October 2016 - Haskell-Cafe

conference rating for Haskell Symposium
by Kazu Yamamoto 18 Oct '16

18 Oct '16

Hello, I would like to know a reasonable conference rating for Haskell Symposium. Many rating pages have only Haskell Workshop. For instance, it is categorized as Rank C: http://lipn.univ-paris13.fr/~bennani/CSRank.html Should we ask such pages to register Haskell Symposium? --Kazu

1 0

code.haskell.org down
by Henning Thielemann 17 Oct '16

17 Oct '16

Currently, both HTTP and SSH to code.haskell.org fail with a timeout. :-(

3 3

Arrows and Computation exercises?
by David Banas 17 Oct '16

17 Oct '16

Hi all, I’m wondering if anyone else happens to be working their way through the exercises in Ross Paterson’s *Arrows and Computation*, and would like to compare notes and/or discuss solutions to his exercises. Here are mine, so far: https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/… <https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/…> Thanks, -db

2 1

[ANN] xls-0.1.0 Parse MS Excel spreadsheets
by Harendra Kumar 17 Oct '16

17 Oct '16

I have uploaded the xls package [1] on Hackage [2]. It works pretty well for the basic use case of parsing all sheets in a single stream of rows composed of cells. The cell values are presented as plain strings i.e. no data type based interpretation. Such stuff can be added if required, it is supported by the underlying C library (libxls). One thing that I would like to have added to the API is a way to list all sheets and select sheets to parse in a workbook. It should be pretty easy to do if anyone wants to do it. 1. https://github.com/harendra-kumar/xls 2. https://hackage.haskell.org/package/xls -harendra

2 3

MTL vs Free-monads, what are your experiences
by Oleg 16 Oct '16

16 Oct '16

Matt wrote: > The mtl technique subsumes the free monad technique. if you have a term: > getCurrentTime :: MonadClock m => m UTCTime The relationship between MTL and Free(r) monad approaches is filled with confusion, as you message has just demonstrated. There is nothing specific to MTL in writing the constraints like MonadClock. There is absolutely nothing that prevents you from defining instance Member ClockEffect r => MonadClock (Eff r) or similar with Free monads, instance MonadClock (Free CurrentTimeF) (which are less efficient, compose less well and require the boilerplate of writing functor instances). In short, defining constraints like MonadClock, MonadState etc. is not specific to any approach to effects. The difference between the monad transformer and Free monad is how you build types that satisfy the MonadClock etc. constraint. In MTL, you build by applying monad transformer to a suitable base monad. In Free monad, you define functors like CurrentTimeF and then build the Free monad. In Freer monad approach, your example getCurrentTime will look like getCurrentTime :: Member ClockEffect r => Eff r UTCTime which looks quite like getCurrentTime :: MonadClock m => m UTCTime in your example. Therefore, there is usually no need to define a separate MonadClock class. But nothing stops you from doing that.

3 2

[ANN] HackerNews 1.0
by David Johnson 14 Oct '16

14 Oct '16

Released new API bindings to the HackerNews API ( https://github.com/HackerNews/API) *Changes/Updates:* - Matching GHCJS implementation, all AJAX calls are tested thanks to phantomjs w/ hspec. - All aeson instances derived generically - Nix-based CI w/ travis ensures successful building on the following platforms: - *nixpkgs-16.09* - ghcjs-7103 - ghc-801 - *Stackage LTS * - LTS 7.3 *Release links*: - http://hackage.haskell.org/package/hackernews - https://github.com/dmjio/hackernews More platforms to be added for testing. - David

1 0

Type alias in constraints
by Sylvain Henry 14 Oct '16

14 Oct '16

Hi, I have been using constraints of the form: xxx :: forall x xs ys zs m. ( Monad m , zs ~ Union (Filter x xs) ys , Catchable x xs , Liftable (Filter x xs) zs , Liftable ys zs ) => Variant xs -> (x -> Flow m ys) -> Flow m zs Where "zs" is used as a type alias. Now with GHC8 and -Wredundant-constraints, GHC complaints that "zs" is redundant (indeed it is). Is there a way to do this properly? If not, could we introduce a new syntax to make this kind of local declaration? We could borrow the syntax of local declarations into list-comprehensions: "let zs ~ ..." Thanks, Sylvain

2 2

Req. transfer of maintainership of network-multicast
by Marco Zocca 14 Oct '16

14 Oct '16

To the Hackage trustees, following a communication with Audrey Tang, I would like to become new maintainer of `network-multicast` . My Hackage account name is `ocramz`. Thank you, Marco

1 0

Help wanted for the architecture backing perf-service.haskell.org
by Sebastian Graf 12 Oct '16

12 Oct '16

Hi all, I'm about to wrap up my work for the Haskell committee (think half a summer of code): feed-gipeda <https://github.com/sgraf812/feed-gipeda> It's basically a daemon which will spawn benchmarking jobs for every commit of registered repositories. Much like Travis CI, but for benchmarks. You can see a simple web server, hosting the gipeda <https://github.com/nomeata/gipeda/>-generated sites, at http://perf-service.haskell.org/ghc/#all. While the Haskell part is working smoothly enough for now, I'd really like some help setting up proper sandboxing environments for the benchmark slaves, in such a way that security isn't as much a concern as it currently is. We can go over the details on a less publicly shared medium, but I doubt the current solution (invoking shell scripts from a non-root user) is safe. So, some concrete points I need help with: 1. Administrative expertise: Which part of the architecture runs has which rights, setting up proper sandboxing environments for benchmark slaves 2. Ops stuff: Creating master and slave containers for a low barrier to entry and reproducible environments 3. Distributed protocols: Someone with experience in stuff like SSH-tunneling/CloudHaskell/other useful things I should make the communication protocol of feed-gipeda aware of 4. Some Haskellers which want to take a look at my code and contribute criticism or even code to it :) Thanks in advance! So long, Sebastian Graf

3 2

Announcing binary-parsers
by 韩冬(基础平台部) 12 Oct '16

12 Oct '16

Hi all, I am happy to announce binary-parsers. A ByteString parsing library built on binary. I borrowed lots of design/tests/document from attoparsec so that i can build its shape very quickly, thank you bos! And thanks to binary's excellent design, the codebase is very small(<500 loc). From my benchmark, it’s insanely fast, it outperforms attoparsec by 10%~30% in aeson benchmark. it’s also slightly faster than scanner(a non-backtracking parser designed for speed) in http request benchmark. I’d like to ask you to give it a shot if you need super fast ByteString parsing. These parsers are just binary's Get monads, so you can seamlessly combine them with combinators from binary package. You can now write more complex Binary instances using comprehensive combinators, with serialisation packages like blaze-texual. all these goodies come for free! Happy hacking! Github: https://github.com/winterland1989/binary-parsers Hackage: http://hackage.haskell.org/package/binary-parsers Winterland

5 9