October 2018 - Beginners

Re: [Haskell-beginners] Lifting over record syntax
by Anthony Clayden 31 Oct '18

31 Oct '18

Hi Jaakko, oh dear you've picked on the most embarrassing part of Haskell. > data Person = Person {name::String, age::Int} deriving Show > > Now, I can create maybe-people like in applicative style: > > Person <$> Just "John Doe" <*> Nothing Hmm? That returns Nothing -- is that what you expect? Perhaps Person <$> Just "John Doe" <*> Just undefined But why not Just $ Person "John Doe" undefined > The problem with the [applicative] approach > is that it depends on the order of the arguments > because it doesn't utilize the record syntax. You'd think that a leading-edge language like Haskell would have really powerful record abstractions. With ways to avoid the shackles of argument ordering. Then it's sad to admit Haskell's records are feeble. Back in Haskell 98 people were saying "surely we can do better!" Well, essentially nothing's happened. So records are no more than pretty(-ish?) syntax for the ordered arguments style. The label names are not first-class. And field labels for building records only work in very restricted syntactic positions, as you point out: > I would like to get the benefits of the the record syntax > but with similar code simplicity as the applicative style has. > Do you have ideas is this possible? > My non-working pseudo-code would look like: > > Person <$> {name=Just "John Doe", age=Nothing} > > But this doesn't work, it's syntax error. You want that bit in braces to be a free-standing expression. That's called "anonymous/extensible records" (search Haskell wiki). "Anonymous" because it's not tied to any particular datatype. It's self-describing: I am a record with two fields labelled `name, age`; and the order of the labelled fields should be arbitrary. "Extensible" because you could start a record with just one labelled field, and extend it with another labelled field. But I'm not sure what the `Maybe` wrapping is doing for you? I'm going to ignore it. I have some good news and some bad news. There is a Haskell you can do n = (name = "John Doe") -- note round parens n_a = (age = undefined | n) -- | for extend the record p = Person n_a For that you need a slightly different data decl data Person = Person (Rec (name :: String, age :: Int)) deriving Show Round parens again; and a mysterious type constructor. But these records are first-class values. (They're a special variety of tuples.) Perhaps you don't need a datatype? type Person = Rec (name :: String, age :: Int) Then you don't need to apply a data constructor. This style of records is 'Trex' -- Typed Records with extensibility https://www.haskell.org/hugs/pages/hugsman/exts.html#sect7.2 The bad news: Trex is available only in a very old Haskell system, Hugs. So old that its download has bitrotted, see note here https://mail.haskell.org/pipermail/hugs-bugs/2018-July/001914.html GHC has had 12 years to come up with something comparable/better. So far, nothing. If you're wanting to get all Applicative and Functorial in GHC, there are heaps of record systems based around Lenses. http://hackage.haskell.org/package/lens-tutorial But that's going way beyond Beginners level; you'll need to call on Template Haskell; and probably add some plumbing of your own. (And under the hood you still have ordered arguments; they're rather better encapsulated.) AntC

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Tagged types
by PICCA Frederic-Emmanuel 29 Oct '18

29 Oct '18

Hello suppose that I have a bunch of type like this data Unchecked data Hdf5 data Cbf data A t > A String The A thing come from a database as A Unchecked now if I read the String and it ends with .h5, I have a A Hdf5 type and If the string end with .cbf, I have a A Cbf. So I would like a function which allow to return a A Hdf5 or a A Cbf depending on the String content. check :: A Unchecked -> A ??? check = ??? Is it possible to do this ? Thanks Frederic PS: At the end I will have more tha one tag.

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I can't start WinGHCi Haskell
by Gustavo Arturo Marquez Flores 14 Oct '18

14 Oct '18

Hi, I want to start with Haskell language programming. But when I launch the WinGHCi program I got he followin message: [image: image.png] What can I do ? How to resolve this problem ? I use Windows 10 plataform. I aprecite your help for resolving this problem to me. Thanks a lot.

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Lifting over record syntax
by Jaakko Luttinen 14 Oct '18

14 Oct '18

Hi! Let's consider this simple data structure: data Person = Person {name::String, age::Int} deriving Show Now, I can create maybe-people like in applicative style: Person <$> Just "John Doe" <*> Nothing or in monad style: do name' <- Just "John Doe" age' <- Nothing return Person {name=name', age=age'} The problem with the first approach is that it depends on the order of the arguments because it doesn't utilize the record syntax. The problem with the second approach is that there's a bit of unnecessary boilerplate (e.g., name' and age' variables). I would like to get the benefits of the the record syntax but with similar code simplicity as the applicative style has. Do you have ideas is this possible? My non-working pseudo-code would look like: Person <$> {name=Just "John Doe", age=Nothing} But this doesn't work, it's syntax error. Any ideas for a nice syntax? Cheers, Jaakko

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why is ScopedTypeVariables not fixing this error?
by Dennis Raddle 09 Oct '18

09 Oct '18

In the following snippet from a program in progress (designed to compute percentile rank for arbitrary lists of values) , I was hoping to declare types of functions within the main function just as a way of helping myself catch type errors. I'm getting the error "Can't match 'a' with 'a1'.... where 'a' is rigid type variable... etc. etc." on the line indicated in the comment below. The usual error I get when I try to do this without ScopedTypeVariables. So, I thought that ScopedTypeVariables was supposed to allow this kind of usage. What am I doing wrong? {-# LANGUAGE ScopedTypeVariables #-} import qualified Data.Map as M import qualified Data.List as L import Data.Map(Map) import Data.Function -- <percent at or below> <percent below> data PercentileData = PercentileData Double Double -- new attempt, October 2018: using new PercentileData construct to -- represent percentile in both ways. (at/below, or below) computePercentile :: Ord a => Map a Double -> Map a PercentileData computePercentile dataIn = error "foo" where pairs :: [(a,Double)] -- THIS IS THE LINE GETTING THE ERROR pairs = L.sortBy (compare `on` snd) $ M.toList dataIn

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[Call for Contributions] Haskell Communities and Activities Report, November 2018 edition (35th edition)
by Mihai Maruseac 06 Oct '18

06 Oct '18

Dear all, We would like to collect contributions for the 35th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 4 November 2018 (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. * 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 FAQ: Q: What format should I write in? A: Any format is ok, I will transpose the submission to the format in use at the time of publication. Previous editions have used a LaTeX format, with the template that is available at: http://haskell.org/communities/11-2018/template.tex And the associated LaTeX style file at http://haskell.org/communities/11-2018/hcar.sty You can use those to edit and preview your entry, but it is very likely that by the time of publication the entire HCAR pipeline would be changed to a more modern one. You will receive a copy of the draft, prior to publication, to ensure that everything looks ok. 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. Q: Can I include Haskell code? A: Yes. If using LaTeX, you can 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 is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments 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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optimisation of code
by PICCA Frederic-Emmanuel 01 Oct '18

01 Oct '18

Hello, I would like to have your advice in order to optimize this code. The purpose is to trigg an action 'a' if a list of files (thousands) exists. A process copy files from one directory to another. allFilesThere :: MonadIO m => [Path Abs File] -> m Bool allFilesThere fs = liftIO $ allM (doesFileExist . fromAbsFile) fs trigOnAllFiles :: MonadIO m => m r -> [Path Abs File] -> m r trigOnAllFiles a fs = go where go = do r <- allFilesThere fs if r then a else ( do liftIO $ threadDelay 1000000 go) It works, but it consums a lot's of resources when all the files does not exists yet. So I would like your advice in order to optimize it :) thanks for your help. Frederic

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