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Today's Topics:
1. Re: Help : foldl skipping one value in list (David Place)
2. problem with lazy IO (Dennis Raddle)
3. Re: Search for docs on operator '<+>'. (Ertugrul Soeylemez)
4. Re: Search for docs on operator '<+>'. (Brent Yorgey)
5. Re: problem with lazy IO (Yitzchak Gale)
6. Re: Help : foldl skipping one value in list
(Alexander V Vershilov)
7. Re: problem with lazy IO (Dennis Raddle)
8. Re: problem with lazy IO (Daniel Fischer)
----------------------------------------------------------------------
Message: 1
Date: Wed, 9 Nov 2011 11:17:10 -0500
From: David Place <d...@vidplace.com>
Subject: Re: [Haskell-beginners] Help : foldl skipping one value in
list
To: Lev Broido <lev.bro...@gmail.com>
Cc: beginners@haskell.org
Message-ID: <5a9c16ca-60f5-46d2-9bb4-c2a49d1c7...@vidplace.com>
Content-Type: text/plain; charset=us-ascii
On Nov 9, 2011, at 12:18 AM, Lev Broido wrote:
> Hi
> I am implementing toy loop search algorithm in graph .
> I encountered strange problem -- it seems , that fold doesn't run on the
> whole list .
> Code is at http://hpaste.org/53776
> Section in question is updVis function
> Problem occurs when _newStateMap_2 is being calculated
> Please advise.
Your code is quite complex for the task. Your analysis that the problem lies
with foldl' not consuming the whole list is extremely unlikely. Probably, the
problem is elsewhere. Try building your solution more from the bottom up and
unit testing the components.
------------------------------
Message: 2
Date: Wed, 9 Nov 2011 09:12:20 -0800
From: Dennis Raddle <dennis.rad...@gmail.com>
Subject: [Haskell-beginners] problem with lazy IO
To: Haskell Beginners <beginners@haskell.org>
Message-ID:
<cakxlvoqar0fdya-aaxz328zq6hfwcucs+ggpoen4_66cu10...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I have the following code, which reads several thousand text files and
creates some Map data. On Windows XP, it crapped out with the error
"resource exhausted: too many open files." So I'm sure I need to make it
stricter. I've never tried to do that before, so I'm not sure where the
strictness is necessary. Maybe all I need is to switch to strict
ByteStrings?
-- Read a bunch of text files. The files are organized in pairs, where the
file names in each pair are
-- "a<number>.txt" and "f<fnumber>.txt". Example: "a00001.txt" and
"f00001.txt".
-- Each file is a list of float values specified in a certain text format
(which happens to be
-- an output format of the audio-related program Csound)
--
-- Input:
-- [(String,String)] : a list of pairs of file names
-- String : directory in which the files exist
readPvsFiles :: [(String,String)] -> String -> IO (Map Int (Map Float
Float))
readPvsFiles filenames dir = do
-- contents :: [(Int,Map Float Float)]
-- where Int is the number in the file name.
-- and Map Float Float is a map of numbers in the
-- in the first file mapped to numbers in the second file
contents <- mapM (oneFilePair dir) filenames
return $ M.fromList contents
-- Read one file pair.
--
-- Input:
-- String : directory of the files
-- (String,String) : the two file names in the pair
-- (Int,Map Float Float) :
-- Int is the number in the file name.
-- and Map Float Float is a map of numbers in the
-- in the first file mapped to numbers in the second file
oneFilePair :: String -> (String,String) -> IO (Int,Map Float Float)
oneFilePair dir (ffile,afile) = do
-- Read the float values from each file (which is a list of floats in
-- a text format.
fvalues <- readTableValues (dir ++ "/" ++ ffile)
avalues <- readTableValues (dir ++ "/" ++ afile)
-- t is the number in the filename.
let t = read . take 6 . drop 1 $ ffile
return (t, M.fromList $ zip fvalues avalues)
-- Open the file via readFile, and parse all the text values in it.
readTableValues :: String -> IO [Float]
readTableValues s = do
b <- readFile s
let bs = lines b
lenLine = head . drop 1 $ bs
n = read (drop 6 lenLine) :: Int
-- valueLines :: [String]. This is a list of the lines
-- in the file that have the float values. One float
-- value in text format per line.
valueLines = take n . drop 25 $ bs
return $ map read valueLines
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Message: 3
Date: Wed, 9 Nov 2011 19:15:18 +0100
From: Ertugrul Soeylemez <e...@ertes.de>
Subject: Re: [Haskell-beginners] Search for docs on operator '<+>'.
To: beginners@haskell.org
Message-ID: <20111109191518.09002...@angst.streitmacht.eu>
Content-Type: text/plain; charset=US-ASCII
"Allen S. Rout" <a...@ufl.edu> wrote:
> Google seems to barf on '<+>'. I've found a haskell "list of
> operators", and it's not mentioned.
>
> Could some kind soul point me at documentation for this thing?
This is the combination operator from the ArrowPlus type class for
arrows, which can be combined in a monoidic fashion. It corresponds to
(<|>) from the Alternative type class and in fact you can give a trivial
Alternative instance for every arrow that is both ArrowPlus and
ArrowZero. Reasonable laws would be:
zeroArrow <+> a = a <+> zeroArrow = a
But I don't think that's official.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
Message: 4
Date: Wed, 9 Nov 2011 13:24:54 -0500
From: Brent Yorgey <byor...@seas.upenn.edu>
Subject: Re: [Haskell-beginners] Search for docs on operator '<+>'.
To: beginners@haskell.org
Message-ID: <20111109182454.ga...@seas.upenn.edu>
Content-Type: text/plain; charset=us-ascii
On Wed, Nov 09, 2011 at 07:15:18PM +0100, Ertugrul Soeylemez wrote:
> "Allen S. Rout" <a...@ufl.edu> wrote:
>
> > Google seems to barf on '<+>'. I've found a haskell "list of
> > operators", and it's not mentioned.
> >
> > Could some kind soul point me at documentation for this thing?
>
> This is the combination operator from the ArrowPlus type class for
> arrows, which can be combined in a monoidic fashion. It corresponds to
> (<|>) from the Alternative type class and in fact you can give a trivial
> Alternative instance for every arrow that is both ArrowPlus and
> ArrowZero. Reasonable laws would be:
>
> zeroArrow <+> a = a <+> zeroArrow = a
>
> But I don't think that's official.
Let me point out again, just so the OP is not confused: it completely
depends where you saw the <+>. There is no "standard" meaning,
although the ArrowPlus one that Ertugrul mentions is common. But you
could have seen it in some module where the author defines <+> to mean
something completely different. Until you tell us where you saw this
<+> we cannot really help.
In fact, Allen, I now recall seeing you on the xmonad mailing list --
perhaps you saw <+> in an xmonad config? In that case, indeed, it is
NOT the ArrowPlus operator, but something defined by xmonad. In the
context of xmonad, <+> is used for combining ManageHooks (actually, it
can be used to combine any two things which have a type that is an
instance of the 'Monoid' type class, but is most commonly used for
ManageHooks).
-Brent
------------------------------
Message: 5
Date: Wed, 9 Nov 2011 20:40:32 +0200
From: Yitzchak Gale <g...@sefer.org>
Subject: Re: [Haskell-beginners] problem with lazy IO
To: Dennis Raddle <dennis.rad...@gmail.com>
Cc: Haskell Beginners <beginners@haskell.org>
Message-ID:
<CAOrUaLad1aC60M=BMe=xaqtgn8z4eafms0rbyumuscyptdp...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Dennis Raddle wrote:
> I have the following code, which reads several thousand text files and
> creates some Map data. On Windows XP, it crapped out with the error
> "resource exhausted: too many open files." So I'm sure I need to make it
> stricter.
Yes.
> I've never tried to do that before, so I'm not sure where the
> strictness is necessary.
When you use readFile, each file remains open until the entire
file has been read. When reading is delayed due to laziness,
the open file handles begin to pile up.
> Maybe all I need is to switch to strict ByteStrings?
That should solve your problem in this case, yes.
You might consider using Text instead of ByteString, though;
you are interpreting those values as text. Text has built-in
functions for parsing floating point numbers. With
ByteString, you'd have to change each line to a String.
I almost always prefer using Text or ByteString over
String nowadays anyway.
To solve the problem when using readFile from the
Prelude, you would need to make sure that each
file is read all the way to the end as you go along.
One trick sometimes used for that is to use the
evaluate function from Control.Exception to force
evaluation of the length of each file:
b <- readFile s
evaluate $ length b
...
That would cause the entire contents of the file
to be read into memory immediately and the
file to be closed, like the behavior of readFile
for strict ByteString and strict Text.
-Yitz
------------------------------
Message: 6
Date: Thu, 10 Nov 2011 04:39:21 +0000
From: Alexander V Vershilov <alexander.vershi...@gmail.com>
Subject: Re: [Haskell-beginners] Help : foldl skipping one value in
list
To: beginners@haskell.org
Message-ID: <20111110043921.gc25...@qnikst.gtn.ru>
Content-Type: text/plain; charset="utf-8"
Hello.
Wed, Nov 09, 2011 at 07:18:14AM +0200, Lev Broido wrote
> Hi
> I am implementing toy loop search algorithm in graph .
> I encountered strange problem -- it seems , that fold doesn't run on the
> whole list .
> Code is at http://hpaste.org/53776
> Section in question is updVis function
> Problem occurs when _newStateMap_2 is being calculated
> Please advise.
>
> Thanks,
> Lev
>
I have not checked all pasted code but for your problem you can try
next solution (using Either to specify is it odd or even argument):
> f = foldl (\x y -> inner x y) (Left [])
> where
> inner (Left xs) _ = Right xs
> inner (Right xs) y = Left (yourfunction y:xs)
--
Alexander V Vershilov
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Message: 7
Date: Wed, 9 Nov 2011 15:48:20 -0800
From: Dennis Raddle <dennis.rad...@gmail.com>
Subject: Re: [Haskell-beginners] problem with lazy IO
To: g...@sefer.org
Cc: Haskell Beginners <beginners@haskell.org>
Message-ID:
<cakxlvoryk3drydfjcpo74ba385ru0brcwvkxyhcy-rzydkg...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hi Yitz,
I tried "evaluate" before posting my question, and for some reason it didn't
change the behavior. But strict bytestrings worked. I haven't tried Text
yet.
It was a pain putting "map (chr . fromIntegral)" in front of everything to
get
regular strings.
On Wed, Nov 9, 2011 at 10:40 AM, Yitzchak Gale <g...@sefer.org> wrote:
>
> To solve the problem when using readFile from the
> Prelude, you would need to make sure that each
> file is read all the way to the end as you go along.
> One trick sometimes used for that is to use the
> evaluate function from Control.Exception to force
> evaluation of the length of each file:
>
> b <- readFile s
> evaluate $ length b
> ...
>
> That would cause the entire contents of the file
> to be read into memory immediately and the
> file to be closed, like the behavior of readFile
> for strict ByteString and strict Text.
>
> -Yitz
>
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Message: 8
Date: Thu, 10 Nov 2011 00:57:07 +0100
From: Daniel Fischer <daniel.is.fisc...@googlemail.com>
Subject: Re: [Haskell-beginners] problem with lazy IO
To: beginners@haskell.org
Message-ID: <201111100057.07359.daniel.is.fisc...@googlemail.com>
Content-Type: Text/Plain; charset="utf-8"
On Thursday 10 November 2011, 00:48:20, Dennis Raddle wrote:
> It was a pain putting "map (chr . fromIntegral)" in front of everything
> to get regular strings.
import qualified Data.ByteString.Char8 as C
foo <- C.unpack `fmap` C.readFile whichever
You don't need to put it *everywhere*
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