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ANN: Version 0.14 of dataenc, version 0.5 of omnicodec
by Magnus Therning 03 Mar '11

03 Mar '11

I've just uploaded a new version of dataenc to hackage[1]. It contains a large change to the API. The old, rather simplistic, lazy API has been removed. It has been replaced by an API based on incremental encoding/decoding. This should make the library easier to use together with left-fold enumerators such as enumerator[2] and iteratee[3]. It's accompanied by a release of omnicodec[4] which makes use of the modified API of dataenc. /M [1] http://hackage.haskell.org/package/dataenc [2] http://hackage.haskell.org/package/enumerator [3] http://hackage.haskell.org/package/iteratee [4] http://hackage.haskell.org/package/omnicodec -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Most software today is very much like an Egyptian pyramid with millions of bricks piled on top of each other, with no structural integrity, but just done by brute force and thousands of slaves. -- Alan Kay

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A simple attoparsec question
by Robert Clausecker 03 Mar '11

03 Mar '11

Hi Haskellers! I'm currently trying to write an assembler for Knuths MMIXAL language. Currently, I'm writing the parser using attoparsec, and have one question: In MMIXAL, a string literal starts with a ", followed by an arbitrary amount of arbitrary characters excluding the newline character and finished with another ". For example: "Hello, World!" is a string, "ſ" is a string too, but "\n" (\n is a newline) isn't. Here's the parser, which parses a s tring and other kinds of constants: parseConstant = Reference <$> try parseLocLabel <|> PlainNum <$> decimal <|> char '#' *> fmap PlainNum hexadecimal <|> char '\'' *> (CharLit <$> notChar '\n') <* char '\'' <|> try $ (char '"' *> (StringLit . B.pack <$> manyTill (notChar '\n') (char '"'))) <?> "constant" The problem is, that attoparsec just silently fails on this kind of strings and tries other parsers afterwards, which leads to strange results. Is there a way to force the whole parser to fail, even if there's an alternative parser afterwards? I hope, you understand my question. Yours, Robert Clausecker

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Re: [Haskell-cafe] Haskell IDE
by Michal Konečný 03 Mar '11

03 Mar '11

On Thu, Mar 3, 2011 at 9:05 AM, Hauschild, Klaus (EXT) <klaus.hauschild.ext(a)siemens.com> wrote: > whats your Haskell IDE of choise? Currently I use leksah. Is the EclipseFP > Plugin for Eclipse a real alternative? I use EclipseFP 2.0.2 on a few fairly large projects and am overall very happy with it despite a few flaws, eg: - it currently signals an error in source code files with non-latin-1 unicode characters - it does not build projects that include C files - I find it fairly slow on very large projects as it continuously rebuilds them as you type, but it is not as slow as this kind of approach may seem to imply I especially appreciate: - how well it highlights compiler errors - the outline of a module I do not use code completion beyond the generic Alt-/, so I do not have an opinion on how well this is supported in eclipsefp. Michal -- |o| Michal Konecny <mikkonecny(a)gmail.com> |o| http://www-users.aston.ac.uk/~konecnym/ |o| office: (+42) (0)121 204 3462 |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aow

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Terminal library : which ?
by Permjacov Evgeniy 03 Mar '11

03 Mar '11

What terminal library you will recomedn? Requirements: crossplatform (win/lin), with direct (i.e. with line/column number pair) cursor positioning and possybly direct symbol output. MUST provide function to get terminal dimensions. (could not find one).

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Re: [Haskell-cafe] Project structure
by John Lato 03 Mar '11

03 Mar '11

> > From: "Hauschild, Klaus (EXT)" > > Hi Haskellers, > > is there a recommended structure for Haskell projects. I like the Maven way > ( > http://maven.apache.org/guides/introduction/introduction-to-the-standard-di…) > for Java projects. How to separate productive from test code, how to > separate source code from other resources? > I like the way the Snap-framework is organized: ./ ./src/ ./test/ ./test/data/ ./test/suite/ ./snap-framework.cabal Then if you have e.g. ./src/Snap/Types.hs the tests go in ./tests/suite/Snap/Types/Tests.hs Basically "src" is for all productive code, "test" is for tests of that code. I sometimes add these to the top level: ./tools ./resources "tools" is for code generators or similar tools that auto-generate stuff to go in ./src which cabal doesn't know about. "resources" would be data files, images, or similar resources that get bundled with the distribution. John L.

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Haskell KMP(Knuth-Morris-Pratt) algorithm
by larry.liuxinyu 03 Mar '11

03 Mar '11

Hi, I read about some KMP implementation in Haskell including: [1] Richard Bird. ``Pearls of Functional algorithm design'' [2] http://twan.home.fmf.nl/blog/haskell/Knuth-Morris-Pratt-in-Haskell.details [3] http://www.haskell.org/haskellwiki/Runtime_compilation [4] LazyString version [1] builds a infinite lazy state transfer trees, while [3] uses index to build overlap table. I created a version which isn't as efficient as in [1]. Just for fun: failure :: (Eq a)=> ([a], [a]) -> ([a], [a]) failure ([], ys) = ([], ys) failure (xs, ys) = fallback (init xs) (last xs:ys) where fallback as bs | as `isSuffixOf` xs = (as, bs) | otherwise = fallback (init as) (last as:bs) kmpSearch2 :: (Eq a) => [a] -> [a] ->[Int] kmpSearch2 ws txt = snd $ foldl f (([], ws), []) (zip txt [1..]) where f (p@(xs, (y:ys)), ns) (x, n) | x == y = if ys==[] then ((xs++[y], ys), ns++[n]) else ((xs++[y], ys), ns) | xs == [] = (p, ns) | otherwise = f (failure p, ns) (x, n) f (p, ns) e = f (failure p, ns) e The function failure just follows the idea that in case (xs, ys) fails matching some letter c in text, where xs++ys = pattern and c!= head ys, it means we must fallback to (xs', ys') so that xs' = longest { s: s is prefix of xs AND s is suffix of xs } The bad thing is that failure can't memorize what it has compute before, for example, as pattern = "ababc" and we fails at ("abab", "c"), then we call function failure to get the new one as ("ab", "abc"). After several matches, we fails again at ("abab", "c"), failure can't just return ("ab", "abc") what it has been compute already. It has too do the same work again. Function f inside kmpSearch2 is in fact a state-transfer function. If we try to use some data structure (for example tree) to memorize the results which failure function calculated, we can finally reach to the idea in [1]. -- LIU http://sites.google.com/site/algoxy/

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Re: [Haskell-cafe] Rank-2 types in classes
by oleg＠okmij.org 03 Mar '11

03 Mar '11

Yves Pare`s wrote: > I'm working on a library which aims to be a generic interface for 2D > rendering. To do that, one of my goals is to enable each implementation of > this interface to run in its own monad (most of the time an overlay to IO), > thus giving me the following class > > class (Monad (IM i x)) => Impl i x where > data IM i x :: * -> * > > (where IM means Implementation Monad) > > I would like to write something like : > > class (forall x. Monad (IM i x)) => Impl i where > data IM i :: * -> * -> * It is not clear if the class constraint is really needed. As an aside, a class constraint is perhaps a bit of mis-feature of type classes: it sure improves convenience by making signatures shorter. But it isn't really necessary. Perhaps there are other, better ways of achieving the convenience (the constraint alias proposal comes to mind). If we drop the class constraint, we can move Monad (IM i x) as the constraint on specific methods of the Impl class. The implicit uiniversal quantification on x is well allowed then. For example: > class Impl (i :: * -> *) where > data IM i :: * -> * -> * > foo :: Monad (IM i x) => Int -> IM i x Int > bar :: Monad (IM i x) => IM i x Int -> IM i x Bool > cast :: IM i x a -> IM i y a > > data Window > > instance Impl IO where > newtype IM IO x a = IMIO (IO a) > foo = IMIO . return > bar (IMIO x) = IMIO (fmap (> 42) x) > cast (IMIO x) = IMIO x > > test :: (Monad (IM i Window), Impl i) => IM i Window Int -> IM i x Bool > test = cast . bar Perhaps this isn't what you had in mind; I more elaborate example would help then.

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Where to put a library
by Richard Senington 03 Mar '11

03 Mar '11

Hi all, I am working on ways to implement local search meta-heuristics in Haskell. I need various test problems to experiment on, and this has resulted in me building various file parsers and data structures for these. Currently I have versions for SAT, TSP and a couple of more private file formats. The file parsers are designed to process files coming out of the TSPLIB and SATLIB repositories. I also want to extend this to cover scheduling and timetabling example problems. Since these are all related I was going to try to put them together into a single library and post them to Hackage, but I am not sure what to put them under. I assume that Data is for data structures, which is not quite right. I was thinking about something like Problems. as a top level point in a hierarchy. Does anyone have any other suggestions, as this seems a little odd as a name to me? Alternatively, does this already exist and I have just missed it? Cheers RS

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Learning list filtering using puzzles
by Karthick Gururaj 03 Mar '11

03 Mar '11

Hi all, I'm very new to Haskell, having started earlier this week. This is my second attempt after two years :) Currently, I can do simple list processing/recursion/functions. I'm using GHC on Ubuntu. I decided to use puzzles as a way to explore list filtering. For example, while "normal" solution for solving a quadratic is to compute the solution algebraically, I tried: solvequad :: Int -> Int -> Int -> [Int] solvequad a b c = [x | x <- [1..10000], a*x*x + b*x + c == 0] This works well, of course assuming the roots are in the given integer range. Next, to solve the puzzle: "A stone weighing 40 kg falls from a certain height and is split into four pieces. With the help of these four pieces, one can weigh any quantity from one kg to 40 kg by using these in a scale (both plates of the scale can be used for placing any stone pieces at a time). Find the weight of each piece." I tried: solvepuzzle = [(a,b,c,d) | a <- [1..40], b <- [a..40], c <- [b..40], d <- [c..40], a+b+c+d == 40, sort [p*a + q*b + r*c + s*d | p <- [-1,0,1], q <- [-1,0,1], r <- [-1,0,1], s <- [-1,0,1], p*a+q*b+r*c+s*d > 0] == [1..40]] It works, but I'm not too happy with the last constraint.. Is there a better way to express: "..one can weigh any quantity from 1kg to 40 kgs.." ? A more interesting candidate - considering the previous ones can be also solved with a language like C easily: consider the well know honestans/swindle-cats puzzle: "there are two gate keepers guarding two doors - one that leads to hell and the other to heaven. One of these keepers always speaks the truth and the other always lies (and will only answer questions that permit a "yes" or "no" for answer). We don't know who is the honest one. If allowed to ask one question to one of the keepers, what would you ask to find out which is the gate to heaven?" I'm trying to see how to formulate this in a way Haskell can lead me to a solution.. This is where I'm at now (in "psedo" Haskell code). --- Oracle knows answers to all questions and will answer truthfully oracle :: Question -> Bool -- Ignoring the definition for now --- invOracle knows answers to all questions but will always lie invOracle :: Question -> Bool invOracle x = not (oracle x) As far as the answer we are looking for, the truth val of "does the first door lead to heaven?" is what we want. I represent this Question as 'q'. So in my notation: keeper1 q - ask the question to keeper 1 oracle q - ask oracle the question (not . oracle) q - ask oracle the question, and then negate it (keeper1 . keeper2) q - Ask keeper 1, "Suppose I ask keeper 2 the question 'q', what would he say?" .. etc Suppose there is a way to generate "all possible questions" that can be asked to the two gate-keepers.. a generator function (say "gen") that takes three parameters: keeper1, keeper2 and q and returns a list of questions (like the examples above). The solution(s) to the puzzle is then (something like..): [x | keeper1 <- [oracle, invOracle], keeper2 <- [oracle, invOracle], keeper1 /= keeper2, x <- gen keeper1 keeper2 q, x q == oracle q] The answer I know is "not (keeper1 (keeper2 q))". Thinking through the puzzle now, I realized even "keeper1 (keeper1 q)" can be an answer. How would I go about defining such a generator function? I'm not looking for a detailed solution, but hints what I can look up/refer. Any other comments on the approach above would be gratefully received :) Cheers, Karthick

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Haskell Weekly News: Issue 171
by Daniel Santa Cruz 03 Mar '11

03 Mar '11

Welcome to issue 171 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of February 20 - 26, 2011 . Announcements Eric Kow [2]announced that the Darcs project is staring to organize the next hacking sprint, which will be "most likely in Paris in this spring. If you fancy a weekend of hacking Haskell on a really neat real world project, come join...!" Jose Pedro Magalhaes [3]informed us of the initial release of htime, a library to mimic the unix "time" utility. Iain Alexander [4]annouced the initial release of data-type, function-combine, and flexiwrap. "data-type contains a number of fundamental types and classes useful for type-level programming, such as type-level booleans, natural numbers, and lists." I'd like to once again recognize some of the members of the community that spend their time fielding questions from the new commers. If you haven't been in the #haskell irc channel, you are just missing out! There are also those who spend their time writting great answers to the many questions in haskell's StackOverflow tag. You are what make this community an inviting place for all the people interested in Haskell. Keep it up! Quotes of the Week kmc: but rarely do beginners come to #haskell and say "i want to append two lists, but i don't understand all this Monoid business, i hear they're like taco salad" Eduard:_Munteanu [In response to "GHC can go jump out a window" and "GHC has already jumped out a window and flied and left you behind"] Yes, GHC even implements optimizations such as defenestration. Top Reddit Stories * What kind of things are easy in Haskell and hard in Scala, and vice versa?: From (programmers.stackexchange.com) scored 47 with 34 comments. Read on [5]reddit or the [6]original post. * Multiday Debugging GHC: From (blog.ezyang.com) scored 37 with 2 comments. Read on [7]reddit or the [8]original post. * Making OpenCL Simple with Haskell [pdf]: From (developer.amd.com) scored 35 with 11 comments. Read on [9]reddit or the [10]original post. * GHC 7.0.2 RC 2 available. Please test!: From (haskell.org) scored 33 with 5 comments. Read on [11]reddit or the [12]original post. * Minecraft data API for Haskell: HaskellNBT: From (blog.acfoltzer.net) scored 27 with 4 comments. Read on [13]reddit or the [14]original post. * Dependently Typed Programming: an Agda Introduction: From (youtube.com) scored 25 with 5 comments. Read on [15]reddit or the [16]original post. * Minecraft network protocol and proxy in Haskell: From (github.com) scored 25 with 1 comments. Read on [17]reddit or the [18]original post. * Threading State - a short tutorial for new Haskell programmers: From (mtnviewmark.wordpress.com) scored 22 with 3 comments. Read on [19]reddit or the [20]original post. * Solution (code) XCKD: Nerd Snipping: From (pgraycode.wordpress.com) scored 20 with 8 comments. Read on [21]reddit or the [22]original post. * Agda 2.2 released: From (permalink.gmane.org) scored 18 with 1 comments. Read on [23]reddit or the [24]original post. Top StackOverflow Answers * [25]Feed elements of a tuple to a function as arguments in Haskell? votes: 14 Function uncurry converts a two-argument (curried) function into a function on pairs. Here's its type signature: uncurry :: (a -> b -> c) -> (a, b) -> c You need to use it on printf, like this: mapM_ (uncurry $ printf "Values: %d %d\n") [(1,100),(2,350),(3,600),(4,200)] Another solution is to use pattern matching to deconstruct the tuple, ... * [26]What is the difference between different orderings of the same monad transformers? votes: 10 Edit: I originally got the cases backwards. Fixed now. The difference between orderings of monad transformer stacks really only matters when you're peeling off layers of the stack. type Procedure a = MaybeT (State ProcedureState) a In this case, you first run the MaybeT, which results in a stateful computation which returns a Maybe a. type Procedure a ... Top StackOverflow Questions * Real-world applications of zygohistomorphic prepromorphisms (votes: 54, answers: 2) [27]read * How to roll a fast BVH representation in Haskell (votes: 10, answers: 2) [28]read * curious about how âloop = loopâ is evaluated in Haskell (votes: 9, answers: 2) [29]read * What is the difference between different orderings of the same monad transformers? (votes: 8, answers: 3) [30]read * haskell function declaration (votes: 6, answers: 3) [31]read About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86711 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86731 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18526 5. http://www.reddit.com/r/haskell/comments/ft6eh/what_kind_of_things_are_easy… 6. http://programmers.stackexchange.com/questions/51245/what-kind-of-things-ar… 7. http://www.reddit.com/r/haskell/comments/fsp4f/multiday_debugging_ghc/ 8. http://blog.ezyang.com/2011/02/multi-day-debugging/ 9. http://www.reddit.com/r/haskell/comments/fqmmu/making_opencl_simple_with_ha… 10. http://developer.amd.com/zones/OpenCLZone/publications/assets/MakingOpenCLS… 11. http://www.reddit.com/r/haskell/comments/fp5e8/ghc_702_rc_2_available_pleas… 12. http://www.haskell.org/pipermail/glasgow-haskell-users/2011-February/020056… 13. http://www.reddit.com/r/haskell/comments/fs2nw/minecraft_data_api_for_haske… 14. http://blog.acfoltzer.net/2010/12/minecraft-data-api-for-haskell-haskellnbt/ 15. http://www.reddit.com/r/haskell/comments/fr5c1/dependently_typed_programmin… 16. http://www.youtube.com/view_play_list?p=44F162A8B8CB7C87 17. http://www.reddit.com/r/haskell/comments/fsgaq/minecraft_network_protocol_a… 18. https://github.com/glguy/minecraft-proxy 19. http://www.reddit.com/r/haskell/comments/fq4k4/threading_state_a_short_tuto… 20. http://mtnviewmark.wordpress.com/2011/02/21/threading-state/ 21. http://www.reddit.com/r/haskell/comments/fr9v7/solution_code_xckd_nerd_snip… 22. http://pgraycode.wordpress.com/2011/02/23/solving-xkcds-nerd-snipping-probl… 23. http://www.reddit.com/r/haskell/comments/fp45s/agda_22_released/ 24. http://permalink.gmane.org/gmane.comp.lang.agda/2566 25. http://stackoverflow.com/questions/5117948/feed-elements-of-a-tuple-to-a-fu… 26. http://stackoverflow.com/questions/5075621/what-is-the-difference-between-d… 27. http://stackoverflow.com/questions/5057136/real-world-applications-of-zygoh… 28. http://stackoverflow.com/questions/5077243/how-to-roll-a-fast-bvh-represent… 29. http://stackoverflow.com/questions/5126759/curious-about-how-loop-loop-is-e… 30. http://stackoverflow.com/questions/5075621/what-is-the-difference-between-d… 31. http://stackoverflow.com/questions/5073058/haskell-function-declaration

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Rank-2 types in classes
by Yves Parès 03 Mar '11

03 Mar '11

Hello, I'm working on a library which aims to be a generic interface for 2D rendering. To do that, one of my goals is to enable each implementation of this interface to run in its own monad (most of the time an overlay to IO), thus giving me the following class class (Monad (IM i x)) => Impl i x where data IM i x :: * -> * (where IM means Implementation Monad) Here, 'x' aims at being a phantom type that ensures a type safe context for some operations. E.g., operations that need to occur in a window will have the type: IM i Window a And will be run by the function: withinWindow :: Window -> IM i Window a -> IM i x a This makes an operation that doesn't instantiate 'x' context-independent. My problem, then, is that in the definition of class Impl the type variable 'x' is useless, since every implementation must leave uninstantiated. I would like to write something like : class (forall x. Monad (IM i x)) => Impl i where data IM i :: * -> * -> * But GHC forbids me to do so. Any suggestion would be warmly welcomed.

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Re: [Haskell-cafe] type safe web forms at compile time Was: Re: Ur vs Haskell
by Alberto G. Corona 02 Mar '11

02 Mar '11

> WASH is full of wonderful ideas . packed in a not so wonderful syntax. It > is worth to evolve it. > > WASH does force form safety in a similar way to Formlets > <http://hackage.haskell.org/package/formlets>: because the form and the > form read code are generated automatically by a class instance. So there is > no need for, type checking safety > > The problem comes when the form is generated and/or maintained( edited by > some people (some forms have lot of formating in the real world) while the > form handling code is .maintained by some other people, the programmers. > > In this real case , type cheching is very important. > > 2011/3/2 Marc Weber <marco-oweber(a)gmx.de> > > Excerpts from Alberto G. Corona's message of Wed Mar 02 20:53:28 +0000 >> 2011: >> > Some time ago I forgot to forward this message to thie ur versus Haskell >> > <http://www.haskell.org/pipermail/haskell-cafe/2011-January/088060.html >> >discussion, >> > (as usual) >> > --- >> > The most impressive feature (of ur) is the compile time checking of >> > conformance between the form and the form results. This can be done in >> >> See WASH (-> hackage). So there is a Haskell implementation. There are >> small >> differences though: urweb has nicer URLS which should be much more SEO >> friendly. >> >> From my point of view its not only about forms. Its also about checking >> SQL queries. And urweb seems to do this very well. >> >> Marc Weber >> > >

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type safe web forms at compile time Was: Re: Ur vs Haskell
by Alberto G. Corona 02 Mar '11

02 Mar '11

Hi. Some time ago I forgot to forward this message to thie ur versus Haskell <http://www.haskell.org/pipermail/haskell-cafe/2011-January/088060.html>discussion, (as usual) --- The most impressive feature (of ur) is the compile time checking of conformance between the form and the form results. This can be done in Haskell using HList magic and some class instances, I guess. ---- Since then I have been playing mentally with this. Recently I found something simple an interesting enough to share. (Although crude). It is a kind of typed form fields data Input a= Input String Type a (String -> Either String a) and a kind of heterogeneous list to aggregate form fields and results with the operator (:*): Input a :* Input b ;* Input c.... a :* b :* c and a (simulated for the purpose of demonstration) send-receive function that type match the form fields and the results: *Main> let form = Input "" Text "stringdata" novalidate :* Input "" Text (1::Integer) novalidate *Main> ask form >>= \(a :* b) -> return $ a ++ b <interactive>:1:0: No instance for (FormDigest (Input [Char] :* Input Integer) ([a] :* [a])) ...... notifying that there is no translation defined , because the result requires two lists of the same type when the form gives a string and an Integer But forcing the correct monomorphic types it does pattern match and return the values. *Main> ask form >>= \ (a :* b) -> print ('s':a) >> print ( fromInteger $ b) "sstringdata" 1 ask is just a simulation of HTTP one time interaction. It returns the input values. The whole loop involves the rendering of the form, with render: *Main> render form <input type="Text" name="var1" value="stringdata"/> <input type="Text" name="var2" value=1/> In a real case the results are read and validated from the the post values.They are (or can be) ordered sequentially acording with Input field names. The FormDigest instances do this work. There is no need to define new FormDigest instances. (although non one to one field-result can be created) The text is in literate haskell. There is a more elaborate example at the end. I know that the instances are non tail recursive and there are factorization pending but this is just a proof of concept: > {-# LANGUAGE > FlexibleInstances, > MultiParamTypeClasses, > TypeOperators > #-} > import Control.Monad.State The Heterogeneous list agregator. I tried to use GADTs but they does not easily pattern match > data x :* xs = x :* xs deriving (Read, Show, Eq) > infixr 5 :* > data Type= Hidden | Text deriving (Show, Read) -- to add all the types the input field, with text formatting, initial value and runtime validator > data Input a = Input String Type a (String -> Either [String] a) > instance(Show a)=> Show (Input a) where > show (Input _ _ x _) = show x rendering of the form need a sequentiation of field names. I use a state monad for this > class RenderForm a where > renderForm :: a -> State Int String > instance (Show a) => RenderForm (Input a) where > renderForm input = do > s1 <- render1 input > n <- get > put $ n + 1 > return s1 HList school here: > instance (Show a,RenderForm xs) => RenderForm (Input a :* xs) where > renderForm (input :* xs)= do > n <- get > put $ n+1 > h <- render1 input > s <- renderForm xs > return $ h++s > render1 (Input msg t x _)= do > n <- get > put $ n+1 > return $ msg > ++ "<input type=\"" > ++ show t ++ "\" name=" > ++ "\"var"++show n ++ "\" value=" > ++ show x ++"/>\n" > > render form= putStrLn $ evalState (renderForm form ) 0 processing of the returned form result, in an ordered String list, according with seuquential names of the fields defined in renderForm. > class FormDigest a b where > formDigest :: a -> [String] -> Either [String] b "Input a" is diggested into a type "a" > instance FormDigest (Input a) (a) where > formDigest (Input _ _ x f) (s: ss)= case f s of > Right x -> Right $ x > Left x -> Left x recursively add instances for any list of inputs Input a's are diggested into a's > instance FormDigest as bs > => FormDigest (Input a :* as) (a :* bs) where > formDigest (input :* fs) es@(s:ss) = > let er = formDigest fs ss > e = formDigest input es > in case (e, er) of > (Right x, Right ys) -> Right $ x :* ys > (Right _, Left errs) -> Left errs > (Left err, Left errs) -> Left (err ++ errs) simulated request-response that returns the entered input values > sendRec xs= do > let strs = showValues xs > return $ formDigest xs strs > class ShowValues a where > showValues :: a -> [String] > instance Show x => ShowValues (Input x) where > showValues i = [show i ] > instance (Show x,ShowValues xs) => ShowValues (Input x :* xs) where > showValues (i :* xs)= show i : showValues xs end of simulated request response > ask :: (ShowValues a, FormDigest a b) => a -> IO b > ask form = do > er <- sendRec form > case er of > Left errs -> error "" -- shoud be: "ask1 errs form "·to render form and errors > Right res -> return res EXAMPLE: > data Emp= Emp{name::String, salary :: Int} deriving Show > emp= Emp "Luis" 10000 toy html operators: > b msg = ("<b> " ++ msg ++ " </b>\n") > p msg = ("<p> " ++ msg ++ " </p>\n") > novalidate n= Right $ read n > main= do > > let form = Input > ( b "please enter the name" > ++ p "mas texto") > Text (name emp) novalidate > :* Input > (b "please enter the salary" > ++ p "jkjkjk") > Text (salary emp) novalidate > render form the matching thing > (n :* s ) <- ask form > print emp > print $ Emp n s

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Examples for the problem
by Robert Clausecker 02 Mar '11

02 Mar '11

Thank you all for the responses. Here's an example: As I alrerady said, I tried to parse the MMIXAL assembly language. Each instruction has up to three operands, looking like this: @+4 (Jump for bytes forward) "foo" (the string foo" '0'>>(1+2) etc. A string literal may contain anything but a newline, (there are no escape codes or similar). But when I add a check for a newline, the parser just fails and the next one is tried. This is undesired, as I want to return an error like "unexpected newline" instead. How is this handled in other parsers?

5 5

HaXml combinators
by DCE 01 Mar '11

01 Mar '11

Hello, I'm trying to use HaXml to do some relatively simple XML processing, but I can't figure out how to get started. My documents look something like this: <doc> <a bar="x"> <b foo="x"/> <b foo="y"/> <a bar="y"> <b foo="z"/> </a> </a> </doc> I want to find any element tagged <a> that has an attribute bar=y, then find the element under it tagged <b> with attribute foo=z, then add an attribute "baz=t" to this <b>. I've tried various incantations like (tag "b" `o` children `o` (tag "a" `with` attrval ...)), but they don't seem to work. And in general, I'm not sure how to accomplish looking up and down the XML tree from a given node. I have figured out how to use mkElemAttr to build a new element, just not how to find the correct elements in the first place. Additionally, I'm confused about when to use foldXml, deep, multi, and so on for processing elements, and whether the 'inplace' combinator is needed for replacing the <b> element with the result of mkElemAttr. Any help would be appreciated.

1 0

SMP parallelism gains inferior than expected
by José Pedro Magalhães 01 Mar '11

01 Mar '11

(Forwarding to haskell-cafe) Hi, I have a program that computes a matrix of Floats of m rows by n columns. Computing each Float is relatively expensive. Each line is completely independent of the others, so I thought I'd try some simple SMP parallelism on this code: myFun :: FilePath -> IO () myFun fp = do fs <- readDataDir fp let process f = readFile' f >>= parse printLine = putStrLn . foldr (\a b -> show a ++ "\t" ++ b) "" runDiff l = [ [ diff x y | y <- l ] | (x,i) <- zip l (map getId fs), myFilter i ] ps <- mapM process fs sequence_ [ printLine x | x <- runDiff ps *`using` parList rdeepseq* ] So, I'm using parList to evaluate the rows in parallel, and fully evaluating each row. Here are the timings on a Dual Quad Core AMD 2378 @2.4 GHz, ghc-6.12.3, parallel-2.2.0.1: -N time (ms) none 1m50 2 1m33 3 1m35 4 1m22 5 1m11 6 1m06 7 1m45 The increase at 7 is justified by the fact that there were two other processes running. I don't know how to justify the small increase at N3, though, but that doesn't matter too much. The problem is that I am not getting the gains I expected (halving at N2, a third at N3, etc.). Is this the best one can achieve with this implicit parallelism, or am I doing something wrong? In particular, is the way I'm printing the results at the end destroying potential parallel gains? Any insights on this are appreciated. Thanks, Pedro

2 1

Rebindable 'let' for observable sharing?
by Henning Thielemann 01 Mar '11

01 Mar '11

Now that almost every syntax can be redirected to custom functions (RebindableSyntax, OverloadedStrings), would it make sense to map 'let' to 'fix' ? Would this open a way to implement observable sharing as needed in EDSLs by a custom 'fix'?

3 2

segfault when using ghc api
by Edward Amsden 01 Mar '11

01 Mar '11

I'm trying to run the following code. I'm not at all sure it's correct, it's based off of a bit of poking around in the ghc api. Running it with a command line argument like "show (5 + 2)" gives me a segmentation fault. Poking around with gdb and following the steps at http://hackage.haskell.org/trac/ghc/wiki/Debugging/CompiledCode yields precisely nothing, as even the "disassemble" command complains "No function contains program counter for selected frame." Any ideas? == code == module Main where import GHC import DynFlags import Data.Dynamic import System evalString :: Typeable a => String -> IO (Maybe a) evalString s = defaultErrorHandler defaultDynFlags $ runGhc Nothing $ do dynflags <- getSessionDynFlags setSessionDynFlags $ dynflags target <- guessTarget "Prelude" Nothing setTargets [target] load LoadAllTargets dyn <- dynCompileExpr s return $ fromDynamic dyn main = do (s:_) <- getArgs e <- evalString s putStrLn $ maybe "oops" id e -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

3 4

HANSEI in Haskell?
by Daryoush Mehrtash 01 Mar '11

01 Mar '11

Is the "Embedded domain-specific language HANSEI for probabilistic models and (nested) inference" described in: http://okmij.org/ftp/kakuritu/index.html#implementation available in Haskell? Is there a reason why the author did the package in Ocaml rather than Haskell? -- Daryoush

3 8

HDBC's future and request for help
by John Goerzen 28 Feb '11

28 Feb '11

Hi folks, HDBC has been out there for quite some time now. I wrote it initially to meet some specific needs, and from that perspective, it has been done for awhile. It is clear, however, that there are some needs it doesn't meet. Most of them relate to specific backend driver items. I'd like to start some discussion in the community about what the future of HDBC and its backend drivers ought to look like. Some models might be: 1. I continue as maintainer for HDBC and HDBC-{postgresql,odbc,sqlite3} and act as patch manager/gatekeeper for patches that are discussed on some public mailing list. 2. Interested parties adopt the backend drivers while I continue to act as maintainer/patch manager/gatekeeper for HDBC itself. 3. Interested parties adopt all of HDBC and maintain it I am not expressing a particular preference for any of these options; just putting them forth. Here are some of the current issues I am aware of: 1. I have no Windows development platform. I can't test the releases on Windows. Many Windows users don't have the skill to diagnose problems. These problems do eventually get fixed when a Windows user with that skill comes along -- and I appreciate their efforts very much! -- but it takes longer than it ought to. 2. The ODBC documentation is monumentally terrible, and the API is perhaps only majestically terrible, and it is not 100% clear to me all the time that I have it right. A seasoned ODBC person would be ideal here. 3. Issues exist with transferring binary data and BLOBs to/from at least PostgreSQL databases and perhaps others. There appear to be bugs in the backend for this, but BLOB support in the API may need to be developed as well. 4. Although the API supports optimizations for inserting many rows at once and precompiled queries, most backends to not yet take advantage of these optimization. 5. I have received dueling patches for whether foreign imports should be marked "safe" or "unsafe" on various backends. There seems to be disagreement in the community about this one. 6. Many interactions with database backends take place using a String when a more native type could be used for efficiency. This project may be rather complex given varying types of column data in a database -- what it expects for bound parameters and what it returns. The API support is there for it though. 7. Various other more minor items. Thoughts? -- John

4 11

Re: [Haskell-cafe] ANN: theoremquest-0.0.0
by Tom Hawkins 28 Feb '11

28 Feb '11

> I find this fairly interesting. Once you're finished grappling with the > logical core, I wouldn't mind helping out with a web interface, time > permitting. I suspect attracting mass appeal, or getting users at all, is > helped massively by having a web interface. Thanks for your interest. Yes, a web interface would be ideal. Currently the library has hooks to translate the transactions to JSON to ease construction of any Javascript client -- thought these translations still need to be implemented. Note the 'RspInJSON' request: http://hackage.haskell.org/packages/archive/theoremquest/0.0.0/doc/html/The…

1 0

trac.haskell.org problem
by Niklas Broberg 28 Feb '11

28 Feb '11

Ah, I suppose that's to do with the trac server itself, that's beyond me. Moving to café: Does anyone know what's up with trac.haskell.org not sending out verification emails? Cheers, /Niklas ________________________________________ From: Jurriaan Hage [jur(a)cs.uu.nl] Sent: Monday, February 28, 2011 8:41 AM To: Niklas Broberg Subject: Re: Again a question about haskell-src-exts. On Feb 28, 2011, at 8:39 AM, Niklas Broberg wrote: > I suppose I do - to the extend I can. What's the issue? :-) The system does not send out verification e-mails, so I can not really register completely and submit new issues. I have tried it with two different e-mail accounts, and somebody else has tried too. They also do not show up in spam folders. Jur

2 1

ANN: Hakyll 3
by Jasper Van der Jeugt 28 Feb '11

28 Feb '11

Hello all, I've just uploaded the 3.0.0.0 version of Hakyll [1] to Hackage [2]. This is a complete rewrite, and completely backward-incompatible with previous versions. Sorry for that. On the other hand, I believe almost all aspects of the library have improved tremendously. Hakyll now uses a declarative EDSL in Haskell to specify compilation rules, and an arrow-based EDSL to describe the compilation process. If you want to have a quick look, I recommend have a glance at the example in the tutorial [3]. More documentation needs to be written, but I wanted to make sure I can incorporate your feedback into that process. [1]: http://jaspervdj.be/hakyll/ [2]: http://hackage.haskell.org/package/hakyll-3.0.0.2 [3]: http://jaspervdj.be/hakyll/tutorial.html Cheers, Jasper

1 0

http://www.cs.cornell.edu/icfp/task.htm
by Hauschild, Klaus (EXT) 28 Feb '11

28 Feb '11

Hi, Currently I'm trying to lern Haskell by doing. After doing some examples I plan to solve an ICFP task (see subject). In short: build an interpreter for a stack based language thata describes a 3D scene. Ray trace this scene in an image. My current source state can be found here: http://code.google.com/p/hgmltracer/source/browse/ My first goal is to develop the interpreter of the GML language. My source contains a data GmlToken with various constructors for the different operators, sequences, int, real, string an so on. Some code for the evaluation is also there und working. But the parser converting a string (contained in the program file provided as command line argument) is hard stuff and I'm stuck. Can any one help me with ideas or concepts for this parser? Thanks

3 4

Re: [Haskell-cafe] [SOLVED] why is ghci trying to load hsc file ??
by briand＠aracnet.com 28 Feb '11

28 Feb '11

On Mon, 28 Feb 2011 02:58:58 +0000 Felipe Almeida Lessa <felipe.lessa(a)gmail.com> wrote: > On Sun, Feb 27, 2011 at 6:48 AM, by way of briand(a)aracnet.com > <briand(a)aracnet.com> wrote: > > the binding-DSL examples do NOT use the above PRAGMA anywhere in the > > code. > > Probably they have > > Extensions: ForeignFunctionInterface > > on their .cabal file. > as I do, hence the reason for my confusion :-( Something's not working right and I can't begin to guess why not. I had to put the actual pragma in my .hsc file to make things work, and now it works. Brian

1 0

Render fonts in HopenGL
by er9999 27 Feb '11

27 Feb '11

Hello, Can anyone give me an example for fonts rendering? I tried to use FTGL library, but it is too complicated on Windows(compile). Is it possible to do it in a simple way? I need more than the given example in red book(Font.hs), something like outline font. By the way, I am new in OpenGL and Haskell, so the real exmaple should be the best. Thanks!

2 1

Type problem
by michael rice 26 Feb '11

26 Feb '11

Why? Shouldn't this work for any type a? Michael ========== f :: [a] -> [a] f l = do x <- l return x ========== *Main> :r [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> f "abcde" "abcde" *Main> f [1,2,3,4,5] [1,2,3,4,5] *Main> "abcde" >>= f <interactive>:1:12: Couldn't match expected type `Char' against inferred type `m b' In the second argument of `(>>=)', namely `f' In the expression: "abcde" >>= f In the definition of `it': it = "abcde" >>= f *Main>

3 3

Simulating open datatypes with type families?
by Patrick Thomson 26 Feb '11

26 Feb '11

I'm currently working on a project that involves a C parser (using Parsec3) that needs to be dynamically extensible - i.e. end-users should be able to add new statement types, expression types, operators, and so on and so forth. Since Haskell ADT's are closed, I thought I would be able to simulate this by using type families, e.g.: {-# LANGUAGE TypeFamilies, DeriveDataTypeable #-} -- the statement type type CStatement = BreakStmt | CaseStmt CExpression CStatement -- rest of the statement types omitted here for brevity -- CStatement derives (Show, Eq, Typeable, Data) statement :: Parser CStatement -- how exactly this is parsed is not relevant, so its definition is also omitted data family StatementLike a data instance StatementLike CStatement = MkStmt CStatement -- okay, this is all well and good. let's create a new ADT, representing the Ruby/Perl-esque 'until' statement and add a StatementLike instance for it. data Until = UntilStmt CExpression CStatement deriving (Eq, Show, Typeable, Data) data instance StatementLike Until = MkUntil Until untilStatement :: Parser Until -- definition omitted -- Here's where it breaks down. I want to be able to have a function of type Parser (StatementLike a) so that I can pattern-match over the a in future code, but this function doesn't typecheck: GHC says that the function can't be that general - it expects either a Parser (StatementLike CStatement) or a Parser (StatementLike Until). newStatement :: Parser (CStatement a) newStatement = (MkUntil <$> untilStatement) <|> (MkStmt <$> statement) Am I expecting the wrong thing of type families? Are typeclasses the better way to do this? Is there any other way to simulate an open, extensible ADT, or am I totally barking up the wrong tree? Any assistance would be much appreciated. Please let me know if I need to provide more information. Thanks in advance, -- Patrick Thomson

2 1

Auto elimination of MVars using a monad or monad transformer.
by Chris Dew 26 Feb '11

26 Feb '11

Hello, just like everyone else, I have a question about monads. I've read the tutorials, written one monad myself (not in this email), but I still consider myself a Haskell beginner. * Does GHC eliminate unneeded MVars during compilation? I'm expecting that it doesn't, as that would mean optimising away ForkIOs, which would be quite a thing to do. I've included example code below. * Is there a monad which allows their automatic elimination of MVars (or their creation only when necessary)? This would be similar to how the IO monad allows you to do purely functional things with a do block, using let. I've had a go at a lifting function, which wraps a pure function into an IO action which forever reads from one MVar and writes to another. What I'm looking for is some form of Monadic context in which many pure functions, MVar fillers and MVar consumers could be linked together, where only the necessary MVars remain (or were created) at compilation time. * Would this be a monad, or a monad transformer? * Can you specialise a monad transformer on a single base (in this case IO) so that you can use forkIO in the bind or return? Thanks, Chris. module Main ( main ) where import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar, takeMVar, ThreadId, threadDelay) import Control.Monad (forever) stepA :: MVar String -> IO () stepA boxa = forever $ do line <- getLine putMVar boxa line stepB :: MVar String -> IO () stepB boxb = forever $ do line <- takeMVar boxb putStrLn line -- This simply wraps a string in brackets. bracket :: String -> String bracket x = "(" ++ x ++ ")" -- This lifts a function into an action which forever performs the function -- between the two MVars given. lft :: (a -> b) -> MVar a -> MVar b -> IO () lft f c d = forever $ do x <- takeMVar c putMVar d (f x) -- Just like C's main. main :: IO () main = do box <- newEmptyMVar box2 <- newEmptyMVar forkIO $ stepA box forkIO $ lft bracket box box2 forkIO $ stepB box2 threadDelay 10000000 -- Sleep for at least 10 seconds before exiting.

2 1

Haskell platform installation failure on OS X 10.6.6 (intel)
by Eugene Kirpichov 26 Feb '11

26 Feb '11

Hello, Below is an experience report from Artem (cc'd) on a failed installation of Haskell platform on a Mac. OS: OS X 10.6.6 CPU: Intel Core i5 Platform downloaded from: http://lambda.haskell.org/hp-tmp/2010.2.0.0/haskell-platform-2010.2.0.0.i38… Sequence of actions: Downloaded the file, mounted the image, ran "Install new GHC", pressed OK until the following screen appeared http://i.imgur.com/GGdbg.png - there the Install button was disabled; pressing "change install location" did not help since only 1 option was available there, and it was already selected. After that, Artem tried to uninstall GHC, but unsuccessfully, since OS told that it was not installed. Then he tried to install the platform, and eventually got the following error around "moving files into place": http://i.imgur.com/YHb22.png I've also heard somewhat similar problem reports on installing haskell-platform on OS X from other people (I myself don't have it) - like being unable to install it without first installing XCode, etc. Can anyone help resolve this issue? I'm sure this is not expected behavior. What further information would help find the cause? -- Eugene Kirpichov Senior Software Engineer, Mirantis Inc. http://www.mirantis.com/

3 4

Haskell Platform install instructions
by Michael Snoyman 26 Feb '11

26 Feb '11

Hi everyone, Some of the dependencies for Yesod (such as haskell-src) require happy. For the "Yesod in Five Minutes" page[1], I've given instructions to cabal install happy and then cabal install yesod. However, as Xavier Shay pointed out to me, this won't actually work on a vanilla install, since happy gets installed in the cabal bin folder, which is not by default on the PATH. Two questions: * Should we document this somewhere in the Haskell Platform install process? I'm sure many old-time users of cabal are well aware that they need ~/.cabal/bin in the PATH, but new users will not be. I would also like to update Yesod's documentation; does anyone have standard phrasing that is applicable to all operating systems? * Should we consider modifying the build tools to automatically look for build dependencies in the cabal bin folder? Michael [1] http://docs.yesodweb.com/five-minutes

3 2

FFI: C-side object not destructed
by Yves Parès 26 Feb '11

26 Feb '11

Hello, I'm trying to use a C++ class in Haskell through C exports. It works all very well, except that when I call the function that deletes the object, it is still valid, I can still call methods on it. Here is my Haskell code: {-# LANGUAGE ForeignFunctionInterface #-} import Foreign import Foreign.C.Types newtype PKlass = PKlass (Ptr PKlass) foreign import ccall unsafe "Klass_Create" kCreate :: CInt -> CInt -> IO PKlass foreign import ccall unsafe "Klass_Destroy" kDestroy :: PKlass -> IO () foreign import ccall unsafe "Klass_GetX" kGetX :: PKlass -> IO CInt foreign import ccall unsafe "Klass_GetY" kGetY :: PKlass -> IO CInt foreign import ccall unsafe "Klass_AddKlass" kAdd :: PKlass -> PKlass -> IO PKlass main = do k <- kCreate 4 5 kDestroy k *kGetY k >>= print -- This shouldn't work* k' <- kCreate 2 8 k'' <- k `kAdd` k' kDestroy k'' *kGetY k'' >>= print -- This neither* So it is very basic, and I can't understand why the supposedly destroyed objects are still here. Enclosed is all the source code (C++ class, C exportation and the Haskell main hereabove). I compile it this way: ghc --make main.hs *.cpp -lstdc++

4 4

Either instance of Monad?
by Yves Parès 26 Feb '11

26 Feb '11

Hello, When importing Control.Monad.Error, it shows me that (Either e) is an instance of MonadFix, but not an instance of Monad. Isn't there a problem?

2 9

Fun with the ST monad
by Andrew Coppin 26 Feb '11

26 Feb '11

OK, so I had a function that looks like transform :: [Word8] -> [Word16] It works nicely, but I'd like to use mutable state inside. No problem! Use the ST monad. Something like transform :: [Word8] -> [Word16] transform xs = runST (work xs) where work :: [Word8] -> ST s [Word16] Ah, yes, well there is one *small* problem... If you do that, the function becomes too strict. The input list is being read from disk by lazy I/O. With the original implementation, the input file gets read at the same time as the output file is written. But runST returns nothing until the *entire* input has been compressed. So writing to disk doesn't start until the entire file has been slurped up into memory. Anybody have any hints on how to get around this? My first thought was to break the ST computation into several seperate pieces. But, /by design/, you cannot do this. And here's why: main = do let r = runST $ newSTRef 0 let x = runST $ n <- readSTRef r; writeSTRef r (n+1); return n let y = runST $ n <- readSTRef r; writeSTRef r (n*2); return n print x print y Now what the hell does this print out? Because it appears, logically, that it ought to vary depending on the order in which x and y are accessed - a clear violation of referential integrity. Fortunately of course, by a clever dodge, this code doesn't actually type-check. That's great, because it has undefined semantics. But it's also a problem, since it's exactly the sort of thing I'd like to do. Come to think of it, how the heck does lazy I/O manage this trick? How come accessing the elements of the list in the wrong order doesn't cause the wrong data to end up in each cell, if each cell is just using unsafePerformIO to read the next byte from a normal file handle?

8 13

best way to deal with #defines when developing ffi's
by briand＠aracnet.com 26 Feb '11

26 Feb '11

Hi all, I'm working on an FFI and as usual there are lots of defines in the header files. What's the best way to make these available to the haskell code ? Ideally it could be done automagically, or at least pseudo-magically so that keeping up with changes to the .h wouldn't be too painful. I'm talking about simple constant defines, nothing tricky like structures or similar. Thanks, Brian

4 3

Proxy for cabal
by Hauschild, Klaus (EXT) 25 Feb '11

25 Feb '11

Hi, Is it possible to configure a proxy server for cabal? When yes, how I configure it? Thanks.

1 1

ANNOUNCE: htime-0.1
by José Pedro Magalhães 25 Feb '11

25 Feb '11

Hello all, I kept being annoyed at the fact that Windows doesn't come with anything similar to the unix "time" utility, so I created a small Haskell program that does something similar. I thought this could perhaps be useful to others, so it's now available on Hackage: http://hackage.haskell.org/package/htime-0.1 Cheers, Pedro

1 0

Re: [Haskell-cafe] HANSEI in Haskell?
by oleg＠okmij.org 25 Feb '11

25 Feb '11

The topic of HANSEI in Haskell does come up from time to time. In fact, there was a Haskell edition of the first version of Hansei: http://okmij.org/ftp/kakuritu/probM.hs It was written to see how the code would look in Haskell, and how memoization (inherent in lazy evaluation of GHC) may help. The code contains some of the tests from the HANSEI distribution. The experience showed that lazy evaluation hurts -- a lot. The problem is not in lack of seq -- adding seq would make things even worse. There is a bit of explanation at the end of probM.hs. The gist of the problem is that approximate inference (sampling, to be precise) trades space for time. The search tree is way too large to fit into memory. Therefore, it is better to recompute the values (the branches of the tree) than to keep storing them. That is precisely the opposite to the trade-off of lazy evaluation. The end result is attempting to allocate gigabytes (really!), even for toy problems. We can get around the problem by using thunks explicitly. But then we lose the remaining benefits of Haskell syntax (the ones that are left after we have to switch to the monadic notation). OCaml becomes quite compelling then. It must be stressed that laziness in general is _crucial_ for solving even moderately complex problems. However, the needed laziness is NOT of the kind provided by GHC. We need memoization specific to a possible world, rather than the one that affects all possible worlds. GHC gives only the latter. The sort of laziness needed for non-deterministic programming calls for first-class store. It can be emulated, albeit imperfectly, for example, as was described in the ICFP09 paper. Hansei uses a quite similar idea. Ideally first-class memory is provided as built-in, since it requires interaction with garbage collection. Greg Morrisett has written extensively on this topic and done a few implementations; see for example, http://www.eecs.harvard.edu/~greg/papers/jgmorris-callcs.ps Sadly, the topic has been forgotten.

4 5

GRIN and Urban Boquist's thesis
by John Lato 24 Feb '11

24 Feb '11

Hello, Does anyone have a current link to Urban Boquist's thesis? It's no longer available at the Chalmers site. On the LHC blog someone gave a link to a mirror[1], however that link is down for me as well. Thanks, John L. [1] http://lhc-compiler.blogspot.com/2010/06/mirroring-boquists-grin-papers.htm…

3 4

Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 90, Issue 48
by Louis Wasserman 24 Feb '11

24 Feb '11

> > Another common usage for Map is as a functional integer-indexed > random access array. > Once I implemented the standard algorithm for random shuffle > of a list using Data.Map Int. It was much nicer than the STArray > version, in my opinion. But when I tried switching to Data.IntMap, > hoping to make it faster, I was devastatingly disappointed. Now > I understand why. A couple thoughts: If you're going to provide an isSingleton method, provide getSingleton :: IntMap m -> Maybe (Int, a) which is trivial based on the current implementation. For my TrieMap package, I've been working on my WordMap implementation here<http://hackage.haskell.org/packages/archive/TrieMap/3.0.1/doc/html/src/Data…>. (Using Words as keys makes my life easier, and it doesn't take too much work to write a wrapper that provides an order-preserving bijection from Int to Word.) It provides O(1) size, but its design reflects my willingness to trade memory and code size for speed. ;) Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

1 0

darcs hacking sprint #6 (March, April, or May 2011)
by Eric Kow 24 Feb '11

24 Feb '11

Hello Haskellers! We are starting to organising the next Darcs Hacking Sprint, most likely in Paris in this spring. If you fancy a weekend of hacking Haskell on a really neat real world project, come join us! If you'd like to attend, please add yourself to http://wiki.darcs.net/Sprints/2011-04 . We will fix the date by March 1st, and there's only about 10+ spaces left, so sign up now! If you want to know what is exactly a Darcs Sprint, have a look at: http://wiki.darcs.net/Sprints http://blog.darcs.net/search/label/sprints The event is open to everybody, even if you're new to Darcs or to Haskell. We have an ample supply of enthusiasm and ProbablyEasy bugs to help you get started. See http://wiki.darcs.net/Recruitment for some ideas and also have a look at http://wiki.darcs.net/Development to see how to start coding on Darcs. Looking forward to see you! Guillaume and Eric -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

1 0

Haskell Weekly News: Issue 170
by Daniel Santa Cruz 24 Feb '11

24 Feb '11

Welcome to issue 170 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of February 13 - 19, 2011. Announcements Simon Michael [2]informed us of reviving of FunGEn as a community project. "FunGEn (Functional Game Engine) is a platform-independent, BSD-licensed, easy-to-install 2D game engine currently based on OpenGL and GLUT." Alexander McPhail [3]announced the release of vector-buffer. "It provides a buffer that can be turned into a Data.Vector.Storable. The mapM* functions map from the oldest element, not the first." Mark P. Jones [4]announce the availability of 2 openings for post-doctoral researches "to help design, develop, and apply a new strongly typed, pure functional language for systems programming." Trystan Spangler [5]made a second release of his Behavior Driven Development for Haskell, and the tool can now be found in Hackage! Jeremy Shaw [6]announce the release of Happstack 6. "We fully recomend that all Happstack users migrate to the new release." Gregory D. Weber [7]published a new release of Sifflet and sifflet-lib. "Sifflet is a visual, functional programming language and support system for students learning about recursion." Johan Tibell [8]announced the release of two new packages: unordered-containers (Efficient hashing-based container types), and hashable (Efficient hashing of common types). Quotes of the Week * kmc: i started to read the "tutorial" and it was incomprehensible. makes the Gentle Introduction to Haskell look like Teach Yourself PHP in 24 Hours * ConorMcBride: Sometimes it's easier to search for good programs in the space of well typed programs, rather than in the space of ascii turds. * monochrom: best practice haskell is "go with your heart" * accel: one day, it was raining -- I was walking home -- I saw Knuth biking home, wearing a helmet, in the rain -- and I freaked out -- rain was hitting him like any other mortal ... it wasn't bounching off his aura of awesomeness Top Reddit Stories * Type level natural numbers ... ready to merge into GHC mainline: From (hackage.haskell.org) scored 62 with 44 comments. Read on [9]reddit or the [10]original post. * Haskell Web Framework Happstack 6 Released: From (groups.google.com) scored 45 with 3 comments. Read on [11]reddit or the [12]original post. * Well-Typed is hiring: From (well-typed.com) scored 41 with 13 comments. Read on [13]reddit or the [14]original post. * Haskell is Better: slides from Mark Lentczner's talk at Ignite Silicon Valley: From (scribd.com) scored 40 with 43 comments. Read on [15]reddit or the [16]original post. * New, faster hashing-based containers library released: From (blog.johantibell.com) scored 40 with 6 comments. Read on [17]reddit or the [18]original post. * Disciple haskell dialect : strict evaluation as default , destructive update of data structures.: From (disciple.ouroborus.net) scored 35 with 25 comments. Read on [19]reddit or the [20]original post. * Yesod: Warp Speed Ahead: From (docs.yesodweb.com) scored 34 with 8 comments. Read on [21]reddit or the [22]original post. * Faster persistent data structures through hashing: From (blog.johantibell.com) scored 28 with 15 comments. Read on [23]reddit or the [24]original post. * Developing iPhone applications in Haskell â a tutorial: From (gergo.erdi.hu), scored 25 with 6 comments. Read on [25]reddit or the [26]original post. * An Introduction to Agda :: PDF: From (dl.dropbox.com) scored 24 with 4 comments. Read on [27]reddit or the [28]original post. Top StackOverflow Answers * [29]How to tell whether Haskell will cache a result or recompute it? votes: 17 primes, in your code, is not a function, but a constant, in haskellspeak known as a CAF. If it would take a parameter (say, ()), you would get two different versions of the same list back if calling it twice, but as it is a CAF, you get the exact same list back both times; As a top-level definition, primes never gets out of scope, thus the head of the list ... * [30]Specific type inference using uncurry function votes: 17 This has nothing to do with ghci. This is the monomorphism restriction being irritating. If you try to compile the following file: add = uncurry (+) main = do print $ add (1,2 :: Int) print $ add (1,2 :: Double) You will get an error. If you expand: main = do print $ uncurry (+) (1,2 :: Int) print $ uncurry (+) (1,2 :: Double) ... * [31]Haskell: Unexpected output for expression [0, 0.1 .. 1] votes: 15 This is a result of the imprecision of floating point values, it isn't particular to Haskell. If you can't deal with the approximation inherent in floating point then you can use Rational at a high performance cost: > import Data.Ratio Data.Ratio> [0,1%10.. 1%1] [0 % 1,1 % 10,1 % 5,3 % 10,2 % 5,1 % 2,3 % 5,7 % 10,4 % 5,9 % 10,1 % 1] Just to hammer ... * [32]valid haskell function names votes: 14 From the Haskell report: Haskell uses the Unicode character set. However, source programs are currently biased toward the ASCII character set used in earlier versions of Haskell. Recent versions of GHC seem to be fine with unicode (at least in the form of UTF-8):... * [33]Haskell range notation to generate list. Unexpected output votes: 13 The notation is meant to mimic the usual way to write simple sequences mathematics. The second element is not the step, but in fact the actual second element of the list. The rest is linearly extrapolated from there. Examples: [1,2..10] = [1,2,3,4,5,6,7,8,9,10] [1,3..10] = [1,3,5,7,9] [4,3..0] = [4,3,2,1,0] [0,5..] = [0,5,10,15,20,25,30,35... Top StackOverflow Questions * How to tell whether Haskell will cache a result or recompute it? (votes: 11, answers: 1) [34]read * How can one distribute Haskell programs to non-technical end users? (votes: 8, answers: 2) [35]read * Haskell range notation to generate list. Unexpected output (votes: 8, answers: 2) [36]read * Appropriate uses of Monad `fail` vs. MonadPlus `mzero` (votes: 8, answers: 2) [37]read * Why does Haskell not have an I Monad (for input only, unlike the IO monad)? (votes: 8, answers: 3) [38]read About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/86330 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/86371 4. http://www.haskell.org/pipermail/haskell/2011-February/022585.html 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18510 6. http://www.mail-archive.com/haskell-cafe@haskell.org/msg87352.html 7. http://www.mail-archive.com/haskell@haskell.org/msg23319.html 8. http://article.gmane.org/gmane.comp.lang.haskell.general/18515 9. http://www.reddit.com/r/haskell/comments/fnc80/type_level_natural_numbers_r… 10. http://hackage.haskell.org/trac/ghc/ticket/4385 11. http://www.reddit.com/r/haskell/comments/fnit9/haskell_web_framework_happst… 12. http://groups.google.com/group/happs/browse_thread/thread/f72d7ff313ea9fa0 13. http://www.reddit.com/r/haskell/comments/flyyg/welltyped_is_hiring/ 14. http://www.well-typed.com/blog/50 15. http://www.reddit.com/r/haskell/comments/flajs/haskell_is_better_slides_fro… 16. http://www.scribd.com/doc/48773692/Ignite-SV-Haskell-is-Better 17. http://www.reddit.com/r/haskell/comments/fo9gm/new_faster_hashingbased_cont… 18. http://blog.johantibell.com/2011/02/new-faster-containers-library.html 19. http://www.reddit.com/r/haskell/comments/fojb4/disciple_haskell_dialect_str… 20. http://disciple.ouroborus.net/ 21. http://www.reddit.com/r/haskell/comments/flp87/yesod_warp_speed_ahead/ 22. http://docs.yesodweb.com/blog/warp-speed-ahead 23. http://www.reddit.com/r/haskell/comments/fm6gg/faster_persistent_data_struc… 24. http://blog.johantibell.com/2011/02/slides-from-my-hashing-based-containers… 25. http://www.reddit.com/r/haskell/comments/fkqe1/developing_iphone_applicatio… 26. http://gergo.erdi.hu/blog/2011-02-13-developing_iphone_applications_in_hask… 27. http://www.reddit.com/r/haskell/comments/fl0wr/an_introduction_to_agda_pdf/ 28. http://dl.dropbox.com/u/361503/Agda%20and%20dependent%20types.pdf 29. http://stackoverflow.com/questions/5032750/how-to-tell-whether-haskell-will… 30. http://stackoverflow.com/questions/4999020/specific-type-inference-using-un… 31. http://stackoverflow.com/questions/5048920/haskell-unexpected-output-for-ex… 32. http://stackoverflow.com/questions/5023485/valid-haskell-function-names/502… 33. http://stackoverflow.com/questions/5012185/haskell-range-notation-to-genera… 34. http://stackoverflow.com/questions/5032750/how-to-tell-whether-haskell-will… 35. http://stackoverflow.com/questions/5004902/how-can-one-distribute-haskell-p… 36. http://stackoverflow.com/questions/5012185/haskell-range-notation-to-genera… 37. http://stackoverflow.com/questions/5023969/appropriate-uses-of-monad-fail-v… 38. http://stackoverflow.com/questions/5032475/why-does-haskell-not-have-an-i-m…

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Performance difference between ghc and ghci
by C K Kashyap 23 Feb '11

23 Feb '11

Hi, Is there a runtime performance difference between a haskell program running under GHCI vs in its compiled form? Especially for a long running program - as in, ignoring the initial setup time. If I understand right, in both case tree reduction is what is happening and performance should be similar. Regards, Kashyap

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Having trouble with instance context
by Kurt Stutsman 23 Feb '11

23 Feb '11

I am trying to create an instance of a class for data types deriving from Enum. When I try to specify this through a context on the instance definition, I get an error. When I do something similar with a function, it seems to work fine. Why does the instance not work? {------------ Example: ---------------} class Test t where func :: t -> Int -- This instance does not compile instance (Enum e) => Test e where func x = fromEnum x -- This function does work newfunc :: (Enum e) => e -> Int newfunc x = fromEnum x Thanks, Kurt

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ANN: unordered-containers - a new, faster hashing-based containers library
by Johan Tibell 23 Feb '11

23 Feb '11

Hi all, I am delighted to announce the release of preview versions of two new packages: unordered-containers 0.1 Efficient hashing-based container types. http://hackage.haskell.org/package/unordered-containers hashable 1.1 Efficient hashing of common types. http://hackage.haskell.org/package/hashable These packages address a need for faster container types in Haskell, especially for string types like ByteString and Text. The package achieves better performance by using hashing and a Patricia trie (also known as a radix tree), instead of a balanced tree, in its implementation. The provided HashMap type is three times faster than Data.Map for lookups and twice as fast for inserts, for all key types I've tried, including ByteString, String, and Int. I'm calling this a preview release, even though the code hash been well tested both for correctness and performance, as the API is quite basic and doesn't provide a HashSet type yet. One thing you'll notice is that the API is split into a value-strict and a value-lazy version, making it easier to use in a strict setting. If you want to contribute, please get copies of the source trees from here: * git clone https://github.com/tibbe/unordered-containers.git * git clone https://github.com/tibbe/hashable.git Alternatively, just fork the projects on GitHub: http://github.com/tibbe As I mentioned in my talk on hashing-based containers earlier this week, I'm working on an even faster version, based on hash-array mapped tries, but it won't be ready until GHC 7.2. Johan

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Re: [Haskell-cafe] Cabal things
by Antoine Latter 23 Feb '11

23 Feb '11

On Wed, Feb 23, 2011 at 8:48 AM, Volker Wysk <pf3(a)volker-wysk.de> wrote: > Hello > > I'm new to Cabal, and I'm trying to Cabalize my library HsShellScript, with > help by Howard Golden. I'm using the Simple Build Infrastructure. > > 1. I'm irritated by the fact, that calling the Cabal with "runhaskell > configure; runhaskell build; runhaskell install" is _not_ sufficent in many > cases. You need additional arguments for "runhaskell configure", and > "runhaskell build" can't build everything, you need other actions in many > cases. I need haddock, and it can't be specified in the .cabal file, or run > by "runhaskell build". > End users installing packages by hand largely use the cabal-install[1] command-line tool, which can be configured to auto-generate documentation for installed packages and put them in a centralized location. It may even do this by default. [1]: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall > > 2. I have some HTML pages of additional documentation, which need to be > installed, and need to be included in the (sdist) generated distribution > package. > > I have no clue how this should be done. > In the package description, you should be able to add an "Extra-Source-Files" section, see: http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.h… That will get them into the source tarball. I'm not sure what else you need to do to get them installed with the package - there is also a "data-files" filed for files that are needed at run-time. > Cheers, Volker > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > >

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help for the usage on mfix
by Gang Yu 23 Feb '11

23 Feb '11

hello cafe, I just want to do cursion to a fixpoint in an IO function, I write sth. like this, handle_ar::(Set String,Set String)->FilePath-> IO (Set String, Set String) handle_ar (def,undef) ar=do let gs = def gu = undef syms <- liftM (map (\x -> (symb x, x))) $ defined_syms ar usyms <- liftM (map (\x -> (symb x, x))) $ undefined_syms ar mfix (\(gs,gu) -> do case find_obj gu usyms of Nothing -> return (gs,gu) Just z -> do let fout = fromList . map fst . filter ((== (objf z)) . objf . snd) ls = fout syms lu = fout usyms handle_obj_ar (gs,gu) (ls,lu)) What I want to express is: gs and gu are initiliazed to def and undef, then do recursion until "find_obj gu usyms" is nothing, i.e, (gs,gu) reaches a fixpoint (suppose handle_obj_ar always change the pair). I am not sure I am on the right track since there is no room for def and undef stand in the function. Anyone can help? thanks a lot. Gang

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Can i split a String by its element ?
by z_axis 23 Feb '11

23 Feb '11

I want to split "2,15,33,0,8,1,16,18" to ([2,15,33],[8,1,16,18]). the "0" will by discarded. However, it seems that there isnot any standard function to do it. Sincerely! ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/Can-i-split-a-String-by-its-element-tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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linear logic
by Vasili I. Galchin 23 Feb '11

23 Feb '11

Hello, What is the category that is used to interpret linear logic in a categorical logic sense? Thank you, Vasili

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monadic plumbing
by Alberto G. Corona 22 Feb '11

22 Feb '11

Recently I had to navigatate trough data structures chained with mutable referenes in th STM monad. The problem is that their values are enveloped in Either or Maybe results. functional compositions in the Either of Maybe , or list monads are not possible when the values are embedded inside effect monads (i.e. STM or IO) . I tried to find some trick to handle it. to summarize, given: foo, : a -> m (Maybe b) bar : b -> m (Maybe c) baz : c -> m (Maybe d) how to compose foo bar and baz? Or, at least, Are something out there to handle it in the less painful way?. I solved the generalized problem (chaining any double monadic combination) with a sort of monadic connector that acts as a " double monadic" operator >>>>== so that return. return (x :: a) >>>>== foo >>>== bar >>>== baz can be possible. Although I don't know if it is the best solution. I wonder why nobody has written about it before: class (Monad m, Monad n) => Bimonad m n where (>>>=) :: n a -> (a -> m(n b)) -> m(n b) (>>>>==) :: (Bimonad m n) => m (n a) -> (a -> m(n b)) -> m (n b) (>>>>==) x f = x >>= \y -> y >>>= f x >>>> f = x >>>>== \ _-> f infixl 1 >>>>==, >>>> The instance for handling the Maybe monad under any other monad is very similar to the definition of the "normal" monad: instance (Monad m) => Bimonad m Maybe where Just x >>>= f = f x Nothing >>>= _ = return $ Nothing

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