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  • What is your favourite cleverly written functional code?

    - by sdcvvc
    What are your favourite short, mind-blowing snippets in functional languages? My two favourite ones are (Haskell): powerset = filterM (const [True, False]) foldl f v xs = foldr (\x g a -> g (f a x)) id xs v -- from Hutton's tutorial (I tagged the question as Haskell, but examples in all languages - including non-FP ones - are welcome as long as they are in functional spirit.)

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  • functional test for rails controller private method

    - by mohit
    I have a private method in my controller. which is used for some database update. this method i am calling from another controller method. and it works fine. But when i am trying to write a test case for that method then It is tripping on accessing (session variable and params) in my functional all other methods are working fine the problem is only with private method? In my setup method in functional test, I am setting session also.?

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  • functional test for rails controller privaet method

    - by mohit
    I have a private method in my controller. which is used for some database update. this method i am calling from another controller method. and it works fine. But when i am trying to write a test case for that method then It is tripping on accessing (session variable and params) in my functional all other methods are working fine the problem is only with private method? In my setup method in functional test, I am setting session also.?

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  • Functional Programming Equivalent of Design Patterns Book?

    - by JasonFruit
    Is there a functional-programming equivalent to the Gang of Four Design Patterns book? That is, is there a book that explains and gives examples of how commonly-needed code structures are implemented functionally? I think seeing that would give me a better idea of how to go about using in practice the functional concepts whose theory I understand.

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  • Imperative Programming v/s Declarative Programming v/s Functional Programming

    - by kaleidoscope
    Imperative Programming :: Imperative programming is a programming paradigm that describes computation in terms of statements that change a program state. In much the same way as the imperative mood in natural languages expresses commands to take action, imperative programs define sequences of commands for the computer to perform. The focus is on what steps the computer should take rather than what the computer will do (ex. C, C++, Java). Declarative Programming :: Declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow. It attempts to minimize or eliminate side effects by describing what the program should accomplish, rather than describing how to go about accomplishing it. The focus is on what the computer should do rather than how it should do it (ex. SQL). A  C# example of declarative v/s. imperative programming is LINQ. With imperative programming, you tell the compiler what you want to happen, step by step. For example, let's start with this collection, and choose the odd numbers: List<int> collection = new List<int> { 1, 2, 3, 4, 5 }; With imperative programming, we'd step through this, and decide what we want: List<int> results = new List<int>(); foreach(var num in collection) {     if (num % 2 != 0)           results.Add(num); } Here’s what we are doing: *Create a result collection *Step through each number in the collection *Check the number, if it's odd, add it to the results With declarative programming, on the other hand, we write the code that describes what you want, but not necessarily how to get it var results = collection.Where( num => num % 2 != 0); Here, we're saying "Give us everything where it's odd", not "Step through the collection. Check this item, if it's odd, add it to a result collection." Functional Programming :: Functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions.Functional programming has its roots in the lambda calculus. It is a subset of declarative languages that has heavy focus on recursion. Functional programming can be a mind-bender, which is one reason why Lisp, Scheme, and Haskell have never really surpassed C, C++, Java and COBOL in commercial popularity. But there are benefits to the functional way. For one, if you can get the logic correct, functional programming requires orders of magnitude less code than imperative programming. That means fewer points of failure, less code to test, and a more productive (and, many would say, happier) programming life. As systems get bigger, this has become more and more important. To know more : http://stackoverflow.com/questions/602444/what-is-functional-declarative-and-imperative-programming http://msdn.microsoft.com/en-us/library/bb669144.aspx http://en.wikipedia.org/wiki/Imperative_programming   Technorati Tags: Ranjit,Imperative Programming,Declarative programming,Functional Programming

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  • System requirement specification vs functional one - separate docs?

    - by user970696
    A lot of sources (e.g. Wikipedia) mentiones System requirement specification and Functional specification as two separate entities. However, Wiegers in his book writes: The software requirements specification is sometimes called a functional specification, a product specification... This is very confusing for me as I thought FS describes just functions while SRS whole system. From this point of view, FS would contain both non functional and functional requirements and everything else.

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  • Is there a name for this functional programming construct/pattern?

    - by dietbuddha
    I wrote a function and I'd like to find out if it is an implementation of some functional programming pattern or construct. I'd like to find out the name of this pattern or construct (if it exists)? I have a function which takes a list of functions and does this to them: wrap(fn1, fn2, fn3, fn4) # returns partial(fn4, partial(fn3, partial(fn2, fn1))) There are strong similarities to compose, reduce, and other fp metaprogramming constructs, since the functions are being arranged together and returned as one function. It also has strong similarities to decorators and Python context managers since it provides a way to encapsulate pre and post execution behaviors in one function. Which was the impetus for writing this function. I wanted the ability that context managers provide, but I wanted to be able to have it defined in one function, and to be able to layer function after function on top.

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  • Why isn't functional language syntax more close to human language?

    - by JohnDoDo
    I'm interested in functional programming and decided to get head to head with Haskell. My head hurts... but I'll eventually get it... I have one curiosity though, why is the syntax so cryptic (in lack of another word)? Is there a reason why it isn't more expressive, more close to human language? I understand that FP is good at modelling mathematical concepts and it borrowed some of it's concise means of expression, but still it's not math... it's a language.

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  • Functional way to get a matrix from text

    - by Elazar Leibovich
    I'm trying to solve some Google Code Jam problems, where an input matrix is typically given in this form: 2 3 #matrix dimensions 1 2 3 4 5 6 7 8 9 # all 3 elements in the first row 2 3 4 5 6 7 8 9 0 # each element is composed of three integers where each element of the matrix is composed of, say, three integers. So this example should be converted to #!scala Array( Array(A(1,2,3),A(4,5,6),A(7,8,9), Array(A(2,3,4),A(5,6,7),A(8,9,0), ) An imperative solution would be of the form #!python input = """2 3 1 2 3 4 5 6 7 8 9 2 3 4 5 6 7 8 9 0 """ lines = input.split('\n') print lines[0] m,n = (int(x) for x in lines[0].split()) array = [] row = [] A = [] for line in lines[1:]: for elt in line.split(): A.append(elt) if len(A)== 3: row.append(A) A = [] array.append(row) row = [] from pprint import pprint pprint(array) A functional solution I've thought of is #!scala def splitList[A](l:List[A],i:Int):List[List[A]] = { if (l.isEmpty) return List[List[A]]() val (head,tail) = l.splitAt(i) return head :: splitList(tail,i) } def readMatrix(src:Iterator[String]):Array[Array[TrafficLight]] = { val Array(x,y) = src.next.split(" +").map(_.trim.toInt) val mat = src.take(x).toList.map(_.split(" "). map(_.trim.toInt)). map(a => splitList(a.toList,3). map(b => TrafficLight(b(0),b(1),b(2)) ).toArray ).toArray return mat } But I really feel it's the wrong way to go because: I'm using the functional List structure for each line, and then convert it to an array. The whole code seems much less efficeint I find it longer less elegant and much less readable than the python solution. It is harder to which of the map functions operates on what, as they all use the same semantics. What is the right functional way to do that?

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  • Is functional GUI programming possible?

    - by eman
    I've recently caught the FP bug (trying to learn Haskell), and I've been really impressed with what I've seen so far (first-class functions, lazy evaluation, and all the other goodies). I'm no expert yet, but I've already begun to find it easier to reason "functionally" than imperatively for basic algorithms (and I'm having trouble going back where I have to). The one area where current FP seems to fall flat, however, is GUI programming. The Haskell approach seems to be to just wrap imperative GUI toolkits (such as GTK+ or wxWidgets) and to use "do" blocks to simulate an imperative style. I haven't used F#, but my understanding is that it does something similar using OOP with .NET classes. Obviously, there's a good reason for this--current GUI programming is all about IO and side effects, so purely functional programming isn't possible with most current frameworks. My question is, is it possible to have a functional approach to GUI programming? I'm having trouble imagining what this would look like in practice. Does anyone know of any frameworks, experimental or otherwise, that try this sort of thing (or even any frameworks that are designed from the ground up for a functional language)? Or is the solution to just use a hybrid approach, with OOP for the GUI parts and FP for the logic? (I'm just asking out of curiosity--I'd love to think that FP is "the future," but GUI programming seems like a pretty large hole to fill.)

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  • Best Functional Approach

    - by dbyrne
    I have some mutable scala code that I am trying to rewrite in a more functional style. It is a fairly intricate piece of code, so I am trying to refactor it in pieces. My first thought was this: def iterate(count:Int,d:MyComplexType) = { //Generate next value n //Process n causing some side effects return iterate(count - 1, n) } This didn't seem functional at all to me, since I still have side effects mixed throughout my code. My second thought was this: def generateStream(d:MyComplexType):Stream[MyComplexType] = { //Generate next value n return Stream.cons(n, generateStream(n)) } for (n <- generateStream(initialValue).take(2000000)) { //process n causing some side effects } This seemed like a better solution to me, because at least I've isolated my functional value-generation code from the mutable value-processing code. However, this is much less memory efficient because I am generating a large list that I don't really need to store. This leaves me with 3 choices: Write a tail-recursive function, bite the bullet and refactor the value-processing code Use a lazy list. This is not a memory sensitive app (although it is performance sensitive) Come up with a new approach. I guess what I really want is a lazily evaluated sequence where I can discard the values after I've processed them. Any suggestions?

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  • Rails Functional test assert_select javascript respond_to

    - by Macint
    Hello, I am currently trying to write functional tests for a charging form which gets loaded on to the page via AJAX(jQuery). It loads the form from the charge_form action which returns the consult_form.js.erb view. This all works, but I am having trouble with my testing. In the functional I can go to the action but I cannot use assert_select to find a an element and verify that the form is in fact there. Error: 1) Failure: test_should_create_new_consult(ConsultsControllerTest) [/test/functional/consults_controller_test.rb:8]: Expected at least 1 element matching "h4", found 0. <false> is not true. This is the view. consult_form.js.erb: <div id="charging_form"> <h4>Charging form</h4> <div class="left" id="charge_selection"> <%= select_tag("select_category", options_from_collection_for_select(@categories, :id, :name)) %><br/> ... consults_controller_test.rb: require 'test_helper' class ConsultsControllerTest < ActionController::TestCase def test_should_create_new_consult get_with_user :charge_form, :animal_id => animals(:one), :id => consults(:one), :format => 'js' assert_response :success assert_select 'h4', "Charging form" #can't find h4 end end Is there a problem with using assert_select with types other than html? Thank you for any help!

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  • GNU Make - Dependencies on non program code

    - by Tim Post
    A requirement for a program I am writing is that it must be able to trust a configuration file. To accomplish this, I am using several kinds of hashing algorithms to generate a hash of the file at compile time, this produces a header with the hashes as constants. Dependencies for this are pretty straight forward, my program depends on config_hash.h, which has a target that produces it. The makefile looks something like this : config_hash.h: $(SH) genhash config/config_file.cfg > $(srcdir)/config_hash.h $(PROGRAM): config_hash.h $(PROGRAM_DEPS) $(CC) ... ... ... I'm using the -M option to gcc, which is great for dealing with dependencies. If my header changes, my program is rebuilt. My problem is, I need to be able to tell if the config file has changed, so that config_hash.h is re-generated. I'm not quite sure how explain that kind of dependency to GNU make. I've tried listing config/config_file.cfg as a dependency for config_hash.h, and providing a .PHONY target for config_file.cfg without success. Obviously, I can't rely on the -M switch to gcc to help me here, since the config file is not a part of any object code. Any suggestions? Unfortunately, I can't post much of the Makefile, or I would have just posted the whole thing.

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  • Ur/Web new purely functional language for web programming?

    - by Phuc Nguyen
    I came across the Ur/Web project during my search for web frameworks for Haskell-like languages. It looks like a very interesting project done by one person. Basically, it is a domain-specific purely functional language for web programming, taking the best of ML and Haskell. The syntax is ML, but there are type classes and monad from Haskell, and it's strictly evaluated. Server-side is compiled to native code, client to Javascript. See the slides and FAQ page for other advertised advantages. Looking at the demos and their source code, I think the project is very promising. The latest version is something 20110123, so it seems to be under active development at this time. My question: Has anybody here had any further experience with it? Are there problems/annoyances compared to Haskell, apart from ML's slightly more verbose syntax? Even if it's not well known yet, I hope more people will know of it. OMG this looks very cool to me. I don't want this project to die!!

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  • Are small amounts of functional programming understandable by non-FP people?

    - by kd35a
    Case: I'm working at a company, writing an application in Python that is handling a lot of data in arrays. I'm the only developer of this program at the moment, but it will probably be used/modified/extended in the future (1-3 years) by some other programmer, at this moment unknown to me. I will probably not be there directly to help then, but maybe give some support via email if I have time for it. So, as a developer who has learned functional programming (Haskell), I tend to solve, for example, filtering like this: filtered = filter(lambda item: included(item.time, dur), measures) The rest of the code is OO, it's just some small cases where I want to solve it like this, because it is much simpler and more beautiful according to me. Question: Is it OK today to write code like this? How does a developer that hasn't written/learned FP react to code like this? Is it readable? Modifiable? Should I write documentation like explaining to a child what the line does? # Filter out the items from measures for which included(item.time, dur) != True I have asked my boss, and he just says "FP is black magic, but if it works and is the most efficient solution, then it's OK to use it." What is your opinion on this? As a non-FP programmer, how do you react to the code? Is the code "googable" so you can understand what it does? I would love feedback on this :) Edit: I marked phant0m's post as answer, because he gives good advice on how to write the code in a more readable way, and still keep the advantages. But I would also like to recommend superM's post because of his viewpoint as a non-FP programmer.

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  • repair broken packages-"dpkg: error: conflicting actions -f (--field) and -r (--remove)"

    - by yinon
    Ubuntu 12.04 LTS. if more information will be needed, tell me and'll give. the main problem is: tzach@tzach-pc:~$ sudo apt-get install docky [sudo] password for tzach: Reading package lists... Done Building dependency tree Reading state information... Done docky is already the newest version. You might want to run 'apt-get -f install' to correct these: The following packages have unmet dependencies: ca-certificates-java : Depends: openjdk-6-jre-headless (>= 6b16-1.6.1-2) but it is not going to be installed or java6-runtime-headless openjdk-7-jre-lib : Depends: openjdk-7-jre-headless (>= 7~b130~pre0) but it is not going to be installed E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution). tzach@tzach-pc:~$ and also: tzach@tzach-pc:~$ sudo apt-get upgrade Reading package lists... Done Building dependency tree Reading state information... Done You might want to run 'apt-get -f install' to correct these. **The following packages have unmet dependencies: ca-certificates-java : Depends: openjdk-6-jre-headless (>= 6b16-1.6.1-2) but it is not installed or java6-runtime-headless openjdk-7-jre-lib : Depends: openjdk-7-jre-headless (>= 7~b130~pre0) but it is not installed E: Unmet dependencies. Try using ******* so we tryied the guide here in messege #9: http://ubuntuforums.org/showthread.php?t=947124 we run all the first 4 commands and the last one-"sudo apt-get autoremove" gave us: tzach@tzach-pc:~$ sudo apt-get autoremove Reading package lists... Done Building dependency tree Reading state information... Done You might want to run 'apt-get -f install' to correct these. The following packages have unmet dependencies: **ca-certificates-java** : Depends: openjdk-6-jre-headless (>= 6b16-1.6.1-2) but it is not installed or java6-runtime-headless **openjdk-7-jre-lib** : Depends: openjdk-7-jre-headless (>= 7~b130~pre0) but it is not installed E: Unmet dependencies. Try using -f. so we run the last command twice: sudo dpkg --remove -force --force-remove-reinstreq ca-certificates-java and sudo dpkg --remove -force --force-remove-reinstreq openjdk-7-jre-lib but both of them gives: tzach@tzach-pc:~$ sudo dpkg --remove -force --force-remove-reinstreq ca-certificates-java [sudo] password for tzach: dpkg: error: conflicting actions -f (--field) and -r (--remove) Type dpkg --help for help about installing and deinstalling packages [*]; Use `dselect' or `aptitude' for user-friendly package management; Type dpkg -Dhelp for a list of dpkg debug flag values; Type dpkg --force-help for a list of forcing options; Type dpkg-deb --help for help about manipulating *.deb files; Options marked [*] produce a lot of output - pipe it through `less' or `more' ! EDIT FOR green7-output of "sudo apt-get -f install": tzach@tzach-pc:~$ sudo apt-get -f install [sudo] password for tzach: Reading package lists... Done Building dependency tree Reading state information... Done Correcting dependencies... Done The following extra packages will be installed: icedtea-7-jre-cacao icedtea-7-jre-jamvm java-common openjdk-7-jre-headless tzdata-java Suggested packages: default-jre equivs sun-java6-fonts ttf-dejavu-extra fonts-ipafont-gothic fonts-ipafont-mincho ttf-telugu-fonts ttf-oriya-fonts ttf-kannada-fonts ttf-bengali-fonts The following packages will be REMOVED: ttf-mscorefonts-installer The following NEW packages will be installed: icedtea-7-jre-cacao icedtea-7-jre-jamvm java-common openjdk-7-jre-headless tzdata-java 0 upgraded, 5 newly installed, 1 to remove and 355 not upgraded. 5 not fully installed or removed. Need to get 0 B/29.6 MB of archives. After this operation, 88.5 MB of additional disk space will be used. Do you want to continue [Y/n]? y debconf: DbDriver "config": /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable dpkg: warning: there's no installed package matching ttf-mscorefonts-installer:amd64 Setting up tzdata (2012e-0ubuntu0.12.04) ... debconf: DbDriver "config": /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable dpkg: error processing tzdata (--configure): subprocess installed post-installation script returned error exit status 1 No apport report written because MaxReports is reached already Errors were encountered while processing: tzdata E: Sub-process /usr/bin/dpkg returned an error code (1) EDIT2 FOR green7: tzach@tzach-pc:~$ sudo apt-get remove --purge tzdata [sudo] password for tzach: Reading package lists... Done Building dependency tree Reading state information... Done You might want to run 'apt-get -f install' to correct these: The following packages have unmet dependencies: ca-certificates-java : Depends: openjdk-6-jre-headless (>= 6b16-1.6.1-2) but it is not going to be installed or java6-runtime-headless libc6 : Depends: tzdata but it is not going to be installed libc6:i386 : Depends: tzdata:i386 libical0 : Depends: tzdata but it is not going to be installed openjdk-7-jre-lib : Depends: openjdk-7-jre-headless (>= 7~b130~pre0) but it is not going to be installed python-dateutil : Depends: tzdata but it is not going to be installed ubuntu-minimal : Depends: tzdata but it is not going to be installed util-linux : Depends: tzdata (>= 2006c-2) but it is not going to be installed E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution). EDIT3 FOR green7: tzach@tzach-pc:~$ sudo apt-get install openjdk-7-jre-headless [sudo] password for tzach: Reading package lists... Done Building dependency tree Reading state information... Done You might want to run 'apt-get -f install' to correct these: The following packages have unmet dependencies: openjdk-7-jre-headless : Depends: tzdata-java but it is not going to be installed Depends: java-common (>= 0.28) but it is not going to be installed Recommends: icedtea-7-jre-cacao (= 7~u3-2.1.1~pre1-1ubuntu3) but it is not going to be installed Recommends: icedtea-7-jre-jamvm (= 7~u3-2.1.1~pre1-1ubuntu3) but it is not going to be installed E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution). some things in the text also supposed to be bolded. but not critic (: Thanks for the editing! Thanks a lot for your assistance.

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  • Developing my own package manager: how do I resolve dependencies?

    - by shantanu
    According to debian control file ant package's dependency list is: Depends: default-jre-headless | java2-runtime-headless | java5-runtime-headless | java6-runtime-headless, libxerces2-java That means it depends on default-jre-headless or java2-runtime-headless or java5-runtime-headless or java6-runtime-headless. But how could i determine that which package need to select as dependency? Note: I am trying to develop a package manager. I can not use provided package because they read sources.list file from a specific directory.

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  • How can I install a package without installing some dependencies?

    - by Alex
    I'm trying to install the package LaTeXila, and the output looks like this: $ sudo apt-get install latexila --no-install-recommends Reading package lists... Done Building dependency tree Reading state information... Done The following extra packages will be installed: latexila-data latexmk luatex tex-common texlive-base texlive-binaries texlive-common texlive-doc-base texlive-latex-base Suggested packages: rubber texlive-latex-extra debhelper Recommended packages: texlive texlive-latex-recommended texlive-luatex lmodern texlive-latex-base-doc The following NEW packages will be installed: latexila latexila-data latexmk luatex tex-common texlive-base texlive-binaries texlive-common texlive-doc-base texlive-latex-base 0 upgraded, 10 newly installed, 0 to remove and 0 not upgraded. Need to get 29.3 MB of archives. After this operation, 74.5 MB of additional disk space will be used. Do you want to continue [Y/n]? I don't want to install the texlive packages. I've installed texlive manually from http://www.tug.org/texlive/. Any suggestions?

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  • For nodejs what are best design practices for native modules which share dependencies?

    - by Mark Essel
    Hypothetical situation, I have 3 node modules all native, A, B, and C.  A is a utilities module which exposes several functions to javascript through the node interface, in addition it declares/defines a rich set of native structures and functions B is a module which is dependent on data structures and source in A, it exposes some functions to javascript, in addition it declares/defines native structures and functions C is a module which is dependent on data structures and source in A & B, it exploses some functions to javascript, in addition it declares/defines native structures and functions So far when setting up these modules I have a preinstall script to install other dependent includes, but in order to access all of another modules source what is the best way to link to it's share library object (*.node) ? Is there an alternative best practice for native modules (such as installing all source from other modules before building that single module)? Reply

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  • Eclipse Plugin project with other project dependencies

    - by James
    I have an Eclipse plugin project, and it depends on other projects that I have in my Eclipse workspace. After adding the project dependencies under "Java Build Path" - "Projects" tab, and also selecting the project in the "Order and Export" I get a java.lang.NoClassDefFoundError. I'm assuming that the other projects have not been properly included into the plugin. Does anyone know how to fix this? Thanks, James

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