December 2014 - Haskell-Cafe

Reminder: Dutch functional programming day (NL-FP 2015)
by Christiaan Baaij 05 Dec '14

05 Dec '14

Hi everyone, This is a reminder that the next annual Dutch Functional Programming day will take place on: Friday, January 9th, 2015 at the the University of Twente, Enschede, The Netherlands. You are all invited to participate, and of course, to give a presentation. The day will end with a joint dinner. Some additional information: - Although it is called the dutch functional programming day, the program will be in english. - Participation for the day itself is free of charge; we expect dinner to cost around 25 euro. In order to make the necessary reservations, we would like to know as soon as possible how many people we may expect. More information, and the online registration form, can be found at: http://wwwhome.cs.utwente.nl/~jankuper/fp-dag/ Hope to meet you all in Twente at the next FP day! Cheers, Christiaan Baaij

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2D symmetry groups implementation in Haskell?
by Stuart Hungerford 05 Dec '14

05 Dec '14

Hi, As per the subject line--does anyone know of any implementations of 2D (planar) symmetry groups in Haskell? (i.e. "rosette", "frieze" and "wallpaper") groups? I've looked in Hackage but as a complete Haskell novice may not being looking in the right places or communities. Many thanks, Stu

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Looking for help finishing off Data.Sequence <*> optimization
by David Feuer 04 Dec '14

04 Dec '14

With help from Carter Schonwald, I've put together a framework to optimize both the zip functions and the (<*>) method for sequences, as shown in https://github.com/haskell/containers/pull/84 The zip optimization is complete, but I'm getting lost in modular arithmetic in the optimization of fs<*>xs. The remaining challenge is to come up with an efficiently splittable representation of fmap (\f -> fmap f xs) fs and a way to split it. The first part, coming up with a representation that (theoretically) does the job, is easy: data CP a b = SingleCP a (Seq b) | FullCP Int Int (Seq a) (Seq b) The SingleCP constructor represents [f] * xs; the FullCP one holds fs, xs, and the start and end points within the original sequence (this representation was half-heartedly suggested by Cale Gibbard, who thinks probably no one uses the Applicative instance for Seq anyway). The trouble is that actually calculating things properly with these numbers is rather confusing for me; I keep going off by one in one way or another, and producing unreadable code. Can anyone help? David Feuer

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Hackage documentation for executables
by Bardur Arantsson 04 Dec '14

04 Dec '14

Hi all, I'm trying to upload documentation for an executable-only package to Hackage, but I can't get Haddock syntax to work in the description field. Here's the previous upload: http://hackage.haskell.org/package/pg-harness-0.1.0 As you can see, the "@" bits haven't been respected, nor have the ">" sections. Is there something I can do to fix this? (I guess I might upload the documentation myself, but I'd obviously prefer if I could avoid that.) (I'm trying to upload a new version, but I'd like to know what to first to avoid this issue.) Regards,

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Array creation in constant time
by Wolfgang Jeltsch 04 Dec '14

04 Dec '14

Hi, I want to create an array without initializing its elements, so that array creation takes constant time (modulo heap management). What are possible approaches for this? I thought about using newArray_ with the IOUArray type. Is this guaranteed to not initialize any elements and run in O(1)? Are there other ways? All the best, Wolfgang

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Stackage survey
by Michael Snoyman 04 Dec '14

04 Dec '14

Please fill out the Stackage survey, targeted at both Haskell library authors and users. It's just a few multiple choice questions, and shouldn't take you more than 1-2 minutes to complete. http://goo.gl/forms/J1xaWV9BOR

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Haskell and SCADA
by Vasili I. Galchin 03 Dec '14

03 Dec '14

http://en.m.wikipedia.org/wiki/SCADA ,,,, this post is in particular for people who are somewhat familliar with SCADA. Question: are SCADA hardware processors running anything like an operating system that would allow software to be written in Haskell?? Vasili

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Re: [Haskell-cafe] Getting Haskell in the top 20
by Henk-Jan van Tuyl 02 Dec '14

02 Dec '14

Using the words "Haskell" and "programming" in emails probably helps getting Haskell a few places up, but I think there is much more needed to get it in the top twenty. Things like a GHC backend that produces Java bytecode and/or Common Intermediate Language, commercial support, or Microsoft delivering Haskell in a Visual Studio environment. These are things that will raise much more interest in Haskell in the commercial world, and therefore would also interest more programmers. Regards, Henk-Jan van Tuyl On Sun, 30 Nov 2014 18:37:34 +0100, Jacques Carette <carette(a)mcmaster.ca> wrote: > Make sure the actual words "Haskell programming" occur in as many places > as possible [posts here, on reddit, on stackoverflow, etc]. Not > variants of those words, but those exact words. Like you did in your > .sig (although that may not be indexed, so it may need to be in the > actual text). > > Get it into people's phrasing that they always speak of "Haskell > programming", not Haskell code, coding, etc, etc. > > Jacques > > On 2014-11-30 11:29 AM, Henk-Jan van Tuyl wrote: >> >> L.S., >> >> In the Tiobe Index[0], F# jumped from position 43 in June to 13 in July >> and is since than in the >> top twenty. What happened in July and how can we get Haskell in the top >> 20? >> >> Regards, >> Henk-Jan van Tuyl >> >> >> [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- 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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Use distributivity proof in a simple example
by Marco Vassena 02 Dec '14

02 Dec '14

My GADT data type is indexed over type level list. data DF (xs :: [ * ]) where Empty :: DF [] Single :: a -> DF '[ a ] Cons :: DF xs -> DF ys -> DF (Append xs ys) Append is a type family that append two type level lists: type family Append (xs :: [ * ]) (ys :: [ * ]) :: [ * ] where Append '[] ys = ys Append (x ': xs) ys = x ': Append xs ys Map is a type level map: type family Map (f :: * -> *) (xs :: [ * ]) :: [ * ] where Map f '[] = '[] Map f (x ': xs) = f x ': Map f xs Now I want to write a function that wraps all the component in Single in a list: wrap :: DF xs -> DF (Map [] xs) wrap Empty = Empty wrap (Single a) = Single [ a ] wrap (Cons x y) = Cons (wrap x) (wrap y) -- Type Error The type-checker doesn't know anything about distributivity: Could not deduce (Append (Map [] xs) (Map [] ys) ~ Map [] zs) from the context (zs ~ Append xs ys) I managed to prove this property by induction on the first type level list. class MapDist xs where mapDist :: (zs ~ Append xs ys) => Proxy xs -> Proxy ys -> Append (Map [] xs) (Map [] ys) :~: Map [] zs instance MapDist '[] where mapDist Proxy Proxy = Refl tailP :: Proxy (x ': xs) -> Proxy xs tailP _ = Proxy instance MapDist xs => MapDist (x ': xs) where mapDist p1 p2 = case mapDist (tailP p1) p2 of Refl -> Refl I have adjusted wrap as follows: toProxy :: DataFormat xs -> Proxy xs toProxy _ = Proxy wrap (Cons x y) = let p1 = toProxy x p2 = toProxy y in case mapDist p1 p2 of Refl -> Cons (wrap x) (wrap y) The problem now is that MapDist xs is not in the context. Adding it to the Cons constructor in the GADT definition does not solve the problem: Could not deduce (MapDist (Map [] xs)) ... How can I solve this issue? Clearly the two instances of MapDist cover all possible cases, and therefore it should be possible to use it for any list xs. I tried avoiding the type class MapDist with a normal function: proof :: forall xs ys zs . Append xs ys ~ zs => Proxy xs -> Proxy ys -> Proxy zs -> Append (Map [] xs) (Map [] ys) :~: Map [] zs However here I have no mean to distinguish the base case ('[]) from the inductive case (x ': xs), thus I cannot complete the proof. Do you have any idea, suggestion or workaround for this kind of situations? Cheers, Marco

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Julia and Charts on Mac
by Dominic Steinitz 02 Dec '14

02 Dec '14

I just had an interesting experience installing the Julia charts package (Gadfly based on ggplot) on my Mac which I thought I would share as I know I and other folk have had trouble getting diagrams / Cairo to work on Macs. It installs homebrew and then all the required packages from its own repo (not sure what the homebrew terminology is for this - possibly a brewery?) so presumably guaranteeing that the installation is consistent and “just works”. I must say I was a bit surprised (I think I would have liked it to tell me what it was about to do) but it was entirely painless and I drew my first chart after less than 5 minutes. Something to bear in mind for making the barrier to entry to Haskell a bit lower. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com PS I am not sure that haskell-cafe is the right list to share this on but as we don’t have a mailing list for numerical stuff or mac stuff...

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