diff --git a/scala.html.markdown b/scala.html.markdown index 5a0cc0ff..a55e1f0e 100644 --- a/scala.html.markdown +++ b/scala.html.markdown @@ -4,6 +4,7 @@ filename: learnscala.scala contributors: - ["George Petrov", "http://github.com/petrovg"] - ["Dominic Bou-Samra", "http://dbousamra.github.com"] + - ["Geoff Liu", "http://geoffliu.me"] filename: learn.scala --- @@ -20,16 +21,16 @@ Scala - the scalable language scala> - This is the so called REPL. You can run commands in the REPL. Let's do just - that: + This is the so called REPL (Read-Eval-Print Loop). You may type any valid + Scala expression into it, and the result will be printed. We will explain what + Scala files look like further into this tutorial, but for now, let's start + with some basics. */ -println(10) // prints the integer 10 -println("Boo!") // printlns the string Boo! - - -// Some basics +################################################# +## 1. Basics +################################################# // Printing, and forcing a new line on the next print println("Hello world!") @@ -37,15 +38,15 @@ println("Hello world!") print("Hello world") // Declaring values is done using either var or val -// val declarations are immutable, whereas var's are mutable. Immutability is +// val declarations are immutable, whereas var's are mutable. Immutability is // a good thing. val x = 10 // x is now 10 x = 20 // error: reassignment to val -var x = 10 +var x = 10 x = 20 // x is now 20 // Single line comments start with two forward slashes -/* +/* Multi line comments look like this. */ @@ -82,262 +83,6 @@ true == false // false */ -// 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 = - - 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 colon explicitly defines the type of a value, in this case a function -// taking an Int and returning an Int. -val add10: Int => Int = _ + 10 - -// Scala allows methods and functions to return, or take as parameters, other -// functions or methods. - -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) - -// 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. */ - - - -// Loops and iteration - -1 to 5 -val r = 1 to 5 -r.foreach( println ) - -r foreach println -// NB: Scala is quite lenient 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 than 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 // Invoking r on String makes it a Regex -val serialKey = """(\d{5})-(\d{5})-(\d{5})-(\d{5})""".r // Using verbatim (multiline) syntax - -val matcher = (value: String) => { - println(value match { - case email(name, domain) => s"It was an email: $name" - case serialKey(p1, p2, p3, p4) => s"Serial key: $p1, $p2, $p3, $p4" - case _ => s"No match on '$value'" // default if no match found - }) -} - -matcher("mrbean@pyahoo.com") // => "It was an email: mrbean" -matcher("nope..") // => "No match on 'nope..'" -matcher("52917") // => "No match on '52917'" -matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917" - - // Strings "Scala strings are surrounded by double quotes" // @@ -379,8 +124,291 @@ val html = """
""" +################################################# +## 2. Functions +################################################# -// Application structure and organization +// 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 = + + 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 colon explicitly defines the type of a value, in this case a function +// taking an Int and returning an Int. +val add10: Int => Int = _ + 10 + + +################################################# +## 3. Flow Control +################################################# + +1 to 5 +val r = 1 to 5 +r.foreach( println ) + +r foreach println +// NB: Scala is quite lenient 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 than 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 } + + +################################################# +## 4. 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 + + +################################################# +## 5. Object Oriented Programming +################################################# + +/* + Aside: Everything we've done so far in this tutorial has been simple + expressions (values, functions, etc). These expressions are fine to type into + the command-line interpreter for quick tests, but they cannot exist by + themselves in a Scala file. For example, you cannot have just "val x = 5" in + a Scala file. Instead, the only top-level constructs allowed in Scala are: + + - objects + - classes + - case classes + - traits + + And now we will explain what these are. +*/ + +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") + + +// Objects and traits coming soon! + + +################################################# +## 6. 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 // Invoking r on String makes it a Regex +val serialKey = """(\d{5})-(\d{5})-(\d{5})-(\d{5})""".r // Using verbatim (multiline) syntax + +val matcher = (value: String) => { + println(value match { + case email(name, domain) => s"It was an email: $name" + case serialKey(p1, p2, p3, p4) => s"Serial key: $p1, $p2, $p3, $p4" + case _ => s"No match on '$value'" // default if no match found + }) +} + +matcher("mrbean@pyahoo.com") // => "It was an email: mrbean" +matcher("nope..") // => "No match on 'nope..'" +matcher("52917") // => "No match on '52917'" +matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917" + + +################################################# +## 7. Functional Programming +################################################# + +// Scala allows methods and functions to return, or take as parameters, other +// functions or methods. + +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) + +// 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 + + +// 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. */ + + +################################################# +## 8. Implicits +################################################# + +Coming soon! + + +################################################# +## 9. Misc +################################################# // Importing things import scala.collection.immutable.List