- Haskell-Cafe - Haskell.org

[ANNOUNCE] tasty-hunit-adapter and tasty-program
by Jan Stolarek 04 Feb '14

04 Feb '14

Dear Haskellers, I recently released two small packages that extend functionality of Roman Cheplyaka's tasty testing framework: 1. tasty-hunit-adapter allows to import existing HUnit tests into tasty. Hackage: http://hackage.haskell.org/package/tasty-hunit-adapter GitHub: https://github.com/jstolarek/tasty-hunit-adapter 2. tasty-program allows to run external program and test whether it terminates successfully. Hackage: http://hackage.haskell.org/package/tasty-program GitHub: https://github.com/jstolarek/tasty-program These packages are inspired by similar packages for test-framework. tasty-program has very basic functionality at the moment, most notably it completely ignores stdout and stderr of a program. If you want this functionality please say so here: https://github.com/jstolarek/tasty-program/issues/1 Jan Stolarek

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Fwd: incorrect MonadPlus law "v >> mzero = mzero"?
by Petr Pudlák 04 Feb '14

04 Feb '14

Since nobody replied so far, I'm also forwarding to cafe. Petr ---------- Forwarded message ---------- From: Petr Pudlák <petr.mvd(a)gmail.com> Date: 2014-01-29 Subject: incorrect MonadPlus law "v >> mzero = mzero"? To: "libraries(a)haskell.org" <libraries(a)haskell.org> Hi, this law apparently fails for a MonadPlus instance that has more than one possible failure value. Consider: runIdentity . runErrorT $ ((ErrorT . Identity $ Left "failure") >> mzero :: ErrorT String Identity ()) evaluates to `Left "failure"`, which isn't equal to ErrorT's mzero `Left ""`. This isn't just the case of ErrorT, it fails for any MonadPlus with multiple failure values. For example lift (tell "foo") >> mzero :: MaybeT (Writer String) () is again distinct from mzero. Actually, no monad transformer with a MonadPlus instance can satisfy the law, because the first part in front of `>> mzero` can introduce side effects in the underlying monad. I'm not sure what should be the proper solution. Perhaps to change the laws to: return x >> mzero = mzero (v >> mzero) >>= f = (v >> mzero)` That is, if an expression ends with `mzero`, it behaves like `mzero`. Petr

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CTO Opportunity
by Charles Weitzer 03 Feb '14

03 Feb '14

My name is Charles Weitzer. I do recruiting for major quantitative hedge funds worldwide. I have a client, a startup quantitative hedge fund located in Northern California, that would like to hire a Chief Technology Officer as soon as possible. Their job description can be found below. * Chief Technology Officer Fast-growing science and technology driven company seeks a Chief Technology Officer. We apply statistical machine learning to develop automated trading systems. We are a growing group of exceptional software and infrastructure professionals and seek an experienced candidate to lead and grow the team. The CTO will manage the implementation of our next generation infrastructure, including production trading systems, software infrastructure for our research department, and our compute and storage hardware systems. The CTO will work directly with the Head of Research and the CEO to set the technical direction of the company. A strong candidate will be well-versed in state-of-the-art technology (particularly distributed systems and cluster computing), and will have a track record of building and leading elite technical teams and successfully delivering complex projects (especially in a startup environment). A background in finance is not required. We are a science-driven systematic trading firm, built on the principle that statistical machine learning provides the best solutions to the scientific problems we must solve. Our CEO is a CS PhD and founded a successful Internet startup, and our Head of Research/Chief Investment Officer is a Berkeley statistics professor; both have a background in finance. Our trading is entirely automated and based on cutting-edge machine learning techniques. We have a collegial, academic working environment, rely on open source software, and have built our own software stack. We have a track record of strong performance and have been raising money and growing fast. Willingness to take initiative, and a gritty determination to productize, are essential. Benefits and compensation are highly competitive. I look forward to speaking with you and anyone you might highly recommend about this opportunity. Thank you, Charles Weitzer CEO/Senior Recruiter Charles Weitzer and Associates, Inc. Global Financial Recruiting Services Charles(a)CharlesWeitzer.com Voice: USA (510) 558-9182

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Dispatch a type-function on the existence (or not) of instances?
by Hans Höglund 03 Feb '14

03 Feb '14

Dear all, I have been curious about the ability to detect the presence of a certain instance (ClassFoo TypeBar) in the type system. Specifically, is it possible to "dispatch" a type on the existence (or not) of such an instance. For example given two functions: withInstance :: (ClassFoo TypeBar) => TypeIfInstanceExists withoutInstance :: TypeIfInstanceDoesNotExists I would be able to consolidate them into something like this: withOrWithoutInstance :: (r ~ InstanceExists ClassFoo TypeBar, a ~ If r TypeIfInstanceExists TypeIfInstanceDoesNotExists) => a I guess what I need is: 1) A type-level "if". 2) The possibility of "converting" a constraint into a type-level bool. I am sure (1) is possible but have no idea about (2). Anyone? Best regards, Hans

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What game libraries should I use?
by Alexander Berntsen 03 Feb '14

03 Feb '14

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 I need: - -2D graphics (preferably with simple shapes available) - -menus (I can make menus myself with shapes though) - -simple audio - -fonts (better than Gloss at least) - -keyboard input - -networking (simple direct connections between two computers) I need to be able to express my game system as declaratively as possible[0]. If the library is based on SDL/OpenGL, that would be nice[1]. This is, however, not necessary. What game libraries are the most mature and adequate for this? The only library I have used previously with Haskell is Gloss. I am not experienced in FRP, though I have read about and somewhat groked the point. I am willing to learn an FRP library. [0] I am writing it as part of a thesis in which I will look at the modularity and expressiveness of purely functional programming compared to object-oriented programming. Writing code that is technically purely functional, but in practice looks like imperative code, is sub-optimal. [1] For the thesis I am writing, I am implementing a C++ version of the same game. This will probably use SDL 2.0 and OpenGL bindings. If the Haskell version could use libraries based on that, this would make it easier for me to write my thesis. I.e., I would not have to spend time justifying that the modularity and expressiveness I am investigating is due to the chosen languages and programming paradigms, and not the libraries I use. - -- Alexander alexander(a)plaimi.net http://plaimi.net/~alexander -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iF4EAREIAAYFAlLrlMoACgkQRtClrXBQc7VkiAEAnEbfjiSLovcXttmjpdD5OSFI uEMBVBJdWonY9ZMPIc8BAIAQn+YMRvGILgb8WmuB9oTWJDVZfqDMyB47qzjyfimO =YGLW -----END PGP SIGNATURE-----

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ANNOUNCE: fpnla - A library for NLA operations
by Mauro Blanco 02 Feb '14

02 Feb '14

Hi. Today we are releasing fpnla <http://hackage.haskell.org/package/fpnla>, which defines a framework for linear algebra operations of BLAS and LAPACK. As its main features it allows: - Definition of multiple representations of vectors and matrices. - Arbitrary combination of strategies and structure representations. - Type-safe manipulation of context information associated to each strategy. - Definition of specialized strategies for a given representation. And also fpnla-examples <http://hackage.haskell.org/package/fpnla-examples>, which contains many example implementations of the operations defined in fpnla using various data structures and algorithms. Regards. -- Mauro Blanco

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New book: "Beginning Haskell"
by Alejandro Serrano Mena 02 Feb '14

02 Feb '14

[Disclaimer: I'm the author of the book] Dear Haskell-cafe, I would like to introduce a new book discussing our favourite language from beginner into upper intermediate level: "Beginning Haskell". The book starts assuming zero knowledge about functional programming and builds step by step to get into the realm of web applications, type-level programming, domain specific languages, distributed computing, unit testing and much more! You can look at the Table of Contents in Amazon [ http://www.amazon.com/Beginning-Haskell-A-Project-Based-Approach/dp/1430262… ]. The book revolves around the idea of a Time Machine Store: in each part some functionality is developed. The first part serves as an introduction and how to model the data with Haskell data types and functions; part 2 discusses many concepts around monads while developing two data-mining algorithms; in the third part storing and interfacing with clients serves as an excuse to introduce input/output, database access and web applications; part 4 is devoted to discussing domain specific libraries for describing offers; and the last part discusses testing and other engineering issues. All the "classical" topics in Haskell are discussed: higher-order functions, type classes -- functors, applicatives, monads --, laziness. The focus also lies on libraries use in modern Haskell daily programming, such as lenses, streaming data libraries, functional dependencies and type families, Software Transactional Memory and Cloud Haskell. One topic that is cross-cutting through the book is how to manage projects with Cabal or EclipseFP: initializing a new package, declaring dependencies, building and integrating tests. The book tries thus to offer a comprehensive view of the Haskell ecosystem, not only of the Haskell programming language. The book is published by Apress [http://www.apress.com/9781430262503] and available as paperback and electronic format. If somebody would like to make a review for some FP or programming-related magazine (such as The Monad Reader) or assess its possibilities for use in classroom, please drop me a line. Hope you like it! :)

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Castle: Permission denied
by Henk-Jan van Tuyl 02 Feb '14

02 Feb '14

L.S., I am trying to install castle, but I get the following messages: Downloading castle-0.1.0.0... Configuring castle-0.1.0.0... realgcc.exe: ./specs: Permission denied What can I do about this? I am running Windows XP, Haskell Platform 2013.2.0.0 Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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DSLs and heterogeous lists
by Corentin Dupont 01 Feb '14

01 Feb '14

Hi again, I have a game in which the user can create/write/read variables, using a small DSL. The type of the variable created can be whatever chooses the user, so I'm using existential types to store those variables in a heterogeneous list. This works fine, but the problem is that the "Typeable" class tag leaks into the DSL... The question is, how to get rid of it? > This is literate Haskell > {-# LANGUAGE GADTs, ScopedTypeVariables #-} > module DSLClass where > import Control.Monad > import Control.Monad.State > import Data.Typeable > This is the (simplified) DSL. With it you can read a variable stored in the game state (creation/writing is not shown). How can we get rid of the "Typeable a" in the ReadFirstVar constructor? > -- first type parameter is used to track effects > data Exp a where > ReadFirstVar :: (Typeable a) => Exp a <----- Ugly > Return :: a -> Exp a > Bind :: Exp a -> (a -> Exp b) -> Exp b This is the definition of a variable. The type is unknow, so I use existantial types. > data Var = forall a . (Typeable a) => Var { v :: a} This game state. It holds the heterogenous list. > data Game = Game { variables :: [Var]} The evaluation of "Exp" can be: > eval :: Exp a -> State Game a > eval ReadFirstVar = do > (Game ((Var v):vs)) <- get > case cast v of > Just val -> return val > Nothing -> error "no cast" > eval (Bind exp f) = do > a <- eval exp > eval (f a) As you can see, I'm obliged to cast the variable type to match it with the expression's type. Is that the right place to do it? Thanks!! Corentin

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Cabal version constraint seems to be ignored.
by Carlo Hamalainen 31 Jan '14

31 Jan '14

Hi, I have two versions of Cabal, both are visible: $ ghc-pkg expose Cabal-1.16.0 $ ghc-pkg expose Cabal-1.18.1.2 I clone and build my package. Since I have GHC 7.6.3 it uses Cabal-1.16.0 (as specified in ghc-imported-from.cabal): $ git clone https://github.com/carlohamalainen/ghc-imported-from $ cd ghc-imported-from $ cabal install It looks like Cabal-1.16.0 was chosen, as expected: $ grep Cabal dist/build/autogen/cabal_macros.h /* DO NOT EDIT: This file is automatically generated by Cabal */ /* package Cabal-1.16.0 */ #define VERSION_Cabal "1.16.0" #define MIN_VERSION_Cabal(major1,major2,minor) (\ But my program doesn't run: $ ghc-imported-from src/Main.hs Main getArgs 11 11 --ghc-options --ghc-pkg-options GhcOptions [] GhcPkgOptions [] Language/Haskell/GhcImportedFrom.hs:132:54: Couldn't match expected type `Distribution.PackageDescription.BuildInfo' with actual type `Cabal-1.16.0:Distribution.PackageDescription.BuildInfo' In the fourth argument of `getGHCOptions', namely `binfo' In a stmt of a 'do' block: getGHCOptions [] c (fromJust $ cradleCabalDir c) binfo In the expression: do { c <- findCradle; pkgDesc <- GhcMonad.liftIO $ parseCabalFile $ fromJust $ cradleCabalFile c; let binfo = head $ cabalAllBuildInfo pkgDesc; getGHCOptions [] c (fromJust $ cradleCabalDir c) binfo } I'm surprised that the error didn't appear at compile time. I guess the first reference to Distribution.PackageDescription.BuildInfo is from Cabal-1.18.1.2? If I hide Cabal-1.18.1.2 then things work ok: $ ghc-pkg hide Cabal-1.18.1.2 $ cabal install $ ghc-imported-from .... (runs as expected) But I can't expect a user to know that they have to hide Cabal-1.18.1.2. Is this actually a GHC bug or have I stuffed up my ghc-imported-from.cabal file? Thanks, -- Carlo Hamalainen http://carlo-hamalainen.net

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