- Haskell-Cafe - Haskell.org

Escaping phantom type
by Miguel Mitrofanov 30 Apr '14

30 Apr '14

Hi cafe! Assume I have some function foo :: A x -> B x Now, the type B is somewhat complicated. It consists of several parts. So, if we call foo twice, like let b1 = foo a b2 = foo a then it's possible that we'd use some parts of b1 and b2 together, mistakenly assuming they are from the same B, not from different ones. I want to exclude this possibility. So I introduce a phantom type, and the signature becomes this: foo :: A x -> exists y. B x y (of course that's not a legitimate Haskell, in reality I'll use some GADT) Now b1 and b2 would have different types, and we can't mix them up by mistake. So far so good. But let's assume that we want to use "foo" recursively. Without the safety precautions it would be easy: generateA :: B x -> OtherArgs -> A x ... b = foo (generateA b otherArgs) Unfortunately, we can't do the same thing with this safety net in place. If we do generateA :: B x y -> OtherArgs -> A x where x doesn't depend on y, then generateA would loose the phantom type, so let b = foo (generateA b otherArgs) in b and let b1 = foo (generateA b1 otherArgs) b = foo (generateA b1 otherArgs) -- sic! in b would be indistinguishable. And that is definitely not what we want. In reality, we want x to depend on y. Unfortunately, there are lots of possibilities regarding the way x would depend on y. I know for sure that there would be many different "generateA" functions, and I don't want to encode them all in types. I've come up with one solution: foo :: (forall y. exists x. ((B x y -> A x), (B x y -> z))) -> z This would work, as in the end I would be interested not in B, but in something that would have nothing to do with this phantom types. But that requires explicit CPS transformation, which makes the code difficult to read. Is there some way to simplify things? To make them easier to use?

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How to create a background program that monitors input (linux)
by EatsKittens 29 Apr '14

29 Apr '14

Essentially, I want to create a program that is simply ran in the background, not in a terminal, that monitors keypresses and mouse presses and takes certain actions on specific keypresses. Where to start?

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Why is this code slow?
by Jakub Arnold 29 Apr '14

29 Apr '14

Hey guys, in my stumbling over Reddit I happened to find the following comment http://www.reddit.com/r/programming/comments/248abo/range_comprehensions_c/…, where the poster shows a simple algorithm which is really slow when implemented in Haskell, compared to a C++ counterpart which does it the same way. I’ve tried to figure this out, but my profiling skills don’t seem to be good enough for this. I’m not sure if this is somehow a limitation of list comprehensions, or if something else is wrong. -- Jakub

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ANNOUNCE: haskintex-0.5.0.0. Evaluating Haskell inside LaTeX.
by Daniel Díaz Casanueva 29 Apr '14

29 Apr '14

Dear cafe readers. I have released today a new version (0.5.0.0) of the haskintex program, which is written in Haskell, for Haskell people, and by Daniel Díaz (that's me!). Haskintex is a program that processes files which contains LaTeX and (maybe!) some Haskell expressions on it. Exactly as you are expecting, it will evaluate those expressions and replace them by their results. Neat, but there is more. If the Haskell expression is of type LaTeX (yes, there is a type for that, look in [1]), then it will be replaced by the LaTeX expression that value represents. This allows magic tricks like plotting a Haskell function in a LaTeX file in just a few seconds! (see [2]). So, what is new in this version? Haskintex can now memorize results of evaluations, to avoid repeating computations. The values are shared in different runs of the program, so you can save lot of time. This feature was proposed by a kind haskintex user in the #hatex IRC channel under the alias of colmcbeardman (I hope I have spelled it correctly). Find more about haskintex in: http://daniel-diaz.github.io/projects/haskintex Happy hacking, Daniel Díaz. -- References [1] http://hackage.haskell.org/package/HaTeX-3.10.0.0/docs/Text-LaTeX-Base-Synt… [2] http://hackage.haskell.org/package/HaTeX-3.10.0.0/docs/Text-LaTeX-Packages-…

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Parse error when #ifdef pragma enabled
by Rob Stewart 28 Apr '14

28 Apr '14

Hi, I'm missing something obvious. I'd like to compile the following code. --8<---------------cut here---------------start------------->8--- {-# LANGUAGE CPP #-} module CPP where #ifdef CUDA import qualified Data.Array.Accelerate.CUDA as CUDA #endif f = "lolcats" --8<---------------cut here---------------end--------------->8--- Without the CUDA pragma, it's all good: $ ghc --make CPP.hs [1 of 1] Compiling CPP ( CPP.hs, CPP.o ) With the pragma thought, I get a compilation error: $ ghc --make CPP.hs -DCUDA [1 of 1] Compiling CPP ( CPP.hs, CPP.o ) CPP.hs:6:39: parse error on input `.' Where's my mistake? Thanks! -- Rob

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ANNOUNCE (2nd call): Applied Functional Programming (AFP) Summerschool 7-18 July 2014, Utrecht, Netherlands
by Atze Dijkstra 28 Apr '14

28 Apr '14

Dear all, we bring the following announcement to your attention a 2nd time as the registration deadline (may 5) approaches. thank you, regards, Atze Dijkstra =========== AFP Summerschool 2014 =========== Applied Functional Programming (AFP) Summerschool July 7-18, 2014 Utrecht University, Department of Information and Computing Sciences Utrecht, The Netherlands Summerschool & registration website: http://www.utrechtsummerschool.nl/courses/science/applied-functional-progra… AFP website with edition 2013 info : http://www.cs.uu.nl/wiki/USCS contact : Uscs-afp(a)lists.science.uu.nl *** The 2014 edition of the Applied Functional Programming (AFP) Summerschool in Utrecht, Netherlands will be held from 7-18 July 2014. The summerschool teaches Haskell on both beginners and advanced levels via lectures and lab exercises. More info can be found via the references above, included here is an excerpt from the summerschool website: ``Typed functional programming in Haskell allows for the development of compact programs in minimal time and with maximal guarantees about robustness and correctness. The course introduces Haskell as well as its theoretical underpinnings such as typed lambda calculus, and Damas-Milner type inference. There is ample opportunity to put this all in practice during lab sessions. Typed functional programming languages allow for the development of robust, concise programs in a short amount of time. The key advantages are higher-order functions as an abstraction mechanism, and an advanced type system for safety and reusability. This course introduces Haskell, a state-of-the-art functional programming language, together with some of its theoretical background, such as typed lambda calculi, referential transparency, Damas-Milner type inference, type level programming, and functional design patterns. We will combine this with applications of functional programming, concentrating on topics such as language processing, building graphical user interfaces, networking, databases, and programming for the web. The goal of the course is not just to teach the programming language and underlying theory, but also to learn about the Haskell community and to get hands-on experience by doing lab exercises or a Haskell project of your own.'' *** regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)uu.nl ............... / |___\

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Why this doesn't work?
by Kai Zhang 28 Apr '14

28 Apr '14

Hi Cafe, I don't understand why this doesn't work: import qualified Data.Vector.Generic as G import Control.Monad.ST test :: G.Vector v a => v a -> v a test x = runST . (>>= G.freeze) . G.thaw $ x But if I replace "." with "$", it can compile. test x = runST $ (>>= G.freeze) . G.thaw $ x I originally though f $ g x can always be written as f . g $ x

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New Functional Programming Job Opportunities
by Functional Jobs 28 Apr '14

28 Apr '14

Here are some functional programming job opportunities that were posted recently: functional software developer at OpinionLab http://functionaljobs.com/jobs/8706-functional-software-developer-at-opinio… Cheers, Sean Murphy FunctionalJobs.com

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BayHac 2014 (May 16-18) Reminder
by Jonathan Fischoff 27 Apr '14

27 Apr '14

This is your three week reminder for the up coming Bay Area Haskell Hackathon, held Friday May 16 to Sunday May 18 at the Hacker Dojo in Mountain View CA. Here is the signup sheet: https://docs.google.com/forms/d/16QEHqAioGQeHHOlnMTEmjdgtO4YNN2_Qc-rbgLOFat… BayHac is time for Haskellers of all skill levels to collaborate on projects, disseminate knowledge and socialize. This year, we responded to requests to have more teaching, and there will be 15 classes in addition to experience reports, demos, and lightning talks. More information can be found on the wiki: http://www.haskell.org/haskellwiki/BayHac2014 Happy Hacking, Jonathan

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Baltimore Haskell Users Group
by Michael Orlitzky 27 Apr '14

27 Apr '14

For those who don't reddit: http://www.reddit.com/r/haskell/comments/23zevt/baltimore_haskell_users_gro… https://groups.google.com/forum/#!forum/bmore-hug And if like me you have no idea how to Google Groups, just send an email to bmore-hug%2Bsubscribe(a)googlegroups.com to subscribe to the list.

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