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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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• 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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• Advantages of compilers for functional languages over compilers for imperative languages

  - by Onorio Catenacci

  As a follow up to this question What are the advantages of built-in immutability of F# over C#?--am I correct in assuming that the F# compiler can make certain optimizations knowing that it's dealing with largely immutable code? I mean even if a developer writes "Functional C#" the compiler wouldn't know all of the immutability that the developer had tried to code in so that it couldn't make the same optimizations, right? In general would the compiler of a functional language be able to make optimizations that would not be possible with an imperative language--even one written with as much immutability as possible?

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• Why don't purely functional languages use reference counting?

  - by Zifre

  In purely functional languages, data is immutable. With reference counting, creating a reference cycle requires changing already created data. It seems like purely functional languages could use reference counting without worrying about the possibility of cycles. Am is right? If so, why don't they? I understand that reference counting is slower than GC in many cases, but at least it reduces pause times. It would be nice to have the option to use reference counting in cases where pause times are bad.

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• Functional data structures in C++

  - by drg

  Does anyone know of a C++ data structure library providing functional (a.k.a. immutable, or "persistent" in the FP sense) equivalents of the familiar STL structures? By "functional" I mean that the objects themselves are immutable, while modifications to those objects return new objects sharing the same internals as the parent object where appropriate. Ideally, such a library would resemble STL, and would work well with Boost.Phoenix (caveat- I haven't actually used Phoenix, but as far as I can tell it provides many algorithms but no data structures, unless a lazily-computed change to an existing data structure counts - does it?)

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• Pure functional bottom up tree algorithm

  - by Axel Gneiting

  Say I wanted to write an algorithm working on an immutable tree data structure that has a list of leaves as its input. It needs to return a new tree with changes made to the old tree going upwards from those leaves. My problem is that there seems to be no way to do this purely functional without reconstructing the entire tree checking at leaves if they are in the list, because you always need to return a complete new tree as the result of an operation and you can't mutate the existing tree. Is this a basic problem in functional programming that only can be avoided by using a better suited algorithm or am I missing something?

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• Is scala functional programming slower than traditional coding?

  - by Fred Haslam

  In one of my first attempts to create functional code, I ran into a performance issue. I started with a common task - multiply the elements of two arrays and sum up the results: var first:Array[Float] ... var second:Array[Float] ... var sum=0f; for(ix<-0 until first.length) sum += first(ix) * second(ix); Here is how I reformed the work: sum = first.zip(second).map{ case (a,b) => a*b }.reduceLeft(_+_) When I benchmarked the two approaches, the second method takes 40 times as long to complete! Why does the second method take so much longer? How can I reform the work to be both speed efficient and use functional programming style?

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• Integration tests in Continuous Integration environment: Database and filesystem state

  - by dario_ramos

  I'm trying to implement automated integration tests for my application. It's a very complex monster. You could say that its database and part of the filesystem are part of its state, because it saves image files in the hard drive, and references to those in the DB. The software needs all those, in a coherent state, to work properly. Back to writing tests: To run any relevant test, I need some image files in the filesystem, and certain records filled in the database. I thought of putting all of these in a separate folder called TestEnvironmentData in the repository, and retrieving them from the Continuous Integration Server (Team City), but a colleague said the repo is quite full as it is, and that I should set up a special directory, and databases, only in the Continuous Integration server. I don't like that because the tests success depend on me manually mantaining stuff in the server, and restoring initial state before every test becomes cumbersome. What do you guys do when you need to write integration tests for an app like this? The main goal is having an automated test harness to approach a large scale refactoring. There's lots of spaghetti code and the app's current architecture is hardly unit testable, that's why I decided on integration tests first. Any alternative approach is welcome.

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• Disable selected automated tests at runtime

  - by squig

  Is is posable to disable selected automated tests at runtime? I'm using VSTS and rhino mocks and have some intergation tests that require an external dependancy to be installed (MQ). Not all the developers on my team have this installed. Currently all the tests that require MQ inherit from a base class that checks if MQ is installed and if is not sets the test result to inconclusive. This works as it stops the tests from running, but marks the test run as unsuccseessful and can hide other failures. Any ideas?

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• How to organize integrity tests and code unit tests?

  - by karlthorwald

  I have several files with code testing code (which uses a "unittest" class). Later I found it would be nice to test database integrity also. I put this into a separate directory tree. (Things like the keys have correct format, parent and child nodes are pointing correctly and such.) I use the same unittest class for the integrity tests. Now I wonder if it makes really sense to keep this separate. To test the integrity of data I often duplicate parts of code that I use to test the code that handles the data. But it is not the same. The code tests use test databases (that get deleted after each test) and the integrity tests connect to the live data and analyze it. The integrity tests I want to call from cron and send an alarm if something happens in the live database. How would you handle that? Are there standards for such a setup? What is your experience? My tendency is to put everything in the same file, which would result in the code tests also being executed by the cron on the production environment.

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• How to organize live data integrity tests and code unit tests?

  - by karlthorwald

  I have several files with code testing code (which uses a "unittest" class). Later I found it would be nice to test database integrity also. I put this into a separate directory tree. (Things like the keys have correct format, parent and child nodes are pointing correctly and such.) I use the same unittest class for the integrity tests. Now I wonder if it makes really sense to keep this separate. To test the integrity of data I often duplicate parts of code that I use to test the code that handles the data. But it is not the same. The code tests use test databases (that get deleted after each test) and the integrity tests connect to the live data and analyze it. The integrity tests I want to call from cron and send an alarm if something happens in the live database. How would you handle that? Are there standards for such a setup? What is your experience? My tendency is to put everything in the same file, which would result in the code tests also being executed by the cron on the production environment.

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• Creating parallel selenium tests in C# and using Nunit as the runner application

  - by damianmartin

  I am writing a new test suite for the company to test a very complex ASP.NET application which is heavily AJAX driven. We have decided to use Selenium (Grid & Remote Control) and Nunit to run these tests. The actually tests are dynamically created at run time from a spreadsheet. Each Column in an excel spreadsheet relates to a new test and each row relates to a selenium command (but in plain English and the dll converts this into Selenium code). My problem i have at the moment is getting the tests running in parallel. There will be 1000+ tests so it is too time consuming to have 1 test run at a time. Selenium Grid and Selenium Remote Control(s) are setup correctly because I can run there demo. From what i have read I need to use Punit but i can not find any documentation on what a test in punit should look like. Nunit tests are [SetUp] [TearDown] [Test]. Can anyone point me in the right direction. Thanks in advance.

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• How to configure Visual Studio 2010 code coverage for ASP.NET MVC unit tests

  - by DigiMortal

  I just got Visual Studio 2010 code coverage work with ASP.NET MVC application unit tests. Everything is simple after you have spent some time with forums, blogs and Google. To save your valuable time I wrote this posting to guide you through the process of making code coverage work with ASP.NET MVC application unit tests. After some fighting with Visual Studio I got everything to work as expected. I am still not very sure why users must deal with this mess, but okay – I survived it. Before you start configuring Visual Studio I expect your solution meets the following needs: there are at least one library that will be tested, there is at least on library that contains tests to be run, there are some classes and some tests for them, and, of course, you are using version of Visual Studio 2010 that supports tests (I have Visual Studio 2010 Ultimate). Now open the following screenshot to separate windows and follow the steps given below. Visual Studio 2010 Test Settings window. Click on image to see it at original size.  Double click on Local.testsettings under Solution Items. Test settings window will be opened. Select “Data and Diagnostics” from left pane. Mark checkboxes “ASP.NET Profiler” and “Code Coverage”. Move cursor to “Code Coverage” line and press Configure button or make double click on line. Assemblies selection window will be opened. Mark checkboxes that are located before assemblies about what you want code coverage reports and apply settings. Save your project and close Visual Studio. Run Visual Studio as Administrator and run tests. NB! Select Test => Run => Tests in Current Context from menu. When tests are run you can open code coverage results by selecting Test => Windows => Code Coverage Results from menu. Here you can see my example test results. Visual Studio 2010 Test Results window. All my tests passed this time. :) Click on image to see it at original size.  And here are the code coverage results. Visual Studio 2101 Code Coverage Results. I need a lot more tests for sure. Click on image to see it at original size.  As you can see everything was pretty simple. But it took me sometime to figure out how to get everything work as expected. Problems? You may face some problems when making code coverage work. Here is my short list of possible problems. Make sure you have all assemblies available for code coverage. In some cases it needs more libraries to be referenced as you currently have. By example, I had to add some more Enterprise Library assemblies to my project. You can use EventViewer to discover errors that where given during testing. Make sure you selected all testable assemblies from Code Coverage settings like shown above. Otherwise you may get empty results. Tests with code coverage are slower because we need ASP.NET profiler. If your machine slows down then try to free more resources.

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• Functional testing of a 3-rd party java program

  - by Dmitri Nesteruk

  I have a 3-rd party java application (I don't own source code) and I want to perform functional testing on it, similar to the way it's done in watin/watij/selenium/nunitforms etc. Can anyone suggest a library that I can use to do this sort of testing. What I'm interested in is clicking the applet's buttons, reading off text values, and the like. Thanks!

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• Big datastructures in functional programming

  - by Denis Gorodetskiy

  I'm newbie in Functional Programming. I have a huge neural network with thousands of neurons and every connection between neurons has its weight. I have to update these weights very often, several thousand times per learning session. Is FP still applicable here? I mean in fp we can't modify variables and only able to return new variables not changing previous values. Does this mean I have to recreate whole network on every weight update?

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• Functional programming and multicore architecture

  - by lonegunman

  I've read somewhere that functional programming is suitable to take advantage of multi-core trend in computin.. I didn't really get the idea. Is it related to the lambda calculus and von neumann architecture?

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• Discovering a functional algorithm from a mutable one

  - by Garrett Rowe

  This isn't necessarily a Scala question, it's a design question that has to do with avoiding mutable state, functional thinking and that sort. It just happens that I'm using Scala. Given this set of requirements: Input comes from an essentially infinite stream of random numbers between 1 and 10 Final output is either SUCCEED or FAIL There can be multiple objects 'listening' to the stream at any particular time, and they can begin listening at different times so they all may have a different concept of the 'first' number; therefore listeners to the stream need to be decoupled from the stream itself. Pseudocode: if (first number == 1) SUCCEED else if (first number >= 9) FAIL else { first = first number rest = rest of stream for each (n in rest) { if (n == 1) FAIL else if (n == first) SUCCEED else continue } } Here is a possible mutable implementation: sealed trait Result case object Fail extends Result case object Succeed extends Result case object NoResult extends Result class StreamListener { private var target: Option[Int] = None def evaluate(n: Int): Result = target match { case None => if (n == 1) Succeed else if (n >= 9) Fail else { target = Some(n) NoResult } case Some(t) => if (n == t) Succeed else if (n == 1) Fail else NoResult } } This will work but smells to me. StreamListener.evaluate is not referentially transparent. And the use of the NoResult token just doesn't feel right. It does have the advantage though of being clear and easy to use/code. Besides there has to be a functional solution to this right? I've come up with 2 other possible options: Having evaluate return a (possibly new) StreamListener, but this means I would have to make Result a subtype of StreamListener which doesn't feel right. Letting evaluate take a Stream[Int] as a parameter and letting the StreamListener be in charge of consuming as much of the Stream as it needs to determine failure or success. The problem I see with this approach is that the class that registers the listeners should query each listener after each number is generated and take appropriate action immediately upon failure or success. With this approach, I don't see how that could happen since each listener is forcing evaluation of the Stream until it completes evaluation. There is no concept here of a single number generation. Is there any standard scala/fp idiom I'm overlooking here?

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• How to call a different post to a different controller in Rails Functional Test

  - by Snehal

  I want to make a post request to a different controller inside my functional test which is intended for a particular controller. However the post method in ActiveController class just takes the method to be called, it doesn't take the controller name to be called. Any ideas how to invoke a different controller?

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• Rails functional testing without migrations

  - by Brian D.

  The name pretty much says it all. Does anyone know how to accomplish functional testing when you are not using migrations in Rails? I'd be open to any advice or third party libraries (if there are any). I thought of creating my own plugin to address this but it seems like a pretty big task and would rather not do this unless necessary. Thanks in advance.

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