Merge pull request #149 from petrovg/adding_scala

Adding Scala

scala.html.markdown

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+---
+language: Scala
+contributors:
+    - ["George Petrov", "http://github.com/petrovg"]
+    - ["Dominic Bou-Samra, "http://dbousamra.github.com"]
+filename: learn.scala
+---
+
+Scala - the scalable language
+
+```c
+
+
+
+/*
+  Set yourself up:
+
+  1) Download Scala - http://www.scala-lang.org/downloads
+  2) unzip/untar in your favourite location and put the bin subdir on the path
+  3) Start a scala REPL by typing scala. You should see the prompt:
+
+  scala>
+
+  This is the so called REPL. You can run commands in the REPL. Let's do just that:
+*/
+
+println(10) // prints the integer 10
+
+println("Boo!") // printlns the string Boo!
+
+
+// Some basics
+
+// Printing, and forcing a new line on the next print
+println("Hello world!")
+// Printing, without forcing a new line on next print
+print("Hello world")
+
+// Declaring values is done using either var or val
+// val declarations are immutable, whereas var's are mutable. Immutablility is a good thing.
+val x = 10 // x is now 10
+x = 20 // error: reassignment to val
+var x = 10 
+x = 20  // x is now 20
+
+//  Single line comments start with two forward slashes
+/* 
+Multi line comments look like this.
+*/
+
+// Boolean values
+true
+false
+
+// Boolean operations
+!true // false
+!false // true
+true == false // false
+10 > 5 // true
+
+// Math is as per usual
+1 + 1 // 2
+2 - 1 // 1
+5 * 3 // 15
+6 / 2 // 3
+
+
+// Evaluating a command in the REPL gives you the type and value of the result
+
+1 + 7
+
+/* The above line results in:
+
+  scala> 1 + 7
+  res29: Int = 8
+
+  This means the result of evaluating 1 + 7 is an object of type Int with a value of 8
+
+  1+7 will give you the same result
+*/
+
+
+// Everything is an object, including a function. Type these in the REPL:
+
+7 // results in res30: Int = 7 (res30 is just a generated var name for the result)
+
+// The next line gives you a function that takes an Int and returns it squared
+(x:Int) => x * x    
+
+// You can assign this function to an identifier, like this:
+val sq = (x:Int) => x * x
+
+/* The above says this
+   
+   sq: Int => Int = <function1>	
+
+   Which means that this time we gave an explicit name to the value - sq is a function that take an Int and returns Int.
+
+   sq can be executed as follows:
+*/
+
+sq(10)   // Gives you this: res33: Int = 100. The result is the Int with a value 100
+
+// Scala allows methods and functions to return, or take as parameters, other functions or methods.
+
+val add10: Int => Int = _ + 10 // A function taking an Int and returning an Int
+List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element
+
+// Anonymous functions can be used instead of named functions:
+List(1, 2, 3) map (x => x + 10)
+
+// And the underscore symbol, can be used if there is just one argument to the anonymous function. It gets bound as the variable
+List(1, 2, 3) map (_ + 10)
+
+TODO // If the anonymous block AND the function you are applying both take one argument, you can even omit the underscore
+List("Dom", "Bob", "Natalia") foreach println
+
+
+
+// Data structures
+
+val a = Array(1, 2, 3, 5, 8, 13)
+a(0)
+a(3)
+a(21)    // Throws an exception
+
+val m = Map("fork" -> "tenedor", "spoon" -> "cuchara", "knife" -> "cuchillo")
+m("fork")
+m("spoon")
+m("bottle")       // Throws an exception
+
+val safeM = m.withDefaultValue("no lo se")
+safeM("bottle")
+
+val s = Set(1, 3, 7)
+s(0)
+s(1)
+
+/* Look up the documentation of map here - http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map 
+ * and make sure you can read it
+ */
+
+
+// Tuples
+
+(1, 2)
+
+(4, 3, 2)
+
+(1, 2, "three")
+
+(a, 2, "three")
+
+// Why have this?
+val divideInts = (x:Int, y:Int) => (x / y, x % y)
+
+divideInts(10,3) // The function divideInts gives you the result and the remainder
+
+// To access the elements of a tuple, use _._n where n is the 1-based index of the element
+val d = divideInts(10,3)
+
+d._1
+
+d._2
+
+
+
+// Combinators
+
+s.map(sq)
+
+val sSquared = s. map(sq)
+
+sSquared.filter(_ < 10)
+
+sSquared.reduce (_+_)
+
+// The filter function takes a predicate (a function from A -> Boolean) and selects all elements which satisfy the predicate
+List(1, 2, 3) filter (_ > 2) // List(3)
+List(
+  Person(name = "Dom", age = 23), 
+  Person(name = "Bob", age = 30)
+).filter(_.age > 25) // List(Person("Bob", 30))
+
+
+// Scala a foreach method defined on certain collections that takes a type returning Unit (a void method)
+aListOfNumbers foreach (x => println(x))
+aListOfNumbers foreach println
+
+
+
+
+// For comprehensions
+
+for { n <- s } yield sq(n)
+
+val nSquared2 = for { n <- s } yield sq(n)
+
+for { n <- nSquared2 if n < 10 } yield n
+
+for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
+
+/* NB Those were not for loops. The semantics of a for loop is 'repeat', whereas a for-comprehension
+   defines a relationship between two sets of data. Research this further  */
+
+
+
+// Loops and iteration
+
+1 to 5
+val r = 1 to 5
+r.foreach( println )
+
+r foreach println     
+// NB: Scala is quite lenien when it comes to dots and brackets - study the rules separately. This 
+// helps write DSLs and APIs that read like English
+
+(5 to 1 by -1) foreach ( println )
+
+// A while loops
+var i = 0
+while (i < 10) {  println("i " + i); i+=1  }
+
+while (i < 10) {  println("i " + i); i+=1  }   // Yes, again. What happened? Why?
+
+i    // Show the value of i. Note that while is a loop in the classical sense - it executes
+     // sequentially while changing the loop variable. while is very fast, faster that Java
+     // loops, but using the combinators and comprehensions above is easier to understand
+     // and parallelize
+
+// A do while loop
+do { 
+  println("x is still less then 10"); 
+  x += 1
+} while (x < 10)
+
+// Tail recursion is an idiomatic way of doing recurring things in Scala. Recursive functions need an 
+// explicit return type, the compiler can't infer it. Here it's Unit.
+def showNumbersInRange(a:Int, b:Int):Unit = { print(a); if (a < b) showNumbersInRange(a+1, b)  }
+
+
+
+// Conditionals
+
+val x = 10
+
+if (x == 1) println("yeah")
+if (x == 10) println("yeah")
+if (x == 11) println("yeah")
+if (x == 11) println ("yeah") else println("nay")
+
+println(if (x == 10) "yeah" else "nope")
+val text = if (x == 10) "yeah" else "nope"
+
+var i = 0
+while (i < 10) { println("i " + i); i+=1  }
+
+
+
+// Object oriented features
+
+// Classname is Dog
+class Dog {
+  //A method called bark, returning a String
+  def bark: String = {
+    // the body of the method
+    "Woof, woof!"
+  }
+}
+
+// Classes can contain nearly any other construct, including other classes, functions, methods, objects, case classes, traits etc.
+
+
+
+// Case classes
+
+case class Person(name:String, phoneNumber:String)
+
+Person("George", "1234") == Person("Kate", "1236")
+
+
+
+
+// Pattern matching
+
+val me = Person("George", "1234")
+
+me match { case Person(name, number) => "We matched someone : " + name + ", phone : " + number }
+
+me match { case Person(name, number) => "Match : " + name; case _ => "Hm..." }
+
+me match { case Person("George", number) => "Match"; case _ => "Hm..." }
+
+me match { case Person("Kate", number) => "Match"; case _ => "Hm..." }
+
+me match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
+
+val kate = Person("Kate", "1234")
+
+kate match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" }
+
+
+
+// Regular expressions
+
+val email = "(.*)@(.*)".r          // The suffix .r invokes method r on String, which makes it a Regex
+
+val email(user, domain) = "henry@zkpr.com"
+
+"mrbean@pyahoo.com" match {
+  case email(name, domain) => "I know your name, " + name
+}
+
+
+
+// Strings
+
+"Scala strings are surrounded by double quotes" //
+'a' // A Scala Char
+'Single quote strings don't exist' // Error
+"Strings have the usual Java methods defined on them".length
+"They also have some extra Scala methods.".reverse // See scala.collection.immutable.StringOps
+
+println("ABCDEF".length)
+println("ABCDEF".substring(2, 6))
+println("ABCDEF".replace("C", "3"))
+
+val n = 45
+println(s"We have $n apples")
+
+val a = Array(11, 9, 6)
+println(s"My second daughter is ${a(2-1)} years old")
+
+// Some characters need to be 'escaped', e.g. a double quote inside a string:
+val a = "They stood outside the \"Rose and Crown\""
+
+// Triple double-quotes allow for strings to span multiple rows and contain funny characters
+val html = """<form id="daform">
+                <p>Press belo', Joe</p>
+             |  <input type="submit">
+              </form>"""
+
+
+
+// Application structure and organization
+
+// Importing things
+import scala.collection.immutable.List
+
+// Import all "sub packages"
+import scala.collection.immutable._
+
+// Import multiple classes in one statement
+import scala.collection.immutable.{List, Map}
+
+// Rename an import using '=>'
+import scala.collection.immutable{ List => ImmutableList }
+
+// Import all classes, except some. The following excludes Map and Set:
+import scala.collection.immutable.{Map => _, Set => _, _}
+
+// Your programs entry point is defined in an scala file using an object, with a single method, main:
+object Application {
+  def main(args: Array[String]): Unit = {
+    // stuff goes here.
+  }
+}
+
+// Files can contain multiple classes and objects. Compile with scalac
+
+
+
+
+// Input and output
+
+// To read a file line by line
+import scala.io.Source
+for(line <- Source.fromPath("myfile.txt").getLines())
+  println(line)
+
+// To write a file use Java's PrintWriter
+
+
+```
+
+## Further resources
+
+[Scala for the impatient](http://horstmann.com/scala/)
+
+[Twitter Scala school(http://twitter.github.io/scala_school/)
+
+[The scala documentation]
+
+Join the [Scala user group](https://groups.google.com/forum/#!forum/scala-user)
+