Combining Monads

Sat, 12 Apr 2003 21:52:32 +0100

Your program doesn't seem to have survived the transmission but try
http://www.cse.ogi.edu/~mpj/pubs/springschool.html or
http://www.cse.ogi.edu/~mpj/pubs/composing.html. For the theory, look at
Triples, Toposes and Theories by Barr & Wells. I think it's chapter 7 but I
haven't got it in front of me.

Dominic Steinitz
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Sent: Saturday, April 12, 2003 5:01 PM
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> Today's Topics:
>
>    1. Combining monads (Mark T.B. Carroll)
>    2. Re: Combining monads (Scott Turner)
>    3. Re: Combining monads (Glynn Clements)
>
> --__--__--
>
> Message: 1
> Date: Sat, 12 Apr 2003 09:52:58 -0400 (EDT)
> From: "Mark T.B. Carroll" <Mark.Carroll@Aetion.com>
> To: haskell-cafe@haskell.org
> Subject: Combining monads
>
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>
> Hello! I am trying and failing to use one monad in another. A tiny example
> is enclosed, where I try to use IO to help along a State-ful computation.
> Is the real mistake in do_next? Is there a minimal rewrite that will get
> this working that I can use as a template for other things? Or, is there a
> good tutorial somewhere on this type of thing? Any pointers are much
> appreciated!
>
> -- Mark
>
> --481034957-302199926-1050155578=:555
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>
> --__--__--
>
> Message: 2
> From: Scott Turner <p.turner@computer.org>
> To: "Mark T.B. Carroll" <Mark.Carroll@Aetion.com>,
haskell-cafe@haskell.org
> Subject: Re: Combining monads
> Date: Sat, 12 Apr 2003 09:14:45 -0500
>
> On Saturday 12 April 2003 09:52, Mark T.B. Carroll wrote:
> > Hello! I am trying and failing to use one monad in another.
> > Is there a minimal rewrite that will get this working ... ?
> Yes. Just add lift.
>      do count <- lift $ get_length name
>
>
> --__--__--
>
> Message: 3
> From: Glynn Clements <glynn.clements@virgin.net>
> Date: Sat, 12 Apr 2003 15:26:03 +0100
> To: "Mark T.B. Carroll" <Mark.Carroll@Aetion.com>
> Cc: haskell-cafe@haskell.org
> Subject: Re: Combining monads
>
>
> Mark T.B. Carroll wrote:
>
> > Hello! I am trying and failing to use one monad in another. A tiny
example
> > is enclosed, where I try to use IO to help along a State-ful
computation.
> > Is the real mistake in do_next? Is there a minimal rewrite that will get
> > this working that I can use as a template for other things? Or, is there
a
> > good tutorial somewhere on this type of thing? Any pointers are much
> > appreciated!
>
> Combining monads isn't straightforward. There is a paper on the topic,
> at:
>
>
http://cm.bell-labs.com/cm/cs/who/wadler/topics/monads.html#combining-monads
>
> However, you can't really combine the IO monad in the way that the
> above paper describes.
>
> The most direct way to fix your program is to use an IORef instead of
> a state monad, e.g.
>
> > import IORef
>
> > update_best_with :: IORef (String, Int) -> (String, Int) -> IO ()
> > update_best_with ref (current_name, current_count) = do
> > (best_name, best_count) <- readIORef ref
> > when (current_count > best_count) $ do
> > writeIORef ref (current_name, current_count)
>
> > do_next :: IORef (String, Int) -> String -> IO ()
> > do_next ref name = do
> > count <- get_length name
> > update_best_with ref (name, count)
>
> > longest_file :: [String] -> IO (String, Int)
> > longest_file names = do
> > ref <- newIORef ("none", 0)
> > mapM_ (do_next ref) names
> > readIORef ref
>
> A more elegant solution would be to generate a list of filename/length
> pairs (e.g. using mapM), then just use maximumBy to select the
> longest, e.g.
>
> > get_length' :: String -> IO (String, Int)
> > get_length' name = do
> > length <- get_length name
> > return (name, length)
>
> > longest_file :: [String] -> IO (String, Int)
> > longest_file names = do
> > pairs <- mapM get_length' names
> > return $ maximumBy cmp pairs
> > where cmp (_, l1) (_, l2) = compare l1 l2
>
> --
> Glynn Clements <glynn.clements@virgin.net>
>
>
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