June 2012 - Haskell-Cafe

Lambda Calculus and Differential Geometry and Big Iron
by Barak A. Pearlmutter 09 Jun '12

09 Jun '12

Seeking PhD students and postdocs interested in an elegant combination of functional programming and big-iron style numeric computing. Functional Programming and Automatic Differentiation PhD Studentships Postdoctoral Positions We are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination in a production-quality optimising compiler. Our research prototype compiler generates object code competitive with the fastest current systems, which are based on FORTRAN. We are seeking PhD students and postdocs with interest and experience in relevant areas, such as programming language theory, numeric computing, machine learning, numeric linear algebra, differential geometry; and a burning drive to help lift big iron numeric computing out of the 1960s and into a newer higher order. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising numeric and machine learning algorithms through the use of type theory and AD operators; and associated type/lambda calculus/PLT/real computation issues. Project headquarters: Hamilton Institute, NUI Maynooth, Ireland, http://www.hamilton.ie/. Applications and queries to: "Barak A. Pearlmutter" <barak%2Bad-fp-job(a)cs.nuim.ie>

1 0

Introducing FP Complete
by Bartosz Milewski 09 Jun '12

09 Jun '12

You might have seen a few post by me mentioning FP Complete and asked yourself the question: Who is this guy and what is FP Complete? I haven't been active in the Haskell community, as I'm a relative newcomer to Haskell. I am better known in the C++ community where I've been promoting functional-style programming and the use of Haskell for modelling difficult aspects of template metaprogramming. I've been involved in discussing the support for concurrency and parallelism in C++; which, by the way, is ages behind what Haskell has to offer. It was therefore natural for me to join a newly founded company, FP Complete, whose goal is to commercialize Haskell. I believe that now is the right time for Haskell to become a strong software industry player, especially that functional programming is being widely recognized as the answer to the recent multicore and GPU explosion. I can't be much more specific about what the role of FP Complete will be in making Haskell an industrial success because our plans are still evolving. We should be ready to makes some announcement shortly. I should mention that the support from the part of the Haskell community that we have so far contacted has been overwhelmingly positive. We have Simon P-J's blessing, we are in close collaboration with Well Typed, we have friends at Galois, Parallel Scientific, Amgen, and many other Haskell shops. Hopefully you will hear from and about us more often in the future. -- [:Bartosz Milewski:]

5 4

XDR library ?
by ARJANEN Loïc Jean David 08 Jun '12

08 Jun '12

Hello, I would like to know if there is an Haskell XDR library, I didn't find one on Hackage. Regards, -- ARJANEN Loïc Jean David http://luigiscorner.wordpress.com --- "Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do." Michael R. Fellows and Ian Parberry

2 1

Using promoted lists
by Yves Parès 08 Jun '12

08 Jun '12

The doc page http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-an… that lists are now usable as types. So I'm trying to make a type level function to test if a type list contains a type. Unless I'm wrong, that calls to the use of a type family. {-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-} data HBool = HTrue | HFalse -- Mandatory as Bool type is not currently promoted to a kind type family Member x (l :: [*]) :: HBool type instance Member x (x ': xs) = HTrue type instance Member x (y ': xs) = Member x xs type instance Member x (y ': '[]) = HFalse But the compiler complains about my instance conflicting. Is what I'm trying to do feasible? Second question: how can type level tuples (also mentioned in the doc page) be exploited? Aren't they redundant with type-level lists?

3 4

Using promoted lists
by oleg＠okmij.org 08 Jun '12

08 Jun '12

Yves Pare`s wrote: > So I'm trying to make a type level function to test if a type list contains > a type. Unless I'm wrong, that calls to the use of a type family. More crucially, list membership also calls for an equality predicate. Recall, List.elem has the Eq constraint; so the type-level membership test should have something similar, a similar equality predicate. As you have discovered, type equality is quite non-trivial. All is not lost however. If we `register' all the types or type constructors, we can indeed write a type-level membership function. In fact, http://okmij.org/ftp/Haskell/TTypeable/ShowListNO.hs does exactly that. It uses the type-level (and higher-order) function Member to test if a given type is a one of the chosen types. If it is, a special instance is selected. Please see http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable http://okmij.org/ftp/Haskell/typeEQ.html#without-over for explanations.

1 0

Have you seen this functor/contrafunctor combo?
by Conal Elliott 08 Jun '12

08 Jun '12

> newtype Q p a = Q (p a -> a) > > instance ContraFunctor p => Functor (Q p) where > fmap h (Q w) = Q (h . w . cmap h) using "cmap" for contravariant map. For instance, p a = u -> a. > instance ContraFunctor p => Applicative (Q p) where > pure a = Q (pure a) > Q fs <*> Q as = Q (\ r -> > let > f = fs (cmap ($ a) r) > a = as (cmap (f $) r) > in > f a) I've checked the functor laws but not the applicative laws, and I haven't looked for a Monad instance. Or extend to a more symmetric definition adding a (covariant) functor f to the contravariant functor p. > newtype Q' p f a = Q' (p a -> f a) A (law-abiding) Functor instance is easy, but I don't know about an Applicative instance. Have you seen Q or Q' before? They look like they ought to be something familiar & useful. -- Conal

2 3

Data.Bitmap question
by Anders Lyckegaard 08 Jun '12

08 Jun '12

Hi, I have a short question on using Data.Bitmap. I have imported an image using STB-image. img <- loadImage "filename" Now I get img which is a Bitmap data structure. How do I access individual pixels? Thank you in advance, Anders

2 2

Inheritance constraints
by Miguel Mitrofanov 08 Jun '12

08 Jun '12

Hi cafe! The ConstraintKinds extension makes it possible to create classes of classes, like this: class F c where cfmap :: c f => (a -> b) -> f a -> f b instance F Functor where cfmap = fmap instance F Monad where cfmap = liftM So, basically we are saying that "c" is a class that has some special properties. Is it possible to express the property "c is a subclass of a specific class d"? Like class F c where Functor f => c f

3 2

Mac and Gtk2hs problem
by Tanja Piechnick 08 Jun '12

08 Jun '12

Hey Haskell-Cafe, I would like to run GTK2HS on my Mac 10.6.8 but after the successful installation with Homebrew of all modules (GTK+, GTK2HS) I get this failure message: Prelude> :l guitest.hs guitest.hs:1:8: Could not find module `Graphics.UI.Gtk': Use -v to see a list of the files searched for. Failed, modules loaded: none. I always changed my PATH. This is the content of my PATH: /usr/bin /bin /usr/sbin /sbin /usr/local/bin /Developer/usr/bin /Users/tanjapiechnick/.cabal/bin $HOME/.cabal/bin Is that correct? Where is the mistake? Thank you for any help! Greetings from Namibia

5 8

ANN: stylish-haskell 0.2
by Jasper Van der Jeugt 08 Jun '12

08 Jun '12

Hello all, I've just released stylish-haskell 0.2 [1]. This release adds a flexible configuration file, which already provides some options for the different processing steps, and it will also make future enhancements easy. You can use a per-project configuration file, as documented in the README [2]. A sample configuration file looks like [3]. Feedback is very welcome, issues and enhancement requests can filed here [4]. [1]: http://hackage.haskell.org/package/stylish-haskell [2]: https://github.com/jaspervdj/stylish-haskell/blob/master/README.markdown [3]: https://github.com/jaspervdj/stylish-haskell/blob/master/.stylish-haskell.y… [4]: https://github.com/jaspervdj/stylish-haskell/issues Peace, Jasper

2 1