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

15 Sep 2011


      Oops, typos:

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

(and forget about lst')

Mike S Craig
(908) 328 8030


On Thu, Sep 15, 2011 at 1:49 AM, Michael Craig  wrote:
...
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 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 
...
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
...
...
...
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
while
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
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