February 2014 - Haskell-Cafe

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 06 Feb '14

06 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 05 Feb '14

05 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 05 Feb '14

05 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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