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

[Michael Craig]

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.

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
>
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