September 2014 - Haskell-Cafe

Taking over the thrift package.
by Sean Parsons 24 Sep '14

24 Sep '14

Hi there. As per the instructions on the Haskell wiki I'm posting here as I'm interested in either taking over the thrift package or helping out with getting more recent builds uploaded as it's wildly out of date: https://hackage.haskell.org/package/thrift Does anyone have any contact details for the current maintainer? Thanks, Sean.

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Combining LoggingT with another library's monad
by Joe Fiorini 24 Sep '14

24 Sep '14

As I've been learning Haskell, I've found I constantly get tripped up on combining monads with transformers to form stacks. This seems to be very common in web development, which is my primary use case for Haskell. I'm using the Wheb framework as it's been the simplest to understand of all the frameworks I've seen. To get a better understanding of monad transformers, I'm trying to implement logging in a small app I wrote. I'm attempting to use Control.Monad.Logger to add LoggingT to my monad stack, that way I can use runStdoutLoggingT to log straight to stdout (without having to deal with implementing my own monadLoggerLog method). I added LoggingT to the top of my monad stack, but ran into lots of problems and moved the LoggingT type to one of my route handlers (you can see my latest attempt & the compiler error here https://gist.github.com/joefiorini/9e89bdf86e28e6b98bf0) I'm at a point now where I believe my problem is either a) I'm just not understanding something about how monad transformers work, in which case I just need to do some more reading or b) the LoggingT transformer is simply not "compatible" with the WhebT transformer, in which case I need to completely change my approach. I'm not looking for someone to solve my problem for me; I'm just trying to get a little guidance. Am I heading in the right direction here or should I change up my approach? If the latter, can anyone give me a little direction as to where to look? Thanks!

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Numerals in input vs numerals in source
by Doug McIlroy 24 Sep '14

24 Sep '14

Consider e1 = read "1" :: Integer e2 = read "1" :: Rational e3 = read "1" :: Double e1 and e3 evaluate to 1 and 1.0 respectively e2 produces an error: "Prelude.read: no parse" Yet (1::Integer, 1::Rational, 1::Double) evaluates to (1, 1%1, 1.0) Is the inconsistent parsing defensible, or just an (irrational!) fact? I believe a reconciliation would break nothing. Doug McIlroy

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code.haskell.org down
by Henning Thielemann 24 Sep '14

24 Sep '14

code.haskell.org seems to be down :-(

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Re: [Haskell-cafe] [ANN] Declarative Game Programming & haskanoid (FRP, SDL, Android)
by Ivan Perez 24 Sep '14

24 Sep '14

Hi Mikolaj, (CC haskell-cafe) Thanks, Mikolaj! Indeed, the original links to the slides were broken (deployment env vs production env, yikes!). That has been fixed now. Bitbucket seeems to be taking some time to refresh the page, though. In the meantime: http://fplab.bitbucket.org/uploads/ppdp2014-tutorial.pdf http://fplab.bitbucket.org/uploads/ppdp2014-tutorial-4up.pdf Best Ivan On 24 September 2014 10:27, Mikolaj Konarski <mikolaj.konarski(a)gmail.com> wrote: > > * Announcement and slides: > > > http://fplab.bitbucket.org/posts/2014-09-23-declarative-game-programm.html > > Hi! Thank you for the post. Both slides links are dead for me, though. >

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Nottingham Research Fellowships (deadline 20th October)
by Graham Hutton 24 Sep '14

24 Sep '14

Dear all, The University of Nottingham is seeking applications for a number of Nottingham Research Fellowships: http://tinyurl.com/notts-fellows These are highly prestigious three-year fellowships, which are normally expected to lead into permenant academic positions. Candidates should have no more than 8 years postdoctoral experience, and fellowships normally start in October 2015. The deadline for submission of initial Expressions of Interest is ** 20th October 2014 **. Applicants in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from outstanding candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from http://fp.cs.nott.ac.uk As an approximate guideline, candidates in the area of the FP lab would normally be expected to have at least 3 years postdoc experience, and a number of strong publications in leading international venues such as ICFP, POPL, LICS, JFP, Haskell Symposium, etc. Best wishes, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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[ANN] Declarative Game Programming & haskanoid (FRP, SDL, Android)
by Ivan Perez 24 Sep '14

24 Sep '14

Hi Café A few weeks ago Henrik Nilsson gave a tutorial at PPDP 14 on Declarative Game Programming, demoing an SDL game using the FRP implementation Yampa. Additional material has been made available now: * Announcement and slides: http://fplab.bitbucket.org/posts/2014-09-23-declarative-game-programm.html * Game: http://github.com/ivanperez-keera/haskanoid The game includes wiimote support, SDL graphics and sound. We have other versions working with a kinect, if anyone is particularly interested. This is, IMHO, very fun to play, since you have to actually run around the room. Also, Keera Studios (my company) is working on an Android version of the game and a video of an early demo running on a Galaxy Tab is available ( http://keera.co.uk/blog/2014/09/24/game-programming-videos-code/ <http://keera.co.uk/blog/?p=690&preview=true>). Our internal, latest version runs now much faster than the ones in either video (new videos will be released soon). I hope we can soon give you a great excuse to be playing games at the office ;) All the best Ivan

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Multicast receiver thread fails to receive last few messages
by Ben Gunton 23 Sep '14

23 Sep '14

I am trying to send multicast messages from one thread, and receive them on another. The sending thread reads lines from a file and sends them. However, the receiving thread does not receive all the messages - it seems to miss the last 10 message or so. If I run other programs to listen for the multicast message, they receive them fine. So I think the issue is something to do with receiving, not sending. I keep the program alive with a threadDelay, so it shouldn't be halting prematurely. If I add a small delay after each send, it works fine. And if I make the input file smaller, it works fine. On the provided data file, on my system (64-bit Ubuntu, GHC 7.8.2), the receiver fails to receive lines 125-140. Compiled with no optimizations. Thanks for any help! The relevant code snippets are below, and the sample data file and full program are attached. -- Loop to receive all the packets let receiveMulticast = do (msg, _) <- recvFrom recvSock 32628 putStrLn . BS.unpack $ BS.take 100 msg receiveMulticast _ <- forkIO receiveMulticast -- Send every line from a file as multicast message inputFile <- openFile "data" ReadMode fileLines <- BS.lines <$> BS.hGetContents inputFile let sendMulticast msg = do sendTo sendSock msg addr -- Receiver FAILS to receive last few messages unless this -- thread delay exists... why?! -- threadDelay (1) mapM_ sendMulticast fileLines hClose inputFile threadDelay (1000*1000*1000) -- Delay for 1000 seconds

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Copyright field vs. License file
by Joachim Breitner 23 Sep '14

23 Sep '14

Dear package maintainers, a lot of our LICENSE file start like tihs: ==> haskell-lens-4.1.2.1/LICENSE <== Copyright 2012-2014 Edward Kmett All rights reserved. ... ==> haskell-cryptocipher-0.6.2/LICENSE <== Copyright (c) 2010-2013 Vincent Hanquez <vincent(a)snarc.org> All rights reserved. ... ==> haskell-http-client-0.3.2.1/LICENSE <== The MIT License (MIT) Copyright (c) 2013 Michael Snoyman Permission is hereby granted, free of charge, to any person obtaining a copy of ... i.e. they mix the copyright information with the license. I know, of course, why that is: "cabal init" prepares it that way, and those not using cabal init probably copy it from an existing file. A notable exception are the GPL-licensed packages; these usually don’t have copyright information in the license. The problem is that this information is not structured, and hard to parse, i.e. to create license reports or (my use case) create distribution packages with much less manual labor. Cabal describes the fields as license-file: filename or license-files: filename list The name of a file(s) containing the precise copyright license for this package. The license file(s) will be installed with the package. If you have multiple license files then use the license-files field instead of (or in addition to) the license-file field. copyright: freeform The content of a copyright notice, typically the name of the holder of the copyright on the package and the year(s) from which copyright is claimed. For example: Copyright: (c) 2006-2007 Joe Bloggs which suggest to put the copyright data into the cabal file only and leave the license file alone. It would also have the advantage that after cabal init, you’d only have to modify one file, and the copyright information is easily visible on hackage. I know that there it is highly unlikely that a significant number of maintainers will change their existing files. But I’d still like to get feedback: Do you agree that this make sense? Should I try to make "cabal init" set it up this way? And would you accept pull requests for this? Thanks, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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Re: [Haskell-cafe] Bound and free variables
by Barak A. Pearlmutter 23 Sep '14

23 Sep '14

On Mon, 22 Sep 2014 19:30:33 -0400, Michael Orlitzky wrote: > It's hard to tell from your example, but if you want, you can > translate the let binding directly to lambda calculus and then go from > there where the rules are simple. > > In Haskell, > > let <name> = <argument> in <expression> > > translates to, > > (\<name> -> <expression>) <argument> Although this list might not be for homework problems, there is an element of terminological confusion and a quest for nomenclature in the question that don't seem entirely unreasonable. Even if we're not going to answer the question per se, we shouldn't give hints that are misleading, or true in Scheme and lambda calculus but false in Haskell. $ ghci Prelude> let b = 3:b in take 3 b [3,3,3] Prelude> (\b -> take 3 b) (3:b) <interactive>:3:21: Not in scope: `b'

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