March 2013 - Beginners

Re: [Haskell-beginners] newtype with a typclass
by Răzvan Rotaru 18 Mar '13

18 Mar '13

On 18 March 2013 18:34, <beginners-request(a)haskell.org> wrote: > > Not directly, no. It is possible to use existentials to do this ? at the > cost that you lose the ability to do anything but what you have declared > about it. For example, if you were to use this to make a map of Show-able > values, the *only* thing you can do with the value is invoke "show" on it > (in particular: you cannot pull out the value directly, because the only > thing you know about it is that "show" works on it; you otherwise have no > idea what it is!). And with such a value, referential transparency means > there is no difference between this and storing the *result* of invoking > "show" on it (that is, a String) instead, and laziness means that it may > well actually store a thunk that will invoke "show" when the value is > needed instead of computing and storing the value, so that there is in fact > no difference between the two except that the type is much more complicated > and both the type and the code are correspondingly more difficult to > understand. > > What problem are you trying to solve by obfuscating things in this manner? > > Also note that most times when someone is trying to do this, they have > confused typeclasses with OOP; they are not, and trying to treat them as > such *will* lead you into dead ends. > > -- > brandon s allbery kf8nh sine nomine > associates > allbery.b(a)gmail.com > ballbery(a)sinenomine.net > unix, openafs, kerberos, infrastructure, xmonad > http://sinenomine.net > Well, I need something similar to Java interfaces. Or at least this is my current way of thinking about the problem. I am trying to build some types for GUI Widgets, and I would like to use Maps to keep their properties. It's meant to replace the record syntax, which I find unusable because of the name clash. I also would like to have fast non-destructive update of this type, by using structural sharing, and Maps as good for this. So, for example a Button would have an id, a label and function for the onclick event, and these three values should be kept in a map, with the keys "id", "label" and "onclick". It's true that keeping Show-able values is not enough, since I won't be able to call the onclick function. Unfortunately, I have no idea how to do this as well. Razvan

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newtype with a typclass
by Răzvan Rotaru 18 Mar '13

18 Mar '13

Hi, It seems that type constructors don't allow usage of typeclasses. Is it not possible to create polymorphic types in Haskell by declaration? For example I want a map with String keys and anything that is Showable as values: newtype MyMap = Map String Show How can I do that? Thanks, Razvan

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Confused about lazy IO
by Jacek Dudek 17 Mar '13

17 Mar '13

Q: When trying to compose the following IO actions: readFile fileName >>= \ contents -> writeFile fileName (f contents) I get an exception: openFile: permission denied. However the following works: readFile fileName >>= \ contents -> (mapM_ putStrLn . lines) contents >> writeFile fileName (f contents) I'm assuming it has something to do with lazy IO, and the second action in the second version forces fileName to be read completely and to be closed. Why do I need to do that? I thought lazy IO was implemented in such a way that you were safe to INTERPRET the IO action as having been fully performed. (And so I would have been safe to interpret that, in the first version, in the first action, the file was released.) If that's not the case, then: (1) What is the proper way to reason about lazy IO? (2) Is there some action less arbitrary that what I cooked up in the second version that explicitly instructs to release a file or some other resource? Regards, Jacek.

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Re: [Haskell-beginners] Beginners Digest, Vol 57, Issue 22
by Răzvan Rotaru 15 Mar '13

15 Mar '13

Thanks for the reply. On 15 March 2013 13:00, <beginners-request(a)haskell.org> wrote: > Perhaps > > data TextfieldProps = TextId | TextLabel | TextValue > -- and similarly for ButtonProps? > > I confess to not fully understanding the GUI modelling attempted here, > which isn't to say that you aren't on the right track. > > To help you and help us help you better, one approach known to be > successful is to sketch out the code (better yet, the type signatures!) of > what you'd like to write, e.g. how are TextfieldProps and ButtonProps used? > What functions act on them? > > -- Kim-Ee > I don't have any code to show you, since I just started trying to write the types. I'll try to explain what I'm trying to do. So, I want model some GUI widgets. This means I should have type for each widget (or a type synomym), holding it's data, or state if you like. A textfield must have an id, a label and the entered text as value. A button must have an id, a label and a function to call when the button is clicked. As previously said, records are not an acceptable solution because of the name clash of properties, and I don't want to prefix each property to avoid this. Next best thing is to use a map to hold the properties. I could use strings as keys, in which case my map would look like this (written as a list of tuples): [ ("id", "name_textfield"), ("label", "Name:"), ("value", "Please enter name here ...")] but then is no checking for the property names (e.g. if I mispelled "id", or I used "onclick" for textfields). I want to find a way to use the type system to check these properties. My idea was to use new types as keys for these maps. I would then have: data TextfieldProperties = Id | Label | Value type Textfield = Map TextfieldProperties String But then I get into the same name clash, because all widgets have Ids, most have labels, etc. And here I got stuck. Then, how to create widgets? Record syntax would have been fine, but since I'm not using records I would have to write some make-... functions, which will receive a list of key value pairs and insert them into the map to create the widget. Also here I can put the default values of properties (for. example if id is not specified, one is generated, or use empty strings for unspecified textfield values). How to use widgets? Well there will be functions to draw them on the screen. I could use a typeclass here. Then their properties must be accessed somehow, and this should be also polymorphic of course: I want one function to get the id of a widget, no matter of what type. Typeclasses can help here as well. Then there would be methods to search the GUI tree for certain widgets... you know, standard stuff you would want to do with GUI widgets. Currently, my blocking point is how to define the types. And I don't want to flatten it all out, and use (Map String String) for any widget. I'm missing out on the type system if I do this. Cheers, Răzvan

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type hierarchies / records
by Răzvan Rotaru 14 Mar '13

14 Mar '13

Dear Haskelleers, I suspect my problem has been discussed many times and under many forms, so please don't shoot me for asking. I could not find an acceptable solution yet, and beeing as stubborn as i am, I still hope the solution is eluding me because of my poor haskell knowlegde. So here I am posting my question on this mailing list. Basically I am trying to GUI Widgets using haskell types (Buttons, Textfields, etc.). 1/ Using Records is not acceptable because of the name clashes between attribute names. I could use prefixes and write functions in typeclasses for accessor, but it's ugly and a lot of work. 2/ I decided to keep the attributes in Maps, so basically a textfield would be data Textfield = Textfield (Map.Map TextfieldProps String) The problem is: how to define TextfieldProps? It should be something like: data TextfieldProps = Id | Label | Value But then, buttons also have Id's and labels. So following is not accepted anymore: data ButtonProps = Id | Label | OnClick Ideas and suggestions are much appreciated. Thank You, Răzvan

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WORLDCOMP Strikes Again for the Last Time
by chrisrogersrogers＠aol.com 13 Mar '13

13 Mar '13

I graduated from University of Florida (UFL) and am currently running a computer firm in Florida. I have attended WORLDCOMP http://www.world-academy-of-science.org in 2010. Except for few keynote speeches and presentations, the conference was very disappointing due to a large number of poor quality papers and cancellation of some sessions. I was instantly suspicious of this conference. Me and my UFL and UGA friends started a study on WORLDCOMP. We submitted a paper to WORLDCOMP 2011 and again (the same paper with a modified title) to WORLDCOMP 2012. This paper had numerous fundamental mistakes. Sample statements from that paper include: (1). Binary logic is fuzzy logic and vice versa (2). Pascal developed fuzzy logic (3). Object oriented languages do not exhibit any polymorphism or inheritance (4). TCP and IP are synonyms and are part of OSI model (5). Distributed systems deal with only one computer (6). Laptop is an example for a super computer (7). Operating system is an example for computer hardware Also, our paper did not express any conceptual meaning. However, it was accepted both the times without any modifications (and without any reviews) and we were invited to submit the final paper and a payment of $500+ fee to present the paper. We decided to use the fee for better purposes than making Prof. Hamid Arabnia (Chairman of WORLDCOMP) rich. After that, we received few reminders from WORLDCOMP to pay the fee but we never responded. We MUST say that you should look at the website http://sites.google.com/site/worlddump1 if you have any thoughts to submit a paper to WORLDCOMP. DBLP and other indexing agencies have stopped indexing WORLDCOMP’s proceedings since 2011 due to its fakeness. The status of your WORLDCOMP papers can be changed from “scientific” to “other” (i.e., junk or non-technical) at anytime. See the comments http://www.mail-archive.com/tccc@lists.cs.columbia.edu/msg05168.html of a respected researcher on this. Better not to have a paper than having it in WORLDCOMP and spoil the resume and peace of mind forever! Our study revealed that WORLDCOMP is a money making business, using UGA mask, for Prof. Hamid Arabnia. He is throwing out a small chunk of that money (around 20 dollars per paper published in WORLDCOMP’s proceedings) to his puppet who publicizes WORLDCOMP and also defends it at various forums, using fake/anonymous names. The puppet uses fake names and defames other conferences/people to divert traffic to WORLDCOMP. That is, the puppet does all his best to get a maximum number of papers published at WORLDCOMP to get more money into his (and Prof. Hamid Arabnia’s) pockets. Monte Carlo Resort (the venue of WORLDCOMP until 2012) has refused to provide the venue for WORLDCOMP’13 because of the fears of their image being tarnished due to WORLDCOMP’s fraudulent activities. WORLDCOMP will not be held after 2013. The paper submission deadline for WORLDCOMP’13 is March 18, 2013 (it will be extended many times, as usual) but still there are no committee members, no reviewers, and there is no conference Chairman. The only contact details available on WORLDCOMP’s website is just an email address! What bothers us the most is that Prof. Hamid Arabnia never posted an apology for the damage he has done to the research community. He is still trying to defend WORLDCOMP. Let us make a direct request to him: publish all reviews for all the papers (after blocking identifiable details) since 2000 conference. Reveal the names and affiliations of all the reviewers (for each year) and how many papers each reviewer had reviewed on average. We also request him to look at the Open Challenge at http://sites.google.com/site/dumpconf We think that it is our professional obligation to spread this message to alert the computer science community. Sorry for posting to multiple lists. Spreading the word is the only way to stop this bogus conference. Please forward this message to other mailing lists and people. We are shocked with Prof. Hamid Arabnia and his puppet’s activities http://worldcomp-fake-bogus.blogspot.com Search Google using the keywords “worldcomp, fake” for additional links. Sincerely, Chris

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Performance problem with Haskell/OpenGL/GLFW
by Jesper Särnesjö 13 Mar '13

13 Mar '13

Hi there! My first post here. :) I have a curious performance problem with a Haskell/OpenGL/GLFW program that I just can't figure out. The program's CPU usage is quite high in general, and increases with the number of pixels redrawn per frame. It uses OpenGL 3.2, does most of its work on the GPU, and should do a constant amount of work per frame on the CPU, so this makes little sense. I've created a smaller program (still 91 lines, though), which renders a single triangle, but still reproduces this problem. If I make the triangle smaller (by editing vertexData), CPU usage goes down, and if I make it bigger, CPU usage goes up. [1] I've also created a C program which does the same thing (as far as possible), but which does not exhibit this problem. CPU usage is far lower, and stays constant regardless of the number of pixels redrawns per frame. [2] The library functions I use (from OpenGLRaw-1.3.0.0 and GLFW-b-0.1.0.5) are only thin wrappers around their C counterparts, so I strongly believe the problem is in my program. Any thoughts? -- Jesper Särnesjö http://jesper.sarnesjo.org/ [1] https://gist.github.com/sarnesjo/5116084#file-test-hs [2] https://gist.github.com/sarnesjo/5116084#file-test-c

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Suspend/resume computation using Cont monad and callCC
by Dmitriy Matrosov 12 Mar '13

12 Mar '13

Hi. I have two functions f and g, and i want them to execute in following order: first function f runs, then suspends and passes control to function g. Function g runs, then suspends and "unpauses" function f. Function f finishes and passes control to function g, which also finishes. Here is illustration ('o' means start of function, dot means suspend and pass control to other function, 'x' means end of function): f g o f-1 | v .-->o | g-1 v .<--. | f-2 v x-->. | g-2 v <----------x (to caller) I want to implement this using Cont monad and callCC. And here is my implementation: import Data.Monoid import Control.Monad.Cont import Control.Monad.Writer type M r = Cont r fM :: M r [String] fM = do let xs' = "I'm in f-1" : [] (ys, k') <- callCC (gM xs') let ys' = "I'm in f-2" : ys zs <- k' ys' let zs' = "I'm in f-3" : zs return zs' gM :: [String] -> (([String], [String] -> M r [String]) -> M r [String]) -> M r ([String], [String] -> M r [String]) gM xs k = do let xs' = "I'm in g-1" : xs ys <- callCC (curry k xs') let ys' = "I'm in g-2" : ys return (ys', \_ -> return ys') type T r = ContT r (Writer String) fT :: T r () fT = do lift $ tell "I'm in f-1\n" k' <- callCC gT lift $ tell "I'm in f-2\n" k' undefined lift $ tell "I'm in f-3\n" gT :: ((() -> T r ()) -> T r ()) -> T r (() -> T r ()) gT k = do lift $ tell "I'm in g-1\n" callCC k lift $ tell "I'm in g-2\n" return (\_ -> return ()) First pair (fM and gM) uses monad result to track execution order, second pair (fT and gT) uses Writer monad. But the tracks produced by these pairs differ: *Main> runCont fM id ["I'm in f-3","I'm in g-2","I'm in f-2","I'm in g-1","I'm in f-1"] *Main> putStr . snd . runWriter . flip runContT return $ fT I'm in f-1 I'm in g-1 I'm in f-2 I'm in g-2 I'm in f-2 <----- Why am i here? I'm in f-3 fM/gM pair produces exactly the track, which i expect (see illustration above, though 'f-3' section does not shown there). But fT/gT pair after 'g-2' section returns to "before f-2" point in function f. And i don't understand why. Thus, my question is why does fT/gT work so? And why do results from these pairs differ? -- Dmitriy Matrosov

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Checking if a Stack is Empty
by doaltan 12 Mar '13

12 Mar '13

Hello, I have this stack data structure : data Stack = Empty | Element Char Stack deriving Show I want to check if it is equal to "Empty" When I try something like this : "a = Empty" or "a = (Empty)" in a haskell file and then write this on ghci : "a = Empty" I get this : <interactive>:1:0: No instance for (Eq Stack) arising from a use of `==' at <interactive>:1:0-11 Possible fix: add an instance declaration for (Eq Stack) In the expression: a == (Empty) In the definition of `it': it = a == (Empty) I don't know how to fix this. Can you help me so that I can check if a stack is Empty without getting this error? Thank you.

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Developing Web Applications with Haskell and Yesod
by KMandPJLynch 12 Mar '13

12 Mar '13

Good morning, I sent a previous email to you in regard to this - if my request is rejected, will I be notified? I'm reading the book "Developing Web Applications with Haskell and Yesod". It is a very interesting read and I'm hoping to be able to be able to put up a simple web app using it as a result. I was wondering if anyone has had experience with it. Thank you On Mar 12, 2013, at 9:21 AM, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Writing a custom pop function for a stack data type (doaltan) > 2. Re: Writing a custom pop function for a stack data type > (Brent Yorgey) > 3. Re: Suspend/resume computation using Cont monad and callCC > (Ertugrul S?ylemez) > 4. Re: Writing a custom pop function for a stack data type > (Emanuel Koczwara) > 5. Re: Performance problem with Haskell/OpenGL/GLFW > (Jesper S?rnesj?) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 12 Mar 2013 11:06:35 +0000 (GMT) > From: doaltan <doaltan(a)yahoo.co.uk> > Subject: Re: [Haskell-beginners] Writing a custom pop function for a > stack data type > To: divyanshu ranjan <idivyanshu.ranjan(a)gmail.com> > Cc: "beginners(a)haskell.org" <beginners(a)haskell.org> > Message-ID: > <1363086395.38137.YahooMailNeo(a)web171402.mail.ir2.yahoo.com> > Content-Type: text/plain; charset="iso-8859-1" > > Actually I'm getting the error with this : > > data Stack = Empty | Elem Char Stack deriving Show > > > pophead :: Stack -> Char > pophead Empty = Empty > pophead (Elem x stack) = x > > > > ________________________________ > From: divyanshu ranjan <idivyanshu.ranjan(a)gmail.com> > To: doaltan <doaltan(a)yahoo.co.uk>; The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> > Sent: Tuesday, 12 March 2013, 12:53 > Subject: Re: [Haskell-beginners] Writing a custom pop function for a stack data type > > > You have declared new data type mystack not Stack, so haskell compiler could not find Stack data type and its constructors. Secondly data type in Haskell need to be start with?capital?letters like > > data Mystack = Empty | Elem Char Mystack deriving Show > then correct Function?definition is? > pophead :: Mystack -> Char > > Regards > Divyanshu? > > > > On Tue, Mar 12, 2013 at 4:12 PM, doaltan <doaltan(a)yahoo.co.uk> wrote: > > Hi I have such a stack data structure:? >> datamystack =Empty |Elem Char mystack derivingShow >> >> I'm trying to get the head of the stack using this: >> pophead :: Stack -> Char >> pophead Empty = Empty >> pophead (Element x stack) = x >> And I'm getting this error for the last sentence of the function : >> Not in > scope: data constructor `Stack' >> Can you tell me how to fix it? >> Thanks. >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> >

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