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nntpnews.net - fa.haskell http://www.nntpnews.net en Fri, 10 Sep 2010 21:40:21 GMT vBulletin 60 http://www.nntpnews.net/fusion/misc/rss.jpg nntpnews.net - fa.haskell http://www.nntpnews.net <![CDATA[Re: [Haskell-cafe] Dependent types]]> http://www.nntpnews.net/showthread.php?t=17058815&goto=newpost Fri, 10 Sep 2010 12:50:59 GMT wrote: > This issue pops up quite quite often - Ryan Ingram's answer...]]> Hi!

On Fri, Sep 10, 2010 at 9:22 AM, Stephen Tetley
< - > wrote:
> This issue pops up quite quite often - Ryan Ingram's answer to it the
> last time it was on the Cafe points to the relevant Trac issue
> numbers:

But I have not yet made it as GADTs. I would need some help here. How
to change MaybePacket to GADTs?

Mitar
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]]> fa.haskell Mitar http://www.nntpnews.net/showthread.php?t=17058815 <![CDATA[[Haskell-beginners] Network programming (select(2)) for Haskell?]]> http://www.nntpnews.net/showthread.php?t=17058814&goto=newpost Fri, 10 Sep 2010 12:03:00 GMT In order to get better at network programming using haskell, I'd like > to try and...]]> Hi Patrick,

On 10/09/2010 14:02, Patrick LeBoutillier wrote:
> In order to get better at network programming using haskell, I'd like
> to try and port it to Haskell, but I can't seem to locate the
> equivalent to select(2) in Haskell. Perhaps a different idiom is to be
> used?

No select(2) syscall is explicitly available at API level.
This is because of how the Haskell (GHC) Runtime is structured.
Both the default single threaded runtime that the multi-threaded
runtime are capable of hosting different haskell light-threads.
IO between lightweight threads is multiplex transparently
by the runtime via select(2) o epoll/kqueue in the upcoming ghc.
Haskell lightweight threads are very cheap and efficient
so the idiomatic way to handle your use case is to spawn different
threads with forkIO (you are forking lightweight threads not OS thread)
and you some Haskell concurrency primitive for thread
communication/synchronization (see MVar and Chan in Control.Concurrent).

Paolo

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]]> fa.haskell Patrick LeBoutillier http://www.nntpnews.net/showthread.php?t=17058814 <![CDATA[[Haskell-cafe] FFI and concurrency]]> http://www.nntpnews.net/showthread.php?t=17058813&goto=newpost Fri, 10 Sep 2010 10:26:35 GMT Hello Johannes, Friday, September 10, 2010, 2:25:58 PM, you wrote: just forkIO in threaded RTS works for me. try: main = do forkIO... Hello Johannes,

Friday, September 10, 2010, 2:25:58 PM, you wrote:

just forkIO in threaded RTS works for me. try:

main = do forkIO expensiveCalc
forkIO expensiveCalc
forkIO expensiveCalc
expensiveCalc

> What's the story with FFI calls and concurrency?

> I have an expensive calculation performed
> by some C function, which I call from Haskell land.
> (This works like a charm.)

> I have several cores available. How could I run
> several of these calculations in parallel?

> Thanks, Johannes.

> _______________________________________________
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--
Best regards,
Bulat private.php?do=newpm&u=

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]]> fa.haskell Johannes Waldmann http://www.nntpnews.net/showthread.php?t=17058813 <![CDATA[[Haskell-beginners] Stack space overflow with foldl']]> http://www.nntpnews.net/showthread.php?t=17058812&goto=newpost Fri, 10 Sep 2010 09:30:19 GMT Hi,

I see a stack overflow with this code, but I don't understand why.

Test.hs
--------------

main :: IO ()
main = do
let list = ([1..3*1000*1000] :: [Int])
let !x = foldl'2 (flip seq) () list
return x

-- foldl'2 is just the __GLASGOW_HASKELL__ version of Data.List.foldl'.
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-- src/Data-List.html
-- The test case behaves the same if I call foldl' instead of foldl'2.
foldl'2 :: (a -> b -> a) -> a -> [b] -> a
foldl'2 f z0 xs0 = lgo z0 xs0
where lgo z [] = z
lgo z (x:xs) = let z' = f z x in z' `seq` (lgo z' xs)

$ ghc Test.hs -o Test
$ time ./Test

real 0m0.087s
user 0m0.084s
sys 0m0.000s

$ ghc Test.hs -o Test -O
$ time ./Test
Stack space overflow: current size 8388608 bytes.
Use `+RTS -Ksize -RTS' to increase it.

real 0m1.780s
user 0m1.764s
sys 0m0.004s

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.3

I looked at the Core output with ghc-core, with or without
optimizations, and I see a $wlgo recursive function that doesn't appear
to end in a tail call. I don't see any let expressions in the
folding code, so I assume no thunks are being created. I can make a
tail call appear by doing either of two things:

1. Replace "lgo z []" with "lgo !z []". This suggestion came from an
email on haskell-beginners that I can't find right now. It was a few
months ago.

2. Instead of using the __GLASGOW_HASKELL__ version of foldl', use the
other version:

foldl' f a [] = a
foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs

My test case is contrived. Originally, I had a program that read
lines from a file as Data.ByteString values, and I was trying to debug
a stack overflow. I added the foldl' call to force the evaluation of
the ByteString lines, but the foldl' call itself overflowed the
stack.

I might have fixed my original stack overflow problem. I was applying
sum to a large list of integers, and sum is lazy. I don't think I have
any real code anymore that overflows the stack, but I'm uncomfortable
because I don't know why my test case doesn't work.

Is the foldl' call in my test case allowed to have linear stack usage?

Thanks,
-Ryan
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]]> fa.haskell Ryan Prichard http://www.nntpnews.net/showthread.php?t=17058812 <![CDATA[[Haskell-cafe] Combining applicative with arrows?]]> http://www.nntpnews.net/showthread.php?t=17054447&goto=newpost Fri, 10 Sep 2010 08:34:22 GMT I just wondered if you can define Applicative instances for arrows? > Basicly...]]> On Fri, Sep 10, 2010 at 10:34:04AM +0200, Nils Schweinsberg wrote:
> I just wondered if you can define Applicative instances for arrows?
> Basicly what I thought of is:
>
> I have a type for my arrow which is "CollectA" (using HXT here):
>
> type CollectA a = SomeArrow XmlTree a

If you do

import Control.Applicative

type CollectA = WrappedArrow SomeArrow XmlTree

then CollectA will be an instance of Applicative.
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]]> fa.haskell Nils Schweinsberg http://www.nntpnews.net/showthread.php?t=17054447 <![CDATA[[Haskell-cafe] Disable LINE Pragma handling in GHC]]> http://www.nntpnews.net/showthread.php?t=17054446&goto=newpost Fri, 10 Sep 2010 06:40:40 GMT Well, in an editor, there some operations that operate on solely on a file, and some at the project level. I would make sense to me if... Well, in an editor, there some operations that operate on solely on a file,
and some at the project level. I would make sense to me if typechecking
errors were reported against the original file (following the pragmas) while
lexing and outlining results would operate without (on the actual file
contents).

JP

On Fri, Sep 10, 2010 at 2:13 PM, Antoine Latter < - > wrote:

> On Fri, Sep 10, 2010 at 1:40 AM, JP Moresmau < - > wrote:
> > Hello fellow Haskellers,
> > In EclipseFP we use the GHC API for IDE related stuff like syntax
> > highlighting and code outlines. However, I ran into something funny
> > yesterday: when a source file contains LINE pragmas
> > (
>

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> ),
> > all the locations for tokens are changed to reflect the pragmas
> information.
> > While this is great in the "normal' GHC usage, this is not so great for
> us,
> > because we're interested in that source code, not in original code. I
> > haven't seen any flag to turn that behavior off in the docs, nor in the
> > Lexer code, but have I missed something? Can I tell GHC to just ignore
> these
> > pragmas? I suppose even using GHC for building and something else
> > (haskell-src-exts?) for code handling would leave us with compilation
> > messages at the wrong place.
> > Thanks,
>
> Would you turn the behavior back on when editing .hsc files or
> something similar?
>

--
JP Moresmau

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]]> fa.haskell JP Moresmau http://www.nntpnews.net/showthread.php?t=17054446 <![CDATA[[Haskell-cafe] ANN: ecu-0.0.0]]> http://www.nntpnews.net/showthread.php?t=17054445&goto=newpost Fri, 10 Sep 2010 04:13:45 GMT to other fields, not just automotive. My question is, is it possible > for a user to...]]> On Thu, Sep 09, 2010 at 11:11:33PM -0500, Tom Hawkins wrote:
> to other fields, not just automotive. My question is, is it possible
> for a user to select which programs to install within a given package?
> I could see where someone would want to install a data analysis tool,
> but not want to bother with separately installing Kvaser canlib. I
> had considered different packages for the individual tools, but that
> would get messy pretty quick, especially since many tools share a lot
> of common code.
>
> And one last question: The Kvaser canlib library has a different name
> depending on if the machine is Linux or Windows. What is the best way
> to configure the build based on the platform? Current I have the
> library name hard coded in the extra-libraries field in the cabal
> file.

You want conditional configurations in the .cabal file. See

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For the OS issue you can do something like

if os(windows)
Build-depends: canlib-windows
else
Build-depends: canlib

or whatever. For user-selectable programs, create some flags and use
the flags to specify which executables are buildable. Then the user
can just pass a flag to cabal install and get the executables they
want.

-Brent
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]]> fa.haskell Tom Hawkins http://www.nntpnews.net/showthread.php?t=17054445 <![CDATA[[Haskell-cafe] "malicious" JS on haskell site]]> http://www.nntpnews.net/showthread.php?t=17054444&goto=newpost Thu, 09 Sep 2010 21:30:50 GMT writes: > Looks like the free web counter was sold to an advertiser as few years ago. I've seen this happen...]]> "Albert Y. C. Lai" < - > writes:

> Looks like the free web counter was sold to an advertiser as few years ago.

I've seen this happen before, and it's just a strategy - first provide
some cross-site neat function, wait around for a while, then replace it
with some ad-serving crap.

Take home lesson: only serve JS that you host yourself.

-k
--
If I haven't seen further, it is by standing in the footprints of giants
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]]> fa.haskell Keith Sheppard http://www.nntpnews.net/showthread.php?t=17054444 <![CDATA[[Haskell-cafe] Style and a problem]]> http://www.nntpnews.net/showthread.php?t=17054443&goto=newpost Thu, 09 Sep 2010 20:55:36 GMT On Fri, Sep 10, 2010 at 1:06 AM, Nils Schweinsberg wrote: > > > Something like...]]> On Fri, Sep 10, 2010 at 12:24:39PM +0300, Wanas wrote:
> On Fri, Sep 10, 2010 at 1:06 AM, Nils Schweinsberg < - > wrote:
>
> > Something like this?
> >
> > import Data.List
> >
> > newList :: Int -> [[Int]]
> > newList n = myNub
> > [ l | l <- undefined -- not really sure how you want
> > -- to generate these lists :)
> > , sum l == sum [1..n]
> > ]
> >
> > myNub :: (Ord a) => [[a]] -> [[a]]
> > myNub = nubBy (\a b -> sort a == sort b)
> >
> >
> > - Nils
> >
> So I've checked out this code, and it doesn't compile on ghci. My version,
> without the "undefined" portion is(which still doesn't work):
>
> import Data.List
>
> newList :: Int -> [[Int]]
> newList n = myNub [ l | l <- [1..n], sum l == sum [1..n] ]

The reason this doesn't compile is that

l <- [1..n]

means l will take on each of the values 1 through n in turn. So l is
of type Int. But then you do 'sum l' which requires l to be a list.
So clearly that does not typecheck. But I'm not quite sure I
understand what you actually want l to be. Do you want it to run
through all lists of a certain length with elements chosen from
[1..n]? In that case you could do something like

import Control.Monad -- for replicateM

...

[ l | l <- replicateM n [1..n], ... ]

which will generate all length-n lists with elements chosen from
[1..n]. If you wanted all lists with lengths from 1 to n and elements
chosen from 1 to n, you could do

[ l | len <- [1..n], l <- replicateM len [1..n], ... ]

As others have pointed out there are likely far more efficient ways to
do what you want, but I'm just trying to help you get this code to
work first, even if it's inefficient.

-Brent

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]]> fa.haskell Wanas http://www.nntpnews.net/showthread.php?t=17054443 <![CDATA[[Haskell-cafe] CAL experience]]> http://www.nntpnews.net/showthread.php?t=17054442&goto=newpost Thu, 09 Sep 2010 19:31:34 GMT Hello, > > as this is really friendly forum, I'd like to ask to perhaps solve my > wonder. From time to time I'm seeing people here...]]> karel.gardas:
> Hello,
>
> as this is really friendly forum, I'd like to ask to perhaps solve my
> wonder. From time to time I'm seeing people here recommending Scala as a
> kind of replacement for non-existent Haskell on Java/JVM platform. My
> wonder is: why the people here do not recommend CAL, which at least to
> me, looks much more closer to Haskell than Scala is. Are there any bad
> experiences with this language and OpenQuark platform? I'm asking since
> I'm currently playing with it and plan to use it for java objects data
> processing.

Since Business Objects was aquired by SAP, development has slowed.

-- Don
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]]> fa.haskell Karel Gardas http://www.nntpnews.net/showthread.php?t=17054442 <![CDATA[[Haskell-cafe] Haskell, arrows and signal processing]]> http://www.nntpnews.net/showthread.php?t=17054441&goto=newpost Thu, 09 Sep 2010 18:40:29 GMT The input and output are infinite streams. I have a few questions: > > 1) Is it possible to change it to use arrows? How would it look like? > 2)...]]> > The input and output are infinite streams. I have a few questions:
>
> 1) Is it possible to change it to use arrows? How would it look like?
> 2) How would one implement an continuous time version?

Yampa can do *exactly* what you're asking for. Unfortunately the code
seems to be a little rough and there are many space leaks (some of
which are alluded to in the source code comments). Definitely make
sure to profile memory usage as you code.
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]]> fa.haskell Anthony LODI http://www.nntpnews.net/showthread.php?t=17054441 <![CDATA[[Haskell] New haskell.org server]]> http://www.nntpnews.net/showthread.php?t=17054440&goto=newpost Thu, 09 Sep 2010 17:47:32 GMT Hi all, We will shortly be replacing the haskell.org server with a more modern, commercially hosted machine. For now, the new server is available...
Hi all,

We will shortly be replacing the haskell.org server with a more modern,
commercially hosted machine. For now, the new server is available as

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The ghc, hugs and nhc98 websites have been copied across, and there is a
copy of the wiki (although that will be resynced when we change
servers). We will also be moving the mailing lists at changeover time.

*** If you know of anything else that haskell.org ***
*** does, and should keep doing, please let us know ***
*** now or it will break soon! ***
*** ***
*** If you have anything else on haskell.org, ***
*** please find it a new home now! ***

Note that if there are other services currently running on haskell.org,
this may be a good time to consider moving them to the community server
instead:

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Meanwhile, we would like to thank Paul Hudak, John Peterson, Mark
Wogahn, the Yale Workstation Support team and Yale University for
hosting, running and maintaining the haskell.org server for countless
years.

We would also like to thank the Haskell Summer of Code team for
providing the funding for the new server, and the community sysadmin
team for agreeing to maintain it in the future.

Thanks also to Thomas Schilling and Ashley Yakeley for getting the wiki
up and running, and restyling it.

Thanks
Ian, on behalf of the Haskell Infrastructure team.

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]]> fa.haskell Ian Lynagh http://www.nntpnews.net/showthread.php?t=17054440 <![CDATA[[Haskell-cafe] installing GHC/Haskell Platform in Linux withoutroot permission]]> http://www.nntpnews.net/showthread.php?t=17054439&goto=newpost Thu, 09 Sep 2010 17:39:00 GMT Unfortunately I don't have ghc installed. Can I install it without root > priv? That...]]> On Thursday 09 September 2010 20:23:13, Ali Razavi wrote:
> Unfortunately I don't have ghc installed. Can I install it without root
> priv?

That ought to work. I just unpacked and installed ghc-6.8.3-i386-unknown-
linux.tar.bz2 in a subdir of $HOME without root privileges.
(I would've tried a newer release, but I had the 6.8.3 vanilla binary just
happening to lie around.)
So in principle it works, if it doesn't work with a later vanilla linux
binary, that would be a surprise (and a bug), but then you could use an
older one to build from source (that works, promised, I have done that a
lot).

>
> On Sep 9, 2010, at 1:40 PM, Don Stewart < - > wrote:
> > ali.razavi:
> >> Is it possible to install either of these (preferably the latter)
> >> somewhere in my home directory without having root permission? I
> >> tried the unknown linux package with configure --prefix set to a
> >> subdir in my home, to no avail. The problem seems to be due to some
> >> library registration which fails.
> >
> > Assuming you have GHC installed, the Haskell Platform Linux generic
> > source installer will install without root privledges.
> >
> > If that doesn't work for you, file a bug with a log of what failed on
> > the HP bug tracker.
> >
> > -- Don

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]]> fa.haskell Ali Razavi http://www.nntpnews.net/showthread.php?t=17054439 <![CDATA[[Haskell-beginners] List Manipulation]]> http://www.nntpnews.net/showthread.php?t=17054438&goto=newpost Thu, 09 Sep 2010 17:13:43 GMT > Up to some extent, I am done (later on I found out that reading this > into a...]]> On Thursday 09 September 2010 19:12:21, Lorenzo Isella wrote:
>
> Up to some extent, I am done (later on I found out that reading this
> into a matrix for hmatrix is a one-liner), but I would like to achieve
> more just performing standard list manipulations.
> For instance, since my data files are always this simple (integers/real
> arrays of numbers)
>
> (1) How can I tell the length of every sublist in my list?

map length

gives the list of lengths

(if all sublists are known to have the same length, of course one length is
enough, e.g. head . map length if there is known to be at least one
sublist).

> (2) How can I e.g. stitch together the 1st element in every sublist in
> order to get the 1st column of the original data file?

map head does this. More general,

import Data.List

and then

transpose

does what you'd expect, so

firstColumn xss = head (transpose xss)

can do it.

But there's a catch.
As a rule, don't use head.

head is a partial function, if it's called with an empty list, it throws an
error. And that error isn't particularly informative. So if you have
multiple calls to head in your programme and one of them throws an error,
it can be a major pain to find out what went wrong (and sometimes an empty
list where a nonempty one is expected isn't an unrecoverable error, in
those cases using head is wrong in an additional way).
It's okay to use head in places where it is known that it can't be called
on an empty list because its argument comes from a function that produces
only nonempty lists (some people disagree with that opinion and think head
should never be called, on principle or because of a feared slippery
slope).
But in general, don't use head.

So, how do we do it without head?

For example,

-- default value if there is no sublist
sublistLength [] = 0
sublistLength (xs : _) = length xs

Or use Maybe to indicate whether it's a real result or not,

sublistLength [] = Nothing
sublistLength (xs : _) = Just (length xs)

{-
import Data.Maybe

sublistLength = fmap length . listToMaybe
-}

-- skip empty sublists
firstColumn xss = [x | (x : _) <- xss]

-- indicate empty sublists with Maybe
import Data.Maybe -- for listToMaybe
firstColumn xss = map listToMaybe xss

That loses a little conciseness, but it buys you safety.

>
> Many thanks
>
> Lorenzo
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]]> fa.haskell Lorenzo Isella http://www.nntpnews.net/showthread.php?t=17054438 <![CDATA[[Haskell-cafe] problems using macports?]]> http://www.nntpnews.net/showthread.php?t=17054437&goto=newpost Thu, 09 Sep 2010 15:39:00 GMT See if Code: --------- Content visible to registered users only. --------- is of any help. _______________________________________________ See if

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is of any help.

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]]> fa.haskell S. Doaitse Swierstra http://www.nntpnews.net/showthread.php?t=17054437