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Conditionals where more than one case is true
by PATRICK BROWNE 17 Apr '17

17 Apr '17

I am having difficulty in evaluating conditionals where more than one case is true. I require a program to express the following conditions: 1) x is faster than y if x is a buffolo and y is a pig. 2) x is faster than y if x is a pig and y is a slug. Both of these conditions are true (expressed in fact predicate). I include my attempt using guards (I have also tried case/if-then-else) I can apreciate that the program works when the first guard condition is met (faster Bob Joe). I am not sure why it fails on the second case (faster Bob Joe) Can I program the requirement using conditionals? Can I pattern match is some way? Is there another way to encode the requirement. Regards, Pat data E = Bob | Joe | Steve | Buffalo | Pig | Slug deriving Show fact Buffalo Bob = True fact Pig Joe = True fact Slug Steve = True faster x y | fact Buffalo x && fact Pig y = True | fact Pig x && fact Slug y = True | otherwise = False -- faster Bob Joe OK -- faster Steve Bob ***Exception: Faster.hs:(5,1)-(7,23): Non-exhaustive patterns in function fact -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Parsing
by mike h 14 Apr '17

14 Apr '17

I have data PackageDec = Pkg String deriving Show and a parser for it packageP :: Parser PackageDec packageP = do literal “package" x <- identifier xs <- many ((:) <$> char '.' <*> identifier) return $ Pkg . concat $ (x:xs) so I’m parsing for this sort of string “package some.sort.of.name” and I’m trying to rewrite the packageP parser in applicative style. As a not quite correct start I have packageP' :: Parser PackageDec packageP' = literal "package" >> Pkg . concat <$> many ((:) <$> char '.' <*> identifier) but I can’t see how to get the ‘first’ identifier into this sequence - i.e. the bit that corresponds to x <- identifier in the monadic version. in ghci λ-> :t many ((:) <$> char '.' <*> identifier) many ((:) <$> char '.' <*> identifier) :: Parser [[Char]] so I think that somehow I need to get the ‘first’ identifier into a list just after Pkg . concat so that the whole list gets flattened and everybody is happy! Any help appreciated. Thanks Mike

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will GHC optimize pattern-matching on integers?
by Patrick Pelletier 13 Apr '17

13 Apr '17

Suppose I am doing a pattern match on a large number of consecutive (or mostly-consecutive) integers: foo 0 = something foo 1 = somethingElse ... foo 1000 = anotherThing Will GHC optimize this to a table lookup, or is it going to test each integer in turn? Am I better off using a Vector or Map instead of pattern matching? Thanks, --Patrick

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Re: [Haskell-beginners] What is the Best Haskell IDE in Windows?
by Gregory Guthrie 12 Apr '17

12 Apr '17

I like the HEAT simple IDE for small one-file programs, which are common in introductory classes. (We made a few local changes and additions - happy to share.) I also like using IntelliJ IDEA with Haskell plugin, and that is probably the most polished and best actual IDE. (The eclipse plugin is abandoned and depreciated.) For larger projects, Leksah is good, although the documentation is sparse (IMHO), and it has a bit of a learning/setup curve. ---------------------------------------------------------------- -----Original Message----- Sent: Wednesday, April 12, 2017 7:00 AM Subject: Beginners Digest, Vol 106, Issue 4 1. What is the Best Haskell IDE in Windows? (Frank Lugala)

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What is the Best Haskell IDE in Windows?
by Frank Lugala 11 Apr '17

11 Apr '17

Can anyone please help?i tried to install Atom, Sublime, Emacs but i can not get them working on windows

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Same code, system, but different arch using Win32 for reading registry.
by Akos Marton 07 Apr '17

07 Apr '17

Dear Haskellers, The setup: Having a binary compiled on either x86 or x64 system (same installation, utilizing stack) in order to preserve compatibility against x64 systems. Actually, not to have 2 separate executable for each, that would be the overall goal. However it surprises me when running the x86 .exe utilizing Win32-2.5.4.1 package with ghc-8.0.2, reading out a registry key fails on x64 system with the following:/ / /me.exe: RegOpenKey: invalid argument (The system cannot find the file specified.)/ That would be fine, however the key does exists. When same code, same system, but the .exe built to be x64 it runs like a charm. A couple of question, which some of them eventually will not make sense, but still: - Can it be ghc code optimization issue? - but this is a runtime check in IO, if so, how? - Yes, I could use a built-in windows system command and parse the input of that; unless absolutely necessary I would not introduce another dependency (system package). Would love to solve it with the currently utilized weapons. - the issue just puzzles me... I would know the answer if possible. - Is it more library (Win32), ghc, binary I generate, issue? What library can I use to detect a system's architecture which works in this scenario? Another thing which convoluted in the issue... The function, /getSystemInfo :: IO SYSTEM_INFO, /can read out the underlying architecture. When compiled on x86 and run on x64 it would tell me: "I am running on x86". That's failure. Most importantly: what is the obvious I am missing? Thank you for your insights! Best, Akos Ps.: Would you/we need sample code to puzzle about I can quickly weld one. - not sure if necessary.

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State monad to help pass around game settings
by Dave Martin 07 Apr '17

07 Apr '17

I'm trying to write a game with a "settings menu" where the user can adjust gameplay options. Right now I pass all the settings around as parameters. I'm trying to figure out how to use the State monad to simplify this task, but I can't figure out how to start. Or maybe my whole design approach is wrongheaded, and not in keeping with best practices. Haskell is my first language. This is the kind of thing I have now: mainM color shape = putStrLn "\n\nMain Menu" >> (putStrLn . unlines) [ "(1) Set", "(2) Display", "(3) Quit"] >> putStr "? " >> getChar >>= \c -> case c of '1' -> set color shape '2' -> display color shape '3' -> return () _ -> mainM color shape set color shape = putStrLn "\n\nSettings" >> (putStrLn . unlines) [ "(1) Color", "(2) Shape", "(3) Main Menu"] >> putStr "? " >> getChar >>= \c -> case c of '1' -> setColor color shape '2' -> setShape color shape '3' -> mainM color shape _ -> set color shape setColor color shape = putStr ("\n\nColor is " ++ color ++ ". New color? ") >> getLine >>= \cs -> set cs shape setShape color shape = putStr ("\n\nShape is " ++ shape ++ ". New shape? ") >> getLine >>= \cs -> set color cs display color shape = putStrLn ("\n\nColor is " ++ color ++ ". Shape is " ++ shape ++ ".") >> mainM color shape

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[Call for Contributions] Haskell Communities and Activities Report, May 2017 edition (32nd edition)
by Mihai Maruseac 02 Apr '17

02 Apr '17

Dear all, We would like to collect contributions for the 32nd edition (10000th edition) of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 2017 (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/05-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2017/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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Suggestions for way to improve / clean up this code?
by Erik Tillett 28 Mar '17

28 Mar '17

I'm looking for sugestions on how to better structure this code. I stuck it in a pastbin: https://pastebin.com/gQFmvq6W/ Would a State monad help? Thanks!

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Parallel processing...
by mike h 27 Mar '17

27 Mar '17

Hi, I have a list of lists - a grid [[ ]] of complex numbers and I map a function over each number in the grid. Each computation on a value at (r, c) is independent of any and all other calculations. This strikes me as being something that can be done in parallel by creating , say 4 quadrants and mapping over each and combining when done, or doing each row in parallel etc. etc. So I'm really just asking for advice or pointers on the Haskell libraries etc that I should start with but I'm not really looking to import an uber-package that will do most of it for me. I want to learn a little more Haskell by working from a few concurrency primitives. Thanks Mike

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