- Haskell-Cafe - Haskell.org

cabal update problem?
by yi lu 17 Aug '17

17 Aug '17

Hi all, I find cabal 2.0.0.0 brings new change both in downloading package list and the hackage mirror. 1) 00-index.tar.gz has be replaced by 01-index.tar.gz 2) If timestamp.json exists and it is new enough, 01-index.tar.gz won't download, even though I don't have the latest copy of 01-index.tar.gz. So my questions are 1) What else has been changed to hackage.haskell.org mirror? I found four more files: mirrors.json root.json snapshot.json timestamp.json 2) Is it a feature or a bug? Yours, Yi

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by Dmitriy Matrosov 17 Aug '17

17 Aug '17

Hi. Is there a way to avoid `UndecidableInstances` in following code: data A f = A {_a1 :: f String} instance Show (f String) => Show (A f) where it does not compile with 1.hs:4:10: error: • The constraint ‘Show (f String)’ is no smaller than the instance head (Use UndecidableInstances to permit this) • In the instance declaration for ‘Show (A f)’ Though, initially, this was {-# LANGUAGE RankNTypes #-} data A f = A {_a1 :: f String} instance forall f a. Show (f a) => Show (A f) where which also does not compile with 1.hs:5:10: error: • Variable ‘a’ occurs more often in the constraint ‘Show (f a)’ than in the instance head (Use UndecidableInstances to permit this) • In the instance declaration for ‘Show (A f)’ The error is different and i don't sure, that this two cases are related. I want these instances to make a type with many records parametrized by `Alternative` type, e.g. data Volume t = Volume { _volName :: t Name , _volSize :: t Size , _volPath :: t Path , _pool :: t Pool } When i try to make instances, which require `*` type, i will end with above cases. -- Dmitriy

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Re: [Haskell-cafe] Precedence of plus and bit-wise-and
by Doug McIlroy 16 Aug '17

16 Aug '17

Sorry for hitting return prematurely. The final sentence below tells the tale. The reason for the low precedence of & and | in PL/I and C is that those were used as Boolean operators in conditions. For example lessequal(a,b) { return(a<b | a==b); } That usage was baked in before the short-cut operators && and || were introduced in C. doug

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Re: [Haskell-cafe] Precedence of plus and bit-wise-and
by Doug McIlroy 16 Aug '17

16 Aug '17

The reason for the low precedence of & and | in PL/I and C is that those were used as Boolean operators in conditions. For example lessequal(a,b) { return(a<b | a==b); }

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Precedence of plus and bit-wise-and
by Levent Erkok 16 Aug '17

16 Aug '17

I was surprised today to find that C and Haskell differ on the precedence of addition and bit-wise and. In Haskell, bit-wise-and (.&.) binds tighter. In C, it's the other way around. While I like Haskell's precedence better, this was a gotcha; and I wondered if this behavior is documented somewhere. I looked through the Haskell wiki, but couldn't find anything pertinent. Discrepancies are always problematic. Without bike-shedding a lot, does anyone know if this was a conscious decision or merely a more natural one to take? Cheers, -Levent.

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GHC 8.2.1 + lld on FreeBSD
by Sumit Raja 15 Aug '17

15 Aug '17

Hi, I've been playing with ghc 8.2.1 and clang + lld on FreeBSD 11. I wasn't able to get the installation as per release notes to select lld instead of gold (will spend more time exploring why when I get a chance) so I modified the settings file after install. I have builds running okay but I keep seeing the following: [1 of 1] Compiling Main ( hs.hs, hs.o ) Linking hs ... <no location info>: error: Warning: Couldn't figure out linker information! Make sure you're using GNU ld, GNU gold or the built in OS X linker, etc. I've set up ld.lld39 as the shipped lld is 4.0. Although not affecting the actual build ("Hello World") but was interested to know why this was happening. Settings file is below: [("GCC extra via C opts", " -fwrapv -fno-builtin"), ("C compiler command", "clang"), ("C compiler flags", " -fno-stack-protector"), ("C compiler link flags", "-L/usr/local/lib -fuse-ld=lld39"), ("C compiler supports -no-pie", "NO"), ("Haskell CPP command","clang"), ("Haskell CPP flags","-E -undef -traditional"), ("ld command", "ld.lld"), ("ld flags", "-L/usr/local/lib"), ("ld supports compact unwind", "YES"), ("ld supports build-id", "YES"), ("ld supports filelist", "NO"), ("ld is GNU ld", "YES"), ("ar command", "/usr/bin/ar"), ("ar flags", "clqs"), ("ar supports at file", "NO"), ("touch command", "touch"), ("dllwrap command", "/bin/false"), ("windres command", "/bin/false"), ("libtool command", "libtool"), ("perl command", "/usr/local/bin/perl"), ("cross compiling", "NO"), ("target os", "OSFreeBSD"), ("target arch", "ArchX86_64"), ("target word size", "8"), ("target has GNU nonexec stack", "True"), ("target has .ident directive", "True"), ("target has subsections via symbols", "False"), ("target has RTS linker", "@HaskellHaveRTSLinker@"), ("Unregisterised", "NO"), ("LLVM llc command", "llc39"), ("LLVM opt command", "opt39") ] Thanks Sumit

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Haskell in Leipzig 2017: last call for papers
by Wolfgang Jeltsch 15 Aug '17

15 Aug '17

Hi! I want to kindly remind you that the submission deadline of Haskell in Leipzig 2017 is on Friday this week. Looking forward to your contributions. ☺ Event: Haskell in Leipzig 2017 Time: October 26–28, 2017 Place: HTWK Leipzig, Germany Homepage: https://hal2017.softbase.org/ About ===== Haskell is a modern functional programming language that allows rapid development of robust and correct software. It is renowned for its expressive type system, its unique approaches to concurrency and parallelism, and its excellent refactoring capabilities. Haskell is both the playing field of cutting-edge programming language research and a reliable base for commercial software development. The workshop series Haskell in Leipzig (HaL), now in its 12th year, brings together Haskell developers, Haskell researchers, Haskell enthusiasts, and Haskell beginners to listen to talks, take part in tutorials, join in interesting conversations, and hack together. To support the latter, HaL will include a one-day hackathon this year. The workshop will have a focus on functional reactive programming (FRP) this time, while continuing to be open to all aspects of Haskell. As in the previous year, the workshop will be in English. Contributions ============= Everything related to Haskell is on topic, whether it is about current research, practical applications, interesting ideas off the beaten track, education, or art, and topics may extend to functional programming in general and its connections to other programming paradigms. Contributions can take the form of * talks (about 30 minutes), * tutorials (about 90 minutes), * demonstrations, artistic performances, or other extraordinary things. Please submit an abstract that describes the content and form of your presentation, the intended audience, and required previous knowledge. We recommend a length of 2 pages, so that the program committee and the audience get a good idea of your contribution, but this is not a hard requirement. Please submit your abstract as a PDF document at https://easychair.org/conferences/?conf=hal2017 until Friday, August 18, 2017. You will be notified by Friday, September 8, 2017. Hacking Projects ================ Projects for the hackathon can be presented during the workshop. A prior submission is not needed for this. Invited Speaker =============== * Ivan Perez, University of Nottingham, UK Invited Performer ================= * Lennart Melzer, Robert-Schumann-Hochschule Düsseldorf, Germany Program Committee ================= * Edward Amsden, Plow Technologies, USA * Heinrich Apfelmus, Germany * Jurriaan Hage, Utrecht University, The Netherlands * Petra Hofstedt, BTU Cottbus-Senftenberg, Germany * Wolfgang Jeltsch, Tallinn University of Technology, Estonia (chair) * Andres Löh, Well-Typed LLP, Germany * Keiko Nakata, SAP SE, Germany * Henrik Nilsson, University of Nottingham, UK * Ertuğrul Söylemez, Intelego GmbH, Germany * Henning Thielemann, Germany * Niki Vazou, University of Maryland, USA * Johannes Waldmann, HTWK Leipzig, Germany Questions ========= If you have any questions, please do not hesitate to contact Wolfgang Jeltsch at wolfgang-it(a)jeltsch.info.

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[ANN] LeanCheck & Extrapolate: enumerative property-based testing & generalization of counter-examples
by Rudy Matela 14 Aug '17

14 Aug '17

Hello Haskell-café, I am happy to announce LeanCheck and Extrapolate, two enumerative property-based testing libraries (a.k.a. QuickCheck-style testing). https://hackage.haskell.org/package/leancheck https://hackage.haskell.org/package/extrapolate LeanCheck is a list-based enumerative property-based testing library with a very small core of only 180 lines of code. LeanCheck is somewhat stable and has actually been around for a while, but this is its first announcement on haskell-café. It works like so: $ ghci > import Test.LeanCheck > check $ \x y -> x + y == y + (x :: Int) +++ OK, passed 200 tests. Extrapolate works on top of LeanCheck, and its able to produce generalized counter-examples in addition to fully defined ones. Like so: $ ghci > import Test.Extrapolate > import Data.List (nub) > check $ \xs -> nub xs == (xs :: [Int]) *** Failed! Falsifiable (after 3 tests): [0,0] Generalization: x:x:_ Conditional Generalization: x:xs when elem x xs Extrapolate is under active development at the moment. To install both libraries, just: $ cabal update $ cabal install leancheck $ cabal install extrapolate They are also available on Stackage, installable via 'stack install': https://www.stackage.org/package/leancheck (lts-9 and nightly) https://www.stackage.org/package/extrapolate (just nightly) -- Rudy

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[ANN] first release of http2-client
by lucas di cioccio 14 Aug '17

14 Aug '17

Hello, I'm pleased to announce the first release of the http2-client [0] library on Hackage. This version is fresh and has limitations (see README.md) but I can successfully query pages with simple queries on a number of popular websites. I could validate server pushes against GoLang's HTTP2 server demo. I could also hammer Warp with dozens of concurrent requests just fine. I would not recommend using the package for more than evaluation at this point. If you are eyeing towards having a solid HTTP2 client soon, please consider trying http2-client and reporting bugs on the GitHub page [1]. I'm welcoming contribution to the code but I'm also looking forward to build some automation to validate against implementations found "in the wild". Starting this work also allowed me to report/fix a few bugs on Warp's HTTP2 implementation. Hence, even if you are not looking forward to be a http2-client user you may find interest in such work as we all benefit from this. [0] http://hackage.haskell.org/package/http2-client [1] https://github.com/lucasdicioccio/http2-client Best, -- Lucas

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Re: [Haskell-cafe] type (++) = (<>)
by Doug McIlroy 12 Aug '17

12 Aug '17

> What do you think of making (++) the same as (<>) This seems to be a call for returning to the old situation in which (++) was an operator of class MonadPlus. Why was that abolished in Haskell 98? Doug

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