EzDevInfo.com

Scala vs. Groovy vs. Clojure [closed]

Can someone please explain the major differences between Scala, Groovy and Clojure. I know each of these compiles to run on the JVM but I'd like a simple comparison between them.

How do you make a web application in Clojure?

I suppose this is a strange question to the huge majority of programmers that work daily with Java. I don't. I know Java-the-language, because I worked on Java projects, but not Java-the-world. I never made a web app from scratch in Java. If I have to do it with Python, Ruby, I know where to go (Django or Rails), but if I want to make a web application in Clojure, not because I'm forced to live in a Java world, but because I like the language and I want to give it a try, what libraries and frameworks should I use?

Debugging in Clojure?

What are best ways to Debug Clojure code, while using the repl?

A gentle tutorial to Emacs/Swank/Paredit for Clojure

I am moving to Emacs to work on Clojure/Lisp. What is all the information I need to setup on Emacs to be able to do the following?

1. automatic matching/generation of corresponding closing brackets
2. autoindent Lisp/Clojure style, not C++/Java style
3. Syntax highlighting
4. Invoking REPL
5. To be able to load a part of code from file into the REPL and evaluate it.

It would be great if I could also get the list of commands to get these things after setting things up on Emacs.

Test whether a list contains a specific value in Clojure

What is the best way to test whether a list contains a given value in Clojure?

In particular, the behaviour of contains? is currently confusing me:

(contains? '(100 101 102) 101) => false

I could obviously write a simple function to traverse the list and test for equality, but there must surely be a standard way to do this?

Mapping a function on the values of a map in Clojure

I want to transform one map of values to another map with the same keys but with a function applied to the values. I would think there was a function for doing this in the clojure api, but I have been unable to find it.

Here's an example implementation of what I'm looking for

(defn map-function-on-map-vals [m f]
  (reduce (fn [altered-map [k v]] (assoc altered-map k (f v))) {} m))
(println (map-function-on-map-vals {:a "test" :b "testing"} #(.toUpperCase %)))
{:b TESTING, :a TEST}

Does anybody know if map-function-on-map-vals already exists? I would think it did (probably with a nicer name too).

Block Comments in Clojure

How do I comment multiple lines in Clojure?

leiningen - how to add dependencies for local jars?

I want to use leiningen to build and develop my clojure project. Is there a way to modify project.clj to tell it to pick some jars from local directories?

I have some proprietary jars that cannot be uploaded to public repos.

Also, can leiningen be used to maintain a "lib" directory for clojure projects? If a bunch of my clojure projects share the same jars, I don't want to maintain a separate copy for each of them.

Thanks

Is there an equivalent for the Zip function in Clojure Core or Contrib?

In Clojure, I want to combine two lists to give a list of pairs,

> (zip '(1 2 3) '(4 5 6))  
((1 4) (2 5) (3 6))

In Haskell or Ruby the function is called zip. Implementing it is not difficult, but I wanted to make sure I wasn't missing a function in Core or Contrib.

There is a zip namespace in Core, but it is described as providing access to the Zipper functional technique, which does not appear to be what I am after.

Is there an equivalent function for combining 2 or more lists, in this way, in Core?

If there is not, is it because there is an idiomatic approach that renders the function unneeded?

How to create default value for function argument in Clojure

I come with this:

(defn string->integer [str & [base]]
  (Integer/parseInt str (if (nil? base) 10 base)))

(string->integer "10")
(string->integer "FF" 16)

But it must be a better way to do this.

Why does Clojure have "keywords" in addition to "symbols"?

I have a passing knowledge of other Lisps (particularly Scheme) from way back. Recently I've been reading about Clojure. I see that it has both "symbols" and "keywords". Symbols I'm familiar with, but not with keywords.

Do other Lisps have keywords? How are keywords different from symbols other than having different notation (ie: colons)?

Interpreting a benchmark in C, Clojure, Python, Ruby, Scala and others [closed]

Disclaimer

I know that artificial benchmarks are evil. They can show results only for very specific narrow situation. I don't assume that one language is better than the other because of the some stupid bench. However I wonder why results is so different. Please see my questions at the bottom.

Math benchmark description

Benchmark is simple math calculations to find pairs of prime numbers which differs by 6 (so called sexy primes) E.g. sexy primes below 100 would be: (5 11) (7 13) (11 17) (13 19) (17 23) (23 29) (31 37) (37 43) (41 47) (47 53) (53 59) (61 67) (67 73) (73 79) (83 89) (97 103)

Results table

In table: calculation time in seconds Running: all except Factor was running in VirtualBox (Debian unstable amd64 guest, Windows 7 x64 host) CPU: AMD A4-3305M

  Sexy primes up to:        10k      20k      30k      100k               

  Bash                    58.00   200.00     [*1]      [*1]

  C                        0.20     0.65     1.42     15.00

  Clojure1.4               4.12     8.32    16.00    137.93

  Clojure1.4 (optimized)   0.95     1.82     2.30     16.00

  Factor                    n/a      n/a    15.00    180.00

  Python2.7                1.49     5.20    11.00       119     

  Ruby1.8                  5.10    18.32    40.48    377.00

  Ruby1.9.3                1.36     5.73    10.48    106.00

  Scala2.9.2               0.93     1.41     2.73     20.84

  Scala2.9.2 (optimized)   0.32     0.79     1.46     12.01

[*1] - I'm afraid to imagine how much time will it take

Code listings

C:

int isprime(int x) {
  int i;
  for (i = 2; i < x; ++i)
    if (x%i == 0) return 0;
  return 1;
}

void findprimes(int m) {
  int i;
  for ( i = 11; i < m; ++i)
    if (isprime(i) && isprime(i-6))
      printf("%d %d\n", i-6, i);
}

main() {
    findprimes(10*1000);
}

Ruby:

def is_prime?(n)
  (2...n).all?{|m| n%m != 0 }
end

def sexy_primes(x)
  (9..x).map do |i|
    [i-6, i]
  end.select do |j|
    j.all?{|j| is_prime? j}
  end
end

a = Time.now
p sexy_primes(10*1000)
b = Time.now
puts "#{(b-a)*1000} mils"

Scala:

def isPrime(n: Int) =
  (2 until n) forall { n % _ != 0 }

def sexyPrimes(n: Int) = 
  (11 to n) map { i => List(i-6, i) } filter { _ forall(isPrime(_)) }

val a = System.currentTimeMillis()
println(sexyPrimes(100*1000))
val b = System.currentTimeMillis()
println((b-a).toString + " mils")

Scala opimized isPrime (the same idea like in Clojure optimization):

import scala.annotation.tailrec

@tailrec // Not required, but will warn if optimization doesn't work
def isPrime(n: Int, i: Int = 2): Boolean = 
  if (i == n) true 
  else if (n % i != 0) isPrime(n, i + 1)
  else false

Clojure:

(defn is-prime? [n]
  (every? #(> (mod n %) 0)
    (range 2 n)))

(defn sexy-primes [m]
  (for [x (range 11 (inc m))
        :let [z (list (- x 6) x)]
        :when (every? #(is-prime? %) z)]
      z))

(let [a (System/currentTimeMillis)]
  (println (sexy-primes (* 10 1000)))
  (let [b (System/currentTimeMillis)]
    (println (- b a) "mils")))

Clojure optimized is-prime?:

(defn ^:static is-prime? [^long n]
  (loop [i (long 2)] 
    (if (= (rem n i) 0)
      false
      (if (>= (inc i) n) true (recur (inc i))))))

Python

import time as time_

def is_prime(n):
  return all((n%j > 0) for j in xrange(2, n))

def primes_below(x):
  return [[j-6, j] for j in xrange(9, x+1) if is_prime(j) and is_prime(j-6)]

a = int(round(time_.time() * 1000))
print(primes_below(10*1000))
b = int(round(time_.time() * 1000))
print(str((b-a)) + " mils")

Factor

MEMO:: prime? ( n -- ? )
n 1 - 2 [a,b] [ n swap mod 0 > ] all? ;

MEMO: sexyprimes ( n n -- r r )
[a,b] [ prime? ] filter [ 6 + ] map [ prime? ] filter dup [ 6 - ] map ;

5 10 1000 * sexyprimes . .

Bash(zsh):

#!/usr/bin/zsh
function prime {
  for (( i = 2; i < $1; i++ )); do
    if [[ $[$1%i] == 0 ]]; then
      echo 1
      exit
    fi
  done
  echo 0
}

function sexy-primes {
  for (( i = 9; i <= $1; i++ )); do
    j=$[i-6]
    if [[ $(prime $i) == 0 && $(prime $j) == 0 ]]; then
      echo $j $i
    fi
  done
}

sexy-primes 10000

Questions

1. Why Scala is so fast? Is it because of static typing? Or it is just using JVM very efficiently?
2. Why such a huge difference between Ruby and Python? I thought these two are not somewhat totally different. Maybe my code is wrong. Please enlighten me! Thanks. UPD Yes, that was error in my code. Python and Ruby 1.9 are pretty equal.
3. Really impressive jump in productivity between Ruby versions.
4. Can I optimize Clojure code by adding type declarations? Will it help?

Java to clojure rewrite

I have just been asked by my company to rewrite a largish (50,000 single lines of code) Java application (a web app using JSP and servlets) in Clojure. Has anyone else got tips as to what I should watch out for?

Please bear in mind that I know both Java AND Clojure quite well.

Update

I did the rewrite and it went into production. Its quite strange as the rewrite ended up going so fast that it was done in about 6 weeks. Because alot of functionality wasn't needed still it ended up more like 3000 lines of Clojure. I hear they are happy with the system and its doing exactly what they wanted. The only downside is that the guy maintaining the system had to learn Clojure from scratch, and he was dragged into it kicking and screaming. I did get a call from him the other day saying he loved Lisp now though.. funny :)

Also, I should give a good mention to Vaadin. Using Vaadin probably accounted for as much of the time saved and shortness of the code as Clojure did.. Vaadin is still the top web framework I have ever used, although now I'm learning ClojureScript in anger! (Note that both Vaadin and ClojureScript use Google's GUI frameworks underneath the hood)

Common programming mistakes for Clojure developers to avoid [closed]

What are some common mistakes made by Clojure developers, and how can we avoid them?

For example; newcomers to Clojure think that the contains? function works the same as java.util.Collection#contains. However, contains? will only work similarly when used with indexed collections like maps and sets and you're looking for a given key:

(contains? {:a 1 :b 2} :b)
;=> true
(contains? {:a 1 :b 2} 2)
;=> false
(contains? #{:a 1 :b 2} :b)
;=> true

When used with numerically indexed collections (vectors, arrays) contains? only checks that the given element is within the valid range of indexes (zero-based):

(contains? [1 2 3 4] 4)
;=> false
(contains? [1 2 3 4] 0)
;=> true

If given a list, contains? will never return true.

Simple explanation of clojure protocols

I'm trying to understand clojure protocols and what problem they are supposed to solve. Does anyone have a clear explanation of the whats and whys of clojure protocols?