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

1 Oct 2013

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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