June 2007 - Haskell-Cafe

Reading/writing packed bytes from file
by Jefferson Heard 20 Jun '07

20 Jun '07

I've read the documentation for some of the marshalling packages out there for Haskell, and I'm left confused as to which one I should be using and how to actually do what I want to do. I have a file, a little over 2gb, of packed data in the format (recordcount) records of: 4-byte int (count), (count) 2-byte unsigned shorts, (count) 4-byte floats all in little-endian order. What I want to do is read each record (lazily), and unpack it into Data.IntMap.IntMap Float where the unsigned shorts become the keys and the 4-byte floats become the values. Then I want to do a lot of interesting processing which we'll skip here, and write back out packed data to a file in the format of 4-byte float, 4-byte float, 4-byte float for each record. I need these output records to be four-byte C floats. I've gotten as far as datatypes and a couple of signatures, but I can't figure out the functions themselves that go with the signatures, and then again, maybe I have the signatures wrong. -- import qualified Data.IntMap as M import qualified Data.ByteString.Lazy.Char8 as B data InputRecord = M.IntMap Float data OutputRecord = (Float, Float, Float) -- open a file as a lazy ByteString and break up the individual records -- by reading the count variable, reading that many bytes times -- sizeof short + sizeof float into a lazy ByteString. readRawRecordsFromFile :: String -> IO [B.ByteString] -- take a bytestring as returned by readRawRecordsFromFile and turn it -- into a map. decodeRawRecord :: B.ByteString -> M.IntMap Float -- Can anyone help with how to construct these functions? I'm going to have to make a few passes over this file, so I'd like the IO to be as fast as Haskelly possible. -- Jeff

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Re: [Haskell] ANN: Uniplate 1.0
by Isaac Dupree 20 Jun '07

20 Jun '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Neil Mitchell wrote: > Hi, > > I am pleased to announce Uniplate (formerly known as Play), a library > for boilerplate removal requiring only Haskell 98 (for normal use) and > optionally multi-parameter type classes (for more advanced features). > This library has been tested with Yhc, Hugs and GHC. > > The Uniplate library can do some of the same things as Scrap Your > Boilerplate (SYB), and has functions that can be used in a similar > manner to everywhere and everything. > > LINKS > > Project home page: http://www-users.cs.york.ac.uk/~ndm/uniplate/ > Hackage release: > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniplate-1.0 > Haddock docs: http://www.cs.york.ac.uk/fp/haddock/uniplate/ > Short Manual: http://www.cs.york.ac.uk/fp/darcs/uniplate/uniplate.htm > Draft Paper: > http://www-users.cs.york.ac.uk/~ndm/downloads/draft-uniplate-15_jun_2007.pdf > > > The manual contains a basic overview of some bits of the library, the > paper goes into more detail, but is intended to still be a readable > introduction to the library. The draft paper feels more readable and up-to-date than the manual. However I have one comment about it. In the section on DERIVE for Uniplate instances, the type appearing directly in one of its own constructors is demonstrated being recognized, as is the type appearing in a list in one of its own constructors. But no mention is made of just what powers the deriving has. I presume lists are a special case (how else would it be possible), but a naive reading gives no particular reason to suspect that the type in a Data.Map or Data.Set or any other collection, in one of its own constructors, would not be allowed. Also is there any restriction on the placement of lists in the constructor? I can't see how data Foo = CFoo Foo [Foo] Foo [Foo] Foo could be translated. Does putting the list somewhere other than last work? Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGdyHsHgcxvIWYTTURAiWuAKCUKf7aGf1rO58gcgxYkqbO7oKNCwCfTRpo 0UsQHx9riP+cZ/S/gloAy3g= =cAxY -----END PGP SIGNATURE-----

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Can't build Lambdabot
by Daniel Fischer 20 Jun '07

20 Jun '07

Hi all, so I got myself a brand new lambdabot-4.0 today, but failed miserably building it. First, build-depends lists fps>=0.7. Now Data.ByteString(.*) is in base(since 6.6.1, I believe). Easy to fix, just delete the entry in lambdabot.cabal. After apparently successful configuration, ./build fails, because 'breakChar' is no longer present (in fps-0.8 or the 6.6.1 base). Okay, change breakChar '\n' to break (== '\n') in BotPP.hs and hope for the best. Trying ./build again, fails with Preprocessing executables for lambdabot-4.0... Building lambdabot-4.0... Plugin.hs:46:7: Could not find module `Text.Regex': it is a member of package regex-compat-0.71, which is hidden which would be easy to fix if regex-compat-0.71 WERE hidden, bu it's NOT, it's definitely exposed. So what's going on here and how can I get myself a working lambdabot? I'm on an i386-linux, if that matters. Thanks, Daniel

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csv library
by Jaap Weel 20 Jun '07

20 Jun '07

This is an announcement of something so tiny (about 100 lines of code including comments), I'd feel bad posting it to the haskell list, so it'll go on haskell-cafe. This was pretty much just a project for me to learn cabal and hackage, but it may very well actually come in useful some day. csv 0.1.0 CSV loader and dumper This library parses and dumps documents that are formatted according to RFC 4180, "The common Format and MIME Type for Comma-Separated Values (CSV) Files". This format is used, among many other things, as a lingua franca for spreadsheets and for certain web services. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/csv-0.1.0 -- /jaap

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hSetBuffering woes
by Eric 20 Jun '07

20 Jun '07

Hi all, I tried to turn off buffering with the command hSetBuffering (from System.IO) but my app still blocks on hGetContents (from Data.ByteString). Does anyone know what's happening? E.

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Recursive instance dictionaries in GHC
by Benja Fallenstein 19 Jun '07

19 Jun '07

Hi, I can't figure out why the following program compiles with this instance declaration > instance Size (Maybe [a]) => Size [a] where size x = size (foo x) but has GHC loop forever with this one: > instance (Foo a b, Size b) => Size a where size x = size (foo x) Anybody here know? Thanks, - Benja > {-# OPTIONS_GHC -fglasgow-exts -fallow-overlapping-instances > -fallow-undecidable-instances #-} > > class Foo a b | a -> b where foo :: a -> b > instance Foo [a] (Maybe [a]) where foo (x:xs) = Just xs; foo [] = Nothing > > class Size a where size :: a -> Int > > instance Size a => Size (Maybe a) where > size (Just x) = 1 + size x; size Nothing = 0 > > --instance (Foo a b, Size b) => Size a where size x = size (foo x) > -- OR > --instance Size (Maybe [a]) => Size [a] where size x = size (foo x) > > main = print $ size "foo"

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OS design & FP aesthetics
by Creighton Hogg 19 Jun '07

19 Jun '07

On 6/18/07, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > > Creighton Hogg wrote: > > Well, since we're on the subject and it's only the Cafe list, what is > > it that you find messy about Linux that you would want to be solved by > > some hypothetical Haskell OS? > > This is drifting off-topic again, but here goes... Yeah, so I'll just split this off into a different thread. There are lots of things to like about Linux. It doesn't cost money. > It's fast. It's reliable. It's flexible. It's secure. Okay, I'm not sure if I'd agree with the reliable & secure points. I mean, relative to what could be done. I'm a rank amateur when it comes to OS work but when I've looked at recent papers Linux really isn't that cutting edge. I mean, it may be reliable in comparison to Windows 98 & has less known exploits than any Windows system, but in terms of how good it *could* be I think there's an awful lot of room for growth. However, > unfortunately it's still Unix. In other words, it's a vast incoherant > mess of largely incompatible ad-hoc solutions to individual problems > implemented independently by unrelated hackers over the 40+ years of > history that this software has been around. New software has to emulate > quirks in old software, and client programs work around the emulated > quirks in the new software to get the functionallity it actually wants. > One vast tangled mess of complexity and disorder. Exhibit A: Package > managers exist. Exhibit B: Autoconf exists. I rest my case. Okay, but these don't seem to really be design flaws so much as the inevitable results of age and the need for backwards compatibility. I'm looking more for technical problems that you would want to see fixed in our magical UberOS. An operating system should have a simple, clear, consistent design. Not > unlike a certain programming language named after a dead mathematition, > come to think of it... > > (Have you ever programmed in C? You can certainly see where Unix gets > its features from - terse, cryptic and messy.) This is another thing we're just going to disagree on. I think C++ is a pretty messy language, but feel that straight up C is rather simple and elegant. I had only used C++ before, but a friend rather easily convinced me that C is in fact a very sexy language when used in its intended design space. Still, I don't have the skill to write a functioning operating system - > much less one that's "ready for the desktop" - so that's that I suppose... > > (I did seriously investigate the task once. Indeed, I got as far as > writing a bootloader. It worked too!) Would you mind sharing the code? I'd be interested.

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Useful IDE features - What abaut automatically adding import statements?
by Marc Weber 19 Jun '07

19 Jun '07

Eclipse does have this which saves you a lot of time: Fix imports. Did I miss anyone mentioning such a feature request? Or is there already a solution around ? Marc Weber

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latex table and backgound color
by Adrian Dusa 19 Jun '07

19 Jun '07

Hello all, I'm a newb on this list, hoping to get (what I suppose is) a straightforward answer to a problem. Is there any possibility to set a background color for different rows in a tabular environment? What I hope to get is easily done in OpenOffice for example: http://www.roda.ro/table.png Many, many thanks in advance, Adrian -- Adrian Dusa Romanian Social Data Archive 1, Schitu Magureanu Bd 050025 Bucharest sector 5 Romania Tel./Fax: +40 21 3126618 \ +40 21 3120210 / int.101

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Building Yi (and wider Cabal stuff)
by Michael T. Richter 19 Jun '07

19 Jun '07

I'm trying to build Yi (from the darcs repository) to take a look at it. The README that comes with it says "it's a standard Cabal project so do what you normally do" (paraphrased slightly). The problem is that I'm not a cabal user just yet and have no idea where to go from here. Just throwing caution to the wind and typing "make" built a setup program which was then executed and failed complaining about not having yi-lib0.3 (which I kind of thought the project should be building for me) and such. Editing the config.mk (which has some options left almost entirely undocumented) got me a step farther (specifically giving it a prefix that actually existed), but then had it choking out that it couldn't find the vty library -- even when I specifically told it to build the gtk version. Is there some painfully obvious thing I'm missing in trying to build this? And is it something that's applicable in the broader Cabal world or is it just this project? Compiling GHC (which is a lot bigger and more complex, I'd have guessed) was painless in comparison, so I'm kind of in the dark here. -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) If there's one thing that computers do well, it's to make the same mistake uncountable times at inhuman speed. (Peter Coffee)

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