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ANN: lucienne-0.0.1
by Alexander Bau 09 Jun '12

09 Jun '12

Hi, some time ago I wrote lucienne[1] using the happstack framework: > Lucienne is a simple server side feed aggregator/reader that helps > you managing your subscribed feeds. It provides multi user support > using basic access authentication. A running mongoDB serves as > database backend. For more information, check [1] and [2]. Best regards, Alex [1] http://hackage.haskell.org/package/lucienne [2] http://www.imn.htwk-leipzig.de/~abau/lucienne

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I don't understand how ST works
by Nicu Ionita 09 Jun '12

09 Jun '12

Hi, After trying the whole afternoon to make a program work using ST and mutable vectors, I must give up and ask for some help. I have a pure function which generates a list of moves. But the whole thing should live in the ST monad, so: > genMoves ... = runST $ do ... Now, as I understand, I have a private universe (under runST) in which I can run "impure code", from which nothing escapes to the outside. Now in that universe I prepare succesively (and use later) a data structure which contains pure and impure values, for example: > data MList = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe MList) } Now comes my question: in the impure values there is always that "s". I was thinking that the whole structure should have s as a parameter: > data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe (MList s)) } but then, when I define functions like: > splitMove :: MList s -> ST s (Maybe (Move, MList s)) > splitMove ml = do > m <- unsafeRead (mvVec ml) 0 > ... I get this message: Moves\MoveList.hs:217:28: Couldn't match type `s' with `PrimState (ST s)' `s' is a rigid type variable bound by the type signature for splitMove :: MList s -> ST s (Maybe (Move, MList s)) at Moves\MoveList.hs:210:1 Expected type: U.MVector (PrimState (ST s)) Move Actual type: U.MVector s Move In the return type of a call of `mlVec' In the first argument of `M.unsafeRead', namely `(mlVec ml)' which really doesn't make sense, as the package primitive defines the instance: instance PrimMonad (ST s) where type PrimState (ST s) = s primitive = ST internal (ST p) = p Should I do the structure agnostic of that s-state? (forall s. ...) This seems really unintuitive to me... Anybody some hint? Thanks, Nicu

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link to section heading in Haddock
by Edward Amsden 09 Jun '12

09 Jun '12

Hi all After glancing through the haddock documentation and some googling, I can't tell if there's a supported way to link to a section heading in haddock documentation. Is there? Thanks, Ed

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Lambda Calculus and Differential Geometry and Big Iron
by Barak A. Pearlmutter 08 Jun '12

08 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>

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Introducing FP Complete
by Bartosz Milewski 08 Jun '12

08 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:]

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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 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.

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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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