April 2016 - Haskell-Cafe

Re: [Haskell-cafe] how ghc finds libgmp.so when linking
by Johannes Waldmann 14 Apr '16

14 Apr '16

more data: when I compile with -optl -L/usr/local/lib, then the correct library is found, when I put ("ld flags", "-L/usr/local/lib") in ghc-7.10.3/settings, then it's not, when I put ("C compiler link flags", "-L/usr/local/lib") in settings, then it is. So, I can leave it at that. But I still don't know how this came to be. I built this ghc-7.10.3 from source. - J.W.

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Haskell, stack and the Atom editor
by Thomas Koster 14 Apr '16

14 Apr '16

Hi friends, I want to switch to the Atom editor for my Haskell development. I have tried several times over the last few years and given up each time because hardly anything works. I can't stand Emacs so don't bother suggesting it. I am currently using Geany, which has served me well, mostly because it is dead simple so nothing can go wrong. In addition to the usual universal text editor functions, all it does is highlight syntax (albeit with an outdated grammar), runs custom commands to build, displays clickable errors in a separate pane and underlines them in the editor pane. But for larger projects, I want more (better autocompletion, better integration with the build system, better VCS support). But back to Atom, I am having extreme difficulty getting even the basics working. I am using GHC 7.10.2, Cabal 1.22.4.0, Stack 1.0.4. I installed Atom 1.6.1 from their deb package. I installed the "language-haskell" package. This works very well, thankfully, but is the *only* thing that I can get working. I avoided "ide-haskell", even though it looks great, because their readme says it can't build with stack and I rely heavily on stack. Instead, I installed the "build" and "linter" packages and created an ".atom-build" file to build my project with stack. It invokes stack correctly, but I can't get it to recognise any errors: linter always says "No issues". I must have tried twenty different regexes in "errorMatch" but none of them seem to work. I also installed "linter-hdevtools", hoping that that may let me see some errors, but I can't get that to work either. The package is loaded but it appears to do absolutely nothing. strace does show that Atom is trying to exec hdevtools through stack, but I get no output at all within Atom as evidence that something worked or failed. hdevtools appears to have its own problems. I had installed hdevtools (0.1.2.1) with stack: stack install hdevtools I ran it myself using: stack exec --no-ghc-package-path hdevtools -- check Main.hs and it gives me the infamous error: Cabal error: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with Cabal. This is strange because "stack exec --no-ghc-package-path env" reports that no such environment variable is defined, as expected. At this point I am ready to give up yet again. Does anybody have any last-minute advice on using Atom to develop with Haskell and Stack? Thanks, Thomas Koster

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[ANN] Vivid 0.2
by amindfv＠gmail.com 14 Apr '16

14 Apr '16

After about a year of blood, sweat, and tears, the new Vivid is out! This gets us to a pretty polished state and updates will now be much more frequent. What is Vivid? It's a library to create music (and other sound) using the SuperCollider synth engine -- a really powerful audio "rendering engine." Vivid lets you create and alter music in real time, or render audio files faster than real time. It's been used in live performances (not only by me!), and I'm very happy with its sound, its power, and its expressivity. Without further ado: http://hackage.haskell.org/package/vivid Happy hacking! Tom

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Lazy monadic queues
by David Feuer 13 Apr '16

13 Apr '16

I've come up with a very simple implementation of lazy monadic queues based loosely on ideas from Leon P. Smith's control-monad-queue package. This implementation ties a similar sort of lazy knot, but avoids the complexity and strictness of continuation-passing style. I'm curious whether something similar is already available on Hackage, and, if not, whether it would be useful enough to package it. The source code can currently be found at https://gist.github.com/treeowl/5c14a43869cf14a823473ec075788a74 David Feuer

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Using existential types from TAPL book in Haskell
by Dominik Bollmann 13 Apr '16

13 Apr '16

Hi all, I'm currently reading Prof. Pierce's TAPL book. Reading the chapter on existential types, I've been wondering how to use existentials in Haskell. In particular, in the TAPL book there is a section on purely functional objects, which are encoded (e.g.) as follows: counter = {*Nat, { state = 5 , methods = { get = \x:Nat. x , inc = \x:Nat. succ x } } } as Counter where Counter = {∃X, { state : X , methods = { get : X -> Nat , inc : X -> X } } } That is, a Counter consists of some internal state as well as methods `get` to retrieve its state and `inc` to increment it. Internally, a Counter is just implemented with a natural number. However, from the outside, we hide this fact by making its state an existential type X. Thus, we can only modify a Counter instance (e.g., counter) using its exposed methods. For example, let {X, body} = counter in body.methods.get (body.state) (see Ch 24.2 on Existential Objects for further details) I've been wondering how to write such counters in Haskell. In particular, how to write a Counter instance like the above counter that uses a concrete representation type Nat, while exposing its state to the outside world abstractly with an existential type X. I've been trying to do this, but so far I haven't succeeded... If anyone could hint me at how to do this, would help me understanding existentials better, I think :-). Cheers, Dominik.

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Simple Interpreter with State Monad
by Jake 13 Apr '16

13 Apr '16

I'm currently in a graphics class where, in order to provide a standard interface to all of our graphics libraries, we have to process small scripts that look like this: line 0 0 0 1 1 1 circle 0 0 10 scale 0 0 3 save pic.png I successfully wrote a parser with attoparsec that parses the file into a list of Commands. Now I'm trying to process that list to produce an IO action, and I thought the State monad would be useful to keep track of the persistent state of the shapes to draw and the transformations on them. I'm confused about how exactly to do this though. What should the type of my State be? Right now I have an execute function that is execute :: Command -> State ParseState (IO ()) where ParseState is a tuple of stuff. ParseState also includes an IO () because I wanted to be able to create multiple pictures one after another, and I couldn't figure out how to access the previous result value from State to add on to it in the next one. So can anyone offer advice on my specific situation, or maybe a simpler example of how to go about writing an interpreter with the State monad? Thanks, Jake

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how ghc finds libgmp.so when linking
by Johannes Waldmann 13 Apr '16

13 Apr '16

Hi. I have some obsolete libgmp in /usr/lib, and the correct one in /usr/local/lib, but ghc does not find it, resp., finds the /usr/lib one earlier, and this is missing some symbols /usr/local/lib/ghc-7.10.3/integ_2aU3IZNMF9a7mQ0OzsZ0dS/libHSinteger-gmp-1.0.0.0-2aU3IZNMF9a7mQ0OzsZ0dS.a(wrappers.o): In function `integer_gmp_mpn_xor_n': wrappers.c:(.text+0x1761): undefined reference to `__gmpn_xor_n' when I put ghc ... -optl -Wl,--verbose I see that: SEARCH_DIR("/usr/i486-linux-gnu/lib32"); SEARCH_DIR("/usr/local/lib32"); SEARCH_DIR("/lib32"); SEARCH_DIR("/usr/lib32"); SEARCH_DIR("/usr/i486-linux-gnu/lib"); SEARCH_DIR("/usr/local/lib"); SEARCH_DIR("/lib"); SEARCH_DIR("/usr/lib"); but that's apparently not what's happening. Is ghc changing this, and how can I work around it? - J.W.

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Finding way around libraries (was parsec reorganization)
by Rustom Mody 13 Apr '16

13 Apr '16

On Wed, Mar 30, 2016 at 10:06 PM, Joe Hillenbrand <joehillen(a)gmail.com> wrote: > Parsec isn't as maintained as Megaparsec > <https://mrkkrp.github.io/megaparsec/>. > > I recommend checking out the tutorials here > <https://mrkkrp.github.io/megaparsec/tutorials.html>. > So under the advice to use megaparsec instead of parsec, I tried the examples here: https://mrkkrp.github.io/megaparsec/tutorials/fun-with-the-recovery-feature… Got a page full of errors starting: fpl.hs:42:20: Not in scope: ‘<$>’ … Perhaps you meant one of these: On a hunch I changed import Control.Applicative (empty) to import Control.Applicative and then it started working. I can only expect that if megaparsec's own official tutorials are thus out of sync with current haskell, then parsec's will be even more so. Also I find no megaparsec on hoogle/hayoo So how does one go about handling the dilemma: - megaparsec : in development but undocumented on hayoo/hoohle - parsec : documented but bitrotten What/how do haskell pros go about handling such Thanks Rusi

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LumiGuide is hiring
by Bas van Dijk 13 Apr '16

13 Apr '16

Dear Haskellers, We're hiring a Haskell engineer to join our software engineering team. LumiGuide develops parking guidance systems for cars and bicycles. Our technology is based on optical detection techniques (Computer Vision). A nice example of one of our projects is the P-route Bicycle <https://bicycledutch.wordpress.com/2015/06/09/bicycle-parking-guidance-syst…> in Utrecht City. Functional Programming is used extensively at LumiGuide. Haskell, GHCJS and the Nix stack (Nix, NixOS, hydra and nixops) are some of the technologies we're using. I recently gave a talk <http://foswiki.cs.uu.nl/foswiki/pub/NlFpDay2016/WebHome/LumiGuide_FP-Dag-20…> at the Dutch Functional Programming Day that highlights some of the products and tools we're developing. We're looking for an experienced Haskell developer who wants to join our team full-time and will work from our office in Nijmegen, The Netherlands. As one of the first engineers you'll have a big impact on the culture and the technological direction of the company. If you're interested, please send a mail to bas(a)lumiguide.nl and tell me a bit about your experience (previous jobs, open source projects, things that you made that you're proud of, etc.). Best Regards, Bas van Dijk CTO LumiGuide

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Re: [Haskell-cafe] Proposal: Add functions to get consecutive elements to Data.List
by David Turner 13 Apr '16

13 Apr '16

Hi there, The idiom I normally use for this is 'zip xs (drop 1 xs)'. By using 'drop 1' instead of 'tail' it handles the empty case without partiality or needing an extra equation. Does that work for you? Cheers, David On 12 Apr 2016 21:55, "Johan Holmquist" <holmisen(a)gmail.com> wrote: I propose adding two new functions to Data.List: zipConsecutives :: [a] -> [(a,a)] zipConsecutives xs = zip xs (tail xs) zipConsecutivesWith :: (a -> a -> b) -> [a] -> [b] zipConsecutivesWith f xs = zipWith f xs (tail xs) (with possibly more efficient implementations) The Trac feature request is at https://ghc.haskell.org/trac/ghc/ticket/11815 . Why These functions are useful when one wants to process consecutive pairs of elements in a sequence, which is not uncommon. The ticket lists some examples, reiterated here: -- diff consecutive elements: diffs = zipConsecutivesWith (flip (-)) -- determine if list is ascending (similar for descending and strict): isAscending = and . zipConsecutivesWith (<=) -- an old friend of ours: fibs = 1 : 1 : zipConsecutivesWith (+) fibs -- get the edges of a closed path defined by points (ps): edges ps = zipConsecutivesWith makeEdge (ps ++ take 1 ps) Why add to Data.List (and base) The definition must either use an extra equation for the empty list case: zipConsecutives [] = [] zipConsecutives xs = zip xs (tail xs) which makes it non practical to define inline at each use site. Or one may omit the empty list equation, which is safe (thanks to laziness), but that may not be immediately obvious. (It wasn't to me anyway.) The tail function is generally considered unsafe so it is not desirable to force the user to use it. The proposed functions would offer an alternative. The Data.List module is often imported unqualified, so new identifiers may cause collissions. I do find it rather unlikely that the proposed names would cause such problems, however. Deadline for discussion: 2016-05-31. _______________________________________________ Libraries mailing list Libraries(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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