December 2005 - Haskell-Cafe

Re: Haskell Speed
by Isaac Gouy 31 Dec '05

31 Dec '05

--- Isaac Gouy <igouy(a)yahoo.com> wrote: > We'll be happy to also show a Haskell program that > uses Data.HashTable - first, someone needs to > contribute that program. Someone did: k-nucleotide Haskell GHC #2 http://shootout.alioth.debian.org/gp4/benchmark.php?test=knucleotide&lang=a… __________________________________ Yahoo! for Good - Make a difference this year. http://brand.yahoo.com/cybergivingweek2005/

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Can this be improved?
by Chris Kuklewicz 30 Dec '05

30 Dec '05

Happy Holidays, I was wondering if it this small snippet of code could be altered to require fewer OPTIONS -fallow-... switches. It creates a show-what-i-mean function called "swim" that takes a variable number of arguments, and treats strings as-is, calling show on the other arguments. > {-# OPTIONS -fglasgow-exts #-} > {-# OPTIONS -fallow-undecidable-instances #-} > {-# OPTIONS -fallow-incoherent-instances #-} > {-# OPTIONS -fallow-overlapping-instances #-} > > -- Want to show strings "as-is" > > class (Show a) => MyShow a where > myShow :: a -> String > myShow = show > > instance MyShow String where > myShow = id > > instance (Show b) => MyShow b > > -- Want to take variable number of arguments > > class PVShow a r where > pvShow :: String -> a -> r > > instance (MyShow b) => PVShow b String where > pvShow oldString b = oldString ++ (myShow b) > > instance (MyShow b,PVShow a r) => PVShow b (a->r) where > pvShow oldString b = pvShow (oldString ++ (myShow b)) > > -- swim is short for Show What I Mean > > swim :: (PVShow a r) => a -> r > swim = pvShow "" > > -- Demonstates of polymorphic and first class nature of swim > > test foo = foo " and " 7 " are " > > -- Tests > > main = do putStrLn $ swim "Hello" " " "World #" [17,18,19] "!" > putStrLn $ test (swim "I also think " [4,5,6]) "cool" "." GHCI session: Main> main Hello World #[17,18,19]! I also think [4,5,6] and 7 are cool. *Main> Could this be improved by TypeEq machinery? Is there a Haskell 98 way to make myShow work this way? -- Chris Kuklewicz

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binary IO
by Pupeno 30 Dec '05

30 Dec '05

It seems I have found a hole in Haskell... :( Before I start to develop a library with functions such us those on http://haskell.org/hawiki/BinaryIo (hGetWord8, hGetWord16le, hGetWord16be, etc), is there some reliable library that can help me ? I basically need a set of functions to read binary data out of a Handle (a higher lever of hGetBuf and hPutBuf). What I am doing is implementing a DNS server, so, my typical need is: First two bytes are an integer. The next bit is a boolean. So are the following three bits. Then I have 4 bits which is also an integer. Etc. Any help is greatly appreciated. Thank you. -- Pupeno <pupeno(a)pupeno.com> (http://pupeno.com)

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module names
by Scherrer, Chad 29 Dec '05

29 Dec '05

Something about the way module names are specified in a file seems really strange to me. When I first started learning Haskell (I had used OCaml previously), I tried things like module Main where import A import B main = A.f >> B.f module A where f = ... module B where f = ... in a single file. This example is straight from chapter 5 of the Report, and no mention is made (that I could find) about modules needing to be in separate files. But this won't load in ghci! (Even if ... is changed to putStr "hi"). Eventually I figured out that it works fine if it's split over three separate files. So here's what I'm trying to figure out: If every file corresponds to exactly one module (is that true?), then why must the module name be given again in the text of the file? When I'm using ghci, I have lots of modules that I sometimes want to load "as Main", and sometimes I only want them loaded as a dependency from another module. Currently, I have to go into each file to change the "module Foo where" line to do this. Chad Scherrer

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Re: [Haskell-cafe] Arrows for invertible programming: Notation, question
by Arjen 29 Dec '05

29 Dec '05

On Fri, 23 Dec 2005, Joel Reymont wrote: > Folks, > > I have been looking at the code for the "Arrows for invertible > programming" paper (http://www.cs.ru.nl/A.vanWeelden/bi-arrows/) and > I have a question about syntax. ghci surely does not like it. > I've updated the web page to say that is does not work with Hugs, GHCi or GHC -fgenerics, sorry for the confusion. It really requires Generic Haskell (GH) 1.42. I never got GH working with GHC 6.4, so you also really need GHC 6.2.2. I think you can run the result of the GH preprocessor (using the right command line options) in GHCi or Hugs, but I'm not sure. If anyone got GH working with GHC 6.4(+), please tell me how you did it. > What does this mean and how do I make it compile? > > mapl{|a, b|arr|} :: (mapl{|a, b|arr|}, ArrowChoice arr, BiArrow arr) > => arr a b > In GH this means: mapl is a polytypic (type-indexed in a and b) function that yields an arr(ow) from a to b. It is overloaded in ArrowChoice and BiArrow on the arr(ow). Because it is polytypic, it is also overloaded `in itself'. Most of the time GH does not require the last part explicitly, but sometimes it triggers a bug if you don't specify it. The semantics of mapl can be found in the paper. regards, Arjen > Thanks, Joel > > -- > http://wagerlabs.com/ >

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Mixing IO and STM
by Quan Ta 29 Dec '05

29 Dec '05

Hello folks, Newbie question: how can I do something like the following? mixing IO and STM. module Test where import System.Random import Control.Concurrent import Control.Concurrent.STM test_Cubby = do tv <- newTVar 0 forkIO (producer tv) >> (consumer tv) where producer tv = do r <- randomRIO (1,10) atomically (do { v <- readTVar tv ; writeTVar tv (v+r) }) print ("insert " ++ show r) producer tv return () consumer tv = do r <- randomRIO (1,10) atomically (do { v <- readTVar tv ; if (v < r) then retry else writeTVar tv (v-r) }) print ("consume " ++ show r) consumer tv return () Thanks.

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RE: [Haskell-cafe] Joels Time Leak
by Simon Peyton-Jones 29 Dec '05

29 Dec '05

Thanks for looking, Adrian, It'd be great if someone was able to find out more about what's going on here. Bandwidth at GHC HQ is always tight, so the more precisely someone can pinpoint what's happening, the faster we can fix it. Joel has done a lot by making a repro case. Maybe others can help to narrow it down? Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Adrian Hey | Sent: 29 December 2005 13:03 | To: haskell-cafe(a)haskell.org | Subject: [Haskell-cafe] Joels Time Leak | | Hello, | | I haven't followed everything that's happened on the Binary IO | thread, but has anybody else actually tried Joels code? .. | | http://wagerlabs.com/timeleak.tgz | | I can reproduce the problem (ghc/Linux), but can't explain it. It | seems very strange that friggin about with an otherwise irrelevant | (AFAICT) MVar fixes the problem. | | Regards | -- | Adrian Hey | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Speed
by Daniel Carrera 29 Dec '05

29 Dec '05

Hi all, I'm taking a look at the "Computer Language Shootout Benchmarks". http://shootout.alioth.debian.org/ It looks like Haskell doesn't do very well. It seems to be near the bottom of the pile in most tests. Is this due to the inherent design of Haskell or is it merely the fact that GHC is young and hasn't had as much time to optimize as other compilers? For example, another very slow language is Ruby. In Ruby's case, there is a design factor that will always make it slow. I wonder if Haskell is in a smilar situation. Yes yes, I know that a high level language trades CPU time by programmer time. I'm still interested in the CPU time question though :) Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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Haskell vs OCaml
by Daniel Carrera 29 Dec '05

29 Dec '05

Hi all, Okay, I've been studying Haskell for a few days and I think I have a feel for the language. I completted a course, and wrote a Revers Polish Notation calculator. Now that I sort of understand Haskell, I was hoping someone could give me an opinion on OCaml from a Haskell POV. Both are functional and have fairly clear syntax. OCaml is supposed to be very fast, and Haskell perhaps is not. Besides that, what else could you say about them? What would make someone prefer one over the other? Any thoughts, opinions and comments would be most appreciated. Thanks! Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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strange stack overflow with Data.Map
by David Roundy 29 Dec '05

29 Dec '05

Hi all, I've got a problem that I'm seeing using either Data.Map or Data.IntMap. > module Main where > import Data.List > import qualified Data.IntMap as Map > stats elems = foldl add_elem Map.empty elems > add_elem m x = Map.insertWith (+) x 1 m > main = print $ stats $ take 1000000 $ repeat 1 This program has a space leak and runs out of stack space. I'm guessing that I'm being bit here by an unnatural amount of laziness in Map.insertWith, but I can't see how to fix this. :( I'm sure it's something elementary... I tried defining add_elem m x = let m' = Map.insertWith (+) x 1 m Just num = Map.lookup x m' in seq num m' to force the (+) to be evaluated strictly, but that didn't help. -- David Roundy http://www.darcs.net

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