April 2012 - Beginners

Re: [Haskell-beginners] Training tasks
by Nikita Beloglazov 20 Apr '12

20 Apr '12

> > Yes, I get that. It's a bad idea. Showing atypical "promotional" cases for > each language encourages faddishness and silver bullet cults; it's > irresponsible and unprofessional. If you're shopping around for a language > then nothing is worse than selected sweet spot cases. Unless they are a > fair-ish benchmark for a language that excels in a clear area, like R, APL > and Awk and you're looking for a glorified DSL. > > I'd say that its better to have the same tasks for each language if you're > evaluating a general purpose tool. Obvious ones would be, from simple to > complex: > > - Some of the unix command line utilities like wc (some of these might be > single liners) > > - OXO, Caesar cipher (10 lines or so) > > - The Markov chain program from the Practice Of Programming > > - Life > > - A simple spheres only ray tracer like the one in Graham's Lisp book > > - A simple calculator program like the one in Hutton > > - The recursive 2D shape program I've mentioned in other posts today > > - A version of the classic Traveller/Elite trading system - a really nice > little toy business rules system that can be code either minimally or to > show off the funkier features of a language to get more flexibility and > more interesting pricing rules. Minimal version would be a few tens of > lines and the config file declaring items. > > If you want to see a language comparison done responsibly, look at > Kernighan and Pike's "Practice Of Programming." Thank you for criticism and tasks examples. But I still think it isn't such a bad idea. Because if somebody get excited about some language (even if it is based on "promotional" cases) and he learns this language more by himself then it's great. I don't think somebody will judge about whole language by few tasks. He may get interested or dislike the language, but I doubt there will be silver bullet cult based on few tasks. By shopping language I mean choosing some new fun language to learn, not to solve particular practical task. Nikita

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Program reliability and multiple data constructors; polymorphism
by umptious 20 Apr '12

20 Apr '12

One of the programming exercises I keep evolving as I learn Haskell is a toy 2D shape editor with four primitives: data Shape = Circle {origin::Pt2, radius::Float} | Square {origin::Pt2, side ::Float} | Rect {origin::Pt2, other ::Pt2} | Composite {shapes::[Shape]} deriving (Show, Read) The intent is Composites can contain Shapes of any kind, including other Composites so that you can apply transformations to a Composite and these will be applied to the contained Shapes recursively. So an arm might contain a hand which constains a dozen or so Rects. Transform the arm and the hand and rects should transform; transform the hand and its rects should transform but the not arm. Big deal. And the above makes that really easy when you know you're talking to a Composite. But now I've hit an intellectual stumbling point and the books and source I have don't seem to address it: I can apply the destructuring command "shapes" defined in the cstr "Composite" to ANY Shape. And if I do that to say a circle, BLAM! Or if I apply "radius" to Rect, BLAM! At runtime. No type checking support (because yes, they're the same type.) To me this seems alarming. It means that I can't reason about the safety of my program based on type checking as I'd like. And none of the answers I can think seem at all elegant: - I could use exception handling, but that means carefully checking which data declarations are potential bombs and using exceptions only when they are involved - hideously error prone - or using exceptions everywhere. Which is just hideous. - I could hack run time type checking using the ctsr info in "show". But again I'd have to know when to use it or use it everywhere. And it seems like a ridiculous kludge to bring to a language that has been designed for elegance. ..So what is the Haskell idiom for dealing with this??? In fact I suppose I'm asking two questions: 1. How do I re-design this program so it is safe (use class and instance maybe, abandoning use of a single data type? but I then have to have separate Lists for each type, even if they derived from a common class?) 2. How can one use compile time checking or (less good) coding practices to eliminate the possibilty of such runtime exceptions? And come to think of it 3. is there a Haskell book which addresses design and structural problems like this one - which I would have thought was both obvious and fundamental - because of the books I've looked at so far seem to do a tolerable job. The best of them present an adequate "on rails" tour, but none of them seem to give you the tools to address issues like this one. Whereas with C++and Stroustrupp, Common Lisp and Graham, the Smalltalk book, and I Erlang and Armstrong I'd know exactly what to do. Admittedly the C++ solutions wouldn't be pretty, but anything the compiled would be safe to run (unless I went to great efforts otherwise..)

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Training tasks
by Nikita Beloglazov 19 Apr '12

19 Apr '12

Hi. I'm building website where people can try and "taste" new languages by solving small or mediums size tasks. Tasks are language specific and should show best features of the language. Website is not meant to teach new language but to give idea what is this language good for. Now I want to add Haskell. I need about 7-10 tasks for now. First three of four tasks are introductory, they should show/check basics of haskell. E.g. given n, return sum of squares of first n even numbers. Other tasks are more complicated and show advantages of functional programming in general or some specific haskell features. I don't have any experience with haskell and I need you help. Could you help me with ideas for tasks? Thank you, Nikita Beloglazov

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Re: [Haskell-beginners] Looking for vim text obejct plugin
by John Tyree 19 Apr '12

19 Apr '12

There are some decent object plugins on github. Look at vim-textobj-user for the parent plugin and then vim-textobj-indent for some useful ones based on indent level. I don't think there's anything that can handle haskell style arguments... although I think they are always either single identifiers or something wrapped in parens, so yiw and yib should handle it usually. -John ------------------------------ Message: 6 Date: Wed, 18 Apr 2012 13:23:00 +0900 From: Mait <mattengi(a)gmail.com> Subject: [Haskell-beginners] Looking for vim text obejct plugin for haskell. To: beginners(a)haskell.org Message-ID: <CABQgwkXyHdad4_Btef10N9nFSWXBo9EKB-Yom3jfmp0cRTfJ_A(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Any good one there everybody knows except me? :) -- Mait ------------------------------ Message: 7 Date: Wed, 18 Apr 2012 13:34:20 +0900 From: Mait <mattengi(a)gmail.com> Subject: Re: [Haskell-beginners] Looking for vim text obejct plugin for haskell. To: beginners(a)haskell.org Message-ID: <CABQgwkWcfa9mL3fSb7RkTuRG8OXhjG3q2Qke=yvP9ZoZoo+mHw(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Especially looking for 'argument' object plugin. function x:xs ^ <- cursor ysaa) or ysia) -> function (x:xs) 2012/4/18 Mait <mattengi(a)gmail.com>: > Any good one there everybody knows except me? :) > > -- > Mait -- Mait

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Looking for vim text obejct plugin for haskell.
by Mait 18 Apr '12

18 Apr '12

Any good one there everybody knows except me? :) -- Mait

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A question of program style
by Michael Alan Dorman 18 Apr '12

18 Apr '12

Hey, Haskellers, I've started writing my first non-toy program in Haskell [1], and things are starting to click, at least a little bit---I find myself working around the periphery, building functions that perform particular operations, testing them in ghci, then figuring out how to link them up. I at least don't feel perpetually stupid any more. Anyway, I find myself with a couple of questions. The first, I guess, is one of style: One Big Monad, or Several Smaller Monads? I gather a lot of programs create a big RWST IO (or something of that ilk) that sits at the heart of the action, and then thread that through the whole program. However, because I am developing in little pieces, I found myself writing, for instance, a routine that used a WriterT IO to find all the .info files in the directory and grab their contents and returning them as a [(filename,contents)], which I could then parse and act upon. I'm thinking of doing roughly the same thing for the processing of scraping web information---creating a separate WriterT IO that goes and grabs the content and delivers it back for parsing. Is there a consensus that one approach is better than the other? I guess I feel like doing it this way is really pushing me to isolate my imperative vs. pure code, but then, it's all feeding back to processes running in the IO monad anyway, so am I just being prissy? Are people likely to look at my code and wonder Just What The Hell Is Going On? The second question is one of libraries, I guess. The process I've spec'd consists of several fairly discrete operations that have a couple of paths through them, some of which are recursive. For instance, if you're given a project name on the command line, you need to hit the web site, scrape some info, get the source, unpack, walk and parse the contents, and then generate the debian files...but if you're called without a project name (to make updating easy), you need to check the current dir to walk and parse the contents, figure out your project name and version, *then* hit the website, scrape some info, check the version numbers, perhaps get the source and unpack in which case you need to walk and parse the contents again, then finally generate the debian files. When I implemented this in Perl, I actually used lazy attributes on a central object to make most of these processing dependencies implicit rather than explicit. It seems like this should be even easier in Haskell, so I was wondering if there was a component out there already that made it easy to express these sorts of dependencies to be "solved"? I appreciate any suggestions or guidance. Thanks, Mike. 1. The program is one for generating the Debian packaging files for Drupal modules (yes, I know one already exists, I've already re-implemented it once in Perl, so this will actually be version #3), which is an easily explained task which nonetheless hits a lot of capabilities (http, html parsing, untar/gz ing, reading and parsing files, generating files from templates, etc) I would like to explore.

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profiling a program that writes midi events
by Dennis Raddle 18 Apr '12

18 Apr '12

I want to profile a program, but it's not the usual kind of "run once, it completes" program. It computes a bunch of midi events and writes them to a MIDI port via the Sound.PortMidi module. It waits between events so that each one happens in the proper place in the music. And the program must not exit while it is writing events. When I run it, it takes about ten seconds until the music starts, so it must be doing most of the computation then. But it takes several minutes as it plays music. How would I profile this kind of program? One possibility is to modify it a bit so it computes all the midi events but doesn't send them, and just exits. However, I have to make sure it actually computes the events all the way (due to language laziness). If I don't write the events to PortMidi, then they are never used. I'm not sure on how to ensure an expression is completely evaluated if it never used. Dennis

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Noob question.(I'm trying 'http://learnyouahaskell.com' tutorial)
by Mait 17 Apr '12

17 Apr '12

ENV Ubuntu 12.04 haskell-platform/precise uptodate 2012.1.0.0~debian ghci --version The Glorious Glasgow Haskell Compilation System, version 7.4.1 I've tried all day with, 'Learn You a Haskell for Great Good!' http://learnyouahaskell.com/ Very attractive book. I'm really enjoying this book. Anyway I have one question, Why I can't declare func type like books? I got always error about 'lacks an accompanying binding', like this, Prelude> :l baby.hs [1 of 1] Compiling Main ( baby.hs, interpreted ) baby.hs:32:1: The type signature for take' lacks an accompanying binding Failed, modules loaded: none. Here is codepad link, http://codepad.org/ttZRBpkL Thanks.

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literate programming
by Francisco Gutierrez 17 Apr '12

17 Apr '12

Dear friends: I am an ULTRA-ULTRA Haskell beginner, just started literally yesterday. I am just toying around with Cordelia Hall and John O'Donnell excellent book on dicrete mathematics with Haskell. Well, they have a program, stdm, to accompany the book. It happens that it is in literate style. In theory, this should be very easy to work with, but after saving it with lhs extension, I try to load it, without success. Could somebody out there help me with this? Best regards, Francisco Gutiérrez From: "beginners-request(a)haskell.org" <beginners-request(a)haskell.org> To: beginners(a)haskell.org Sent: Monday, April 16, 2012 2:45 PM Subject: Beginners Digest, Vol 46, Issue 23 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: splitAt implementation (using foldr) and infinite lists (Lorenzo Bolla) 2. Re: splitAt implementation (using foldr) and infinite lists (Ozgur Akgun) 3. Re: Design of Webmachine in Haskell (Petar Radosevic) 4. Re: Design of Webmachine in Haskell (Michael Snoyman) 5. Re: splitAt implementation (using foldr) and infinite lists (Dmitriy Matrosov) 6. Training tasks (Nikita Beloglazov) 7. Cross-platform .hs files on Linux and Windows (Vinay Sajip) 8. Re: Cross-platform .hs files on Linux and Windows (Lorenzo Bolla) ---------------------------------------------------------------------- Message: 1 Date: Mon, 16 Apr 2012 15:21:55 +0100 From: Lorenzo Bolla <lbolla(a)gmail.com> Subject: Re: [Haskell-beginners] splitAt implementation (using foldr) and infinite lists Cc: beginners(a)haskell.org Message-ID: <20120416142155.GC30186@dell> Content-Type: text/plain; charset=us-ascii On Mon, Apr 16, 2012 at 12:52:02PM +0400, Dmitriy Matrosov wrote: > Hi all. > > If i implement take using foldr > > take' :: Int -> [a] -> [a] > take' n = foldr (\x z -> if fst x <= n then snd x : z else []) [] > . zip [1..] > > it'll work fine with infinite lists. But if i implement splitAt similarly > > splitAt' :: Int -> [a] -> ([a], [a]) > splitAt' n = foldr (\x (z1, z2) -> if fst x <= n then (snd x : z1, z2) > else ([], snd x : z2)) > ([], []) > . zip [1..] > > and call it like this > > *Main> fst $ splitAt' 4 [1..] > ^CInterrupted. Try something like this: splitAt' n = foldr (\x zs -> if fst x <= n then (snd x : fst zs, snd zs) else ([], snd x : snd zs)) ([], []) . zip [1..] I'm no Haskell expert, but I suspect that when pattern-matching z2, it tries to evaluate it and it hangs... My version does not hang... hth, L. -- Lorenzo Bolla http://lbolla.info ------------------------------ Message: 2 Date: Mon, 16 Apr 2012 15:55:20 +0100 From: Ozgur Akgun <ozgurakgun(a)gmail.com> Subject: Re: [Haskell-beginners] splitAt implementation (using foldr) and infinite lists To: Lorenzo Bolla <lbolla(a)gmail.com> Cc: beginners(a)haskell.org Message-ID: <CALzazPAs7X+JXD5nu4+E+HiyCeQhbMwJ7qJ+beC0uCzips6gFA(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" You can also use lazy pattern matching. http://en.wikibooks.org/wiki/Haskell/Laziness#Lazy_pattern_matching On 16 April 2012 15:21, Lorenzo Bolla <lbolla(a)gmail.com> wrote: > > splitAt' :: Int -> [a] -> ([a], [a]) > > splitAt' n = foldr (\x ~(z1, z2) -> if fst x <= n then (snd x : z1, > z2) > > else ([], snd x : z2)) > > ([], []) > > . zip [1..] > Ozgur

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splitAt implementation (using foldr) and infinite lists
by Dmitriy Matrosov 17 Apr '12

17 Apr '12

Hi all. If i implement take using foldr take' :: Int -> [a] -> [a] take' n = foldr (\x z -> if fst x <= n then snd x : z else []) [] . zip [1..] it'll work fine with infinite lists. But if i implement splitAt similarly splitAt' :: Int -> [a] -> ([a], [a]) splitAt' n = foldr (\x (z1, z2) -> if fst x <= n then (snd x : z1, z2) else ([], snd x : z2)) ([], []) . zip [1..] and call it like this *Main> fst $ splitAt' 4 [1..] ^CInterrupted. it will hang. I don't understand, why it tries to compute second list (z2), if it only needs first?

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