- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] To [] Or Not To []
by Doug McIlroy 13 Mar '17

13 Mar '17

This stirred ancient memories in praise of wrong programs: > > * If your program sorts a list, then your program is wrong. > > This seems a very strange claim. > > The whole thing is an abuse of the word "wrong". > A program can be all of ugly, inefficient, unidiomatic, > &c &c without being WRONG. Fifty-plus years ago, when computing was 1000 times slower and cost $600/hour, it was typical for professional programmers to mediate between scientists and computers so that those expensive machines would be used efficiently. At Bell Labs, though, Dick Hamming insisted on open-shop computing because "It is more important to have the right problem done the wrong way, than to have the wrong problem done the right way." Doug

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Re: [Haskell-cafe] reading and writing Data.Ratio.Rational
by Mike Ledger 12 Mar '17

12 Mar '17

I can never seem to remember to reply-all ... On Wed, Mar 8, 2017 at 3:24 PM, Mike Ledger <eleventynine(a)gmail.com> wrote: > There is also the exact-real and numbers packages which provide exact > reals with their respective CReal types. > > On Wed, Mar 8, 2017 at 6:22 AM, Olaf Klinke <olf(a)aatal-apotheke.de> wrote: > >> Before asking on this list, I wrote my own version which did just what >> David suggested: Parse the integer before the decimal point, parse the part >> behind the decimal point and divide the latter by 10^length, then sum both >> parts. This function has type >> >> Fractional t => String -> t >> >> I thought that this was maybe very inefficient and therefore looked for >> prior art. If someone feels it is worth adding to some package, I'm happy >> to provide the code. >> >> Olaf >> >> >> > Am 06.03.2017 um 21:46 schrieb David Feuer <david.feuer(a)gmail.com>: >> > >> > I think your best bet is probably to get your hands dirty and parse it >> yourself: first grab an integer, then optionally a decimal point, etc. >> > >> > On Mar 6, 2017 3:43 PM, "Olaf Klinke" <olf(a)aatal-apotheke.de> wrote: >> > Ah, thanks! That is something to build on. >> > >> > Olaf >> > > Am 24.02.2017 um 15:35 schrieb Patrick Chilton <chpatrick(a)gmail.com>: >> > > >> > > Prelude> import Numeric >> > > Prelude Numeric> fst $ head $ readFloat "0.1234" :: Rational >> > > 617 % 5000 >> > > >> > > On Fri, Feb 24, 2017 at 1:17 PM, Olaf Klinke <olf(a)aatal-apotheke.de> >> wrote: >> > > Dear cafe, >> > > >> > > when processing text files containing numbers of the form "xxxx.yyyy" >> I used to parse them into Double using that type's Read instance. >> Obviously, even with arithmetic no more complicated than the field >> operations the result might have ugly rounding errors like 12.000000000002 >> due to the fact that numbers like 0.7 are not dyadic rationals. The math in >> my program is not complicated and the numbers are not large, so I don't >> care about Rationals having potentially huge memory footprints. >> > > >> > > So here's my question. A literal like 0.7 has type Fractional a => a, >> but the Read instance of Rational rejects the string "0.7". Must it be this >> way? Do I have to go via toRational.(read :: String -> >> Data.Scientific.Scientific)? >> > > >> > > Note that the documentation of Data.Scientific explicitly states that >> using (/) is unsafe, so I'd rather stay with the field Rational. >> > > >> > > For the output as decimal expansion, there is of course long division >> as described here [1], but I wonder whether either this exists in some >> library or there is even a more efficient solution. >> > > >> > > Any pointers are appreciated. >> > > Thanks, >> > > Olaf >> > > >> > > [1] http://stackoverflow.com/questions/30931369/how-to-convert- >> a-rational-into-a-pretty-string >> > > _______________________________________________ >> > > Haskell-Cafe mailing list >> > > To (un)subscribe, modify options or view archives go to: >> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > > Only members subscribed via the mailman list are allowed to post. >> > > >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > To (un)subscribe, modify options or view archives go to: >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > Only members subscribed via the mailman list are allowed to post. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > >

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Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 11 Mar '17

11 Mar '17

And I was merely pointing that out: That there are recommended use cases in the article, while the abstract given by the author on this list was quite negative. (Possibly it had to be, in order to gain the necessary attention.) I was not arguing against the syntactic preference Haskell gives to singly linked lists over other data structures. If Haskell was designed from scratch, what would take the place of []? Possibly Data.Sequence? Mildly complex data processing programs are somewhat special: You read in and parse one or more input files, do some transformation on the parsed data and write some results back to disk. As the article explained, the Haskell compiler will be able to optimise away the intermediate list structure (at least in simple cases). If data dependency is local, you'll be fine with data input to String rather than Text or ByteString, with map rather than fmap or foldl', and you'll be fine to use a mapM_ on a list for output. Olaf > I think you are missing the point of the article, as that is pretty much > exactly the type of situation where it recommends lists be used. > > On Mar 10, 2017 7:45 PM, "JK" <jerzy.karczmarczuk(a)unicaen.fr> wrote: > >> Hi. >> Le 11/03/2017 à 00:28, Olaf Klinke a écrit : >> >> Actually, your negative abstract turns out to be for a quite positive article. List are like iterators, that's the essence, and as long as you are iterating, it's fine to use []. The prominence of singly linked lists in Haskell has tought me to write my data processing programs in a streaming style. >> >> *Yes.* I've been teaching not just "data processing" - after all almost >> everything we program is "data processing", no?... but such concrete stuff >> as physics simulation (diff. eqs.), some other numerics (asymptotic >> expansions, etc.) signal processing (including sound generation), and I >> liked to present several examples in a *dataflow style* with plenty of >> co-recursive contraptions. Haskell lazy lists were natural, concise, and >> easy to manipulate. We enjoyed it, wrong or not. >> >> Nothing is perfect, not only mister Nobody; calling ANY approach to >> programming "wrong" is sectarian. A professional coder working on a >> concrete project may say bad words about anything he wishes, but for a >> teacher this is a pedagogical sin, and inefficient programs can be more >> inspiring than some "correct" doctrines. >> >> Jerzy Karczmarczuk

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Is this a "monad" for contravariant functors?
by Zemyla 10 Mar '17

10 Mar '17

There originally seems like there shouldn't be one, because it's impossible to, for any permutation of the arguments of (>>=), get the covariances/contravariances right on the arguments. However, based on the fact that there is a mapping: a -> ((a -> r) -> r) and what seems like an isomorphism: (a -> r) -> (((a -> r) -> r) -> r) (((a -> r) -> r) -> r) -> (a -> r) then this looks like it might be some kind of monad-like structure: class Contravariant f => Contramonad f where contrareturn :: a -> f (f a) contrabind :: (a -> f (f b)) -> f b -> f a I have no idea what the laws should be for it, though. So is this worth pursuing, or have I just stumbled down a blind alley?

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regex and Regular Expressions Libraries
by Chris Dornan 10 Mar '17

10 Mar '17

Dear Haskell Cafe, With the regex announcement, I wanted to try get other folks' perspective on what has been happening with the Haskell regular expression libraries. I have been in turn impressed at how good the engineering of the Regex packages while gobsmacked by how difficult the traditional Text.Regex API is to use. In this blog post, http://engineers.irisconnect.net/posts/2017-03-07-regex.html I rather cheekily speculate that the Haskellers perhaps have been a bit disdainful of regular expressions (not important in a language capable of doing proper parsing, etc.). What do you think? Chris

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CEF3 bindings and game dev in Haskell
by Maksymilian Owsianny 10 Mar '17

10 Mar '17

Hey I'm interested in making gamedev in haskell more viable and one thing that I believe is missing in that part of the environment is a good GUI story. So I decided to create proper bindings to cef3, you can check out what I've done so far [here][1]. In this current version I basically took the work done by Daniel Austin [here][2] split it into parts to prevent my machine from dying from lack of memory and packed it up with some nix glue for convenient usage. Now all this doesn't look like much but it sill took me much more time then I'm willing to admit in public. That's one of the eternal truths of software development, that everything always takes an order of magnitude more time then you suspect it would. Anyway, what I would like to do here is add support for all platforms. Create some nice scripts that help with automatizing building, developing and deployment of code with this library for users. Possibly write a script that automatically generates bindings from c headers to bring this lib up to speed with the current version of cef. Write some proper guides on using this lib for creating game GUI or standalone desktop application. All of that is nice and well to wish for but it is quite a lot of frankly not very fun work. If I had to choose I would rather spend my time working on either something fun or something that pays the rent, so usually at this point I stop and walk away. But this time I decided to go about it differently, and ask for community support. I have set up a BountySource campaign [here][3] so I could focus on making this happen. Alternatively, if that's your thing, I have set up a bitcoin address [4]. Depending on how well this campaign will go I'm also planing on creating a full 2D game engine... but yeah, that's in the future. So yeah, if you'd like to help me build this please support me. Thanks, Max. [1]: https://github.com/haskell-ui/cef3-raw [2]: https://github.com/fluffynukeit/bindings-cef3 [3]: https://salt.bountysource.com/teams/haskell-ui [4]: 1LskxSUyLDLpMCBfUUnqcRT8mEFMW511Eq PS. I have also created a reddit thread for this proposal here <https://www.reddit.com/r/haskell/comments/5yi1b7/cef3_bindings_and_game_dev…>.

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To [] Or Not To []
by Johannes Waldmann 09 Mar '17

09 Mar '17

Dear Cafe, I wrote a text (well, you might call it a rant) on the tragic over-use of lists in Haskell, based on my experience in programming and teaching. http://www.imn.htwk-leipzig.de/~waldmann/etc/untutorial/list-or-not-list/ * If your program accesses a list by index (with the !! operator), then your program is wrong. * If your program uses the length function, then your program is wrong. * If your program sorts a list, then your program is wrong. * If you wrote this sort function yourself, then it is doubly wrong. * The ideal use of a list is such that will be removed by the compiler. * The enlightened programmer writes list-free code with Foldable. Have fun reading, and avoiding lists from here on! - J.W.

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Parsing a bunch of strings without backtracking
by Olaf Klinke 09 Mar '17

09 Mar '17

The tree with internal nodes of type Char and leaves of type v is a representation of a finite state machine, a common tool in parsing certain grammars. I'm not sure your type Compact v captures exactly this, since a value v may be followed by more Chars down in the tree. You'd want type Compact v = [Either (More v) v] -- akin to Data.Tree.Forest data More v = More Char (Compact v) -- akin to Data.Tree.Tree What you may want to look at is a so-called trie [1,2]. In textbooks, it is common to assume that there is a charcter '$' not element of your alphabet. Then '$' is appended to each string before compressing all into a tree structure. This way, one can put both a string and a prefix of it into the same tree. A special kind of trie is the suffix trie. It holds all suffixes of a string and facilitates fast full-text search. More efficient variants are extensively used in bioinformatics [3]. Olaf [1] http://hackage.haskell.org/package/word-trie-0.3.0/docs/Data-Trie.html [2] https://en.wikipedia.org/wiki/Trie [3] http://mummer.sourceforge.net

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Haskell-related PhD studentship
by Meng Wang 09 Mar '17

09 Mar '17

Dear Haskellers, I am seeking a PhD student on Haskell-related projects, particularly in the area of bidirectional programming (lenses), property-based testing (QuickCheck) and refactoring. This would be a good opportunity for someone who is interested in applying Haskell ideas to real problems. The student will be part of the Programming Languages and Systems (PLAS) group in the School of Computing at the University of Kent. A major focus within the group is functional programming (FP), where it has had an international reputation for more than twenty five years. The group currently has 13 FTE academic staff, 4 RAs and 12 doctoral students. . The funding covers maintenance, EU student fees and research related expenses. But non-EU students are welcome to apply too. For Sep 2017 starting, the application deadline is 7 April 2017. If you are interested, please contact me at m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk>. More information can be found at https://www.cs.kent.ac.uk/research/studyingforaphd/phd-wang-2017.html. Best wishes, Meng Dr. Meng Wang | Lecturer in Computer Science School of Computing, University of Kent Canterbury, Kent, CT2 7NF, UK m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk> | ☎ +44 (0)1227 827699 www.cs.kent.ac.uk/~mw516<http://www.cs.kent.ac.uk/~mw516>

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[ANN] DataHaskell mailing list
by Marco Zocca 09 Mar '17

09 Mar '17

Dear all, DataHaskell now has a mailing list hosted on haskell.org : https://mail.haskell.org/cgi-bin/mailman/listinfo/data-haskell This is an addition to our current Gitter chatroom ; the purpose is to host longer-form archived discussions on libraries, documentation and more. With kind regards, Marco Zocca (https://github.com/ocramz)

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