- Haskell-Cafe - Haskell.org

Using lenses
by Simon Peyton-Jones 03 Oct '13

03 Oct '13

(I sent this to 'libraries' but Kim-Ee suggested adding Café, where so many smart people hang out.) Friends Some of you will know that I've promised to give a talk about Edward's lens library<http://hackage.haskell.org/package/lens> at the Haskell Exchange<http://skillsmatter.com/event/scala/haskell-exchange> in London next Wednesday (9th). I did this to give everyone (including me) a break from GHC hackery, and also to force me to learn about this lens voodoo that everyone is twittering about. Edward generously gave me quite a bit of one-to-one attention last week (my hair is still standing on end), but this message is to ask your help too. Specifically, I'd like to give some compelling use-cases. If you are using the lens library yourself, could you spare a few minutes to tell me how you are using it? I expect to cover Lens and Traversal but not Prism. The use-case everyone starts with is nested records, but I'd like to go beyond that. The next levels seem to be: · Lenses as views of data that isn't "really there" e.g. regarding a record with rectangular coordinates as having polar coordinates too. · Lenses and Traversals that focus on elements of finite maps (Control.Lens.At) What else? I'm sure you are using them in all sorts of cool ways that I would never think of, and I'd love to know. Please don't tell me anything secret! To give everyone the benefit I may just concatenate all the replies and send to you all, so please say if you don't want me to do that with yours. And don't burn too many cycles on this...I don't want to waste your time, and I can always get back to you if I can't understand what you say. Sooner is better than later...Weds is coming. Simon "Edward's prophet" PJ

9 8

ANN: Nomyx 0.3 beta, the game where you can change the rules
by Corentin Dupont 03 Oct '13

03 Oct '13

Hello everybody! I released the third beta of Nomyx <http://www.nomyx.net>, the only game where You can change the rules! The second beta was a success. Great players (byorgey, nomeata, Toxaris...) proposed amazing rules, effectively building a nice universe. For example, a banking system in ecu was proposed, with cash monies and even a way to buy pies! Other rules like different democratic systems were proposed. You can check those rules in this thread: http://www.nomyx.net/forum/viewtopic.php?f=4&t=1520 Let's start a new game! I propose to set it in the medieval era. Dear rulers, I'll let your imagination speak! Please login to the game here: www. nomyx.net:8000/Nomyx. The corresponding forum thread is here: http://www.nomyx.net/forum/viewtopic.php?f=4&t=1523 Some background: this is an implementation of a Nomic [2] game in Haskell(I believe the first complete implementation of a Nomic game on a computer). In a Nomyx game you can change the rules of the game itself while playing it. The players can submit new rules or modify existing ones, thus completely changing the behaviour of the game through time. The rules are managed and interpreted by the computer. They must be written in the Nomyx language <http://hackage.haskell.org/package/Nomyx-Language>, which is a subset of Haskell. At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Here is a video introduction and first tutorial of the game: http://vimeo.com/58265498 The example file gives a lot of examples of rules that you can submit: www. nomyx.net:8000/src/Language/Nomyx/Examples.hs changes in V0.3: - creation of admin role - inputs made by the rules now include checkboxes, buttons, textarea - outputs made by the rules are now managed (create, update, delete) - refactored the GUI - bumped to GHC 7.6 - compilation errors are displayed in context - bug fixes Cheers, Corentin [1] www.nomic.net

1 1

indentation with let and do
by Corentin Dupont 03 Oct '13

03 Oct '13

Hi the list, why do this function doesn't compile (parse error): test :: Bool -> IO () test foo = do let bar = case foo of True -> "Foo"; False -> "Bar" return () while this one does (just adding one space in front of True and False): test :: Bool -> IO () test foo = do let bar = case foo of True -> "Foo"; False -> "Bar" return () Thanks!! Corentin

5 5

"plugins" fails on a simple example
by Petr Pudlák 03 Oct '13

03 Oct '13

Hi, I'm playing with "plugins", trying to evaluate a simple expression: |import Control.Monad import System.Eval.Haskell main =do let fExpr ="1 + 2 :: Int" r <- eval_ fExpr ["Prelude"] [] [] [] ::IO (Either [String] (Maybe Int)) case rof Right (Just f) ->do print $ f Left err -> putStrLn $"Error:" ++ unlines err _ -> putStrLn $"Unknown error."| However, it fails with |Error: on the commandline: Warning: -fglasgow-exts is deprecated: Use individual extensions instead| Am I doing something wrong? How can I turn off the flag? I'm using GHC 7.6.3. Thanks, Petr

2 4

[ANN] lvish 1.0 -- successor to monad-par
by Ryan Newton 03 Oct '13

03 Oct '13

Hi all, I'm pleased to announce the release of our new parallel-programming library, LVish: hackage.haskell.org/package/lvish It provides a "Par" monad similar to the monad-par package, but generalizes the model to include data-structures other than single-assignment variables (IVars). For example, it has lock-free concurrent data structures for Sets and Maps, which are constrained to only grow monotonically during a given "runPar" (to retain determinism). This is based on work described in our upcoming POPL 2014 paper: http://www.cs.indiana.edu/~rrnewton/papers/2013_07_LVish_quasiDet_working_d… Cheers, -Ryan

4 5

Re: [Haskell-cafe] Using lenses
by Niklas Haas 03 Oct '13

03 Oct '13

Another great use of lenses is the lens-aeson library (not to be confused with aeson-lens). It's technically based around prisms, though, so it's outside the scope of your talk; but you may wish to at least reference it - it makes working with JSON really elegant!

1 0

Re: [Haskell-cafe] Using lenses
by Nicolas Trangez 03 Oct '13

03 Oct '13

Simon, On Thu, 2013-10-03 at 08:07 +0000, Simon Peyton-Jones wrote: > If you are using the lens library yourself, could you spare a few minutes to tell me how you are using it? I'm not a heavy 'lens'-user (yet), and this might not be the most pretty use-case from a theoretic point of view, but anyway: I use 'lens' in Kontiki [1], my implementation of Raft [2], a fairly recent consensus protocol for distributed systems (think Paxos & multi-Paxos, Zookeeper,...). The Raft protocol description is fairly imperative ("When in state S, if you receive a message M, broadcast message M'(M) to all other nodes, append entry E(M) to your log, and update the state to S'(S)"), so in order to keep the code close to this description, I created a TransitionT monad transformer (where the underlying monad should, in some cases, provide access to the persisted log of the system, most likely requiring IO access, but this is abstracted over), which is basically RWST. The R environment gives access to system configuration (e.g. the set of nodes (or at least their names/IDs) part of the cluster), W is used to output/list Commands to be executed as part of a state transition, S gives access to the current state, and a transition should yield a new state (or the old one if e.g. a message is received which should be ignored in the current state). See [3] for an overview of the types involved. Finally, after this long introduction, the 'lens' part: thanks to the 'lens' operators, and my types involved in the TransitionT monad (specifically the R environment and S state) being lens'ified, accessing R & S becomes fairly unobtrusive and more imperative-looking (which makes sense when using RWS). As an example, in [4], you see some parts of the current (Candidate) state being accessed: currentTerm <- use cCurrentTerm votes <- use cVotes Not using 'lens', this would be something (OTOH) like currentTerm <- fmap _cCurrentTerm get which is less familiar from an imperative POV. Then also updating the current state 'in-place' (not for real, obviously): cVotes %= Set.insert sender And accessing the configuration (R) environment, e.g.: nodeId <- view configNodeId See other handler functions in that module, or in the Follower and Leader modules in the same directory, for more examples. Regards, Nicolas [1] https://github.com/NicolasT/kontiki [2] https://ramcloud.stanford.edu/wiki/download/attachments/11370504/raft.pdf [3] https://github.com/NicolasT/kontiki/blob/master/src/Network/Kontiki/Types.hs [4] https://github.com/NicolasT/kontiki/blob/master/src/Network/Kontiki/Raft/Ca…

1 0

[ANN] yi-0.7
by Dmitry Ivanov 03 Oct '13

03 Oct '13

I'm pleased to announce a new release of Yi, text editor written and extensible in Haskell. This release introduces new vim keybindings emulation. If all goes well, the old one will be deprecated in future. The advantages of the new emulation include support for visual block operations, more consistent repeat (dot command) behavior, lots of tests (but not nearly enough, because vim feature set is HUGE) and more. If you are a vim user, please try it out and help us make it better. Full changelog is available at https://github.com/yi-editor/yi/blob/v0.7/CHANGELOG Places to dicuss using and developing yi: mailing list: https://groups.google.com/group/yi-devel issue tracker: https://github.com/yi-editor/yi/issues IRC channel: #yi@freenode (it's very slow compared to #haskell, please don't quit after 10 mins) Regards, Dmitry

1 0

Roshask
by Tijn van der Zant 02 Oct '13

02 Oct '13

Hi, I want to use Haskell to program my robot using ros. I've found roshask https://github.com/acowley/roshask and I was wondering if someone on this list has experience with it and can tell me how good it is. Thanx! --Tijn

2 2

Haskell Logo Macbook Decal
by Ryan Trinkle 02 Oct '13

02 Oct '13

Hi everyone, I put together a Haskell logo decal designed to fit over the Apple logo on a MacBook, and had a few made by macdecals.com. They came out great! They're available here: http://www.macdecals.com/macbook-haskell . My share of the proceeds will go to haskell.org, and macdecals.com has generously offered to match that contribution, as well. Ryan

5 4