[Haskell-beginners] How Best to Deal with Nested Monads?

[Michael Craig]

Alright, I return from the land of transformers with a solution:


import Control.Monad.Trans.Class
import Control.Monad.Trans.Maybe

main = sequenceMTAll (lst' >>= map (getB' >=> getC'))

lst' = lift lst :: MaybeT m [a]
getB' = MaybeT . getMB
getC' = lift . getC

sequenceMTAll :: (Monad m) => [MaybeT m a] -> m [a]
sequenceMTAll (x:xs) = do
    y <- runMaybeT x
    case y of
      Nothing -> sequenceMTAll xs
      Just z -> sequenceMTAll xs >>= return . (z:)
sequenceMTAll [] = return []


(Of course in real code I'd just modify lst, getMB, getC, etc. to fit the
new types. The crux here is sequenceMTAll.)

Am I abusing Maybe too much?

Mike S Craig
(908) 328 8030


On Thu, Sep 15, 2011 at 1:15 AM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:

> On Wed, Sep 14, 2011 at 09:21:04PM -0400, Michael Craig wrote:
> > Brent: Thanks for reminding me about (>=>). Far more readable! But
> regarding
> > the sequence thing: I can think of all sorts of reasons why we'd want to
> do
> > a single traversal. How about when lst is long or infinite? In general,
> it's
> > more useful to produce output incrementally than all at once at the
> > end.
>
> Yes, producing output incrementally is great!  My point is that
> usually laziness will take care of it for you, without having to
> worry about it specifically.
>
> In this particular case, most monads will not actually allow
> incremental processing anyway.  For example, suppose m = Maybe.  Then
> when mapping getMB over lst, any particular element could cause the
> whole computation to fail.  So we cannot output anything based on the
> first elements in the list until we have processed the entire list,
> because until we get to the very end of the list we do not know
> whether to begin by outputting 'Just' or 'Nothing'.
>
> -Brent
>
> >
> > Mike S Craig
> > (908) 328 8030
> >
> >
> > On Wed, Sep 14, 2011 at 8:18 PM, Brent Yorgey <byorgey at seas.upenn.edu
> >wrote:
> >
> > > On Wed, Sep 14, 2011 at 06:48:29PM -0400, Michael Craig wrote:
> > > > Say we've got these types
> > > >
> > > > lst :: m [a]
> > > > getMB :: a -> m (Maybe b)
> > > > getC :: b -> m c
> > > >
> > > > and we want to map getMB and getC over the elements of lst, all the
> while
> > > > discarding elements x where getMB x == Nothing.
> > > >
> > > > (This could be generalized more by replacing Maybe with some monad
> m',
> > > but
> > > > let's run with Maybe because it's easy to talk about.)
> > > >
> > > > The best I've got (after some help on IRC) is this
> not-so-easy-to-read
> > > > oneliner:
> > > >
> > > > lst >>= (\x -> mapM (liftM (liftM getC) (getMB x)) >>= sequence
> > > > . catMaybes
> > >
> > > How about this:
> > >
> > >  lst >>= (mapM getMB >=> (return . catMaybes) >=> mapM getC)
> > >
> > > Everyone always forgets about (>=>).
> > >
> > > > This is hard to read, but it's also bad because we run sequence twice
> > > (once
> > > > inside of mapM). If we want to do multiple things to each element of
> lst,
> > > it
> > > > would be nice to process each element completely before moving on to
> the
> > > > next.
> > >
> > > I wouldn't worry about running sequence twice.  Processing things by
> > > chaining whole-structure transformations is the Haskell Way (tm).  All
> > > that business about "doing only one traversal" is for people
> > > programming in strict languages to worry about. The compiler can often
> > > turn a chain of wholesale transformations into a single traversal
> > > anyway.  In short, I see no particular reason why it is "nice" to
> > > process each element completely before moving on.  Isn't it nicer to
> > > be able to think in a more modular style?
> > >
> > > -Brent
> > >
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners at haskell.org
> > > http://www.haskell.org/mailman/listinfo/beginners
> > >
>
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