- Haskell-Cafe - Haskell.org

Opaque types vs automated instance deriving
by Alexey Karakulov 21 Jun '11

21 Jun '11

Hi all, I encountered a problem when trying to > derive makeBinary ''DiffTime with help of *derive* package. The error was: Not in scope: data constructor `MkDiffTime' Which makes a sense, since it's not exported in Data.Time.Clock. I bypassed the problem (yes, I'm too lazy to write instances by hands) with > instance Binary UTCTime where > put = putGeneric > get = getGeneric But it must be less efficient (and more verbose) than compile-time deriving. If there was such a module like Data.Time.Clock.Internal, I could import it to get hidden constructors (and maybe I'll have to fork the library for this purpose). OTOH, is it possible to change the derive TH function so it can bypass module encapsulation mechanism and access un-exported things?

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Haskell on iPad? (Scheme and Ocaml are there)
by John Velman 21 Jun '11

21 Jun '11

There are (at least) two Scheme interpreters for iPad at the iTunes store: PixieScheme and GambitREPL. Both allow entry of scripts, by typing or pasting. The Gambit community is very busy trying to expand the usefulness of their interpreter. Both have pretty good interfaces. There is also an Ocaml app, but I don't know or want to know Ocaml, and the interface looks very unfriendly. I'd really like to have something like this in Haskell, in the "education" pot, as is the GambitREPL. Hugs is written in C, if I recall correctly. Would it be possible to compile Hugs for the iPad processor, taking out enough system calls to make it acceptable? John Velman

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Job position for haskell/clojure dev. in San Dimas, CA
by vagif.verdi＠gmail.com 20 Jun '11

20 Jun '11

Anyone interested in full time employment working with haskell and clojure in San Dimas, CA (local job only, NO telecommute) please let me know. Regards, Vagif Verdi

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AusHac2011 - The Australasian Haskell Hackathon - July 8-10
by Alex Mason 20 Jun '11

20 Jun '11

It's almost that time of year again! Next month is the second annual AusHac event, at UNSW in Sydney, and you are invited! But if you want to come, we need to hear from you soon, as we need to organise access to the university network for you. If you're an Australian Haskell hacker, enthusiast, newbie or professional, we would love to see you there. Below is the announcement, if you have any questions, please just hit reply, and Ivan or I will get back to you. We would absolutely love to see you there if you can make it, we'd love for this year to be even more awesome than last year. -- Alex Mason ------------------------------------------------------------------------------ ==== Attention all Australasian Haskellers! ==== After last year’s fantastic turnout for the first Australian Haskell Hackathon - AusHac 2010 - we’ve decided to organise another. As it’s rather boring to have a Hackathon with only two people, we encourage and welcome anyone interested in Haskell in Australia, New Zealand and surrounding areas (or further afield if you want!) to come and join us. Last year we had more than 25 Haskellers come from Brisbane, Canberra, Melbourne, Sydney and even two from as far away as New Zealand. AusHac 2011 will be held from Friday July 8th until Sunday July 10th, again at UNSW’s Computer Science building in Sydney. There’s no need to come for the entire weekend, just for whenever you’re able to. Last year, people worked on several projects including: • Accelerate: a high performance library for array computations using various backends (CUDA, LLVM, etc.) being written at UNSW. • Leksah: the Haskell IDE written in Haskell • DDC: the compiler for the Haskell-like language Disciple, which has strict evaluation by default, region, effect and closure typing and other interesting language features. • Hubris: the Haskell/Ruby bridge for calling Haskell from Ruby and vice-versa. • haskell-mpi: bindings to MPI, the message passing interface used frequently on high performance super-computers to allow programs to run on many nodes in parallel with easy communication. • There was even some work on LLVM to produce HTML output showing register allocations (as an aid to those writing new code generators etc.), which has now been included upstream. This work did not use Haskell, but it did happen at AusHac 2010. • And much more! This is a great chance to collaborate with others and get feedback on your work, while having heaps of fun and meeting other Haskellers, to verify they do actually exist! And don’t worry if you can’t think of a project to work on; just come along and pitch in with whatever strikes your fancy! If you’re interested in coming and have a project you’d like to work on, check out the wikipage http://www.haskell.org/haskellwiki/AusHac2011 where you can add your project ideas, or just take a look and see what others will be working on that you can help out with. People of all skill levels are welcome, from complete beginners to Oleg-level sage masters. We’re more than happy to help you learn Haskell if you’re just starting out or help you solve that little niggly bit in your code. If you’re interested in coming along, fill out our registration form http://axman6.wufoo.com/forms/aushac-2011-sign-up/. Registration is required so we know numbers and for you to get access to the university Wifi. Even if you’re not sure, fill it in to express your interest, obligation free. We’d rather know we have room for too many people than not enough. So come along! And if for some reason you don’t enjoy yourself, registration comes with a money-back guarantee! Hope to see you there, -- Alex Mason and Ivan Miljenovic, AusHac organisers ------------------------------------------------------------------------------

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Text report tools?
by Dmitri O.Kondratiev 20 Jun '11

20 Jun '11

Hi, I am looking for an easy way to generate text reports. For starters I need a very simple report that may contain: - Some headers - Lists of text strings where each string can include instances of basic Haskell types. Each string should be printed on a separate line (terminated with LF). Do I understand this right that for such simple reports (as above) Text.Printf is the main tool? As a next step, a simple report engine controlled by templates or DSL would help. Thanks!

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Probably type checker error.
by Serguey Zefirov 20 Jun '11

20 Jun '11

Right now I write a quite heavy transformation of Haskell source code and found some strange behaviour of typechecker. Some prerequisites: -- dummy class. My own class is much bigger, but I -- could reproduce that behaviour with that class. class ToWires a -- a type with phantom type arguments. data E ins outs = E -- a function that relates E and its inputs. projectInsType :: E ins outs -> ins projectInsType = error "projectInsType gets evaluated." -- "register" function. register :: a -> a -> a register def a = def -- a simple addition. add2 :: (ToWires a, Num a) => (a,a) -> a add2 (a,b) = a+b First I have a function: func :: (ToWires a, Num a) => Maybe a -> a func mbA = currentSum where x = case mbA of Just a -> a Nothing -> 0 nextSum = add2 (x,currentSum) currentSum = register 0 nextSum It typechecks and works just fine after some transformation. The transformation I work on transform code into something like that: func_E :: (ToWires a, Num a) => E (Maybe a) a func_E = r where r = E -- here we relate mbA and r. mbA = projectInsType r x = case mbA of Just a -> a Nothing -> 0 nextSum = add2 (x,currentSum) currentSum = register 0 nextSum Note the absence of input of func in transformed func_E. I relate mbA with it's proper type using binding "mbA = projectInsType r". Then suddently ghc loses all of the context associated with mbA. And find type error at the calling of add2. If I drop ToWires from contexts of func_E and add2 types, all works fine. If I change add2 to simple addition (x + currentSum), all works fine. Full source code is in attachment. I found it using ghc 6.12.1. I asked colleagues, they told me that the same error manifests itself in ghc 7.0.3. Should I fill a bug report or maybe I misunderstood something?

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ANNOUNCEMENT: TinyLaunchbury 1.0.1
by Elliot Stern 20 Jun '11

20 Jun '11

Sankel Software is pleased to announce the release of TinyLaunchbury 1.0.1 [1], a small implementation of John Launchbury's natural semantics for call-by-need. We hope that is is useful in helping to understand how call-by-need works. There is also a new blog post[2], explaining the semantics, the code and why the semantics are useful to understand. Elliot Stern Sankel Software [1] http://hackage.haskell.org/package/TinyLaunchbury [2] http://netsuperbrain.com/blog/posts/haskells-evaluation-isnt-magic/

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Conditional IO ?
by Dmitri O.Kondratiev 20 Jun '11

20 Jun '11

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing: handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ? What should be an 'otherwise' case ? Thanks!

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Software patents covered in GHC?
by Shakthi Kannan 20 Jun '11

20 Jun '11

Hi, I would like to know if there are any software algorithms used in the Glasgow Haskell Compiler that have been patented? If yes, in which countries do they apply? Just curious to know. SK -- Shakthi Kannan http://www.shakthimaan.com

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Text.CSV questions
by Dmitri O.Kondratiev 20 Jun '11

20 Jun '11

Hi, I try to parse csv file with Text.CSV, like this: import Text.CSV import System main = do [inpFileName] <- getArgs putStrLn ("Parsing "++inpFileName++"...") let result = parseCSVFromFile inpFileName print result === As a result I get: No instance for (Show (IO (Either Text.Parsec.Error.ParseError CSV))) arising from a use of `print' Possible fix: add an instance declaration for (Show (IO (Either Text.Parsec.Error.ParseError CSV))) === Question: How to add a Show instance for "(IO (Either Text.Parsec.Error.ParseError CSV)))" ? Thanks!

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