => NatTrans s t where
>         nmap :: forall a. s a -> t a
>         -- Such that: nmap . fmap f = fmap f . nmap
>
>     -- In 7.10, this Functor constraint becomes redundant
>     instance (Functor m, MonadIO m) => NatTrans IO m where
>         nmap = liftIO
>
>     main :: IO ()
>     main = void $ runMaybeT $ nmap $ print (10 :: Int)
>
> Now if I have a functor of one kind and need another, I reach for nmap in
> the
> same way that I reach for fmap to transform the mapped type.
>
> --
> John Wiegley
> FP Complete                         Haskell tools, training and consulting
> http://fpcomplete.com               johnw on #haskell/irc.freenode.net
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>



-- 
Sincerely yours,
-- Daniil
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20131007/b0bd2f11/attachment.htm>