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Today's Topics:
1. maybe use for functors or arrows? (Michael Mossey)
2. Re: maybe use for functors or arrows? (Stephen Tetley)
3. Re: maybe use for functors or arrows? (Stephen Tetley)
4. Re: maybe use for functors or arrows? (Heinrich Apfelmus)
5. Re: System.Process.createProcess [solved] (John Obbele)
6. Re: Re: maybe use for functors or arrows? (Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Sat, 26 Jun 2010 22:58:07 -0700
From: Michael Mossey <[email protected]>
Subject: [Haskell-beginners] maybe use for functors or arrows?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Is there a way to write the function
process :: [(Location,Item)] -> [(Location,ValuableItem)]
given a function indicating which Item's to keep
transform :: Item -> Maybe ValuableItem
using functors and arrows? The value for location stays with any item that
is kept.
What I have is
process inp = catMaybes (map g inp)
where g (l,i) = case transform i of
Nothing -> Nothing
Just v -> Just (l,v)
This looks like an arrow situation to me because you want to make a
function that acts on the second value in a tuple, and a little bit like a
Maybe functor.
Thanks,
Mike
------------------------------
Message: 2
Date: Sun, 27 Jun 2010 08:38:31 +0100
From: Stephen Tetley <[email protected]>
Subject: Re: [Haskell-beginners] maybe use for functors or arrows?
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
Hi Michael
The /case transform i of .../ code should be able to be rephrased with
fmap as it is "putting back" the first element of the tuple if
/transform/ produces a (Just) value.
fmap (\a -> (a,v))
Personally, I would look to avoiding building the list of type ::Maybe
(Location,ValuableItem) during the first traversal (via map) to later
filter them out with a second traversal via catMaybes.
The optimization technique "stream fusion" can often collapse
compositions of list functionals (map, filter, etc). into one
traversal. But it isn't available by default - you have to install the
stream-fusion library. Also a composition of functionals isn't
necessarily clearer than a direct implementation.
Best wishes
Stephen
------------------------------
Message: 3
Date: Sun, 27 Jun 2010 08:46:05 +0100
From: Stephen Tetley <[email protected]>
Subject: Re: [Haskell-beginners] maybe use for functors or arrows?
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
On 27 June 2010 08:38, Stephen Tetley <[email protected]> wrote:
>
> fmap (\a -> (a,v))
>
Likely, that fmap is the wrong way round:
fmap (\a -> (l,a))
(The type checker would have spotted the mistake in the really code...)
------------------------------
Message: 4
Date: Sun, 27 Jun 2010 09:59:42 +0200
From: Heinrich Apfelmus <[email protected]>
Subject: [Haskell-beginners] Re: maybe use for functors or arrows?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
Michael Mossey wrote:
> Is there a way to write the function
>
> process :: [(Location,Item)] -> [(Location,ValuableItem)]
>
>
> given a function indicating which Item's to keep
>
> transform :: Item -> Maybe ValuableItem
>
> using functors and arrows? The value for location stays with any item
> that is kept.
>
> What I have is
>
> process inp = catMaybes (map g inp)
> where g (l,i) = case transform i of
> Nothing -> Nothing
> Just v -> Just (l,v)
>
> This looks like an arrow situation to me because you want to make a
> function that acts on the second value in a tuple, and a little bit like
> a Maybe functor.
import Control.Arrow ((***))
process =
catMaybes . map (uncurry (liftM2 (,)) . (return *** transform))
Whether this is more readable is another question.
Regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
------------------------------
Message: 5
Date: Sun, 27 Jun 2010 13:05:19 +0200
From: John Obbele <[email protected]>
Subject: Re: [Haskell-beginners] System.Process.createProcess [solved]
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Sat, Jun 26, 2010 at 08:59:01PM -0400, Brandon S Allbery KF8NH wrote:
> On 6/26/10 18:57 , John Obbele wrote:
> > After digging things a little, it seems I am messing with the
> > System.Process.createProcess function : none of my spawned
> > process terminates and eventually my OS freezes.
>
> You need to reap zombies manually; take a look at
> System.Process.waitForProcess. (On POSIX-ish systems, including Linux, you
> can use the signal handling functions to auto-reap; on Linux simply setting
> SIGCHLD to SIG_IGN is good enough, but other systems (*BSD, OSX) require you
> to also set the SA_NOCLDWAIT flag and for portability you should include it.
> (Which, I note, doesn't seem to be supported in System.Posix.Signals. O
> Haskell gods: Bug?)
Oki, I've test with waitForProcess and it's working fine. Thanks
for your help :)
And concerning the signals part, I've never heard of managing
child processes with them. I will go reread some docs about
it, I may learn some new interesting things today ...
regards,
/john
------------------------------
Message: 6
Date: Sun, 27 Jun 2010 14:36:05 +0100
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Re: maybe use for functors or arrows?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Sun, Jun 27, 2010 at 09:59:42AM +0200, Heinrich Apfelmus wrote:
> Michael Mossey wrote:
>> Is there a way to write the function
>>
>> process :: [(Location,Item)] -> [(Location,ValuableItem)]
>>
>>
>> given a function indicating which Item's to keep
>>
>> transform :: Item -> Maybe ValuableItem
>>
>> using functors and arrows? The value for location stays with any item that
>> is kept.
>>
>> What I have is
>>
>> process inp = catMaybes (map g inp)
>> where g (l,i) = case transform i of
>> Nothing -> Nothing
>> Just v -> Just (l,v)
>>
>> This looks like an arrow situation to me because you want to make a
>> function that acts on the second value in a tuple, and a little bit like a
>> Maybe functor.
>
> import Control.Arrow ((***))
>
> process =
> catMaybes . map (uncurry (liftM2 (,)) . (return *** transform))
One could also do
process = catMaybes . map (sequenceA . second transform)
where
Data.Traversable.sequenceA :: (Traversable t, Applicative f) => t (f a) -> f
(t a)
is used to "lift" the Maybe from the inside of the tuple to apply to
the whole tuple, specifically
sequenceA :: (a, Maybe b) -> Maybe (a,b)
Unfortunately this will not work without a Traversable instance for
((,) a), which doesn't already exist, but it ought to, and it's easy
to make:
import Data.Foldable
import Data.Traversable
instance Foldable ((,) a) where
foldMap = foldMapDefault
instance Traversable ((,) a) where
sequenceA (a,fb) = fmap ((,) a) fb
-Brent
>
> Whether this is more readable is another question.
>
>
> Regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
> _______________________________________________
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