[Haskell-cafe] Lifting IO actions into Applicatives

John Wiegley johnw
Tue Oct 1 18:00:08 UTC 2013

>>>>> 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

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