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

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 wrote:

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Yitzchak Gale writes:

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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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Sincerely yours, -- Daniil