- Haskell-Cafe - Haskell.org

Using Intero on latest MacOS
by Evžen Wybitul 27 Jun '17

27 Jun '17

I’m using the latest developer beta of MacOS High Sierra. There are some problems with new Clang/LLVM; I can’t install terminfo, because of this problem: https://ghc.haskell.org/trac/ghc/ticket/13805 <https://ghc.haskell.org/trac/ghc/ticket/13805>. It seems it's been already fixed (and that it can be fixed by passing an argument to CFLAGS) - I don’t really know what CFLAGS are, though, and I also don’t know how can I update my local Clang/LLVM to include the new patch. Is somebody in a similar situation? I’d welcome any help.

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Re: [Haskell-cafe] Dynamically linking frameworks on macOS (Ryan Trinkle)
by Thomas Jakway 27 Jun '17

27 Jun '17

You could provide a wrapper script around ghc to pass -dynamic automatically. Or when installing append alias ghc="ghc -dynamic" to their bashrc. Not elegant but it's probably easier than any other solution. On 06/24/2017 08:00 AM, haskell-cafe-request(a)haskell.org wrote: > Message: 1 > Date: Fri, 23 Jun 2017 11:31:37 -0400 > From: Ryan Trinkle<ryan.trinkle(a)gmail.com> > To: Haskell Cafe<haskell-cafe(a)haskell.org> > Subject: [Haskell-cafe] Dynamically linking frameworks on macOS > Message-ID: > <CAHNEpix8NeMNo%3Dur3FX_Hs4Li8RTx4t0KT7aUutVGcN4_J5XVQ(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi everyone, > > I'm trying to ensure that beginner-level code that uses reflex-dom can be > built by simply calling 'ghc myFile.hs', provided that all the necessary > libraries are available in ghc-pkg. This works on linux, but on macOS, > it's currently necessary for the user to add '-dynamic' to the command > line. In particular, without -dynamic, the linker fails to find > "_OBJC_CLASS_$_NSURL", which is a symbol in the Foundation system > framework. Apple doesn't support (and strongly discourages) statically > linked executables[1], so it's not terribly surprising that this doesn't > work. > > Is there anything I can do to avoid the need for -dynamic here? > > > Thanks, > Ryan > > [1]https://developer.apple.com/library/content/qa/qa1118/_index.html

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OpenCV Haskell: Missing C libraries (macOS)
by Geraldus 26 Jun '17

26 Jun '17

Hi folks! I'm trying to build opencv <http://lumiguide.github.io/haskell-opencv/doc/index.html> as dependency on my macOS Sierra. I stuck with missing _contrib and _legacy C libraries. I've installed OpenCV with contrib packages via brew. Here are the steps I've done. Installation I have installed opencv via brew (with some patches <http://www.pyimagesearch.com/2017/05/15/resolving-macos-opencv-homebrew-ins…> of brew formula): brew install opencv3 --with-contrib --with-python3 python Python 3.6.1 (default, Apr 4 2017, 09:40:21) [GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin Type "help", "copyright", "credits" or "license" for more information.>>> import cv2>>> cv2.__version__'3.2.0' I have not installed Python 2.7 bindings. Python 3.6 only. Now stack build need some attention too: stack build opencv-0.0.1.0: configure Progress: 1/2 -- While building package opencv-0.0.1.0 using: Configuring opencv-0.0.1.0... setup: Missing dependencies on foreign libraries: * Missing C libraries: opencv_calib3d, opencv_imgproc, opencv_contrib, opencv_legacy, This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. Following suggestion I've tried specify include and lib dirs: stack build --extra-lib-dirs=/usr/local/opt/opencv3/lib --extra-include-dirs=/usr/local/opt/opencv3/include opencv-0.0.1.0: configure Progress: 1/2 -- While building package opencv-0.0.1.0 using: Configuring opencv-0.0.1.0... setup: Missing dependencies on foreign libraries: * Missing C libraries: opencv_contrib, opencv_legacy, This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. I have no idea how to find where missing libraries are located. Would appreciate any help! Arthur!

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stack, git, directory structure
by Dennis Raddle 26 Jun '17

26 Jun '17

I have a few basic questions about my project organization using stack. I'm going to have several executables and a hierarchical library. For instance, let's say I'm going to call my overall system "cac" (computer aided composition). I'll have imports like import Cac.Util import Cac.Search import Cac.Search.Algo1 -- some modules that assist things import XDoc import XDoc.Parse and so on. I'll have executables named 'foo', 'bar', etc. So would the following be correct? - call the directory where I put all stack packages $PROJ - call the top level of my specific project $PROJ/cac, which will have $PROJ/cac/cac.cabal - I'll have the following files and dirs: - modules: - $PROJ/cac/src/Cac.hs - $PROJ/cac/src/Cac/ - $PROJ/cac/src/Cac/Util.hs - $PROJ/cac/src/Cac/Search.hs - $PROJ/cac/src/Cac/Search/ - $PROJ/cac/src/XDoc.hs - $PROJ/cac/src/XDoc/ - $PROJ/cac/src/XDoc/Parse.hs - executables: - $PROJ/app/foo.hs - $PROJ/app/bar.hs Also, if I'm going to use git, would my .git/ directory be at $PROJ/cac/.git/ ? thanks D

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External scripts for use in testing
by Ivan Lazar Miljenovic 26 Jun '17

26 Jun '17

I'm trying to write a test suite for a package, but to properly ensure it's working I need to have some sample script/program for it to interact with. What would be the best way to ship this script/program? I was considering using extra-source-files but the Paths_* support from Cabal doesn't seem to list those. So should I ship them as a data file (which could have an issue if they aren't marked executable and installed globally)? Or blindly trust that the test suite is being run from within the source directory and try to guess/determine it's location from that? Furthermore, what about if it's an actual (small) program that requires compilation? Or am I completely out of luck at this point? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Munich Haskell Meeting, 2017-06-27 @ 19:30
by Christian Neukirchen 25 Jun '17

25 Jun '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, June 27 at Hirschgarten (self-service area) at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jun-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Proposal: Instance apartness guards
by Anthony Clayden 24 Jun '17

24 Jun '17

After years of pondering this idea (in various forms), and several rounds of discussion on several forums, I've written it up. "This proposal tackles the thorny topic of Overlapping instances, for both type classes and Type Families/Associated types, by annotating instance heads with type-level apartness Guards. Type-level disequality predicates appear in Sulzmann & Stuckey 2002; in the type-level case selection in HList 2004; and in various guises in Haskell cafe discussions in following years. This proposal builds on the apartness testing implemented as part of the Closed Type Families work." All feedback welcome. https://github.com/AntC2/ghc-proposals/blob/instance-apartness-guards/propo… AntC

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Implicit Multi Threading in Switch Case
by Yotam Ohad 24 Jun '17

24 Jun '17

Hi, After reading "Push-Pull Functional Reactive Programming <https://pdfs.semanticscholar.org/fb7a/879d639641341e025197b40afad9e21f0ce5.…>" I had an idea about deciding which, of two events, comes first. Instead of using forkIO, I tried something like the following: infi :: Maybe Int infi = infi stop :: Maybe Int stop = Nothing test :: Int test = case (infi, stop) of (Nothing, Just _) -> 1 (_, _) -> 2 Here, infi is an action that never ends, and stop a function that ends immediately. I thought that the compiler would see that stop evaluates immediately to Nothing and thus will return 2, but it tries to evaluate infi and get stuck. I think it happens because I am using a tuple to hold both values (but not really sure about it). Do you know a way to make this arrangement work? Yotam

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Why can't GHC infer record fields?
by Clinton Mead 23 Jun '17

23 Jun '17

I asked this below question on Stack Overflow <https://stackoverflow.com/questions/44714711/silly-duplicated-record-fields…> and I got an answer suggesting that GHC doesn't support type inference of overloaded record fields. But I'm wondering why this is the case. Consider the following: {-# LANGUAGE DuplicateRecordFields #-} data A = A { name :: String } data B = B { name :: String } main = print $ name (A "Alice") When compiled, I get the following message (on GHC 8.0.2) duplicatedrecords.hs:7:16: error: Ambiguous occurrence ‘name’ It could refer to either the field ‘name’, defined at duplicatedrecords.hs:5:14 or the field ‘name’, defined at duplicatedrecords.hs:3:14 But if I modify the main line as follows: main = print $ name ((A "Alice") :: A) Compilation proceeds successfully. I'm not sure why I need the :: A here, it seems to be clear that (A "Alice") is of type A due to the A constructor. It's worth noting that the following compiles fine: data A = A { a_name :: String }data B = B { b_name :: String } class Name t where name :: t -> String instance Name A where name = a_nameinstance Name B where name = b_name main = print $ name (A "Alice") We can even go further as follows, allowing different result types: {-# LANGUAGE TypeFamilies #-} data A = A { a_name :: String }data B = B { b_name :: Int } class Name t where type family T t name :: t -> T t instance Name A where type T A = String name = a_name instance Name B where type T B = Int name = b_name main = print $ name (A "Alice") It seems like GHC just has to mechanically add a class for each unique record name and an instance for each record in each data type. This will mean however that name x == name y not implying that the types of x and y are the same but I'd expect that when using this extension anyway. Just wondering if there's anything tricky I'm missing here regarding the implementation or that it just needs someone to implement it?

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exceptions, errors, random
by Dennis Raddle 23 Jun '17

23 Jun '17

I am wondering what exception/error and random classes I should use for my application. The application will be doing Monte Carlo backtracking search. I want it to be multithreaded and run on all four cores on my MacBook. I need detailed error messages in order to investigate errors. So I will throw errors or exceptions, then catch them in lower functions and annotate them with the parameters or computations within the lower function, then re-throw until finally the exception pops into the terminal. Because it's multi-threaded, at least some parts will needs to be in IO. Also, because it's a Monte Carlo algorithm it will need to make pseudorandom choices. Perhaps I should put everything in the IO monad? This will give me access to threads and the standard generator seed. But how about throwing, catching/rethrowing exceptions in IO? I have done this in pure code before: not sure if it's better to do it that way, in which case I could put some kind of state monad to hold the random seed together with an error monad and IO at the core of the monad stack. Any advice welcome. D

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