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More Flexible Monad Transformer OR Bad Coding Style
by aditya siram 09 Aug '10

09 Aug '10

Hi all, I was experimenting with monad transformers and realized that the stacking order of the monads can remain unknown until it is used. Take for example the following code: import "mtl" Control.Monad.State import "mtl" Control.Monad.Writer import "mtl" Control.Monad.Identity test :: (MonadWriter [Char] m, Num t, MonadState t m) => m () test = do put 1 tell "hello" main = do x <- return $ runIdentity $ runStateT (runWriterT test) 1 -- test :: WriterT String (StateT Int Identity) y <- return $ runIdentity $ runWriterT $ runStateT test 1 -- test :: StateT Int (WriterT String Identity) z <- runWriterT $ runStateT test 1 -- test :: StateT Int (WriterT String IO) (((), Int), String) print x print y print z *Main> main (((),"hello"),1) (((),1),"hello") (((),1),"hello") Until test is called in 'main' we don't know the order of monads. In fact even the base monad is not know. All we know is that it uses the State and Writer monad. In each call to 'test' in main we can determine the stacking order and the base monad yielding different results. This seems to be a more flexible way of using monad transformers but I haven't seen this in code before so is there anything wrong with this style? -deech

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Some Haskell mode extensions
by Christopher Done 09 Aug '10

09 Aug '10

Hi there, I've written some very simple Emacs modules for making using Haskell in Emacs a little bit nicer: http://github.com/chrisdone/haskell-mode-exts You can download the project with git, or pick and choose individual files: http://github.com/chrisdone/haskell-mode-exts/raw/master/haskell-navigate-m… http://github.com/chrisdone/haskell-mode-exts/raw/master/haskell-align-impo… http://github.com/chrisdone/haskell-mode-exts/raw/master/haskell-sort-impor… http://github.com/chrisdone/haskell-mode-exts/raw/master/haskell-installed-… More documentation about installed module completion here, I need to do another module for syntax-aware import editing, but this module is already quite usable: http://www.chrisdone.com/posts/2010-08-07-emacs-haskell-installed-packages.… I have a few more in the works and more planned, e.g. (some of which may already have been implemented somewhere, but): 1) syntax aware import line editing (think: tabbing between sections, toggling qualified on/off, etc.) 2) controlling cabal through inferior-haskell-mode (already done this, but I need to make it portable) 3) tidying up of inferior-haskell (it wastes a lot of space) 4) more interactiveness with inferior-haskell e.g. "you need -XExtension" should really just prompt me whether I want to add it to the file or enable it in ghci, and then do it. (When you're in the middle of working, adding extensions to the top of the file is laborious.) 5) automatic integration with etags and some improvement to its little issues 6) a high-level .cabal file editing mode 7) unit testing integration And so on. There's loads that can be done. I'm just implementing the stuff I want, and if some of it's useful in general, maybe it can be absorbed into the general haskell-mode distrib. The haskell-emacs mailing list is down at the moment, so I thought I'd post here. Cheers!

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Re: [Haskell] ANNOUNCE: jhc 0.7.4
by Antoine Latter 09 Aug '10

09 Aug '10

On Sat, Jul 10, 2010 at 1:47 AM, John Meacham <john(a)repetae.net> wrote: > Announcing jhc 0.7.4! There have been a few major changes, the main one being > that there is now support for a garbage collector. This drastically increases > the number of programs which are feasable to compile with jhc. > > http://repetae.net/computer/jhc/ > Hi John, This looks fantastic and fun to play with. A few issues: * running DrIFT on src/E/TypeCheck.hs fails with an illegal bytesequence in hGetContents. I'm guessing that this is only an issue when building DrIFT with GHC 6.12+, and that the file contains bytes illegal in UTF8. I deleted everything funny looking in the file and then it went smooth * The way you use sed doesn't work with the BSD sed that ships with my Mac Book. Installing GNU sed and using it works. Similarly, BSD find doesn't know about '-name', so make hl-clean results in sadness. * jhci works great, but jhc crashes when I try to compile something: >>>>> jhc test1.hs jhc 0.7.4 (tokfekyuvi-27) Finding Dependencies... Using Ho Cache: '/Users/alatter/.jhc/cache' Main [test1.hs] Typechecking... [1 of 1] Main (.............................................) test1.hs:9 - Warning: defaulting: t93 => Jhc.Basics.Integer Compiling... [1 of 1] Main <..................................................> Collected Compilation... -- typeAnalyzeMethods -- BoxifyProgram -- Boxy WorkWrap -- LambdaLift E jhc: <stdout>: hPutChar: invalid argument (Illegal byte sequence) <<<<< Again, this seems like the handle is in UTF8 mode and we're trying to output something non-UTF8. * cabal install has a --jhc flag, but it doesn't seem to work: >>>>> cabal install byteorder --jhc Resolving dependencies... cabal: internal error: impossible <<<<< I have jhc installed in a non-standard location (under ${HOME}/usr) so I may need to have some environment variables set up. This is wil Cabal 1.8.0.4 and cabal-install 0.8.2 Keep up the good work! I have an implementation for STRefs I threw together this afternoon for jhc if you're interested. I can't test it properly, though, with the compiler crash above. Antoine

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Odd parallel haskell observations
by Alexander Kotelnikov 09 Aug '10

09 Aug '10

Hello. I am exploring haskell features for parallel and cocurrent programming and see something difficult to explain. In brief - asking RTS to use more threads results in awfull drop of performance. And according to 'top' test programm consumes up to N CPUs power. Am I doing something wrong? I attached the code, but I am just issuing thousands of HTTP GET requests in 1-4 forkIO threads. And since it looks like local apache is faster than haskell program (which is a pity) I expected that using more OS threads should improve performance. Just in case: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 import Data.List import System.IO import qualified System.IO.UTF8 import System.Environment (getArgs) import Network.HTTP import Network.URI import System.Time import System.IO.Unsafe import Control.Monad import Control.Exception import Control.Concurrent import Control.Concurrent.MVar secDiff :: ClockTime -> ClockTime -> Float secDiff (TOD secs1 psecs1) (TOD secs2 psecs2) = fromInteger (psecs2 - psecs1) / 1e12 + fromInteger (secs2 - secs1) -- single get get :: Int -> IO(String) get id = do res <- simpleHTTP $ getRequest "http://127.0.0.1" case res of Left err -> return(show err) Right rsp -> return(show $ rspCode rsp) -- perform GET per each list element using c threads doList :: [Int] -> Int -> IO() doList ids 0 = return() doList [] c = return() doList ids c = do forkChild $ forM_ todo get doList later (c-1) where (todo, later) = splitAt (length ids `div` c) ids {- Copied from http://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Conc… Terminating the program -} children :: MVar [MVar ()] children = unsafePerformIO (newMVar []) waitForChildren :: IO () waitForChildren = do cs <- takeMVar children case cs of [] -> return () m:ms -> do putMVar children ms takeMVar m waitForChildren forkChild :: IO () -> IO ThreadId forkChild io = do mvar <- newEmptyMVar childs <- takeMVar children putMVar children (mvar:childs) forkIO (io `finally` putMVar mvar ()) -- end of copied code main = do [c', n'] <- getArgs let c = read c' :: Int n = read n' :: Int start <- getClockTime doList [1..n] c waitForChildren end <- getClockTime putStrLn $ show(c) ++ " " ++ show(secDiff start end) ++ "s" 20:31 sacha@loft4633:/tmp 21> ghc --make -threaded get.hs [1 of 1] Compiling Main ( get.hs, get.o ) Linking get ... 20:31 sacha@loft4633:/tmp 22> ./get 1 10000 1 3.242352s 20:31 sacha@loft4633:/tmp 23> ./get 2 10000 2 3.08306s 20:31 sacha@loft4633:/tmp 24> ./get 2 10000 +RTS -N2 2 6.898871s 20:32 sacha@loft4633:/tmp 25> ./get 3 10000 3 2.950677s 20:32 sacha@loft4633:/tmp 26> ./get 3 10000 +RTS -N2 3 7.381678s 20:32 sacha@loft4633:/tmp 27> ./get 3 10000 +RTS -N3 3 14.683548s 20:32 sacha@loft4633:/tmp 28> ./get 4 10000 4 3.332165s 20:33 sacha@loft4633:/tmp 29> ./get 4 10000 +RTS -N4 -s ./get 4 10000 +RTS -N4 -s 4 57.17923s 2,147,969,912 bytes allocated in the heap 49,059,288 bytes copied during GC 736,656 bytes maximum residency (98 sample(s)) 486,744 bytes maximum slop 5 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 949 collections, 948 parallel, 76.73s, 25.67s elapsed Generation 1: 98 collections, 98 parallel, 7.70s, 2.56s elapsed Parallel GC work balance: 2.17 (6115428 / 2822692, ideal 4) MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 1.43s ( 27.76s) 6.31s ( 2.12s) Task 1 (worker) : 0.00s ( 28.13s) 10.62s ( 3.56s) Task 2 (worker) : 0.37s ( 28.63s) 11.06s ( 3.69s) Task 3 (worker) : 0.00s ( 28.95s) 6.29s ( 2.10s) Task 4 (worker) : 20.73s ( 28.95s) 9.68s ( 3.24s) Task 5 (worker) : 0.00s ( 28.95s) 0.60s ( 0.20s) Task 6 (worker) : 21.81s ( 28.95s) 11.91s ( 3.97s) Task 7 (worker) : 18.59s ( 28.95s) 13.04s ( 4.36s) Task 8 (worker) : 17.24s ( 28.96s) 14.92s ( 4.99s) SPARKS: 0 (0 converted, 0 pruned) INIT time 0.00s ( 0.00s elapsed) MUT time 79.23s ( 28.95s elapsed) GC time 84.43s ( 28.23s elapsed) EXIT time 0.00s ( 0.01s elapsed) Total time 162.49s ( 57.19s elapsed) %GC time 52.0% (49.4% elapsed) Alloc rate 27,513,782 bytes per MUT second Productivity 48.0% of total user, 136.5% of total elapsed gc_alloc_block_sync: 15006 whitehole_spin: 0 gen[0].steps[0].sync_large_objects: 7617 gen[0].steps[1].sync_large_objects: 35 gen[1].steps[0].sync_large_objects: 1400

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Suggestions For An Intro To Monads Talk.
by aditya siram 09 Aug '10

09 Aug '10

Hi all, I am doing an "Intro To Monads" talk in September [1]. The audience consists of experienced non-Haskell developers but they will be familiar with basic functional concepts (closures, first-class functions etc.). I am looking for suggestions on how to introduce the concept and its implications. I'd also like to include a section on why monads exist and why we don't really see them outside of Haskell. Has anyone here done a talk like this? And if so what parts of your presentation were successful and what would you stay away from. Thanks for the feedback. -deech [1] It's in St.Louis, Missouri at the St.Louis Perl Mongers meeting<http://St.Louis%20Perl%20Mongers%20meeting>so come on by if you're around!

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preparing for Haskell 2011
by Malcolm Wallace 09 Aug '10

09 Aug '10

Dear all, Although the Haskell 2010 Language Report has only been published recently, it will soon be time for the Committee to make decisions on the next version, Haskell 2011. I am aiming for the committee to make decisions around the end of Sept or beginning of October 2010. Can I therefore encourage any people who have made proposals, either informally on mailing lists, or formally in the Haskell-prime ticket system, to consider what they need to do to bring those proposals to a state where the committee can vote on them. Perhaps you have not made such a proposal yourself, but are very keen that someone else's proposal be adopted. Work with the proposer to polish it! Here is what you need to know about the proposal process: http://hackage.haskell.org/trac/haskell-prime/wiki/Process Please note especially that the key requirement that will bring your proposal to the attention of the committee for a decision is the "Report delta", which describes the changes with exact precision. Regards, Malcolm

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Socket not released
by Jonathan Geddes 09 Aug '10

09 Aug '10

Cafe, I'm writing a network application that uses static configuration a la xmonad and yi. When the app receives a certain command it recompiles its source, closes the socket it is using and runs its newly compiled "predecessor" as a new process. The problem I'm having is that the port that the parent process was using is not available to the child process. Even though the parent process has terminated, the port is unusable until the child process also terminates. Can anyone give me a clue about what's going on here? I'm using Network and System.Process modules and ruining on Ubuntu linux. I'm using the sClose function to close the socket. Thanks for any tips. --Jonathan

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Re: [Haskell] ANNOUNCE: jhc 0.7.4
by Simon Marlow 09 Aug '10

09 Aug '10

On 27/07/2010 01:54, John Meacham wrote: > For each type I can statically generate an "optimal" layout based on its > structure. For instance, maybe benefits from two of these optimizations, > first of all, nullary constructors (Nothing) need never appear in the > heap, so they are given values that pack directly into a word, this > happens for all nullay constructors in general. Then, since there is > only one constructor left (Just) we can discard the tag field, because > if something of type Maybe, is in WHNF, and is not Nothing then in must be > a Just. so the Just is a single heap allocated word (which are packed > end to end in a page with no overhead) > > I am refining the optimization algorithm, on 64 bit machines I have > another few bits to play with for instance that I can take advantage of. > > A particularly nice case is that characters can be fully integreated > into the word, so the entire space usage of > > "Hello" is 10 words as lists benefit from the same packing benefits as > Maybe. I should point out that in GHC "Hello" requires 15 words: 3 for each list cell, and the Chars are free because they're statically allocated in the RTS. A Just requires 2 words (one more than JHC, I guess). Tantalisingly, it's almost possible to represent a Just with zero words - the pointer to the Just points directly to the element - but there's nowhere to put the tag bits for the element pointer (the tag bits are for the Just). A Nothing in GHC takes zero words, as do all nullary constructors. I wrote up a summary of the GHC heap representation on Stack Overflow recently: http://stackoverflow.com/questions/3254758/memory-footprint-of-haskell-data… Cheers, Simon > compare this to 30 or so words used in a traditional "everything is on > the heap and tagged" model. > > > The manual has a section describing the RTS, it doesn't describe the GC > though, but talks about how I pack things in the pointers. > > http://repetae.net/computer/jhc/manual.html#the-run-time-system > > John >

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Typo or on purpose? http://haskell.org/ghc/docs/6.12.2/html/users_guide/lang-parallel.html
by Alexander Kotelnikov 09 Aug '10

09 Aug '10

This + 1 in (n1 + n2 + 1) what is it doing there? import Control.Parallel nfib :: Int -> Int nfib n | n <= 1 = 1 | otherwise = par n1 (pseq n2 (n1 + n2 + 1)) where n1 = nfib (n-1) n2 = nfib (n-2) BTW, same code can be found in "Seq no more: Better Strategies for Parallel Haskell" article.

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a random numbers generator with a good 'split'
by deb＠pudlak.name 09 Aug '10

09 Aug '10

Hi cafe. recently I was looking at various possibilities of generating random numbers in Haskell. Apparently, the main problem is the 'split' function in the System.Random.RandomGen class. Since it is rarely needed in imperative programs, not much is known how to implement it safely. It's documentation says: > The split operation allows one to obtain two distinct random number > generators. This is very useful in functional programs (for example, > when passing a random number generator down to recursive calls), but > very little work has been done on statistically robust implementations > of split ([System.Random, System.Random] are the only examples we know > of). I consulted it with a friend, who has done much work in the area of random number generators. He suggested that this might be implemented using a random number generator with a very large period, such as Mersene Twister (MT19937 has a period of 2^19937-1). A pair of splitted generators states could be positioned sufficiently far enough from each other so that in practice they never reach each other, and in such a way that successive splits (in practice) never meet. I searched Hackage, where I found a few implementations of Mersene Twister, but AFAIK no one actually implements neither RandomGen nor 'split' alone. I wonder why -- is RandomGen inconvenient or obsolete in some way? Or just nobody tried? With best regards, Petr Pudlak PS: Sorry if you get multiple copies, I had problems sending the mail.

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