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Today's Topics:
1. Re: ghci reports "The last statement in a 'do' construct
must be an expression" error (Nico Rolle)
2. Re: xml parsing (Antoine Latter)
3. Modifications inside a Reader? (Brian Troutwine)
4. Re: xml parsing (Yitzchak Gale)
5. Re: map increases length of list (Jack Kennedy)
6. Re: map increases length of list (Daniel Fischer)
----------------------------------------------------------------------
Message: 1
Date: Wed, 17 Jun 2009 18:13:03 +0200
From: Nico Rolle <nico.ro...@googlemail.com>
Subject: Re: [Haskell-beginners] ghci reports "The last statement in a
'do' construct must be an expression" error
To: Daniel Fischer <daniel.is.fisc...@web.de>
Cc: beginners@haskell.org
Message-ID:
<45fccde20906170913t1fe143c3q26f8008cd37b1...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
it really was an indentation error
i configured my editor to expand tabs into spaces but somehow he did a real
tab on line 9 and 10
thank you
2009/6/16 Daniel Fischer <daniel.is.fisc...@web.de>
> Am Dienstag 16 Juni 2009 22:35:17 schrieb Nico Rolle:
> > Hi
> >
> > Heres my code snippet.
> > It reports that my error is in line 9 right after the main definition.
> > All functions that i call work under normal circumstances.
> > Thanks
>
> Must be the indentation, probably the xs is indented further than the
> following line
> (though that's not the case for the code copy-pasted from the mail to an
> editor).
>
> But note that this will most likely print out 0 twice.
> The let pnp = ... bindings don't cause any computation to occur.
> To measure the time the computations take, you must force them to occur
> between the two
> calls to getCurrentTime.
>
> >
> >
> > module Benchmark
> > where
> >
> > import ReadCSV
> > import Operators
> > import Data.Time.Clock (diffUTCTime, getCurrentTime)
> >
> > main = do
> > xs <- readCSV "dataconvert/lineitem.tbl" '|'
> > start <- getCurrentTime
> > let pnp = projection [5] xs
> > let snp = selection (\x -> (x!!0) > (Int 17000)) pnp
> > end <- getCurrentTime
> > putStrLn $ show (end `diffUTCTime` start)
> > start2 <- getCurrentTime
> > let pp = pProjection [5] xs
> > let sp = pSelection (\x -> (x!!0) > (Int 17000)) pp
> > end2 <- getCurrentTime
> > putStrLn $ show (end2 `diffUTCTime` start2)
> > return xs
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 2
Date: Wed, 17 Jun 2009 22:32:50 -0500
From: Antoine Latter <aslat...@gmail.com>
Subject: Re: [Haskell-beginners] xml parsing
To: Benedikt Ahrens <benedikt.ahr...@gmx.net>
Cc: beginners@haskell.org
Message-ID:
<694519c50906172032r23e7b0b2xe6439e9e2de46...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On Wed, Jun 17, 2009 at 1:05 PM, Benedikt Ahrens<benedikt.ahr...@gmx.net> wrote:
> Hello,
>
> I want to write a program which among other should read some xml file.
> The xml tags will mostly be custom ones defined by a dtd.
>
> My question is: which parsing library to use?
>
> On
> http://en.wikibooks.org/wiki/Haskell/XML
> three libraries are mentioned, but a comparison and recommendation of
> which one to choose is missing.
>
> As you might imagine, I would like to use a library which is actively
> maintained? Has any of these libraries developed into a "standard"
> library?
>
> Thanks in advance for your help.
> ben
One option I've used before which isn't on the list is on hackage here:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml
I have a parser for these XML files:
http://cgit.freedesktop.org/xcb/proto/tree/src
in this Haskell package on hackage:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xcb-types
I'm no Haskell guru, and the code may not be the best example - but
the library is pretty easy to get started with.
Antoine
------------------------------
Message: 3
Date: Wed, 17 Jun 2009 20:32:53 -0700
From: Brian Troutwine <goofyheadedp...@gmail.com>
Subject: [Haskell-beginners] Modifications inside a Reader?
To: beginners@haskell.org
Message-ID:
<971980cc0906172032v5b9a9ad2l5ea0ea4fc878b...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hello all.
I'm writing a UDP echo server, full source given below. The current
implementation echoes back the "payload" of every incoming message but
I would prefer that only unique payloads be echoed back. To that end
I've started in with Data.BloomFilter but am not sure how to update it
accordingly. I imagine that Reader is probably the wrong monad to be
using though I'm unsure how I might modify my program to use State.
Could someone lead me along a bit?
Also, any general comments on the style of my program?
Thanks,
Brian
--
import Prelude hiding (putStrLn, catch)
import Network.Socket hiding (send, sendTo, recv, recvFrom)
import Network.Socket.ByteString
import Data.ByteString.Char8 hiding (head)
import Control.Monad.Reader
import Control.Monad (forever)
import Control.Exception (bracket)
import Data.BloomFilter
import Data.BloomFilter.Easy
import Data.BloomFilter.Hash (cheapHashes)
data Globals = Globals {
socketG :: Socket
, bloomF :: Bloom ByteString
}
type Echo = ReaderT Globals IO
run :: Echo ()
run = forever $ do
s <- asks socketG
(msg, addr) <- liftIO $ recvFrom s 1024
let [_, _, _, payload] = split ':' msg
liftIO $ sendTo s payload addr
main :: IO ()
main = bracket build disconnect loop
where
disconnect = sClose . socketG
loop st = runReaderT run st
build :: IO Globals
build = do
addrinfos <- getAddrInfo
(Just (defaultHints {addrFlags = [AI_PASSIVE]}))
Nothing (Just "1514")
let serveraddr = head addrinfos
sock <- socket (addrFamily serveraddr) Datagram defaultProtocol
bindSocket sock (addrAddress serveraddr)
return $ Globals sock $ emptyB (cheapHashes 3) 1024
------------------------------
Message: 4
Date: Thu, 18 Jun 2009 12:26:58 +0300
From: Yitzchak Gale <g...@sefer.org>
Subject: Re: [Haskell-beginners] xml parsing
To: Antoine Latter <aslat...@gmail.com>
Cc: Benedikt Ahrens <benedikt.ahr...@gmx.net>, beginners@haskell.org
Message-ID:
<2608b8a80906180226j5a9382f1ke6fa26738d9e4...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Benedikt Ahrens wrote:
>> I want to write a program which among other should read some xml file.
>> The xml tags will mostly be custom ones defined by a dtd.
>> My question is: which parsing library to use?
Antoine Latter wrote:
> One option I've used before which isn't on the list is on hackage here:
> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml
Yes, that is a great library for quick-and-dirty xml work.
It's very easy to use. And it's very well maintained, because
it was written at Galois and is actively used there.
But if you need to do more serious XML work, including
support and verified compliance for the alphabet soup of XML
specs, you need to use one of the other libraries.
For example, the xml library will work fine with custom tags.
But if you want to verify against the DTD, you'll have to
look elsewhere. It is easy to search an XML tree by hand
with the xml library, but I don't think it will do an XPath
search for you. Etc.
By the way, perhaps the xml library should be added to
that wikibook page.
Hope this helps,
Yitz
------------------------------
Message: 5
Date: Thu, 18 Jun 2009 08:24:20 -0500
From: Jack Kennedy <j...@realmode.com>
Subject: Re: [Haskell-beginners] map increases length of list
To: Alan Mock <docm...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<1008bfc90906180624s3c6757a9kb2bdcac14c2cb...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I think I understand the reason why, but still I find it disturbing that in
this first expression, x has 6 elements:
Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map degreesToRadians
y)
([0,60,120,180,240,300],[0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,1.0471975333333332,2.094395066
6666664,3.1415926,4.188790133333333,5.235987666666667,6.2831852])
But if I add a comparison to y, x now has 7 elements:
Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map degreesToRadians
y, *x==y*)
([0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,1.0471975333
333332,2.0943950666666664,3.1415926,4.188790133333333,5.235987666666667,6.2831852],True)
I. J. Kennedy
On Wed, Jun 17, 2009 at 4:55 PM, Alan Mock <docm...@gmail.com> wrote:
> That's because [0,60,..359] is not the same as [0,60..359] :: [Double]. So
> what you're passing to degreesToRadians is
> [0.0,60.0,120.0,180.0,240.0,300.0,360.0] and not [0,60,120,180,240,300]. I
> don't know why the Double version adds another number, though.
>
>
> On Jun 17, 2009, at 4:35 PM, Aaron MacDonald wrote:
>
> For some reason, the map function returns a list that has one more element
>> than my input list.
>>
>> My input list is a range defined by [0, 60..359] (should translate into
>> [0,60,120,180,240,300]).
>>
>> The function I'm giving to map is defined this way:
>> -----
>> degreesToRadians :: Double -> Double
>> degreesToRadians degrees = degrees * (pi / 180)
>> -----
>>
>> This is how I'm calling map overall:
>> -----
>> > map degreesToRadians [0,60..359]
>>
>> [0.0,1.0471975511965976,2.0943951023931953,3.141592653589793,4.1887902047863905,5.235987755982989,6.283185307179586]
>> -----
>>
>> As you can hopefully see, there are seven elements instead of six. Getting
>> the length confirms this:
>> -----
>> > length [0,60..359]
>> 6
>> > length $ map degreesToRadians [0,60..359]
>> 7
>> -----
>>
>> I do not seem to get this behaviour with the length if I either substitute
>> the degreesToRadians function or substitute the [0,60..359] range.
>>
>> P.S. Is there a built-in function to convert degrees to radians and
>> vice-versa?
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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------------------------------
Message: 6
Date: Thu, 18 Jun 2009 15:49:02 +0200
From: Daniel Fischer <daniel.is.fisc...@web.de>
Subject: Re: [Haskell-beginners] map increases length of list
To: beginners@haskell.org
Message-ID: <200906181549.02790.daniel.is.fisc...@web.de>
Content-Type: text/plain; charset="iso-8859-15"
Am Donnerstag 18 Juni 2009 15:24:20 schrieb Jack Kennedy:
> I think I understand the reason why, but still I find it disturbing that in
> this first expression, x has 6 elements:
>
> Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map
> degreesToRadians y)
You give no type for x, so the default is chosen, that is Integer.
The expression map degreesToRadians y forces y to have type [Double] (you could
give y a
more general type if you specified a type signature).
> ([0,60,120,180,240,300],[0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,1.047
>1975333333332,2.094395066
> 6666664,3.1415926,4.188790133333333,5.235987666666667,6.2831852])
>
> But if I add a comparison to y, x now has 7 elements:
>
> Prelude> let x = [0,60..359]; y = [0,60..359] in (x, y, map
> degreesToRadians y, *x==y*)
> ([0.0,60.0,120.0,180.0,240.0,300.0,360.0],[0.0,60.0,120.0,180.0,240.0,300.0
>,360.0],[0.0,1.0471975333
> 333332,2.0943950666666664,3.1415926,4.188790133333333,5.235987666666667,6.2
>831852],True)
Now, the expression x==y forces x to have the same type as y, which still is
[Double].
But, lo and behold:
Prelude> let degreesToRadians :: Double -> Double; degreesToRadians d = d*pi/180
Prelude> let x :: (Num a, Enum a) => [a]; x = [0, 60 .. 359]; y :: (Num a, Enum
a) => [a];
y = [0, 60 .. 359]
Prelude> (x,y,map degreesToRadians y, x == y)
([0,60,120,180,240,300],[0,60,120,180,240,300],
[0.0,1.0471975511965976,2.0943951023931953,3.141592653589793,4.1887902047863905,5.235987755982989,6.283185307179586],True)
>
> I. J. Kennedy
>
------------------------------
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