April 2014 - Haskell-Cafe

Looking for maintainer of pwstore-fast
by Michael Snoyman 09 Apr '14

09 Apr '14

I just tried emailing Peter Scott, the maintainer of pwstore-fast[1], but the listed email address is no longer working. He also hasn't been responding to Github pings. Does anyone know how to get in touch with him? [1] http://hackage.haskell.org/package/pwstore-fast

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Heart Bleed bug in OpenSSL
by Vasili I. Galchin 09 Apr '14

09 Apr '14

http://heartbleed.com/ Ok .. I just scanned this .. but is this problem a "logic" bug in the OpenSSL C/C++ code or is a type correctness issue? Thanks, Vasili

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Re: [Haskell-cafe] ANNOUNCE: GHC version 7.8.1
by Felipe Lessa 09 Apr '14

09 Apr '14

Em 09-04-2014 11:23, Austin Seipp escreveu: > An updated SHA256SUMS.sig is attached. Thanks for Edsko de Vries for > pointing it out! If anybody is, like myself, trying to find a link for the SHA256SUMS file on the download page, here it is: https://www.haskell.org/ghc/dist/7.8.1/SHA256SUMS For some reason it seems that it's not linked anywhere on that page. Cheers, and thanks for the release! =) -- Felipe.

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Library for creating interactive diagrams?
by Kai Zhang 09 Apr '14

09 Apr '14

Hi cafe, Is there any active project about creating interactive diagrams, like D3js http://d3js.org/ ? The Diagrams library has already done a good job of making static images. Would it be possible to extend this library to support declaring interactive diagrams and then use JS and HTML as backend to draw them? Example: circle 1 $ with & hover .~ (change style) PS: I don't want a D3js binding.

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GHC Optimization seriously hurting performance by unsharing.
by Michael Karcher 09 Apr '14

09 Apr '14

Hello Haskell folks, Can you give me a hint whether the GHC optimizer behaviour demonstrated below should be considered a bug? If it is no bug, what should I keep in mind to make sure not to hit that trap again? This posting is (obviously) formatted as literal Haskell file, so feel free to try. This is a case where -O2 completely kills performance, because it unshares an object that should be shared. I stumbled across this behaviour of GHC 7.6.3 when trying to solve RMQ from hackerrank. Lets start with a typical header, the reason for the IORef module is discussed later. > module Main where > import Data.Set (member, fromAscList) > import Control.Monad (replicateM_) > import Data.IORef Now lets get to the code. > main = do The original problem statement requested to read a set of numbers from the standard input. To have this testcase self-contained, we use a "virtual file" instead, which is an (IORef String). The key point is that we need to get the string with the space-sperated test element from an IO operation, whereas it does not matter whether it is "getLine" or "readIORef" > ior <- newIORef $ unwords $ map show $ [1..20000] Now read the space-separated test input from an IO action. > testdata <- fmap (map read.words) $ readIORef ior and build a set from the number array we obtained to speed up membership queries. You don't want to search 20.000 numbers on each iteration. > let bigset = fromAscList testdata finally query the membership of the number 5 several times. This happens to be reasonably fast if compiled without optimization, but goes down to dog slow if compiled with optimization, as the set building process is hoisted into the loop and thus performed 10 times. > replicateM_ 10 . print . member 5 $ bigset this is the solution (or the workaround) to the problem. If you use this line, things get fast, because the loop gets created after evaluting the set to WHNF, thus forcing sharing replicateM_ 10 . print . member 5 $! bigset Enabling both of the replicateM_ lines also speeds up the first one, as the use of bigset in both lines prevents hoisting set construction into the first loop. Regards, Michael Karcher

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ANNOUNCE slot-lambda
by ducis 08 Apr '14

08 Apr '14

http://hackage.haskell.org/package/slot-lambda It lets your write lambdas with 'slots' without inventing names for the parameters. [s| ı + ı |] = \x y -> x+y The unicode character ı(305) representing a 'slot' can be input in vim with the digraph 'i.' . Use _ı to refer to the immediate left ı, and _0, _1, _2, ... to refer to the 1st, 2nd, 3rd, ... arguments respectively. e.g. [s| ı : ı : _ı : ı : _ı : _ı : _0 : [] |] 'a' 'b' 'c' = (\x y z -> x:y:y:z:z:z:x:[]) 'a' 'b' 'c' = "abbccca" I originally intended using '_' to represent the 'slots' but that doesn't parse as legal haskell. Actually I wonder how to achieve that without rolling my own haskell parser or forking haskell-src-exts.

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Towards a better time library (announcing tz)
by Mihaly Barasz 08 Apr '14

08 Apr '14

I would like to propose reforming the 'time' [1] library. Initially, I was just planning to announce my new 'tz' [2] library, but realized that I have a more important agenda. Namely: Haskell needs a better time library! Let me summarize what are ‒ in my view ‒ the biggest deficiencies of 'time': 1. Inefficient data structures and implementations. 2. Ad-hoc API which is hard to remember and frustrating to work with. 3. Conceptually wrong representations and/or missing concepts. The wonderful thyme [3] package (by Liyang HU) improves a lot on #1 by choosing better data structures and careful implementations and on #2 by lensifying the API. But, it was the #3 that caused me the most frustration lately; most importantly the time zone handling. There is a TimeZone data type in 'time', but it is a misnomer, it basically represents a fixed time difference (with a label and a DST flag). 'time' basically adapts the broken approach from libc: you can work with one time zone at a time, which is defined globally for your program (via the TZ environment variable). So, the transformation between UTCTime and LocalTime which should have been a pure function can only be done in IO now. Like this: do tz <- getTimeZone ut return $ utcToLocalTime tz ut Oh, and just to hammer down on the point #1 from the list above. This code runs in about 6100 ns on my machine. The drop-in replacement from tz: utcToLocalTimeTZ [4] (which is actually pure) runs in 2300 ns. While this is a significant improvement, it's easy to miss the point where the bulk of the inefficiency comes from the data structures. In my main project we represent times as Int64 (raw nanoseconds since UNIX epoch; and similar representation for zoned times). And to convert those to and from different time zones we need 40 ns. That's right, a 150 _times_ improvement! (There are many other interesting benchmark results that I could mention. An exciting bottom line: we can actually beat the libc in many use-cases!) The 'tz' package is still very much in flux. I will try to solidify the API soon, but until then it should be considered more of a proof of concept. There is some missing functionality, for example. On the other hand, there are the 'timezone-series' [5] and 'timezone-olson' [6] packages that together provide about the same functionality as 'tz' (minus the efficiency), and I'd like to explore if we could remove some of the overlap. But, all kind of suggestions and requests are welcome! More importantly, I'd like to hear the opinions of the community about the general issue of a better time library! Do we need one? How should we proceed about it? I think, Haskell could potentially have one of the best time libraries, but the current de-facto standard is mediocre at best. Unfortunately, designing a good time library is very far from trivial, as many existing examples demonstrate. And I definitely don't know enough for it. (I understand time zone info files, now that I wrote tz, but that's just a tiny fraction of what's needed.) So, if you think you can contribute to the design (have important use-cases in mind, know good examples of API, have some experience working with dates and time, etc. etc.) ‒ speak up! Mihaly Footnotes: [1] http://hackage.haskell.org/package/time [2] http://hackage.haskell.org/package/tz [3] http://hackage.haskell.org/package/thyme [4] http://hackage.haskell.org/package/tz-0.0.0.1/docs/Data-Time-Zones.html#v:u… [5] http://hackage.haskell.org/package/timezone-series [6] http://hackage.haskell.org/package/timezone-olson

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Haskell + emacs + Ghc-mod 4.0.1
by Arnaud Bailly 08 Apr '14

08 Apr '14

Hello, I upgrade my emacs configuration with latest ghc-mod and all of sudden nothing works (again). When I try to load a .hs file from a project I got the first line highlighted with the error "cannot satisfy -package spec" and I cannot got type, info or doc from that file. Also I do not have anymore syntax errors highlighting I used to see. Any help greatly appreciated. I would also appreciate any pointer explaining how to setup and maintain a proper Haskell dev environment. Haskell is not the language I use at day work but I have been using it for years and every so often my emacs configuration is broken and I have a hard time getting it back to sane state. Thanks in advance Arnaud Here is my emacs configuration: ;; haskell coding (require 'haskell-mode) (require 'haskell-cabal) (autoload 'ghc-init "ghc" nil t) (add-hook 'haskell-mode-hook (lambda () (ghc-init))) (eval-after-load "haskell-mode" '(progn (define-key haskell-mode-map (kbd "C-,") 'haskell-move-nested-left) (define-key haskell-mode-map (kbd "C-.") 'haskell-move-nested-right) (define-key haskell-mode-map (kbd "C-c v c") 'haskell-cabal-visit-file) (define-key haskell-mode-map (kbd "C-c C-c") 'haskell-compile) (define-key haskell-mode-map (kbd "C-x C-d") nil) (setq haskell-font-lock-symbols t))) (add-hook 'haskell-mode-hook 'turn-on-haskell-decl-scan) (add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (eval-after-load "which-func" '(add-to-list 'which-func-modes 'haskell-mode)) (eval-after-load "haskell-cabal" '(define-key haskell-cabal-mode-map (kbd "C-c C-c") 'haskell-compile)) Here is the output of ls -l ~/Library/Haskell/bin: lrwxr-xr-x 1 arnaud staff 45 Dec 27 22:36 HsColour -> ../ghc-7.6.3/lib/hscolour-1.20.3/bin/HsColour lrwxr-xr-x 1 arnaud staff 50 Dec 31 07:11 aeson-pretty -> ../ghc-7.6.3/lib/aeson-pretty-0.7/bin/aeson-pretty lrwxr-xr-x 1 arnaud staff 52 Dec 31 07:11 biblio2yaml -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/biblio2yaml lrwxr-xr-x 1 arnaud staff 49 Jan 10 16:25 cabal -> ../ghc-7.6.3/lib/cabal-install-1.18.0.2/bin/cabal lrwxr-xr-x 1 arnaud staff 46 Jan 15 13:51 cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/cabal-dev lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 cpphs -> ../ghc-7.6.3/lib/cpphs-1.17.1/bin/cpphs lrwxr-xr-x 1 arnaud staff 43 Dec 28 14:17 doctest -> ../ghc-7.6.3/lib/doctest-0.9.10/bin/doctest lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 fake-ghc-cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/fake-ghc-cabal-dev lrwxr-xr-x 1 arnaud staff 62 Jan 26 11:15 ghc-events -> ../../../projects/data-scientist/.cabal-sandbox/bin/ghc-events lrwxr-xr-x 1 arnaud staff 42 Apr 3 15:04 ghc-mod -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-mod lrwxr-xr-x 1 arnaud staff 43 Apr 3 13:35 ghc-modi -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-modi lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 ghc-pkg-6_8-compat -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/ghc-pkg-6_8-compat lrwxr-xr-x 1 arnaud staff 47 Dec 31 07:11 hakyll-init -> ../ghc-7.6.3/lib/hakyll-4.4.2.0/bin/hakyll-init lrwxr-xr-x 1 arnaud staff 45 Mar 31 13:57 hasktags -> ../ghc-7.6.3/lib/hasktags-0.68.7/bin/hasktags lrwxr-xr-x 1 arnaud staff 36 Mar 17 06:09 hell -> ../ghc-7.6.3/lib/hell-0.0.1/bin/hell lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 hlint -> ../ghc-7.6.3/lib/hlint-1.8.55/bin/hlint lrwxr-xr-x 1 arnaud staff 47 Apr 3 15:07 hspec-discover -> ../ghc-7.6.3/lib/hspec-1.9.1/bin/hspec-discover lrwxr-xr-x 1 arnaud staff 58 Dec 31 07:11 make-pandoc-man-pages -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/make-pandoc-man-pages lrwxr-xr-x 1 arnaud staff 62 Jan 15 04:25 operational-TicTacToe -> ../ghc-7.6.3/lib/operational-0.2.2.1/bin/operational-TicTacToe lrwxr-xr-x 1 arnaud staff 43 Jan 15 05:14 pandoc -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/pandoc lrwxr-xr-x 1 arnaud staff 56 Dec 31 07:11 pandoc-citeproc -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/pandoc-citeproc lrwxr-xr-x 1 arnaud staff 44 Dec 27 19:34 pdf2line -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdf2line lrwxr-xr-x 1 arnaud staff 43 Dec 27 19:34 pdfdump -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdfdump lrwxr-xr-x 1 arnaud staff 42 Jan 15 05:26 word2vec -> ../ghc-7.6.3/lib/word2vec-0.1/bin/word2vec lrwxr-xr-x 1 arnaud staff 43 Dec 31 07:11 yaml2json -> ../ghc-7.6.3/lib/yaml-0.8.5.2/bin/yaml2json lrwxr-xr-x 1 arnaud staff 34 Mar 13 08:58 yate -> ../ghc-7.6.3/lib/yate-0.1/bin/yate -- Arnaud Bailly FoldLabs Associate: http://foldlabs.com

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Question about typeclass & constraints
by jean-christophe mincke 08 Apr '14

08 Apr '14

Hello Consider the following data, class and instance: class PP m where create :: a -> m a data A a = A a instance PP A where create a = A a And then: class CB a where fb :: a -> Int data B m a = B (m a) If I try to define an instance of PP for B with the following implementation: instance (PP m) => PP (B m) where create a = let _ = fb a in B (create a) GHC issues the (expected) error: Could not deduce (CB a) arising from a use of 'fb' from the context (PP m). So the workaround I am using is to define a new class PP' where the type a is made more explicit. That allows the constraint (CB a) to be stated in an instance declaration. class PP' m a where create' :: a -> m a instance (PP m) => PP' m a where create' = create instance (PP m, CB a) => PP' (B m) a where create' a = let _ = fb a in B (create a) So, my question is: is there a way to achieve the same results without having to create the new class PP'? Thank you for your help J-C

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ANN: Melbourne Haskell Users Group
by Lyndon Maydwell 08 Apr '14

08 Apr '14

Hi Haskellers, A few people have been meeting up in the Melbourne CBD to talk about Haskell for the past couple of months. This month I've asked Bernie Pope (who organised the Melbourne Functional Programming Union) to give a talk about two libraries he has written in Haskell that compile Python code to Haskell, and Python Byte-Code. I thought I'd take the opportunity to announce the this meetup and the group in general to the wider Haskell community in case there is anyone in Melbourne who would be interested in coming along. The talk will be on Thursday, April 24. Details can be found at: http://www.meetup.com/Melbourne-Haskell-Users-Group/events/174401572/ Hope to see you there! - Lyndon

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