March 2018 - Beginners

Join'ing a State Monad, example from LYH
by Olumide 09 May '18

09 May '18

Dear List, I've been stumped for a few months on the following example, from chapter 13 of LYH http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions runState (join (State $ \s -> (push 10,1:2:s))) [0,0,0] I find the following implementation of join in the text is hard to understand or apply join :: (Monad m) => m (m a) -> m a join mm = do m <- mm m In contrast, I find the following definition(?) on Haskell Wikibooks https://en.wikibooks.org/wiki/Haskell/Category_theory#Monads join :: Monad m => m (m a) -> m a join x = x >>= id easier to understand, and although I can apply it to the following Writer Monad example, in the same section of LYH, runWriter $ join (Writer (Writer (1,"aaa"),"bbb")) I cannot apply it to the State Monad example. Regards, - Olumide

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phantom type
by PICCA Frederic-Emmanuel 18 Apr '18

18 Apr '18

Hello, Here my problem. I extract some information from a database and put them into a type with 50 parameters Type MyData = MyData Int String ... But I need to be able to differenciate two different case. depending on the String content. something like if the String start with "ref-" it is a Caracterization and if not it is a Collect So I end up with this solution data Unchecked data Caracterization data Collect data MyData t = MyData Int String ... now I defined a function getMyDataFromDatabase :: ... -> Mydata Unchecked and isCollect in order to verify that I have a Collect and not something Else isACollect :: MyData Unchecked -> Either Text (MyData Collect). which use coerc in order to avoid copying all the members. It is great. now Here is my problem. I create a method myMethod :: MyData Collect -> IO () but I want my method to work for MyData Collect and Mydata Caracterization but not Unchecked. So It seems to me that I can not just do myMethod :: MyData t -> IO () what is the right way to solve this problem ? thanks for your help. Frederic

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Stack and Cabal
by mike h 29 Mar '18

29 Mar '18

Hi, I have dependencies on mtl >= 2.2.2 and containers >= 0.5.11.0 should these go into my .cabal file or into my local stack.yaml? Which is best practice? Can anyone recommend a stack/cabal ‘cheat sheet’? Thanks Mike

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Two type declarations
by Justin Thong 28 Mar '18

28 Mar '18

What is the difference between these two type declarations? The second one is wrong but I can't convince myself why it should be wrong. Is it because *Int *not a constraint class and it is only an instance of one? My curiousity is why #1 can't be written in the form of #2. I apologise if I am using wrong terminology as type, class and constraint class are used with not much distinction. To add context, this problem is to find a function that will find an element by passing in a list and an index argument. elementAt''' ::[a]-> Int ->a -- #1 elementAt''' [] _= error "list is empty" elementAt''' list index | (index < 1) = error "index has to be positive number" | otherwise= list !! (index-1) elementAt'''' ::(Int b)=>[a]-> b ->a -- #2 elementAt'''' [] _= error "list is empty" elementAt'''' list index | (index < 1) = error "index has to be positive number" | otherwise= list !! (index-1) Thank you. I just began learning Haskell. Yours sincerely, Justin *I check my email at 9AM and 4PM everyday* *If you have an EMERGENCY, contact me at +447938674419(UK) or +60125056192(Malaysia)*

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Rounding to 2 decimal places
by Raghav Malik 27 Mar '18

27 Mar '18

Hi, I have a complex double (x :+ y) where 'x' and 'y' are computed using sin and cos functions on an angle expressed in radians. However, the output on the command line (using GHCi) shows up with a lot of decimal places. How can I limit the values of 'x' and 'y' to having only two decimal places? Thanks, Raghav

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how to split a list
by PICCA Frederic-Emmanuel 27 Mar '18

27 Mar '18

Hello, I try to achieve this but I can not find a convenient (elegant solution) let l =[1,2, 3, 4, 6, 7, 9, 10] I want this [[1, 2, 3, 4][6, 7],[9, 10]] In fact I want to split a list with all consecutive series. I imagine that it existe a one-liner for this but I did not find it :)) Cheers and thanks for your help Frederic

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[Call for Contributions] Haskell Communities and Activities Report, May 2018 edition (34th edition)
by Mihai Maruseac 26 Mar '18

26 Mar '18

Dear all, We would like to collect contributions for the 34th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 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: The usual format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2017/hcar.sty that you can use to preview your entry. If you do not know LaTeX or don't want to use it or don't have time to translate your entry into it, then please use plain text, it is better to have an entry in plain-text which we will translate than not have it at all. 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. 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 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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GSoC Project
by Sanya Negi 20 Mar '18

20 Mar '18

Hello! I am extremely interested in the project " offline mode for stack " for GSoC, for which I have learnt basic stack and have been learning more. Please guide me as to what other specific things I have to delve into and provide me pointers and inputs on how to break this problem into pieces to form my proposal. I can't seem to get in touch with the projects mentor Emanuel Borsboom. And as the deadline is reaching, I really need to start forming my proposal now. Soliciting a prompt and positive response. Sanya Negi

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how to know if a value has already been evaluated?
by Ut Primum 18 Mar '18

18 Mar '18

In Haskell, the value of a variable is not always evaluated. This is fundamental when working with infinite lists. Is there a way of determining whether, at a certain point of the execution of a program, a value has been calculated or not? In particular, I would like to write a function that works on an infinite list that: - if the first value of a list, x, has already been evaluated, returns [x] - Else waits until it is evaluated, but without forcing its evaluation. (eventually, its evaluation will be forced later by other functions). Until then, the result of this function must be considered as [ ]. As soon as x is computed, it can return [x]. Is something like this possible in Haskell?

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ReadP & Error Reporting
by Hilco Wijbenga 10 Mar '18

10 Mar '18

Hi all, I recently discovered ReadP (Text.ParserCombinators.ReadP) and I wondered if I could use it to write simple parsers _with decent error reporting_. Let's say I want to parse "[ab] [12]" (i.e. either 'a', or 'b', followed by whitespace, followed by '1', or '2'). I believe something like this does the trick: parse = do name <- ReadP.choice [ReadP.char 'a', ReadP.char 'b'] ReadP.skipMany1 (ReadP.satisfy (\ch -> ch == ' ' || ch == '\t')) value <- ReadP.choice [ReadP.char '1', ReadP.char '2'] return (name, value) How do you add error handling to this? Obviously, I could change the signature to return an Either but I don't see how I "return" anything other than Right. E.g., I would want to give a warning like "Expected 'a', or 'b'" if anything other than 'a', or 'b' is used as first character. Similarly for '1' and '2'. The obvious(?) way would be to add an error result as the last option. The best I can come up with is adding ReadP.pfail to the list of choices but that doesn't allow me to add a useful error. Is ReadP simply not powerful enough for this? Or am I overlooking something obvious? I did some googling and there is plenty about creating parsers with parser combinators but I could not find anything about error handling. Cheers, Hilco

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