Re: [Haskell-cafe] mdo with multiple values

15 Jan 2014

      Hello Joachim,

I don't really understand what you're doing here. There's the obvious

  mapM_ (act2 >=> act1)

But presumably act1 performs some monadic action that doesn't depend on its
input, and you need that to be performed before act2?

To me, it feels like there's some sort of implicit coupling between act1
and act2, and you would be better off extracting that, perhaps by changing
act1 to have the type act1 :: M T -> () .

If that's not possible, your approach seems pretty simple to me.

John L.
On Jan 15, 2014 1:57 AM, "Joachim Breitner" 
wrote:
...
Dear List,
a little puzzle.
Given a monad M with a MonadFix instance, and these two functions:
        act1 :: T -> M ()
        act2 :: a -> M T
I morally want to write this function:
        foo :: [a] -> M ()
        foo = mdo
          mapM_ act1 xs
          xs <- mapM act2
          return ()
Unfortunately, that will not work: mapM_ will force xs before any of it
can be generated. But morally it should be possible, as the lists passed
to mapM_ and mapM have the same, already known list.
So here is my solution (which is a bit more general, because I happen to
need some that in one place):
mapFstMapSnd :: MonadFix m => [(a -> m (), m a)] -> m ()
        mapFstMapSnd xs = const () `liftM` go xs (return [])
          where
            go [] cont = cont
            go ((f,s):xs) cont = mdo
                f v
                (v:vs) <- go xs $ do
                    vs <- cont
                    v <- s
                    return (v:vs)
                return vs
Using that, I can write
        foo = mapFstSnd . map (x -> (act1, act2 x))
Are there better solutions? Simpler ones? Or possibly ones that do not
require a partial pattern?
Hmm, and thinking while writing lets me come up with
data FunSplit m where
            FunSplit :: forall m a . (a -> m ()) -> m a -> FunSplit m
mapFstMapSnd :: forall m. MonadFix m => [FunSplit m] -> m ()
        mapFstMapSnd xs = const () `liftM` go xs (return ())
          where
            go :: [FunSplit m] -> m b -> m b
            go [] cont = cont
            go (FunSplit f s:xs) cont = mdo
                f v
                (v,vs) <- go xs $ do
                    vs <- cont
                    v <- s
                    return (v,vs)
                return vs
foo :: [SPut] -> SPut
        foo = mapFstMapSnd . map go
            where go x = FunSplit act1 (act2 x)
Any suggestions for improvement?
Thanks,
Joachim
--
Joachim Breitner
  e-Mail: mail@joachim-breitner.de
  Homepage: http://www.joachim-breitner.de
  Jabber-ID: nomeata@joachim-breitner.de
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Re: [Haskell-cafe] mdo with multiple values

John Lato