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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

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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?

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Using promoted lists
by oleg＠okmij.org 07 Jun '12

07 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.

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Have you seen this functor/contrafunctor combo?
by Conal Elliott 07 Jun '12

07 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

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Data.Bitmap question
by Anders Lyckegaard 07 Jun '12

07 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

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Inheritance constraints
by Miguel Mitrofanov 07 Jun '12

07 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

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Mac and Gtk2hs problem
by Tanja Piechnick 07 Jun '12

07 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

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ANN: stylish-haskell 0.2
by Jasper Van der Jeugt 07 Jun '12

07 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

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Status of Haskell on Android? Possible ideas to make this happen?
by Alp Mestanogullari 07 Jun '12

07 Jun '12

Hi Cafe! I know, this topic comes back on the list from time to time. I know, there probably isn't anything new since the last time it was asked here, on reddit or on SO. However, I remember seeing that iPwn Studios were thinking about getting GHC to build applications for android, in addition to their ghc-iphone project. Is this correct? Is there anything new on that front? Is anyone else working/thinking-to-work on this? Aside from these questions, I'm wondering about the "easiest" way to get a ghc-android compiler up and running. I have found a few things, but they all look really, really painful and cumbersome, like http://stackoverflow.com/a/5152910/193424 or http://stackoverflow.com/a/7301024/193424 . Recently, I heard about Renderscript: http://developer.android.com/guide/topics/renderscript/index.html Of course, we could write a binding or some kind of EDSL à-la Atom to have some funny apps/games running on Android by generating Renderscript code under the hood. However, something in the "Renderscript Runtime Layer" section caught my attention: > You define your Renderscript code in .rs and .rsh files in the src/ directory of your Android project. The code is compiled to intermediate bytecode by the llvm compiler > that runs as part of an Android build. When your application runs on a device, the bytecode is then compiled (just-in-time) to machine code by another llvm compiler that > resides on the device. The machine code is optimized for the device and also cached, so subsequent uses of the Renderscript enabled application does not recompile the > bytecode. But hey, GHC *does* have an LLVM backend! So can't we somehow break that chain of compilers so that we can plug GHC's LLVM codegen on top of it and then use their llvm-on-the-device thing? I know I am skipping over many, many technical details like how we could have an analog to their "reflected layer" on the java side but I would really be interested in hearing your opinions/objections/thoughts on this. -- Alp Mestanogullari

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GHC 7.4 and TypeSynonymInstances
by Ivan Lazar Miljenovic 07 Jun '12

07 Jun '12

Has there been a change in the behaviour/requirement of TypeSynonymInstances as of GHC-7.4.1? (Not sure if this behaviour occurs with 7.2.1 as I don't have it installed) I had an instance for String for a class which ghc accepted whilst using FlexibleInstances; however, when trying to load it in 7.0.* it stated that TypeSynonymInstances was needed (which I forgot to add since it didn't seem to need it). All I could find in the release notes was that as of 7.2.1, both extensions were needed, not that one implied the other: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-2-1.html#i… Is this behaviour deliberate, or a bug? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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