November 2012 - Beginners

list splitting - nice implementation?
by Emmanuel Touzery 18 Nov '12

18 Nov '12

Hello, i wonder what would be the idiomatic way to achieve that algorithm in haskell: [1,4,56,450,23,46,52] => [1,4,56,450] [1,4,56,450,23,46,52] => [23,46,52] in other words split the list when one element gets smaller than the previous one. Tge rest of the time the list is sorted. There would be only two lists, not N. I always need the first or second sublist, I don't need both at once. But of course a more complete algorithm handling the N case and/or returning both sublists would be good. i could code this by hand, but i'm trying to use as much as possible builtin higher-order functions. However in this case so far I've only come up with this: import Data.List isSorted :: Ord a => [a] -> Bool isSorted l = (sort l) == l secondPart :: Ord a => [a] -> [a] secondPart l = head $ filter isSorted (tails l) firstPart :: Ord a => [a] -> [a] firstPart l = last $ filter isSorted (inits l) It is concise alright, but it seems contrived and also in terms of performance I don't think it's OK (for small lists sure but for big lists?). Anyway, somehow I think something as simple as this must be doable very concisely and with optimal performance using only builtin higher-order functions. Any idea? Thanks! Emmanuel

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Consuming Rule Based Parsing
by Christopher Howard 16 Nov '12

16 Nov '12

I haven't learned anything substantive yet regarding Haskell approaches to parsing, so I would appreciate any pointers towards appropriate modules, abstractions, tutorials, or such like. Specifically: For a particular application, I want to play around with a rules (grammer) based parsing which consumes an incoming data stream. That is, I'm not parsing a single type of document with a set structure. Rather, the idea is that I have a list of symbols (characters, numbers, whatever) and pattern match the front part of the list according to some (grammer) rule (which itself could be a conjunction or disjunction of rules) in a set of rules. If the match succeeds, then I consume that part of the list, and then analyze the remaining list, starting again with the first rule in my rule set. If the match fails, then I try the next rule in my rule set. (Sort of like Prolog's context free grammars and difference lists.) Backtracking would also be cool (to see what other results I can get). Is there a particular Haskell abstraction or system suitable for what I have described? -- frigidcode.com

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Representing most types by the same letter
by Christopher Howard 14 Nov '12

14 Nov '12

Hi. I'm trying to learn Haskore, and it seems rather odd: Why are most types in the system represented by the same letter, the letter 'T'? For example, instead of a Scale type, a Duration type, and a Pitch type, there is a T type in the Scale module, a T type in the Duration module, and a T type in the Pitch module. It makes the documentation rather challenging to read, because I have to read the link for each appearance of the letter to see what type it is referring to. Is there some theory behind this practice? -- frigidcode.com

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cabal-install: storing Hackage login
by Christopher Howard 13 Nov '12

13 Nov '12

Hi. I am trying to use cabal-install build reporting functionality. I've put my username / password in my ~/.cabal/config file. Nevertheless, when I call the "cabal report" command, it asks me for my Hackage username and Hackage password. Is there something else I need to turn on in the config file so it will use the username/password combo in the config file? -- frigidcode.com

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Reminder and Forward: Wednesday 14 November 2012 NYHUG Inaugural Meetup: Ozgun Ataman on Practical Data Processing and Gershom Bazerman on Putting Cloud Haskell to Work
by Jay Sulzberger 12 Nov '12

12 Nov '12

Below is a version of the announcement sent to the haskell-cafe and the haskell mailing lists by Gershom Bazerman on 9 October 2012. oo--JS. <blockquote what="official New York Haskell Users Group announcement" rsvp="Yes, see below." more="http://en.wikipedia.org/wiki/Haskell_%28programming_language%29 [page was last modified on 11 November 2012 at 00:56] http://www.haskell.org/haskellwiki/Haskell http://www.reddit.com/r/haskell http://www.aosabook.org/en/ghc.html http://en.wikipedia.org/wiki/Type_inference [page was last modified on 15 September 2012 at 15:37] http://en.wikipedia.org/wiki/Hindley%E2%80%93Milner [page was last modified on 9 October 2012 at 17:40] http://www.users.waitrose.com/~hindley/ http://www.cs.ru.nl/~henk/ http://www.usingz.com/ http://en.wikipedia.org/wiki/Curry-Howard [page was last modified on 14 September 2012 at 22:01] http://www.paultaylor.eu/stable/prot.pdf ftp://ftp.cs.cmu.edu/usr/rwh/www/home/courses/logic/www/handouts/curry-howa… http://www.students.science.uu.nl/~3448584/TypeSystems.pdf http://www.math.ias.edu/~vladimir/Site3/Univalent_Foundations_files/2011_Go… http://math.andrej.com/2012/11/08/how-to-implement-dependent-type-theory-i/ http://math.andrej.com/2012/11/11/how-to-implement-dependent-type-theory-ii/ http://math.andrej.com/2012/10/03/am-i-a-constructive-mathematician/" exclamation-of-old-lisper-upon-first-seeing-curry-howard-theorem: "But this is clearly wrong! Look I have a function string-length which has type String->Int, but 'String->Int' is not a theorem of any usual intuitionistic propositional logic." insight-of-newbie-coming-to-haskell-from-lisp: "Lisp's now buried background would require, if a student of Lisp were to ask for an outline of this background, a book of perhaps a hundred pages. Haskell, and more generally, all the New Crazy Types programming systems, would require three hundred pages." edits="some adventitious markup and some blank space removed"> Subject: Wednesday 14 November, NY Haskell Inaugural Meetup: Practical Data Processing _and_ Putting Cloud Haskell to Work Date: Fri, 12 Oct 2012 18:05:02 -0400 From: Gershom B <gershomb(a)gmail.com> To: lisp(a)lispnyc.org ====================================================== What: New York Haskell Users Group Meeting When: Wednesday, November 14, 2012 7 PM - 9 PM Where: Pivotal Labs, 841 Broadway, 8th floor (Just south of Union Square) http://maps.google.com/maps?q=841+Broadway,+New+York RSVP: http://www.meetup.com/NY-Haskell/ ====================================================== * 7 PM - Practical Data Processing With Haskell * 8 PM - Putting Cloud Haskell to Work for Distributed Computing -------------------------------------------------------------------------------------- 7 PM - Practical Data Processing With Haskell: Ozgun Ataman will give an introductory talk on Haskell, diving right in with how you can start using Haskell for practical data manipulation tasks today. You'll be introduced to a typical setup for Haskell development and given a demonstration of how a common data format (CSV, JSON, etc.) can be parsed, processed and finally output using Haskell. The talk will include a small actual Haskell program to be modified live and some commentary around using Haskell in practical applications. 8 PM - Putting Cloud Haskell to Work for Distributed Computing: Gershom Bazerman will give an overview of the new distributed-process library that implements Cloud Haskell, providing computation across heterogeneous nodes through a message passing interface. He will discuss what this new tool provides out of the box, and what you'll have to bring to the table. The talk will include some code samples using the new library, as well as some experience about what it's like to use Haskell for distributed computing in the real world. Speaker Biographies: Ozgun Ataman is the founder of Soostone, a management consulting and analytics company built on Haskell. He is the author of many open-source Haskell libraries, and is a contributor to the Snap web framework. Gershom Bazerman is a developer at S&P Capital IQ. He is most well known in the Haskell community as the author of the JMacro library for programmatic generation of JavaScript, and for one well-received April fools joke. -------------------------------------------------------------------------------------- We're kicking off the New York Haskell Users Group with two great talks, one for people just getting started and one for those with some experience already. Both should be accessible and enjoyable no matter how much (or little) you already know. Food and refreshments will be provided, courtesy of the generosity of Pivotal Labs, and after the talks, we're planning to keep the discussion going over food and drink at a nearby establishment. </blockquote> Distributed poC TINC: Jay Sulzberger <secretary(a)lxny.org> Corresponding Secretary LXNY LXNY is New York's Free Computing Organization. http://www.lxny.org

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MonadError example
by Giacomo Tesio 11 Nov '12

11 Nov '12

Hi, I'm looking from an example of MonadError usage for exception handling. After reading this http://mvanier.livejournal.com/5343.html I'd like to see an example with two different error types handled in the same function. I mean I have f :: a -> Either e1 b g :: b -> Either e2 c and I want to write a new function z :: a -> Either e3 c using f and g. (with Either e1, Either e2 and Either e3 all instances of MonadError) e3 could provide constructors like E1Occurred e1 and E2Occurred e2 (to mimic inner exceptions) I'd like to see how readable would be z. Can you point me to a snippet or even a real codebase showing such thing? Giacomo

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Depended switching in netwire
by Nathan Hüsken 09 Nov '12

09 Nov '12

Hey, I have main wire for a game: mainGameWire :: WireP Input GameState and a wire for an info screen (to display a message): infoScreenWire :: Sting -> WireP Input GameState I functions to test if a game has been won or lost: gameWon :: GameState -> Bool gameLost :: GameState -> Bool Now I want to switch between the info screen and the main game. A different message should be displayed depending on if the player one or lost. One working way is this: mainWire = F.fix (\start -> infoScreenWire "Press Enter to start" --> (unless gameWon . ((unless gameLost . mainGameWire) --> (infoScreenWire "You LOST!") --> start)) --> (infoScreenWire "You WON!") --> start) This works, but is pretty clumsy. So I am trying to use switchBy: lostWire = startScreenWire "YouLost" --> inhibit mainGameWire' wonWire = startScreenWire "YouWon" --> inhibit mainGameWire' mainGameWire' = (unless gameWon --> inhibit [wonWire]) . (unless gameLost --> [lostWire]) mainGameWire mainWire = switchBy id mainGameWire' This does NOT work because: a) WireP Input GmaeState is not a monoid. b) The exception type for the main Game wire is "WireP Input GameState" I guess a) is easily solveable ... but for b) ... do I really have to change the exception type for mainGameWire (and all the wires it uses ...)? What is the "proper" way of doing this type of switching/how should switchBy be used? Thanks! Nathan

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dynamic set of objects in netwire
by Nathan Hüsken 08 Nov '12

08 Nov '12

Hey, I need (in netwire) a way to handle a dynamic set of objects. Therefor I need a way of growing and shrinking the set. Since I have little experience with Haskell and FRP, I would appreciate some input. Shrinking is not problem, as I can just remove wires that inhibit. But how to let the set grow? So far, my main loop would look like this: I have input data data Input = UpdatePhysic | KeyDown Int ... and the main Wire which creates objects on certain inputs and updates the physics when UpdatePhysics is send: mainWire :: WireP Input mainWire = proc input -> do userControlledObject <- userControlledObjectWire -< input if input == UpdatePhysics then do rec letCollData = ... physicsObjects <- physicsObjectsWire -< collData else empty -< My idea for the dynamics set wire is this: data Subwire e m a b = forall a'. Subwire (a -> a') (Wire e m a' b) dynamicSet :: (Monad m) => (c -> Subwire e m a b) -> Wire e m (Either a c) b The Idea is that the dynamicSet in every invocation either creates wires or steps them: mainWire :: WireP Input mainWire = proc input -> do userControlledObject <- userControlledObjectWire -< input rec let collisionData = ... objects <- dynamicSet -< if input == UpdatePhysics then Left collisionData else Right input So far so good, what bothers me is, that when I have several dynamicSets (different kind of objects) I have to make several if input == ... then ... else ... constructs, which feels repeative. I would welcome some input on how to structure the main loop and/or dynamic set... Regards, Nathan

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Re: [Haskell-beginners] exceptions and stacks
by Emmanuel Touzery 08 Nov '12

08 Nov '12

so I have found the culprit, by defining this function: Import Debug.Trace headMarker :: String -> [a] -> a headMarker marker [] = trace marker (head []) headMarker _ array = head array and replacing calls to head to "headMarker "XX"", "headmarker "YY"" and so on. it works but I would really rather have a compiler/runtime based solution which would not force me to modify my code to track the bug (and would be simpler to use in case the problem comes from a library). If anybody has an idea what I did wrong with the -xc and the -RTS I'm all ears... I'd love to test it and find out how to make that work reliably. Emmanuel On Tue, Nov 6, 2012 at 10:50 PM, Emmanuel Touzery <etouzery(a)gmail.com>wrote: > you mean in combination with the RTS/profiling trick? > If I must do something like that, I'll just mark my code with debug lines > or code the else situations (like making a headMarker with an extra > parameter "String marker" and it will output the marker on stdoud if the > array is empty), seems less work... > > I was really hoping being able to fix it without having the modify the > code, it's disappointing, I think. > > > On Tue, Nov 6, 2012 at 9:47 PM, Christian Maeder <Christian.Maeder(a)dfki.de > > wrote: > >> Usually, these head calls are in your code. Do "fgrep head" on your >> sources and replace all suspicious occurrences. Use "let h : _ = l in ..." >> and "h" instead of "head l" for proper positions. >> >> HTH Christian >> >> Am 06.11.2012 12:44, schrieb Emmanuel Touzery: >> >> Hello, >>> >>> I'm getting really frustrated by error handling in haskell. By this >>> I mean "assert"-like errors. Which means, something that should never >>> happen, happens. This will happen in the development and deployment of >>> software of any size. >>> When coding the feature you'll say, I don't need to handle the >>> "Nothing" case, because it will never happen, or almost never, and this >>> is not critical production software. >>> >>> But if one day it happens you find yourself with a several thousand >>> lines program and all you have to debug is the input data (most of the >>> time anyway, sometimes not even that) and this error message (for >>> instance): >>> >>> *** Exception: Prelude.head: empty list >>> >>> And you don't know even, did you call "head" or maybe a library you >>> are using called it. >>> >>> This is really poor compared to what Java, python and others >>> offers. I can understand that it's difficult to provide a stack trace, >>> especially with lazy evaluation, but at least the function name and >>> maybe a line number... Actually I don't know why it would be so >>> difficult to also give me the values of the parameters of the >>> function... using print and if it's a pure function it intuitively >>> doesn't seem difficult. >>> >>> I've tried ghci with :trace and ":set -fbreak-on-exception" and I >>> got nowhere. It breaks in things which are not important (I have no idea >>> where it breaks, it claims an exception was thrown, maybe but not in my >>> code and it never reaches my code for sure). After hitting ":continue" >>> many times, I get to my exception and there I didn't manage to get >>> useful information (:hist never printed me anything). >>> >>> But I would like not to have to use ghci at all. And to avoid >>> having to write all over the place in my code these "else" and case >>> situations for things I assume are impossible (these are cases I would >>> not write in other modern languages). >>> >>> This is a feature for which I would be ready to pay a 50% speed >>> penalty and 50% memory use penalty. >>> >>> I understand if it's the haskell way of writing all those else >>> clauses... But I think in that case it really makes it compare >>> negatively to other languages. It would be a real minus in my opinion >>> (including to code readability). >>> >>> I have read (quickly) those but didn't find the solution there: >>> http://www.haskell.org/ghc/**docs/7.0.1/html/users_guide/** >>> ghci-debugger.html<http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/ghci-debugger.html> >>> http://book.realworldhaskell.**org/read/error-handling.html<http://book.realworldhaskell.org/read/error-handling.html> >>> >>> Any advice will be welcome! I really hope I missed something. >>> >>> Emmanuel >>> >>> >

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Prolog
by Christopher Howard 07 Nov '12

07 Nov '12

Hi. I still have a lot to learn in Haskell (who doesn't?) but I was thinking about learning Prolog on the side. I was wondering what you thought about the language. I was interested in it primarily because I understood it was a purely declarative language and that everything is defined in the programs as relations. However, at the Wikipedia page it states that Prolog is not actually purely declarative, referencing an article which points out that it is necessary to think about Prolog programs procedurally because of the way that Prolog resolves queries. Is this not as big a deal as it sounds like? Or is there some other similar language I should learn instead? (One of the "successors" of Prolog?) I'm fairly open minded, so long as all the tools are FOSS. Though of course, a language with better community support, documentation, etc. is preferable. -- frigidcode.com indicium.us

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