May 2011 - Haskell-Cafe

Work on Collections Processing Arrows?
by David Barbour 13 May '11

13 May '11

Has anyone developed a typeclass model for (Control.Monad.mapM) and related functions for arrows? I've a preliminary model, using Adam Megacz's Generalized Arrows, of the form: class (GArrowDrop a (**) u) => GArrowMap_ a (**) u c where mapA_ :: a d u -> a (c d) u class (GArrow a (**) u c) => GArrowMap a (**) u c where mapA :: a d r -> a (c d) (c r) class (GArrowMap a (**) u c) => GArrowJoin a (**) u c where join :: a d (c r) -> a (c d) (c r) class (GArrow a (**) u) => GArrowUnion a (**) u c where union :: a ((c r) ** (c r)) (c r) class (GArrowMap a (**) u c, GArrow a (++) v) => GArrowPartition a (**) u (++) v c where partition :: a d (q ++ r) -> a (c d) ((c q) ** (c r)) Motivations: regular arrows (including GArrows) expose simple products for behavior, but those have a static 'width' and don't seem suitable for processing large values. ArrowApply would give me a dynamic amount of processing, but seems excessively expressive. In my own case, 'c' might be representing an asynchronous or distributed, reactive collection, so the ability to restrict expressiveness is important for performance. I'm wondering if you know of any other work along the same lines. Thank you, Dave

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Re: [Haskell-cafe] cabal-latest
by Jason Dagit 13 May '11

13 May '11

On Thu, May 12, 2011 at 7:24 AM, Guy <guytsalmaves-h(a)yahoo.com> wrote: > http://www.haskell.org/cabal/release/cabal-latest/ points to version > 1.8.0.4. Is this correct? > Replying to this on Haskell-Cafe and cabal-dev lists. The latest version is 1.10.x and ships with the HP. I bet that link is just needs to be updated. Jason

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old versions of cabal install
by Evan Laforge 13 May '11

13 May '11

If you search for "cabal install", the first link is http://hackage.haskell.org/trac/hackage/wiki/CabalInstall, which in turn leads to http://haskell.org/cabal/download.html. It looks very official and canonical and all that, but that page has an old version of cabal install that will no longer compile with ghc7 because it requires old libraries. Of course it won't tell you that right off, first it will install a whole bunch of old stuff, charge you a fiver, and then emit some "constraint can't be satisfied" msgs which if you have some experience with haskell and cabal already should eventually tip you off that something's wrong. Is there someone out there who has permissions to either take that page down, or replace it with a link to the new version at http://hackage.haskell.org/package/cabal-install? I updated http://hackage.haskell.org/trac/hackage/wiki/CabalInstall to point to the platform and to the hackage page, but the cabal page at http://haskell.org/haskellwiki/Cabal-Install still links to http://haskell.org/cabal/download.html. Should I just replace the haskellwiki page with a link to the hackagewiki one? I would try to port over any up to date info of course.

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ANN: AERN-Real-Double-2011.1
by Michal Konečný 13 May '11

13 May '11

Dear all, I am pleased to announce the first release of (a large part of) a newly designed AERN. AERN (approximating exact real numbers) is a set of Haskell libraries providing arbitrary precision interval arithmetic, polynomial arithmetic and distributed lazy exact real computation. Anyone is welcome to join in the development of AERN via http://code.google.com/p/aern/ . This release contains the following features: * Type classes for rounded arithmetic and rounded interval arithmetic with explicit control of rounding direction and the possibility to increase the rounding precision arbitrarily for types that support it. For example, type classes for types with rounded addition, multiplication, exponentiation etc. * Over 150 QuickCheck properties for these type classes, such as associativity of multiplication modulo rounding. * Interval arithmetic using Double endpoints, rounded outwards or inwards. Outwards rounding allows to safely approximate exact real arithmetic while a combination of both outwards and inwards rounding allows one to safely approximate exact interval arithmetic. Inverted intervals with Kaucher arithmetic are also supported. * For most of the type classes there are also in-place versions that act in the ST monad on mutable variables containing numbers or intervals. This facilitates some explicit memory allocation optimisation, especially for data on the C heap. The best starting point for experimenting with these features are the demo programs included in the bundle and the documentation for the AERN-Real-Double package: http://www-users.aston.ac.uk/~konecnym/aern/AERN-Real-Double (The HackageDB page does not have Haddock documentation because AERN fails to build with ghc 7.0.* due to a compiler bug.) The following features are planned for future releases: * Interval arithmetic with MPFR arbitrary precision endpoints. * Polynomial interval arithmetic with an optimised C core * Distributed exact real and geometric computation with lazy query-driven communication. For those familiar with the old AERN design, the main changes in the new design are: * use of many smaller type classes instead of few large ones * use of associated types instead of multi-parameter type classes * a much more systematic approach to representing the numerical order on real numbers and the information refinement (ie the inverse set inclusion) order on intervals * ability to specify different effort indicator types for each type of operation and each numerical type instead of Int. (For example, Double has () as its effort indicator for almost all operations while MPFR will have Precision as the effort indicator type.) * a more systematic approach to specifying QuickCheck properties and applying them to form test suites * a unified implementation of interval elementary operations (only sqrt and exp at the moment) that is efficient when applied to Double intervals as well as polynomial intervals * new support for in-place rounded numerical operations The current development team will be grateful for any feedback and suggestions. Best regards, 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.aston

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how to force hackage to use ghc 6.12.3
by Michal Konečný 13 May '11

13 May '11

Hi, I have been releasing packages on hackage that do not build with ghc 7.0.* due to a bug that will be fixed in ghc 7.2.1. (One of the packages is http://hackage.haskell.org/package/AERN-Basics) I was hoping hackage will try also ghc 6.12.3 but it does not. Is there some way I can change the cabal file to help hackage to compile it with 6.12.3 and generate haddock? The package requires base >= 4 so I cannot try base < 4. Michal -- |-| Dr. Michal Konecny, Computer Science, Aston University |-| Room MB212D | Tel +44 121 204 3462 | Fax +44 121 204 3681 |-| http://www.aston.ac.uk/~konecnym |-| OpenPGP key http://www.aston.ac.uk/~konecnym/ki.aston -- |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.aston

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ANN: Newt - command-line template instantiation tool & library
by Rogan Creswick 13 May '11

13 May '11

I'm happy to announce Newt: a trivial tool for creating boilerplate. I frequently need to create projects with slight customizations -- I have a particular layout for cabal projects, and make files for LaTeX papers, etc... However, there are often fields that need to be updated in many places. (At times, even file or directory names need changed.) Newt makes this trivial. My objective was to provide a tool that allows you to very quickly take a project, mark the portions that must change with angle brackets, and use that as a source for myriad derivations of that project, or share the project for others to create their own instances. Newt scans the input (either a file, directory or stdin) for tags marked with "<<<tagName>>>" [1], then replaces those entries with values specified on the command line, producing either a new file, modifying the input template in place (--inplace), writing to stdout, or writing to a directory. A newt template for a cabal project is available on github: - http://github.com/creswick/Newt/tree/master/tests/testFiles/dirTemplates/ca… Newt is available on hackage[2], and the source is hosted on github[3]. The readme provides more details and examples: - http://github.com/creswick/Newt/blob/master/README.md --Rogan [1] The tag syntax can be changed with command-line flags, or by creating an instance of the Tag typeclass. See --help. [2] http://hackage.haskell.org/package/newt [3] http://github.com/creswick/Newt/

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Hash table constructors return table in IO Monad. Why?
by michael rice 13 May '11

13 May '11

Is there some reason why ALL the ways to create a hash table return one in the IO Monad, but all the functions for retrieving a value from a hash table take as an argument a hash table NOT in the IO Monad? Michael ================= Prelude Data.HashTable> let ht = fromList id [(5,'a'),(6,'b')]Prelude Data.HashTable> fmap ((flip Data.HashTable.lookup) 6) htPrelude Data.HashTable> Prelude Data.HashTable> :t fmap ((flip Data.HashTable.lookup) 6) htfmap ((flip Data.HashTable.lookup) 6) ht :: IO (IO (Maybe Char))

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ANNOUNCE: iterIO-0.1 - iteratee-based IO with pipe operators
by David Mazieres 12 May '11

12 May '11

Hi, everyone. I'm pleased to announce the release of a new iteratee implementation, iterIO: http://hackage.haskell.org/package/iterIO IterIO is an attempt to make iteratees easier to use through an interface based on pipeline stages reminiscent of Unix command pipelines. Particularly if you've looked at iteratees before and been intimidated, please have a look at iterIO to see if it makes them more accessible. Some aspects of iterIO that should simplify learning and using iteratees are: * Every aspect of the library is thoroughly document in haddock including numerous examples of use. * Enumerators are easy to build out of iteratees. * There is no difference between enumerators and "enumeratees" (i.e., inner pipeline stages). The former is just a type-restricted version of the latter. * Parsing combinators provide detailed error reporting and support LL(*) rather than LL(1) parsing, leading to fewer non-intuitive parsing failures. A couple of tricks avoid consuming excessive memory for backtracking. * Super-fast LL(1) parsing is also available through seamless integration with attoparsec. * A universal exception mechanism works across invocations of mtl monad transformers, thereby unifying error handling. * All pipe operators have uniform semantics, eliminating corner cases. In particular, if the writing end of a pipe fails, the reading end always gets EOF, allowing it to clean up resources. * One can catch exceptions thrown by any contiguous subset of stages in a pipeline. Moreover, enumerator exception handlers can resume downstream stages that haven't failed. * The package is full of useful iteratees and enumerators, including basic file and socket processing, parsec-like combinators, string search, zlib/gzip compression, SSL, HTTP, and "loopback" enumerator/iteratee pairs for testing a protocol implementation against itself. Please enjoy. I'd love to hear feedback. David

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parsing currency amounts with parsec
by Eric Rasmussen 12 May '11

12 May '11

Hi everyone, I am relatively new to Haskell and Parsec, and I couldn't find any articles on parsing numbers in the following format: Positive: $115.33 Negative: ($1,323.42) I'm working on the parser for practical purposes (to convert a 3rd-party generated, most unhelpful format into one I can use), and I'd really appreciate any insight into a better way to do this, or if there are any built-in functions/established libraries that would be better suited to the task. My code below works, but doesn't seem terribly efficient. Thanks! Eric ------------------------------------------------- {- parses positive and negative dollar amounts -} integer :: CharParser st Integer integer = PT.integer lexer float :: CharParser st Double float = PT.float lexer currencyAmount = try negAmount <|> posAmount negAmount = do char '(' char '$' a <- currency char ')' return (negate a) posAmount = do char '$' a <- currency return a currency = do parts <- many floatOrSep let result = combine orderedParts where combine = sumWithFactor 1 orderedParts = reverse parts return result floatOrSep = try float <|> beforeSep beforeSep = do a <- integer char ',' return (fromIntegral a :: Double) sumWithFactor n [] = 0 sumWithFactor n (x:xs) = n * x + next where next = sumWithFactor (n*1000) xs

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Haskell Weekly News: Issue 181
by Daniel Santa Cruz 12 May '11

12 May '11

Welcome to issue 181 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 1 to 7, 2011. Announcements Last week I failed to note the announcement [http://goo.gl/ApClU] of a new version of Leksah (0.10.0). If you haven't yet, give Leksha a test run and send some feedback! Ketil Malde started a thread [http://goo.gl/Prlu2] about Robert Harper's thoughts on monads and laziness. Hakim Cassimally extended an invitation [http://goo.gl/QIjcF] to anyone living in the Northwest UK for Tuesday May 31st to Geekup's first ever "Functional Programming Night". Andy announced [http://goo.gl/uCEn1] a new release of Manatee (0.2.1). Quotes of the Week * hpc: [SPOILER] You can check wikipedia for the definition of fibonacci sequences if you get stuck after half an hour * pmetzger: it is always possible to make an algorithm slower. just give it to a CS freshman to code up. * kmc: ghc -XMixfixOperators -XDependentTypes -XMakeMeASandwich * pmetzger: I have my password tattooed to my forehead. that way when I forget it others can tell me what it is. * alexbagel: i find that my first 26 variables are always easy to name. after that it becomes a bit harder. * Eric Kow: Haskell is a good fit for me because I tend to practise Doh!-oriented programming Top Reddit Stories * More points for lazy evaluation Domain: augustss.blogspot.com, score: 56, comments: 24 On Reddit: http://goo.gl/qecQ1 Original: http://goo.gl/OnJXx * iterIO: iteratee-based IO with pipe operators, courtesy David Mazieres Domain: haskell.org, score: 43, comments: 15 On Reddit: http://goo.gl/YzPJn Original: http://goo.gl/6WQAk * Monad comprehensions are finally in GHC Domain: self.haskell, score: 37, comments: 32 On Reddit: http://goo.gl/1Rjnw * Horner's Rule Domain: patternsinfp.wordpress.com, score: 34, Comments: 2 On Reddit: http://goo.gl/A1wYo Original: http://goo.gl/zeC1f * Patent 5,893,120 reduced to mathematical formulae (via Haskell) Domain: paulspontifications.blogspot.com, score: 31, comments: 13 On Reddit: http://goo.gl/PmB4K Original: http://goo.gl/QknmV * Custom Haskell mug! Domain: imgur.com, score: 31, comments: 6 On Reddit: http://goo.gl/lD388 Original: http://goo.gl/bzZR8 * Differences between 'data' and 'newtype' ... in pictures Domain: stackoverflow.com, score: 31, comments: 2 On Reddit: http://goo.gl/5YaSo Original: http://goo.gl/KQiJv * Haskell: not pure enough? Domain: blog.ezyang.com, score: 30, comments: 19 On Reddit: http://goo.gl/6v28x Original: http://goo.gl/zsfSq * Galois is hiring computer scientists Domain: corp.galois.com,score: 27, comments: 28 On Reddit: http://goo.gl/ToeJ0 Original: http://goo.gl/osjJY * Fancy comparison of Data values Domain: pnyf.inf.elte.hu, score: 26, comments: 18 On Reddit: http://goo.gl/BlFVl Original: http://goo.gl/RuMKT Top StackOverflow Questions * Ordering of parameters to make use of currying votes: 43, answers: 3 Read on SO: http://goo.gl/7LnBJ * Difference between 'data' and 'newtype' in Haskell votes: 29, answers: 2 Read on SO: http://goo.gl/KQiJv * Haskell: why the convention to name a helper function "go"? votes: 24, answers: 3 Read on SO: http://goo.gl/CE1O7 * How do you design programs in Haskell or other functional programming languages? [migrated] votes: 24, answers: 0 Read on SO: http://goo.gl/IW7Z5 * Haskell Graphics Library that works in GHCi on MacOS X votes: 19, answers: 3 Read on SO: http://goo.gl/VZGAK 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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