- Haskell-Cafe - Haskell.org

Needing strictness?
by michael rice 22 Oct '09

22 Oct '09

Busy night. I wanted the question and response to be on the same line (see below), so I used putStr instead of putStrLn, but Haskell jumps the gun and goes to the second line to get the response before the question ever gets printed. Is this an example of needing strictness? How would I do that, or something else, to get the result I want? putStrLn "Some yes/no question (y/n)?" ans <- getLine Michael

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ANN: GPipe 1.02 and Vec-Transform 1.0.1
by Tobias Bexelius 21 Oct '09

21 Oct '09

I've uploaded new versions of these two packages to hackage: GPipe-1.0.2: * Removed rasterize and made rasterizeFront work for other primitives than triangles instead. * Fixed notB Vertex and Fragment instances Vec-Transform-1.0.1: * Added rotationLookAt Cheers Tobias Bexelius _________________________________________________________________ Windows Live: Dina vänner får dina uppdateringar från Flickr, Yelp och Digg när de skickar e-post till dig. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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How to use "bracket" properly ?
by zaxis 21 Oct '09

21 Oct '09

winSSQ count noRed noBlue = do { yesRed <- [1..33] \\ noRed; yesBlue <- [1..16] \\ noBlue; bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1); return () } will report: Couldn't match expected type `IO ()' against inferred type `[()]' In a stmt of a 'do' expression: bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\ hd1 -> pickSSQ count yesRed yesBlue hd1) However, the following works fine: winSSQ count noRed noBlue = do let yesRed = [1..33] \\ noRed let yesBlue = [1..16] \\ noBlue bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ count yesRed yesBlue hd1) Why ? -- View this message in context: http://www.nabble.com/How-to-use-%22bracket%22-properly---tp25953522p259535… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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list -> sublists
by satorisanitarium 21 Oct '09

21 Oct '09

I just started learning haskell and I just came to my first wtf moment. I'm trying to do something like this: calling: foo 3 [1,2,3,2,4,1,6,3,6,2,3,5,2,5,2,1,6,4] returns: [[1,2,3],[2,4,1,6,3],[2,3]] but i have no idea how to put something into a sublist. How to make a list of sublists out of a list, whether they be a list of numbers or a string. TX in advance. -- View this message in context: http://www.nabble.com/list--%3E-sublists-tp25975341p25975341.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Test cases for type inference
by Peter Verswyvelen 21 Oct '09

21 Oct '09

For learning, I would like to develop my own implementation of type inference, based on the paper "Typing Haskell in Haskell". At first sight, the source code of THIH contains a small number of tests, but I was wandering if a large test set exist? Thanks, Peter

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systemtimmetypeable: Workaround for lack of deriveable data instance in either System.Time or Data.Time.
by Thomas Hartman 21 Oct '09

21 Oct '09

At http://osdir.com/ml/haskell-cafe@haskell.org/2009-10/msg00197.html I griped that the lack of Data-Derivable time values was causing me headache in happstack. In the proposed cabal package http://patch-tag.com/r/tphyahoo/systemtimetypeable/snapshot/current/content… I submit a workaround that, while probably not the ideal thing, has proved helpful to me. Basically, I use the type MyTime, which is data-deriveable, when working with Macid in happstack; and convert from System.Time and/or Data.Time with the accompanying utility functions when necessary. Perhaps the utility functions are useful on their own as well. I found it surprisingly hard to get from Data.Time values to System.Time values. Anyways, I am interested in what others think and, of course, patches and suggestions welcome. thomas.

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Re: [Haskell-beginners] map question
by Gregory Propf 21 Oct '09

21 Oct '09

Remember that there is asymmetry between (+) and (-). The former has the commutative property and the latter does not so: (+) 3 4 = 7 and (+) 4 3 = 7 but (-) 3 4 = -1 and (-) 4 3 = 1 --- On Thu, 9/17/09, Tom Doris <tomdoris(a)gmail.com> wrote: From: Tom Doris <tomdoris(a)gmail.com> Subject: Re: [Haskell-beginners] map question To: "Joost Kremers" <joostkremers(a)fastmail.fm> Cc: beginners(a)haskell.org Date: Thursday, September 17, 2009, 6:06 AM This works: map (+ (-1)) [1,2,3,4] 2009/9/17 Joost Kremers <joostkremers(a)fastmail.fm> Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude> map (+ 1) [1,2,3,4] [2,3,4,5] Prelude> map (* 2) [1,2,3,4] [2,4,6,8] Prelude> map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude> map (2 /) [1,2,3,4] [2.0,1.0,0.6666666666666666,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude> map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners -----Inline Attachment Follows----- _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-cafe] Error handling package
by Michael Snoyman 20 Oct '09

20 Oct '09

(Sorry, accidently took off cafe.) On Mon, Oct 19, 2009 at 12:44 AM, Henning Thielemann < lemming(a)henning-thielemann.de> wrote: > > On Mon, 19 Oct 2009, Michael Snoyman wrote: > > Does the explicit-exception package provide what you need? >> >> http://hackage.haskell.org/package/explicit-exception >> >> >> I don't think so, but correct me if I'm wrong. I want to make it easy to >> chain together >> computations which could fail in different ways. For example, something >> like this: >> >> attemptReadInt :: String -> Attempt Int >> attemptLookup :: String -> [(String, String)] -> Attempt String >> attemptLookupInt :: String -> [(String, String)] -> Attempt Int >> attemptLookupInt k m = attemptLookup k m >>= attemptReadInt >> >> Now, in the explicit-exception package, I could- in this simple example- >> define >> something like: >> >> data MyErrors = KeyNotFound | InvalidInt >> > > > type Attempt = Exceptional MyErrors > True; that's what I meant by I could do this in my simple example. > > > But this solution would not scale. >> > > You want to add other exceptions? The idea of my package is to make > exceptions explicit in the type. Otherwise you would use > extensible-exceptions. Or you could define MyErrors using an existential > type. Which is my point. I'm trying to provide a package for non-explicit exceptions. To compare to other programming languages, I think your package is providing the equivalent of Java checked exceptions, while mine is providing (safe) unchecked exceptions. I say safe because you still need to explicitly decide to turn an Attempt into a possible runtime exception which will bring down your program. Defining MyErrors using an existential type would essentially recreate the entire attempt package; I don't see that purpose in everyone wanted unchecked exceptions needing to reinvent the wheel in non-compatible ways. If multiple libraries use attempt, they can easily have their possible-error-returning functions chain together safely. > > > Additionally, there's two immediate features I think I would miss from my >> package: >> >> 1) fail works properly, so an Attempt would be a valid monad response from >> people who >> use that function. >> > > As far as I understand, 'fail' is used/abused for reporting failed pattern > matches in do notation. If a failed pattern match indicates a programming > error, it should be a really error, and not something that must be handled > at run-time. > That's a lot of very debateable statements you just made. It might be that it's strongly encouraged to only use fail for failed pattern matching, but in practice you could use it for any monadic failure. Also, there's nothing stopping a user from re-throwing pattern match exceptions received in an Attempt. Michael

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Error handling package
by Michael Snoyman 20 Oct '09

20 Oct '09

While working on the next release of data-object, I wanted to represent some operations that might fail. The typical candidates were: 1) Maybe 2) Either 3) Monad Monad is always iffy because of the often times poorly defined fail. Maybe doesn't provide any means of reporting what the problem was. Either is not defined as a Monad in the base library. Also, the usual candidate of Either String only provides for an error message. It would be nice to have more in-depth information available. So I've put together a package I call "attempt". It defines a data type called (surprise) Attempt with a Success and Failure constructor. The trick here is that it using the new extensible exceptions under the surface, so you can report any kind of exception you want. It also provides a "FromAttempt" type class for possible conversion targets from an Attempt, provides an attempt function with some helpers for performing the equivalent of Control.Exception.catches, and provides two samples of functions I would want to implement with this library: attemptJoin and attemptLookup. My questions for the list are: 1) Is the overall approach sound, or do people have better ideas? 2) Are there any other FromAttempt instances I should provide out of the box? 3) I was considering adding specialized versions of the fromAttempt function, ie ioFromAttempt, maybeFromAttempt. Thoughts? 4) Should I follow the naming scheme attemptJoin, attemptLookup, etc, or just call the functions join, lookup and force people to import the module qualified? 5) Any other suggestions for attempt functions? I've considered head/tail/etc. 6) Include ToAttempt? The code is available on github at http://github.com/snoyberg/attempt/blob/master/Data/Attempt.hs . I appreciate the review. Also, I have not yet documented the code, but I will do so before uploading to Hackage; I just didn't want to document a changing target. Thank you, Michael

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debugtracehelpers Re: traceM and traceShowM
by Thomas Hartman 20 Oct '09

20 Oct '09

I have this and a couple other handy functions in DebugTraceHelpers: http://patch-tag.com/r/tphyahoo/DebugTraceHelpers/snapshot/current/content/… hackage: http://hackage.haskell.org/packages/archive/DebugTraceHelpers/0.12/doc/html… 2009/10/16 Martijn van Steenbergen <martijn(a)van.steenbergen.nl>: > Hello, > > I propose the addition of the following two functions to module Debug.Trace: > >> traceM :: Monad m => String -> m () >> traceM msg = trace msg (return ()) >> >> traceShowM :: (Show a, Monad m) => a -> m () >> traceShowM = traceM . show > > These functions allow tracing in any do-block. I often define them myself; I > think they are useful in general. > > Deadline: 23 October 2009. > > Martijn. > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries >

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Make two windows work together
by xu zhang 20 Oct '09

20 Oct '09

Hi there, I am trying to make a sub-window of one window and make them work together. I mean, if I zoom or transform some content in one of the window, the other one works in the same way. What I have done so far is just open a new window which have the same drawing view as the original one, but when I try to change some content in one window, the other one does not work at the same time. They are separate. Can anyone give me some hint on how to make them work at the same time not separate anymore? Thank you so much!

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GHC devs
by Andrew Coppin 20 Oct '09

20 Oct '09

Random question of the day: How many developers are working on GHC? I had always *assumed* that there was something like a hundred core developers, with a much larger number of people casually testing and occasionally submitting the odd patch or two. However, I watched a video of a talk the other day (no, I don't remember which one) and it made me reconsider this view. Somebody muttered something like "GHC is really too big now for just 3 developers to manage", which makes it sound as if there are only 3 active developers... Surely that can't be correct. (There would never be any releases, for one thing...) Does anybody know anything concrete about this?

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Re: [Haskell-cafe] Parsec bug, or...?
by S.Doaitse Swierstra 20 Oct '09

20 Oct '09

After some private exchange of info between Uwe and me it became clear that it may not have been immediately clear that the error messages in my original posting where actually part of a more involved process. By removing the optional part in the pCommand (i.e. making the part starting with `opt` invisible, we can show the full power of the built- in error correction. import Text.ParserCombinators.UU.Parsing pCommand [] = pure [] pCommand (x:xs) = ((:) <$> pSym x <*> pCommand xs) -- `opt` (x:xs) pCommands = amb . foldr (<|>) pFail . map pCommand $ ["banana", "chocolate", "frito", "fromage"] t :: String -> ([String], [Error Char Char Int]) t input = parse ( (,) <$> pCommands <*> pEnd) (listToStr input) Now one also gets error messages like: *Main> t "frxi" (["frito"],[ Deleted 'x' at position 2 expecting one of ['i', 'o'], Inserted 't' at position 4 expecting 't', Inserted 'o' at position 4 expecting 'o']) for free. However, since the decision what element to take is based on a limited look-ahead, one also gets: *Main> t "xfxrxix" (["fromage"],[ Deleted 'x' at position 0 expecting one of ['b', 'c', 'f', 'f'], Deleted 'x' at position 2 expecting 'r', Deleted 'x' at position 4 expecting 'o', Deleted 'i' at position 5 expecting 'o', Deleted 'x' at position 6 expecting 'o', Inserted 'o' at position 7 expecting 'o', Inserted 'm' at position 7 expecting 'm', Inserted 'a' at position 7 expecting 'a', Inserted 'g' at position 7 expecting 'g', Inserted 'e' at position 7 expecting 'e']) *Main> which is something not completely expected; the current look-ahead is however three symbols ahead, and once a decision is taken this is not reconsidered (for cost reasons). This is currently a consequence of the rather simplistic inner organisation of the intermediate library. In the next version we hope to have gotten rid of this artefact. Best, Doaitse > > Yes, for my particular problem the complexity is very limited. I > wouldn't even have used parsec for this, in spite of the comment I had > made earlier about this, if I were not already using it in a different > part of the project to parse individual records ("buy security <foo> > for this price on this date", etc), so it was natural to add a bit > more parsec code to also deal with the commands saying what I want to > see from the data. It's all still pretty trivial, but starting already > to be useful to me... it's really quite gratifying what a small amount > of haskell code suffices to make a useful and flexible program. > > best regards, > Uwe > > On 10/15/09, S. Doaitse Swierstra <doaitse(a)swierstra.net> wrote: >> >> On 15 okt 2009, at 16:58, Uwe Hollerbach wrote: >> >>> Hi, all, thanks for the further inputs, all good stuff to think >>> about... although it's going to be a little while before I can >>> appreciate the inner beauty of Doaitse's version! :-) >> >> The nice thing is that you do not have to understand the inner >> workings ;-} I basically builds a greedy parser for each word to be >> recognised which can stop and assume the rest is there if it can no >> longer proceed (the `opt` is greedy in its left alternative) . Hence >> it recognises the longest possible prefix. >> Since my parsers pursue all alternatives in parallel you >> automatically >> get what you want, without having to indicate prefix lengths, calls >> to >> try, etc. >> >> The "amb" combinator has type >> >> amb :: Parser a -> Parser [a] >> >> and collects the result from all alternatives its argument parser is >> constructed from; you might say it convert an ambiguous parser to a >> parser with a list as result, hence preventing the rest of the input >> being parsed over and over again. I am currently working on bringing >> back more abstract interpretation in the implementation (i.e. what we >> have had for almost 10 years in the uulib library), but I do not >> expect you to see a lot of that from the outside. >> >> If you want to work with left-recursive parsers (which does not seem >> to be the case), you may revert to more complicated solutions such as >> found in the "christmastree" (Changing Haskell's Read Implementation >> Such That by Manipulationg Abstract Syntax Trees Read Evaluates >> Efficiently) package if you need to generate parsers online, or to >> happy-based solutions in case your grammar is fixed. >> >> >> If you have any questions do not hesitate to ask, >> Doaitse >> >> >>> I had considered >>> the approach of doing a post-parsec verification, but decided I >>> wanted >>> to keep it all inside the parser, hence the desire to match prefixes >>> there (and lack of desire to write 'string "p" <|> string "pr" <|> >>> string "pre" ...'. >>> >>> By way of background, the actual stuff I'm wanting to match is not >>> food names, but some commands for a small ledger program I'm working >>> on. I needed something like that and was tired of losing data to >>> quicken every so often. I realize of course that there are other >>> excellent ledger-type programs out there, but hey, I also needed >>> another hacking project. I'll put this onto hackage in a while, once >>> it does most of the basics of what I need. No doubt the main >>> differentiator between mine and those other excellent ledger >>> programs >>> out there will be that mine has fewer features and more bugs... >>> >>> thanks again, all! >>> >>> Uwe >> >>

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ANN: strptime bindings
by Eugene Kirpichov 20 Oct '09

20 Oct '09

Hi all. I had to parse dates from bytestrings efficiently and tried to use the parsedate library, but I had to port it to parsec3 to make it usable with bytestrings, and I could not get decent performance out of the resulting version due to ghc having difficulties with parsec3's generality, so I wrote bindings to strptime. Here they are: http://hackage.haskell.org/package/strptime They work for Strings and strict and lazy bytestrings. They are several orders of magnitude faster than parsedate and than Data.Time.readTime,parseTime (a program that ran in 40s with parsec3'd parsedate runs in less than 1s). -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-beginners] map question
by oleg＠okmij.org 20 Oct '09

20 Oct '09

> Something I wonder from time to time if it would be a good thing or > not is to allow > a `f g` b > to mean > f g a b You don't have to wonder: http://www.haskell.org/haskellwiki/Infix_expressions Granted, you have to use different characters rather than the backquote. On the other hand, you can play with infix expressions right now, on any Haskell98 system.

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Problems with Haskell
by Philippos Apolinarius 20 Oct '09

20 Oct '09

Before anything else, I want to point out that I have no intention to confront your community, or denigrate Haskell. A few days ago I answered an email from a Clean programmer on something related to Clean. He was worried that Clean team could give up its good work, and Clean could disappear; therefore, he was thinking about switching to Haskell. Since I thought that my email could be of interest for the Clean community, I posted it in the -- small-- Clean list :-(Clean is not as popular as Haskell). I received a lot of furious and offensive private emails for suggesting the Clean programmer to stick with Clean. However, I also received a very polite, humorous, and illuminating private email from a person who seems to work at Microsoft. His name is Simon Peyton-Jones. He urged me to post my comments on a Haskell cafe. He also filed one of my comments as a bug in a Haskell bug track. Here is a couple of snippets from his email: --- I think it's v bad that a straightforward program runs so slowly, and it's certainly true that this is an area we could pay more attention to. --- Meanwhile, I'm curious: are the arrays in Philippos's program strict? Or lazy? If strict, that's a pretty big difference. Therefore, here are my comments, with a lot of code. A few months ago I came accross an unpublished article about a novel genetic programming system. The system was coded in Larceny Scheme. I translated it to Clean and to Haskell. Unhappily, I cannot post the program here because it is very large, and the authors of the original Lisp program don't want me to divulge it before they see in in a printed page of a Journal. Therefore, I wrote an empty genetic programming framework, just to compare languages. Comparing Clean and Haskell, I noticed: 1 -- Clean compiler almost never let me do very stupid things, like trying to unbox a tree, or to write in a closed file (I will post an example of this in a near future). For instance, Clean compiler would never swallow something like the code below: import Control.Monad.ST import Data.Array.ST import Data.Array.Base import System.Random data Op = AND | OR | NOT; data Tree= L Double | T Op [Tree] main = print $ runST (do arr <- newArray (1,2000000) (L 0.0) :: ST s (STArray s Int Tree) go arr 2000000 0.0 ) go :: STArray s Int Tree -> Int -> Double -> ST s Double go a i acc | i < 1 = return acc | otherwise=do b <- unsafeRead a i {- readArray a i -} writeArray a i (setDouble ((getDouble b)+3.0)) c <- readArray a i go a (i-1) (acc+ (getDouble c)) -- What I really need is a random index in Haskell. getDouble (L r)= r getDouble _ = 0.0 setDouble r= L r 2 -- Safety does not cost much in Clean. For instance, removing array boundary check does not seem to affect Clean. I believe that it does not affect Haskell either, but I have not verified this point. 3 -- Haskell seems to loop more often than Clean. For instance, Haskell may loop if I change function mutate to mutate e (L i) xs = (e, xs) mutate e t (y:ys) = ins t (rnLen t y, ys) where ins (T p (L i:xs)) (0, st)=(T p (e:xs), st) ins (T p (t:xs)) (n,(r1:rs)) | n > 0= let (T p mt, s2)= ins (T p xs)(n-1, rs) in (T p (t:mt), s2) ins (T p (t:xs)) (n,(r1:rs)) | rn 2 r1== 0= (T p (e:xs), rs) | rn 2 r1== 1= let (xpr, st)= mutate e t rs in (T p (xpr:xs), st) This might be a bug in my implementation of show Tree. It would be great if you people could "show" me what I did wrong. 4 -- On the plus side, there are libraries in Haskell that seem to behave better than the Clean equivalent libraries. This could be explained by the fact that there are a lot of people coding Haskell libraries, while Clean team seems to be reluctant in accepting libraries from outsiders. For instance, lethevert made a very important improvement in ObjectIO (changing fonts in edit text field), but it was never incorporated into Clean (yes, I wrote a lot of emails to Clean team about it). Last year, when I was learning Clean, I discovered that many of my professors and teachers are presbyopic. Therefore, it would be a good policy to use very large fonts for homework. I only succeeded in doing it thanks to lethevert. In the case of the program below, Haskell System.Random library seems to work much better than Clean MersenneTwister. 5 --- Any improvement in the program below will be welcome. The program is already very fast thanks to suggestions I received from the bug track people, and from Peyton-Jones. Function "show" seems to loop if I replace mutate in item 3 for mutate in the code below. {- ghc gp.hs -O2 --make -} {- Execute: gp.exe +RTS -sstderr -} import Control.Monad.ST import Data.Array.ST import Data.Array.Base import System.Random data Op = AND | OR | NOT; data Tree= L Int | T Op [Tree] psz= 20000 thr=4.0 gates= [AND, NOT, OR] table= [ (False, False, False), (True, False, True), (True, True, False), (False, True, True)] prt NOT= "NOT" prt OR= "OR" prt AND= "AND" instance Show Tree where show (L i) = "L"++(show i) show (T p xs) = (prt p)++(show xs) main = do xs <- (rnList (1, 20000)) print $ runST $ do arr <- newArray_ (1,psz) :: ST s (STArray s Int Tree) (arr, xs1) <- gen0 arr psz xs g1 <- evolve 30 arr (L 0) xs1; return $ g1 gen0 a i xs = if i<=0 then return(a,xs) else do (tree, rs) <- return(rndXpr gates 5 xs) writeArray a i tree gen0 a (i-1) rs mutate e (L i) xs = (e, xs) mutate e t (y:ys) = ins t (rnLen t y, ys) where ins (T p (L i:xs)) (0, st)=(T p (e:xs), st) ins (T p (t:xs)) (n,(r1:rs)) | n > 0= let (T p mt, s2)= ins (T p xs)(n-1, rs) in (T p (t:mt), s2) ins (T p (t:xs)) (n,(r1:rs)) | rn 2 r1== 0= (T p (e:xs), rs) | rn 2 r1== 1= let (xpr, st)= mutate e t rs in (T p (xpr:xs), st) fxy NOT r= T NOT [L (rn 2 r)] fxy AND st = T AND [L 0, L 1] fxy OR st = T OR [L 0, L 1] rndXpr fs beastSz xs= loop beastSz xs where rth s r= s!!(rn (length s) r) loop n (r1:r2:rs) | n<1 = (fxy (rth fs r1) r1, rs) |otherwise= mutate (fxy (rth fs r1) r2) f1 rs where (f1, ys)= loop (n-1) rs run (L 0) x y= x -- Interpreter run (L 1) x y= y run (T AND xs) x y = and [run c x y | c <- xs] run (T OR xs) x y= or [run c x y | c <- xs] run (T NOT (t:_)) x y= not (run t x y) rn n r= (abs r) `rem` n rnLen (T _ s) r= rn (length s) r rnList :: (Int, Int) -> IO [Int] rnList r=getStdGen>>=(\x->return(randomRs r x)) frag (L i) st = (L i, st) frag (T p xs) (r1:r2:rs) | rn 2 r2==0= (xpr, rs) | otherwise= frag xpr rs where xpr= xs!!(rnLen (T p xs) r1) crossover e1 e2 rs = ([c2, c1], rs4) where (g1, rs1) = frag e1 rs (g2, rs2) = frag e2 rs1 (c1, rs3) = mutate g1 e2 rs2 (c2, rs4) = mutate g2 e1 rs3 nGates (L i)= 0.0 nGates (T p xs) = (-0.1) + sum [nGates g | g <- xs] fitness tt gt = (ng + 1.0 + sum [ft t | t <- tt]) where ng= nGates gt ft (out, t1, t2) | run gt t1 t2 == out= 1.0 ft _ = 0.0 evolve n p b xs | n < 1 = do (arr, xs1) <- gen0 p psz xs evolve 30 arr b xs1 evolve n p b (r1:r2:rs) = do n1 <- return $ 1+(rn psz r1) n2 <- return $ 1+(rn psz r2) g1 <- readArray p n1; g2 <- readArray p n2 ([c1,c2], rs) <- return $ crossover g1 g2 rs insrt c1 p 1 psz insrt c2 p 1 psz res <- best 1 b p fitn <- return $ fitness table res if fitn > thr then return res else evolve (n-1) p res rs best i res p | i >= psz= return res best i fg p= do g <- readArray p i if (fitness table fg) > (fitness table g) then best (i+1) fg p else best (i+1) g p insrt g v i sz | i >= sz = return () insrt g v i sz = do a <- readArray v i fg <- return $ fitness table g fa <- return $ fitness table a if fa > fg then insrt g v (i+1) sz else do writeArray v i g __________________________________________________________________ Connect with friends from any web browser - no download required. Try the new Yahoo! Canada Messenger for the Web BETA at http://ca.messenger.yahoo.com/webmessengerpromo.php

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Re: Documentation (was: ANN: text 0.5, a major revision of the Unicode text library)
by John Lato 19 Oct '09

19 Oct '09

> From: Derek Elkins <derek.a.elkins(a)gmail.com> > > On Sun, Oct 11, 2009 at 8:55 AM, Iain Barnett <iainspeed(a)gmail.com> wrote: >> >> On 11 Oct 2009, at 13:58, John Lato wrote: >> >>> For anyone writing introductions to generic programming, take this as >>> a plea from Haskellers everywhere. If one of the RWH authors can't >>> understand how to make use of these techniques, what hope do the rest >>> of us have? >>> >>> John Lato >>> >>> P.S. Some might wryly note that I'm the maintainer of a package which >>> is also known for incomprehensible documentation. To which I would >>> reply that our effort is much newer, I consider it a problem, and it's >>> being worked on, contrasted to the state of GP where similarly >>> impenetrable documentation has been and continues to be the norm. >>> >> >> You could say that about most documentation (for Haskell and beyond). >> Apparently, programmers like programming better than documenting. The effect >> of this is that less people use their programming, making their efforts >> redundant. >> >> Silly really, considering programmers are (allegedly:) intelligent. > > Apparently, programmers like programming better than reading as > well... in my experience. I won't disagree. But I think the real difficulty is that the intersection of programmers who can come up with really good ways to solve problems (not even all programmers, unfortunately) and people who are good at writing documentation is vanishingly small. It seems to me that when someone works in a problem domain (e.g. Generic Programming), they gain a very deep understanding of that area and are used to working at a certain level within it. When introducing the topic to newcomers (even ostensibly smart programmers) the introduction can't assume prior knowledge of the problem domain, but the authors are so used to thinking at one level they often take for granted knowledge that the audience doesn't have. I don't think this problem is particular to programming, but it is common in Haskell. Most likely because Haskell, with a reputation as a research language, has a lot of computer science types doing research in wide-ranging topics. Somebody's expertise in category theory, for example, might not directly carry over to generic programming (or maybe it does; I'm not an expert in either).

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ANNOUNCE: cereal-0.2
by Trevor Elliott 19 Oct '09

19 Oct '09

Hello Everyone, cereal is a variation on the binary package that provides strict parsing, handleable exceptions with a named call stack, and a new parsing isolation feature. The major differences from binary are a new class called Serialize replacing the Binary class, getting and putting using strict ByteStrings and a number of combinators for parsing lists, arrays and other containers. Version 0.2 is available from hackage: * http://hackage.haskell.org/package/cereal-0.2 --trevor

1 0

Hackage down
by Dougal Stanton 19 Oct '09

19 Oct '09

Has not been responding for at least the last 12 hours. Is there somewhere to look for status reports on sysadmin details like this, so we can tell if - it's a scheduled down time - it's a problem but the admins know about it - etc etc. D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

7 6

Re: [Haskell-cafe] Error handling package
by Michael Snoyman 19 Oct '09

19 Oct '09

On Mon, Oct 19, 2009 at 3:39 PM, Jose Iborra <pepeiborra(a)gmail.com> wrote: > You may want to take a look at anoother option in Hackage, the > control-monad-exception package. > > http://pepeiborra.github.com/control-monad-exception/ > > The control-monad-exception library provides the building blocks for > > * Explicitly Typed exceptions (checked or not) > * which are composable > * and even provide stack traces (experimental feature) Jose, Thank you very much, thinks looks like just the thing I was looking for. The only thing that slightly concerns me is the transformers dependency, but it's not a serious worry. I'd love to try out the package right now, but with hackage being down I can't get the dependencies yet ;). I'll try it out as soon as I can. > > > > On 19/10/2009, at 01:00, Michael Snoyman wrote: > > (Sorry, accidently took off cafe.) >> >> On Mon, Oct 19, 2009 at 12:44 AM, Henning Thielemann < >> lemming(a)henning-thielemann.de> wrote: >> >> On Mon, 19 Oct 2009, Michael Snoyman wrote: >> >> Does the explicit-exception package provide what you need? >> >> http://hackage.haskell.org/package/explicit-exception >> >> >> I don't think so, but correct me if I'm wrong. I want to make it easy to >> chain together >> computations which could fail in different ways. For example, something >> like this: >> >> attemptReadInt :: String -> Attempt Int >> attemptLookup :: String -> [(String, String)] -> Attempt String >> attemptLookupInt :: String -> [(String, String)] -> Attempt Int >> attemptLookupInt k m = attemptLookup k m >>= attemptReadInt >> >> Now, in the explicit-exception package, I could- in this simple example- >> define >> something like: >> >> data MyErrors = KeyNotFound | InvalidInt >> >> >> type Attempt = Exceptional MyErrors >> >> True; that's what I meant by I could do this in my simple example. >> >> >> But this solution would not scale. >> >> You want to add other exceptions? The idea of my package is to make >> exceptions explicit in the type. Otherwise you would use >> extensible-exceptions. Or you could define MyErrors using an existential >> type. >> >> Which is my point. I'm trying to provide a package for non-explicit >> exceptions. To compare to other programming languages, I think your package >> is providing the equivalent of Java checked exceptions, while mine is >> providing (safe) unchecked exceptions. I say safe because you still need to >> explicitly decide to turn an Attempt into a possible runtime exception which >> will bring down your program. >> >> Defining MyErrors using an existential type would essentially recreate the >> entire attempt package; I don't see that purpose in everyone wanted >> unchecked exceptions needing to reinvent the wheel in non-compatible ways. >> If multiple libraries use attempt, they can easily have their >> possible-error-returning functions chain together safely. >> >> > I believe that control-monad-exception solves this tension between > composability and explicit exceptions. > You can have explicit exceptions which are composable: > > > data DivideByZero = DivideByZero deriving (Show, Typeable) > > data SumOverflow = SumOverflow deriving (Show, Typeable) > > > instance Exception DivideByZero > > instance Exception SumOverflow > > > data Expr = Add Expr Expr | Div Expr Expr | Val Double > > > eval (Val x) = return x > > eval (Add a1 a2) = do > > v1 <- eval a1 > > v2 <- eval a2 > > let sum = v1 + v2 > > if sum < v1 || sum < v2 then throw SumOverflow else return sum > > eval (Div a1 a2) = do > > v1 <- eval a1 > > v2 <- eval a2 > > if v2 == 0 then throw DivideByZero else return (v1 / v2) > > GHCi infers the following types > > > :t eval > > eval :: (Throws DivideByZero l, Throws SumOverflow l) => Expr -> EM l > Double > > > :t eval `catch` \ (e::DivideByZero) -> return (-1) > > .... :: Throws SumOverflow l => Expr -> EM l Double > > > :t runEM(eval `catch` \ (e::SomeException) -> return (-1)) > > .... : Expr -> Double > > >> Additionally, there's two immediate features I think I would miss from my >> package: >> >> 1) fail works properly, so an Attempt would be a valid monad response from >> people who >> use that function. >> >> As far as I understand, 'fail' is used/abused for reporting failed pattern >> matches in do notation. If a failed pattern match indicates a programming >> error, it should be a really error, and not something that must be handled >> at run-time. >> >> That's a lot of very debateable statements you just made. It might be that >> it's strongly encouraged to only use fail for failed pattern matching, but >> in practice you could use it for any monadic failure. Also, there's nothing >> stopping a user from re-throwing pattern match exceptions received in an >> Attempt. >> > > I am with Henning on 'fail'. > It must not be used as a replacement for throw, only for failed pattern > matches which are programming errors and thus unchecked exceptions. > I'm not arguing how it *should* be used, I'm arguing how it *might* be used, and I think calling error is not appropriate. An unchcked exception which can be caught if necesary (I'm assuming that's what runEMParanoid does) sounds like that right thing. Michael

1 0

Documentation (was: ANN: text 0.5, a major revision of the Unicode text library)
by John Lato 19 Oct '09

19 Oct '09

For anyone writing introductions to generic programming, take this as a plea from Haskellers everywhere. If one of the RWH authors can't understand how to make use of these techniques, what hope do the rest of us have? John Lato P.S. Some might wryly note that I'm the maintainer of a package which is also known for incomprehensible documentation. To which I would reply that our effort is much newer, I consider it a problem, and it's being worked on, contrasted to the state of GP where similarly impenetrable documentation has been and continues to be the norm. > > From: "Bryan O'Sullivan" <bos(a)serpentine.com> > > I think maybe someone else will have to take a crack at a Data instance for > Text, because the documentation for Data.Data is not written in English. In > its syntax and structure, it closely hews to what we think of as English, > but it is the kind of documentation that can only be understood by someone > who already knows what it is going to say. > > This is an exemplar of my experience with the cottage industry of generic > programming in Haskell: I'd really quite like to use the stuff, but for > goodness's sake, o beloved researchers, please aim your expository papers at > non-specialists once in a while. An endless chain of papers of the form "my > technique, which you won't understand, is better than this other technique, > which you haven't read about and won't anyway understand, in subtle ways > that you won't understand" does not feel to me like progress. > > Yours in some misery and frustration, > Bryan.

5 4

Too much strictness in binary-0.5.0.2
by Khudyakov Alexey 18 Oct '09

18 Oct '09

Hello I run into problems with new binary package. Following function reads a list of elements one by one until end of stream. List is very long (won't fit into memory). In binary-0.5.0.1 and earlier it read list lazily. Now it seems that it tries to read whole list to memory. Program does not produce any output and memory usage steadily grows. > getStream :: Get a -> Get [a] > getStream getter = do > empty <- isEmpty > if empty > then return [] > else do x <- getter > xs <- getStream getter > return (x:xs) How could I add laziness to this function to revert to old behavior. -- Khudyakov Alexey

3 3

Merging modules
by pat browne 18 Oct '09

18 Oct '09

Hi, I want to establish the strengths and weakness of the Haskell module system for the ontology merging task (I know it was not designed for this!). I wish to make a new module (MERGEDONTOLOGY) from three input modules one of which is common to the other two. The desired merge should contain the following data types: Woman FinancialBank HumanBeing RiverBank Which are all identifiable without any qualifying module names. Below is the detailed requirement and my first attempt at this task in Haskell. I would be grateful for any information on how to merge these modules giving a set of unique unqualified types (i.e. without reference to their originating modules). Regards, Pat ======================== Informal Specification ======================== The diagram and text below explain the requirement MERGEDONTOLOGY { Woman, RiverBank, FinancialBank, HumanBeing} /\ /\ / \ / \ / \ / \ / \ / \ ONTOLOGY1 ONTOLOGY2 {Woman, Bank, Person} {Woman, Bank, Human} /\ /\ \ / \ / \ / \ / \ / \ / \ / {Woman , Person} COMMMON This example includes both synonyms and homonyms. 1)The Woman sort (or data type) should be the same in all modules, there is only one Woman sort and it is named as such in each module. Hence there should be only one MERGEDONTOLOGY.Woman. 2)There is only one sort MERGEDONTOLOGY.HumanBeing, but there are 3 synonyms for it called ONTOLOGY2.Human, ONTOLOGY1.Person, and COMMON.Person. The last sentence considers ONTOLOGY1.Person and COMMON.Person as synonyms; they have different qualifiers but the intention is that they represnt the same thing. Hence should be mapped to same MERGEDONTOLOGY.HumanBeing. To do this (in Maude) COMMON.Person was renamed to ONTOLOGY2.Human which in turn was renamed to MERGEDONTOLOGY.HumanBeing. 3)The homonyms are ONTOLOGY1.Bank and ONTOLOGY2.Bank should become distinct sorts MERGEDONTOLOGY.RiverBank and MERGEDONTOLOGY.FinancialBank in the final ontology at the top of the diagram. ================================================ My first attemt at merging using Haskell modules ================================================= module COMMON where data Woman = WomanC data Person = PersonC module ONTOLOGY1 where import COMMON data Bank = BankC module ONTOLOGY2 where import COMMON data Bank = BankC module MERGEDONTOLOGY where import ONTOLOGY1 import ONTOLOGY2 If I use qualified names all the constructors are interpreted correctly MERGEDONTOLOGY> :t COMMON.WomanC COMMON.WomanC :: COMMON.Woman MERGEDONTOLOGY> :t COMMON.PersonC COMMON.PersonC :: COMMON.Person MERGEDONTOLOGY> :t ONTOLOGY2.BankC ONTOLOGY2.BankC :: ONTOLOGY2.Bank MERGEDONTOLOGY> :t ONTOLOGY1.BankC ONTOLOGY1.BankC :: ONTOLOGY1.Bank MERGEDONTOLOGY> :t COMMON.Woman However, I wish that these types and constructors be fully defined in the context of the MERGEDONTOLOGY. I have tried type synonyms and newtypes. type RiverBank = ONTOLOGY1.Bank type FinancialBank = ONTOLOGY2.Bank newtype RiverBank = BankC Int I have not explored import modes or qualified imports.

2 4

x -> String
by Andrew Coppin 18 Oct '09

18 Oct '09

Is there any way that you can turn an arbitrary Haskell value into a string? I rephrase: There *is* a way to turn arbitrary values into strings. I know there is, because the GHCi debugger *does* it. The question is, does anybody know of an /easy/ way to do this? Basically, I'm writing a mutable container implementation. It can hold any type of data, but it would massively aid debugging if I could actually print out what's in it. On the other hand, I don't want to alter the entire program to have Show constraints everywhere just so I can print out some debug traces (and then alter everything back again afterwards once I'm done debugging). Anybody know of a way to do this? (As it happens, the values I'm testing with are all Showable anyway, but the type checker doesn't know that...)

8 13

Happstack + network package issue on the Mac
by Sean Leather 18 Oct '09

18 Oct '09

We have run into an issue that it seems other people have already found but has apparently not been resolved, yet. It manifests as with the error "HTTP request failed with: Unsupported socket". The problem is described in two places with different workarounds. http://code.google.com/p/happstack/issues/detail?id=88 http://groups.google.com/group/HAppS/msg/0c9a0d0fd7c6aff0 One workaround is to rewrite part of the network package, and the other is to downgrade the package. It could be the case that the rewrite came from the older version, though I'm not sure. I'm curious if it is possible to fix this, either in Happstack or the network package, once and for all. The workarounds are rather painful when so many things depend on the network package. Thanks, Sean

5 9

Reverse dependencies in Hackage
by Roel van Dijk 17 Oct '09

17 Oct '09

A few weeks ago I wrote a patch that adds reverse dependencies to hackage [1]. I have know hosted a small test hackage which demonstrates this feature. It can be found here: http://bifunctor.homelinux.net/~roel/hackage Browse to your favorite packages and find out how much other packages depend on them! I also noticed that work is underway to implement a new hackage-server [2] based on happstack. If people find this feature useful I could also write a patch against the hackage-server code base [3]. Regards, Roel van Dijk [1] - http://hackage.haskell.org/trac/hackage/ticket/576 [2] - http://sparky.haskell.org:8080/ [3] - http://code.haskell.org/hackage-server/

3 2

Monadic correctness
by Andrew Coppin 17 Oct '09

17 Oct '09

Suppose we have newtype Foo x instance Monad Foo runFoo :: Foo x -> IO x What sort of things can I do to check that I actually implemented this correctly? I mean, ignoring what makes Foo special for a moment, how can I check that it works correctly as a monad.

5 7

Genuine Need For Persistent Global State?
by Alan Carter 17 Oct '09

17 Oct '09

Hi, I've been looking at the patches given by Tom at Beware the Jabberwolk, for building Linux kernel modules in Haskell. Once I'd got Tom's stuff building, the next thing was to build a little driver which actually does something. Step by step I was making progress, and I've now got a little function which can see all the characters which are catted to the device file. That's when I got stuck. Trouble is, my function is (ultimately) being called from the C kernel stuff. It isn't on the bottom of a call graph coming from a Haskell main. A driver really needs to know where it's at. So I seem to need some kind of global, persistent state, and Control.Monad.State seems to be out because I can't pass a State around my call graph. I've thought about trying to create some State when I initialize, pass some kind of pointer back to the C shim which actually does the calling into Haskell, and then passing it back into Haskell on the writes and reads, but that seems dangerous because the Haskell garbage collector would (understandably) get the wrong idea and delete it. Also it's ugly - having to use C for something Haskell can't do. I've been looking at the Halfs Haskell file system, which surely must have solved its own version of this problem, but whatever it does in its FSState seems very complicated and is beyond my comprehension :-( So my questions are: 1) Am I right in thinking that I have a genuine need for global, persistent state? 2) Halfs works. Am I right in thinking it has (somehow) solved this problem? 3) Is there a simple way to maintain global persistent state that I can stash between calls into a function, and access without needing to pass the state in? If so, is there an example anywhere? Thanks in advance, Alan -- ... the PA system was moaning unctuously, like a lady hippopotamus reading A. E. Housman ..." -- James Blish, "They Shall Have Stars"

4 6

There is no Text.Regex module
by zaxis 17 Oct '09

17 Oct '09

In Archlinux i can use Text.Regex as below: import Text.Regex let [y,m,d] = map (\x -> read x::Int) $ splitRegex (mkRegex "-") dateStr However, in Ubuntu 9.10 it doesnot work reporting no Text.Regex module. So i download and install regex-base and regex-posix from Hackage. But it still doesnot work ! $dpkg -l ghc6 名称版本简介 ================================================================================== ghc6 6.10.4-1ubuntu2 GHC - the Glasgow Haskell Compilation system SIncerely! -- View this message in context: http://www.nabble.com/There-is-no-Text.Regex-module-tp25935308p25935308.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 3

Haskell Weekly News: Issue 136 - October 17, 2009
by jfredett＠gmail.com 17 Oct '09

17 Oct '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091017 Issue 136 - October 17, 2009 --------------------------------------------------------------------------- Welcome to issue 136 of HWN, a newsletter covering developments in the [1]Haskell community. Over the last week, fpisfun from reddit announced a [2]new subreddit for simple, direct applications of Haskell to common problems which normally might be solved by a perl script or bit of bash. There are a number of examples already there, and the numbers keep growing. It's not just command-line utilities either, there is also [3]this post on calculating the bend needed for a bay window curtainrod. There is plenty of great material if you're new to Haskell and looking for some basic examples of different simple projects, examples of using monads and functors appropriately, or just want to see some 'real' programs in Haskell, it's worth a look! Also, a small correction, due to some issues with the tools, unicode characters get replaced with their ASCII 'equivalents' so that an 'e' with an umlaut becomes an 'e' character. Thanks to Wolfram Kahl for pointing out that this change is not as innocent as I thought, apologies thusly to Guenther Schmidt (I'm told thats a better approximation of the correct spelling) for the error, I apologize in advance for any other similar errors. I'm working on improving the unicode support in the tools, but it's a nasty bug to catch. This was a long editorial, so I won't take any more of your time, Haskellers, the Haskell Weekly News! Announcements Call for Contributions - Haskell Communities and Activities Report, November 2009 edition. voigt [4]called for contributions to the November 2009 issue of the Haskell Communities and Activities Report. See post for details. Call for Participation: VSTTE 2009. Jean-Christophe Filliatre [5]announced a call for participation for the VSTTE Workshop. WFLP 2010 Call for papers. Pablo Nogueira [6]announced a call for papers for the 2010 WFLP. BAHUG Next meeting: October 21st at MIT (32G-882). Ravi Nanavati [7]announced the next meeting of the Boston Area Haskell User's Group. Which will be in the CSAIL Reading Room at MIT. This editor hopes to actually make it to this one, so maybe I'll see you there! fp-southwales, the South Wales Functional Programming User Group. Andy Gimblett [8]announced the formation of fp-southwales, a user group for anybody interested in functional programming in the area of south Wales, UK. The Monad.Reader (14) - Call for copy. Brent Yorgey [9]announced the call for copy for the next issue of `The Monad.Reader`, Issue #14 Extensions to Vec uploaded (useful for GPipe programs). Tobias Bexelius [10]announced two extensions to the Vec package: Vec-Transform and Vec-Boolean. Vec-Transform provides some 4x4 transform matrices such as perspective projection and rotation. Vec-Boolean provides Data.Boolan instances for the Vec data types. Bindings to FFMpeg library. Vasyl Pasternak [11]announced the next release of the hs-ffmpeg library. Downloadable from Hackage along with the ffmpeg-tutorials, which show capabilities of this library. The installation process is a bit tricky so Vasyl put up a [12]blog post which describes the installation process. Haskell Hackathon in Zurich, Switzerland. Johan Tibell [13]announced a Hackathon in Zurich, to be held in March in the Google office. No concrete date has been decided, but if your interested, make sure to add your name [14]here. Reverse Dependencies in Hackage (demo). Roel van Dijk [15]announced his patch to add reverse-dependencies to Hackage, see the linked post for information on where to see an example version of hackage. Discussion Relational Algebra. Guenther Schmidt [16]asked about whether or not a EDSL for relational algebra in Haskell. GHC devs. Andrew Coppin [17]asked about how many GHC Developers there are, and was somewhat surprised with the answer. Fuzzy Logic / Linguistic Variables. Neal Alexander [18]demonstrated an implementation of fuzzy logic taken from the book: 'Programming Game AI by Example' by Mat Buckland. In the book, the code was in C++, rewriting it in Haskell made for much more succint, readable code. Graph Library Using Associated Types. Lajos Nagy [19]asked about using Associated types in a Graph library. Is proof by testing possible? Muad'Dib [20]asked us Haskell Mentat's about whether it was possible to prove a function correct in a finite number of tests. The discussion brought forth not only answers but some /very cool/ results about the application of compactness (in the topological sense) to testing. Blog noise [21]Haskell news from the [22]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Don Stewart (dons): [23]LACSS 2009: Domain Specific Languages and Haskell. * FP Lunch: [24]Factorising folds for faster functions. * Mikael Vejdemo Johansson (Syzygy-): [25][MATH 198] Lecture 4 and a question for the community. More of Syzygy's lectures on category theory, Syzygy also asks about what it means to 'integrate' a datatype. * Neil Brown: [26]Emulating Shared Mutable Variables with Message-Passing Processes. * Galois, Inc: [27]Tech Talk: Writing Linux Kernel Modules with Haskell. * Galois, Inc: [28]Domain Specific Languages for Domain Specific Problems. * FP-Syd: [29]Sydney FP Group: FP-Syd #18.. * Neil Brown: [30]The octopus, the boids and GHC 6.12.1rc1. * Brent Yorgey: [31]Call for submissions: Monad.Reader issue 15. * Brent Yorgey: [32]diagrams 0.2.1, and future plans. * Don Stewart (dons): [33]Self-optimizing data structures: using types to make lists faster. * Martijn van Steenbergen: [34]Context Synonyms. * Joachim Breitner: [35]arbtt: Now with Documentation. * Dan Piponi (sigfpe): [36]Vectors, Invariance, and Math APIs. * Creighton Hogg: [37]How to teach Category Theory?. Quotes of the Week * sproingie: How do you beta-reduce a problem like Maria * Baughn: There's also the language, Quine, whose interpreter is implemented as a symlink to /bin/cat * benmachine: [on the best editor] opinions vary. from correct opinions i.e. mine to other ones. [editor's comment: Obviously, this question is settled, the best editor is *garbled*] * pozic: statistics. a wonderful tool to control people. * FunctorSalad: I think the 'asks', 'gets' etc family of names is sort of cute, like the program is talking about itself in the third person * mmorrow:: Right, that's just beggin for a State monad. About the Haskell Weekly News New editions are posted to [38]the Haskell mailing list as well as to [39]the Haskell Sequence and [40]Planet Haskell. [41]RSS is also available, and headlines appear on [42]haskell.org. To help create new editions of this newsletter, please see the information on [43]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [44]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://www.reddit.com/r/EnHaskell 3. http://www.reddit.com/r/EnHaskell/comments/9syi3/calculating_the_bend_for_a… 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17572 5. http://article.gmane.org/gmane.comp.lang.haskell.general/17570 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64933 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64928 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64858 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64717 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64673 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64901 12. http://progandprog.blogspot.com/2009/10/video-processing-on-haskell-easy.ht… 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64947 14. http://haskell.org/haskellwiki/ZuriHac#Possible_Attendees 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/64915 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64899 17. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64867 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64860 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64812 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64757 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://donsbot.wordpress.com/2009/10/16/lacss-2009-domain-specific-language… 24. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=267 25. http://blog.mikael.johanssons.org/archive/2009/10/math-198-lecture-4-and-a-… 26. http://chplib.wordpress.com/2009/10/14/emulating-shared-mutable-variables-w… 27. http://www.galois.com/blog/2009/10/13/haskellkernelmodules/ 28. http://www.galois.com/blog/2009/10/13/domain-specific-languages-for-domain-… 29. http://www.mega-nerd.com/erikd/Blog/FP-Syd/fp-syd-18.html 30. http://chplib.wordpress.com/2009/10/12/the-octopus-the-boids-and-ghc-6-12-1… 31. http://byorgey.wordpress.com/2009/10/12/call-for-submissions-monad-reader-i… 32. http://byorgey.wordpress.com/2009/09/24/diagrams-0-2-1-and-future-plans/ 33. http://donsbot.wordpress.com/2009/10/11/self-optimizing-data-structures-usi… 34. http://martijn.van.steenbergen.nl/journal/2009/10/11/context-synonyms/ 35. https://www.joachim-breitner.de/blog/archives/342-arbtt-Now-with-Documentat… 36. http://blog.sigfpe.com/2009/10/vectors-invariance-and-math-apis.html 37. http://abstractabsurd.blogspot.com/2009/10/how-to-teach-category-theory.html 38. http://www.haskell.org/mailman/listinfo/haskell 39. http://sequence.complete.org/ 40. http://planet.haskell.org/ 41. http://sequence.complete.org/node/feed 42. http://haskell.org/ 43. http://haskell.org/haskellwiki/HWN 44. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

1 0

ANNOUNCE IDynamic-0.1
by Alberto G. Corona 16 Oct '09

16 Oct '09

IDynamic is variant of Data.Dynamic that can be indexed, serialized., stored, transmitted trough communications etc. So it can be used in abstract data containers, persistence, communications etc. http://hackage.haskell.org/package/IDynamic I Just uploaded it, so the documentation has not been created by Hackage. However I uincluded an example in the pachage. {-# OPTIONS -XTypeSynonymInstances #-} module Main where import Data.IResource import Data.IDynamic import Data.Typeable instance IResource Int where keyResource x= "I" serialize = show deserialize = read defPath _= "saved/" instance IResource String where keyResource x= take 5 x serialize = show deserialize = read defPath _= "saved/" main= do putStrLn "see the code to know the meaning of he results" registerType :: IO Int -- register both datatypes (Int, and String) registerType :: IO String let x= 1 :: Int let list= [IDynamic x, IDynamic "hello, how are you"] --typical Dynamic usage let assoc= zip (map keyResource list) list print $ lookup (keyResource (5 ::Int)) assoc -- lookup a IDynamic object from a container and show it mapM writeResource list mds ← readResource $ IDynamic "hello" -- save both objects case mds of Nothing → error "must have been Just!" --why? Just ds → do putStrLn $ serialize ds -- serialize the dynamic object and print it let str= fromIDyn ds :: String -- casting putStrLn str -- print the string read let y= fromIDyn ds :: Int -- casting error print y

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Fwd: [Haskell-cafe] Re: Best Editor In Windows
by Alberto G. Corona 16 Oct '09

16 Oct '09

I use Leksah. It´s an IDE . It works fine in Windows. http://leksah.org/ http://leksah.org/Leksah-0.6.1.0.exe By the way, keep the good work Leksah people !. 2009/10/16 Stefan Monnier <monnier(a)iro.umontreal.ca> > The only thing I haven't figured out is how to do tab-completion of > > words in the ghci buffer. Do I need to use a different key > > combination? I couldn't find that in the documentation. > > I think it's just a missing feature. > > > Stefan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

Generating random enums
by michael rice 16 Oct '09

16 Oct '09

What is the minimum I need to do to get this function to generate a three direction tuple? Michael ===================== import System.Random import Data.Ord data Dir = North | South | East | West deriving (Show, Read, Eq, Enum, Ord, Bounded) threeDirs :: StdGen -> (Dir,Dir,Dir) threeDirs gen = let (firstDir, newGen) = random gen (secondDir, newGen') = random newGen (thirdDir, newGen'') = random newGen' in (firstDir, secondDir, thirdDir) ===================== GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l dir.hs [1 of 1] Compiling Main ( dir.hs, interpreted ) dir.hs:15:29: No instance for (Random Dir) arising from a use of `random' at dir.hs:15:29-42 Possible fix: add an instance declaration for (Random Dir) In the expression: random newGen' In a pattern binding: (thirdDir, newGen'') = random newGen' In the expression: let (firstDir, newGen) = random gen (secondDir, newGen') = random newGen (thirdDir, newGen'') = random newGen' in (firstDir, secondDir, thirdDir) Failed, modules loaded: none. Prelude>

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Fwd: [Haskell-cafe] x -> String
by Alberto G. Corona 16 Oct '09

16 Oct '09

yep add Show to your abstrac container. , for example: data SDynamic= forall a.Show a => SDynamic a instance Show SDynamic where show (SDynamic a)= show a 2009/10/16 David Virebayre <dav.vire%2Bhaskell(a)gmail.com<dav.vire%252Bhaskell(a)gmail.com> > On Fri, Oct 16, 2009 at 8:19 PM, Andrew Coppin > <andrewcoppin(a)btinternet.com> wrote: > > Is there any way that you can turn an arbitrary Haskell value into a > string? > > > I rephrase: There *is* a way to turn arbitrary values into strings. I > know > > there is, because the GHCi debugger *does* it. The question is, does > anybody > > know of an /easy/ way to do this? > > Ghci only displays values with a Show instance. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 0

package name in TypeRep?
by Johannes Waldmann 16 Oct '09

16 Oct '09

Dear all, I want a function that produces the URL of the haddocumentation of a given type, generically. My solution is that the argument (type) is an instance of Data.Typeable and I am parsing the output of show of the TypeRep, cf. http://dfa.imn.htwk-leipzig.de/cgi-bin/cvsweb/common/Gateway/Help.hs?rev=1.… That stops working when I split my project into several packages. Is there a way to obtain the package name from the TypeRep? Because I seem to need it to procuce the correct URL. Or else, is there a way to merge all the per-package haddock outputs? Perhaps a pseudo-package that imports all and re-exports everything? Best - Johannes.

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Merging classes
by pat browne 16 Oct '09

16 Oct '09

Hi, Following on from my earlier *Merging modules* email, where I tried to simulate ontology merging with Haskell modules; http://article.gmane.org/gmane.comp.lang.haskell.cafe/64943 I *think* that the type of merging that I am attempting is difficult with modules so I gave classes a try. I did not get very far (see below). Perhaps I need a combination of type classes and modules. In fairness this is more of a specification task than a programming task and perhaps I am barking up the wrong tree. Any help appreciated. Regards, Pat ============================================== attempting an ontology merge with type classes For specification of requirement see http://article.gmane.org/gmane.comp.lang.haskell.cafe/64943 ============================================== -- Data types for COMMON data Woman = WomanC data Person = PersonC -- Additional data type for ONTOLOGY1 data Bank = BankC -- Additional data type for ONTOLOGY2 -- but how do I let type Bank have the same name in both classes and -- still be distinct (homonyms) in the merged class -- data Bank = BankC class COMMON a b where class (COMMON a b) => ONTOLOGY1 a b where class (COMMON a b) => ONTOLOGY2 a b where class (ONTOLOGY1 a b, ONTOLOGY2 a b) => MERGEDONTOLOGY a b where -- not really sure how to proceed from here instance COMMON Woman Person where

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How to reliably open and close resources in MonadIO in the presence of exceptions?
by Bas van Dijk 16 Oct '09

16 Oct '09

> import Control.Monad.Trans ( MonadIO, liftIO ) > import Control.Exception ( throwIO, ArithException(DivideByZero), onException ) > import Foreign.Marshal.Alloc ( malloc, free ) > import Foreign.Storable ( poke, peek ) Suppose we define a function which receives a computation in an arbitrary monad that is capable of lifting IO actions and returns a computation in the same monad: > foo :: MonadIO m => m () -> m () In 'foo', I would like to open some resource (in this case: allocate memory) and after performing some lifted IO actions and some arbitrary computations in 'm' I want to close the resource (in this case: freeing the allocated memory). Of course the resource should also be closed when an exception is thrown in some lifted IO computation. If I was performing a computation directly in IO I could use: alloca :: Storable a => (Ptr a -> IO b) -> IO b which allocates the memory, performs some action on it and frees the memory after the action terminates (either normally or via an exception). Unfortunately the type of 'alloca' does not allow it to be used in our arbitrary monad 'm'. So it looks like I'm forced to manually allocate the memory using 'malloc' and finally free it using 'free'. To handle exceptions in lifted IO computations I have to put a (`onException` free ptr) around every lifted IO computation: For example: > foo action = do ptr <- liftIO malloc > liftIO $ putStrLn "aaa" `onException` free ptr > liftIO $ poke ptr (3 :: Int) `onException` free ptr > liftIO $ putStrLn "bbb" `onException` free ptr > action > x <- liftIO $ peek ptr `onException` free ptr > liftIO $ print x `onException` free ptr > liftIO $ putStrLn "ccc" `onException` free ptr > liftIO $ free ptr I find this rather ugly. What's worse is that exceptions, thrown in lifted IO computations in 'action' can't be handled! For example, the following won't free the memory: > bar :: MonadIO m => m () > bar = foo action > where > action = liftIO $ throwIO DivideByZero So my question is: How can I reliably open and close resources in MonadIO? And as a supplementary question: Is it possible to define a generalized alloca: genAlloca :: (MonadIO m, Storable a) => (Ptr a -> m b) -> m b Thanks, Bas

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ANN: Haskell Hackathon in Zurich, Switzerland
by Johan Tibell 16 Oct '09

16 Oct '09

Hi all! We are in the early stages of planning a Haskell hackathon/get together, ZuriHac, to be held this March at the Google office, in Zurich, Switzerland. Right now we're looking at four possible dates (Friday-Sunday): March 5-7, March 12-14, March 19-21, or March 26-28. If you might be interested in attending, please add your name on the wiki page: http://haskell.org/haskellwiki/ZuriHac#Possible_Attendees If you don't have an account on the wiki feel free to just respond to this email. If there are any dates that absolutely don't work for you then please note them on the wiki. I'll come back with a finalized date and more details soon. Hope to see you in Zurich! Johan Tibell (tibbe)

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Why there is not standard Monoid instance for ZipList a?
by Vladimir Reshetnikov 16 Oct '09

16 Oct '09

I find the following instance very convenient: ------------------------------------------------------------------------ import Data.Monoid import Control.Applicative instance Monoid a => Monoid (ZipList a) where mempty = pure mempty mappend = liftA2 mappend ------------------------------------------------------------------------ Any reason why it is not in the standard library? Thanks, Vladimir

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ANN: WFLP 2010 Call for papers
by Pablo Nogueira 16 Oct '09

16 Oct '09

[Apologies for multiple receptions of this message] ******************************************************************** Preliminary Call For Papers 19th International Workshop on Functional and (Constraint) Logic Programming Madrid, Spain, January 17, 2010 http://babel.ls.fi.upm.es/events/wflp2010/ ********* colocated with Principles of Programming Languages POPL 2010 http://www.cse.psu.edu/popl/10/ ******************************************************************** IMPORTANT DATES Abstract Submission: November 9, 2009 Full Paper Submission: November 15, 2009 Acceptance Notification: December 15, 2009 Preliminary Proceedings: January 5, 2010 Workshop: January 17, 2010 SCOPE The aim of the Workshop on Functional and (Constraint) Logic Programming is to bring together researchers interested in functional programming and (constraint) logic programming with special emphasis on the integration of both paradigms and of other declarative programming extensions. It promotes the cross-fertilizing exchange of ideas and experiences among researchers and students from the different communities interested in the foundations, applications, and combinations of high-level declarative programming languages and related areas. The previous WFLP editions are: WFLP 2009 (Brasilia, Brazil), WFLP 2008 (Siena, Italy), WFLP 2007 (Paris, France), WFLP 2006 (Madrid, Spain), WCFLP 2005 (Tallinn, Estonia), WFLP 2004 (Aachen, Germany), WFLP 2003 (Valencia, Spain), WFLP 2002 (Grado, Italy), WFLP 2001 (Kiel, Germany), WFLP 2000 (Benicassim, Spain), WFLP'99 (Grenoble, France), WFLP'98 (Bad Honnef, Germany), WFLP'97 (Schwarzenberg, Germany), WFLP'96 (Marburg, Germany), WFLP'95 (Schwarzenberg, Germany), WFLP'94 (Schwarzenberg, Germany), WFLP'93 (Rattenberg, Germany), and WFLP'92 (Karlsruhe, Germany). LOCATION WFLP 2010 will be held on January 17, 2010 in Madrid, Spain, colocated with the 37th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages (POPL 2010). WFLP 2010 solicits papers in all areas of functional and (constraint) logic programming, including but not limited to: * Foundations: formal semantics, logic variables, binding and abstract syntax, rewriting and narrowing, unification, constraint solving, dynamics, type theory, meta-theory, effects, etc. * Language Design: security, services, modules, type systems, multi-paradigm languages, concurrency and distribution, objects, libraries, generic programming, interoperability, etc. * Implementation: abstract machines, parallelism, compile-time and run-time optimizations, foreign-language interfaces, memory management, multi-threading, exploiting parallel hardware, etc. * Transformation and Analysis: abstract interpretation, specialization, partial evaluation, program transformation, program calculation, program proof, meta-programming, generative programming, etc. * Software Development: algorithms, data structures, design patterns, components and composition, specification, proof assistants, verification and validation, model checking, debugging, testing, profiling, tracing, etc. * Paradigm Integration: integration of declarative programming with other paradigms or features such as imperative, object-oriented, aspect-oriented, concurrent, real-time programming, event-driven architectures, etc. * Applications: education, industry, commercial uses, domain-specific languages, visual/graphical user interfaces, embedded systems, WWW applications, XML processing, artificial intelligence, knowledge representation and machine learning, deductive databases, advanced programming environments and tools, etc. SUBMISSIONS and PROCEEDINGS Authors are invited to submit papers presenting original unpublished work. Papers must be at most 15 pages long. Exceptionally, authors may surpass the page limit by providing well-marked appendices intended as reviewing aids. Appendices will not appear in the final publication. Submission categories include regular research papers, system descriptions, and short papers describing on-going work (at most 8 pages). Submissions must be formatted in Lecture Notes in Computer Science (LNCS) style. This requirement needs not apply to appendices. Papers should be submitted in pdf or postscript format electronically via the web-based submission site http://www.easychair.org/conferences/?conf=wflp2010 Preliminary proceedings will be available at the workshop. Selected authors will be invited to submit a full version of their papers after the workshop. Contributions accepted for the post-workshop proceedings have usually appeared in journals -- those of the 2009 edition have been published in LNCS, previous editions in ENTCS, etc. PROGRAMME CHAIR Julio Marino (Universidad Politecnica de Madrid, Spain) PROGRAMME COMMITTEE Maria Alpuente (Universidad Politecnica de Valencia, Spain) Sergio Antoy (Portland State University, USA) Rachid Echahed (CNRS-IMAG, France) Santiago Escobar (Universidad Politecnica de Valencia, Spain) Moreno Falaschi (Universita di Siena, Italy) Murdoch Gabbay (Heriot-Watt University, UK) Maria Garcia de la Banda (Monash University, Australia) Victor Gulias (Lambdastream SL, Spain) Michael Hanus (CAU Kiel, Germany) Herbert Kuchen (Univ. of Muenster, Germany) Francisco Lopez-Fraguas (Universidad Complutense de Madrid, Spain) James Lipton (Wesleyan University, USA) Mircea Marin (University of Tsukuba, Japan) Juan Jose Moreno-Navarro (Ministry of Science & Innovation, Spain) Brigitte Pientka (McGill University, Canada)

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Re: Relational Algebra
by oleg＠okmij.org 16 Oct '09

16 Oct '09

> In hindsight it occurred to me that the algorithm can be abstractly > expressed in terms of relational algebra for which I need an EDSL. > > One concrete implementation could then be an interpreter / compiler to > SQL, if I choose to use a database backend, and another implementation > could be a interpretor to in-memory data structures (maps, whathaveyou), > or a debugger to see if I have got the abstract algorithm right. Perhaps the CoddFish project (what a great name!) may be relevant here: http://wiki.di.uminho.pt/twiki/bin/view/Research/PURe/CoddFish Perhaps it is the first project that systematically exploited the connection between functional dependencies in type classes and functional dependencies among database attributes.

1 0

Very evil
by Andrew Coppin 16 Oct '09

16 Oct '09

{-# LANGUAGE EmptyDataDecls #-} module Main (main) where import Unsafe.Coerce data Anything newtype Key x = Key Int deriving Eq type Dict = [(Key Anything, Anything)] put :: Key x -> x -> Dict -> Dict put k' v' = raw (unsafeCoerce k') (unsafeCoerce v') where raw k0 v0 [] = [(k0,v0)] raw k0 v0 ((k,v):zs) | k == k0 = (k0 ,v0) : zs | otherwise = (k ,v ) : raw k0 v0 zs get :: Key x -> Dict -> Maybe x get k' zs = unsafeCoerce (raw (unsafeCoerce k') zs) where raw k0 [] = Nothing raw k0 ((k,v):zs) | k == k0 = Just v | otherwise = raw k0 zs main = do let k1 = Key 1 :: Key Int let k2 = Key 2 :: Key Double let k3 = Key 3 :: Key String let k4 = Key 4 :: Key Bool let d0 = [] let d1 = put k1 123 d0 let d2 = put k2 123 d1 let d3 = put k3 "123" d2 let d4 = put k4 True d3 print (get k1 d4) print (get k2 d4) print (get k3 d4) print (get k4 d4) Unsafe coerce, anyone? This particular example appears to run without incident, but the GHC docs suggest that this is very... well, unsafe. (In particular, the docs claim this will fall over on function types.) I presume there's some less-evil way of doing this?

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Fwd: [BostonHaskell] Next meeting: October 21st at MIT (32G-882)
by Ravi Nanavati 16 Oct '09

16 Oct '09

---------- Forwarded message ---------- From: Ravi Nanavati <ravi_n(a)alum.mit.edu> Date: Thu, Oct 15, 2009 at 11:07 PM Subject: [BostonHaskell] Next meeting: October 21st at MIT (32G-882) To: bostonhaskell(a)googlegroups.com I'm pleased to announce the October meeting of the Boston Area Haskell Users' Group. Based on the feedback from the meeting polls and the constraints of our speakers, the October meeting has been scheduled for Wednesday, October 21st from 7pm - 9pm. Like the past three meetings, it will be held in the MIT CSAIL Reading Room (32-G882, i.e. a room on the 8th floor of the Gates Tower of the MIT's Stata Center at 32 Vassar St in Cambridge, MA). Based on the topic poll (and because he volunteered), our first speaker will be Brian Sniffen with an introduction to Parsec. Following his presentation will be our traditional break (with the now-traditional baked goods). Our second speaker will be Edward Kmett, who will be picking up the part of his monoids and parsing presentation that we didn't get to in August or September (third time's the charm!): "A Parallel Parsing Trifecta: Iteratees, Parsec, and Monoids". Edward's slides from August are here: http://comonad.com/reader/wp-content/uploads/2009/08/IntroductionToMonoids.… I'd recommend looking at them if you weren't at the August meeting (or want a refresher on the material) since the two parts of his presentation were designed to go concurrently. There's also a preview of the presentation as part of this post on Edward's blog: http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ The October attendance poll is at: http://doodle.com/u7762nfxpghsrn2r As always, responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, since I have talked my wife into baking some more goodies and bringing drinks again. Helping with BostonHaskell (besides volunteering to speak at future meetings): 1. Sponsorship of refreshments is still being eagerly solicited. 2. Please let me know if you're willing to arrive early (6:30pm) to help me move refreshments from my car. Based on past experience, I could use 3-4 volunteers. 3. Please let me know if you're willing to arrive early (6:30pm) to stand by the door and let people into the 8th floor. I'm guessing 1 or 2 volunteers is enough here. 4. If someone is willing to edit the HaskellWiki page, post to reddit or otherwise publicize the meeting please just go ahead and do that (especially as I'm sending this out a bit later than intended). 5. I'm also still having trouble embedding this poll in a page on our Google Group, so if anyone has any ideas about that, please feel free to try (any group member can edit pages) and/or offer other suggestions. If you have any questions about the meeting please send them to the BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the October meeting! Thank you, - Ravi Nanavati --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "BostonHaskell" group. To post to this group, send email to bostonhaskell(a)googlegroups.com To unsubscribe from this group, send email to bostonhaskell%2Bunsubscribe(a)googlegroups.com For more options, visit this group at http://groups.google.com/group/bostonhaskell?hl=en -~----------~----~----~----~------~----~------~--~---

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ANN: Reverse Dependencies in Hackage (demo)
by Roel van Dijk 15 Oct '09

15 Oct '09

Hello, I have implemented reverse dependencies in Hackage. You can play with the demo here: http://bifunctor.homelinux.net/~roel/hackage I already send a message to Haskell-Cafe, but I made a few changes and would like some feedback. You can sort the tables of reverse dependencies by clicking on the column headers. Large tables (base or the total overview) might take some time. Sorting is done client-side with a bit of JavaScript. I added a link to an overview of the reverse dependencies of every package in hackage. It is sorted by the total number of rev. deps by default, so the most dependent-on packages (within hackage) are at the top. In the future such a list might be expanded with other information, like the download count. Things I would like feedback on: - Rev. deps overview for all packages; is it useful? - If you look at the reverse dependencies for a package like digest you'll see a list of packages that depend on digest. Next to the names of the packages that depend on digest are their respective reverse dependencies. The idea is that you can see which are the most "important" (for some value of important) packages that depend on digest. Is that useful? - Is this the best way to present the information? Currently I am working on a patch for the new hackage-server. Regards, Roel

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Parsec bug, or...?
by Uwe Hollerbach 15 Oct '09

15 Oct '09

a brain fart? Hi, cafe, I've been playing a little bit with a small command processor, and I decided it'd be nice to allow the user to not have to enter a complete command, but to recognize a unique prefix of it. So I started with the list of allowed commands, used filter and isPrefixOf, and was happy. But then I increased the complexity a little bit and it got hairier, so I decided to rewrite the parser for this bit in parsec. The function I came up with is parsePrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs) which I call as parseFoo = parsePrefixOf 1 "foo" and it recognizes all of "f", "fo", and "foo" as "foo". OK so far, this also seems to work fine. But during the course of writing this, I made a stupid mistake at one point, and the result of that seemed odd. Consider the following program. It's stupid because the required prefix of "frito" is only 2 characters, which isn't enough to actually distinguish this from the next one, "fromage". (And if I change that to 2 to 3 characters, everything works fine.) So here's the complete program module Main where import Prelude import System import Text.ParserCombinators.Parsec as TPCP myPrefixOf n str = string (take n str) >> opts (drop n str) >> return str where opts [] = return () opts (c:cs) = optional (char c >> opts cs) myTest = myPrefixOf 1 "banana" <|> myPrefixOf 1 "chocolate" <|> TPCP.try (myPrefixOf 2 "frito") <|> myPrefixOf 3 "fromage" myBig = spaces >> myTest >>= (\g -> spaces >> eof >> return g) parseTry input = case parse myBig "test" input of Left err -> return (show err) Right val -> return ("success: '" ++ val ++ "'") main = getArgs >>= (\a -> parseTry (a !! 0)) >>= putStrLn If I compile this, say as program "opry", and run it as shown below, I expect the results I get for all but the last one: % ./opry b success: 'banana' % ./opry c success: 'chocolate' % ./opry fr success: 'frito' % ./opry fri success: 'frito' % ./opry fro "test" (line 1, column 3): unexpected "o" expecting "i", white space or end of input Sooo... why do I get that last one? My expectation was that parsec would try the string "fro" with the parser for "frito", it would fail, having consumed 2 characters, but then the TPCP.try which is wrapped around all of that should restore everything, and then the final parser for "fromage" should succeed. The same reasoning seems to me to apply if I specify 3 characters as the required initial portion for "frito", and if I do that it does succeed as I expect. So is this a bug in parsec, or a bug in my brain? thanks... Uwe

7 11

more improvable randomness
by Michael Mossey 15 Oct '09

15 Oct '09

I wrote some code to model the keystroke-to-keystroke delay in a person typing, with pseudorandomness. There are two kinds of delays.. one is a very small delay as the person reaches for a new key (call this 'reach' delays), the other is a larger delay that represents a pause to think or to take a break (call this 'break' delays). My thought is that I could create an infinite series of reach delays and an infinite series of break delays and zipWith (+). The inifinite series of break delays would look like: - The overall pattern is a large number of zeros, in the range (N,M) followed by a single positive number, chosen from a distribution D... and then repeat. Here's some code. I used the Gen monad to gain access to the 'frequency' function, which is useful, but I had trouble figuring out how to put the whole algorithm into Gen. I also looked at Rand. The problem is that if I want to create the list by 'sequence'ing a list of monads, they need to have state that remembers how many zeros are left to go. breakSeries :: Int -> Int -> StdGen -> [Float] breakSeries lowerB upperB gen = let (n,gen1) = randomR (lowerB,upperB) gen (gen2,gen3) = split gen1 delay = generate 1 gen2 breakM in replicate n 0 ++ [delay] ++ breakSeries lowerB upperB gen3 breakM :: Gen Float breakM = frequency [ (10, choose( 1::Float , 2)) , (10, choose( 4::Float , 6)) ] test = (print . take 100 . breakSeries 2 4 ) =<< newStdGen

2 1

Re: [Haskell-cafe] Need help with ghc static compile for cgi using mysql
by Austin King 15 Oct '09

15 Oct '09

For now, I've given up on cheap hosting (via statically compiled CGI). I've created a GHC 6.10.4, cabal-install, Ubuntu 9.04 ec2 instance + MySQL and it works well. Running this plus EBS and Elastic IP will run a little over $60 a month. Ouch. The work going into Haskell Platform is amazing, but... Haskell needs a better commodity hosting story, to allow for casual, fun web hacking. For the record, I tried nearlyfreespeech.net and dreamhost. I wasn't able to install ghc and cabal due to: * FreeBSD GHC support (old version) * FreeBSD library support (no HDBC) * Linux - Missing 'dev' libraries like zlib and not having root access

4 3

CBN, CBV, Lazy in the same final tagless framework
by oleg＠okmij.org 15 Oct '09

15 Oct '09

Actually it is possible to implement all three evaluation orders within the same final tagless framework, using the same interpretation of types and reusing most of the code save the semantics of lam. That is where the three orders differ, by their own definition. In call-by-name, we have lam f = S . return $ (unS . f . S) In call-by-value, we have lam f = S . return $ (\x -> x >>= unS . f . S . return) In call-by-need, we have lam f = S . return $ (\x -> share x >>= unS . f . S) In CBV, the function first evaluates its argument, whether the value will be needed or not. In call-by-need, we arrange for the sharing of the result should the computation be evaluated. Here is an illustrative test t2 :: HOAS exp => exp IntT t2 = (lam $ \z -> lam $ \x -> let_ (x `add` x) $ \y -> y `add` y) `app` (int 100 `sub` int 10) `app` (int 5 `add` int 5) t2SN = runName t2 >>= print {- *CB> t2SN Adding Adding Adding Adding Adding Adding Adding 40 -} In CBN, the result of subtraction was not needed, and so it was not performed OTH, (int 5 `add` int 5) was computed four times. t2SV = runValue t2 >>= print {- *CB> t2SV Subtracting Adding Adding Adding 40 -} In CBV, although the result of subtraction was not needed, it was still performed. OTH, (int 5 `add` int 5) was computed only once. t2SL = runLazy t2 >>= print {- *CB> t2SL Adding Adding Adding 40 -} Now, Lazy is better than both CBN and CBV: subtraction was not needed, and it was not performed. All other expressions were needed, and evaluated once. The complete code is at http://okmij.org/ftp/tagless-final/CB.hs

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Re: [Haskell-cafe] Finally tagless - stuck with implementation of "lam"
by Jacques Carette 15 Oct '09

15 Oct '09

(sorry for the slow reply on this topic...) Robert Atkey and Oleg presented some very interesting code in response to your query. But some of you might (and should!) be asking "why on earth did Jacques use unsafePerformIO?", especially when neither Robert nor Oleg did. Simply put: I answered your question exactly as asked, while they answered the question you *should* have asked. At the exact 'type' involved in your question, there is no good answer; if you want an instance of lam at a monadic type 'directly', you need to 'extract' from the monad. But the point is that that isn't really the right question. Rather than requiring complete parametric polymorphism in the answer-type, if you allow a little non-uniformity through a simple type family, the shift in types is sufficient to no longer require a monadic 'extract' at all. But even once you've gotten to that point, that's not enough, because at that point you still have the issue of what calling convention (by value, name or need) to use. And figuring that out is rather fun, so you got detailed answer from Robert and Oleg explaining that part in detail. Robert carefully used IntT and :-> to label the different types, and Oleg's code (http://okmij.org/ftp/tagless-final/CB.hs) showed how these were really just 'labels', which are useful mnemonics for humans, but not essential. If you dig into our JFP paper describing "finally tagless" in all its gory details, you'll see that we use a poor man's version of type families there too (see section 4.3). Jacques

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Relational Algebra
by Günther Schmidt 15 Oct '09

15 Oct '09

Hi all, has anybody already developed an EDSL for relational algebra? HaskellDB does that, but its current implementation only allows for generating SQL, are there other implementations? Günther

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