- Haskell-Cafe - Haskell.org

Unmaintained packages and hackage upload rights
by Roman Cheplyaka 09 Feb '14

09 Feb '14

In the recent past I took over two unmaintained packages: bert and ansi-terminal. I don't mind spending a bit of time to keep our ecosystem from bitrotting. However, both times I had to go through an irritating procedure of contacting hackage admins, asking them to grant me upload rights, explaining why the maintainers can't do that themselves and why I think the packages are abandoned. Instead of a feeling that I'm doing something good and useful, I have a feeling that I'm bothering people with my own problems. It also adds unnecessary latency to my work. So from now on I'll simply fork the packages I need to fix. Others are of course welcome to use my forks. (This email was prompted by regex-tdfa which doesn't build on GHC 7.8, and whose maintainer hasn't responded. My fork is at http://hackage.haskell.org/package/regex-tdfa-rc .) Roman

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Puppet module for installing cabal packages?
by Andrew Pennebaker 09 Feb '14

09 Feb '14

I'm using Vagrant <http://www.vagrantup.com/> and Puppet<http://puppetlabs.com/puppet/what-is-puppet> to automate setting up development, testing, and production environments. For now, I'm executing shell commands to install my favorite Cabal packages (`cabal update`, `cabal install <package>`), but I'd prefer to use something a bit more high level. Would anyone be interested in creating a Puppet module for Cabal? This could encourage more use of awesome packages like Darcs and ShellCheck by non-Haskellers. -- Cheers, Andrew Pennebaker www.yellosoft.us

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[ANN] deka - decimal arithmetic
by Omari Norman 09 Feb '14

09 Feb '14

Announcing deka, a library for decimal arithmetic. Why decimal arithmetic is important is discussed at length at http://speleotrove.com/decimal/ but for a short explanation, go into ghci and type: print $ 0.1 + 0.1 + 0.1 This is 0.3, right? But on my machine I get 0.3 plus a small fraction, which is simply not acceptable for applications where it is essential to get exact results, such as financial applications. deka is a binding to the decNumber C library, a thoroughly tested and fast implementation of decimal arithmetic. deka is fully tested and I encourage you to run the tests using the usual "cabal test" method. deka is available at http://hackage.haskell.org/package/deka and is on Github at https://github.com/massysett/deka Because deka is a binding to the decNumber C library, first you will need to install decNumber. An easy to install version with the GNU autotools is at: https://github.com/massysett/decnumber/releases decNumber's website is at http://speleotrove.com/decimal/decnumber.html A literate Haskell file illustrating the use of deka is available at https://github.com/massysett/deka/blob/master/lib/Data/Deka/Docs/Examples.l…

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An arrow transformer
by Roman Cheplyaka 08 Feb '14

08 Feb '14

I wrote (and found useful) the following arrow transformer: data Partial ar a b = Partial (ErrorArrow () ar a b) | Total (ar a b) toPartial :: ArrowChoice ar => Partial ar a b -> ErrorArrow () ar a b toPartial (Partial f) = f toPartial (Total sf) = liftError sf instance ArrowChoice ar => Category (Partial ar) where id = Total id Total f . Total g = Total $ f . g f . g = Partial $ toPartial f . toPartial g instance ArrowChoice ar => Arrow (Partial ar) where arr = Total . arr first (Total f) = Total $ first f first (Partial f) = Partial $ first f It allows to work with partial functions/arrows and at the same time tracks which of them are actually total. I wonder if it is already defined somewhere or is perhaps an instance of a more general construction. Roman

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ANN: Wheb -- A WAI framework
by Kyle Hanson 08 Feb '14

08 Feb '14

Hello all, I was rather frustrated with the use of Template Haskell as the main entry point for the big framework projects (Yesod, Snap, etc.) While these frameworks offer template-free options, they seem like second class citizens. So I started work on Wheb with the goal that out of the box I could start a project quickly, without Template Haskell and without the dreaded long list of Language Pragma's at the top. I was inspired by the simplicity of the Scotty project which showed how easy a Haskell server could be to write. I have included a couple of plugin examples for handling Auth and Sessions. Wheb is database and template agnostic, but I plan to write some plugins soon to make it easier to use the major libraries with Wheb. I just started work on it last weekend but wanted to share my progress. Take a look if it interests you! http://hackage.haskell.org/package/Wheb-0.0.1.1 https://github.com/hansonkd/Wheb-Framework Thanks, Kyle Hanson

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STM and unsafePerformIO/bracket
by Rob Leslie 07 Feb '14

07 Feb '14

I’ve run into a difficulty I’d appreciate some advice to solve. I am trying to make a call to C (through the FFI) within an STM transaction to compute what is effectively a pure result. By itself this doesn’t seem to be a problem, except that the C function depends on some global state in a way that compromises thread safety, and so I would like to serialize the process of setting up that state and calling the FFI. I have tried wrapping my call up with: > unsafePerformIO $ bracket (takeMVar lock) (putMVar lock) $ \_ -> do > setupFFIGlobalState > callFFI This seems to work as intended, with one significant exception. If the surrounding STM transaction is aborted (i.e. because of a TVar conflict detected by another thread) while inside the bracket, then apparently the second argument to ‘bracket’ may never be evaluated, leaving my ‘lock’ mutex forever empty, and eventually causing deadlock. Here is a working example of the problem: http://lpaste.net/99399 When I run that, I get: taking lock... lock acquired taking lock... stm-test: thread blocked indefinitely in an MVar operation Is this expected behavior? I realize performing any IO within an STM transaction is inherently unsafe, but I am a little surprised that ‘bracket’ fails here. Is there a better way to do what I’m trying to accomplish? How can I provide a pure interface to my foreign function that will work within an STM transaction? Thanks, -- Rob Leslie rob(a)mars.org

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Filing bugs for haskell packages
by Szymon Pyżalski 07 Feb '14

07 Feb '14

Hello! I am new to haskell and I found a bug in package http://hackage.haskell.org/package/regex-compat-0.92/docs/Text-Regex.html On the hackage page it says that the maintainer e-mail of this package is: libraries(a)haskell.org. So i sent a bug report there, but bounced back to me because of quota. So apparently nobody reads these e-mails. How can I file a bug report for this package? I don't feel good enough in haskell to try and fix it myself. Greetings Szymon Pyżalski

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2nd Call for Papers - TFPIE 2014
by Peter Achten 06 Feb '14

06 Feb '14

All, Please find below the call for papers for the 3rd International Workshop on Trends In Functional Programming in Education, TFPIE 2014. Apologies in advance for multiple copies you may receive. Best regards, James Caldwell Call for Papers ___________________________________________________________________________________ *3rd International Workshop on Trends in Functional Programming in Education (TFPIE 2014)* May 25, 2014 Utrecht University Soesterberg, The Netherlands (http://www.cs.uwyo.edu/~jlc/tfpie14/ <http://www.cs.uwyo.edu/%7Ejlc/tfpie14/>) The 3rd International Workshop on Trends in Functional Programming in Education, TFPIE 2014, will be co-located with the Symposium on Trends in Functional Programming (TFP 2014) <http://www.cs.uu.nl/wiki/TFP2014/> at Soesterberg, at the "Kontakt der Kontinenten" <http://www.kontaktderkontinenten.nl/conferentiehotel/home.aspx?lang=en-US> hotel in the Netherlands on Sunday, May 25th. TFP will follow from May 26-28. The goal of TFPIE is to gather researchers, teachers and professionals that use, or are interested in the use of, functional programming in education. TFPIE aims to be a venue where novel ideas, classroom-tested ideas and work-in-progress on the use of functional programming in education are discussed. The one-day workshop will foster a spirit of open discussion by having a review process for publication after the workshop. The program chair of TFPIE 2014 will screen submissions to ensure that all presentations are within scope and are of interest to participants. Potential presenters are invited to submit an extended abstract (4-6 pages) or a draft paper (up to 16 pages) in EPTCS style. The authors of accepted presentations will have their preprints and their slides made available on the workshop's website/wiki. Visitors to the TFPIE 2014 website/wiki will be able to add comments. This includes presenters who may respond to comments and questions as well as provide pointers to improvements and follow-up work. After the workshop, presenters will be invited to submit (a revised version of) their article for review. The PC will select the best articles for publication in the journalElectronic Proceedings in Theoretical Computer Science (EPTCS) <http://published.eptcs.org/>. Articles not selected for presentation and extended abstracts will not be formally reviewed by the PC. TFPIE workshops have previously been held in St Andrews, Scotland (2012) and in Provo Utah, USA (2013). *Program Committee* James Caldwell, (Program Chair) University of Wyoming Peter Achten, Radboud University, Nijmgen Edwin Brady, University of St Andrews, St Andrews Jurriaan Hage, Universiteit Utrecht Philip Holzenspies, University of Twente Daniel R. Licata, Wesleyan University Marco T Morazan, Seton Hall University Christian Skalka, University of Vermont David Van Horn, Northeastern University *Submission Guidelines* There will be two types of presentations at TFPIE 2014. Regular papers and "best lecture" presentations. The best lecture talks are intended to allow for presentations or short lectures of purely pedagogical material. Papers and abstracts can be submitted via easychair at the following link: https://www.easychair.org/conferences/?conf=tfpie2014 *Papers* TFPIE 2014 welcomes submissions describing techniques used in the classroom, tools used in and/or developed for the classroom and any creative use of functional programming (FP) to aid education in or outside Computer Science. Topics of interest include, but are not limited to: * FP and beginning CS students * FP and Computational Thinking * FP and Artificial Intelligence * FP in Robotics * FP and Music * Advanced FP for undergraduates * FP in graduate education * Engaging students in research using FP * FP in Programming Languages * FP in the high school curriculum * FP as a stepping stone to other CS topics * FP and Philosophy * * *Best Lectures* In addition to papers, this year we are requesting "best lecture" presentations. What's your best lecture topic in an FP related course? Do you have a fun way to present FP concepts to novices or perhaps an especially interesting presentation of a difficult topic? In either case, please consider sharing it. Best lecture topics will be selected for presentation based on a short abstract describing the lecture and its interest to TFPIE attendees. *Important Dates* * 1 February 2014: TFPIE submissions open on easychair. * 7 April 2014: TFP and TFPIE registration opens * 21 April 2014: Submission deadline for draft TFPIE papers and abstracts * 27 April 2014: Notification of acceptance for presentation * 25 May 2014: Presentations in Soesterberg, Netherlands * 29 June 2014: Full papers for EPTCS proceedings due. * 16 August 2014: Notification of acceptance for proceedings * 8 September 2014: Camera ready copy due for EPTCS Submission of an abstract implies no obligation to submit a full paper; abstracts with no corresponding full versions by the full paper deadline will be considered as withdrawn. At least one author from each accepted presentation must attend the workshop.

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problem with happy and type identity
by Richard Cobbe 06 Feb '14

06 Feb '14

I'm working on a small project that involves an Alex scanner and a Happy parser, and I'm getting an error from the type-checker that I do not understand. Can anyone help shed some light on what's going on? I'm running Haskell Platform 2013.2.0.0, on MacOS 10.8.5 with XCode 4.6.3. I've reduced the problem to a very small example, which I've attached as a tar file. It's a cabal package, and it contains a library with some test cases. If I run "cabal configure && cabal build" then the library builds with no problems whatsoever. But if I run cabal clean && cabal configure --enable-tests && cabal build then I get the following error message: RunTests.hs:16:27: Couldn't match expected type `sample-0.4:Ast.Entry' with actual type `Entry' In the return type of a call of `Entry' In the second argument of `(~?=)', namely `Entry "mumble"' In the expression: parse "entry" ~?= Entry "mumble" And this doesn't make any sense to me, because the two types are supposed to be the same. If I take Happy and Alex out of the picture by replacing the Parser module with a hand-written parser, the error goes away. This is hardly feasible in the actual project, of course :-) so I'm hoping someone can shed some light on what I'm doing wrong here. Thanks! Richard (I apologize for the attachment, by the way, but since filesystem layout is potentially important here, and since it's only slightly larger than 1K, I figured it was the best way. If there's a better way to do this in the future (which doesn't assume I can throw it on the web somewhere), then I'd love to hear about it.)

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Generalized null / zero
by Alvaro J. Genial 06 Feb '14

06 Feb '14

1. Is there a more general version of `null`? (e.g. for a Monad, Functor, Applicative, Traversable or the like.) The closest I can come up with is, in decreasing clunkiness: zero :: (MonadPlus m, Eq (m a)) => m a -> Bool zero = m == mzero zero :: (Alternative f, Eq (f a)) => f a -> Bool zero = m == empty zero :: (Monoid m, Eq m) => m -> Bool zero = m == mempty Though requiring Eq seems ugly and unnecessary, in theory. 2. In that vein, is there an existing function for "a value or a default if it's zero"? E.g.: orElse :: (Monoid m) => m -> m -> m a `orElse` b = if zero a then b else a Thank you, Alvaro http://alva.ro

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Thr Neat templating system
by Alvaro J. Genial 06 Feb '14

06 Feb '14

Howdy, I wrote a template system that I've used in a few personal projects and I thought it might be worth it to clean the core of it up a bit and show it to the world. I named it `neat`, for no other reason than I found it so; you can find the sources, an explanation of how it works and basic instructions on how to try it and experiment with it here: https://github.com/ajg/neat >From the synopsis: "Neat is a slightly unusual templating tool that does not interpret templates at runtime; rather, it compiles templates statically from the source (input) language into the target (output) language. In that sense neat can be thought of as a very flexible preprocessor masquerading as something fancier. Of course, the result can then be embedded or compiled directly in order to accept arbitrary data at runtime and transform it as desired, dynamically." I haven't released it as a proper package because I don't know whether it'll be useful; for now it's just a pet project; it is MIT-licensed. I'd like to get your ideas, suggestions, criticism, and other feedback. Thank you, Alvaro http://alva.ro

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Is it possible to declare assignment operators in Haskell?
by Alexander Alexeev 06 Feb '14

06 Feb '14

Hello! I'm wondering, is there any way to declare assignment operators in Haskell? For example, strict assignment operators: x .= y -- equals: let x = y `deepseq` y x := y -- equals: x <- y `deepseq` y As far as I can tell, Template Haskell is not able to do anything like this, because I can't event quote expressions like `let x = y` or `x <- y`. Is there any other way (plugin for GHC, etc)? Or maybe I'm doing something wrong and TH is quite able to handle this task? I'm pretty sure that I could use a preprocessor, but I wonted to find a better solution. -- Best regards, Alexander Alexeev http://eax.me/

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Announcing Haskell bindings to libsystemd-journal
by Oliver Charles 05 Feb '14

05 Feb '14

Hi all, Just a brief announcement - I've just released Haskell bindings to libsystemd-journal, the systemd library for writing directly to the journal. I've tried to be moderately type safe without giving up flexibility or ease of use. The documentation is at: http://hackage.haskell.org/package/libsystemd-journal-1.0.0/docs/Systemd-Jo… Note that some of the hyperlinks on this page don't work, and I can't be bothered fighting with Hackage to make them work. Sorry! If you need that, just install the library and you can read documentation locally. To install this library, you'll need libsystemd-journal (which you probably have if you're reading this) and then you can just run: cabal install libsystemd-journal Happy logging! - ocharles

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Google Summer of Code 2014 project suggestion
by Bjarki Geir Benediktsson 05 Feb '14

05 Feb '14

Hi I have a suggestion for a project for Google Summer of Code 2014 Currently Cabal-Install is not a fully featured package manager and can be improved considerably by adding package management features to it. Features that could be implemented include: Interaction with ghc-pkg to be able to modify (upgrade/downgrade) or remove packages Support for having multiple versions of the same package installed Automatic installation of build tools when needed (Alex, Happy) this is not an exhaustive list of what could be added to make Cabal-Install behave more like a package manager and it could be extended to interact with system package manager where available Do you agree this would be a good project for GSoC 2014 ? Do you think some important features have been overlooked ? Do you think it would be better to write a replacement for Cabal-Install from scratch rather than extending it? I would appreciate feedback. Regards Bjarki Geir Benediktsson

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ANNOUNCE: Applied Functional Programming (AFP) Summerschool 7-18 July 2014, Utrecht, Netherlands
by Atze Dijkstra 05 Feb '14

05 Feb '14

=========== 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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hyperloglog build error
by Konstantine Rybnikov 05 Feb '14

05 Feb '14

Hi! I wanted to use hyperloglog [0] package, but got these errors: https://gist.github.com/k-bx/70654962d4d98ee52e22 I am now trying to "match by hands" why that instance doesn't exist for me, but maybe some of your could help, since I'm far away from all near-lens libraries (but I promise to learn those!). Thank you! [0]: https://github.com/ekmett/hyperloglog/

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Mixing own and derived instances with Generic Deriving Mechanism
by Maarten Faddegon 04 Feb '14

04 Feb '14

Dear Pedro, Cafe, Thanks again for helping me out last December. I have been playing a bit more with deriving show and now ran into an interesting problem mixing my own instances with derived instances. Hope you can enlighten me there! > {-# LANGUAGE DeriveGeneric #-} > module Test where > import GHC.Generics > import Generics.Deriving.Show The Generic Deriving Mechanism adds the keyword 'default' to class definitions. With this keyword we can define a type-generic definition of that method when not given. For example, if we define our own MyData type, we can derive the GShow methods: > data MyData = MyData MyFancyType deriving Generic > instance GShow MyData We can also still give our own definition, for example if we want values of the MyFancyType to always be shown as the same string: > data MyFancyType = MyFancy1 | MyFancy2 deriving Generic > instance GShow MyFancyType where > gshow _ = "Fancy!" There is something strange here though: when we use gshow directly on a MyFancyType value our own instance definition is used, evaluating as expected to "Fancy!". > ex1 = gshow MyFancy1 But as soon as we are inside a derived method, we will continue using derived instances even though we defined our own. The example below evaluates to "MyData MyFancy1", rather than "MyData Fancy!": > ex2 = gshow (MyData MyFancy1) The default methods of GShow are defined in terms of methods from GShow' which operate on the type-representation. From this representation I do not see a way to recover the information that a type has a GShow instance. Am I correct (I hope not :) or is there a way out? Cheers, Maarten Faddegon

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Big Data & Haskell
by Marc Weber 04 Feb '14

04 Feb '14

Is Haskell ready? Are there library which can - distribute data on multiple machines (similar to cassandra) - implement multi index things, such as having fields - name (eg of stores) - geo location (x/y) keys so that you can find items nearby a location fast - additional data to be searched as needed Maybe have server which gets "code" sent by a master to be compiled defining the data to host or such? I know that this could be done - just wondering whether it has been done already. Marc Weber

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[ANNOUNCE] tasty-hunit-adapter and tasty-program
by Jan Stolarek 04 Feb '14

04 Feb '14

Dear Haskellers, I recently released two small packages that extend functionality of Roman Cheplyaka's tasty testing framework: 1. tasty-hunit-adapter allows to import existing HUnit tests into tasty. Hackage: http://hackage.haskell.org/package/tasty-hunit-adapter GitHub: https://github.com/jstolarek/tasty-hunit-adapter 2. tasty-program allows to run external program and test whether it terminates successfully. Hackage: http://hackage.haskell.org/package/tasty-program GitHub: https://github.com/jstolarek/tasty-program These packages are inspired by similar packages for test-framework. tasty-program has very basic functionality at the moment, most notably it completely ignores stdout and stderr of a program. If you want this functionality please say so here: https://github.com/jstolarek/tasty-program/issues/1 Jan Stolarek

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Fwd: incorrect MonadPlus law "v >> mzero = mzero"?
by Petr Pudlák 04 Feb '14

04 Feb '14

Since nobody replied so far, I'm also forwarding to cafe. Petr ---------- Forwarded message ---------- From: Petr Pudlák <petr.mvd(a)gmail.com> Date: 2014-01-29 Subject: incorrect MonadPlus law "v >> mzero = mzero"? To: "libraries(a)haskell.org" <libraries(a)haskell.org> Hi, this law apparently fails for a MonadPlus instance that has more than one possible failure value. Consider: runIdentity . runErrorT $ ((ErrorT . Identity $ Left "failure") >> mzero :: ErrorT String Identity ()) evaluates to `Left "failure"`, which isn't equal to ErrorT's mzero `Left ""`. This isn't just the case of ErrorT, it fails for any MonadPlus with multiple failure values. For example lift (tell "foo") >> mzero :: MaybeT (Writer String) () is again distinct from mzero. Actually, no monad transformer with a MonadPlus instance can satisfy the law, because the first part in front of `>> mzero` can introduce side effects in the underlying monad. I'm not sure what should be the proper solution. Perhaps to change the laws to: return x >> mzero = mzero (v >> mzero) >>= f = (v >> mzero)` That is, if an expression ends with `mzero`, it behaves like `mzero`. Petr

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CTO Opportunity
by Charles Weitzer 04 Feb '14

04 Feb '14

My name is Charles Weitzer. I do recruiting for major quantitative hedge funds worldwide. I have a client, a startup quantitative hedge fund located in Northern California, that would like to hire a Chief Technology Officer as soon as possible. Their job description can be found below. * Chief Technology Officer Fast-growing science and technology driven company seeks a Chief Technology Officer. We apply statistical machine learning to develop automated trading systems. We are a growing group of exceptional software and infrastructure professionals and seek an experienced candidate to lead and grow the team. The CTO will manage the implementation of our next generation infrastructure, including production trading systems, software infrastructure for our research department, and our compute and storage hardware systems. The CTO will work directly with the Head of Research and the CEO to set the technical direction of the company. A strong candidate will be well-versed in state-of-the-art technology (particularly distributed systems and cluster computing), and will have a track record of building and leading elite technical teams and successfully delivering complex projects (especially in a startup environment). A background in finance is not required. We are a science-driven systematic trading firm, built on the principle that statistical machine learning provides the best solutions to the scientific problems we must solve. Our CEO is a CS PhD and founded a successful Internet startup, and our Head of Research/Chief Investment Officer is a Berkeley statistics professor; both have a background in finance. Our trading is entirely automated and based on cutting-edge machine learning techniques. We have a collegial, academic working environment, rely on open source software, and have built our own software stack. We have a track record of strong performance and have been raising money and growing fast. Willingness to take initiative, and a gritty determination to productize, are essential. Benefits and compensation are highly competitive. I look forward to speaking with you and anyone you might highly recommend about this opportunity. Thank you, Charles Weitzer CEO/Senior Recruiter Charles Weitzer and Associates, Inc. Global Financial Recruiting Services Charles(a)CharlesWeitzer.com Voice: USA (510) 558-9182

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Dispatch a type-function on the existence (or not) of instances?
by Hans Höglund 03 Feb '14

03 Feb '14

Dear all, I have been curious about the ability to detect the presence of a certain instance (ClassFoo TypeBar) in the type system. Specifically, is it possible to "dispatch" a type on the existence (or not) of such an instance. For example given two functions: withInstance :: (ClassFoo TypeBar) => TypeIfInstanceExists withoutInstance :: TypeIfInstanceDoesNotExists I would be able to consolidate them into something like this: withOrWithoutInstance :: (r ~ InstanceExists ClassFoo TypeBar, a ~ If r TypeIfInstanceExists TypeIfInstanceDoesNotExists) => a I guess what I need is: 1) A type-level "if". 2) The possibility of "converting" a constraint into a type-level bool. I am sure (1) is possible but have no idea about (2). Anyone? Best regards, Hans

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What game libraries should I use?
by Alexander Berntsen 03 Feb '14

03 Feb '14

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 I need: - -2D graphics (preferably with simple shapes available) - -menus (I can make menus myself with shapes though) - -simple audio - -fonts (better than Gloss at least) - -keyboard input - -networking (simple direct connections between two computers) I need to be able to express my game system as declaratively as possible[0]. If the library is based on SDL/OpenGL, that would be nice[1]. This is, however, not necessary. What game libraries are the most mature and adequate for this? The only library I have used previously with Haskell is Gloss. I am not experienced in FRP, though I have read about and somewhat groked the point. I am willing to learn an FRP library. [0] I am writing it as part of a thesis in which I will look at the modularity and expressiveness of purely functional programming compared to object-oriented programming. Writing code that is technically purely functional, but in practice looks like imperative code, is sub-optimal. [1] For the thesis I am writing, I am implementing a C++ version of the same game. This will probably use SDL 2.0 and OpenGL bindings. If the Haskell version could use libraries based on that, this would make it easier for me to write my thesis. I.e., I would not have to spend time justifying that the modularity and expressiveness I am investigating is due to the chosen languages and programming paradigms, and not the libraries I use. - -- Alexander alexander(a)plaimi.net http://plaimi.net/~alexander -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iF4EAREIAAYFAlLrlMoACgkQRtClrXBQc7VkiAEAnEbfjiSLovcXttmjpdD5OSFI uEMBVBJdWonY9ZMPIc8BAIAQn+YMRvGILgb8WmuB9oTWJDVZfqDMyB47qzjyfimO =YGLW -----END PGP SIGNATURE-----

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ANNOUNCE: fpnla - A library for NLA operations
by Mauro Blanco 03 Feb '14

03 Feb '14

Hi. Today we are releasing fpnla <http://hackage.haskell.org/package/fpnla>, which defines a framework for linear algebra operations of BLAS and LAPACK. As its main features it allows: - Definition of multiple representations of vectors and matrices. - Arbitrary combination of strategies and structure representations. - Type-safe manipulation of context information associated to each strategy. - Definition of specialized strategies for a given representation. And also fpnla-examples <http://hackage.haskell.org/package/fpnla-examples>, which contains many example implementations of the operations defined in fpnla using various data structures and algorithms. Regards. -- Mauro Blanco

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New book: "Beginning Haskell"
by Alejandro Serrano Mena 02 Feb '14

02 Feb '14

[Disclaimer: I'm the author of the book] Dear Haskell-cafe, I would like to introduce a new book discussing our favourite language from beginner into upper intermediate level: "Beginning Haskell". The book starts assuming zero knowledge about functional programming and builds step by step to get into the realm of web applications, type-level programming, domain specific languages, distributed computing, unit testing and much more! You can look at the Table of Contents in Amazon [ http://www.amazon.com/Beginning-Haskell-A-Project-Based-Approach/dp/1430262… ]. The book revolves around the idea of a Time Machine Store: in each part some functionality is developed. The first part serves as an introduction and how to model the data with Haskell data types and functions; part 2 discusses many concepts around monads while developing two data-mining algorithms; in the third part storing and interfacing with clients serves as an excuse to introduce input/output, database access and web applications; part 4 is devoted to discussing domain specific libraries for describing offers; and the last part discusses testing and other engineering issues. All the "classical" topics in Haskell are discussed: higher-order functions, type classes -- functors, applicatives, monads --, laziness. The focus also lies on libraries use in modern Haskell daily programming, such as lenses, streaming data libraries, functional dependencies and type families, Software Transactional Memory and Cloud Haskell. One topic that is cross-cutting through the book is how to manage projects with Cabal or EclipseFP: initializing a new package, declaring dependencies, building and integrating tests. The book tries thus to offer a comprehensive view of the Haskell ecosystem, not only of the Haskell programming language. The book is published by Apress [http://www.apress.com/9781430262503] and available as paperback and electronic format. If somebody would like to make a review for some FP or programming-related magazine (such as The Monad Reader) or assess its possibilities for use in classroom, please drop me a line. Hope you like it! :)

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Castle: Permission denied
by Henk-Jan van Tuyl 02 Feb '14

02 Feb '14

L.S., I am trying to install castle, but I get the following messages: Downloading castle-0.1.0.0... Configuring castle-0.1.0.0... realgcc.exe: ./specs: Permission denied What can I do about this? I am running Windows XP, Haskell Platform 2013.2.0.0 Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

3 2

DSLs and heterogeous lists
by Corentin Dupont 01 Feb '14

01 Feb '14

Hi again, I have a game in which the user can create/write/read variables, using a small DSL. The type of the variable created can be whatever chooses the user, so I'm using existential types to store those variables in a heterogeneous list. This works fine, but the problem is that the "Typeable" class tag leaks into the DSL... The question is, how to get rid of it? > This is literate Haskell > {-# LANGUAGE GADTs, ScopedTypeVariables #-} > module DSLClass where > import Control.Monad > import Control.Monad.State > import Data.Typeable > This is the (simplified) DSL. With it you can read a variable stored in the game state (creation/writing is not shown). How can we get rid of the "Typeable a" in the ReadFirstVar constructor? > -- first type parameter is used to track effects > data Exp a where > ReadFirstVar :: (Typeable a) => Exp a <----- Ugly > Return :: a -> Exp a > Bind :: Exp a -> (a -> Exp b) -> Exp b This is the definition of a variable. The type is unknow, so I use existantial types. > data Var = forall a . (Typeable a) => Var { v :: a} This game state. It holds the heterogenous list. > data Game = Game { variables :: [Var]} The evaluation of "Exp" can be: > eval :: Exp a -> State Game a > eval ReadFirstVar = do > (Game ((Var v):vs)) <- get > case cast v of > Just val -> return val > Nothing -> error "no cast" > eval (Bind exp f) = do > a <- eval exp > eval (f a) As you can see, I'm obliged to cast the variable type to match it with the expression's type. Is that the right place to do it? Thanks!! Corentin

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Cabal version constraint seems to be ignored.
by Carlo Hamalainen 31 Jan '14

31 Jan '14

Hi, I have two versions of Cabal, both are visible: $ ghc-pkg expose Cabal-1.16.0 $ ghc-pkg expose Cabal-1.18.1.2 I clone and build my package. Since I have GHC 7.6.3 it uses Cabal-1.16.0 (as specified in ghc-imported-from.cabal): $ git clone https://github.com/carlohamalainen/ghc-imported-from $ cd ghc-imported-from $ cabal install It looks like Cabal-1.16.0 was chosen, as expected: $ grep Cabal dist/build/autogen/cabal_macros.h /* DO NOT EDIT: This file is automatically generated by Cabal */ /* package Cabal-1.16.0 */ #define VERSION_Cabal "1.16.0" #define MIN_VERSION_Cabal(major1,major2,minor) (\ But my program doesn't run: $ ghc-imported-from src/Main.hs Main getArgs 11 11 --ghc-options --ghc-pkg-options GhcOptions [] GhcPkgOptions [] Language/Haskell/GhcImportedFrom.hs:132:54: Couldn't match expected type `Distribution.PackageDescription.BuildInfo' with actual type `Cabal-1.16.0:Distribution.PackageDescription.BuildInfo' In the fourth argument of `getGHCOptions', namely `binfo' In a stmt of a 'do' block: getGHCOptions [] c (fromJust $ cradleCabalDir c) binfo In the expression: do { c <- findCradle; pkgDesc <- GhcMonad.liftIO $ parseCabalFile $ fromJust $ cradleCabalFile c; let binfo = head $ cabalAllBuildInfo pkgDesc; getGHCOptions [] c (fromJust $ cradleCabalDir c) binfo } I'm surprised that the error didn't appear at compile time. I guess the first reference to Distribution.PackageDescription.BuildInfo is from Cabal-1.18.1.2? If I hide Cabal-1.18.1.2 then things work ok: $ ghc-pkg hide Cabal-1.18.1.2 $ cabal install $ ghc-imported-from .... (runs as expected) But I can't expect a user to know that they have to hide Cabal-1.18.1.2. Is this actually a GHC bug or have I stuffed up my ghc-imported-from.cabal file? Thanks, -- Carlo Hamalainen http://carlo-hamalainen.net

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TFP 2014 - 2nd call for papers
by Peter Achten 31 Jan '14

31 Jan '14

--------------------------------- 2ND C A L L F O R P A P E R S --------------------------------- ======== TFP 2014 =========== 15th Symposium on Trends in Functional Programming May 26-28, 2014 Utrecht University Soesterberg, The Netherlands http://www.cs.uu.nl/wiki/TFP2014/WebHome *** Submission for TFP 2014 is now open: please direct your browser to *** http://www.cs.uu.nl/wiki/TFP2014/PaperSubmission The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below), described in draft papers submitted prior to the symposium. A formal post-symposium refereeing process then selects a subset of the articles presented at the symposium and submitted for formal publication. Selected revised papers will be published as a Springer Lecture Notes in Computer Science (LNCS) volume. TFP 2014 will be the main event of a pair of functional programming events. The other is the International Workshop on Trends in Functional Programming in Education (TFPIE). TFPIE will take place on May 25th. Its website is located at http://www.cs.uwyo.edu/~jlc/tfpie14/ The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in Edinburgh (Scotland) in 2003, in Munich (Germany) in 2004, in Tallinn (Estonia) in 2005, in Nottingham (UK) in 2006, in New York (USA) in 2007, in Nijmegen (The Netherlands) in 2008, in Komarno (Slovakia) in 2009, in Oklahoma (USA) in 2010, in Madrid (Spain) in 2011, St. Andrews (UK) in 2012 and Provo (Utah, USA) in 2013. For further general information about TFP please see the TFP homepage. INVITED SPEAKERS TFP is pleased to announce talks by the following two invited speakers: John Hughes of Chalmers, Goteborg, Sweden, is well-known as author of Why Functional Programming Matters, and as one of the designers of QuickCheck (together with Koen Claessen); the paper on QuickCheck won the ICFP Most Influential Paper Award in 2010. Currently he divides his time between his professorship and Quviq, a company that performs property-based testing of software with a tool implemented in Erlang. Dr. Geoffrey Mainland received his PhD from Harvard University where he was advised by Greg Morrisett and Matt Welsh. After a two year postdoc with the Programming Principles and Tools group at Microsoft Research Cambridge, he is now an assistant professor at Drexel University. His research focuses on high-level programming language and runtime support for non-general purpose computation. SCOPE The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not submitted for simultaneous publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or more experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Using functional techniques to reason about imperative/object-oriented programs Debugging for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2014 program chair, Jurriaan Hage at J.Hage(a)uu.nl. BEST PAPER AWARDS To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. SPONSORS TFP is financially supported by NWO (Netherlands Organisation for Scientific Research), Well-Typed and Erlang Solutions. PAPER SUBMISSIONS Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (16 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate whether the main author or authors are research students. In the case of a FULL STUDENT paper, the draft paper will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. IMPORTANT DATES Submission of draft papers: March 17, 2014 Notification: March 24, 2014 Registration: April 7, 2014 TFP Symposium: May 26-28, 2014 Student papers feedback: June 9th, 2014 Submission for formal review: July 1st, 2014 Notification of acceptance: September 8th, 2014 Camera ready paper: October 8th, 2014 PROGRAM COMMITTEE Peter Achten Radboud University Nijmegen Emil Axelsson Chalmers Lucilia Camarao de Figueiredo Universidade Federal de Ouro Preto Laura Castro University of A Coruna Frank Huch Christian-Albrechts-University of Kiel Matthew Fluet Rochester Institute of Technology Jurriaan Hage (chair) University of Utrecht Yukiyoshi Kameyama University of Tsukuba Andrew Kennedy Microsoft Research Tamas Kozsik Eotvos Lorand University Ben Lippmeier University of New South Wales Luc Maranget INRIA Jay McCarthy (co-chair) Brigham Young University Marco T. Morazan Seton Hall University Ricardo Pena Universidad Complutense de Madrid Alexey Rodriguez LiquidM Sven-Bodo Scholz Heriot-Watt University Manuel Serrano INRIA Sophia Antipolis Simon Thompson University of Kent Tarmo Uustalu Inst of Cybernetics David Van Horn University of Maryland Janis Voigtlaender University of Bonn

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Re: [Haskell-cafe] manage effects in a DSL
by oleg＠okmij.org 31 Jan '14

31 Jan '14

> - What do you mean by "The type Cont Int a describes an impure computation, > which may abort with an Int value, for example". > Aborting with an Int value is akin to exceptions? I see that using Cont monad wasn't a good idea. The point can be made simply, with an Error monad or applicative. Here is a simple function to multiply a list of integers: > muls :: [Integer] -> Either e Integer > muls [] = pure 1 > muls (h:t) = (h*) <$> muls t We give the signature (which the compiler accepts) with the polymorphic return type Either e Integer, fully polymorphic over e. Therefore, muls throws no exceptions. Here is a bit more optimized function, which stops all multiplications once zero is encountered. > mulse :: [Integer] -> Either Integer Integer > mulse [] = pure 1 > mulse (0:t) = Left 0 > mulse (h:t) = (h*) <$> mulse t We cannot make the return type (Either e Integer), it must be (Either Integer Integer). The function can indeed throw an exception. If we handle the exception > muls' :: [Integer] -> Either e Integer > muls' l = either pure pure $ mulse l the effect is encapsulated and the type is polymorphic again. > - for me it's not clear when to choose an "applicative" or a "monadic" DSL? > Betsides, what's the rational behind the name "let_" (never seen it before)? The first rule of thumb is to use the simplest structure that does the job. Another good rule is to distinguish the DSL from its implementation. For example, the DSL below http://okmij.org/ftp/tagless-final/index.html#call-by-any has no monads. It is a simple lambda-calculus with constants. Its implementation in Haskell, the embedding, does use monads, sometimes quite specific monads (e.g., IO, to print the trace of evaluation). But these monads are not exposed to the DSL programmer. That article also shows off let_. > data Eff = Effect | NoEffect > -- first type parameter is used to track effects > data Exp :: Eff -> * -> * where > ReadAccount :: Exp r Int --ReadAccount can be used in whatever > monad (with or without effect) > WriteAccount :: Exp NoEffect Int -> Exp Effect () --WriteAccount > takes an effect-less expression, and returns an effectfull expression This is exactly what I had in mind (and probably Jacques as well). > Other pointers where this idiom is realised?? Lots of places; ST monad comes to mind, and the Region monad. This trick is often used to track, for example, information flow security (the label then reflects the sensitivity level of data processed by the computation, Low or High). See, for example, http://www.cse.chalmers.se/~russo/seclib.htm http://www.scs.stanford.edu/~deian/pubs/stefan:2011:flexible-slides.pdf or just search for information flow control or information flow security in Haskell. Edward Yang has a lot to say on this topic.

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liboleg
by Suhail Shergill 30 Jan '14

30 Jan '14

hi, i would like to take over as maintainer of the liboleg package on hackage <http://hackage.haskell.org/package/liboleg>. i have gotten permission from the author (oleg) and the current maintainer (don) regarding the same via email. my hackage username is shergill. if there are no objections, could i be added to the maintainer group for liboleg and be granted upload rights? -- Suhail

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Deprecating bytestring version 0.10.2.0
by Christian Laustsen 30 Jan '14

30 Jan '14

Hi everyone, Following some problems I experienced with Network.HTTP.Client.TLS, I found out the problem was specific to bytestring library version 0.10.2.0 and most likely fixed with this commit https://github.com/haskell/bytestring/commit/86df1f17b2332940df69f484182c5c… . The default version shipped with GHC 7.6.3 (0.10.0.2) works fine, and the version after 0.10.2.0 (that is, version 0.10.4.0) also works. It was suggested that I mention on here that I think we should deprecate the 0.10.2.0 version of bytestring specifically, since that is the one that introduces the problem, and there can be problems with having to specify specific versions of bytestring as a dependency when compiling across GHC versions. -- Christian // Tehnix

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Builder vs Builder
by Andrew Cowie 30 Jan '14

30 Jan '14

Does anyone know whether blaze-builder is deprecated in favour of bytestring's new Builder module, or are they the same, or... I know Simon is the evil wizard cackling in his dank lair in both cases, and I do remember him saying he was trying to get his work "upstream" into bytestring, but I haven't heard anyone suggesting we should be using Data.ByteString.Lazy.Builder instead of Blaze.ByteString.Builder I can only imagine the integration with the allocation routines is much better now that it's in bytestring, but on the other hand it's hard to upgrade bytestring — it being so low in the stack — so perhaps the original blaze remains the right choice from a bugfix and stability perspective. Thoughts? AfC Sydney

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GHC iOS ARMv7/ARMv7s fat support completed
by Luke Iannini 30 Jan '14

30 Jan '14

Hi folks, Happy to report that I've finished an approach to armv7/armv7s fat compilation, just in time for 7.8's imminent release. You'll find the necessary scripts here: https://github.com/ghc-ios/ghc-ios-scripts and the latest instructions for building GHC for iOS usage here: https://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS I've also added support for a perf-cross BuildFlavour, which will give a higher-performance and profiling-ready build that matches what we'll be putting together as the official 7.8 GHC iOS binaries: https://ghc.haskell.org/trac/ghc/ticket/8700 Cheers Luke

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Is it safe to create global variables using unsafePerformIO?
by Alexander Alexeev 30 Jan '14

30 Jan '14

Hello! Lets consider the following code: import Control.Concurrent import Control.Concurrent.STM import System.IO.Unsafe (unsafePerformIO) {-# NOINLINE counter #-} counter :: TVar Int counter = unsafePerformIO $ newTVarIO 0 incCounter :: IO Int incCounter = do r <- atomically $ do t <- readTVar counter let t' = t + 1 writeTVar counter t' return t' return r main :: IO () main = do n1 <- incCounter print n1 n2 <- incCounter print n2 n3 <- incCounter print n3 This program prints: 1 2 3 So we have a "global variable". Do I right understand that newTVarIO creates TVar and RTS memoizes it since 'counter' function is pure? If it's true, could it happen that under some circumstances memoized value will be deleted from memory? Or Haskell keeps all memoized values forever? Another issue which I'm afraid of --- would the given code be safe in multithread application? For example, is it possible to encounter a race condition if two threads will try to create a new counter in the same time? Is there any other problems which should be taken in account? -- Best regards, Alexander Alexeev http://eax.me/

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Lazy MWC Random?
by Sacha Sokoloski 29 Jan '14

29 Jan '14

Dear Haskellers, I'm in a situation where I'd like to generate an infinite list of random elements (basically, I'm simulating stochastic systems). I feel like MWC Random is the fastest RNG available, but when I try to pull the infinite list out of RandST, it obviously doesn't halt, because ST is strict. Someone posted a way around this in this stack overflow thread: https://stackoverflow.com/questions/16248600/parallel-computations-with-fas… Which would fix my problem. My question is though, why isn't ST.Lazy included as a PrimMonad instance anyway? The best answer I can come up with is that, since evaluating the Generator is time dependent, it's best to make it strict to make sure that one's program isn't tapping into /dev/random at arbitrary times. In this way the best stackoverflow solution is quite good. It requires one to strictly generate a Seed (since that's the only way to do it), but then converts the ST Monad to the Lazy version to Lazify everything else. However, my understanding of PrimMonad is simply that it's a class of low level monads i.e. IO and ST, so if there's some deeper reason to this, it's beyond me. Another question that I'm puzzling over: In the stack overflow solution, they also make an effort to only have to generate the seed a single time. Is this important performance wise? What I suppose this must hinge upon, is whether in saving an ST s Gen to a Seed, the conversion from an immutable to mutable array requires a copy or not. Is that the full extent of the complexity of this? Is the stackoverflow solution ultimately the most efficient? Is using MWC Random to generate infinite lists and efficient solution anyway? Thanks for any insight, - Sacha Sokoloski

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New Functional Programming Job Opportunities
by Functional Jobs 29 Jan '14

29 Jan '14

Here are some functional programming job opportunities that were posted recently: Haskell Developer at Zalora http://functionaljobs.com/jobs/8678-haskell-developer-at-zalora Cheers, Sean Murphy FunctionalJobs.com

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ANNOUNCE: GLUT 2.5.1.0
by Sven Panne 29 Jan '14

29 Jan '14

A new version of the GLUT package is available on Hackage, with the following new features: * Added support for left/right SHIFT/CTRL/ALT special keys. * Added skipStaleMotionEvents state flag, making it possible to skip all but the last motion event. * Added leaveFullScreen call. * Added support for position, application life cycle and multi-touch window callbacks. * Various object drawing improvements: * Added teacup and teaspoon objects, so we support the full Newell tea set now. * Added geometryVisualizeNormals state flag. * Added support for drawing objects via VBOs: vertexAttribCoord3, vertexAttribNormal, and vertexAttribTexCoord2. * Added support for menu fonts via new constructor MenuWithFont. Apart from glutInitErrorFunc and glutInitWarningFunc, which are not extremely useful IMHO, all freeglut features are now exposed on the Haskell side. Cheers, S.

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To LLVM or not to LLVM...
by Ian Ross 29 Jan '14

29 Jan '14

A package I just released (arb-fft) gets about a 17% performance boost from using GHC's LLVM backend (on my machine at least). That seems a big enough gain that I put "-fllvm" into the ghc-options field in the Cabal file for the package. Unfortunately, that means the package won't install unless you have the LLVM tools installed. What's the best thing to do? Use the native code generator by default and add a Cabal flag to install using LLVM? Just take the "-fllvm" out and add a note to the Cabal file description to tell people to install using LLVM if they have it? A quick survey of packages on Hackage reveals only very few instances where the "-fllvm" flag appears in Cabal files, which makes me suspect that I'm doing it wrong. -- Ian Ross Tel: +43(0)6804451378 ian(a)skybluetrades.net www.skybluetrades.net

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Re: [Haskell-cafe] manage effects in a DSL
by oleg＠okmij.org 29 Jan '14

29 Jan '14

Lindsey Kuper wrote: > Lately I have gotten a lot of mileage out of thinking of monads as > embedded DSLs. This is in fact how Moggi suggested to view monads. I strongly suggest the remarkable paper Eugenio Moggi and Sonia Fagorzi A Monadic Multi-stage Metalanguage FOSSACS2003, pp. 358--374 http://www.disi.unige.it/person/MoggiE/ftp/fossacs03.pdf Here are two most salient quotations: An important principle of Haskell [PHA+ 97] is that pure functional evaluation (and all the optimization techniques that come with it) should not be corrupted by the addition of computational effects. In Haskell this separation has been achieved through the use of monads (like monadic IO and monadic state). When describing MMML we adopt this principle not only at the level of types, but also at the level of the operational semantics. In fact, we distinguish between simplification (described by local rewrite rules) and computation (that may cause side-effects). ... We outline a general pattern for specifying the operational semantics of monadic metalanguages, which distinguishes between transparent simplification and programmable computation. This is possible because in a monadic metalanguage there is a clear distinction between term-constructors for building terms of computational types, and the other term-constructors that are computationally irrelevant. For computationally relevant term-constructors we give an operational semantics that ensures the correct sequencing of computational effects, e.g. by adopting some well-established technique for specifying the operational semantics of programming languages (see [WF94]), while for computationally irrelevant term-constructors it suffices to give local simplification rules, that can be applied non-deterministically (because they are semantic preserving). I should point out a large design space of the embedded effectful DSL. First of all, an embedded DSL does not have to be monadic. Monad essentially corresponds to the let-form in embedded DSL: let_ :: Exp a -> (a -> Exp b) -> Exp b If we are implementing an offline DSL compiler, such a let_ form is inappropriate since it presupposes the interleaving of compilation and code generation. Indeed, according to the type of let_, the code for the body of let_ may differ depending on the result of Exp a (that is, the result of compiling and then running Exp a). Normally, we first generate code, _then_ we compile it and then we run it. Therefore, many DSLs will not have let_ form. Rather, they will have app :: Exp (a -> b) -> Exp a -> Exp b That is, DSL is an applicative rather than monadic. DSL may be even simpler, see for example, Sec 3.4 of http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fu… for a simple DSL of communicating processes, which is not even applicative. (Its realization in Haskell may use monads, but this is an implementation detail. The full power of the implementation language is not exposed to the DSL programmer.) If the DSL is effectful, we may annotate the Exp type with a set of effects that an expression may perform. The most familiar example is the continuation monad Cont r a, which is annotated with the answer-type r. If Cont r a is polymorphic over r, the computation is pure. The type Cont Int a describes an impure computation, which may abort with an Int value, for example. If we continue on this road we quickly come to extensible-effects. It may happen that one annotation is not enough: we need two, to describe the (type)state of the computation before and after an operation. The simplest example is locking, with the type state showing the state of the lock (acquired or released). See Sec 5.2 Tracking state and control in a parameterised monad in the above typefun paper. The computational structure in question is not a monad -- it is more general than that. To summarize: the design space is large. The relevant computational structure is not necessary a monad: it could be less powerful or more powerful. One should resist jumping on the bandwagon of monads or free monads, which are so (over)hyped these days. It's best to first decide what sort of programs a DSL programmer should be able to write. The computational structure (monad or something else) will be clearer then.

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manage effects in a DSL
by Corentin Dupont 28 Jan '14

28 Jan '14

Hi Haskell-Caféists! I have a small DSL for a game. Some instructions have effects (change the game state), some not. -> In short, my question is: how can I semantically separate instructions with effect from the others? i.e. how can I mark down and track those effects? Here is a simplified version of the DSL I use. First some boilerplate: > {-# LANGUAGE GADTs #-} > import Control.Monad > import Control.Monad.State > import Control.Monad.Free This is the DSL: > data Exp a where > ReadAccount :: Exp Int > WriteAccount :: Exp Int -> Exp () > SetVictory :: Exp Bool -> Exp () > OnTimer :: Exp () -> Exp () > Return :: a -> Exp a > Bind :: Exp a -> (a -> Exp b) -> Exp b It can read and write to an account (belonging to the state of the game), set a victory condition, and trigger some event every minute. > instance Monad Exp where > return = Return > (>>=) = Bind > instance Functor Exp where > fmap f e = Bind e $ Return . f With that you can write: > victoryRule :: Exp () > victoryRule = SetVictory $ do > m <- ReadAccount > return (m > 100) "victoryRule" sets the victory condition to be: "if there is more than 100 gold in the account, you win." This is the game state: > data Game = Game { bankAccount :: Int, > victory :: Exp Bool, > timerEvent :: Exp ()} The evaluation of "Exp" can be: > eval :: Exp a -> State Game a > eval (SetVictory v) = modify (\g -> g{victory = v}) > eval ReadAccount = get >>= return . bankAccount > eval _ = undefined -- etc. If you evaluate "victoryRule", you change the Game state by setting the victory field. Then, each time you will evaluate the victory field, you will know if you won or not (depending on your account...). This is all well and good, but imagine if you write: > victoryRule' :: Exp () > victoryRule' = SetVictory $ do > m <- ReadAccount > WriteAccount (return $ m + 1) > return (m > 100) Ho no! Now each time a player is refreshing his screen (on the web interface), the victory condition is re-evaluated to be displayed again, and the bank account is increased by 1! This is not what we want. We should allow only effect-less (pure) instructions in the victory field, like readAccount, but not WriteAccount. How would you do that? I tried with the Free monad to delimit those effects. I re-write each primitives, marking them with the special type "Effect", when needed. > type Effect = Free Exp > -- readAccount remain the same: it has no effect > readAccount :: Exp Int > readAccount = ReadAccount > --writeAccount is marked as having an effect > writeAccount :: Exp Int -> Effect (Exp ()) > writeAccount ei = Pure $ WriteAccount ei > --onTimer is able to trigger an effect every minute > onTimer :: Effect (Exp ()) -> Effect (Exp ()) > onTimer e = Pure $ OnTimer $ iter join e > --victoryRule can be re-written like this, note that effects are rejected now > victoryRule'' :: Effect (Exp ()) > victoryRule'' = Pure $ SetVictory $ do > m <- readAccount > --writeAccount (return $ m + 1) --will be rejected at compile time (good)! > return (m > 100) > --increase my bank account by 1 every minute > myTimer :: Effect (Exp ()) > myTimer = onTimer $ do > m <- lift readAccount > writeAccount (return $ m + 1) I don't know if I got it right at all... How does it sound? It only bothers me that in this context "Pure" really means "Impure" :) Do you think of any other solution? Cheers, Corentin

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Munich Haskell Meeting
by Heinrich Hördegen 28 Jan '14

28 Jan '14

Dear all, I hope that you had good start into 2014. Munich's Haskell enthusiasts (or other declarative languages) meet for their first get-together in 2014 on Thu, 30th of January at 19h30 at Cafe Puck. If you plan to join, please go here and hit the button: http://www.haskell-munich.de/dates With my best wishes for this year, Heinrich

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[ANN]: arb-fft 0.1, Fast Fourier transform library
by Ian Ross 28 Jan '14

28 Jan '14

Dear Cafe, I'm happy to announce the first release of arb-fft, a pure Haskell FFT implementation for arbitrary length vectors: http://hackage.haskell.org/package/arb-fft This is probably more of pedagogical interest than anything else, since there's a long series of blog articles describing the development of the package, indexed at http://skybluetrades.net/haskell-fft-index.html The package has some interesting features beyond the usual "textbook" powers-of-two FFT algorithm. In particular, it uses a mixed-radix decomposition of composite input lengths, uses Rader's algorithm for large prime factors and has an empirical benchmarking scheme using Criterion for FFT plan selection. The performance of arb-fft is within a factor of 10 of FFTW for most input sizes, which isn't too bad for a pure Haskell with only a relatively limited amount of work done on optimisation. Commentary is very welcome, as are offers to help with any of the tasks listed in the last blog article: http://skybluetrades.net/blog/posts/2014/01/27/data-analysis-fft-14.html Hackage: http://hackage.haskell.org/package/arb-fft GitHub: https://github.com/ian-ross/arb-fft Blog article index: http://skybluetrades.net/haskell-fft-index.html -- Ian Ross Tel: +43(0)6804451378 ian(a)skybluetrades.net www.skybluetrades.net

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Announcement - HGamer3D release candidate 0.3.0
by Peter Althainz 28 Jan '14

28 Jan '14

Dear All, I'm happy to announce HGamer3D, version 0.3.0 as a release candidate on Github. Please look here: http://www.hgamer3d.org and https://github.com/urs-of-the-backwoods/HGamer3D . The new version features: - Linux ! (in addition to Windows) - HBind tool to generate and administrate bindings Of course I would be largely interested in feedback for the final release of 0.3.0. Best regards Peter Althainz

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Target Selection in HaRe
by AlanKim Zimmerman 28 Jan '14

28 Jan '14

The Haskell Refactorer now makes use of the GHC API to load and typecheck the code to be refactored. It uses ghc-mod internally to identify a project cabal file, and extract the targets in it. The current code attempts to load all the targets into the module graph, to make sure that when a project is refactored the ancillary parts such as tests and benchmarks are refactored too, e.g. when renaming a function. The problem is that GHC is unable to load more than one main file. I am trying to decide on the best way of resolving this in terms of a user of HaRe, where it should 'just work' most of the time. The actual refactoring is done by calling the HaRe executable with command line arguments. Options that seem viable are 1. require the names of the target(s) to be loaded to be passed in as command line arguments. This means the IDE integration is going to have to provide a way of deciding the scope of the refactoring. 2. Create a config file that lives in the project directory and specifies the targets to be loaded 3. Try to build up a union of the module graph for all the targets, excluding all main modules. The problem with this is that it then becomes difficult to refactor a main module. 4. A different option, or blend of the above.e.g. load the union but specify the specific main module. Does anyone have any preferences in terms of this? Alan

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Haddock changes pushed upstream
by Mateusz Kowalczyk 27 Jan '14

27 Jan '14

Hi all, As some of you might know, I hacked on Haddock over summer as part of GSOC. After some tedious fighting with GHC validation process and help of the guys on ghc-devs, the changes were finally pushed upstream couple of hours ago. While I'm waiting for the maintainer to make a call on the official release, you can already get the changes if you compile GHC HEAD. For those not so eager about compiling HEAD, 7.8 should be coming out the changes will be in that. You can read the brief changelog at [1] and the updated documentation at [2]. We were careful to not make changes to any existing syntax that would result in a large amount of packages breaking. If you're a package maintainer, here are some things to consider: * If your documentation looks fine how it is now, you're probably fine. Read the changelog[1] as it mentions some issues that were fixed. Amongst others, Haddock will now link qualified function names properly so that's something to look out for. * none of your documentation should get parse failures: any previously-failing documentation should now be displayed. This means that if you have parse failures, now is the time to go and fix them up or your documentation probably won't look pretty. On an upside, this means that we won't have whole Hackage packages missing documentation due to a minor mistake in one of the comments. * You no longer need to break up lists of the same type with newlines. Previously you'd get two lists merged on a single line in an incoherent mess. Now it'll be rendered properly. I know that even GHC documentation is guilty of this mistake. * You can now escape the markup properly. Before, you were very limited on how the markup could be escaped. For example, <<foo ba\>r>> would result in a parse failure. * Bold markup added. The syntax is two underscores around what you want bold. Note, we still do not support multi-line markup. It's possible but it's a design choice not to support it. * You previously have gotten a document coverage report. We now have --print-missing-docs flag which will also tell you where the undocumented entities are: ``` ✓ misaki shana % haddock -h -U F.hs --print-missing-docs -o . Haddock coverage: 33% ( 2 / 6) in 'F' Missing documentation for: Baz (F.hs:5) Bar (F.hs:6) Foo (F.hs:7) add ``` * You can now nest paragraphs inside of lists. For most people this simply means that you can now have multiple levels of lists. We allow arbitrary nesting depth. The rules are: 4 spaces of indentation, new paragraphs have to be preceded by an empty line. ``` 1. Foo 1. Nested inside of Foo 2. Another thing inside of Foo >>> example inside of Foo example result 2. Back after Foo ``` * You can now have headings inside of your documentation rather than only for splitting up sections of your module. Example usage: ``` = Heading level one >>> some code example hello == Heading level two * Hello Haddock! ``` Up to 6 ‘=’s are currently supported. LaTeX back-end only honours up to three ‘=’s but it is not an error to have >3, they'll simply be treated as 3. * You can now display enabled module extensions in the generated documentation with {-# OPTIONS_HADDOCK show-extensions #-}. * Picture syntax now documented. * You can now have the title attribute for your hyperlinks. The syntax is <link title> I think that about covers it all. Perhaps I will write a more visual guide to the new changes but please don't hold your breath! Feel free to ask questions. If you have bugs to report or features to ask for, please head to the Haddock Trac[3]. [1]: http://www.haskell.org/haddock/CHANGES.txt [2]: http://www.haskell.org/haddock/doc/html/ [3]: http://trac.haskell.org/haddock -- Mateusz K.

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hoogle - find functions as arrows?
by Gregory Guthrie 27 Jan '14

27 Jan '14

I was surprised that a Hoogle search for: (a -> b) -> (a -> c) -> a -> (b, c) No results found Didn't find this: (&&&)<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Ar…> :: Arrow a => a b c -> a b c' -> a b (c, c')<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Ar…> Since functions are arrows; yes? I suppose I can see why it might not, but it seems that it would be nice if it did.

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evaluating CAFs at compile time
by Evan Laforge 27 Jan '14

27 Jan '14

So I have a large CAF which is expensive to generate. Theoretically it should be possible to totally evaluate at compile time, resulting in a bunch of constructor calls, ideally totally de-thunked and in the read-only segment of the binary. In the absence of a "eval at compile time" pragma, it seemed like TH should be able to do this, and searching haskell-cafe I turned up an old post by Wren where he is doing basically that, and eventually discovered the Lift class in http://hackage.haskell.org/package/template-haskell-2.8.0.0/docs/Language-H… However, if I understand Lift correctly (and not really understanding much of TH), you need to create instances for every type you wish to generate, which seems like it would be a pain. Even if they can be automatically derived, it would spread TH dependency gunk throughout the whole program. Is this true? Is there a library that does the equivalent of a "eval at compile time" pragma? (Wren's proposed QAF library seems to have never made it to hackage, but maybe given Lift and the proper instances it turns out to be trivial.) Would it be possible or desirable for an actual pragma that wouldn't introduce a TH dependency? Also, I assume it would produce a giant set of constructor applications which ghc would then optimize as well as it can... but it seems like that might not include full strictness, since even 'x = (4, undefined)' is obliged to not diverge as long as you don't look at the snd field, so even a large literal expression is actually unevaluated code if there are some non-strict data types in there. And... is it actually possible for ghc to do clever optimization with constant values, i.e. lay them out fully evaluated in read-only memory? I know that something like 'x = "abc" ++ "def"' will wind up as 'unpackCString# "abcdef"', but I'm curious what happens to more complicated data structures. Strictness seems to make a difference, e.g. with nonstrict fields the core has separate bindings for the contained values, while with strict ones the values get inlined directly into the consumer of the data type and the constructor is nowhere to be seen. But if the type is recursive (data X = X Int (Maybe X)), then we wind up with CAFs applying it, though they have lots of provocative flags that indicate ghc knows it's dealing with constructors: Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=True, ConLike=True, WorkFree=False, Expandable=True, Guidance=IF_ARGS [] 10 30}] I assume it still can't hoist the undefined to the entire expression though, because of non-strictness. I would think that if all data types are strict, then it could transform 'caf = X 42 (StrictJust (X 24 undefined))' to 'caf = undefined', but that doesn't seem to happen either. Tangentially, I've noticed that the 'unpackCString# "abcdef"' optimization is limited to String, replacing it with Text produces "abc" + giant wodge of code that is presumably appending "def" at runtime. I'm sure I've seen some discussions around here about wanting to optimize string literals to 'Text 0 len (giant chunk of binary data)', but I don't think they talked about possible compile time evaluation... presumably it could also solve that problem?

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Arithmetic overflow
by Rob Leslie 25 Jan '14

25 Jan '14

Greetings, I have a question about arithmetic overflow in Haskell -- actually, probably specifically in GHC. It’s my understanding from the Haskell 2010 Language Report that “The results of exceptional conditions (such as overflow or underflow) on the fixed-precision numeric types [e.g. Int] are undefined; an implementation may choose error (⊥, semantically), a truncated value, or a special value such as infinity, indefinite, etc.” In the documentation for Data.Int from the current release of the Haskell Platform, I read that “All arithmetic is performed modulo 2^n, where n is the number of bits in the type.” However, there seems to be at least one exception to this promise, namely: λ> (minBound :: Int) `quot` (-1) *** Exception: arithmetic overflow Is this a bug? More specifically, is it intended to be safe to rely on the modulo arithmetic behavior of GHC in spite of the Language Report? And if so, should the above witnessed behavior still be expected? Thanks, -- Rob Leslie rob(a)mars.org

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Re: [Haskell-cafe] Why is the Show instance of Text not used by showTerm?
by JP Moresmau 24 Jan '14

24 Jan '14

Karl, thanks, but the flag is set to True... And setting it to False doesn't change the output. I'll continue my investigations! JP On Fri, Jan 24, 2014 at 6:12 PM, Karl Voelker <karl(a)karlv.net> wrote: > On Fri, Jan 24, 2014, at 07:38 AM, JP Moresmau wrote: > > Hello cafe, I'm continuing my adventures with the GHC API. I'm following > the code given at > http://www.haskell.org/haskellwiki/GHC/As_a_library#Running_interactive_sta… run statements like in GHCi, but the behavior sometimes puzzles me. > > If I load a simple module: > > module Test where > import qualified Data.Text as T > > And then run the statement T.pack \"test\", I expect to see "test" back. > But I get Data.Text.Internal.Text _ 0 4. If I run "show $ T.Pack \"test\"" > I get the expected result. Text _ 0 4 seems to be the output of the > showText debugging function in Data.Text, that I don't see used in the Show > instance. And looking at the GHC code (Debugger.hs) it does perform a show > of the expression given. Why doesn't it work? What subtlety am I missing? > > > The example you linked to calls showTerm ( > http://www.haskell.org/ghc/docs/latest/html/libraries/ghc/src/Debugger.html…) > which seems to have two different paths depending on the > Opt_PrintEvldWithShow dynflag. One path appears to use show, and the other > does not. > > -Karl > -- JP Moresmau http://jpmoresmau.blogspot.com/

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