January 2007 - Haskell-Cafe

IO in lists
by Magnus Therning 31 Jan '07

31 Jan '07

Not a very descriptive subject, I know, but here's what I'd like to do. I can take getChar and create an infinate list: listChars = getChar : listChars but how do I go about creating a finite list, e.g. a list that ends as soon as 'q' is pressed? I was thinking of something like listChars1 = do c <- lift getChar if c == 'q' then [return c] else [return c] ++ listChars1 However, that triggers an interesting behaviour in ghci: *Main> :t listChars1 <interactive>:1:0: Can't find interface-file declaration for listChars1 Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error Compiling it doesn't work either: % ghc -o listio listio.hs listio.o: In function `Main_main_info': (.text+0x1bd): undefined reference to `Main_listChars1_closure' listio.o: In function `Main_main_srt': (.rodata+0x10): undefined reference to `Main_listChars1_closure' collect2: ld returned 1 exit status I was also looking briefly at ListT, bout couldn't quite see how to use basic list operations on it, e.g. ':' '++' etc. listChars2 :: ListT IO Char listChars2 = do c <- lift getChar if c == 'q' then lift $ return c else (lift $ return c) ++ listChars2 /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Microsoft has a new version out, Windows XP, which according to everybody is the 'most reliable Windows ever.' To me, this is like saying that asparagus is 'the most articulate vegetable ever.' -- Dave Barry

10 28

Replacing [a] with (Set c a) in Monad instance.
by Daniel McAllansmith 30 Jan '07

30 Jan '07

Hello. Given: newtype Dist a = D {unD :: [(a,Int)]} instance Monad Dist where return x = D [(x,1)] d >>= f = D [(y,q*p) | (x,p) <- unD d, (y,q) <- unD (f x)] fail _ = D [] How would one change Dist to wrap an instance of the (Data.Edison.Set c a) typeclass so that the Monad instance could be implemented in terms of e.g. singleton, unionWith, empty, etc? Thanks Daniel

3 3

Re: How did you stumble on Haskell?
by Chad Scherrer 30 Jan '07

30 Jan '07

I started using Python around 1998, and I loved the elegance of it, but I was frustrated with the performance. Around 2001 I was poking around for something that would give me better performance but still allow some nice capabilities for abstraction. I started using OCaml after reading a commentary from one of the Shootout guys that he had been impressed with it. I loved the performance and the parametrized modules, but the lack of overloading was frustrating. Once I was looking for a nice rule of thumb for when to use foldl vs. foldr, and was disappointed to hear "Just use foldl - it's tail recursive." Maybe there's a place for folding right in a strict language, but I haven't seen it. Once I was deep into OCaml, Haskell kept coming up, since so many people using OCaml also like Haskell. Seeing some elegance in "A Gentle Introduction to Haskell" and not immediately understanding it got me hooked right away. And there's a time to fold right! Chad

1 0

proposal: HaBench, a Haskell Benchmark Suite
by Kenneth Hoste 30 Jan '07

30 Jan '07

Hello, Following up and the threads on haskell and haskell-cafe, I'd like to gather ideas, comments and suggestions for a standarized Haskell Benchmark Suite. The idea is to gather a bunch of programs written in Haskell, and which are representative for the Haskell community (i.e. apps, libraries, ...). Following the example of SPEC (besides the fact that the SPEC benchmarks aren't available for free), we would like to build a database containing performance measurements for the various benchmarks in the suite. Users should be able to submit their results. This will hopefully stimulate people to take performance into account when writing a Haskell program/library, and will also serve as a valuable tool for further optimizing both applications written in Haskell and the various Haskell compilers out there (GHC, jhc, nhc, ...). This thread is meant to gather peoples thought on this subject. Which programs should we consider for the first version of the Haskell benchmark suite? How should we standarize them, and make them produce reliable performance measurement? Should we only use hardware performance counters, or also do more thorough analysis such as data locality studies, ... Are there any papers available on this subject (I know about the paper which is being written as we speak ICFP, which uses PAPI as a tool). I have created a HaskellWiki page (http://www.haskell.org/haskellwiki/ HaBench) in order to centralize ideas and suggestions. Feel free to add anything, and if you're willing to contribute to this project (in any way), add your name to the wiki for future reference. greetings, Kenneth (a.k.a. boegel) -- Statistics are like a bikini. What they reveal is suggestive, but what they conceal is vital (Aaron Levenstein) Kenneth Hoste ELIS - Ghent University kenneth.hoste(a)elis.ugent.be http://www.elis.ugent.be/~kehoste

11 18

Re: [Haskell] [Fwd: Re: Computer Language Shootout]
by dons＠cse.unsw.edu.au 30 Jan '07

30 Jan '07

simonmarhaskell: > Forwarding on behalf of Andrzej Jaworski <himself(a)poczta.nom.pl>: > > -------- Original Message -------- > From: Andrzej Jaworski <himself(a)poczta.nom.pl> > > Dear fellows, > > It is ironic that just after SPJ disclosed Comments from Brent Fulgham on > Haskell and the shootout the situation has radically changed for the worse. > Without knowing that I committed a blunder referring to the damn benchmark > http://shootout.alioth.debian.org/gp4/benchmark.php?test=all&lang=all > together with multicore support arguments when trying to convert a prominent > OCaml programmer to Haskell. Now they know more of us:-) > > What a language it is that jumps 30% up and down on benchmark while other > languages gracefully stay in line? Can any of you explain the reason for > this disgraceful degradation in Computer Language Shootout? The degradation is due to two things: * several entries have been disqualified (some fairly, some unfairly) Fix: fix is to submit more * the shootout haskellers stopped submitting once it was clear we'd need bytestrings to do signifcantly better. Fix: bytestring is now on the shootout machine, so we can resume I don't think there is any fundamental issue exposed, other than that we stopped updating entries, while other languages decided they should try harder. If people want to contribute, improve (or write) ByteString based versions for the slow running String programs, upload them to the wiki, and to the shootout page, http://www.haskell.org/haskellwiki/Shootout For things that have been disqualified, correct the errors, and resubmit. -- Don

7 11

are Monads with slightly stricter types in instances still Monads?
by Julien Oster 30 Jan '07

30 Jan '07

Hello, The type of the monadic bind function in the Monad class is Monad m => m a -> (a -> m b) -> m b Now, would it be possible to create a monad with a slightly stricter type, like StrictMonat m => m a -> (a -> m a) -> m a and, accepting that all steps of the computation would be bound to operate on the same type, would this be without any undesirable implications? For the sake of understanding monads better, I tried to write several custom monads which may or may not be useful. Among those were: * The Tracker Monad - tracks every result of every step of the sequential computation in a (normal, stricly typed) list inside of the monad * The Goto Monad - sequential computation that allows restarts of the computation at arbitrarily set labels within it But Haskell doesn't like those. Rightly so, because the bind function would have the stricter type mentioned above. [Otherwise the Tracker monad would have to store values of different types in its list and the Goto monad would encounter restarts at labels that process different types of the value than what has been computed so far. Both doesn't make sense.] I still have to prove wether those two monads follow the monadic laws at all, but that's part of my exercise. But let's say they follow the laws (I'm pretty sure that at least the Tracker monad does), is there anything else that would prevent the stricter typing from being legal or useful? Maybe I'm missing something simple. And would I still be able to use Haskell's "do" syntax? My first guess is yes, because it really just seems to translate into normal syntax. Thanks, Julien

2 1

Re: [Haskell-cafe] How to write elegant Haskell programms?
by John Hughes 30 Jan '07

30 Jan '07

> >I think that whole "program flow" thing is something you get used to. In > > >>true, pure functional programming (i.e. Haskell) "program flow" is a >>meaningless term, basically. Haskell is a declarative language, not an >>imperative one. You have to learn to give up that control and trust the >>runtime to Do The Right Thing. (In this regard it's similar to logic >>programming languages.) >> >> >> > >I think it's important/useful to point out that "program flow" in a pure >functional language is really a matter of data dependency. The compiler is >only free to arbitrarily order computations if there are no data >dependencies. Furthermore, monads are not special in any way (they are after >all just a useful set of combinators; e.g. >http://sigfpe.blogspot.com/2006/08/you-could-have-invented-monads-and.html) >they only wind up sequencing computations because they set up a data >dependency between the two arguments of the bind operator. > >-Jeff > And actually they don't even do that, always. A useful example in practice: then Gen monad in QuickCheck does *not* necessarily set up any data dependencies, so do in the Gen monad does not force sequencing. The fact that it's non-strict is what enables us to generate infinite random data-structures with it. John Hughes

1 0

GHC-Cygwin
by Daniil Elovkov 30 Jan '07

30 Jan '07

Hello Haskellers My desire is simple. I want GHC-Cygiwn. I spent quite a lot of time struggling, trying to build ghc-6.6-Cygwin with ghc-6.4.1-mingw32. After having patched a lot of #ifdefs and the like I finally did it. Threaded RTS doesn't seem to work, but that's ok. And all that only to eventually realise that it crashes compiling non-trivial programs. The message is 3 [main] ghc-6.6 3648 _cygtls::handle_exceptions: Error while dumping state (probably corrupted stack) I will probably fight it again some time, but now while I'm taking a rest, whould anybody be so kind, who has a working GHC-Cygwin, package it and put somewhere so that I can just take it. I suppose that many people will appreciate it. Thank you.

3 4

Simple HTTP lib for Windows?
by Greg Fitzgerald 30 Jan '07

30 Jan '07

I'd like to write a very simple Haskell script that when given a URL, looks up the page, and returns a string of HTML. I don't see an HTTP library in the standard libs, and the one in Hackage requires Windows machines have GHC and MinGW to be installed and in the PATH. Is there a simple way to get the contents of a webpage using Haskell on a Windows box? Thanks, Greg

8 16

How to write elegant Haskell programms? (long posting)
by Michael Roth 30 Jan '07

30 Jan '07

Hello list, I'm new to Haskell and I'm trying to learn how to write elegant code using Haskell. I decided to convert the following small tool, written in ruby: =========================================================== #! /usr/bin/env ruby require 'pathname' BASENAMES = %w{ mail.log thttpd.log } ARCHIVEDIR = Pathname.new '/var/log/archive' LOGDIR = Pathname.new '/var/log' class Pathname def glob glob_pattern Pathname.glob self.join(glob_pattern) end def timestamp stat.mtime.strftime '%Y%m%d' end end for basename in BASENAMES for oldname in LOGDIR.glob "#{basename}.*.gz" newname = ARCHIVEDIR.join "#{basename}.#{oldname.timestamp}.gz" puts "mv #{oldname} #{newname}" File.rename oldname, newname end end =========================================================== My solution in Haskell is: =========================================================== import System.Directory (getDirectoryContents, getModificationTime, renameFile) import System.Locale (defaultTimeLocale) import System.Time (ClockTime, toUTCTime, formatCalendarTime) import Text.Regex (mkRegex, matchRegex) import Maybe import Control.Monad logdir, archivedir :: String logfiles :: [String] logfiles = [ "mail.log", "thttpd.log" ] logdir = "/var/log" archivedir = "/var/log/archive" basename :: String -> String basename filename = head . fromMaybe [""] $ matchRegex rx filename where rx = mkRegex "^(.+)(\\.[0-9]+\\.gz)$" isLogfile :: String -> Bool isLogfile filename = basename filename `elem` logfiles timestamp :: ClockTime -> String timestamp time = formatCalendarTime defaultTimeLocale "%Y%m%d" (toUTCTime time) makeOldname :: String -> String makeOldname fn = logdir ++ '/' : fn makeNewname :: String -> String -> String makeNewname bn ts = archivedir ++ '/' : bn ++ '.' : ts ++ ".gz" move :: String -> String -> IO () move oldname newname = do putStrLn $ "mv " ++ oldname ++ ' ' : newname renameFile oldname newname main :: IO () main = do files <- liftM (filter isLogfile) (getDirectoryContents logdir) let oldnames = map makeOldname files times <- mapM getModificationTime oldnames let newnames = zipWith makeNewname (map basename files) (map timestamp times) zipWithM_ move oldnames newnames =========================================================== Ok, the tool written in Haskell works. But, to me, the source doesn't look very nice and even it is larger than the ruby solution, and more imporant, the programm flow feels (at least to me) not very clear. Are there any libraries available to make writing such tools easier? How can I made the haskell source looking more beautiful? Michael Roth

5 6