- Haskell-Cafe - Haskell.org

Python is lazier than Haskell
by Henning Thielemann 29 Apr '11

29 Apr '11

I like to apply for the quote of the week. :-) "If Haskell is great because of its laziness, then Python must be even greater, since it is lazy at the type level." Dynamically typed languages only check types if they have to, that is if expressions are actually computed. Does this prove that laziness at type level as in Python is a great thing or does this prove that laziness at the value level as in Haskell is a bad thing or does this prove nothing? Hope for controversial discussion ...

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Iteratee: manyToOne
by Dmitry Olshansky 29 Apr '11

29 Apr '11

Hello, does somewhere exist function with type like this - manyToOne :: [Iteratee a m b] -> Iteratee a m [b] ? I.e. I need to process one input through many Iteratees indepentently in constant space and collect results. It is similar by type with sequenceM but as far as I understand sequenceM won't use the same input for all Iteratees. Dima

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-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell -- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, -- 2010, page 25 #Haskell -- This version assumes 3 disjoint ordered sets represented as sorted arrays.ts.
by caseyh＠istar.ca 29 Apr '11

29 Apr '11

-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell -- O(log|X|+log|Y|+log|Z|) performance -- Question: is there a way to get the type signature as the following: -- smallest :: (Ord a) => Int -> [Array Int a] -> a module SelectionProblem where import Data.Array import Data.List -- Works on 2 finite ordered disjoint sets represented as sorted arrays. smallest :: (Ord a) => Int -> (Array Int a, Array Int a) -> a smallest k (xa,ya) = ~~~~search k (xa,ya) (0,m+1) (0,n+1) ~~~~~~~~where ~~~~~~~~(0,m) = bounds xa ~~~~~~~~(0,n) = bounds ya -- Removed some of the "indexitis" at the cost of calling another function. search :: (Ord a) => Int -> (Array Int a, Array Int a) -> (Int,Int) -> (Int,Int) -> a search k (xa,ya) (lx,rx) (ly,ry) ~~~~| lx == rx = ya ! (k+ly) ~~~~| ly == ry = xa ! (k+lx) ~~~~| otherwise = case (xa ! mx < ya ! my) of ~~~~~~~~~~~~~~~~~~~~~~(True) -> smallest2h k (xa,ya) ((lx,mx,rx),(ly,my,ry)) ~~~~~~~~~~~~~~~~~~~~~~(False) -> smallest2h k (ya,xa) ((ly,my,ry),(lx,mx,rx)) ~~~~~~~~~~~~~~~~~~where ~~~~~~~~~~~~~~~~~~~~~~mx = (lx+rx) `div` 2 ~~~~~~~~~~~~~~~~~~~~~~my = (ly+ry) `div` 2 -- Here the sorted arrays are in order by their middle elements. -- Only cutting the leading or trailing array by half. -- Here xa is the first array and ya the second array by their middle elements. smallest2h :: (Ord a) => Int -> (Array Int a, Array Int a) -> ((Int,Int,Int),(Int,Int,Int)) -> a smallest2h k (xa,ya) ((lx,mx,rx),(ly,my,ry)) = ~~~~case (k<=mx-lx+my-ly) of ~~~~~~(True) -> search k (xa,ya) (lx,rx) (ly,my) ~~~~~~(False) -> search (k-(mx-lx)-1) (xa,ya) (mx+1,rx) (ly,ry) ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- -- Works on 3 finite ordered disjoint sets represented as sorted arrays. smallest3 :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> a smallest3 k (xa,ya,za) = ~~~~-- On each recursive call the order of the arrays can switch. ~~~~search3 k (xa,ya,za) (0,bx+1) (0,by+1) (0,bz+1) ~~~~~~~~where ~~~~~~~~(0,bx) = bounds xa ~~~~~~~~(0,by) = bounds ya ~~~~~~~~(0,bz) = bounds za -- Removed some of the "indexitis" at the cost of calling another function. search3 :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> ~~~~~~~~~~~~(Int,Int) -> (Int,Int) -> (Int,Int) -> a search3 k (xa,ya,za) (lx,rx) (ly,ry) (lz,rz) ~~~~| lx == rx && ly == ry = za ! (k+lz) ~~~~| ly == ry && lz == rz = xa ! (k+lx) ~~~~| lx == rx && lz == rz = ya ! (k+ly) ~~~~| lx == rx = search k (ya,za) (ly,ry) (lz,rz) ~~~~| ly == ry = search k (xa,za) (lx,rx) (lz,rz) ~~~~| lz == rz = search k (xa,ya) (lx,rx) (ly,ry) ~~~~| otherwise = case (xa ! mx < ya ! my, xa ! mx < za ! mz, ya ! my < za ! mz) of ~~~~~~~~~~~~~~~~~~~~~~(True, True, True) -> smallest3h k (xa,ya,za) ((lx,mx,rx),(ly,my,ry),(lz,mz,rz)) -- a<b<c ~~~~~~~~~~~~~~~~~~~~~~(True, True, False) -> smallest3h k (xa,za,ya) ((lx,mx,rx),(lz,mz,rz),(ly,my,ry)) -- a<c<b ~~~~~~~~~~~~~~~~~~~~~~(False, True, True) -> smallest3h k (ya,xa,za) ((ly,my,ry),(lx,mx,rx),(lz,mz,rz)) -- b<a<c ~~~~~~~~~~~~~~~~~~~~~~(False, False, True) -> smallest3h k (ya,za,xa) ((ly,my,ry),(lz,mz,rz),(lx,mx,rx)) -- b<c<a ~~~~~~~~~~~~~~~~~~~~~~(True, False, False) -> smallest3h k (za,xa,ya) ((lz,mz,rz),(lx,mx,rx),(ly,my,ry)) -- c<a<b ~~~~~~~~~~~~~~~~~~~~~~(False, False, False) -> smallest3h k (za,ya,xa) ((lz,mz,rz),(ly,my,ry),(lx,mx,rx)) -- c<b<a ~~~~~~~~~~~~~~~~~~where ~~~~~~~~~~~~~~~~~~~~~~mx = (lx+rx) `div` 2 ~~~~~~~~~~~~~~~~~~~~~~my = (ly+ry) `div` 2 ~~~~~~~~~~~~~~~~~~~~~~mz = (lz+rz) `div` 2 -- Here the sorted arrays are in order by their middle elements. -- Only cutting the leading or trailing array by half. -- Here xa is the first array, ya the second array, and za the third array by their middle elements. smallest3h :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> ~~~~~~~~((Int,Int,Int),(Int,Int,Int),(Int,Int,Int)) -> a smallest3h k (xa,ya,za) ((lx,mx,rx),(ly,my,ry),(lz,mz,rz)) = ~~~~case (k<=mx-lx+my-ly+mz-lz) of ~~~~~~(True) -> search3 k (xa,ya,za) (lx,rx) (ly,ry) (lz,mz) ~~~~~~(False) -> search3 (k-(mx-lx)-1) (xa,ya,za) (mx+1,rx) (ly,ry) (lz,rz) ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- -- To convert a list into an array indexed from 0. xa = listArray (0, length xs - 1) xs ya = listArray (0, length ys - 1) ys za = listArray (0, length zs - 1) zs xs = [0,17..90] ys = [1,13..69] zs = [7,24..91] ua = listArray (0, length us - 1) us va = listArray (0, length vs - 1) vs wa = listArray (0, length ws - 1) ws us = [0,17..100] vs = [101,121..200] ws = [201,221..300] -- *SelectionProblem> sort (xs++ys++zs) -- [0,1,7,13,17,24,25,34,37,41,49,51,58,61,68,75,85]

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Trouble installing FileManipCompat
by Tony Miller 29 Apr '11

29 Apr '11

Hi, I'm trying to install FileManipCompat(well, really HStringTemplateHelpers, but the error is from FileManipCompat) and I'm getting this error: [1 of 1] Compiling System.FilePath.FindCompat ( System/FilePath/FindCompat.hs, dist/build/System/FilePath/FindCompat.o ) System/FilePath/FindCompat.hs:175:20: Not in scope: data constructor `State' I've attached the complete output of `cabal --verbose install HStringTemplateHelpers`.

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ANNOUNCE: cereal-0.3.3.0
by Trevor Elliott 28 Apr '11

28 Apr '11

Hot on the heels of the last release, cereal-0.3.3.0 [1] adds support for parsing and rendering lazy ByteStrings. Most running functions in Data.Serialize.Get and Data.Serialize.Put now have lazy analogues, and Data.Serialize has gained encodeLazy and decodeLazy. This new functionality was made possible by the incremental parsing support added in cereal-0.3.1.0 by Lemmih. [1] http://hackage.haskell.org/package/cereal-0.3.3.0

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: zeno-0.2
by Benedict Eastaugh 28 Apr '11

28 Apr '11

Hi Will, I attended your presentation on Zeno at AngloHaskell last year and was really intrigued; it's great to see you've got a release up on Hackage, and I look forward to trying it out. Are you planning on making your source code repository publicly available on GitHub or somewhere similar? If nothing else it would be interesting to follow further developments between releases. All the best, Benedict.

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Re: [Haskell-cafe] Python is lazier than Haskell
by Dan Doel 28 Apr '11

28 Apr '11

(Sorry if you get this twice, Ertugrul; and if I reply to top. I'm stuck with the gmail interface and I'm not used to it.) On Thu, Apr 28, 2011 at 11:27 AM, Ertugrul Soeylemez <es(a)ertes.de> wrote: > I don't see any problem with this. Although I usually have a bottom-up > approach, so I don't do this too often, it doesn't hurt, when I have to. I do. It's low tech and inconvenient. Whenever I program in Haskell, I miss Agda's editing features, where I can write: foo : Signature foo x y z = ? Then compile the file. The ? stands in for a term of any type, and becomes a 'hole' in my code. The editing environment will then tell me what type of term I have to fill into the hole, and give me information on what is available in the scope. Then I can write: foo x y z = { partialImpl ? ? } and execute another command. The compiler will make sure that 'partialImpl ? ?' has the right type to fill in the hole (with ?s again standing in for terms of arbitrary type). If the type checking goes through, it expands into: foo x y z = partialImpl { } { } and the process repeats until my function is completely written. And of course, let's not forget the command for automatically going from: foo x y z = { x } to foo Con1 y z = { } foo Con2 y z = { } foo Con3 y z = { } ... I don't think there's anything particularly Agda-specific to the above. In fact, the inference required should be easier with Haskell/GHC. Features like this would be pretty killer to have for Haskell development; then I wouldn't have to prototype in Agda. :) -- Dan

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There is no null; Maybe/Option types
by Christopher Done 28 Apr '11

28 Apr '11

Hi Cédric, I saw your post on Maybe types wondering what the point of it is and why anyone would want it. I thought I'd reply by email as I don't want to infringe on your freedom of expression. If you find that I'm saying things you already know, please scroll down to the bottom, which addresses your final question. I'm CC'ing the Haskell mailing list. I wrote a lot and I might as well share it. Quotes below are from: http://beust.com/weblog/2010/07/28/why-scalas-option-and-haskells-maybe-typ… > If you read the voluminous material that describes the concepts behind the > Option class, there are two main benefits: > > - It saves you from NullPointerException > - It allows you to tell whether null means “no object” or “an object > whose value is null” > > The second point is true, as you probably already understand: - Nothing is different from False. - They also have distinct, completely un-unifiable types. Nothing is type Maybe something, False is type Bool. - Maybe Int is also distinct from Maybe String. The latter point is true in a sense, but I think the problem and the solution aren't the same for Haskell and Java. I can't speak for Java because I don't know it, I know C#, which I know from a lot of experience does have nullable objects and thus null pointer exceptions. I assume everything below can be applied to Java, please tell me if not. - *No* value in Haskell can be null/empty. - *All* values in C# can be null/empty. - Therefore if C#'s values were completely explicit, and they might be defined, in Haskell, as: - data CObject a = TheValue a | Null - I'm assuming you know a bit of Haskell or can just figure it out easily enough. Read as: the type CObject, for all types, a, can be represented by either a value of type a, or null. - So a value of type int can be inhabited by either 123 or null. - Notice, of course, that this is the same as the definition of Maybe: data Maybe a = Just a | Nothing. - Therefore, in Haskell's context, it's not that Maybe saves you from null pointers, it's that it *allows you to express* nullability in a language without implicit nullability, which is default for all values in C#. I suspect the crux of the problem is that Java and C# have implicit nullability. It's also worth noting that the Maybe type in Haskell *can* *also* cause something similar "null pointer exceptions"; if you use the function fromJust, which just takes from a Maybe value and throws an exception if there's nothing in it. And, indeed, this function is a very common cause of exceptions in Haskell. A simple Google shows 870 results of this exception: http://www.google.com/#q="Exception:+Maybe.fromJust%3A+Nothing"<http://www.google.com/#q=%22Exception:+Maybe.fromJust%3A+Nothing%22>Also similar is use of the head function, which takes the first element of a list or throws an exception is the list is empty: http://www.google.com/#q="Exception:+Prelude.head%3A+empty+list"<http://www.google.com/#q=%22Exception:+Prelude.head%3A+empty+list%22>. 4,610 results. Most of the (non-IO) runtime errors I get using Haskell software is due to head or fromJust, it's actually quite annoying. That is the technical aspect and some background, for what it's worth. val result = map.get( "Hello" ) > > result match { > case None => print "No key with that name!" > case Some(x) => print "Found value" + x > } > > See what’s going on here? You avoid a NullPointerException by… testing > against null, except that it's called None. What have we gained, exactly? > > The worst part about this example is that it forces me to deal with the > null case right here. Sometimes, that's what I want to do but what if such > an error is a programming error (e.g. an assertion error) that should simply > never happen? > I believe here lies the problem. It's true; if your method expects a value not to be null then you shouldn't have to deal with it. - If the value is never ever supposed to be null, then you should be given a non-nullable value. - If the value is supposed to be nullable at some point, then your function is explictly giving the opportunity for optionality and should support it. If it barfs on a null value then it's not really satisfying the deal made with the caller that it supports nullable values. Unfortunately this is impossible in Java and C# because you can't have non-nullables. You also can't tell everyone to never use null and use a Maybe value, so C# or Java are more or less screwed on this point. > And you know what construct does exactly this? NullPointerException! Try > to reference a null pointer and that exception will be thrown. It will make > its way up the stack frames since you probably never catch it anywhere (nor > should you) and it will show you a clear stack trace telling you exactly > what happened and where. > The solution proposed for Java is for the method to barf on bad input at runtime. The solution of OCaml/Haskell is to barf at the caller of the function at development time; you're using the function incorrectly, and the developer's realised that he's using it incorrectly and immediately fixes it; the code never made it into testing or production. How much time did this save? I don't think anyone would argue that finding out later rather than now that we screwed up is desirable. > In this case, I just want to assume that I will never receive null and I > don't want to waste time testing for this case: my application should simply > blow up if I get a null at this point. > You can do that with fromJust, but it's generally considered bad practice, at least in the Haskell development world. Using fromJust brings enforcing constraints away from the compiler and into the hands of the developer who is flawed, busy and confused most of the time. These types of functions are called partial functions. A total function is a function that handles every case of a value given to it. So: > :t head head :: [a] -> a > :t listToMaybe listToMaybe :: [a] -> Maybe a > head [] *** Exception: Prelude.head: empty list > listToMaybe [] Nothing The former is partial, the latter is total. Code that uses partial functions is inherently partial in its entirety, leaving enforcing invariants on the programmer which could otherwise have been done by the compiler. *Static*: In Fantom, the fact that a variable can be null or not is captured > by a question mark appended to its type: Already covered; we both know Option/Maybe do this. > > - *Runtime*: The second aspect is solved by Fantom's "safe invoke" > operator, ?.. This operator allows you to dereference a null pointer > without receiving a null pointer exception: > // hard way > Str? email := null > if (userList != null) > { > user := userList.findUser("bob") > if (user != null) email = user.email > } > > // easy way > email := userList?.findUser("bob")?.email > > Note how this second example is semantically equivalent to the first > one but with a lot of needless boiler plate removed. > > So, can someone explain to me how Option addresses the null pointer > problem better than Fantom's approach? > Use of Fantom's save invoke and Maybe are more or less the same. -- Hard way email = if userList /= Nothing then let user = findUser "bob" (fromJust userList) in if user /= Nothing then getEmail (fromJust user) else Nothing else Nothing Note that I have to use fromJust because there is no implicit nullability. -- Easy way email = userList >>= findUser "bob" >>= getEmail This is semantically equivalent to the first one. There is also syntactic sugar for the above, for when you're not just chaining, but doing something complicated: -- Sugared way email = do list <- userList user <- findUser "bob" list email <- getEmail user return email If I'm using list or user or email more than once, this style is very useful. This is exactly equivalent to the above, it de-sugars into using the >>=function, which is pretty much the same as your use of ?., in this context. Cases and functions are just another way of accessing maybe values when you explicitly want to handle the null case. λ> maybe 4 (+2) (Just 5) 7 λ> maybe 4 (+2) Nothing 4 λ> case Just 5 of Just x -> x * 2; Nothing -> 0 10 I'm not saying it's the only way or the best way. It's certainly nice, though. P.S. Question: what would this code do and what would the compiler tell you about it? Str? email := null email = userList.findUser("bob").email

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HipmunkPlayground compile error
by Andrew Pennebaker 28 Apr '11

28 Apr '11

$ system_profiler SPSoftwareDataType | grep "System Version" System Version: Mac OS X 10.6.7 (10J869) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 $ cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library $ cabal install hipmunkplayground ... Updating documentation index /Users/andrew/Library/Haskell/doc/index.html cabal: Error: some packages failed to install: GLFW-0.4.2 failed during the building phase. The exception was: ExitFailure 1 HipmunkPlayground-5.2.0.3 depends on GLFW-0.4.2 which failed to install. Cheers, Andrew Pennebaker www.yellosoft.us

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GHC repositories
by Andrew Pennebaker 28 Apr '11

28 Apr '11

Now that GHC has a git repository<http://hackage.haskell.org/trac/ghc/wiki/Repositories>, could that change be reflected in the docs<http://www.haskell.org/ghc/download> ? It's hard to work with folks on Trac to debug GHC when I can't find the latest version (changesets are now git, not svn). Cheers, Andrew Pennebaker www.yellosoft.us

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Haskell Weekly News: Issue 179
by Daniel Santa Cruz 28 Apr '11

28 Apr '11

Welcome to issue 179 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of April 16 to 23, 2011. Announcements Alex Mason announced (http://goo.gl/2qtyM) the second annual Australian Haskell Hackathon to be heald from Friday July 8th to Sunday July 10th, at the UNSW's Computer Science building in Sydney. Come share with your fellow Australasian Haskellers! "The Hac" team invited us (http://goo.gl/8EQfI) to HacPhi 2011, a Haskell hackathon/get-together to be held July 29 to 31 at the University of Pennsylvania, in Philadelphia. Daniel Wagner wanted to stress that "everyone is welcome--you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu." Robert Clausecker asked "Is Hugs dead?" (http://goo.gl/639dC) Sad to see projects of this sort fade into the past. Quotes of the Week * copumpkin: THE POWER OF MIXFIX COMPELS YOU (to not use it much) Top Reddit Stories * Why Darcs users care about consistency Domain: koweycode.blogspot.com, Score: 59, Comments: 3 On Reddit: http://goo.gl/oBU0r Original: http://goo.gl/lnUCU * Review: Learn You a Haskell for Great Good! Domain: jerf.org, Score: 57, Comments: 15 On Reddit: http://goo.gl/BjJq7 Original: http://goo.gl/P7u3x * The Haskell Heap : Inside P4 Domain: blog.ezyang.com, Score: 37, Comments: 3 On Reddit: http://goo.gl/EoV5D Original: http://goo.gl/ropku * First-class modules for Haskell : Shields and Peyton Jones, 2001 :: PDF Domain: research.microsoft.com, Score: 27, Comments: 1 On Reddit: http://goo.gl/7renV Original: http://goo.gl/7lHV4 * Darcs hacking sprint 6 report Domain: blog.darcs.net, Score: 27, Comments: 13 On Reddit: http://goo.gl/ljO1O Original: http://goo.gl/xRr6r * Ugly memoization Domain: augustss.blogspot.com, Score: 25, Comments: 8 On Reddit: http://goo.gl/D6FoV Original: http://goo.gl/IUL1M * Monoids for Maybe Domain: byorgey.wordpress.com, Score: 23, Comments: 2 On Reddit: http://goo.gl/yVU7O Original: https://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/ * Don't forget to think about GC flags when doing performance tuning Domain: stackoverflow.com, Score: 20, Comments: 3 On Reddit: http://goo.gl/6xzEj Original: http://goo.gl/Xwkro * Haskell videos on video.google.com, which shuts down for good on April 29th 2011 Domain: google.com, Score: 20, Comments: 5 On Reddit: http://goo.gl/MizlZ Original: http://goo.gl/CxnBK * Evaluation on the Haskell Heap Domain: blog.ezyang.com, Score: 20, Comments: 2 On Reddit: http://goo.gl/EX5aE Original: http://goo.gl/cU7Nt Top StackOverflow Questions * What are the primary theoretical difficulties with adding ML-style modules to Haskell? votes: 27, answers: 3 Read on SO: http://goo.gl/8An0y * Optimizing numerical array performance in Haskell votes: 25, answers: 1 Read on SO: http://goo.gl/ykzlV * lenses, fclabels, data-accessor - which library for structure access and mutation is better votes: 17, answers: 1 Read on SO: http://goo.gl/c74uO * In Haskell performing 'and' and 'or' for boolean functions votes: 15, answers: 5 Read on SO: http://goo.gl/0U5pI * Automatic memoizing in functional programming languages votes: 11, answers: 4 Read on SO: http://goo.gl/hg8Ei * Alternative implementations of Haskell's standard library type classes votes: 9, answers: 3 Read on SO: http://goo.gl/LDp9u * Scheme vs Haskell for an Introduction to Functional Programming? [migrated] votes: 8, answers: 0 Read on SO: http://goo.gl/lbeX3 * Writing loops for interactive IO: problems with do-notation and layout votes: 5, answers: 1 Read on SO: http://goo.gl/HVRrR * Python-"is"-like equality operator for Haskell/GHC votes: 5, answers: 4 Read on SO: http://goo.gl/7Z4mA * Optimising a Haskell XML parser votes: 5, answers: 1 Read on SO: http://goo.gl/fsqqw 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

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Good reads?
by Christopher Svanefalk 27 Apr '11

27 Apr '11

[I originally posted this over in haskell-beginners. However, since this list has a lot more traffic, and I am not sure how many people read both lists, I thought I would post here as well. I apologize for the doublepost! Just want to make sure I can get some replies from the great people here who might not read the beginners list] Hey all, I have mostly been brought in imperative/OOP languages. In connection with my college studies, however, I have taken great interest in the functional paradigm, and would very much like to dig deeper into it. I am currently reading through Peyton-Jones "Haskell: The Craft of Functional Programming" (2nd ed.), as well as a great paper published by one of my professors (http://www.cse.chalmers.se/~rjmh/Papers/whyfp.html) However, what other works, in your opinions, should I look into to get a more complete understanding of functional programming? Thanks in advance! Chris

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REMINDER: Haskell Communities and Activities Report, May 2011 edition
by Janis Voigtländer 27 Apr '11

27 Apr '11

Dear all, It is not yet too late to contribute to the 20th edition of the ================================================================ Haskell Communities & Activities Report http://tinyurl.com/haskcar Submission deadline: this weekend (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ If you haven't already, please write an entry for your new project or update your old entry. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2011-April/022720.html I look forward to your contributions, Janis (current editor) -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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[GSoC] Text/UTF-8: Call for Benchmarks
by Jasper Van der Jeugt 27 Apr '11

27 Apr '11

Hello all, I'm very glad that I have been accepted again this year for the Google Summer of Code [1] program for haskell.org. My project aims to improve the text [2] library by converting it to internally use UTF-8 instead of UTF-16. UTF-8 and UTF-16 both have advantages and disadvantages, which actually makes it a pretty complicated choice. I've written about this a little in my [3] (especially see Tom Harper's master dissertation if you're interested in the subject). To support a decision here on UTF-8 vs. UTF-16, lots of benchmarks will be needed. Hence, this is the first focus of the GSoC project: collecting a large benchmark suite which models real-world usage of the text library. This is why I'd like to ask everyone who has written/knows libraries or applications that use the text library extensively to inform me of these efforts. The reverse dependencies list on Hackage is a good starting point for me but it doesn't point out how popular these packages are and how intensively they use the text library. I will then convert a subset of this code to a benchmark suite using criterion. Open source code means more reliable benchmarks, because I can publish the code I used for them. However, I'm also willing to sign non-disclosure agreements if this means I can try out what effects the changes have on large systems. There's several ways to contact me: you can reply to this thread, or you can mail me privately using `jaspervdj+text(a)gmail.com`. Thanks in advance for any help! [1]: http://code.google.com/soc/ [2]: http://hackage.haskell.org/package/text [3]: http://jaspervdj.be/files/2011-gsoc-text-utf8-proposal.html Cheers, Jasper

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ANNOUNCE: New version Feldspar
by Emil Axelsson 27 Apr '11

27 Apr '11

On behalf of the Feldspar team, I'm happy to announce version 0.4.0.2 of feldspar-language and feldspar-compiler: http://hackage.haskell.org/package/feldspar-language http://hackage.haskell.org/package/feldspar-compiler Feldspar is an embedded domain-specific language for generating code for embedded applications, in particular for digital signal processing. For more information, see our web pages: http://feldspar.inf.elte.hu/feldspar/ http://feldspar.inf.elte.hu/ A brief list of changes since the last version: * Support for GHC 7. * Including a fresh tutorial for new users. * Improved C code generation (efficiency and readability). - Efficient vector append. - More unified representation of arrays in the C code. - Etc. * A new sequential construct for unfolding vectors using an accumulating parameter. * Support for structs. * New optimization: common sub-expression elimination. * Lots of restructuring under the hood. * Bugfixes. / Emil

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How to update the RNG per call (State monad) when generating QuickCheck arbitraries?
by Daniel Kahlenberg 27 Apr '11

27 Apr '11

Maybe this is a beginners question... But here my problems description: > import Random > import Control.Monad > import qualified Control.Monad.State as S > import Test.QuickCheck.Gen > import Test.QuickCheck.Arbitrary Each thing can have one of three types: > data Ontology = Thing1 Bool > | ThingK Bool > deriving (Show, Eq) > instance Arbitrary Ontology where > arbitrary = > oneof [ return $ Thing1 True > , return $ ThingK True > , return $ Thing1 False > , return $ ThingK False ] Liked to have a state monad runner for my arbitrary things as in "Real World Haskell", "Chapter 14. Monads" ("Random values in the state monad"). [RWH]: > -- file: ch14/Random.hs > type RandomState a = S.State StdGen a [RWH]: < -- file: ch14/Random.hs < getRandom :: Random a => RandomState a < getRandom = < S.get >>= \gen -> < let (val, gen') = random gen in < S.put gen' >> < return val [RWH]: < -- file: ch14/Random.hs < getTwoRandoms :: Random a => RandomState (a, a) < getTwoRandoms = liftM2 (,) getRandom getRandom [RWH]: < -- file: ch14/Random.hs < runTwoRandoms :: IO (Int, Int) < runTwoRandoms = do < oldState <- getStdGen < let (result, newState) = S.runState getTwoRandoms oldState < setStdGen newState < return result Thought getRandom function would be the best place to inject my unGen function call, but cannot get it to type-check: > getRandom :: Random a => RandomState [a] > getRandom = > S.get >>= \gen -> > let (val, gen') = liftM2 (,) (unGen arbitrary gen 12) (random gen) in > S.put gen' >> > return val A function not almost fulfilling my needs but the Random Number Generator isn't updated... I liked to have different [Ontology] occasions per call: > genArray :: [Ontology] > genArray = unGen arbitrary (mkStdGen 42) 12 :: [Ontology] Does anyone have the patience to help me out at least one step further? Cheers Daniel

2 3

plugins and cabal build
by Michael Snoyman 27 Apr '11

27 Apr '11

Hi all, I'm experimenting with using the plugins package for yesod devel server. The basic approach is to use cabal for building the object files, and then load them with plugins. I can get everything to work when I compile with "ghc --make", but I believe name mangling is getting in the way with the "cabal build" route. Can someone give some guidance on how to properly mangle the function names so that plugins can load them? Thanks, Michael

3 3

Creating polygons in qtHaskell
by Øystein Kolsrud 27 Apr '11

27 Apr '11

Hi! I'm using qtHaskell and I'm trying to create a QGraphicsPolygonItem object. From what I see I need to create a QPolygonF item that I can supply to the constructor, but to populate the QPolygonF I need to use methods of the QVector class that QPolygonF inherits and I can't figure out how to do that. It seems that QVector is not part of the qtHaskell library. What I would like to do is something like this (where mypoints is a list of QPointF): makePolygon = do polygon <- qPolygonF () mapM (pushBack polygon) mypoints qGraphicsPolygonItem_nf polygon However, pushBack doesn't seem to be available. Does anyone know a way to do this? -- Mvh Øystein Kolsrud

1 0

ANNOUNCE: iteratee-compress 0.2.0.0
by Maciej Piechotka 26 Apr '11

26 Apr '11

Iteratee-compress provides compressing and decompressing enumerators including flushing (using John Lato's implementation). Currently only gzip and bzip is provided but LZMA is planned. Changes from previous version: - Add BZip support Next goals: - LZMA support - Generic interface for flushing To think about: - Should inner iteratee be able to request flushing Regards

2 2

Re: [Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)
by John Lato 26 Apr '11

26 Apr '11

Joining slightly late... > From: John Millikin <jmillikin(a)gmail.com> > > John Lato's "iteratee" package is based on IterateeMCPS.hs[1]. I used > IterateeM.hs for "enumerator", because when I benchmarked them the non-CPS > version was something like 10% faster on most operations. > Based on tests I did before iteratee-0.4, the fastest implementation was Oleg's alternate design which is in the comments of IterateeM.hs. I don't think any current packages use that however, with good reason. I'd be interested to see the results of a shootout between iteratee and enumerator. I would expect them to be basically equivalent most of the time, with maybe two or three operations with a small (but consistent) difference one way or the other. John L.

2 2

Haskell User Group starter kit
by Eric Y. Kow 26 Apr '11

26 Apr '11

Hi all, We may be coming to a time where we start to see Haskell User Groups or Functional Programming clubs popping up all the over the place. Maybe it's a good time to put our heads together and figure out what we can do to nurture these groups, to keep them going strong rather than fizzling out. One idea might be create some sort of Haskell User Group starter kit so that individual groups can hit the ground running with some concrete ideas. http://www.haskell.org/haskellwiki/Haskell_User_Group My hope is that it would provide just enough material for a group to reach critical mass. So just enough stuff to account for the different kinds of activities a HUG may want to do (eg. Coding Dojos vs Talks vs Reading Group), but not so much that HUGs feel intimidated or stifled. Have you started a HUG recently? Or are you thinking of starting one? Maybe you could contribute just one idea to the kit? More welcome, of course :-) -- 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)

2 1

Re: [Haskell-cafe] How to update the RNG per call (State monad) when generating QuickCheck arbitraries?
by Bryan O'Sullivan 26 Apr '11

26 Apr '11

On Tue, Apr 26, 2011 at 9:16 AM, Daniel Kahlenberg <d.kahlenberg(a)gmail.com>wrote: > > hold on I'd like to have the genArray call generating distinctive > results in one IO execution The problem you're seeing is due to the fact that you're not taking the final RNG state from the first execution of your code and passing it as the initial state to the second. Since you're initialising each one with the same RNG state, you're getting the same results in each case.

1 0

typing hmatrix expressions
by Carter Schonwald 26 Apr '11

26 Apr '11

hello alberto, i've had some funny issues with using the cmap function, and i'd like understand what i'm doing wrong namely it wont correctly instantiate for vectors or matrices and I don't understand why. basically my question is: what is the correct type for the hmatrix expression mapMat f =liftMatrix $! cmap f I'd like to just give it a type like Storable a => (a->a)-> Matrix a -> Matrix a but i get an error message like Electrical.lhs:89:29: No instance for (Container Vector e0) arising from a use of `cmap' Possible fix: add an instance declaration for (Container Vector e0) In the second argument of `($!)', namely `cmap f' In the expression: liftMatrix $! cmap f In an equation for `mapMat': mapMat f = liftMatrix $! cmap f which i feel shouldn't be happening. is the type of cmap in some way too polymorphic, or am I just not turning on the right ghc type extensions for this to work, or is this something that would have been well typed prior to ghc 7? Or am i just not correctly understanding this type error and i'm making a simple mistake? thanks! -Carter

1 0

converting prefixes of CString <-> String
by Eric Stansifer 26 Apr '11

26 Apr '11

I have been reading Foreign.C.String but it does not seem to provide the functionality I was looking for. Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert Haskell Strings to CStrings. (If I understand correctly, c2h . h2c === id, but h2c . c2h is not the identity on all inputs; or perhaps c2h is not defined for all CStrings. Probably this is all locale dependent.) I have an infinite Haskell String transferred byte-wise over a network; I would like to convert some prefix of the bytes received into a prefix of the String I started with. However, if I understand correctly, if "s" is a Haskell String it is not necessarily true that "c2h (take n (h2c s))" is a prefix of s for all n. So I have two questions: Given a CString of the form "cs = take n (h2c s)", how do I know whether "c2h cs" is a prefix of s or not? Is there a way to recognize whether a CString is "valid" as opposed to truncated in the middle of a code point, or is this impossible? Better yet, given a CString "cs = take n (h2c s)", is there a way to find the maximal prefix cs' of cs such that c2h cs' is a prefix of s? If s == s1 ++ s2, is it necessarily true that s == (c2h (h2c s1)) ++ (c2h (h2c s2))? If so, then I can perform my conversion a bit at a time, otherwise I'd need to start from the beginning of the cstring each time I receive additional data. In practice, I think my solution will come down to restricting my program to only using the lower 128 characters, but I'd like to know how to handle this problem in full generality. Thanks, Eric

2 3

Why does Cabal not choose the latest package?
by Emil Axelsson 26 Apr '11

26 Apr '11

Hello! I've had some of the usual problems with packages depending on multiple versions of another package. It seems the root of the hole problem was that I once attempted to run cabal install cabal-install This brought in a number of older packages (Cabal-1.8.0.6, containers-0.3.0.0, directory-1.0.1.2, filepath-1.1.0.4, process-1.0.1.5) to my local package DB, which led to inconsistencies when trying to install new packages. However, if I run cabal install cabal-install-0.10.2 things work just fine. So the problem is that it wants to install an older version of cabal-install. A session demonstrating the problem is found below. Why doesn't cabal always install the latest packages? Thanks! / Emil [emax@laptop:~]> ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 [emax@laptop:~]> cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library [emax@laptop:~]> cabal install cabal-install Resolving dependencies... Downloading containers-0.3.0.0... ... [emax@laptop:~]> cabal install cabal-install-0.10.2 Resolving dependencies... Configuring cabal-install-0.10.2... Preprocessing executables for cabal-install-0.10.2... Building cabal-install-0.10.2... ...

2 1

-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell
by caseyh＠istar.ca 26 Apr '11

26 Apr '11

-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, -- 2010, page 25 #Haskell -- This version assumes 3 disjoint ordered sets represented as lists. -- So either: x<y XOR x>y -- Since it uses lists it is no faster than the divide and conquer approach. -- I might try to convert this version to sorted arrays for -- O(log|X|+log|Y|+log|Z|) performance -- If I can figure out how to do it without suffering from "indexitis". smallest3'' :: Ord a => Int -> ([a], [a], [a]) -> a smallest3'' k ([],[],ts) = ts !! k smallest3'' k (zs,[],[]) = zs !! k smallest3'' k ([],ws,[]) = ws !! k smallest3'' k ([],ws,ts) = smallest'' k (ws,ts) smallest3'' k (zs,[],ts) = smallest'' k (zs,ts) smallest3'' k (zs,ws,[]) = smallest'' k (zs,ws) smallest3'' k (zs,ws,ts) = ~~~~case (a<b, b<c, a<c) of ~~~~~~(True, True, True) -> smallest3h'' k ((zs,p,ys),(ws,q),(ts,o,rs)) -- a<b<c ~~~~~~(True, False, True) -> smallest3h'' k ((zs,p,ys),(ts,o),(ws,q,us)) -- a<c<b ~~~~~~(False, True, True) -> smallest3h'' k ((ws,q,vs),(zs,p),(ts,o,rs)) -- b<a<c ~~~~~~(False, True, False) -> smallest3h'' k ((ws,q,vs),(ts,o),(zs,p,xs)) -- b<c<a ~~~~~~(True, False, False) -> smallest3h'' k ((ts,o,ss),(zs,p),(ws,q,us)) -- c<a<b ~~~~~~(False, False, False) -> smallest3h'' k ((ts,o,ss),(ws,q),(zs,p,xs)) -- c<b<a ~~~~where ~~~~~~p = (length zs) `div` 2 ~~~~~~q = (length ws) `div` 2 ~~~~~~o = (length ts) `div` 2 ~~~~~~(xs, a : ys) = splitAt p zs ~~~~~~(us, b : vs) = splitAt q ws ~~~~~~(rs, c : ss) = splitAt o ts ~~~~~~smallest3h'' k ((zs,p,ys),(ws,q),(ts,o,rs)) = ~~~~~~~~case (k<=p+q+o) of ~~~~~~~~~~(True) -> smallest3'' k (zs,ws,rs) ~~~~~~~~~~(False) -> smallest3'' (k-p-1) (ys,ws,ts) smallest'' :: Ord a => Int -> ([a], [a]) -> a smallest'' k ([],ws) = ws !! k smallest'' k (zs,[]) = zs !! k smallest'' k (zs,ws) = ~~~~case (a<b, k<=p+q) of ~~~~~~(True, True) -> smallest'' k (zs,us) ~~~~~~(True, False) -> smallest''(k-p-1) (ys,ws) ~~~~~~(False, True) -> smallest'' k (xs,ws) ~~~~~~(False, False)-> smallest''(k-q-1) (zs,vs) ~~~~where ~~~~~~p = (length zs) `div` 2 ~~~~~~q = (length ws) `div` 2 ~~~~~~(xs, a : ys) = splitAt p zs ~~~~~~(us, b : vs) = splitAt q ws

2 1

Haskell from SML - referrential Transparency?!
by Gregory Guthrie 26 Apr '11

26 Apr '11

I am pretty new to Haskell, so need some clarification. I am porting some code from SML, and getting a result that surprises me. I basically have some functions which work like this: f1 = fa fb fc test1 = do print "test1:" f1 But I ran a few tests, and got odd results - so I ran the same test function twice, and got different results - that was my surprise. I did this: f1 = fa fb fc f2 = fa fb fc test2 = do print "test1:" f1 f2 and I get different results from the two executions (f1,f2), even though they have exactly the same definition. Reversing their order, gives the exact same results (i.e. the results are still different, and in the same original order as f2;f1). Even doing (f1;f1) gives two different results. Seems to me that by referential transparency, I should always get the same result from the function(s). So, I added some Debug.trace to the argument functions which are used, and I get a trace from the first call(s), but none from the second one(s), although I do get the result from each. It is as if because of the laziness, it someone cached some of the intermediate results, so did not re-invoke the functions. Anyway, totally confused. I must be missing something significant here. Thanks for any clarification! (The original code is a bit long, so I did not include here...)

8 11

Data.Enumerator.Text.utf8 not constant memory?
by Skirmantas Kligys 26 Apr '11

26 Apr '11

Hi, I am learning iteratees, and as a starter project I wanted to use expat- enumerator to parse a 2 gigabyte XML file. I expected to be able to do what SAX does in Java, i.e. to avoid loading the whole 2 gigabytes into memory. For warm-up, I wrote an iteratee to count lines in the file, and it does load the whole file into memory! After profiling, I see that the problem was Data.Enumerator.Text.utf8, it allocates up to 60 megabytes when run on a 40 megabyte test file. Any suggestions how to fix Text.utf8, or what people do for parsing UTF-8 encoded text files with iteratees? Thanks! Here is my code and profiling results: http://i.imgur.com/XEI1v.png http://hpaste.org/46037/counting_lines_with_iteratees http://hpaste.org/46038/counting_lines_with_iteratees

3 5

errors while installing yesod 0.8
by Michael Litchard 26 Apr '11

26 Apr '11

Trying to install yesod 0.8 breaks when it's time to install monad-control. Google wasn't very helpful, nor was the error message I received mlitchard@apotheosis:~/monad-control$ cabal install Resolving dependencies... Configuring monad-control-0.2.0.1... cabal: Error: some packages failed to install: monad-control-0.2.0.1 failed during the configure step. The exception was: ExitFailure 11 note: I've been trying to use the cab command to manage my packages, I get the same error as above when I use cab instead of cabal.I mention this just in case there is some unforseen problem having to do with cab/cabal interaction. Has anyone experienced this problem, or know what I can do to get more useful error messages that might reveal the cause of the breakage?

5 20

Storing failing testcases for QuickCheck
by Sönke Hahn 25 Apr '11

25 Apr '11

Hi! I would like to have a library that would allow to use QuickCheck in the normal manner, but it would save test data for failing properties on the filesystem (maybe using the shiny new acid-state?). On consecutive test runs, the saved test data would be used first (before generating new arbitrary test data). I think, this would come in quite handy when doing test driven development. Any comments on this? Does something like this already exist? Cheers, Sönke

3 3

Trouble with currying and uncurrying...
by Angel de Vicente 25 Apr '11

25 Apr '11

Hi, I'm reading "The Craft of Functional Programming" and I found something I don't understand in page 185. It says: "Suppose first that we want to write a curried version of a function g, which is itself uncurried and of type (a,b) -> c. curry g This funtion expects its arguments as a pair, but its curried version, curry g, will take them separately - we therefore have to form them into a pair before applying g to them: curry :: ((a,b) -> c) -> (a -> b -> c) curry g x y = g (x,y) curry multiplyUC will be exactly the same function as multiply." OK, I have tried it and it works, but I don't understand the syntax for curry. Until now I have encountered only functions that take the same number of arguments as the function definition or less (partial application), but this syntax looks a bit new to me. curry is supposed to have as its argument one function of type (a,b) -> c and produce another function, but then the second line gives three arguments to curry, the function itself and the variables x and y. What I'm missing here? Thanks a lot, Ángel de Vicente -- http://www.iac.es/galeria/angelv/ High Performance Computing Support PostDoc Instituto de Astrofísica de Canarias --------------------------------------------------------------------------------------------- ADVERTENCIA: Sobre la privacidad y cumplimiento de la Ley de Protección de Datos, acceda a http://www.iac.es/disclaimer.php WARNING: For more information on privacy and fulfilment of the Law concerning the Protection of Data, consult http://www.iac.es/disclaimer.php?lang=en

4 5

A parallelization problem
by wren ng thornton 25 Apr '11

25 Apr '11

Hello all, I have some loopy code I'd like to make parallel, but I'm not sure the best way to go about it. Everywhere else in this program I'm using STM, but that seems pretty heavy-handed for this task, and it looks like the perfect place for `par` and friends. So the 10,000ft view of the code is something like: foldl' step (0,state0) xs step (n,stateN) x = (n+1, k stateN) where k = case lookup n stateN of Nothing -> error "oh noes" Just substateN -> insert (n+1) $ foldl' maybeInsert empty (views substateN x) maybeInsert state (k,v) | p (k,v) = state | otherwise = insert k v state That is, for each x that comes in we add a bunch of things to a map datastructure. The thing that makes the inner loop parallelizable is that we happen to know that none of the new keys will (a) conflict with old keys (as indicated by the fact that we're nesting maps), (b) conflict with the other new keys, nor (c) depend on the other new key/value pairs. Thus, we know that we can reassociate the order of insertions without changing the semantics. So the question is, how do I do it? One issue not apparent in this high-level look at the code is that I'm actually using ST for doing the inner loop efficiently, which is why I'm not just using fromList. So far I'm using an IntMap for storing the state. The STM approach would be to store the IntMap in a TMVar and fire off threads to compute and insert each one of the views. But even as lightweight as Haskell's threads are, I worry that the overhead would swamp the benefits. (Of course, it may be viable to push the ST down far enough to use (fromList . catMaybes). Then we could just construct the list and call par on all the elements. That wouldn't do much to parallelize the insertions themselves, but that shouldn't matter too much I hope. Perhaps I've just answered my own question; any other ideas?) -- Live well, ~wren

1 0

generics and sql
by nadine.and.henry＠pobox.com 25 Apr '11

25 Apr '11

Dear Group, Greetings. I have a feeling that what I am trying to do is easy, I just don't know how to say it in Haskell. Let's start with: > {-# LANGUAGE > DeriveDataTypeable, GeneralizedNewtypeDeriving #-} > > import Database.HDBC > import Data.Typeable (Typeable) > import Data.Data > data C = C { str :: String, dbl:: Double } > deriving (Eq, Ord, Typeable, Data) > > a :: C > a = C "twelve" 12.0 > Now I load this up in ghci and I can do the following: toSql . str $ a -- result: SqlString "twelve" toSql . dbl $ a -- result: SqlDouble 12.0 but what I would really like to do is something like: gmapQ toSql $ a which results in: <interactive>:1:7: Could not deduce (Convertible d SqlValue) arising from a use of `toSql' from the context (Data d) bound by a type expected by the context: Data d => d -> SqlValue at <interactive>:1:1-11 Possible fix: add (Convertible d SqlValue) to the context of a type expected by the context: Data d => d -> SqlValue or add an instance declaration for (Convertible d SqlValue) In the first argument of `gmapQ', namely `toSql' In the expression: gmapQ toSql In other words, I'm looking for a function with a signature: (Whatever Instances I neeed here) => a -> [SqlValue] I have tried various incantations of type signatures, but thus far I can't get it right. Can someone point me in the right direction? Thanks. Henry Laxen -- Nadine & Henry Laxen Belle, Venus, Aphrodite 10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod Suite 115-396 Via Alta # 6 Reno, Nevada Chapala, Jalisco, Mexico 89503-1896 CP 45900 The rest is silence. (Hamlet)

3 5

How to use cabal's data-files feature and run in-place?
by Richard Cobbe 24 Apr '11

24 Apr '11

I'm running Haskell Platform 2011.2.0.1 on a MacOS 10.6.7 machine (the machine is 64-bit, but I'm running the 32-bit platform). I'm writing an application for personal use, and I'd like to use Cabal to package it up and handle installation. This way, when I'm working on the program, I won't break the reasonably stable installed version and can continue to use it. (I'm not committed to cabal by any means; suggestions for other ways to accomplish the same thing are more than welcome!) I've got some data files I'd like to package with the application, and Cabal's data-files field is the obvious way to do that. The immediate problem, then, is how to access the files -- I can use the generated Paths_pkg module for the "installed" copy of the app, but then I can't run the thing in-place in ghci in the development directory. I did some googling and came across a blog post (http://neilmitchell.blogspot.com/2008/02/adding-data-files-using-cabal.html) which suggested that I provide my own Paths_pkg.hs file that points to the files' location in the development copy. When I build with Cabal, the generated form of this module would override my hand-written one, and it should therefore work in the installed case as well. Unfortunately, that's not happening. Cabal is clearly generating the module; I can see it in dist/build/autogen. But my copy is overriding the autogenerated one, even for cabal builds -- at least, that's what I'm seeing when I run the binary out of dist/build/<package>/<executable>. Is there a way to be able to use data files in both contexts? FWIW, I'm running cabal as runhaskell Setup.hs configure --user runhaskell Setup.hs build Thanks much, Richard

6 8

http://trac.haskell.org/ not redering correctly
by Erik de Castro Lopo 24 Apr '11

24 Apr '11

Hi all, Trac pages aren't rendering correctly. It seems the HTTP server can't find the CSS files. See below. Cheers, Erik erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/ticket.css -O /dev/null --2011-04-17 20:11:53-- http://trac.haskell.org/trac/chrome/common/css/ticket.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:11:56 ERROR 404: Not Found. erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/trac.css -O /dev/null --2011-04-17 20:28:36-- http://trac.haskell.org/trac/chrome/common/css/trac.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:28:40 ERROR 404: Not Found. -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

3 5

Berlin Haskell Meeting
by Sönke Hahn 23 Apr '11

23 Apr '11

Hi all! The next Berlin Haskell Meeting will be next wednesday: Date: Wednesday, April 27th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin I hope, this is not on too short notice. Cheers, Sönke

1 0

Pattern matching on lazy bytestrings: how does it work?
by Tom Brow 23 Apr '11

23 Apr '11

I noticed today that I can pattern match against lazy bytestrings when using the OverloadedStrings extension: import Data.ByteString.Char8 () > > import Data.ByteString.Lazy.Char8 > > >> f :: ByteString -> Bool > > f "abc" = True > > f _ = False > > >> main = do > > print $ f $ fromChunks ["abc"] > > print $ f $ fromChunks ["a","bc"] > > When I run the above, I get: True > > True > > Given that pattern matching is based on data constructors, how is it possible that (Chunk "abc Empty) and (Chunk "a" (Chunk "bc" Empty)) match the same pattern? Tom

3 2

naming convention for maybes?
by Evan Laforge 23 Apr '11

23 Apr '11

Here's a simple issue that's been with me for a while. As do many people, I use plural variable names for lists, so if a Block as called 'block' then [Block] is 'blocks'. The other pattern that comes up a lot is 'Maybe Block'. When I have to name it, I call it 'maybe_block', e.g. maybe_block <- lookup something case maybe_block of Just block -> ... However, this maybe_ prefix is rather long and unwieldy. I have considered things like 'm' or 'mb' but they don't suggest Maybe to me. An 'm' prefix or suffix is already implying 'monad'. If '?' were allowed in identifiers I could use it as a suffix. I could just append 'q' and get used to it... lispers did it with 'p' after all. I suppose 'mby' could be ok, but for some reason it just looks ugly to me. 'opt' looks ok, I suppose, but 'optional' doesn't cover the full range of Maybe's usage (i.e. it's strange to call a failed lookup result "optional"). Does anyone else have a nice convention for this? Hopefully something short but intuitive and nicely reading like the plural convention?

7 7

Is Hugs dead?
by Robert Clausecker 23 Apr '11

23 Apr '11

Some weeks ago, I mirrored the hugs repo to github. (https://github.com/fuzxxl/Hugs) This was, when I found out, that the last commit was about 2 years ago. Also, since some of the dependencies moved, I was unable to build hugs. Now my question is: Is Hugs dead? What's the status of development of hugs? Yours, Robert Clausecker

6 5

Re: [Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)
by oleg＠okmij.org 23 Apr '11

23 Apr '11

Daniel Schuessler wrote: > The thing I don't understand yet is the last line: Why is it OK to discard the > leftover input from the (f x) Iteratee and yield just the leftover input from > the first one (m0)? First of all, the question is about an older version of Iteratee. For example, the following code http://okmij.org/ftp/Haskell/Iteratee/Iteratee.hs defines Iteratee a bit differently so the question does not apply. > data Iteratee a = IE_done a > | IE_cont (Maybe ErrMsg) (Stream -> (Iteratee a,Stream)) > > instance Monad Iteratee where > return = IE_done > IE_done a >>= f = f a > IE_cont e k >>= f = IE_cont e (docase . k) > where > docase (IE_done a, stream) = case f a of > IE_cont Nothing k -> k stream > i -> (i,stream) > docase (i, s) = (i >>= f, s) No left-over is discarded any more. Your question is about the previous design, called `The second design' described in Iteratee.hs. The corresponding comment block answers your question, please search for ``Justification for the case IE_done x s >>= f''. Please see http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs for the `production' case of Iteratee in a base monad. The file IterateeM.hs talks about one more design, and its drawbacks. It's all about finding the optimal trade-off, I guess.

4 4

ANN: hledger 0.14
by Simon Michael 23 Apr '11

23 Apr '11

I'm pleased to announce hledger 0.14. This release includes work by Dmitry Astapov, Eric Kow, Max Bolingbroke, Omari Norman and myself. Thanks especially to Dmitry for helping me with release wrangling. binaries for linux, mac, windows: http://hledger.org/DOWNLOADS.html (64-bit linux binaries to follow) or: cabal update && cabal install hledger, cabal install hledger- web, ... home and docs: http://hledger.org irc: irc://irc.freenode.net/#ledger Release notes: * remove the specific process dependency that caused too many cabal install problems * treat arguments as possibly-encoded platform strings, do not assume UTF-8 * hledger now always reads and writes data as UTF-8, ignoring the system locale (#34) * look at the LEDGER_FILE env var for the journal path, otherwise LEDGER, like ledger * handle a blank LEDGER_FILE or LEDGER value more gracefully (use the default file path) * the default journal file path is now ~/.hledger.journal, to avoid breaking mac filevault (#41) * amounts with different prices are now aggregated, like ledger * zero amounts now have no sign or commodity, like ledger * parsing: assume current year when transaction dates have no year and there is no default year * parsing: more careful validation of eg leap years in transaction dates * parsing: better international number format support, allowing comma as decimal point and flexible digit groups (#32) * parsing: support @@ syntax specifying total price * parsing: infer the conversion price in transactions involving two unpriced commodities * parsing: support per-posting cleared status * parsing: more reporting interval syntax: biweekly, bimonthly, every N days/weeks/months/quarters/years, every Nst/nd/rd/th day of month/week * add: avoid offering account names for completion in inappropriate contexts * add: remember default account even if user submits a different amount. * convert: account-field directive specifies a field containing the base account name * convert: effective-date-field directive specifies a field containing the effective date * convert: date-format directive specifies custom date formats * convert: allow amount fields containing "AMT @@ PRICE" * histogram: honour the specified start or end dates * print: don't show a trailing space when description is blank * web: allow filter patterns with spaces if quoted, like command line * web: make edit form more cross-browser compatible, fixing it in firefox (#38) * web: move hidden add/edit/import forms below main content to help text-mode browsers a bit (#33) Release contributors: Simon Michael, Dmitry Astapov, Eric Kow, Max Bolingbroke, Omari Norman. Stats: 137 days, 113 commits, 11 end-user features and 15 end-user bugfixes since last release. 189 unit & functional tests and 59% unit test coverage (hledger, hledger-lib packages). 5540 lines of code (all packages). Cheers, -Simon

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Global memoization in Haskell
by Albert Zeyer 22 Apr '11

22 Apr '11

Hi, I thought about implementing memoization which could be applied globally to Haskell as an optimization. I also asked the question (in a bit different way) here: http://stackoverflow.com/questions/5749039/automatic-memoizing-in-functiona… When you want to do that, the most important bit about the implementation is your replacement algorithm (which throws away old data -- because you need some sort of memory limit). As it was pointed out on StackOverflow, page replacement algorithms address exactly this issue. Though, when applied to memoization (esp. in pure functional languages), the problem becomes more specific and so maybe even more clever methods could be used. Earlier, I also came up with an own very simple implementation (using LRU) which can be seen here: http://www.az2000.de/docs/memoization/ I wonder if someone already tried to implement memoization as a global optimization for Haskell. And if so, how has it performed? I would be interested in implementing such thing myself as an addon for GHC and doing some benchmarks. Could you maybe: - Give me some hints where to start in GHC? I never really have worked on GHC so far. - Suggest some good benchmark code? I am mostly interesting in how this performs in huge real-world applications, so having some toy benchmark problems would be nice but some more huge benchmark problems would be better. Best would be some pool of both small and big benchmark programs. Thanks, Albert

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Question about the Monad instance for Iteratee (from the enumerator package)
by Daniel Schüssler 22 Apr '11

22 Apr '11

Hello, for reference, said instance is: > instance Monad m => Monad (Iteratee a m) where > return x = yield x (Chunks []) > > m0 >>= f = ($ m0) $ fix $ > \bind m -> Iteratee $ runIteratee m >>= \r1 -> > case r1 of > Continue k -> return (Continue (bind . k)) > Error err -> return (Error err) > Yield x (Chunks []) -> runIteratee (f x) > Yield x extra -> runIteratee (f x) >>= \r2 -> > case r2 of > Continue k -> runIteratee (k extra) > Error err -> return (Error err) > Yield x' _ -> return (Yield x' extra) The thing I don't understand yet is the last line: Why is it OK to discard the leftover input from the (f x) Iteratee and yield just the leftover input from the first one (m0)? Cheers, Daniel

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QuickCheck, (Ord a)=> [a] -> Property problem
by larry.liuxinyu 22 Apr '11

22 Apr '11

Hi, I found there is similar question as: http://groups.google.com/group/haskell-cafe/browse_thread/thread/7439262e9a… I am still think it's very strange. For example: prop_foo :: (Ord a) => [a] -> Property prop_foo xs = not (null xs) ==> maximum xs == minimum xs This is an extreme case that the property is always wrong. However, QuickCheck produces: *Main> test prop_foo OK, passed 100 tests. Why this happen? If I use verboseCheck, I can find the sample test data are as the following: *Main>verboseCheck prop_foo ... 97: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),()] 98: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),(),(),(),(),()] 99: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),()] OK, passed 100 tests. So Arbitrary () is generated as the instance of Ord a. I have to change the prototype as prop_foo :: (Num a) => [a] -> Property This works at least, However, since 'a'<'b', they are order-able, what if I want to test prop_foo works for char? I am using Haskell Platform (version 6.10.4), with QuickCheck version 1.2.0.0 Thanks -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/home

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ANN: HacPhi 2011
by wagnerdm＠seas.upenn.edu 22 Apr '11

22 Apr '11

Greetings, I am very pleased to officially announce HacPhi 2011, a Haskell hackathon/get-together to be held July 29-31 at the University of Pennsylvania in Philadelphia supported by contributions from Amgen, Jane Street, and Microsoft Research. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's HacPhi was a lot of fun, drawing an international crowd, and many people have already expressed interest in coming back this year. Come meet your Haskelly brothers-in-arms! I want to stress that everyone is welcome---you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the HacPhi wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! - The Hac ? team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://haskell.org/haskellwiki/Hac_?/Register [2] http://haskell.org/haskellwiki/Hac_? [3] http://haskell.org/haskellwiki/Hac_?/Talks

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impoosible dependencies
by Kevin Quick 21 Apr '11

21 Apr '11

Hmmm... $ cabal update $ cabal install hakyll Resolving dependencies... cabal: dependencies conflict: ghc-6.12.3 requires unix ==2.4.0.2 however unix-2.4.0.2 was excluded because ghc-6.12.3 requires unix ==2.4.1.0 $ Any advice (other than upgrading to 7.0.3, which is not an option at the moment)? -- -KQ

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Indexed monads and MonoLocalBinds
by Mikhail Vorozhtsov 21 Apr '11

21 Apr '11

Hi Cafe, Here is another example of why 'let' should be sometimes generalised. I've been recently playing with indexed monads (for JSON processing) and found out that the following code fails to typecheck: >{-# LANGUAGE MonoLocalBinds #-} >data M t t' a = M >ipure :: a -> M t t a >ipure a = M >iseq :: M t t' a -> M t' t'' b -> M t t'' b >iseq a b = M >np :: M () Bool () >np = M >test = p `iseq` np `iseq` p > where p = ipure () Test.hs:13:27: Couldn't match expected type `Bool' with actual type `()' Expected type: M Bool t''0 b0 Actual type: M () () () In the second argument of `iseq', namely `p' In the expression: p `iseq` np `iseq` p In practice that means that I need to provide a signature for almost every monadic local binding that is used more than once, which is unbearable, especially when monad transformers and complex indices are used.<http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7>

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Hackage haddock not running?
by Chris Smith 21 Apr '11

21 Apr '11

I've noticed a lot of packages recently missing documentation links on Hackage. It looks like the job to build these hasn't been running since perhaps about Saturday. Are appropriate people aware of this? -- Chris Smith

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Haskell Weekly News: Issue 178
by Daniel Santa Cruz 21 Apr '11

21 Apr '11

Welcome to issue 178 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of April 10 to 16, 2011. You can find the HTML version at: http://bit.ly/feMLgB Announcements Simon Marlow commented (http://goo.gl/XythI) on plans to have a Hackathon at MSR Cambridge this summer. Dr. Heinrich Hordegen informed us (http://goo.gl/lQvqw) of the creation of a Munich funtional programmers' user group. Help us spread the word to anyone interested in functional programming in general. Eric Y. Kow announced (http://goo.gl/PYdKI) the creation of the parallel-haskell mailing list. "The group exists to bring together the various groups in the Haskell community that are working on parallelism. It is intended to provide some visibility into the various efforts and to encourage collaboration." Don Stewart announced (http://goo.gl/0nWn2) the release of the Haskell Platform 2011.2.0.1. "This release adds support for GHC 7.0.3, and significant improvements for Mac OS X users." Quotes of the Week * Gregory Collins: "Easy to use" and "high performance" don't have to be contradictory, but you shouldn't neglect the latter. * xplat: haskell is good for portability. it can run well on any sort of x86 unix, which is all there is after all. * Spockz: it's just that my first name isn't Simon so I'm not allowed to use unsafePerformIO :P * djahandarie: Category theorists are morphisms for turning cofree theorems into free theorems. * monochrom: lambdabot is partially broken and partially correct when it comes to utf-8 * mm_freak: > case analysis of syntax -> bullshit <lambdabot> True Top Reddit Stories * Simon Peyton-Jones – A Taste Of Haskell Domain: ontwik.com, Score: 57, Comments: On Reddit: http://goo.gl/fCxtx Original: http://goo.gl/4LpJQ * Really, HaskellWiki, really? Domain: i.imgur.com, Score: 43, Comments: 13 On Reddit: http://goo.gl/HTigS Original: http://i.imgur.com/fPpby.png * Haskell Platform 2011.2.0.1 is out -- major improvements for Mac users Domain: hackage.haskell.org, Score: 39, Comments: 9 On Reddit: http://goo.gl/BJWwp Original: http://goo.gl/5vbN8 * Tracing the compilation of Hello Factorial in GHC Domain: blog.ezyang.com, Score: 36, Comments: 1 On Reddit: http://goo.gl/WNA2Z Original: http://goo.gl/fsVtS * acid-state: use regular Haskell data structures without worrying about data loss or durability Domain: haskell.org, Score: 32, Comments: 5 On Reddit: http://goo.gl/2hgGj Original: http://goo.gl/tKpV2 * Persistence of Memory: persistent vs ephemeral data structures Domain: existentialtype.wordpress.com, Score: 28, Comments: 5 On Reddit: http://goo.gl/Ss70R Original: http://goo.gl/XHdJV * Bugs from using the wrong variable Domain: blog.ezyang.com, Score: 26, Comments: 11 On Reddit: http://goo.gl/x7RId Original: http://goo.gl/3HJs2 * Cost of calling Haskell from C? Domain: stackoverflow.com, Score: 20, Comments: 1 On Reddit: http://goo.gl/6OiRH Original: http://goo.gl/XfmRe * Comparing Snap and Yesod Domain: stackoverflow.com, Score: 19, Comments: 4 On Reddit: http://goo.gl/P0Gr6 Original: http://goo.gl/dvbEe * Announcing: Heist v0.5.1 Domain: snapframework.com, Score: 17, Comments: On Reddit: http://goo.gl/cIja8 Original: http://goo.gl/d7FWG Top StackOverflow Questions * Comparing Haskell's Snap and Yesod web frameworks votes: 28, answers: 4 Read on SO: http://goo.gl/AhFH5 * Haskell library for 2D drawing votes: 19, answers: 4 Read on SO: http://goo.gl/61C4e * One REPL to bind them all? votes: 19, answers: 3 Read on SO: http://goo.gl/JUFS2 * Performance considerations of Haskell FFI / C? votes: 16, answers: 4 Read on SO: http://goo.gl/tldoX * What is Haskell's style of polymorphism? votes: 13, answers: 1 Read on SO: http://goo.gl/z3JzT 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

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Data.Binary.IncrementalGet remake
by Sergey Mironov 20 Apr '11

20 Apr '11

Hello cafe. Haskell wiki told me about continuation-based parser Data.Binary.IncrementalGet [1] from binary-strict package. I found the idea very useful and tried to use it. Original library by Lennart Kolmodin raises some questions. The lib's main data structures are: data IResult a = IFailed S String | IFinished S a | IPartial (B.ByteString -> IResult a) newtype Get r a = Get { unGet :: S -> (a -> S -> IResult r) -> IResult r } instance Monad (Get r) where return a = Get (\s -> \k -> k a s) m >>= k = Get (\s -> \cont -> unGet m s (\a -> \s' -> unGet (k a) s' cont)) fail err = Get (\s -> const $ IFailed s err) Here, "S" is parser's state. It works well, but currently doesn't support lookAhead. I tried to add such function and gave up to do it having current implementation, but write simpler one instead. Please see IncrementalGet2.hs (link [2] below). Remake is briefly tested, has no ghc-specific optimizations, but allows user to peek data from stream. What bothering me is the fact that I could actually miss idea (or part of idea) of the original. If it is so, please let me know :) For example, what is the point of using separate result type r in original Get r a? [1] - Original IncrementalGet lib. http://www.haskell.org/haskellwiki/DealingWithBinaryData#Incremental_parsing [2] - Main file of remake https://github.com/ierton/binary-incremental-get2/blob/a9f9afaa0cbc0435a8ac… [3] - whole github project https://github.com/ierton/binary-incremental-get2 -- Sergey sorry for weak English

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