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• Explaining persistent data structures in simple terms

  - by Jason Baker

  I'm working on a library for Python that implements some persistent data structures (mainly as a learning exercise). However, I'm beginning to learn that explaining persistent data structures to people unfamiliar with them can be difficult. Can someone help me think of an easy (or at least the least complicated) way to describe persistent data structures to them? I've had a couple of people tell me that the documentation that I have is somewhat confusing. (And before anyone asks, no I don't mean persistent data structures as in persisted to the file system. Google persistent data structures if you're unclear on this.)

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• Clojure - tail recursive sieve of Eratosthenes

  - by Konrad Garus

  I have this implementation of the sieve of Eratosthenes in Clojure: (defn sieve [n] (loop [last-tried 2 sift (range 2 (inc n))] (if (or (nil? last-tried) (> last-tried n)) sift (let [filtered (filter #(or (= % last-tried) (< 0 (rem % last-tried))) sift)] (let [next-to-try (first (filter #(> % last-tried) filtered))] (recur next-to-try filtered)))))) For larger n (like 20000) it ends with stack overflow. Why doesn't tail call elimination work here? How to fix it?

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• Ruby on Rails testing: How can I test or at the very least see a form_for's error_messages_for?

  - by williamjones

  I'm working on creating a tests, and I can't figure out why the creation of a model from a form_for is failing in the test but works in real browsers. Is there a straightforward way for me to see what the problems are in the model creation? Even better would be, is there a straightforward way for me to test the error outputs that I access via error_messages_for? In that case, I'd like to also add in tests that make sure that malformed forms are outputting the correct errors.

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• Naming Suggestions For A Function Providing Chaining In A Different Way

  - by sid3k

  I've coded an experimental function which makes passed objects chainable by using high order functions. It's name is "chain" for now, and here is a usage example; chain("Hello World") (print) // evaluates print function by passing "Hello World" object. (console.log,"Optional","Parameters") (returnfrom) // returns "Hello World" It looks lispy but behaves very different since it's coded in a C based language, I don't know if there is a name for this idiom and I couldn't any name more suitable than "chain". Any ideas, suggestions?

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• Ruby: intelligent patch/update

  - by Shyam

  Hi, After being blown away by the greatness of irb and rails console, I am loving the dynamical parts of the language more and more. Now, I wonder if it would be possible for a Ruby application to update itself on-the-fly (if the write permission allows it). Can a Ruby application fetch an update and refresh itself? I have seen this functionality a lot in Apple applications and I know updates and fixes are something you must anticipate for when deploying an application. Thank you for your feedback, comments and answers!

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• What is a monad?

  - by kronoz

  Having briefly looked at Haskell recently I wondered whether anybody could give a brief, succinct, practical explanation as to what a monad essentially is? I have found most explanations I've come across to be fairly inaccessible and lacking in practical detail, so could somebody here help me?

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• What is the single best book on Automated Testing of Websites

  - by user22883

  I am looking for the one good book to read for Automated Testing in general of a website.

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• More FP-correct way to create an update sql query

  - by James Black

  I am working on access a database using F# and my initial attempt at creating a function to create the update query is flawed. let BuildUserUpdateQuery (oldUser:UserType) (newUser:UserType) = let buf = new System.Text.StringBuilder("UPDATE users SET "); if (oldUser.FirstName.Equals(newUser.FirstName) = false) then buf.Append("SET first_name='").Append(newUser.FirstName).Append("'" ) |> ignore if (oldUser.LastName.Equals(newUser.LastName) = false) then buf.Append("SET last_name='").Append(newUser.LastName).Append("'" ) |> ignore if (oldUser.UserName.Equals(newUser.UserName) = false) then buf.Append("SET username='").Append(newUser.UserName).Append("'" ) |> ignore buf.Append(" WHERE id=").Append(newUser.Id).ToString() This doesn't properly put a , between any update parts after the first, for example: UPDATE users SET first_name='Firstname', last_name='lastname' WHERE id=... I could put in a mutable variable to keep track when the first part of the set clause is appended, but that seems wrong. I could just create an list of tuples, where each tuple is oldtext, newtext, columnname, so that I could then loop through the list and build up the query, but it seems that I should be passing in a StringBuilder to a recursive function, returning back a boolean which is then passed as a parameter to the recursive function. Does this seem to be the best approach, or is there a better one?

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• Mathematica equivalent of Ruby's inject

  - by Ben Alpert

  Is there a Mathematica function like inject in Ruby? For example, if I want the product of the elements in a list, in Ruby I can write: list.inject(1) { |prod,el| prod * el } I found I can just use Product in Mathematica: Apply[Product, list] However, this isn't general enough for me (like, if I don't just want the product or sum of the numbers). What's the closest equivalent to inject?

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• posmax: like argmax but gives the position(s) of the element x for which f[x] is maximal

  - by dreeves

  Mathematica has a built-in function ArgMax for functions over infinite domains, based on the standard mathematical definition. The analog for finite domains is a handy utility function. Given a function and a list (call it the domain of the function), return the element(s) of the list that maximize the function. Here's an example of finite argmax in action: http://stackoverflow.com/questions/471029/canonicalize-nfl-team-names/472213#472213 And here's my implementation of it (along with argmin for good measure): (* argmax[f, domain] returns the element of domain for which f of that element is maximal -- breaks ties in favor of first occurrence. *) SetAttributes[{argmax, argmin}, HoldFirst]; argmax[f_, dom_List] := Fold[If[f[#1]>=f[#2], #1, #2]&, First[dom], Rest[dom]] argmin[f_, dom_List] := argmax[-f[#]&, dom] First, is that the most efficient way to implement argmax? What if you want the list of all maximal elements instead of just the first one? Second, how about the related function posmax that, instead of returning the maximal element(s), returns the position(s) of the maximal elements?

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• Make a function which returns the original list except the argument

  - by Alex

  I want make a function which takes a list of string and a string and returns NONE if there is no string in the string list, otherwise it returns SOME of the list of string which is the same as the original list of string except it doesn't contain the initial string (pattern): fun my_function (pattern, source_list) = case source_list of [] => NONE | [x] => if pattern = x then SOME [] else NONE | x::xs => if pattern = x then SOME (xs) else SOME (x) :: my_function (pattern, xs) (* this is wrong, what to do here?*) val a = my_function ("haha", ["12", "aaa", "bbb", "haha", "ccc", "ddd"]) (* should be SOME ["12", "aaa", "bbb", "ccc", "ddd"]*) val a2 = my_function ("haha2", ["123", "aaa", "bbb", "haha", "ccc"]) (*should be NONE*) val a3 = my_function ("haha3", ["haha3"]) (* should be SOME []*) I'm confused by the 3rd case: x::xs => .... What should do there? Note that I'd like not to use any sml library function.

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• How to start the web cam by programmatically?

  - by Nitz

  Hello Guys How to start any web cam through programmatically? my main requirement is it should start webcam? and that should be any application - software not a website. we can use any language. So how can start the web cam using programing language? btw... [I am not talking about the power of the webcam][Any web cam means any companies web cam]

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• Can anyone explain Monads?

  - by Steve Willard

  I think I understand what 'Maybe Monads' are, but I'm not sure about the other types.

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• Zipping with padding in Haskell

  - by Travis Brown

  A couple of times I've found myself wanting a zip in Haskell that adds padding to the shorter list instead of truncating the longer one. This is easy enough to write. (Monoid works for me here, but you could also just pass in the elements that you want to use for padding.) zipPad :: (Monoid a, Monoid b) => [a] -> [b] -> [(a, b)] zipPad xs [] = zip xs (repeat mempty) zipPad [] ys = zip (repeat mempty) ys zipPad (x:xs) (y:ys) = (x, y) : zipPad xs ys This approach gets ugly when trying to define zipPad3. I typed up the following and then realized that of course it doesn't work: zipPad3 :: (Monoid a, Monoid b, Monoid c) => [a] -> [b] -> [c] -> [(a, b, c)] zipPad3 xs [] [] = zip3 xs (repeat mempty) (repeat mempty) zipPad3 [] ys [] = zip3 (repeat mempty) ys (repeat mempty) zipPad3 [] [] zs = zip3 (repeat mempty) (repeat mempty) zs zipPad3 xs ys [] = zip3 xs ys (repeat mempty) zipPad3 xs [] zs = zip3 xs (repeat mempty) zs zipPad3 [] ys zs = zip3 (repeat mempty) ys zs zipPad3 (x:xs) (y:ys) (z:zs) = (x, y, z) : zipPad3 xs ys zs At this point I cheated and just used length to pick the longest list and pad the others. Am I overlooking a more elegant way to do this, or is something like zipPad3 already defined somewhere?

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• Yet another Haskell vs. Scala question

  - by Travis Brown

  I've been using Haskell for several months, and I love it—it's gradually become my tool of choice for everything from one-off file renaming scripts to larger XML processing programs. I'm definitely still a beginner, but I'm starting to feel comfortable with the language and the basics of the theory behind it. I'm a lowly graduate student in the humanities, so I'm not under a lot of institutional or administrative pressure to use specific tools for my work. It would be convenient for me in many ways, however, to switch to Scala (or Clojure). Most of the NLP and machine learning libraries that I work with on a daily basis (and that I've written in the past) are Java-based, and the primary project I'm working for uses a Java application server. I've been mostly disappointed by my initial interactions with Scala. Many aspects of the syntax (partial application, for example) still feel clunky to me compared to Haskell, and I miss libraries like Parsec and HXT and QuickCheck. I'm familiar with the advantages of the JVM platform, so practical questions like this one don't really help me. What I'm looking for is a motivational argument for moving to Scala. What does it do (that Haskell doesn't) that's really cool? What makes it fun or challenging or life-changing? Why should I get excited about writing it?

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• How to test for the appearance of a Toast message

  - by Adrian

  Would anyone know how to test for the appearance of a Toast message on an Activity? I'm using code similar to what the OP posted on this question for testing my program flow from one activity to the next. I'd also like to be able to test for toast messages on particular activities.

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• When is a scala partial function not a partial function?

  - by Fred Haslam

  While creating a map of String to partial functions I ran into unexpected behavior. When I create a partial function as a map element it works fine. When I allocate to a val it invokes instead. Trying to invoke the check generates an error. Is this expected? Am I doing something dumb? Comment out the check() to see the invocation. I am using scala 2.7.7 def PartialFunctionProblem() = { def dream()() = { println("~Dream~"); new Exception().printStackTrace() } val map = scala.collection.mutable.HashMap[String,()=>Unit]() map("dream") = dream() // partial function map("dream")() // invokes as expected val check = dream() // unexpected invocation check() // error: check of type Unit does not take parameters }

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• Chain call in clojure?

  - by Konrad Garus

  I'm trying to implement sieve of Eratosthenes in Clojure. One approach I would like to test is this: Get range (2 3 4 5 6 ... N) For 2 <= i <= N Pass my range through filter that removes multiplies of i For i+1th iteration, use result of the previous filtering I know I could do it with loop/recur, but this is causing stack overflow errors (for some reason tail call optimization is not applied). How can I do it iteratively? I mean invoking N calls to the same routine, passing result of ith iteration to i+1th.

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• F#: how to find Cartesian power

  - by Nike

  I have a problem with writing a Cartesian power function. I found many examples about calculating Cartesian Product, but no one about Cartesian power. For example, [1;2] raised to power 3 = [ [1;1;1] ; [1;1;2] ; [1;2;1] ; [1;2;2] ; [2;1;1] ; [2;1;2] ; [2;2;1]; [2;2;2] ] I use following code to calculate Cartesian Product: let Cprod U V = let mutable res = [] for u in U do for v in V do res <- res @ [[u;v]] res And trying to calculate Cartesian power. I use following code to calculate Cartesian Product: let Cpower U n = let mutable V = U for i=0 to n-1 do V <- Dprod U V V Visual Studio said: Error The resulting type would be infinite when unifying ''a' and ''a list'. I will thankful for any help and links.

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• Is there a language that encourages good coding practices?

  - by Darrell Brogdon

  While I love PHP I find its biggest weakness is that it allows and even almost encourages programmers to write bad code. Is there a language that encourages good programming practices? Or, more specifically, a web-related language that encourages good practices. I'm interested in languages who have either a stated goal of encouraging good programming or are designed in such a way as to encourage good programming.

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• Closures and universal quantification

  - by Apocalisp

  I've been trying to work out how to implement Church-encoded data types in Scala. It seems that it requires rank-n types since you would need a first-class const function of type forAll a. a -> (forAll b. b -> b). However, I was able to encode pairs thusly: import scalaz._ trait Compose[F[_],G[_]] { type Apply = F[G[A]] } trait Closure[F[_],G[_]] { def apply[B](f: F[B]): G[B] } def pair[A,B](a: A, b: B) = new Closure[Compose[PartialApply1Of2[Function1,A]#Apply, PartialApply1Of2[Function1,B]#Apply]#Apply, Identity] { def apply[C](f: A => B => C) = f(a)(b) } For lists, I was able to get encode cons: def cons[A](x: A) = { type T[B] = B => (A => B => B) => B new Closure[T,T] { def apply[B](xs: T[B]) = (b: B) => (f: A => B => B) => f(x)(xs(b)(f)) } } However, the empty list is more problematic and I've not been able to get the Scala compiler to unify the types. Can you define nil, so that, given the definition above, the following compiles? cons(1)(cons(2)(cons(3)(nil)))

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• How to sort a list in Scala by two fields?

  - by Twistleton

  how to sort a list in Scala by two fields, in this example I will sort by lastName and firstName? case class Row(var firstName: String, var lastName: String, var city: String) var rows = List(new Row("Oscar", "Wilde", "London"), new Row("Otto", "Swift", "Berlin"), new Row("Carl", "Swift", "Paris"), new Row("Hans", "Swift", "Dublin"), new Row("Hugo", "Swift", "Sligo")) rows.sortBy(_.lastName) I try things like this rows.sortBy(_.lastName + _.firstName) but it doesn't work. So I be curious for a good and easy solution. Thanks in advance! Pongo

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• Please Help me add up the elements for this structure in Scheme/Lisp

  - by kunjaan

  I have an input which is of this form: (((lady-in-water . 1.25) (snake . 1.75) (run . 2.25) (just-my-luck . 1.5)) ((lady-in-water . 0.8235294117647058) (snake . 0.5882352941176471) (just-my-luck . 0.8235294117647058)) ((lady-in-water . 0.8888888888888888) (snake . 1.5555555555555554) (just-my-luck . 1.3333333333333333))) (context: the word denotes a movie and the number denotes the weighted rating submitted by the user) I need to add all the quantity and return a list which looks something like this ((lady-in-water 2.5) (snake 2.5) (run 2.25) (just-myluck 2.6)) How do I traverse the list and all the quantities? I am really stumped. Please help me. Thanks.

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• Haskell: OpenGL, how to prevent immediate window closing

  - by Yasir Arsanukaev

  Hello. Whenever I close OpenGL window it makes ghci console from which the app was started to immediately disappear. Is there a way to prevent this behaviour? A sample app. Thanks.

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• Scheme: what are the benefits of letrec?

  - by Ixmatus

  While reading "The Seasoned Schemer" I've begun to learn about letrec. I understand what it does (can be duplicated with a Y-Combinator) but the book is using it in lieu of recurring on the already defined function operating on arguments that remain static. An example of an old function using the defined function recurring on itself (nothing special): (define (substitute new old lat) (cond ((null? l) '()) ((eq? (car l) old) (cons new (substitute new old (cdr l)))) (else (cons (car l) (substitute new old (cdr l)))))) Now for an example of that same function but using letrec: (define (substitute new old lat) (letrec ((replace (lambda (l) (cond ((null? l) '()) ((eq? (car l) old) (cons new (replace (cdr l)))) (else (cons (car l) (replace (cdr l)))))))) (replace lat))) Aside from being slightly longer and more difficult to read I don't know why they are rewriting functions in the book to use letrec. Is there a speed enhancement when recurring over a static variable this way because you don't keep passing it?? Is this standard practice for functions with arguments that remain static but one argument that is reduced (such as recurring down the elements of a list)? Some input from more experienced Schemers/LISPers would help!

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