wren ng thornton wrote:
Anders Kaseorg wrote:
Isaac Dupree wrote:
Do you see the difference? The effects are sequenced in different
places.
The return/join pair moves all the effects *outside* the operations such
as catch... thus defeating the entire purpose of morphIO.
Yes; my question is more whether Wren has a more clever way to get an
isomorphism (forall b. (m a -> IO b) -> IO b) <-> IO (m a) that would
make the simpler interface work out. (Or maybe I misunderstood what
he was getting at.)
Yeah no, that's what I was getting at. Since it doesn't quite work out,
I should probably rethink my appeal to parametricity re Kleisli arrows.[1]
No, my parametricity was correct, just the implementations were wrong:
{-# LANGUAGE RankNTypes #-}
module MorphIO where
import Prelude hiding (catch)
import Control.Monad
import qualified Control.Exception as E
import Control.Exception (NonTermination(..))
class Monad m => MonadMorphIO m where
morphIO :: (forall b. (m a -> IO b) -> IO b) -> m a
class Monad m => MonadJoinIO m where
-- | Embed the IO into the monad m
joinIO :: IO (m a) -> m a
-- | Extract the IO computation to the top level,
-- rendering the m pure from IO.
partIO :: m a -> IO (m a)
joinIO' m = morphIO (m >>=)
morphIO' f = joinIO (f partIO)
instance MonadMorphIO IO where
morphIO f = f id
instance MonadJoinIO IO where
joinIO = join
partIO = fmap return -- N.B. fmap return /= return
catch m h = morphIO $ \w -> w m `E.catch` \e -> w (h e)
catch' m h = morphIO' $ \w -> w m `E.catch` \e -> w (h e)
test = E.throwIO NonTermination `catch` \NonTermination -> return
"moo"
test' = E.throwIO NonTermination `catch'` \NonTermination -> return
"moo"
--
Live well,
~wren
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