September 2013 - Haskell-Cafe

Cabal install build of scion-browser 0.2.18 fails when linking Persistent-sqlite1.21
by Stephen Taylor 22 Sep '13

22 Sep '13

Hi I'm a Haskell newbie. I just installed Haskell on Windows 7 x64 (Haskell Platform 2013.2.0.0 containing GHC 7.6.3), and have been using cabal-install to build/install from Hackage all the "helper executables" required and/or supported by EclipseFP, the Eclipse Haskell Plugin. All the helper exe's installed without any issues, except for one: scion-browser 0.2.18. All its dependencies seemed to build fine, and it built fine up until the near the end of the link phase when the link to Persistent-sqlite 1.2.1 failed. Here is the error message (path information elided): .... Loading package persistent-sqlite-1.2.1 ... ghc.exe: *Unknown PEi386 section name `.drectve'* (while processing:\persistent-sqlite-1.2.1\libHSpersistent-sqlite-1.2.1.a) ghc.exe: panic! (the 'impossible' happened) (GHC version 7.6.3 for i386-unknown-mingw32): loadArchive "persistent-sqlite-1.2.1\libHSpersistent-sqlite-1.2.1.a": failed Failed to install scion-browser-0.2.18 I understand what the message is saying, but I don't know how to go about resolving it. I have of course cleaned and retried a few times. But I get the same error. Any and all suggestions on how to resolve or work around this issue so that I can provide EclipseFP with a usable scion-browser, would be greatly appreciated. Thanks

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Minh Thu, please add me to your LinkedIn network
by Kyle Hanson via LinkedIn 22 Sep '13

22 Sep '13

LinkedIn ------------ Kyle Hanson requested to add you as a connection on LinkedIn: ------------------------------------------ Minh Thu, I'd like to add you to my professional network on LinkedIn. - Kyle Accept invitation from Kyle Hanson http://www.linkedin.com/e/uc6lxc-hlw276p4-5e/XvIdBwmueHfd6vFMPXXdLaqreCbl5o… View profile of Kyle Hanson http://www.linkedin.com/e/uc6lxc-hlw276p4-5e/rso/60831967/_LLG/name/2272454… ------------------------------------------ You are receiving Invitation emails. This email was intended for Minh Thu Vo. Learn why this is included: http://www.linkedin.com/e/uc6lxc-hlw276p4-5e/plh/http%3A%2F%2Fhelp%2Elinked… (c) 2012, LinkedIn Corporation. 2029 Stierlin Ct, Mountain View, CA 94043, USA.

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type-level integers, type-level operators, and most specific overlapping instance
by TP 22 Sep '13

22 Sep '13

Hi everybody, I encouter some problem in my code in the following simple example: two instances overlap for the multiplication sign `*`. The `OverlappingInstances` extension is of no help because it seems GHC does not look at the instance context to decide which instance is the most specific. file:///usr/share/doc/ghc-doc/html/users_guide/type-class- extensions.html#instance-overlap How to make GHC realize that in the second instance below, the instance context is not satisfied, such that the first instance has to be used? (Indeed, a Scalar is a `Tensor 'Zero`, so the strict inequality `'Zero :< n` is not satisfied in the context). -------------------------------------------- {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} import Prelude hiding ((*)) data Nat = Zero | Succ Nat deriving ( Show, Eq, Ord ) type family (m::Nat) :< (n::Nat) :: Bool -- Here Bool is a kind. type instance m :< 'Zero = 'False type instance 'Zero :< ('Succ n) = 'True type instance ('Succ m) :< ('Succ n) = m :< n data Tensor :: Nat -> * where Tensor :: Tensor order Mult :: Scalar -> Tensor order -> Tensor order deriving instance Show (Tensor order) type Scalar = Tensor 'Zero class Multiplication a b c | a b -> c where (*) :: a -> b -> c instance Multiplication Scalar (Tensor n) (Tensor n) where s * t = Mult s t instance (('Zero :< n) ~ 'True) => Multiplication (Tensor n) Scalar (Tensor n) where t * s = Mult s t main = do let s = Tensor :: Scalar let a = s*s print a -------------------------------------------- that yields at compilation: Overlapping instances for Multiplication Scalar Scalar (Tensor 'Zero) arising from a use of `*' Matching instances: instance [overlap ok] Multiplication Scalar (Tensor n) (Tensor n) -- Defined at test_overlapping_instance_with_typelevelinteger_test.hs:31:10 instance [overlap ok] ('Zero :< n) ~ 'True => Multiplication (Tensor n) Scalar (Tensor n) -- Defined at test_overlapping_instance_with_typelevelinteger_test.hs:34:10 In the expression: s * s In an equation for `a': a = s * s In the expression: do { let s = ...; let a = s * s; print a } Thanks in advance, TP

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ANNOUNCE: New OpenGL packages
by Sven Panne 21 Sep '13

21 Sep '13

New versions of the OpenGL packages are available on Hackage: * OpenGLRaw 1.4.0.0 * GLURaw 1.4.0.0 * OpenGL 2.9.0.0 * GLUT 2.5.0.0 The mid-term goal is to make all these packages conform to the latest OpenGL 4.4 specification, and while we're not yet there, this release is nevertheless an important stepping stone towards that goal. The packages contain a few non-backwards compatible changes, something which is never nice for a public API, but it has been necessary: OpenGL has come a long way from its initial fixed function pipeline to its current form flexible form centered around shaders and buffers. Because of this, a few design decisions on the Haskell side were not a good fit anymore and simply had to change. Nevertheless, almost all changes needed in the applications and libraries using the OpenGL packages should be mechanical and straightforward. If not: hopengl(a)haskell.org is the place to get help if needed. Hopefully the new packages will make it into the next Haskell Platform release (2013.4.0.0), at least if I find out how to make it through the proposal process... ;-) Cheers, S. P.S.: Here a list of the changes for each package: ====================================================================== Changes in the OpenGLRaw package ====================================================================== * Added support for the following extensions: GL_ARB_occlusion_query2 GL_ARB_timer_query GL_ARB_draw_indirect GL_ARB_gpu_shader5 GL_ARB_tesselllation_shader GL_ARB_transform_feedback3 GL_ARB_ES2_compatibility GL_ARB_get_program_binary GL_ARB_separate_shader_objects GL_ARB_shader_atomic_counters GL_ARB_compute_shader GL_ARB_ES3_compatibility GL_ARB_framebuffer_no_attachments GL_ARB_shader_storage_buffer_object GL_ARB_query_buffer_object * Added GLfixed type from OpenGL 4.1. * Moved GLhandle type to Graphics.Rendering.OpenGL.Raw.ARB.ShaderObjects where it belongs and fixed its representation on Mac OS X. * Added new Graphics.Rendering.OpenGL.Raw.Type module which exports all GL types. Core31 and Core32 export only their respective subset now. * Correctly typed bitfield tokens as, well, GLbitfield instead of GLenum. * Consistently use ‘Ptr a’ for ‘void*’ which are not opaque. * Use ccall instead of stdcall on x86_64-windows. * Use the OpenGLES framework on iOS. ====================================================================== Changes in the GLURaw package ====================================================================== * Use ccall instead of stdcall on x86_64-windows. * Use the OpenGLES framework on iOS. ====================================================================== Changes in the OpenGL package ====================================================================== * Added sync object support. * Added full support for OpenGL 4.4 query objects, extending and changing the previous query object API a bit. * Split ObjectName class into ObjectName + GeneratableObjectName classes. Added single-name variants deleteObjectName and genObjectName, they are a very common use case. * Made BufferObject and TextureObject abstract. Now all GL objects names are abstract and have to be explicitly generated. The only exception is DisplayList, which is required to be non-abstract by the spec. * Shader is not a class anymore, but a data type with an ObjectName instance and a creation action. Added ShaderType and shaderType. * Added partial support for tessellation/geometry/compute shaders. * Program is not a GeneratableObjectName, but has a createProgram action now. Added attachShader and detachShader for incremental changes. * Deprecated shaderSource and added shaderSourceBS instead. Using ByteString is more efficient and forces the caller to think about encodings, e.g. via Data.Text.Encoding. * Added support for shader/program binaries and shader precision queries. * Revamped texture targets, adding partial support for texture buffer objects and texture arrays. * OpenGL 3.1 deprecated separate polygon draw modes, so use GL_FRONT_AND_BACK internally in polygonMode whenever possible. * Added missing Eq/Ord/Show instances for lots of data types. * Simplified TransformFeedbackVaryings API, making it a bit more similar to the one for activeAttribs and activeUniforms. * Exported ContextProfile’. * Renamed BlitFramebufferMask (to BlitBuffer) and its constructors. * Renamed BufferRangeAccessBit (to MapBufferUsage) and its constructors * Removed currentMatrix, genLists, deleteLists and isList, they have been deprecated for ages. * Full internal support for UTF-8. * Do not expose internal #hidden modules. * Lots of Haddock fixes and improvements. * Renamed IndexedTransformFeedbackBuffer to IndexedTransformFeedbackBuffer. * Fixed clip plane query. ====================================================================== Changes in the GLUT package ====================================================================== * Deprecated the misspelled ContinueExectuion constructor and added a new one correctly called ContinueExecution instead. * Use ccall instead of stdcall on x86_64-windows. * Do not expose internal #hidden modules.

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using "default" declaration for overloaded numeric operations
by TP 21 Sep '13

21 Sep '13

Hi, I try to develop an embedded domain specific language in Haskell. I don't want to type "::Rational" all the time, so I try to use a default declaration for my types. This works correctly in this simple example: ------------------------------------------ default (Integer, Double) -- "default default" mag :: Float -> Float -> Float mag x y = sqrt( x^2 + y^2 ) main = do print $ mag 1 1 ------------------------------------------ Indeed we obtain sqrt(2) as a result. If we replace the default declaration by: default () , we obtain errors at compilation as expected: the type of `2` is ambiguous. Now let us consider a more complicated example: ------------------------------------------ {-# LANGUAGE FlexibleInstances #-} import Prelude hiding ((^^)) import Data.Ratio default (Integer, Rational, Double) class (Num a) => Foo a where (^^) :: Num b => b -> a -> b instance Foo Rational where (^^) x r = x ^ n where n = 2*numerator r -- dummy calculation instance Foo Integer where (^^) x n = x ^ n mag :: Float -> Float -> Float mag x y = sqrt( x ^^ 2 + y ^^ 2 ) main = do print $ mag 1 1 ------------------------------------------ I would expect it to work correctly, but we obtain errors concerning the ambiguous type of `2`. Why? How to make my default declaration work? And if I want to give the priority to `Rational` instead of `Integer`? Indeed, this should be possible because we don't get any error below. $ ghci > import Data.Ratio > 2::Rational 2 % 1 Thanks in advance, TP

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Demarcating monad transformers.
by Nickolay Kudasov 20 Sep '13

20 Sep '13

Hi Café, Below I describe what I call «demarcating monad transformer». It works great for my purposes, though the construction feels a bit awkward. Perhaps, this is just an instance of a general case. For details and example, see [1] (single module package). Recently I got a challenge of manipulating transformed monadic values. In my case the value was of type t (Free f) a (or MonadFree m => t m a). The challange itself was to provide a function: transform :: (Functor f, MonadTrans t) => (forall b. f b -> t (Free f) b) -> t (Free f) a -> t (Free f) a The type t (Free f) a for my purposes can be read as «a program with low-level API specified with functor f and extra features specified with monad transformer t». transform takes a “basic transformation” phi and an abstract program p and applies phi whenever p “executes” command of free functor f. It turns out that this function is impossible (try StateT). The point is that you can't “get inside” of transformed monadic value. So I came up with idea of «demarcating» monad transformer. By «demarcating» I mean separating pure monadic computations (lifted m a) from transformed ones (t m a). Such separation can be made explicit with the help of free monads: data DemarcateF t m next = forall b. DemarcateMonad (m b) (b -> next) -- "pure" monadic computation | forall b. DemarcateTrans (t m b) (b -> next) -- transformed monadic computation instance Functor (Demarcate t m) where ... -- getting monad for freenewtype Demarcate t m a = Demarcate { unDemarcate :: Free (DemarcateF t m) a } instance Monad (Demarcate t m) where ...instance MonadTrans (Demarcate t) where ... With that I can define transform functions: -- transform arbitrary monadic computationtransformDemarcateM :: (forall b. m b -> Demarcate t m b) -> Demarcate t m a -> Demarcate t m a -- transform free monadic actionstransformDemarcateFree :: (forall b. f b -> Demarcate t (Free f) b) -> Demarcate t (Free f) a -> Demarcate t (Free f) atransformDemarcateFree phi = transformDemarcateM (iterM phi) The complete code is available at [1]. Check out examples/simple.hs for a use case. Now the questions are: - has anyone else encountered such a challenge? - is this solution an instance of a more general pattern? - is it sensible to use Demarcate t m a when m is not a free monad? - how Demarcate may affect the performance? Thanks in advance, Nick [1] <https://github.com/fizruk/demarcate>https://github.com/fizruk/demarcate

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Which builder to choose?
by Alejandro Serrano Mena 20 Sep '13

20 Sep '13

Hi, I'm looking at the packages blaze-builder and bytestring, and both provide builders for ByteString. Which one should I use? In which situations is one more convenient than the other? Thanks for the help.

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Fwd: Functional Programming in Bioinformatics, Birds-of-a-Feather Session at CUFP
by Ashish Agarwal 19 Sep '13

19 Sep '13

Functional Programming in Bioinformatics Birds-of-a-Feather Session at CUFP 2013 September 23, 2013 at 6:00 PM Boston, MA United States Bioinformatics is a discipline applying computational methods to biological problems. Due to advances in experimental techniques, especially in next-generation DNA sequencing, the amount of data being generated by biologists is exponentially increasing. The resulting data deluge promises to accelerate fundamental advances in human health, agriculture, and basic research. However, progress is hindered due to the lack of software tools for managing, analyzing, visualizing, and sharing such data. The goal of this meeting is to bring together researchers and practitioners of functional programming, logic, type theory, and related fields who are interested in applying their skills to biology and medicine. We also welcome biologists who are curious about state-of-the-art software engineering techniques. The meeting will be casual with no presentations. Our goal is simply to meet and discuss our interests, and propose future events and collaborations. We will meet at Connolly’s Publik House within the conference venue, the Hilton Boston Logan Airport Hotel, on Monday September 23rd at 6:00 PM. Everyone is welcome to attend this meeting regardless of whether you registered for any ICFP or CUFP events. RSVP is not required, but to help with planning, please email “agarwal1975 at gmail.com” if you know you will attend. Please also join the Well-Formed-Bio mailing list [1], which we welcome everyone to use for discussions around the topic of formal software methods applied to biology. [1] https://groups.google.com/forum/#!forum/well-formed-bio [2] http://cufp.org/conference/sessions/2013/birds-feather-functional-programmi… Organizers: Ashish Agarwal, Jeff Hammerbacher

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ANN: tf-random 0.1 (splittable pseudorandom number generator)
by Michał Pałka 19 Sep '13

19 Sep '13

Dear all, I am happy to announce the release of tf-random 0.1. tf-random is a high-quality splittable pseudorandom number generator intended to address the random number quality problems of the standard Haskell StdGen generator. tf-random uses a cryptographic hash function under the hood to generate pseudorandom numbers, which is based on the ThreeFish block cipher. The design of the generator is explained in the paper 'Splittable Pseudorandom Number Generators Using Cryptographic Hashing' by Claessen, Pałka, which will be presented on the upcoming Haskell Symposium. The performance of tf-random was found to be about 8% slower than StdGen on typical QuickCheck properties, which make heavy use of splitting, but up to 2x faster in linear generation on a x86-64 processor. The package is available from hackage at: http://hackage.haskell.org/package/tf-random The code has not yet been thoroughly tested, so expect bugs. For example, it has never been tried on a big-endian architecture, and I would appreciate if somebody could check if the results returned by it are the same (with a given random seed) as on x86. In addition to the generator, the package also contains alternative versions of the RandomGen and Random classes. The most notable difference is that method 'next' from RandomGen returns a Word32 drawn uniformly from its the full range. The reason for this change is that I found it impossible to write efficient Random instances that are parametrized over the range of the generator (genRange). The instances currently defined in the random package make an (illegitimate) assumption that the range of StdGen is used, for that matter. I hope that this release will spark a discussion about the future of the standard PRNG and the RandomGen API in Haskell. Best regards, Michał PS. Please copy me when replying as I am not on the list.

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ANN Vigilance: Get notified when periodical tasks fail to run successfully
by Michael Xavier 19 Sep '13

19 Sep '13

Hey Cafe, Just wanted to announce a project I've been tinkering with for a while finally got to a state where I felt comfortable releasing it. Vigilance is a Dead Man's Switch system that notifies you when periodical tasks that fail to check in when you expected them to. An example of this could be registering the daily backups you do of your servers and have them send emails or send HTTP POST requests if backups ever fail to check in. Vigilance provides an executable for doing check-ins and inspecting your watches as well as a simple REST API if you need something embeddable for existing projects. HackageDB: http://hackage.haskell.org/package/vigilance Github: http://github.com/michaelxavier/vigilance Introductory blog post: http://michaelxavier.net/posts/2013-09-17-Announcing-Vigilance-An-Extensibl… -- Michael Xavier http://www.michaelxavier.net

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