- Beginners - Haskell.org

Multiple parameters vs anonymous syntax
by Wink Saville 17 Oct '17

17 Oct '17

I'm going through "Haskell Programming from first principles" and in section 7.3 Anonymous Functions there is an exercise on converting multiple parameters to anonymous functions, and it asks: 1. Which (two or more) of the following are equivalent? mTh1 x y z = x * y * z mTh2 x y = \z -> x * y * z mTh3 x = \y -> \z -> x * y * z mTh4 = \x -> \y -> \z -> x * y * z So I created a file, anon.hs (attached): module Anon where mTh1 x y z = x * y * z mTh2 x y = \z -> x * y * z mTh3 x = \y -> \z -> x * y * z mTh4 = \x -> \y -> \z -> x * y * z I load that into ghci and check the function types: $ ghci anon.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Anon ( anon.hs, interpreted ) Ok, 1 module loaded. *Anon> :t mTh1 mTh1 :: Num a => a -> a -> a -> a *Anon> :t mTh2 mTh2 :: Num a => a -> a -> a -> a *Anon> :t mTh3 mTh3 :: Num a => a -> a -> a -> a *Anon> :t mTh4 mTh4 :: Integer -> Integer -> Integer -> Integer Why is mTh4 different from the rest? On the flip side If I enter "mTh4 = \x -> \y -> \z -> x * y * z" directly in ghci command line then it has same type as the others: $ ghci GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Prelude> mTh4 = \x -> \y -> \z -> x * y * z Prelude> :t mTh4 mTh4 :: Num a => a -> a -> a -> a -- Wink

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Safe TCP accept loop
by Sumit Raja 16 Oct '17

16 Oct '17

Hello, I’ve looked at https://wiki.haskell.org/Concurrency_demos/Graceful_exit and used(2) Using throwTo without the use of block and unblock. It runs on 8.2.1 with my limited testing. I can’t find out much about what the > 7.x GHC replacement for block/unblock is other than mask. What does the unblock do in acceptConnections'? Does this method of handling an accept loop still need masking of async exceptions ? If so where does this need to be done? Thanks Sumit

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monad question
by mike h 14 Oct '17

14 Oct '17

I have cap :: String -> String cap = toUpper rev :: String -> String rev = reverse then I make tupled :: String -> (String, String) tupled = do r <- rev c <- cap return (r, c) and to be honest, yes it’s been a long day at work, and this is coding at home rather than coding (java) at work but I’m not sure how tupled works!!! My first shot was supplying a param s like this tupled :: String -> (String, String) tupled s = do r <- rev s c <- cap s return (r, c) which doesn’t compile. But how does the first version work? How does the string to be processed get into the rev and cap functions?? Thanks Mike

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Return type from class method
by PATRICK BROWNE 09 Oct '17

09 Oct '17

Hi, Is there a way rewriting the definition of (+) so that testPlusArg returns a (Moving Double). My current intuition is that the signature [(+) :: a -> a -> a] says that the type should be the same as the arguments. And indeed (:t testPlus) confirms this. But the type of testPlusArg is a Double. Can I make it (Moving Double) ? Thanks, Pat {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeSynonymInstances #-} module Moving where data Time = Time Double type Moving v = Time -> v class Number a where (+) :: a -> a -> a instance Number (Moving Double) where (+) a b = \t -> ((a t) Prelude.+ (b t)) a,b :: Moving Double a (Time x) = 2.0 b (Time x) = 2.0 testPlus ::(Moving Double) testPlus = (a Moving.+ b) testPlusArg = (a Moving.+ b) (Time 2.0) -- 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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Kyiv Haskell Learning Group Meetups
by Kostiantyn Rybnikov 09 Oct '17

09 Oct '17

Hi! We're starting a Haskell Learning Group in Kyiv, Ukraine. First Meetup will be at 17 October (Tuesday) and will continue on weekly basis at the Institute of Mathematics. See full info about the first meetup at https://www.meetup.com/Kyiv-Haskell-Learning-Group/events/243945548/ Info about the course itself (in Ukrainian): https://github.com/KyivHaskell/haskell-study-startup See you there!

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Library for rendering text
by Jona Ekenberg 06 Oct '17

06 Oct '17

Dear mailing list, I want to create a text editor, preferably using open gl to render text. Are there any easy to use libraries for this? I like the gloss library, but the functions for rendering text seems quite basic. I could add upon this, but I figured I should ask here first; are there any existing libraries suitable for rendering text? Kind regards, Jona

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Automatically "deducing" of wrappers
by Baa 06 Oct '17

06 Oct '17

Hello, List! This is a little bit theoretical question but not only... Let's suppose I have a lot of functions like: llfunc :: A -> B -> C -> D -> C and I have types C and HLC and "constructor" of HLC from C: makeHLC :: C -> Maybe HLC -- or Either String HLC and I want to create "automatically" (i.e. with minimal coding!) functions like: hlfunc :: A -> B -> HLC -> D -> HLC -- see llfunc i.e. for each ll* (low-level) to create hl* (high-level). For example, I can have even HLC -> C function. I may have a lot of llfunc's. Idea is to replace a coding of hlfunc's with some automatically or semi-automatically deduce/creating of them. For example, C may be `FilePath`, HLC may be some kind of `Path a` (wrapper for paths). And I have a lot of FilePath-based legacy functions. I suppose it's possible to solve such problem in D language (I made somthing similar several years ago). And possible it's easy in Nim (Nimrod) because of them: a) types exist b) meta-programming features. IMHO should be a way to do similar in Haskell, but it does not look easy. IMHO I can code arguments + return types with Generic, and next step may be type class with custom methods C -> HLC/HLC -> C and default method like `perform` which will get Generic and automatically makes instance of this type class or... And how will it look like? Is it even possible in Haskell? === Best regards, Paul

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[Call for Contributions] Haskell Communities and Activities Report, November 2017 edition (33rd edition)
by Mihai Maruseac 06 Oct '17

06 Oct '17

Dear all, We would like to collect contributions for the 33rd edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 5 November 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/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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Coverage percents - BUG?
by Baa 04 Oct '17

04 Oct '17

Hello, all! Currently I'm trying to add coverage report to my application tests, and I see such line in the HPC report: .. 100% boolean coverage (0/0) .. but 0/0 =/= 100% (an undefined limit ;) IMHO better is to be reported as 0. Is it a bug in HPC? === Best regards, Paul

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Lifting Numbers
by PATRICK BROWNE 03 Oct '17

03 Oct '17

Hi, I am trying to compile, run, and understand the following code from [1]. type Moving v = Time -> v class Number a where (+), (-), (*) :: a -> a -> a sqr, sqrt :: a -> a sqr a = a * a instance Number v => Number (Moving v) where (+) a b = \t -> (a t) + (b t) (-) a b = \t -> (a t) - (b t) (*) a b = \t -> (a t) * (b t) sqrt a = \t -> sqrt (a t) I followed the compiler advice to produce the following version which compiles: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeSynonymInstances #-} module MovingPoint where type Time = Float -- Type synonym assumed, could it be data type?? type Moving v = Time -> v class Number a where (+), (-), (*) :: a -> a -> a sqr :: a -> a sqrt :: a -> a instance (Floating v) => Number (Moving v) where (+) a b = \t -> (a t) Prelude.+ (b t) (-) a b = \t -> (a t) Prelude.- (b t) (*) a b = \t -> (a t) Prelude.* (b t) sqr a = \t -> (a t) Prelude.* (a t) sqrt a = \t -> Prelude.sqrt (a t) I do not know how to invoke any of the operations. In general I do know how to execute lambdas. I do not understand the bracketed pairs e.g. (a t). Any help on understanding and running the program would be appreciated. Thanks, Pat [1] Ontology for Spatio-temporal Databases http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.113.9804&rep=rep1&… -- 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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