- Haskell-Cafe - Haskell.org

Job opening: further Haskell Developers at Anchor
by Andrew Cowie 08 Jul '14

08 Jul '14

Openings for additional Haskell developers... About Anchor ------------ Anchor Systems, based in Sydney Australia, has successfully been providing managed hosting and infrastructure to clients around the world for almost 15 years. The critical difference in the service we offer our customers often boils down to providing operations expertise and infrastructure configuration balancing the short term needs to get a new installation running with the long term imperatives of security, scalability, and flexibility. The engineering department is building the analytics capabilities, internal systems, cloud infrastructure, deployment tooling, and operations practices that will be needed to take the company to the next level. We have every intention of changing the hosting industry to make developing applications — and operating them at scale — easier for people facing massive growth. Haskell is the working language for all internal development, and we're exploring it for web facing code too. The benefits of type safety, resilience when refactoring, and stability over time added to the power of functional programming has already paid dividends. We have a significant programme of work ahead, and we're looking to add further capability as we face these challenges. Interested in helping? Requirements ------------ You'll need to have demonstrable experience programming in Haskell, familiarity with open source development practices, and the ability to tolerate other people refactoring your code. Exposure to the harsh realities of IT operations and experience of the thrill of Linux systems administration will stand you in good stead. Knowledge of the history, progress, and problems in configuration management will be helpful. A history of contributing to libraries on Hackage would be relevant. More relevant would be a track record of working well in the open (regardless of language or project). Good documentation is as important as good code (a type signature is not sufficient), and command of written English is necessary both to collaborate with your peers and to describe and promote your results. A Bachelor's in science, engineering, or xenobiology will be well thought of. Your specific field of study certainly doesn't matter, but how you use computing to solve problems in your chosen field does. A degree in engineering will be a distinct asset. An undergraduate degree in computer science will not be held against you. Applying -------- If you're interested, send a PDF copy of your CV to <jobs(a)anchor.com.au>, along with the URL to your GitHub account. We also invite you to include a pointer to something that shows you in a good light: a mailing list thread where you argued a contentious position, a piece of particularly thorny code in which you found an elegant solution to a problem, or even an essay or article you've written. We don't do international relocations (sorry everyone who wants to work remotely from Hawaii; I want to work remotely from Hawaii too), but we readily sponsor work visas for new hires should they need one. So whether you've just moved to Australia or have been here for generations, give us a shout next time you're in Sydney. AfC Sydney -- Andrew Frederick Cowie Head of Engineering Anchor Systems 201 Elizabeth Street Level 11, Sydney NSW 2000, Australia http://www.anchor.com.au | afcowie(a)anchor.com.au | +61 2 8296 5133

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strange behavior of GHC 7.8.2
by Kazu Yamamoto 08 Jul '14

08 Jul '14

Hi cafe, I noticed that strange behavior of GHC 7.8.2. Consider the following example which requires the "ghc" package: ---- {-# LANGUAGE GeneralizedNewtypeDeriving #-} module A where import GHC import MonadUtils import Control.Monad.Trans.RWS.Lazy (RWST(..)) newtype M a = M (RWST () () () IO a) deriving (Functor,Applicative,Monad,MonadIO) ---- The "ghc" library depends on "transformers" 0.3.0.0. If "transformers" *0.4.1.0* is NOT installed, GHCi can handle the code above well: ---- % ghci -package ghc A.hs ... Loading package transformers-0.3.0.0 ... linking ... done. ... Ok, modules loaded: A. [*A] > ---- However, if "transformers" *0.4.1.0* is installed, an error happens: ---- % cabal install transformers % ghci -package ghc A.hs ... Loading package transformers-0.3.0.0 ... linking ... done. ... [1 of 1] Compiling A ( A.hs, interpreted ) A.hs:10:49: No instance for (MonadIO (RWST () () () IO)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (MonadIO M) Failed, modules loaded: none. [Prelude] > ---- A you can see, "transformers" 0.3.0.0 is certainly linked. How can I interpret this behavior? If this is a bug of GHC 7.8.2, I will file this to GHC's trac. P.S. We noticed this because "doctest" of ghc-mod fails only for GHC 7.8.2. --Kazu

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withFile variant which takes a ByteString?
by Bardur Arantsson 08 Jul '14

08 Jul '14

Hi all, Is there a variant of withFile which takes a ByteString? (I'm aware that I could use the lower-level System.Posix FD-based functions, but if someone already has a package out there, I'd rather not.) Regards,

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Checking minimum package bounds
by Omari Norman 07 Jul '14

07 Jul '14

Often a topic of discussion is making sure that the upper bounds on one's packages are up to date, or even whether upper bounds are a good idea at all. I don't see much discussion on lower bounds. Lower bounds that are needlessly restrictive can get in the way too. For instance, I could just slap "text >= 1.1 && < 1.2" on my package, but this makes things needlessly hard for someone trying to make my package work in conjunction with an older one, especially if all I use are basic functions like "pack" that have been around at least since, say. text-0.11. Does anyone have a best practice for testing and verifying lower bounds? Testing the lower bounds and the upper ones is a challenge. Does anybody bother? I have my sunlight package at http://hackage.haskell.org/package/sunlight but it is really a nasty hack. I am considering cleaning it up so it is less of a nasty hack but before I do that I wondered if anyone else has ever thought about this problem. Thanks. Omari

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ML Family workshop: First Call for Participation
by oleg＠okmij.org 07 Jul '14

07 Jul '14

Higher-order, Typed, Inferred, Strict: ACM SIGPLAN ML Family Workshop Thursday September 4, 2014, Gothenburg, Sweden Call For Participation http://okmij.org/ftp/ML/ML14.html Early registration deadline is August 3. Please register at https://regmaster4.com/2014conf/ICFP14/register.php This workshop specifically aims to recognize the entire extended ML family and to provide the forum to present and discuss common issues, both practical (compilation techniques, implementations of concurrency and parallelism, programming for the Web) and theoretical (fancy types, module systems, metaprogramming). We also encourage presentations from related languages (such as Scala, Rust, Nemerle, ATS, etc.), to exchange experience of further developing ML ideas. The workshop is conducted in close cooperation with the OCaml Users and Developers Workshop http://ocaml.org/meetings/ocaml/2014/ taking place on September 5. Program * Andreas Rossberg 1ML -- core and modules as one (Or: F-ing first-class modules) * Jacques Garrigue and Leo White Type-level module aliases: independent and equal * Felix Klock and Nicholas Matsakis Demo: The Rust Language and Type System * Tomas Petricek and Don Syme Doing web-based data analytics with F# * Thomas Braibant, Jonathan Protzenko and Gabriel Scherer Well-typed generic smart-fuzzing for APIs * Ramana Kumar, Magnus O. Myreen, Michael Norrish and Scott Owens Improving the CakeML Verified ML Compiler * Leo White and Frederic Bour Modular implicits * Nada Amin and Tiark Rompf Implicits in Practice * Anil Madhavapeddy, Thomas Gazagnaire, David Scott and Richard Mortier Metaprogramming with ML modules in the MirageOS * Katsuhiro Ueno and Atsushi Ohori Compiling SML# with LLVM: a Challenge of Implementing ML on a Common Compiler Infrastructure * Akinori Abe and Eijiro Sumii A Simple and Practical Linear Algebra Library Interface with Static Size Checking * John Reppy SML3d: 3D Graphics for Standard ML In addition, the joint poster session with the OCaml workshop will take place in the afternoon on September 5. The session will include posters: * Nicolas Oury Core.Sequence: a unified interface for sequences * Thomas Gazagnaire, Amir Chaudhry, Anil Madhavapeddy, Richard Mortier, David Scott, David Sheets, Gregory Tsipenyuk, Jon Crowcroft Irminsule: a branch-consistent distributed library database * Michel Mauny and Benoit Vaugon Nullable Type Inference * Edwin Toeroek LibreS3: design, challenges, and steps toward reusable libraries * Fabrice Le Fessant A Case for Multi-Switch Constraints in OPAM Program Committee Kenichi Asai Ochanomizu University, Japan Matthew Fluet Rochester Institute of Technology, USA Jacques Garrigue Nagoya University, Japan Dave Herman Mozilla, USA Stefan Holdermans Vector Fabrics, Netherlands Oleg Kiselyov (Chair) University of Tsukuba, Japan Keiko Nakata Tallinn University of Technology, Estonia Didier Remy INRIA Paris-Rocquencourt, France Zhong Shao Yale University, USA Hongwei Xi Boston University, USA

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Control.Exception.bracket is broken(?)
by Yuras Shumovich 07 Jul '14

07 Jul '14

Hello, >From docs: "If an exception is raised, then bracket will re-raise the exception (after performing the release)." But what if the release action will throw an exception? There are 3 options: 1) suppress the exception from release action and rethrow the original one 2) shutdown the program 3) suppress the original exception and throw the exception from release action. Current behavior is #3 -- suppress the original exception. I think it the worse choice. What other languages do: - java has try-with-resources statement which is the nearest equivalent of haskell's "bracket". It uses the option #1 (rethrow the original exception) - in c++ it is common to use RAII for resource management, so someone's destructor performs resource cleanup. But if destructor throws exception during stack unrolling, the program is terminated (option #2) I prefer #2 because suppressing exception is a really bad idea. What is your choice? And the related thing. Release action should not use any interruptible action. Otherwise async exception can occur while processing other exception, so one of them will be suppressed. Is it correct? Control.Exception module contains a list of uninterruptible actions (see "Interruptible operations" section), and e.g. "hClose" is not there. Is "hClose" interruptible or not? If async exception interrupts "hClose", the file descriptor will leak. So, should we use uninterruptibleMask in release action for "bracket"? Thanks, Yuras

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how to turn on RTS options?
by Johan Larson 06 Jul '14

06 Jul '14

I need to change some runtime options. I updated the options line in my cabal file: ghc-options: -Wall -fno-warn-orphans -O2 -rtsopts But when I rebuild and run, it's as though runtime options are still disabled. C:\johan\Haskell\projects\bio-acai>cabal build .\bio-acai.cabal has been changed. Re-configuring with most recently used options. If this fails, please run configure manually. Resolving dependencies... Configuring bio-acai-0.1.0.0... Building bio-acai-0.1.0.0... Preprocessing executable 'acai' for bio-acai-0.1.0.0... Preprocessing test suite 'tests' for bio-acai-0.1.0.0... C:\johan\Haskell\projects\bio-acai>dist\build\acai\acai.exe +RTS -H128m -K64m *acai.exe: Most RTS options are disabled. Link with -rtsopts to enable them.* My best guess is that the "ghc-options" line is the wrong place to try to turn on RTS option handling. But what's the right one? -- Johan Larson -- Toronto, Canada

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[ANN] codex 0.1.0 - generate tags file from dependencies
by Alois Cochard 06 Jul '14

06 Jul '14

Hi, Just to let you know that I released a new version of `codex` (a tool which generate a tags[1] file for a given cabal project using the sources of all the dependencies of that project). This release include some bug fixes as well as two new features: - Support modules added using `cabal sandbox add-source` as workspace dependencies - Support `emacs` format by disabling sorting on merge of tags files Thanks to everyone who reported bugs or submitted patch! [1]Those tags file basically contain references to functions/types definition in source code and allow "jump to definition" like functionality in text editors. source: http://github.com/aloiscochard/codex hackage: http://hackage.haskell.org/package/codex -- *A\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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async simplify code
by grant weyburne 06 Jul '14

06 Jul '14

Hi Cafe, Is there any way to simplify this async code to somehow merge the withAsync code into the STM code? Looking at the code for tstABorCD it is not easy to see that it is really just (A&&B)||(C&&D). Here is the code: http://lpaste.net/106945 Thanks for any pointers, Grant

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Restarting doc build on hackage
by Erik Hesselink 05 Jul '14

05 Jul '14

Hi cafe, Does anyone know if there is a way to restart a documentation build on hackage? We have a package where the documentation failed to build due to a dependency lacking upper bounds. That has recently been fixed (using the new in-place cabal file editing feature), and we'd like to get documentation for our package now. Is there any way to do this (apart from uploading a new version without changes)? Erik

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Signal Processing Library
by Vivian McPhail 05 Jul '14

05 Jul '14

Hi, There is also a signal processing package on Hackage [1] which performs auto_correlation. FFT is has been moved from hmatrix to hmatrix-gsl [2]. Hope this helps. Cheers, Vivian [1] http://hackage.haskell.org/package/hsignal <http://hackage.haskell.org/package/hstatistics> [2] http://hackage.haskell.org/package/hmatrix-gsl <http://hackage.haskell.org/package/hstatistics> Date: Fri, 4 Jul 2014 10:19:51 -0700 > From: Jeremy Wright <jeremy(a)codestrokes.com> > To: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> > Subject: [Haskell-cafe] Signal Processing Library > Message-ID: <BAY179-W29288B298009D61A9BC5A2B9000(a)phx.gbl> > Content-Type: text/plain; charset="iso-8859-1" > > Hello: > > I'm working on my Master's Thesis, and I need a signal processing library > that can compute FFTs and autocorrelation on discrete data. Doesn't anyone > have experience with such a package in Haskell? I appreciate your time. > > Sincerely, > Jeremy Wright > >

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How to state semantics in denotational design?
by Hans Höglund 05 Jul '14

05 Jul '14

Dear all, This may seem a strange question due to my unfamiliarity with formal semantics, but bear with me. I am trying to apply denotational design principles to my library music-score, and often find myself writing down semantics of data structures in pseudo-Haskell such as > type Duration = Double > type Time = Point Duration > type Span = Time^2 > type Note a = (Span, a) The actual implementation of the data structures may or may not be identical to the semantics. For example, it makes sense to implement Span as (Time^2) or (Time x Duration), as these types are isomorphic. My question is basically: 1) Is my approach (using pseudo-Haskell) a sound way to state denotational semantics? 2) How can I state semantics (in code and/or documentation), and be sure that my implementation follow the semantics? I understand that the correctness of the implementation w.r.t. to the semantics can be verified using manual proofs, which worries me as I want to be able to refactor the semantics and be make sure that the implementation is still correct without having to repeat all the proofs. Is there a "trick" to encode the semantics in actual Haskell and have the type system and/or QuickCheck do the verification for me? Or am I misunderstanding the concept of denotational design? Sincerly, Hans - Hans Höglund Composer, conductor and developer hans [at] hanshoglund.se hanshoglund.com https://twitter.com/hanshogl https://soundcloud.com/hanshoglund http://github.com/hanshoglund

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Signal Processing Library
by Jeremy Wright 04 Jul '14

04 Jul '14

Hello: I'm working on my Master's Thesis, and I need a signal processing library that can compute FFTs and autocorrelation on discrete data. Doesn't anyone have experience with such a package in Haskell? I appreciate your time. Sincerely, Jeremy Wright

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New Functional Programming Job Opportunities
by Functional Jobs 04 Jul '14

04 Jul '14

Here are some functional programming job opportunities that were posted recently: Developer / DevOps Engineer at Klarna http://functionaljobs.com/jobs/8721-developer-devops-engineer-at-klarna Cheers, Sean Murphy FunctionalJobs.com

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How to deal with dependency bounds for an application
by Alois Cochard 04 Jul '14

04 Jul '14

Hey, I have wrote a little tool and some people had issues installing it with cabal and had to use "--reorder-goals" to be able to install it [1]. Obviously it's because I have very few dependency bounds (actually near none aside base). I don't have to be convince that it's a bad idea, the motivation for adding bounds are pretty clear... but... I'm not sure how to define them? Should I know what is the lowest possible combination of my dependencies who would compile... because if I'm too strict it might be difficult for some user to install? or maybe I should target as lowest what is in current haskell-platform? I'm looking for some advice and feedback from maintainers. Thanks in advance! [1]: https://github.com/aloiscochard/codex/issues/6 -- *A\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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Proposal: New mailing lists -- haskell-jobs & haskell-academia
by Alexander Berntsen 04 Jul '14

04 Jul '14

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 The proposal is simply as follows. 1. Make a list, haskell-jobs. This would be primarily for job postings and looking-for-job posts. A secondary purpose is discussing jobs. E.g. "Anyone know any jobs in the Foo area?", and meta-discussions like "How well does Haskell jobs pay in your experience?". 2. Make a list, haskell-academia. This would be primarily for studentship postings and call for papers. A secondary purpose is discussing academia. E.g. "Anyone know any unis that use Haskell extensively in the Foo area?", and meta-discussions like "How well is Haskell taught at unis in your experience?" The motivation for this is twofold. 1. We get a lot of job postings and studentship postings. Many of these will be ignored by readers, up until they suddenly find themselves looking for a job or studentship. It would be useful if they could then easily filter their email on lists -- or go to the online archives for that specific list. The latter is a big Win, because they needn't even be subscribed in the first place. 2. Job/studentship postings are effectively spam, unless you are actually looking for that kind of information. This proposal will reduce the amount of spam people get. That's always nice. Two weeks should be a satisfactory discussion period. - -- Alexander alexander(a)plaimi.net https://secure.plaimi.net/~alexander -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iF4EAREIAAYFAlO1W6MACgkQRtClrXBQc7Vu1wD/YSAY6AwqzBR74y7zF7OXjaHv 1PwRWglP/hIMOKeI5pAA/iIfvderfxrVLuVwG37ldQ3ar9Xb6+WG1zPfW3vW/PRv =zNvy -----END PGP SIGNATURE-----

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Type-level Nat to Integer
by Gautier DI FOLCO 04 Jul '14

04 Jul '14

Hi all, I'm trying to change type-level Nat to Integer as following: {-# LANGUAGE DataKinds, KindSignatures, GADTs, PolyKinds #-} data Nat = Z | S Nat class NatToInt n where natToInt :: n -> Int instance NatToInt Z where natToInt _ = 0 instance NatToInt (S n) where natToInt = 1 + natToInt (undefined :: n) I understand why it fails (Z and S have not the right kind). How to specify that NatToInt is Nat-specific? Moreover, if you have any advanced explanations/links which could give me a deeper understanding on what going on, I'll take them. Thanks in advance for your help.

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Making the Haskell 2010 report latex repo
by Christopher Done 04 Jul '14

04 Jul '14

The Haskell 2010 report is here: http://darcs.haskell.org/haskell2010-report Also mirrored in git here: git@github.com:hvr/haskell2010-report.git This version doesn't build because it uses old "Char" instead of "Data.Char"-style imports. So my version here: https://github.com/chrisdone/haskell2010-report Fixes that: https://github.com/chrisdone/haskell2010-report/commit/6a773abb7201f4854ba5… But that's as far as I get. Here is what happens when I run $ cd tools; make; cd .. $ cd report; make I get: http://lpaste.net/raw/5699097250156773376 As is typical in the LaTeX, a torrent of uninteresting information is spewed out. Finally, at the end, it says: > (./haskell.ind (./index-intro.tex) > ! Argument of \OT1\" has an extra }. > <inserted text> > \par > l.61 \item |hyperindexformat{\"} > , 51, 73, 107, 112 > ? Does anyone familiar with the language of LaTeX have any idea what's going on? Has anyone successfully been able to build it? If so, please state the exact steps to do so and I will be very grateful. Ciao!

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Fwd: pair hacking on the Platform this weekend?
by Mark Lentczner 03 Jul '14

03 Jul '14

Okay - My mind is rattling around like a cage against this codebase... I'm rattling around too many design decisions and feeling blue 'cause I'm writing shell scripts! I'm looking for another hacker in the SF Bay area this weekend to work with me on the Mac installation in particular, and the Platform build in general. A few hours any day would be much appreciated. Any takers? - Mark

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Declarative database migrations
by Timur Amirov 03 Jul '14

03 Jul '14

Hello! Looking for tools to use discovered that I miss something like Rails database migrations in haskell. Is there a package for it (describe the whole schema and/or particular migrations)? -- Timur Amirov Berlin, Germany

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Re: [Haskell-cafe] Specifying cabal sandbox flag for local package
by Vlatko Basic 03 Jul '14

03 Jul '14

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Re: [Haskell-cafe] Specifying cabal sandbox flag for local package
by Vlatko Basic 03 Jul '14

03 Jul '14

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Final Call for Papers: OCL 2014 Submissions Due in One Week
by Achim D. Brucker 03 Jul '14

03 Jul '14

(Apologies for duplicates) CALL FOR PAPERS 14th International Workshop on OCL and Textual Modeling Applications and Case Studies (OCL 2014) Co-located with ACM/IEEE 17th International Conference on Model Driven Engineering Languages and Systems (MODELS 2014) September 28-30 (tbc), 2014, VALENCIA, SPAIN http://www.software.imdea.org/OCL2014/ Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- metamodeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing applications and case studies of textual modeling as well as test suites and benchmark collections for evaluating textual modeling tools. Venue ===== The workshop will be organized as a part of MODELS 2014 Conference in Valencia, Spain. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, and 2013 in Miami. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Two types of papers will be considered: * short papers (6 pages) and * full papers (10 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://www.easychair.org/conferences/?conf=ocl2014) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Authors of selected papers will be invited to submit an extended version of their workshop paper to a special issue of the Electronic Communications of the EASST (http://journal.ub.tu-berlin.de/eceasst) Important Dates =============== Submission of papers: July 11, 2014 Notification: August 8, 2014 Workshop date: one day during September 28-30, 2014 Organizers ========== Achim D. Brucker, SAP AG, Germany Carolina Dania, IMDEA Software Institute, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Programme Committee (partly confirmation pending) =================== Michael Altenhofen, SAP AG, Germany Thomas Baar, University of Applied Sciences Berlin, Germany Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Fabian Buettner, Ecole des Mines de Nantes, France Achim D. Brucker, SAP AG, Germany Jordi Cabot, INRIA-Ecole des Mines de Nantes, France Yoonsik Cheon, University of Texas, USA Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Madrid, Spain Carolina Dania, IMDEA Software Institute, Madrid, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Atos Research, Madrid, Spain Geri Georg, Colorado State University, Fort Collins, Colorado, USA Martin Gogolla, University of Bremen, Germany Pieter Van Gorp, Eindhoven University of Technology, The Netherlands Heinrich Hussmann, LMU Munchen, Germany Tihamer Levendovszky, Vanderbilt University, USA Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Shane Sendall, Snowie Research SA, Switzerland Michael Wahler, ABB Switzerland Ltd Corporate Research, Switzerland Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ Paris-Sud, France Steffen Zschaler, King’s College, London, UK -- Dr. Achim D. Brucker, SAP AG, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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Haskell Weekly News: Issue 298
by Daniel Santa Cruz 03 Jul '14

03 Jul '14

Welcome to issue 298 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from June 15 to 28, 2014 Quotes of the Week * Kinnison * imagines a radio station playing only things like 'Life tru a Lens', stuff by 'Dire States' or 'Monadonna' Top Reddit Stories * Today I published an introductory book on Haskell Data Analysis Domain: haskelldata.com, Score: 99, Comments: 26 Original: [1] http://goo.gl/R2kRFu On Reddit: [2] http://goo.gl/Ka0oP5 * Backpack: An ML-like module system for Haskell Domain: plv.mpi-sws.org, Score: 76, Comments: 44 Original: [3] http://goo.gl/7Zkxbg On Reddit: [4] http://goo.gl/78H37f * Cgrep, a context-aware grep for source code. Domain: awgn.github.io, Score: 67, Comments: 13 Original: [5] http://goo.gl/q1VdEA On Reddit: [6] http://goo.gl/2NNTrO * Teenage Haskell Domain: twdkz.wordpress.com, Score: 63, Comments: 7 Original: [7] http://goo.gl/cvJ0ap On Reddit: [8] http://goo.gl/aISHlG * GHC 7.10 gains -XBinaryLiterals language syntax extensions Domain: github.com, Score: 63, Comments: 14 Original: [9] http://goo.gl/qfWvAg On Reddit: [10] http://goo.gl/NOVgme * Haskelier - Haskell for the Advanced Layman Domain: haskelier.tumblr.com, Score: 59, Comments: 26 Original: [11] http://goo.gl/pLsRB1 On Reddit: [12] http://goo.gl/5HBSWD * Barely Functional: Writing a Real Program in Haskell Domain: ben.kirw.in, Score: 57, Comments: 36 Original: [13] http://goo.gl/4yWLuL On Reddit: [14] http://goo.gl/B1LX8f * hmatrix 0.16 now BSD3 and with static dimension checking Domain: github.com, Score: 51, Comments: 10 Original: [15] http://goo.gl/C5zKzk On Reddit: [16] http://goo.gl/A94AkD * Formatting: type-safe printf-like library by chrisdone Domain: hackage.haskell.org, Score: 51, Comments: 24 Original: [17] http://goo.gl/DHM0Yn On Reddit: [18] http://goo.gl/zUQN02 * Haskell 2014 accepted papers, with links (pull requests welcome) Domain: github.com, Score: 50, Comments: 5 Original: [19] http://goo.gl/B2f434 On Reddit: [20] http://goo.gl/XSmJGo * Real World Haskell - Outdated Parts? Domain: self.haskell, Score: 50, Comments: 12 Original: [21] http://goo.gl/5YhIOT On Reddit: [22] http://goo.gl/5YhIOT * I'm trying my hardest to learn this language and only get more and more discouraged. Domain: self.haskell, Score: 48, Comments: 111 Original: [23] http://goo.gl/9vuPBe On Reddit: [24] http://goo.gl/9vuPBe * Using f-algebras to produce a statically typed functional programming language Domain: burz.github.io, Score: 47, Comments: 19 Original: [25] http://goo.gl/9YCz4k On Reddit: [26] http://goo.gl/DvWBPS * 'ghc-make' released Domain: neilmitchell.blogspot.de, Score: 47, Comments: 19 Original: [27] http://goo.gl/JlMS1a On Reddit: [28] http://goo.gl/kO2FQP * A simple alternative to De Bruijn indexing, from ICFP 2013 Domain: pchiusano.io, Score: 46, Comments: 24 Original: [29] http://goo.gl/7PBIM2 On Reddit: [30] http://goo.gl/PGmQ6W * Haskoin: A Haskell implementation of the Bitcoin protocol. Domain: github.com, Score: 44, Comments: 0 Original: [31] http://goo.gl/n0eCEI On Reddit: [32] http://goo.gl/bvsE24 * What is the state of "The JavaScript Problem"? What is the currently preferred way to solve in a real world application? Domain: self.haskell, Score: 42, Comments: 55 Original: [33] http://goo.gl/dIww4i On Reddit: [34] http://goo.gl/dIww4i * ANNOUNCE: STM-Containers. A hash map and hash set for STM Domain: hackage.haskell.org, Score: 41, Comments: 13 Original: [35] http://goo.gl/6nuWAc On Reddit: [36] http://goo.gl/rAzIYr Top StackOverflow Questions * Why does flooring infinity does not throw some error? votes: 14, answers: 1 Read on SO: [37] http://goo.gl/f7gUB8 * How long can the name of a type constructor be? votes: 12, answers: 1 Read on SO: [38] http://goo.gl/mGiblp * Difference between $ and () votes: 11, answers: 5 Read on SO: [39] http://goo.gl/lTB24e * Need advice on optimising Haskell data processing votes: 11, answers: 3 Read on SO: [40] http://goo.gl/NnEqgn * Reconciling lens usage with database access votes: 11, answers: 1 Read on SO: [41] http://goo.gl/Zczjjf * Why constraints on data are a bad thing? votes: 11, answers: 3 Read on SO: [42] http://goo.gl/l0WXtP * Can I use template haskell to define missing functions? votes: 10, answers: 2 Read on SO: [43] http://goo.gl/AFFS2T * Is there a reason we can't populate types with DataKinds? votes: 10, answers: 2 Read on SO: [44] http://goo.gl/1gpZej * Why is a built-in function applied to too few arguments considered to be in weak head normal form? votes: 10, answers: 2 Read on SO: [45] http://goo.gl/6iFG8a Until next time, [46]+Daniel Santa Cruz References 1. http://haskelldata.com/ 2. http://www.reddit.com/r/haskell/comments/296l80/today_i_published_an_introd… 3. http://plv.mpi-sws.org/backpack/ 4. http://www.reddit.com/r/haskell/comments/28v6c9/backpack_an_mllike_module_s… 5. http://awgn.github.io/cgrep/ 6. http://www.reddit.com/r/haskell/comments/28moo0/cgrep_a_contextaware_grep_f… 7. http://twdkz.wordpress.com/2014/06/26/teenage-haskell/ 8. http://www.reddit.com/r/haskell/comments/29abyz/teenage_haskell/ 9. https://github.com/ghc/ghc/commit/1c0b5fdc9f2b6ea8166cc565383d4cd20432343c 10. http://www.reddit.com/r/haskell/comments/29b2jj/ghc_710_gains_xbinarylitera… 11. http://haskelier.tumblr.com/ 12. http://www.reddit.com/r/haskell/comments/28d7ma/haskelier_haskell_for_the_a… 13. http://ben.kirw.in/2014/06/24/barely-functional-1-rlp/ 14. http://www.reddit.com/r/haskell/comments/28zwoc/barely_functional_writing_a… 15. https://github.com/albertoruiz/hmatrix 16. http://www.reddit.com/r/haskell/comments/28peug/hmatrix_016_now_bsd3_and_wi… 17. http://hackage.haskell.org/package/formatting 18. http://www.reddit.com/r/haskell/comments/291wel/formatting_typesafe_printfl… 19. https://github.com/yallop/haskell2014-papers 20. http://www.reddit.com/r/haskell/comments/28yu1f/haskell_2014_accepted_paper… 21. http://www.reddit.com/r/haskell/comments/2938yb/real_world_haskell_outdated… 22. http://www.reddit.com/r/haskell/comments/2938yb/real_world_haskell_outdated… 23. http://www.reddit.com/r/haskell/comments/288egd/im_trying_my_hardest_to_lea… 24. http://www.reddit.com/r/haskell/comments/288egd/im_trying_my_hardest_to_lea… 25. http://burz.github.io/2014/06/15/feval.html 26. http://www.reddit.com/r/haskell/comments/28rbwr/using_falgebras_to_produce_… 27. http://neilmitchell.blogspot.de/2014/06/announcing-ghc-make.html 28. http://www.reddit.com/r/haskell/comments/28tttf/ghcmake_released/ 29. http://pchiusano.io/2014-06-20/simple-debruijn-alternative.html 30. http://www.reddit.com/r/haskell/comments/28vfd0/a_simple_alternative_to_de_… 31. https://github.com/haskoin/haskoin 32. http://www.reddit.com/r/haskell/comments/28y454/haskoin_a_haskell_implement… 33. http://www.reddit.com/r/haskell/comments/28o7my/what_is_the_state_of_the_ja… 34. http://www.reddit.com/r/haskell/comments/28o7my/what_is_the_state_of_the_ja… 35. http://hackage.haskell.org/package/stm-containers 36. http://www.reddit.com/r/haskell/comments/295lbr/announce_stmcontainers_a_ha… 37. http://stackoverflow.com/questions/24338673/why-does-flooring-infinity-does… 38. http://stackoverflow.com/questions/24335869/how-long-can-the-name-of-a-type… 39. http://stackoverflow.com/questions/24271129/difference-between-and 40. http://stackoverflow.com/questions/24278006/need-advice-on-optimising-haske… 41. http://stackoverflow.com/questions/24462070/reconciling-lens-usage-with-dat… 42. http://stackoverflow.com/questions/24465586/why-constraints-on-data-are-a-b… 43. http://stackoverflow.com/questions/24233346/can-i-use-template-haskell-to-d… 44. http://stackoverflow.com/questions/24439618/is-there-a-reason-we-cant-popul… 45. http://stackoverflow.com/questions/24447324/why-is-a-built-in-function-appl… 46. https://plus.google.com/105107667630152149014/about

1 0

GLUT under Windows using ghc 7.8.2?
by Dominick Samperi 03 Jul '14

03 Jul '14

It appears that The Haskell Platform includes libglut32.a and other important libs, so it is self-contained, provided users can work with the version of ghc that is provided: currently 7.6.3 under Windows. In particular, 'cabal install glut' works fine with the default version of ghc. But if I try using ghc-7.8.2 I get: Configuring GLUT-2.5.1.1... cabal: Missing dependencies on a foreign library: * Missing C library: glut32 ... All of the following do not change the result: 1. Adding --extra-lib-dirs=c:\glut-3.7.6-bin to the command line. 2. Using freeglut instead of glut 3. Copying libglut.a from The Haskell Package mingw/lib directory to c:\ghc-3.8.2\mingw\lib (dangerous because I am mixing architectures here). How does cabal decide that the C library glut32 is missing? Where is it looking? Why doesn't it look in the places I specify? Thanks, Dominick

1 1

handling non-string command line arguments
by Johan Larson 02 Jul '14

02 Jul '14

I've been looking at choices for parsing command line arguments, including getOpt. The examples I can find focus on string arguments, whereas I am interested in numbers. In the application at hand, it is particularly important to issue clear error messages when arguments don't parse as numbers or are out of range. I could use getOpt as a first pass with string arguments and then turn the strings into validated numbers in a second pass, but that's a bit awkward. Alternately I could use the options records with Options -> IO Options functions. But both of these solutions treat type mismatches differently from other options errors. Has anyone found a cleaner solution? -- Johan Larson -- Toronto, Canada

6 6

Problem with type in a function
by Leza Morais Lutonda 02 Jul '14

02 Jul '14

Hi, I have the following function plotf :: Ploteable a => ([a] -> IO ()) -> (a -> a) -> [a] -> IO () plotf plot fn xs = plot $ map fn xs that compiles correctly, but when I use it with: plotf win sin [1..100::Double] -- where win :: [Double] -> IO () I'm geting the error: No instance for (Ploteable Double) arising from a use of ‘plotf’ In the expression: plotf win sin [1 .. 100 :: Double] In an equation for ‘it’: it = plotf win sin [1 .. 100 :: Double] What am I missing? Thanks! 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

2 2

haskell in several upcoming live performances in the UK
by Renick Bell 02 Jul '14

02 Jul '14

Hello everyone. I hope the Haskell Cafe will tolerate this musical interruption. I suppose it's not so often that you can hear two different performers use Haskell in their performances in one concert, but if you live in the UK you have several opportunities to do so in the next few days. Both Alex McLean and I will be playing at Algorave NIME on Thursday night in London at Corsica Studios. The event starts at 21:00. Alex will be playing in the duo Canute with Matthew Yee-King, and I will be playing a solo set. http://www.nime2014.org/public-events/concerts-and-installations/ After that, it's the UK algorave tour, with stops in Brighton (07/04), Sheffield (07/06), Leeds (07/07), Manchester (07/08), and York (07/09): http://algorave.com/ Both Alex and I will perform in Sheffield and Manchester. I will be playing all of the above dates. See Alex fling Haskell in Emacs and me in vim, tmux, and xmonad! If you haven't heard Alex's music recently, he did a great track the other day: https://soundcloud.com/yaxu/at-last There's this set he did in Antwerp in May: https://www.youtube.com/watch?v=fgRDKxkHCSw My sets are likely to resemble this one that I did last Friday in Portugal: https://soundcloud.com/renick/live-at-xcoax-2014-porto-portugal-june-27-2014 Or this one from the Linux Audio Conference in May: https://soundcloud.com/renick/live-at-the-linux-audio-conference-2014-karls… If you come to one of the performances, please come up and say hello. I really want to meet the UK Haskell crew! Hope to see you there, dancing to Haskell or just relaxing and enjoying the music. Best, Renick -- Renick Bell - http://renickbell.net - http://twitter.com/renick - http://the3rd2nd.com

1 0

use of -Werror
by Johan Larson 02 Jul '14

02 Jul '14

Is there a consensus within the Haskell community on the use of -Werror in packages uploaded to Cabal? Most places where I've worked, building with -Werror has been considered a laudable ideal, but fixing all the old warnings in the codebase was too much work, so it wasn't done. I suppose -Werror could cause problems in heterogeneous build environments. -- Johan Larson -- Toronto, Canada

7 12

Haskell Wiki user account creation
by Haitham Gad 01 Jul '14

01 Jul '14

Hi, I read the following note at the log-in page of haskell wiki: NOTE: Automatic wiki account creation has been disabled. If you would like an account please email "nominolo" (at the email service from Google) or on the haskell-cafe mailing list. Does anyone know the exact email address I should be sending to (is it nominolo(a)gmail.com)? Thanks, Haitham

4 6

Need help - my haskell code is over 50 times slower than equivalent perl implementation
by C K Kashyap 01 Jul '14

01 Jul '14

Dear cafe, I have created a reproducible app here - https://github.com/ckkashyap/haskell-perf-repro Essentially I am trying to open a number of files and printing out their sized by reading them in and computing it's length. I have the equivalent perl program also there. I'd appreciate it very much if someone could take a look at my Haskell implementation and point out what I am doing wrong. Since it's over 50 times slower than the perl code, I assume I am doing something obviously incorrect. Regards, Kashyap

10 23

Algorithm for labeling same-colored regions in a 2D array?
by Steve Trout 01 Jul '14

01 Jul '14

(Apparently joining the list through the Google Groups interface doesn't work, but the post still shows up there. Sorry if you see this twice!) For fun, I'm writing a rules engine for the board game Go (in Haskell obviously). I have a board, which is a grid of spaces that can contain a black piece, a white piece, or nothing. Part of what i need to do is to detect regions of the same "color" (black, white, or empty) and get information about them like their size, the color of the bordering spaces, etc. Regions are bounded by other colors or the board edges in cardinal directions. I'm just starting on this so I haven't picked my data structures yet, but Vector (Vector _) or Map (Int, Int) _ are what I've toyed with. I have a vague idea of how I would go about this in an imperative language with mutation. I'd iterate over the board, keeping lists of points contained in the same region, combining (relabeling) regions if they happen to connect later on. (I realize there are probably better algorithms in those languages, but that would be my first attempt.) I know i could cobble something like this together in the State monad, but I have a feeling that there ought to be a better (more "Haskellish"/pure-functional) way. What would a Haskellish algorithm for this kind of thing look like? I wouldn't mind a complete answer or just a nudge in the right direction. Thanks, -Steve

3 2

Making new Unboxed Algebraic Types? (A Repa Problem)
by Hans Georg Schaathun 30 Jun '14

30 Jun '14

Hi, could someone offer som advice on handling composite types with the Repa library? I can see a /long/ way around my problem, but I wonder what is the /best/ way ... The problem is that I am trying to parallellise a probabilistic algorithm using Repa. To handle pseudo-randomness in parallel, I couple each data element with a generator state, in a tuple. The generator state is of type TFGen (tf-random package) or StdGen¹ (random package), neither of which is an Unbox instance. Hence, I have been using boxed repa arrays. The code does not execute in parallel, and if I understand correctly this is because of the Boxed type. There is a workaround², but that requires an NFData instance. To make a NFData instance and work through the code to support it should be as simple as it is tedious, but it would still leave the other drawbacks of a boxed type. (In particular, I do sequential fold because the Repa fold function did not support boxed types.) I could of course rewrite the published libraries to use tuples instead of algebraic types, and benefit from the pre-made Unbox instances, but sounds messy and error-prone (as well as boring). If I have to I have to. Is there a quick and robust way to make Unbox instances from composite datatypes? Or any other advice on how to approach the matter? Both random ideas, references, and concrete solutions are welcome. [The other element of the tuple is an vector of unboxed constituent type. I am not sure how that's going to work, but I'll cross that bridge when I get there. For the time being it is the generator that stops type checking.] TIA ¹ Yes, I know that StdGen is statistically unsound. ² http://stackoverflow.com/questions/16097418/parallel-repa-code-doesnt-creat… -- :-- Hans Georg

4 4

Monad laws
by Rafael Almeida 30 Jun '14

30 Jun '14

Hello, You guys have been great taking my questions. Thank you. Now I have another one :) What can go wrong if I make an Monad instance but don't follow Monad rules (identity and associativity)? Sure it would be misleading for someone using the non-conforming class. They may make code that assume those laws, although they don't hold. However, could it make the compiler generate bad code or some standard functions to behave badly or even break? []'s Rafael

10 19

Re: [Haskell-cafe] Making new Unboxed Algebraic Types? (A Repa Problem)
by Hans Georg Schaathun 30 Jun '14

30 Jun '14

On Tue, Jul 01, 2014 at 01:43:32AM +0400, Dmitry Dzhus wrote: > This implies that the amount of generator states used in your parallel > computation is equal to data array size N. This is not the case in > general (unless the computation is split into N processes running in > parallel by the scheduler). On the contrary, commonly you would even > want to minimize the amount of generator states used (for resource > efficiency reasons). Fair point. The large number of states may be a disadvantage. > Generator states may be implicitly passed into sequential > subcomputations (scheduled by the parallel planner). This of course > calls for some hacking on Repa harness that runs the computations in > parallel (computeP). Such a solution would probably not be determinstic, and it would definitely make the output depend on the run-time environment (such as the number of threads). I am interested in the particular problem of pseudo-random, /deterministic/, parallel programs. I think the large number of generator states is necessary to achieve deterministic, parallel behaviour. If someone knows otherwise, I would be curious to learn. > Some time ago I was also struggling find a way to parallelize a > probabilistic algorithm (Monte-Carlo molecular simulation) and > eventually I settled with even simpler solution based on strategies > instead of Repa: > https://github.com/dzhus/dsmc/blob/master/src/Control/Parallel/Stochastic.hs. > It can be easily ported to any stateful mapping computation that needs > parallelization (including your PRNGs of choice). See the whole package > for usage examples. Thanks a lot for the pointer. I shall have a look. In fact I am curious to learn about strategies as well. (Contrary to low-level hacking of Repa features which I am not at all curious about :-) -- :-- Hans Georg

1 0

ANNOUNCE: stm-containers
by Nikita Volkov 30 Jun '14

30 Jun '14

A post with introduction to the library, motivation behind it and its performance analysis: http://nikita-volkov.github.io/stm-containers/ Hackage: http://hackage.haskell.org/package/stm-containers GitHub: https://github.com/nikita-volkov/stm-containers

3 2

Re: [Haskell-cafe] [Haskell] I've just published an introductory book on Haskell Data Analysis
by Henning Thielemann 30 Jun '14

30 Jun '14

Am 30.06.2014 19:24, schrieb Heinrich Apfelmus: > The word "blog" was hyperlinked to a blog post of mine [1] which has > absolutely nothing to do with finance or data analysis. I don't know > how, on the basis of this evidence, Packt came to the conclusion that > I would have any expertise in "Haskell financial data modelling and > predictive analysis". The blog post contains a screenshot showing Dollar and Euro amounts. FRP might be "Financial Rewarding Programming". It matches the scope of the book pretty much, doesn't it? :-)

1 0

ANN: rest
by Adam Bergmark 30 Jun '14

30 Jun '14

We at Silk are proud to announce the release of our Haskell REST framework `rest'. Please see the related blog post for more information: http://engineering.silk.co/post/90354057868/announcing-rest-a-haskell-rest-… Cheers, Adam

2 2

Testing web-interfacing applications
by Mateusz Kowalczyk 30 Jun '14

30 Jun '14

Hi, Whenever I write a program which has to interface with the web (scraping, POSTing, whatever), I never know how to properly test it. What I have been doing up to date is fetching some pages ahead of time, saving locally and running my parsers or whatever it is I'm coding at the moment against that. The problem with this approach is that we can't test a whole lot: if we have a crawler, how do we test it it goes to the next page properly? Testing things like logging in and such seems close to impossible, we can only test if we are making a good POST. Let's stick to a crawler example. How would you test that it follows links? Do people set up local webservers with few dummy pages they download? Do you just inspect that GET and POST ‘look’ correct? Assume that we don't own the sites so we can't let the program run tests in the wild: page content might change (parser tests fail), APIs might change (unexpected stuff back), our account might be locked (guess they didn't like us logging in 20 times in last hour during tests) &c. Of course there is nothing which can prevent upstream changes but I'm wondering how we can test the more-or-less static stuff without calling out into the world. -- Mateusz K.

2 1

Munich Haskell Meeting
by Heinrich Hördegen 30 Jun '14

30 Jun '14

Dear all, today again is out monthly Haskell Meeting in Munich. I completly forgot inviting you! Sorry! Today at 19h30, we will meet here: http://www.max-emanuel-brauerei.de/de/Kontakt.html I will try to reserve a table for 10 to 12 persons. If the weather is fine, it will be outside. Please be early, because I cannot defend the table very long if nobody shows up! Cheers, Heinrich

1 0

Conceptualizing Functor vs Performance
by Yuri Lensky 30 Jun '14

30 Jun '14

I am having trouble consolidating a performance-based data representation decision I would like to make against the concept of a "functor". The question can be reduced to the following: Conceptually I think that Data.Vector /should/ be a functor. More specifically, I see no CONCEPTUAL reason that it shouldn't be possible to define some "efficient" container-like object (for example for fast Matrix operations for unboxable numbers, but something that still works for a symbolic variable type) that chooses to represent its data as a Data.Vector.Unboxed if possible, but chooses some more generic structure (perhaps a strict list or a generic Data.Vector) if that is not the case. I haven't found such a functor, and can't seem to implement it on my own. The point is that in theory there is no true constraint on the type of the object being contained so it SHOULD be a Functor/Traversable/etc., the representation simply changes to something more efficient only if possible, and decays to a more generic type gracefully if necessary. Perhaps there is some type hackery that can be done with TypeFamilies? I.E. define some container type "data C a = (ListType a) a", but I haven't found a generic way to do this. Finally, I understand that "efficient" data structure can be problem-dependent, but I have no problem defining many different such "generic" types for different applications. The simplest example would be something like (invalid Haskell): (Data.Vector.Unboxed.Vector v a) => data instance (ListType a) = v (Data.Vector.Generic.Vector v a) => data instance (ListType a) = v data instance (ListType a) = ([]) Of course implementing fmap is a whole separate issue. The point is I do not think this is the right way/possible, but am wondering what the "true" solution is. Thanks.

2 2

Smallcheck Depth
by Omari Norman 30 Jun '14

30 Jun '14

The docs for smallcheck, and the original paper, say that the depth is the depth of nested constructor applications. For some reason this behavior has changed in newer versions of Smallcheck and maybe someone can shed some light on why. For instance, I would think the depth of True and False is 0, as there are no nested constructors. Sure enough, in Smallcheck 0.2.1, (series 0) :: [Bool] gives [True, False]. Yet in smallcheck 1.1.1, (list 0 series) :: [Bool] gives []. Thanks. Omari

2 1

Implementing parental using blood type
by Rafael Almeida 29 Jun '14

29 Jun '14

Hello, I'm trying to practice my Haskell. So I had the idea of making a program which, given a mother's and a child blood type, it can determine whether a certain father is possible or not. Please take a look on this gist with my implementation: https://gist.github.com/aflag/14429dfb2e89791a44e2#file-parentaltesting Would you care to comment on it? It looks a bit cumbersome to my eyes. I'm trying to find the most intuitive and elegant way of do it. I'm not so much worried with performance. []'s Rafael

4 6

Requesting maintainership: bzlib-conduit and process-conduit
by Michael Snoyman 29 Jun '14

29 Jun '14

I'd like to request to be granted maintainership of two conduit-related packages: bzlib-conduit and process-conduit. Both packages are maintained by the same author. I opened up a pull request[1] on bzlib-conduit back in January, and haven't heard back yet. There's also an issue with process-conduit[2] which I'd like to address, but given the lack of response of bzlib-conduit, haven't sent a pull request yet. I also emailed the author directly five days ago regarding the bzlib-conduit pull request, and have not received a response. [1] https://github.com/tanakh/bzlib-conduit/pull/7 [2] https://github.com/tanakh/process-conduit/issues/14

3 4

HSpec vs Doctest for TDD
by Gautier DI FOLCO 29 Jun '14

29 Jun '14

Hi all, I'm a huge fan of TDD (Test-Driven Development) et I used to use tools such as RSpec (Ruby). So naturally, I looked to HSpec, but it seems not idiomatic in Haskell. I have a bunch of questions: - Do you do TDD? - Which tools do you use? - Is doctest "better" (in some senses) than HSpec? Why? - Are HSpec and Doctest complementary? If so, in which case do you use one or another? - Is there some Haskell-specific good practices do to TDD? Thanks in advance for your lights.

23 39

Empty case alternative
by Frantisek Farka 28 Jun '14

28 Jun '14

Hello cafe, I was looking into Haskell Report 2010 and notice something in the definition of case statement: https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.… Apparently an empty alternative within case statement is a plausible production. ('alt' production rule, third option). If I get it correctly, following example shows the empty alternative: > > cStmt e = case e of { 'a' -> True ; {-- empty here --} ; 'b' -> False } > What I miss is any ratio behind this. What is the empty alternative good for? Any idea? Franta

3 2

Moderators, please block zhangliu529@sina.com
by Nikita Volkov 28 Jun '14

28 Jun '14

Every time I send a message to Cafe I get an autoreply from this user with the following text: 您发的邮件我已收到，我将尽快给您答复。 It's extremely annoying. Please, exclude this user from the mailing list.

1 0

"Generating" type synonyms without Template Haskell
by Tom Ellis 27 Jun '14

27 Jun '14

I'd like to know how to conveniently generate specialisations of a product type without resorting to Template Haskell. Concretely, I have a type like data MyProduct a b c = MyProduct { foo :: a , bar :: b , baz :: c } and I use it in type signatures in various different specialisations. Generally the "base" type of each component stays fixed but it is wrapped in zero or more type constructors. For example a. MyProduct Int Bool String b. MyProduct (Maybe Int) (Maybe Bool) (Maybe String) c. MyProduct [IO Int] [IO Bool] [IO String] d. MyProduct (P Int) (P Bool) (P String) for various P that are not always functors in my uses. I thought I might be able to reduce this duplication by parametrisation, that is data MyProduct f a b c = MyProduct { foo :: f a , bar :: f b , baz :: f c } However, I can't always substitute a type constructor for `f` because that doesn't work for cases a. or c.. I thought a type family might work but it seems they have to be fully applied, like type synonyms. Is there an approach to this problem that will allow me to avoid Template Haskell here? Thanks, Tom

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POSIX capabilities support
by Vasili I. Galchin 27 Jun '14

27 Jun '14

Hello Haskellers, I have looked through the various POSIX hack age modules to see if there is capabilities support. ??? Regards, Vasya

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ICFP 2014 Call for Participation
by David Van Horn 26 Jun '14

26 Jun '14

[ Please note that much of the block reservation of hotel rooms currently being held for ICFP participants will be released next week. The beginning of September is a very busy conference week in Göteborg, so there is high pressure on hotel rooms in that period. If you plan to attend ICFP 2014, it would be best to make your hotel reservations now before the block reservation expires. ] ===================================================================== Call for Participation ICFP 2014 19th ACM SIGPLAN International Conference on Functional Programming and affiliated events August 31 - September 6, 2013 Gothenburg, Sweden http://icfpconference.org/icfp2014/ ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: 1 conference, 1 symposium, 10 workshops, tutorials, programming contest results, student research competition * Accepted Papers: http://www.icfpconference.org/icfp2014/accepted.html * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2014/local.html * Registration is available via: https://regmaster4.com/2014conf/ICFP14/register.php Early registration is due 3 August, 2014. * Programming contest, 25-28 July, 2014: http://icfpcontest.org/ * Follow @icfp_conference on twitter for the latest news: http://twitter.com/#!/icfp_conference There are several events affiliated with ICFP: Sunday, August 31 ACM SIGPLAN Workshop on Generic Programming ACM SIGPLAN Workshop on Higher-order Programming with Effects Monday, September 1 – Wednesday, September 3 ICFP Thursday, September 4 ACM SIGPLAN Commercial Users of Functional Programming: Day 1, Tutorials ACM SIGPLAN Haskell Symposium: Day 1 ACM SIGPLAN Workshop on Functional High-Performance Computing ACM SIGPLAN ML Family Workshop Friday, September 5 ACM SIGPLAN Commercial Users of Functional Programming: Day 2, Tutorials ACM SIGPLAN Haskell Symposium: Day 2 ACM SIGPLAN OCaml Workshop ACM SIGPLAN Erlang Workshop ACM SIGPLAN Workshop on Haskell and Rewriting Techniques Saturday, September 6 ACM SIGPLAN Commercial Users of Functional Programming: Day 3, Talks ACM SIGPLAN Haskell Implementors Workshop ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Conference Organizers General Chair: Johan Jeuring, Utrecht University Program Chair: Manuel Chakravarty, University of New South Wales Local Arrangements Chair: Björn von Sydow, Chalmers University Industrial Relations Chair: Anil Madhavapeddy, University of Cambridge Workshop Co-Chairs: Tom Schrijvers, Ghent University Sam Tobin-Hochstadt, Indiana University Programming Contest Co-Chairs: Duncan Coutts, Well Typed LLP Nicolas Wu, University of Oxford Student Research Competition Chair: Meng Wang, Chalmers University Publicity Chair: David Van Horn, University of Maryland Video Chair: Iavor Diatchki, Galois Malcolm Wallace, Standard Chartered Bank Industrial partners: Platinum partners Jane Street Capital Gold partners Google Microsoft Research Mozilla Oracle Labs Standard Chartered Bank Silver partners Bloomberg Credit Suisse CyberPoint Erlang Solutions Facebook Galois Klarna Lexifi Twitter Bronze partners IntelliFactory Opera Software QuviQ =====================================================================

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