July 2005 - Haskell - Haskell.org

Haskell-mode 2.0
by Stefan Monnier 02 Feb '09

02 Feb '09

I have recently taken over maintainership of Haskell-mode, and after making a bunch of changes, I figured it would be a good idea to make a new release. You can find this new release at: http://www-perso.iro.umontreal.ca/~monnier/elisp/ If you think it should also be found at http://www.haskell.org/haskell-mode/, then please tell John Peterson to get back to me (my email to him seem to go straight to the bitbucket. John?). This release has several changes that have all been only lightly tested, especially w.r.t compatibility with various Emacsen. See appended the NEWS file that describes some of the notewirthy changes. Stefan Changes since 1.45: * keybindings C-c <char> have been replaced by C-c C-<char> so as not to collide with minor modes. * The following modules are now automatically activated without having to add anything to haskell-mode-hook: haskell-font-lock (just turn on global-font-lock-mode). haskell-decl-scan (just bind `imenu' to some key). * In recent Emacsen, haskell-doc hooks into eldoc-mode. * haskell-hugs and haskell-ghci are superceded by inf-haskell. * Indentation rules have been improved when using layout inside parens/braces. * Symbols like -> and \ can be displayed as actual arrows and lambdas. See haskell-font-lock-symbols. * Tweaks to the font-lock settings. Among other things paren-matching with things like \(x,y) should work correctly now. * New maintainer <monnier(a)gnu.org>.

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Mixing monadic and non-monadic functions
by Sean E. Russell 17 Sep '05

17 Sep '05

Hello, I posted this question to comp.lang.functional, and someone suggested that I try this group instead. I'm struggling with monads. Well, not monads themselves, but mixing them with non-monadic functions. Here's my base case: someFunc :: String -> IO [a] ... ax <- someFunc a bx <- someFunc b assertBool "fail" $ length ax == length bx I don't like the assignments; the typing is redundant, if I have a lot of asserts like this, and the "variables" are temporary. What I'd much rather have is: ... assertBool "fail" $ (length $ someFunc a) == (length $ someFunc b) which is more readable, to my eye. The only solution which has been suggested that may work is liberal use of the liftM variants, but this gets *really* tedious and obtuse. Is there an elegant way to do what I want to do, or am I stuck with procedural-style assignments and bunches of temp vars? Thanks! -- ### SER ### Deutsch|Esperanto|Francaise|Linux|XML|Java|Ruby|Aikido ### http://www.germane-software.com/~ser jabber.com:ser ICQ:83578737 ### GPG: http://www.germane-software.com/~ser/Security/ser_public.gpg

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problems with RegEx
by Srinivas Nedunuri 03 Aug '05

03 Aug '05

hello, I'm trying to use the Text.Regex library from Hugs. However, Hugs doesn't recognize any of the methods in there. For example if I try to use the mkRegex function I get the error message "Undefined variable "mkRegex". I have imported Text.Regex into the module. I looked at the contents of the module in the HUGS library but couldnt make out too much. It looked basically empty although Text.Regex.Posix had some includes. Any help would be appreciated cheers

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Files and lazyness
by Diego y tal 01 Aug '05

01 Aug '05

I was developing a web site using haskell programs as cgi's, and I found a strange behavior that I would like to know whether it is normal. I have reduced the "problem" to the next program: fEntrada = "fich.txt" fSalida = "fich.txt" creaFich :: IO() creaFich = writeFile fEntrada "me molo" main :: IO () main = do x <- readFile fEntrada -- print x -- In the second try, uncomment this line writeFile fSalida "" writeFile fSalida x Running the next commands (suposing that $ is the prompt of a linux shell and main> is the prompt of hugs) main> creaFich main> main $ cat fich.txt will give us different results if we comment or uncomment the second line of the main body, although the meaning of the program is the same. I understand that this is caused by the lazyness, that doesn't evaluate the expression "x <- readFile fEntrada" until it's necessary, but.. is it normal that we have to think about this "problem" when programming? In my case, this behavior caused my program to fail (and it was really complicated to find why) and the only solution I found was indeed printing to a scratch file the string I was reading inmediatly after reading it (really, after telling hugs to read it). I find this a great disadvantage as oppose of the imperative paradigm, overall because is like having to control something (near to concurrence if you ask me) that has not been asked for! I 'll be thankful of any comments or replies you send to me. Greetings, Diego y tal <deigote(a)gmail.com>

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strict vs. lazy state threads
by Wolfgang Jeltsch 31 Jul '05

31 Jul '05

Hello, http://haskell.org/ghc/docs/latest/html/libraries/base/Control.Monad.ST.html says about the module Control.Monad.ST: This library provides support for strict state threads, as described in the PLDI '94 paper by John Launchbury and Simon Peyton Jones Lazy State Threads. I cannot find a paper Lazy State Threads in the list of papers on Simon Peyton Jones' website but I assume that the library documentation refers to Lazy *Functional* State Threads. The problem is that I cannot find an exact specification about what strict and lazy means in conjunction with state threads. I thought that the (only) difference between strict and lazy state threads is that with strict state threads the result is _|_ if the final state is _|_ while with lazy state threads the result may be non-_|_ even if the final state is _|_. Operationally, this would mean that with strict state threads the final state would always be demanded which wouldn't be true for lazy state threads. Is this correct? But if it's correct, I don't understand why the library documentation claims that the above-mentioned paper describes strict state threads since from my studies of the paper I came to the conclusion that it describes lazy state threads. Any helpful comments are appreciated. Regards, Wolfgang

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RE: [Haskell] combining IntMaps
by Scherrer, Chad 31 Jul '05

31 Jul '05

Thanks! It's interesting the way your AVL tree library is set up -- there seems to be a much broader degree of functionality than that provided by Data.Set. But I'm trying to see, is there a significant difference in the fundamental data structure, or is the main point that the additional functionality could not have otherwise been provided in an efficient way without going into the guts of Data.Set? Chad Scherrer -----Original Message----- From: Adrian Hey [mailto:ahey@iee.org] Sent: Wednesday, July 20, 2005 10:57 PM To: Scherrer, Chad; haskell(a)haskell.org Subject: Re: [Haskell] combining IntMaps On Wednesday 20 Jul 2005 4:05 am, Scherrer, Chad wrote: > I'm using the (IntMap Int) type to implement functions (Int -> Int), > by treating non-keys as values that map to zero. I'd like to be able > to add two of these pointwise, and delete the key from the resulting > map when the sum of the values is zero. My specification is > > addMaps :: IntMap Int -> IntMap Int -> IntMap Int addMaps m = > IntMap.filter (/= 0) . IntMap.unionWith (+) m > > But I'm not really happy with this because it traverses both maps for > the union, and then traverses the result to get rid of all the zeros. > (This function is a performance bottleneck in my current code). Examples like this are interesting because they show just how difficult it is produce a comprehensive library for even one common or garden data structure. I thought my AVL library was reasonably complete when I released it, but I've subsequently thought of plenty of stuff that's still missing (arguably), and you've just given me more. Anyway, you might like to try using AVL trees which I have just upgraded to provide the necessary functions.. http://homepages.nildram.co.uk/~ahey/HLibs/Data.Tree.AVL/ You should be able to produce a reasonable alternative to Data.IntMap with this. I'd be interested to know how it performs. I won't do the whole thing myself, but here's a start (uses GHCs unboxed Ints). {-# OPTIONS -fglasgow-exts #-} import Data.COrdering import Data.Tree.AVL import GHC.Base data IntAssoc = IntAssoc Int# Int# --Perhaps use boxed values instead?? newtype IMap = IMap (AVL IntAssoc) emptyIMap :: IMap emptyIMap = IMap empty lookUp :: IMap -> Int -> Int lookUp (IMap avl) (I# skey) = genReadDefault 0 avl cmp where cmp (IntAssoc key v) = case compareInt# skey key of LT -> Lt EQ -> Eq (I# v) GT -> Gt set :: Int -> Int -> IMap -> IMap set (I# k) (I# v) (IMap avl) = IMap avl' where avl' = if v ==# 0# then genDel cmp avl else genPush ccmp ia avl ia = IntAssoc k v cmp (IntAssoc k' _) = compareInt# k k' ccmp (IntAssoc k' _) = case compareInt# k k' of LT -> Lt EQ -> Eq ia GT -> Gt addMaps :: IMap -> IMap -> IMap addMaps (IMap avl0) (IMap avl1) = IMap (genUnionMaybe ccmp avl0 avl1) where ccmp (IntAssoc k0 v0) (IntAssoc k1 v1) = case compareInt# k0 k1 of LT -> Lt EQ -> let s = v0 +# v1 in if s ==# 0# then Eq Nothing else Eq (Just (IntAssoc k0 s)) GT -> Gt Regards -- Adrian Hey

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combining IntMaps
by Scherrer, Chad 30 Jul '05

30 Jul '05

I'm using the (IntMap Int) type to implement functions (Int -> Int), by treating non-keys as values that map to zero. I'd like to be able to add two of these pointwise, and delete the key from the resulting map when the sum of the values is zero. My specification is addMaps :: IntMap Int -> IntMap Int -> IntMap Int addMaps m = IntMap.filter (/= 0) . IntMap.unionWith (+) m But I'm not really happy with this because it traverses both maps for the union, and then traverses the result to get rid of all the zeros. (This function is a performance bottleneck in my current code). I thought I could do something like addMaps' :: IntMap Int -> IntMap Int -> IntMap Int addMaps' = IntMap.foldWithKey f where f k x = IntMap.update (maybeAdd x) k maybeAdd x v = let s = v + x in if s == 0 then Nothing else Just s But this is no good, because IntMap.update only affects those keys where lookup succeeds, so IntMap.update (maybeAdd 3) 8 IntMap.empty returns IntMap.empty, rather than a map with 8 -> 3 as I had hoped. What would you suggest to help addMaps run faster? Do I need to pry open the IntMap implementation to do better? Thanks so much, Chad Scherrer PS - I mistakenly crosslisted this to the GHC users list, because I didn't realize how the mailing lists were set up. Please excuse my noobiness.

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4-year PhD Positions at UPM -- MOBIUS EU Project
by MOBIUS PhD Grants 30 Jul '05

30 Jul '05

---------------------------------------------------------------------- Announcement of up to three PhD Scholarships to pursue PhD studies in the CLIP group (http://clip.dia.fi.upm.es) Technical University of Madrid (Spain) in the context of the MOBIUS EU project (http://mobius.inria.fr) ---------------------------------------------------------------------- The CLIP group at the Technical University of Madrid (UPM) invites applications for up to three, fully funded, 4-year, PhD Scholarships for qualified graduate students. Candidates should have graduated in Computer Science, Computer Engineering, or related fields prior to the start of their study at UPM. The working language of the group is English, and candidates can be of any nationality. The Ph.D. work will center around the research areas of the CLIP group within the European research project "MOBIUS", under the direction of German Puebla and Manuel Hermenegildo. The general aim of "MOBIUS: Mobility, Ubiquity and Security" is to develop a framework for establishing trust and security of Java programs, using the Proof Carrying Code paradigm. The project will start September 1, 2005 and will run for four years. Within the MOBIUS project, the research of the CLIP group focuses on Abstraction Carrying Code, an Abstract Interpretation based approach to mobile code safety. This involves the development of new advanced program analysis techniques for security and verification, including resource consumption, both in terms of time and memory. The offered scholarships provide excellent opportunities for international collaboration since MOBIUS is a European project with 16 partners from 10 countries. Also, the CLIP group is active and international, with around 15 full-time members from 6 different countries. The grants include academic fees, medical care, plus a tax-free monthly payment which ranges from 1,100 to 1,600 euros, depending on experience. The following qualifications are not mandatory, but increase the chances of success: * Experience with Java, Java bytecode, and JML * Knowledge of (Constraint) Logic Programming * Interest in software verification and security Applications and information: ---------------------------- The deadline for applications is September 26, 2005. Applications received after this deadline may be considered if the scholarships have not been filled at the time. Applications should be sent to mobius-phd-grants(a)clip.dia.fi.upm.es and include a curriculum vitae, a listing of grades from previous studies, a brief description of research interests with regards to the CLIP group and MOBIUS, and, if at all possible, letters of recommendation from teachers or managers that the student has worked with. The latest information on the scholarships and the application process can be found at http://www.clip.dia.fi.upm.es/Job_Openings/mobius-phd-grants.html ---------------------------------------------------------------------- -- =============================================================== | German Puebla | http://clip.dia.fi.upm.es/~german | | Facultad de Informatica | Universidad Politecnica de Madrid | ===============================================================

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RE: [Haskell] stack overflow - nonobvious thunks?
by Scherrer, Chad 29 Jul '05

29 Jul '05

Thanks! I was confused at first because ghci still runs into memory problems, though no longer stack overflows. ghc runs it beautifully, though. -Chad -----Original Message----- From: Amanda Clare [mailto:afc@aber.ac.uk] Sent: Thursday, July 28, 2005 3:20 AM To: Dean Herington Cc: Scherrer, Chad; haskell(a)haskell.org Subject: Re: [Haskell] stack overflow - nonobvious thunks? Dean's version certainly seems the neatest, but just for interest you can also do it with a cps fold instead of foldl' too: table xs = assocs $! cpsfold f empty xs where f x m k = case Map.lookup x m of Just v -> v `seq` (k $ Map.adjust (+1) x m) Nothing -> k $ Map.insert x 1 m cpsfold f a [] = a cpsfold f a (x:xs) = f x a (\y -> cpsfold f y xs) As far as I understand it this just makes sure the "seq" happens before the folding continues. When compiled with ghc, both solutions are very well behaved, and seem to take the same small amount of memory whether for 10000000 or 100000000. Amanda Dean Herington wrote: > The following version seems to do the trick (and still remain quite > readable). It worked for 100000000 as well. > > import Data.Map as Map > import System.Random > import Data.List (foldl') > > table :: (Ord a) => [a] -> [(a,Int)] > table xs = Map.assocs $! foldl' f Map.empty xs > where f m x = let m' = Map.insertWith (+) x 1 m > Just v = Map.lookup x m' > in v `seq` m' > > unif :: [Int] > unif = randomRs (1,10) $ mkStdGen 1 > > f :: Int -> [(Int, Int)] > f n = table $ take n unif > main = print $ f 10000000 > > - Dean

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RE: [Haskell] stack overflow - nonobvious thunks?
by Scherrer, Chad 29 Jul '05

29 Jul '05

Adrian, Does your AVL library have an "insertWith'"-type function mentioned by Udo? If I lookup and insert into the table separately, forcing evaluation at each step, I can do table' :: (Ord a) => [a] -> [(a, Int)] table' xs = Map.assocs $! foldl' f Map.empty xs where f m x = (Map.insert x $! 1 + Map.findWithDefault 0 x m) $! m This helps with the stack overflow problem, but now I'm hitting a different wall: *Main> table $ take 10000000 unif [(1,999662),(2,1000220),(3,998800),(4,1000965),(5,999314),(6,1001819),(7 ,1000997),(8,999450),(9,999877),(10,998896)] *Main> table $ take 100000000 unif <interactive>: out of memory (requested 1048576 bytes) I thought I may have found a good approach using an idea from one of Amanda Clare's pages http://users.aber.ac.uk/afc/stricthaskell.html If I write eqSeq x y = if x==x then y else y this forces evaluation of x further than seq alone. Then I can write table :: (Ord a) => [a] -> [(a, Int)] table xs = Map.assocs $! foldl' f Map.empty xs where f m x = m `eqSeq` Map.insertWith (+) x 1 m Same result as Udo's suggestion - out of memory. I still don't see why this function should need any more than a few kilobytes, even for very large n like this. -Chad -----Original Message----- From: u.stenzel(a)web.de [mailto:u.stenzel@web.de] Sent: Wednesday, July 27, 2005 11:02 AM To: Scherrer, Chad Cc: haskell(a)haskell.org Subject: Re: [Haskell] stack overflow - nonobvious thunks? Scherrer, Chad wrote: > f m x = Map.insertWith (+) x 1 m insertWith is inserting the "nonobvious thunks". Internally it applies (+) to the old value and the new one, producing a thunk. There is no place you could put a seq or something to force the result. You basically need insertWith', which isn't there. I think, your best best is to manually lookup the old value, combine with the new, force the result, then insert that, overwriting the old value. On top of that you still need foldl' to avoid building long chains of Map.insert. Udo. -- The Second Law of Thermodynamics: If you think things are in a mess now, just wait! -- Jim Warner

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