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Today's Topics:
1. Re: "computation", "action" (Michael Mossey)
2. Re: "computation", "action" (Jason Dusek)
3. HaL4: Haskell-Treffen in Halle/Saale, 12. Juni 2009 -
Programm und Anmeldung (Henning Thielemann)
4. Catching Network Connection Error (aditya siram)
5. parMap on multicore computers (Miguel Pignatelli)
----------------------------------------------------------------------
Message: 1
Date: Tue, 19 May 2009 15:34:03 -0700
From: Michael Mossey <[email protected]>
Subject: Re: [Haskell-beginners] "computation", "action"
To: beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Andrew Wagner wrote:
>
>
> One words that seem to come up in Haskell are "algorithm" and
> "procedure."
>
>
> I've not seen these used in Haskell. They sound distinctly imperative to
> me, for some reason. But, until you define the words connotatively, all
> I can say is how they sound to me.
>
Yes, I meant to write they *don't* come up in Haskell.
Regarding how words "sound", that's a good thing to talk about, in my
opinion. It's related to the philosophy of epistemology. Words have
connotations. They are used in certain contexts, with certain implications.
Thanks,
Mike
------------------------------
Message: 2
Date: Tue, 19 May 2009 16:27:00 -0700
From: Jason Dusek <[email protected]>
Subject: Re: [Haskell-beginners] "computation", "action"
To: Michael P Mossey <[email protected]>
Cc: [email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
2009/05/19 Michael P Mossey <[email protected]>:
> For example, a monad is sometimes called a "computation" or an
> "effect", but I've not seen it called an "algorithm."
There is a difference between a monad and monadic value --
just as there is a difference between "List" as a structure
and a particular list as an instantiation of that structure.
> Can someone elaborate on these terms and why they are used?
> What is the difference between a computation and an algorithm?
This is a little like saying, what is the difference between
an algorithm and a program? The term "algorithm" denotes not
only a sequence of instructions to perform but also a solution
(in a well-defined sense) to a particular problem. A
computation is probably anything whatever that warms the
computer.
> The word "effect" has a fairly obvious meaning (like an IO
> side-effect) but I suspect there's more to it.
I believe the usual aphorism is "Monads sequence effects.".
--
Jason Dusek
------------------------------
Message: 3
Date: Wed, 20 May 2009 15:12:14 +0200 (CEST)
From: Henning Thielemann <[email protected]>,
[email protected]
Subject: [Haskell-beginners] HaL4: Haskell-Treffen in Halle/Saale, 12.
Juni 2009 - Programm und Anmeldung
To: [email protected], [email protected],
[email protected]
Message-ID:
<alpine.deb.2.00.0905201426420.13...@anubis.informatik.uni-halle.de>
Content-Type: text/plain; charset="iso-8859-15"
Kindly excuse the German noise, please ...
Liebe Haskell-Freunde!
Das Programm fuer unser Haskell-Treffen steht jetzt so gut wie fest und
man kann sich online anmelden.
---------------------------------------------
HaL4 : Haskell - Tutorial + Workshop + Party
am Freitag, dem 12. Juni 2009, in Halle/Saale
---------------------------------------------
Das traditionsreiche HaL-Treffen bietet eine gute Mischung
von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre
mit vielen Möglichkeiten zu Diskussion und Unterhaltung
bei der anschließenden Party. Der Workshop wird in diesem Jahr
ergänzt durch Tutorien für Haskell-Ein- und Umsteiger.
Diesmal findet das Treffen in Halle/Saale im Institut für Informatik
der Martin-Luther-Universität Halle-Wittenberg statt. Wir bieten:
10 Uhr:
Entwurfsmuster vom Kopf auf die Fuesse gestellt:
data, fold und laziness statt Kompositum, Visitor, Iterator.
Johannes Waldmann, HTWK Leipzig.
11 Uhr:
CAL/openquark: (openquark.org)
Haskell + Java => CAL, eine praktische Einfuehrung
(Eclipse als CAL-IDE, Excel lesen, typsicheres SQL = LINQ fuer Java?)
Alf Richter, iba Consulting, Leipzig
12 Uhr:
Wie man das Semikolon ueberlaedt:
Code-Strukturierung und -Wiederverwendung durch Monaden
15 Uhr:
Hayoo! Haskell API Search
Timo Huebel, FH Wedel
16 Uhr:
Haskell als reine Spezifikationssprache
Baltasar Trancon y Widemann, Universit Bayreuth
17 Uhr:
Funktional-logische Programmierung mit Curry
Jan Christiansen, Universitaet Kiel
19 - 22 Uhr: Grillparty
Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen
Diskussionen und bitten um Anmeldung bis zum 31. Mai.
Weitere Informationen auf http://www.iba-cg.de/hal4.html
Beste Gruesse
Henning Thielemann
------------------------------
Message: 4
Date: Fri, 22 May 2009 01:20:39 -0500
From: aditya siram <[email protected]>
Subject: [Haskell-beginners] Catching Network Connection Error
To: beginners <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi all,
I am trying to Network.HTTP to connect to a website and catch the error if
the network is down. Here's what I have so far:
data Errors = DocumentParseError
| WebsiteUnreachableError
| ISBNNotFoundError
respHTML' :: ISBN -> IO (Either Errors (IO String))
respHTML' isbn =
do
rsp <- simpleHTTP (getRequest $ "
http://isbndb.com/api/books.xml?results=details&access_key=";
++ key
++ "&index1=isbn&value1="
++ isbn :: Request_String)
return $ Right $ getResponseBody rsp
`E.catch` (\(e :: E.SomeException ) ->
return $ Left $ WebsiteUnreachableError)
I get the following error when I load it into GHCI:
Couldn't match expected type `[Char]'
against inferred type `Either Errors b'
Expected type: IO String
Inferred type: IO (Either Errors b)
In the expression: return $ Left $ WebsiteUnreachableError
In the second argument of `E.catch', namely
`(\ (e :: E.SomeException)
-> return $ Left $ WebsiteUnreachableError)'
Failed, modules loaded: none.
Control.Exception.catch has the following signature :
catch :: forall a. IO a -> (IOError -> IO a) -> IO a
I would expect that the handler function needs to return an IO (Either
Errors (IO String)). Why is it trying to return an IO String?
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Message: 5
Date: Fri, 22 May 2009 23:38:18 +0200
From: Miguel Pignatelli <[email protected]>
Subject: [Haskell-beginners] parMap on multicore computers
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Hi all,
I'm experimenting a bit with the parallelization capabilities of
Haskell.
What I am trying to do is to process in parallel all the lines of a
text file, calculating the edit distance of each of these lines with a
given string.
This is my testing code:
import System.IO
import Control.Monad
import Control.Parallel
import Control.Parallel.Strategies
edist :: String -> String -> Int
-- edist calculates the edit distance of 2 strings
-- see for example
http://www.csse.monash.edu.au/~lloyd/tildeFP/Haskell/1998/Edit01/
getLines :: FilePath -> IO [Int]
getLines = liftM ((parMap rnf (edist longString)) . lines) . readFile
main :: IO ()
main = do
list <- getLines "input.txt"
mapM_ ( putStrLn . show ) list
I am testing this code in a 2xQuadCore linux (Ubuntu 8.10) machine (8
cores in total).
The code has been compiled with
ghc --make -threaded mytest.hs
I've been trying input files of different lengths, but the more cores
I try to use, the worst performance I am getting.
Here are some examples:
# input.txt -> 10 lines (strings) of ~1200 letters each
$ time ./mytest +RTS -N1 > /dev/null
real 0m4.775s
user 0m4.700s
sys 0m0.080s
$ time ./mytest +RTS -N4 > /dev/null
real 0m6.272s
user 0m8.220s
sys 0m0.290s
$ time ./mytest +RTS -N8 > /dev/null
real 0m7.090s
user 0m10.960s
sys 0m0.400s
# input.txt -> 100 lines (strings) of ~1200 letters each
$ time ./mytest +RTS -N1 > /dev/null
real 0m49.854s
user 0m49.730s
sys 0m0.120s
$ time ./mytest +RTS -N4 > /dev/null
real 1m11.303s
user 1m36.210s
sys 0m1.070s
$ time ./mytest +RTS -N8 > /dev/null
real 1m19.488s
user 2m6.250s
sys 0m1.270s
What is going wrong in this code? Is this a problem of the "grain
size" of the parallelization?
Any help / advice would be very welcome,
M;
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