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Re: [Haskell-cafe] Arrows for invertible programming: Notation, question
by Arjen 30 Dec '05

30 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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Re: [Haskell-cafe] GHC for Windows - no installer
by Paul Moore 29 Dec '05

29 Dec '05

On 12/29/05, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > Yes, it should work > http://www.haskell.org/ghc/docs/latest/html/users_guide/sec-install-wind > ows.html Thanks. Sorry I missed that link, and thanks for the pointer. Paul.

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How to read this syntax?
by David F. Place 29 Dec '05

29 Dec '05

Hi, I am trying to read _Arrows, Robots, and Functional Reactive Programming_ by Hudak, et al. http://www.haskell.org/yampa/AFPLectureNotes.pdf In section 2.1 there are a number of equations of the form: g’ :: SF A C g’ = arr g = arr f1 >>> arr f2 and i’ :: SF (A,C) (B,D) i’ = arr i = arr (f1 . fst) &&& arr (f2 . snd) = (arr fst >>> arr f1) &&& (arr snd >>> arr f2) I can't find any reference to the syntax where the function left hand side is just "space." I'd greatly appreciate any insight. -------------------------------- David F. Place mailto:d@vidplace.com

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Re: [Haskell-cafe] strange stack overflow with Data.Map
by David Roundy 29 Dec '05

29 Dec '05

On Thu, Dec 29, 2005 at 04:24:02PM +0100, Jean-Philippe Bernardy wrote: > On 12/29/05, David Roundy <droundy(a)abridgegame.org> wrote: > > On Thu, Dec 29, 2005 at 01:42:29PM +0100, Christian Maeder wrote: > > > David Roundy wrote: > > > >>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... > > > try: > > > > > > stats = foldl' add_elem Map.empty > > > add_elem m x = myinsertWith (+) x 1 m > > > > > > myinsertWith f k v m = > > > case Map.lookup k m of > > > Nothing -> Map.insert k v m > > > Just w -> let r = f v w in r `seq` Map.insert k r m > > > > Thanks, that does it! I had tried something like that, but without the > > foldl', and had tried foldl', but without the myinsertWith. The reason I > > didn't spend much time using this implementation is that for large maps, it > > will take twice as long as Map.insertWith, so I assumed (incorrectly) that > > Map.insertWith should be written so that it behaves in a non-leaky manner. > > I'm not sure I understand this ... Do you say that the leaky program > is faster than the non-leaky one ? No, what I mean is that Map.insertWith only traverses the Map once, while myinsertWith has to traverse it twice, so for a large map, each call to myinsertWith will take twice as long as Map.insertWith. Or to put it a different way, if myinsertWith were part of the Map module, it could be twice as fast. > > Should the Map modules have an additional Map.insertWith' that behaves > > strictly, or might it be the case that you always want strict behavior when > > using insertWith? > > IMO, there is always a possibility for needing the lazy version. I > also have to say that making a strict version for each "...With" does > not fill my heart with joy. :) Such cases call for cheapness analysis, > or optimistic evaluation, or some other kind of solution implemented > at the compiler level, IMHO. I agree, there is a possibility for wanting the lazy version. If you only want to provite one insertWith, I suppose you'd want to figure out whether the lazy or strict version is more commonly needed (or "safer"), and make people wanting the other version implement it themselves. In my opinion, the strict version seems nicer, largely because HNF is cheap for most "large" computations I do, so strictness wouldn't cost much. On the subject of excessive functions, what are the uses of insertWithKey? It seems like anything you do with insertWithKey you could just as efficiently do with insertWith... it seems pretty trivial to write insertWithKey f k x = insertWith (f k) k x There may be a use to all the WithKey variants, but I'd much rather have strict varients... -- David Roundy http://www.darcs.net

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GHC for Windows - no installer
by Paul Moore 29 Dec '05

29 Dec '05

I'd like to have GHC available to me, but some of the PCs I work on are extremely tight on space, or "locked down" in such a way that I can't install GHC on them. To get around this, I was considering copying GHC from my development PC onto a pen drive, and then using that (setting PATH when I needed access to GHC). My question is, will this work? Can the Windows install of GHC be copied around, or does it rely on registry entries or other system configuration in such a way that it has to be "properly" installed? Thanks, Paul.

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