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Teaching haskell at the 42 french school
by Quentin Le Guennec 05 Oct '15

05 Oct '15

Hi, as an introduction, it would be a good advice to first look, if you did not heard of it yet, at what the 42 school is: http://www.42.fr/42-revolutionary-computer-training-free-and-open-to-all/ (TL;DR: *"unique pedagogical approach and accessibility to all, completely free of charge, 42 is the most daring response yet to the challenge of information-technology skill development", "**Innovative teaching methods in technology training: Peer-to-Peer learning"*) The school aims to teach a general knowledge of computer science, but does not provide a Haskell course yet. The exercices and projects are built by students, for students. I love haskell and I'd be really happy to teach that wonderful language, and I'd love to hear your suggestions. I was thinking of the haskell's hard learning way. (I gotta say, the school's pedagogy is pretty sadistic). - Recode List, (,), Either, Maybe and their functor/applicative/monad/etc instances (implying recode those data types first) - String manipulation (basically recode Data.List). Any ideas for prevent students from just copy pasting the hackage code source? - Red black trees are fun, aren't they? - Some kind of AI to solve, for example, Rubik's cube One other thing, this may be very sadistic, but I would like to ask what do you think of allowing ONLY pointfree? It may end-up by having different answers and promote beautiful and concise code. Please expose your ideas, everything is feasible, bonus added to traps, hard to solve problems, and everything that may lead the student to think a lot. Thanks. -- Quentin Le Guennec

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[ANN] PETSc Haskell bindings
by Marco Zocca 05 Oct '15

05 Oct '15

Dear all, development of the Haskell bindings for the PETSc scientific computation library is now public; the repository can be found at https://github.com/ocramz/petsc-hs The interface is still very much experimental and in a state of flow but a number of basic functions are working. I look forward to all feedback; Kind regards, Marco Zocca

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Concept for "equal but unknown"
by martin 05 Oct '15

05 Oct '15

Hello all, there is a problem which popped up a number of times recently and I wondered if there is a general concept to tackle this kind of problems. It goes like this: I book a hotel room. At this time it becomes known that the hotel has one fewer room available, but it is not known which room it is. Likewise I know, that I have a reservation for "a room", but I don't know which one it is. It is however known, that the room in my reservation is the same room the hotel has removed from the list of available rooms. Another example: In air-cargo the cargo company has reserverd space in an aircraft for two containers. One for high-priority parcels and one for low-priority parcels. In the cargo hub, they plan to pack these two containers. At this point in time it is only known that there will be two containers one labelled "high-prio" and the other labelled "low-prio" but their physical identity is not known. Only when you actually do the packing you have a physical container at hand, which has identity. At this moment it becomes clear to which physical containers the reservations refer to and which physical containers are used for packing. I think I can handle this in an imparative style using an extra indirection (think pointers), though even there I would be happy if someone told me something like "this is an example of covariant unification" or something like this. But in haskell I cannot see a solution at all? So: how can I express that two things are equal to each other without knowing the exact value and how do I manage to make both assume a value once one of them becomes known. Do these kind of problems have a name?

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Approach to generalising Functor, is it worth pursuing?
by Clinton Mead 04 Oct '15

04 Oct '15

I've been thinking about generalising Functor, as I had something which was "Functor like" but didn't seem to fit into the existing definition. The example I'll present isn't the problem I was trying to solve, but it probably illustrates the solution better. The questions I have is: 1. Is this useful? 2. Has this been done? If the answers are Yes and No, I'll continue on into making this into a package. Anyway, here's what I've done: As we know, the type signature of fmap is the following: fmap :: Functor f => (a -> b) -> (f a -> f b) I've put brackets around the last two arguments to show that fmap can be also seen as a function which takes a function and returns a new function, in a different "space", for want of a better word. fmap should also follow this rule: fmap (f . g) == (fmap f . fmap g) So we can map an ordinary function into the "Maybe" space, but in a way that composing functions in the putting them in the space is the same as putting them in the space then composing them. I thought it would be nice if we could fmap to Kleisli Arrows, like so: fmap :: Monad m => (a -> b) -> Kleisli m a b Of course this is just "arr". But "arr" follows fmap's rules, so I thought it would be nice to make it a functor. So I was looking for more generalised versions of "Functor", and I found the 'categories' package, which had the following definition: class (Category <https://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Category.html…> r, Category <https://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Category.html…> t) => Functor f r t | f r -> t, f t -> r where fmap :: r a b -> t (f a) (f b) This solves half my problem, as now I can set r = (->) and t = Kleisli m and I've got the categories right. But it forces the "output" category to have a data constructor. So: fmap :: Monad m => (a -> b) -> Kleisli m a b Just won't fit. I'd need to make it: fmap :: Monad m => (a -> b) -> Kleisli m (Id a) (Id b) and then unwrap the results. I found this ugly though. So I've gone with another approached. I've linked the code here: http://ideone.com/TEg4MN and also included it inline at the bottom of this mail. Defining functors now becomes a bit wordy, but basically what I've defined is two functor instances. The first is just a copy of the existing functor instances to maintain the status quo behaviour. But secondly I've defined the Kleisli instance as discussed above. The "main" line, does two calls to fmap. The outermost (on the left) is just an ordinary fmap call on lists. The innermost, fmaps the function "triple" into a Kleisli Arrow, allowing it to be composed with the the Kleisli arrow already defined, 'evenOrNothing'. Type inference works this all out magically without signatures being required. Like I said, is approach new and useful? Improvements would be appreciated also, I'm only over the last few months really started focusing on learning Haskell properly (after leaving my previous job of 8 years), so I'm sure I'm still doing plenty of things not quite right. Thanks, Clinton --- {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleContexts #-} module Main ( main ) where import qualified Control.Arrow import Prelude hiding (Functor, fmap, (.)) import Control.Category (Category, (.)) import qualified Data.Functor import qualified Control.Arrow import Control.Arrow (Kleisli(Kleisli), runKleisli) type family F (r :: (* -> * -> *)) (t :: (* -> * -> *)) x type family InputParam f x type family ResultParam f x type family InputCategory f :: (* -> * -> *) type family ResultCategory f :: (* -> * -> *) class Functor f where fmap :: ( Category r, Category t, f ~ F r t c, f ~ F r t d, r ~ InputCategory f, t ~ ResultCategory f, a ~ InputParam f c, b ~ InputParam f d, c ~ ResultParam f a, d ~ ResultParam f b ) => r a b -> t c d data OrdinaryFunctor :: (* -> *) -> * type instance F (->) (->) (f a) = OrdinaryFunctor f type instance InputParam (OrdinaryFunctor f) (f a) = a type instance ResultParam (OrdinaryFunctor f) a = f a type instance InputCategory (OrdinaryFunctor f) = (->) type instance ResultCategory (OrdinaryFunctor f) = (->) instance (Data.Functor.Functor f) => Functor (OrdinaryFunctor f) where fmap = Data.Functor.fmap data KleisliFunctor :: (* -> *) -> * type instance F (->) (Kleisli m) a = KleisliFunctor m type instance InputParam (KleisliFunctor f) a = a type instance ResultParam (KleisliFunctor m) a = a type instance InputCategory (KleisliFunctor m) = (->) type instance ResultCategory (KleisliFunctor m) = Kleisli m instance (Monad m) => Functor (KleisliFunctor m) where fmap = Control.Arrow.arr triple = (*3) evenOrNothing = Kleisli (\x -> if (even x) then Just x else Nothing) main = print $ fmap (runKleisli (evenOrNothing . fmap triple)) [3..6]

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HGamer3D 0.6.2
by Peter Althainz 03 Oct '15

03 Oct '15

Dear All, HGamer3D has some new features with version 0.6.2, see www.hgamer3d.org. Builds on Windows and Linux. with version 0.6 a switch was made towards a simpler structure and a new 3d library. regards Peter

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Re: [Haskell-cafe] Parsing Atom feed link
by Adam Bergmark 02 Oct '15

02 Oct '15

Hi, This sounds like a good change to me! Are you willing to provide a patch (and maybe a testcase??) for this? - Adam On Sun, Sep 27, 2015 at 11:17 PM, fr33domlover <fr33domlover(a)riseup.net> wrote: > On Sat, 26 Sep 2015 10:15:24 +0200 > Francesco Ariis <fa-ml(a)ariis.it> wrote: > > > > > Since you have done the homework and this is rfc compliant, have you > > tried sending an email to the maintainer [1]? > > It seems an uncontroversial change and they are active (last version > > released in 2015), I bet they are be willing to fix this. > > > > [1] http://hackage.haskell.org/package/feed > > I sent the message to the maintainer too. I'm tempted to add a quick > temporary fix to my code, but hopefully this issue will get fixed soon > anyway. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Concept for "equal but unknown"
by Olaf Klinke 02 Oct '15

02 Oct '15

To me this sounds like the Reader monad: You have a value that depends on something that is yet to be decided upon. data SomeCargo = SomeCargo { decide_attribute1 :: ContainerID -> Attr1 decide_attribute2 :: ContainerID -> Attr2 ... } data ConcreteCargo = ConcreteCargo { attribute1 :: Attr1 attribute2 :: Attr2 } withID :: SomeCargo -> ContainerID -> ConcreteCargo If the type ContainerID is searchable [1] then you can have an Eq instance even for the SomeCargo type. That is, two containers are equal if whenever the same ID gets assigned, they have the same properties. The technical term would be "indistinguishable in all contexts". -- Olaf [1] http://math.andrej.com/2008/11/21/a-haskell-monad-for-infinite-search-in-fi…

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Call for Contributions - Haskell Communities and Activities Report, November 2015 edition (29th edition)
by Mihai Maruseac 02 Oct '15

02 Oct '15

Dear all, We would like to collect contributions for the 28th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 October 2015 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. The final report might have an index with such projects, provided we get enough such submissions. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac and Alejandro Serrano Mena FAQ: Q: What format should I write in? A: The usual format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2015/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2015/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. _However_, if you don't want/have time to format the entry for LaTeX, you can submit it in any other format possible and we will be happy to convert it for the final report. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then, PNG being preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Haskell Tooling Infrastructure
by Alan & Kim Zimmerman 02 Oct '15

02 Oct '15

Now that I am finally turning back to HaRe, the question of infrastructure again comes up. Basically I woudl like to focus in the tool itself, but it can only be useful if it can deal with real world environments. This means properly managing all the different project and compiler environments, and being integrated into IDEs. The current version uses ghc-mod for the lower level interface, and a simple command line interface for the IDE integration. But I have a definite sense that as a community every tool maker is solving the same problems over and over, from ghc-mod to ide-backend to ghci-ng, and onwards. To start a discussion around some kind of architeccture that tool writers can develop agains, ide's can integrat against and ghc/cabal can eventually internalise, I have put some basic points down at https://github.com/RefactoringTools/HaRe/wiki/Requirements-for-IDE-GHC-inte… Regards Alan

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Candidate Haskell Platform 7.10.2-a for use with OS X El Capitan
by Gershom B 01 Oct '15

01 Oct '15

The new OS X El Capitan release (out today) broke the method that Haskell Platform uses to install itself, due to new security features. In fact, if you upgrade your OS, you are also likely to (slightly) break your existing install. There is now a candidate new installer, available at http://downloads.haskell.org/~platform/7.10.2/Haskell%20Platform%207.10.2-a… Note that it is unsigned and you will have to give it permission to run, the usual way, by right clicking on it, and choosing "open" then explicitly allowing it to run. The central difference between this and the prior installer is just that it installs symlinks to ghc tools in the /usr/local tree rather than the /usr tree since the latter is now off limits due to new OS X security features. If you have a prior install and just want to "fix" it after your OS upgrade, no need to reinstall the whole platform. Just download and run the activate-hs shellscript from here: https://github.com/mzero/haskell-platform/blob/master/hptool/os-extras/osx/… Again, after a bit more testing we'll get this signed and released as the new official platform installer for OS X, but in the meantime, I wanted to make sure people were in the loop. Regards, Gershom

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