- Haskell-Cafe - Haskell.org

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 23 Jun '17

23 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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Having trouble with this recursive Parsec parser
by Cody Goodman 23 Jun '17

23 Jun '17

Hi all, I'm having trouble with a recursive parsec parser that keeps recursing on one type of end tag and terminates on another type of end tag. I'm sure I'm stuck on something silly, but here's what I have so far. Git repo with stack/cabal project: https://github.com/codygman/megaparsectest/blob/master/library/Example.hs Code I have so far (I deleted a few different approaches I tried because I thought they would clutter things up and make it harder to assist me with this issue): {-# LANGUAGE QuasiQuotes #-} -- | An example module. module Example where import Text.Megaparsec import Text.RawString.QQ import Text.Megaparsec.String -- input stream is of the type ‘String’ import qualified Text.Megaparsec.Lexer as L import Control.Monad (void, join) ex :: String ex = [r| begin field1 string begin field11 int field12 string end subsection; // optional end; |] data Field = Field String String deriving Show data Block = Fields [Field] | Block [Field] deriving Show sc :: Parser () sc = L.space (void spaceChar) lineCmnt blockCmnt where lineCmnt = L.skipLineComment "//" blockCmnt = L.skipBlockComment "/*" "*/" field :: Parser Field field = dbg "field" $ do sc Field <$> someTill ((oneOf' (['a'..'z'] ++ ['0'..'9']))) spaceChar <*> some ((oneOf' (['a'..'z'] ++ ['0'..'9']))) endEof = do sc *> string "end" *> char ';' *> sc *> eof pure "" endIdent = do string "end" *> sc ident <- someTill ((oneOf' (['a'..'z'] ++ ['0'..'9']))) (char ';') sc *> eof pure ident block = error "TODO implement" -- Thanks, -- Cody

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Issue 60 :: Haskell Weekly
by Taylor Fausak 22 Jun '17

22 Jun '17

\ Haskell Weekly \/\/ Issue 60 <https://haskellweekly.news/issues/60.html> Welcome to another issue of Haskell Weekly! Haskell is a purely functional programming language that focuses on robustness, concision, and correctness. This is a weekly summary of what's going on in its community. ## Featured ## - Understanding ResourceT <https://www.fpcomplete.com/blog/2017/06/understanding-resourcet> > Instead of just teaching you how to use the library, this post will demonstrate why you need it and how it works internally, to help you avoid some of the potential pitfalls of the library. - Announcing Weeder: dead export detection <https://neilmitchell.blogspot.com/2017/06/announcing-weeder-dead-export-det…> > Most projects accumulate code over time. To combat that, I've written Weeder which detects unused Haskell exports, allowing dead code to be removed (pulling up the weeds). - Dhall is now a template engine <http://www.haskellforall.com/2017/06/dhall-is-now-template-engine.html> > Now you can use Dhall as a template engine with the newly released dhall-text library which provides a dhall-to-text executable for templating text. - Front Row is hiring a senior backend Haskell engineer <https://frontrow.workable.com/j/463B843754> > Come change how 6.5+ million K-12 US students learn Math, Language Arts, Social Studies and more. Use data, advanced type systems, great product design and deep pedagogy to change lives. - Testing PostgreSQL for fun <https://hackernoon.com/testing-postgresql-for-fun-af891047e5fc> > In this post I'll show how to make database programming fun by using packages that make testing easy. I'll walk through a example of building a durable queue backed with PostgreSQL. - Exceptional Servant handling <http://www.parsonsmatt.org/2017/06/21/exceptional_servant_handling.html> > There are various good reasons why you might want to strip ExceptT from your Servant handlers. There are various good reasons why you wouldn't want to do that. I'm in the first camp --- I don't want ExceptT over IO. - On competing with C using Haskell <https://two-wrongs.com/on-competing-with-c-using-haskell> > By writing very straight-forward pure, functional Haskell code without any special trickery, you can get performance equivalent to what you'd get by calling out to a C function, which is also in the same order of magnitude as the same algorithm implemented in a program completely written in C. - How well does it work? Profiling in Haskell <https://mmhaskell.com/blog/2017/6/19/profiling-in-haskell> > This is where benchmarking and profiling come in. We're going to take a specific problem and learn how we can use some Haskell tools to zero in on the problem point. - The chain in blockchain <http://haroldcarr.com/posts/2017-06-19-the-chain-in-blockchain.html> > This post develops a simple blockchain with the goal of understanding the basics of the chain. The most straightforward part of a blockchain is the chain itself, a sequence of blocks. - Mezzo: type-safe music composition <https://github.com/DimaSamoz/mezzo/blob/1d46c5edd5d7d360a027a614c618bb2830d…> > Its novelty is in the fact that it can enforce various rules of music composition statically, that is, at compile-time. This effectively means that if you write "bad" music, your composition will not compile --- think of it as a very strict spell-checker for music. - Linear types 101 and its relevance to streams <https://m0ar.github.io/safe-streaming/2017/06/19/linear-types-101.html> > It creates a resource aware type system, which not only knows types of values but also number of uses; a linearly typed value is guaranteed to only have one reference to it at any time, duplication or ignoring will not compile. ## Package of the week ## This week's package of the week is async, a library that allows you to run IO operations asynchronously and wait for their results. <https://www.stackage.org/lts-8.19/package/async-2.1.1.1> ## In brief ## - An alternate definition of Haskell's Functor type class <https://github.com/rampion/kinder-functor/blob/20e5c57/README.md> - Configurable data types <http://mazzo.li/posts/customizable-data-types.html> - Generic unification <https://ro-che.info/articles/2017-06-17-generic-unification> - The linearity monad <https://www.cis.upenn.edu/~jpaykin/papers/pz_linearity_monad_2017.pdf) (pdf> - A terminal interface for Tetris <https://github.com/samtay/tetris/blob/5d9c74c/README.md>

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