- Haskell-Cafe - Haskell.org

GSoC Retrospective
by Dominic Steinitz 20 Sep '15

20 Sep '15

In past years, Gwern did an outstanding job of summarising the GSoC proposals and how well they had done: http://www.gwern.net/Haskell%20Summer%20of%20Code <http://www.gwern.net/Haskell%20Summer%20of%20Code> Here are the proposals for this year. Perhaps students or their mentors could comment on how successful their projects have been? I realise this is a very poor substitute for Gwen’s analyses but it would better than nothing. A Strict language pragma for GHC - Adam Sandberg Eriksson <https://github.com/adamse> STM Data Structures Implementation <https://gist.github.com/Alllex/439480b7e80303f19ddc> - Alex Semin Implementation of Layered Gramamar of Graphics <https://gist.github.com/AjayRamanathan/c84a4641836700a2547b> - chinu <https://github.com/AjayRamanathan/Plot> Implementing Version Comparison for Cabal Packages - Craig Roche <https://github.com/cdxr> Improving Hackage Discoverability <http://dzackgarza.com/mockup/main.html> - D. Zack Garza <https://github.com/dzackgarza> Darcsden improvements <http://darcs.net/GSoC/2015-Darcsden> - Daniil Frumin <https://github.com/co-dan> Pursuit enhancements <https://gist.github.com/hdgarrood/0a389937149453c69e03> - Harry Garrood <https://github.com/hdgarrood> Improvements For HBLAS And Adding LAPACK Bindings. - JuejiYang A standalone functional parser for CommonMark <https://gist.github.com/Jubobs/7a9298eeaf02bcefbc35> - Julien Cretel <https://github.com/Jubobs> Refactor program with HLint suggestions <http://mpickering.github.io/gsoc2015.html> - Matthew Pickering <https://github.com/mpickering> Replication back-end for acid-state - Max Voit Native Haskell Type Encoding for LiquidHaskell <https://gist.github.com/spinda/b261167303515cc8a1d9> - Michael Smith <https://github.com/spinda> Exhaustiveness Checker for PureScript - Nicolas Del Piano <https://github.com/nicodelpiano> Fast splittable pseudorandom number generator for System.Random <https://gist.github.com/nkartashov/e46fd146b1df2d79aaf3> - Nikita Kartashov <https://github.com/nkartashov> Interactive widgets in IHaskell <https://github.com/sumitsahrawat/gsoc/blob/master/2015/ihaskell.pdf> - Sumit Sahrawat <https://github.com/sumitsahrawat> Improvements to yesod-devel - urbanslug <https://github.com/urbanslug> Implement nix-like package management features in cabal <https://gist.github.com/fugyk/37510958b52589737274> - Vishal Agrawal <https://github.com/fugyk> Haddock improvements - Łukasz Hanuszczak <https://github.com/mrhania> Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

6 9

MonadPlus and Alternative folds
by David Feuer 20 Sep '15

20 Sep '15

It kind of seems like there should be fold-like things to match some and many. foldrAlt :: Alternative f => (a -> b -> b) -> b -> f a -> f b foldrAlt c n m = foldr1Alt c n m <|> pure n foldr1Alt :: Alternative f => (a -> b -> b) -> b -> f a -> f b foldr1Alt c n m = c <$> m <*> foldrAlt c n m foldlMP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b foldlMP' f b m = foldl1MP' f b m <|> pure b foldl1MP' :: MonadPlus f => (b -> a -> b) -> b -> f a -> f b foldl1MP' f b m = m >>= \r -> let !b' = f b r in foldlMP' f b' m --These look a bit iffy foldlAlt :: Alternative f => (b -> a -> b) -> b -> f a -> f b foldlAlt f b m = ($ b) <$> foldrAlt (\x r acc -> r (f acc x)) id m foldl1Alt :: Alternative f => (b -> a -> b) -> b -> f a -> f b foldl1Alt f b m = ($ b) <$> foldr1Alt (\x r acc -> r (f acc x)) id m Do these or similar exist somewhere? If not, should they exist somewhere? I'm particularly interested in whether either of these will be fast, or will admit a fast implementation, for attoparsec parsers. Thanks, David Feuer

3 6

Re: [Haskell-cafe] MonadPlus and Alternative folds
by Olaf Klinke 20 Sep '15

20 Sep '15

Is there an instance of Alternative where some and many are not bottom? The usual example monads, [] and Maybe, don't serve as a source of intuition. In general I agree with David: If f is a functor such that every type (f a) is a monoid, then there should be folds. The usual question here is: Your functions have types more general than the known foldr and foldl. Do your functions specialize to the known ones? It seems not. For example, foldrAlt (+) 0 [1,2,3] diverges, and so does foldlMP' (+) 0 [1,2,3]. I have doubts that knowing how to append two lists gives you access to the recursive structure of a list. Note that mplus is a _constructor_ of lists, while recursion is built on _destructors_. The same seems to apply to all other functions provided by Applicative, Alternative and MonadPlus. The foldr of Data.Foldable, by contrast, accepts a foldable structure and produces something which is not a foldable structure. Hence it must have access to the internals of that foldable structure. Olaf

2 1

Why Haskell is beautiful to the novice
by Olaf Klinke 20 Sep '15

20 Sep '15

Dear cafe, please correct me if questions like this should not go via this mailing list. Presumably everyone on this list agrees that Haskell stands out as a beautiful and pleasant language to have. The recent nitpicking and real-world problems like cabal hell don't change that. However, statements supporting Haskell's beauty usually involve: "In Haskell it is so much clearer/easier/faster to ... than in another language." That is, the beauty of Haskell presents itself to those who can compare it to other imperative or not strongly typed languages that one learned before Haskell. My question is, for what reason should anyone not acquainted with any programming language find Haskell beautiful? Maybe it does not look beautiful at all to the novice. The novice can not draw on the comparison, unless he takes the effort to learn more than one language in parallel. The novice likely also does not have the mathematical background to see the beautiful correspondence between the language and its semantics. (My reason to love FP is because it is executable domain theory.) One might argue that it is not the language itself that is beautiful, but rather the concepts (data structures, algorithms, recursion) and Haskell does a great job to preserve their beauty into the implementation. Do you agree? Disclaimer: I am about to start teaching a first course in computer science in secondary school. I can teach whatever I want, since this is the first CS course the school ever had. I want to teach beautiful things. I love functional programming. I need not start teaching programming right away. But I am reluctant to expose the pupils to something whose beauty escapes them completely. -- Olaf

29 79

persistent sqlite: getting record by ID
by miro 19 Sep '15

19 Sep '15

Please, can you help me with following? I’m trying to get a record from sqlite by id. I have found following peace of code,…but I’m missing the place that where can I specify the table I’m actually querying (like PersonId). get $ Key $ (PersistInt64 $ fromIntegral id) Cheers, Miro

2 1

Looking for better than Dragon Book
by Michael Litchard 19 Sep '15

19 Sep '15

I've got the Tiger Book, ML version, which I will use to help me implement Tiger in Haskell. Below is a class curriculum I've found. But besides the Tiger Book, it uses the Dragon Book. I know there are more modern books out there. Looking for suggestions as what might be a good fit. http://cs.nyu.edu/courses/fall13/CSCI-GA.2130-001/

4 4

London Haskell eXchange & Haskell Infrastructure Hackathon
by Oliver Charles 18 Sep '15

18 Sep '15

Hi all, In October Skills Matter & Well-Typed are organising a few Haskell events in London that will (I expect!) be of interest to people here. On the 8-9th of October is the Haskell eXchange, a 2-day conference on various topics about Haskell. The programme includes a variety of topics from the theoretical to the practical, with a mixture of talks, keynotes, lightning talks and workshop. A promo-code is valid until Sept 19th for 25% off - use HASKELL-EXCHANGE-25. For more details and booking, see https://skillsmatter.com/conferences/7069-haskell-exchange-2015. Shortly after the Haskell eXchange, Well-Typed will co-organize and participate in a two-day Haskell Hackathon <https://skillsmatter.com/conferences/7316-haskell-hackathon-2015>, which takes place directly after the Haskell eXchange, 10-11th October. This Hackathon aims at bringing together Haskell developers – both beginners and experts – who want to help improve the Haskell infrastructure, predominantly Hackage and Cabal. We’ll aim to arrange some introductory overview talks, to e.g. provide an overview over the code bases and the most important open issues. Participation is free, but please register online: https://skillsmatter.com/conferences/7316-haskell-hackathon-2015. Hope to see you there! ocharles

1 0

Replication class
by David Feuer 18 Sep '15

18 Sep '15

A number of types support some version of replication. Lists and vectors offer replicateM, while sequences have replicateA (the same thing, but generalized to Applicative). Is there, or should there be, a class for structures that support this operation?

2 2

Anchors in Haddock
by Sven Panne 18 Sep '15

18 Sep '15

Haddock offers a way to add anchors to the documentation and to link to them, see https://www.haskell.org/haddock/doc/html/ch03s08.html#idm140354810720016. I've got

1 1

Linking to anchors in Haddock
by Sven Panne 18 Sep '15

18 Sep '15

Haddock offers a way to add anchors to the documentation and to link to them, see https://www.haskell.org/haddock/doc/html/ch03s08.html#idm140354810720016. I've got 2 problems with that: * A link to an anchor "Foo.Bar#Baz" is always rendered with the text "Foo.Bar" in the HTML backend. Is there a way to change that? With the HTML backend I actually want something like: <a href="path-to-Foo.Bar#Baz">My own link text</a> * Using relative hype [My own link text](Foo-Bar.html#Baz) instead has problems, too: The "Foo-Bar.html" part is fragile and depends on Haddock flags. Furthermore, when using the LaTeX backend, the resulting LaTeX code is broken (the '#' needs an escaping backslash). So is there a non-fragile way working with several backends which makes it possible to specify the link text? Staring at the Haddock docs and the Haddock source code didn't help, and several experiments gave no clue, either. :-/

1 0