April 2016 - Haskell-Cafe

cannot perform arithmetic with GHC.TypeLits?
by Baojun Wang 02 May '16

02 May '16

Hi List, When I try to build below program, the compiler complains Couldn't match type ‘n’ with ‘1 + (n - 1)’ … Why GHC (7.10.3) cannot do type level natural number arithmetic? `` n /= 1 + (n-1)`` at type level? -- | main.hs {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE DeriveGeneric #-} module Main where import qualified Data.ByteString as B import Data.ByteString(ByteString) import Data.Serialize -- require cereal import GHC.TypeLits data Scalar :: Nat -> * -> * where Nil :: Scalar 0 a Cons :: a -> Scalar m a -> Scalar (1+m) a instance (Serialize a) => Serialize (Scalar 0 a) where get = return Nil put _ = return $ mempty instance (Serialize a) => Serialize (Scalar n a) where get = do x <- get :: Get a xs <- get :: Get (Scalar (n-1) a) return $! Cons x xs put (Cons x xs) = do put (x :: a) put xs -- /tmp/vect1/app/Main.hs:31:15: Couldn't match type ‘n’ with ‘1 + (n - 1)’ … ‘n’ is a rigid type variable bound by the instance declaration at /tmp/vect1/app/Main.hs:27:10 Expected type: Scalar n a Actual type: Scalar (1 + (n - 1)) a Relevant bindings include xs :: Scalar (n - 1) a (bound at /tmp/vect1/app/Main.hs:30:5) get :: Get (Scalar n a) (bound at /tmp/vect1/app/Main.hs:28:3) In the second argument of ‘($!)’, namely ‘Cons x xs’ In a stmt of a 'do' block: return $! Cons x xs Compilation failed. - baojun

5 6

Call for Contributions - Haskell Communities and Activities Report, May 2016 edition (30th edition)
by Mihai Maruseac 01 May '16

01 May '16

Dear all, It's that time of the year again (https://ro-che.info/ccc/16) :) We would like to collect contributions for the 30th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 2016 ============================================================ The short story follows: * If you are working on _any_ project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is small, unfinished or used only by yourself or you think it is not important enough, please reconsider and submit an entry anyway! * The submission file can be in _any_ format you like. Although the final report uses LaTeX, if it is easier for you to submit a Markdown file, text file, Doc file or anything else, please do so. We'll be happy to do the translation and get a new entry in the HCAR :) * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me or contact a project leader to get them to submit an entry. We have received feedback in the past that some projects were missing from the edition, but, sadly, that was too late at the time, but those projects will be contacted for this edition. * Feel free to pass on this call for contribution to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai FAQ: Q: What format should I write in? A: The recommeneded format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2016/template.tex There is also a LaTeX style file at http://haskell.org/communities/06-2016/hcar.sty that you can use to preview your entry. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. _However_, if you don't want/have time to format the entry for LaTeX, you can submit it in any other format possible and we will be happy to convert it for the final report. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then, PNG being preferred for simplicity. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

3 2

[ANN] Pre-announcing ZuriHac 2016
by Alexander Bernauer 29 Apr '16

29 Apr '16

Hello fellow Haskellers, it's my pleasure to announce that this year's ZuriHac, the Zurich Haskell hackathon, will happen from July 22nd to 24th. The registration will open this Saturday at 12pm (UTC+2). More information will follow. Greetings, Alex on behalf of the orga team

1 1

[ANN] Darcs 2.12.0 release
by Guillaume Hoffmann 29 Apr '16

29 Apr '16

Hi all, The Darcs team is pleased to announce the release of Darcs 2.12.0. # Downloading # One way of installing Darcs 2.12.0 is with stack: $ stack install darcs-2.12.0 Or first install the Haskell Platform (http://www.haskell.org/platform) and install Darcs with cabal-install: $ cabal update $ cabal install darcs-2.12.0 You can also download the tarball from http://darcs.net/releases/darcs-2.12.0.tar.gz and build it by hand. The 2.12 branch is also available as a darcs repository from http://darcs.net/releases/branch-2.12 # What's new since Darcs 2.10.3 # ## New features ## * `darcs show dependencies`: export patch dependency graph as dot file (Ale Gadea) ## User Interface ## * improvements in `darcs record` output with irrelevant files (Ben Franksen) * `darcs log -v --machine-readable`: show internal representation of patches (including explicit dependencies). Remove patch viewing via the "darcs annotate" command. (Guillaume Hoffmann) * `whatsnew -s` (and `status`) show conflicting files (Guillaume Hoffmann) * honor "quiet" flag in command outputs (Ben Franksen) * a single `show patch-index` command (Guillaume Hoffmann) * remove deprecated aliases of show (Guillaume Hoffmann) * remove hack to enable arbitrary protocols via env variables (Guillaume Hoffmann) ## Performance ## * handle file moves natively when importing from git (Owen Stephens) ## Developer-related ## * require GHC 7.6 (base > 4.6) and support GHC 8 (Ganesh Sittampalam) * switch to sandi from dataenc (Daniil Frumin) * refactoring to prepare stash (Ganesh Sittampalam) * change argument order of type witness RL (Ben Franksen) * Darcs.Repository / repository cloning refactoring (Ben Franksen) * use of async when cloning with packs (Ben Franksen) * SSH code refactoring (Ben Franksen) * Patch and RealPatch renamed to RepoPatchV1 and RepoPatchV2 (Guillaume Hoffmann) * merged hashed-storage into libdarcs and hashed-storage tests into darcs-test (Guillaume Hoffmann) * reorganize Darcs.Repository.* modules (Guillaume Hoffmann) ## Issues resolved in Darcs 2.12 ## * issue1807: clarify help of PAGER, DARCS_PAGER (Guillaume Hoffmann) * issue2258: improve patch index error message with suggestion (Guillaume Hoffmann) * issue2269: push hijack test to suspend time (Eric Kow) * issue2276: Keep track of patch hijack decisions (Eric Kow) * issue2138: report conflicting files in whatsnew -s (Guillaume Hoffmann) * issue2393: remove whatsnew functionality from annotate (Guillaume Hoffmann) * issue2400: use async package to keep track of unpack threads (Ben Franksen) * issue2459: fall back to writing the file if createLink fails (Ben Franksen) * issue2479: root dir most not be among the sources of a move (Ben Franksen) * issue2481: expose API for 'darcs diff' command (Ganesh Sittampalam) * issue2486: obliterate --not-in-remote -q should be more quiet (Ben Franksen) * issue2489: dequote filepaths while importing from git (Guillaume Hoffmann) * issue2494: output of darcs record with file arguments (Ben Franksen) Thanks! Guillaume

1 0

[ANNOUNCE] GHC 8.0.1 release candidate 4 available
by Ben Gamari 29 Apr '16

29 Apr '16

Hello Haskellers, The GHC team is happy to announce the availability of fourth release candidate of the Glasgow Haskell Compiler's 8.0.1 release. Source and binary distributions can be found at, http://downloads.haskell.org/~ghc/8.0.1-rc4/ This is the last of four candidates leading up to the 8.0.1 release, addressing nearly all of the known issues of the previous candidates. These include, * A type-checker panic triggered by use of Typeable on a kind-polymorphic type constructor (#11334) * A type-checker explosion where -XTypeInType would gobble up massive amounts of memory when used in a data instance (#11407) * A variety of other typechecker issues (#11811, #11797, #11813, #11814) * A build issue seen on OS X (#11828) * Template Haskell can now produce instances with OVERLAPP{ING,ABLE,ED} pragmas * Autoconf has improved version checks for libdw (#11820) * Typeable and hs-boot files now interact nicely (#11824) * The build system now checks for the broken Apple `nm` utility (#11823, #11744) * Various issues involving unexpected laziness resulting in exception handlers not being invoked (#11555) * GHC now fails more gracefully when used with an older cabal-install release (#11558) * TypeInType now has proper documentation in the users guide (#11614) * The story surrounding type `RuntimeRep`s (formerly known as `Levity`) is now far better developed, closing several doors to unsafe behavior that TypeInType previously opened (#11473, #11723) * A long-standing bug in the constant-folding rules for `mod` for the `Word` type has been resolved (#11702) * Various issues introduced by OverloadedRecordFields have been fixed (#11662, #11401) * A regression in the typechecker resulting in the rejection of code in the `free` and `microlens` packages has been fixed (#11608) * A bug in the LLVM code generator which caused some programs to emit unbuildable LLVM IR has been fixed (#11649) * A bug where pattern synonyms defined in terms of a pattern match on a record would be rejected if the fields weren't written in the same order as they were defined has been resolved (#11633) * A bug in the runtime system's treatment of weak references which could result in segmentation faults in some cases has been fixed (#11108) * a variety of optimizations improving compiler performance have been merged * and many others Mac OS X users should be aware that the recent XCode 7.3 release ships with a broken `nm` utility which breaks the GHC build (#11744, #11823). The build system will check for this condition and request that the tree is configured to use the `nm-classic` utility if found. OS X users running XCode 7.3 are encouraged to open a ticket with Apple so that this issue may be fixed in future XCode releases. If no major issues pop up we expect that this will be the last release candidate before the final release, which we hope to push out in the coming weeks. Thanks to everyone who has contributed code, testing, builds, and bug reports thusfar! Cheers and happy testing, - Ben

2 1

Fw: Bikeshedding request for GHCi's :type
by Takenobu Tani 28 Apr '16

28 Apr '16

Dear cafe, I'm forwarding mail from ghc-devs ML [1]. I think `:type` command is very important for beginners and middle users. Please feedback your opinion to them ;) [1]: https://mail.haskell.org/pipermail/ghc-devs/2016-April/011933.html > 2016-04-26 22:08 GMT+09:00 Richard Eisenberg <eir(a)cis.upenn.edu>: > Hi devs, > > Over the weekend, I was pondering the Haskell course I will be teaching next year and shuddered at having to teach Foldable at the same time as `length`. So I implemented feature request #10963 ( https://ghc.haskell.org/trac/ghc/ticket/10963) which allows for a way for a user to request a specialization of a type. It all works wonderfully, but there is a real user-facing design issue here around the default behavior of :type and whether or not to add new :type-y like commands. I have outlined the situation here: https://ghc.haskell.org/trac/ghc/wiki/Design/GHCi/Type > > I'd love some broad input on this issue. If you've got a stake in how this all works, please skim that wiki page and comment on #10963. > > Thanks! > Richard Cheers, Takenobu

3 2

Re: [Haskell-cafe] Fw: Bikeshedding request for GHCi's :type
by Anton Felix Lorenzen 28 Apr '16

28 Apr '16

As a middle user, I often depend on `:type` when learning a library. I don't have strong preferences over one proposal or the other, but I would like to add, that there should always be `:type` in its current form (renamed or not). I wouldn't like getting lots of for me irrelevant information, like examples, when I only want a one-line type signature. That said, I like the general idea, although I don't know, whether a beginner would like having to learn the type signatures again, when getting to a more advanced level. - Anton Lorenzen

1 0

ANN: Haskell Prime 2020 committee has formed
by Herbert Valerio Riedel 28 Apr '16

28 Apr '16

Dear Haskell Community! Some time ago I asked for nominations to reboot the Haskell Prime process[1], and now I'm pleased to finally announce the formation of the new Haskell Language 2020 Committee! The goal of the Haskell Language committee together with the Core Libraries Committee is to work towards a new Haskell 2020 Language Report. I'd like to remind everyone that the Haskell Prime Process[4] relies on *everyone* in the community to help by contributing proposals which the committee will then evaluate and if suitable help formalise for inclusion. Everyone interested in participating is also invited to join the haskell-prime mailing list. Four years (or rather ~3.5 years) from now may seem like a long time. However, given the magnitude of the task at hand, to discuss, formalise, and implement proposed extensions (taking into account the recently enacted three-release-policy[3]) to the Haskell Report, the process shouldn't be rushed. Consequently, this may even turn out to be a tight schedule after all. However, it's not excluded there may be an interim revision of the Haskell Report before 2020. Based on this schedule, GHC 8.8 (likely to be released early 2020) would be the first GHC release to feature Haskell 2020 compliance. Prior GHC releases may be able to provide varying degree of conformance to drafts of the upcoming Haskell 2020 Report. The Haskell Language 2020 committee starts out with 20 members which contribute a diversified skill-set. These initial members also represent the Haskell community from the perspective of practitioners, implementers, educators, and researchers. - Andres Löh - Antonio Nikishaev - Austin Seipp - Carlos Camarao de Figueiredo - Carter Schonwald - David Luposchainsky - Henk-Jan van Tuyl - Henrik Nilsson - Herbert Valerio Riedel - Iavor Diatchki - John Wiegley - José Manuel Calderón Trilla - Jurriaan Hage - Lennart Augustsson - M Farkas-Dyck - Mario Blažević - Nicolas Wu - Richard Eisenberg - Vitaly Bragilevsky - Wren Romano The Haskell 2020 committee is a language committee; it will focus its efforts on specifying the Haskell language itself. Responsibility for the libraries laid out in the Report is left to the Core Libraries Committee (CLC)[5]. Incidentally, the CLC still has an available seat[2]; if you would like to contribute to the Haskell 2020 Core Libraries you are encouraged to apply for this opening. As this is a general announcement broadcasted to multiple mailing lists, a separate email discussing the next steps of the new committee will be sent to the haskell-prime mailing list shortly. [1]: https://mail.haskell.org/pipermail/haskell-prime/2015-September/003936.html [2]: https://mail.haskell.org/pipermail/libraries/2015-November/026497.html [3]: https://prime.haskell.org/wiki/Libraries/3-Release-Policy [4]: https://prime.haskell.org/wiki/Process [5]: https://prime.haskell.org/wiki/Libraries -- hvr

1 0

Sending email
by David Escobar 28 Apr '16

28 Apr '16

Hi everyone, I'm trying to use the *Network.Mail.SMTP* library to send email: *{-# LANGUAGE OverloadedStrings #-}* *module Main where* *import Control.Exception* *import qualified Data.Text as T* *import qualified Data.Text.Lazy as LT* *import Network.Mail.SMTP* *main :: IO ()* *main = do* * sendEmail (“Person sender”, “sender(a)somewhere.com <sender(a)somewhere.com>”)* * [(“Person recipient“, “recipient(a)somewhere.com <recipient(a)somewhere.com>”)]* * "Test email"* * "Some message goes here."* *sendEmail :: (T.Text, T.Text) -> [(T.Text, T.Text)] -> T.Text -> T.Text -> IO ()* *sendEmail (fromName, fromEmail) toAddresses subject' body' = do* * let toNameAddrs = map (\(toName, toEmail) -> Address (Just toName) toEmail) toAddresses* * msg = simpleMail (Address (Just fromName) fromEmail)* * toNameAddrs* * []* * []* * subject'* * [ plainTextPart $ LT.fromStrict body' ]* * result <- try $ sendMailWithLogin' "smtp.gmail.com <http://smtp.gmail.com>"* * 465 -- SSL port* * “sender_login”* * “sender_password”* * msg :: IO (Either SomeException ())* * case result of* * Left e -> putStrLn $ "Exception caught: " ++ (displayException e)* * Right _ -> putStrLn "Sent email successfully."* The program compiles, but when I run it, I get: *Exception caught: <socket: 49>: Data.ByteString.hGetLine: end of file* I tried using the TLS port of 587, but then I just get an authentication failure. Am I using the wrong library or is it just the wrong configuration. Thanks.

4 10

Summer of Haskell - Now Accepting Applications!
by Gershom B 27 Apr '16

27 Apr '16

(Note: I am posting this on behalf of Edward Kmett who has spotty availability at the moment) We've posted an official Summer of Haskell website: https://summer.haskell.org/ It contains the full timeline for the program this summer, and most importantly, a form for submitting student applications, also linked here: https://docs.google.com/forms/d/1JLIa58u7AcWN31bH_LcCBujN9uzAwfd3HxrT32qz9_… The student application period ends next Friday, May 6. Last year we had a very successful project brainstorming thread. This led to our largest and most successful Summer of Code, ever. There is a thread on reddit now devoted to the purpose of further brainstorming for this year: https://www.reddit.com/r/haskell/comments/4gh1tu/summer_of_haskell_now_acce… If you have a proposal that you think a student could make a good dent in over the course of a summer, especially one with broad impact on the community, please feel free to discuss it there. If you are a potential student, please feel free to skim the proposals for ideas, or put forth ones of your own! If you are a potential mentor, please feel free to comment on proposals that interest you, put forth ideas looking for students and express your interest, to help us pair up potential students with potential mentors. Ultimately, the project proposals that are submitted get written by students, but if we can give a good sense of direction for what the community wants out of the summer, we can improve the quality of proposals, and we can recruit good mentors to work with good students on good projects. Resources: We have a wiki on https://ghc.haskell.org/trac/summer-of-code/ It is, of course, a Wiki, so if you see something out of order, take a whack at fixing it. We have an active #haskell-gsoc channel on irc.freenode.net that we run throughout the summer. Potential mentors and students alike are welcome. We have a Trac full of suggested Google Summer of Code proposals both current and from years past, but it could use a whole lot of eyeballs and an infusion of fresh ideas: https://ghc.haskell.org/trac/summer-of-code/report/1 Many of our best proposals in years have come from lists of project suggestions that others have blogged about. Many of our best students decided to join the summer of code based on these posts. The Trac isn't the only source of information on interesting projects, and I'd encourage folks to continue posting their ideas. If you or your employer would be interested in helping to fund additional students, please feel free to reach out to me. Thank you, -Edward Kmett

2 2