From 651e1e90d2da18ee0dd225ef78631c90d519af9d Mon Sep 17 00:00:00 2001 From: Geoff Liu Date: Fri, 14 Nov 2014 15:01:16 -0800 Subject: Some work on the Scala page --- scala.html.markdown | 152 ++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 118 insertions(+), 34 deletions(-) diff --git a/scala.html.markdown b/scala.html.markdown index dc039f0c..529347be 100644 --- a/scala.html.markdown +++ b/scala.html.markdown @@ -54,14 +54,17 @@ var y = 10 y = 20 // y is now 20 /* - Scala is a statically typed language, yet note that in the above declarations, we did not specify - a type. This is due to a language feature called type inference. In most cases, Scala compiler can - guess what the type of a variable is, so you don't have to type it every time. We can explicitly - declare the type of a variable like so: + Scala is a statically typed language, yet note that in the above declarations, + we did not specify a type. This is due to a language feature called type + inference. In most cases, Scala compiler can guess what the type of a variable + is, so you don't have to type it every time. We can explicitly declare the + type of a variable like so: */ val z: Int = 10 val a: Double = 1.0 -val b: Double = 10 // Notice automatic conversion from Int to Double, result is 10.0, not 10 + +// Notice automatic conversion from Int to Double, result is 10.0, not 10 +val b: Double = 10 // Boolean values true @@ -94,8 +97,8 @@ true == false // false This means the result of evaluating 1 + 7 is an object of type Int with a value of 8 - Note that "res29" is a sequentially generated variable name to store the results of the - expressions you typed, your output may differ. + Note that "res29" is a sequentially generated variable name to store the + results of the expressions you typed, your output may differ. */ "Scala strings are surrounded by double quotes" @@ -142,27 +145,69 @@ val html = """
// 2. Functions ///////////////////////////////////////////////// -// The next line gives you a function that takes an Int and returns it squared -(x:Int) => x * x +// Functions are defined like so: +// +// def functionName(args...): ReturnType = { body... } +// +// If you come from more traditional languages, notice the omission of the +// return keyword. In Scala, the last expression in the function block is the +// return value. +def sumOfSquares(x: Int, y: Int): Int = { + val x2 = x * x + val y2 = y * y + x2 + y2 +} -// You can assign this function to an identifier, like this: -val sq = (x:Int) => x * x +// The { } can be omitted if the function body is a single expression: +def sumOfSquaresShort(x: Int, y: Int): Int = x * x + y * y -/* The above says this +// Syntax for calling functions is familiar: +sumOfSquares(3, 4) // => 25 - sq: Int => Int = +// In most cases (with recursive functions the most notable exception), function +// return type can be omitted, and the same type inference we saw with variables +// will work with function return values: +def sq(x: Int) = x * x // Compiler can guess return type is 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. +// Functions can have default parameters: +def addWithDefault(x: Int, y: Int = 5) = x + y +addWithDefault(1, 2) // => 3 +addWithDefault(1) // => 6 - sq can be executed as follows: -*/ -sq(10) // Gives you this: res33: Int = 100. +// Anonymous functions look like this: +(x:Int) => x * x -// 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 +// Unlike defs, even the input type of anonymous functions can be omitted if the +// context makes it clear. Notice the type "Int => Int" which means a function +// that takes Int and returns Int. +val sq: Int => Int = x => x * x + +// Anonymous functions can be called as usual: +sq(10) // => 100 + +// If your anonymous function has one or two arguments, and each argument is +// used only once, Scala gives you an even shorter way to define them. These +// anonymous functions turn out to be extremely common, as will be obvious in +// the data structure section. +val addOne: Int => Int = _ + 1 +val weirdSum: (Int, Int) => Int = (_ * 2 + _ * 3) + +addOne(5) // => 6 +weirdSum(2, 4) // => 16 + + +// The return keyword exists in Scala, but it only returns from the inner-most +// def that surrounds it. It has no effect on anonymous functions. For example: +def foo(x: Int) = { + val anonFunc: Int => Int = { z => + if (z > 5) + return z // This line makes z the return value of foo! + else + z + 2 // This line is the return value of anonFunc + } + anonFunc(x) // This line is the return value of foo +} ///////////////////////////////////////////////// @@ -187,7 +232,7 @@ 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 + // fast, faster that Java loops, but using the combinators and // comprehensions above is easier to understand and parallelize // A do while loop @@ -290,13 +335,24 @@ d._2 And now we will explain what these are. */ +// classes are similar to classes in other languages. Constructor arguments are +// declared after the class name, and initialization is done in the class body. class Dog(br: String) { + // Constructor code here var breed: String = br - //A method called bark, returning a String - def bark: String = { - // the body of the method - "Woof, woof!" - } + + // Define a method called bark, returning a String + def bark = "Woof, woof!" + + // Values and methods are assumed public. "protected" and "private" keywords + // are also available. + private def sleep(hours: Int) = + println(s"I'm sleeping for $hours hours") + + // Abstract methods are simply methods with no body. If we uncomment the next + // line, class Dog would need to be declared abstract + // abstract class Dog(...) { ... } + // def chaseAfter(what: String): String } val mydog = new Dog("greyhound") @@ -304,17 +360,45 @@ println(mydog.breed) // => "greyhound" println(mydog.bark) // => "Woof, woof!" -// Classes can contain nearly any other construct, including other classes, -// functions, methods, objects, case classes, traits etc. +// The "object" keyword creates a type AND a singleton instance of it. It is +// common for Scala classes to have a "companion object", where the per-instance +// behavior is captured in the classes themselves, but behavior related to all +// instance of that class go in objects. The difference is similar to class +// methods vs static methods in other languages. Note that objects and classes +// can have the same name. +object Dog { + def allKnownBreeds = List("pitbull", "shepherd", "retriever") + def createDog(breed: String) = new Dog(breed) +} -// Case classes -case class Person(name:String, phoneNumber:String) +// Case classes are classes that have extra functionality built in. A common +// question for Scala beginners is when to use classes and when to use case +// classes. The line is quite fuzzy, but in general, classes tend to focus on +// encapsulation, polymorphism, and behavior. The values in these classes tend +// to be private, and only methods are exposed. The primary purpose of case +// classes is to hold immutable data. They often have few methods, and the +// methods rarely have side-effects. +case class Person(name: String, phoneNumber: String) + +// Create a new instance. Note cases classes don't need "new" +val george = Person("George", "1234") +val kate = Person("Kate", "4567") + +// With case classes, you get a few perks for free, like getters: +george.phoneNumber // => "1234" + +// Per field equality (no need to override .equals) +Person("George", "1234") == Person("Kate", "1236") // => false + +// Easy way to copy +// otherGeorge == Person("george", "9876") +val otherGeorge = george.copy(phoneNumber = "9876") -Person("George", "1234") == Person("Kate", "1236") +// And many others. Case classes also get pattern matching for free, see below. -// Objects and traits coming soon! +// Traits coming soon! ///////////////////////////////////////////////// @@ -423,7 +507,7 @@ for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared // 8. Implicits ///////////////////////////////////////////////// -Coming soon! +// Coming soon! ///////////////////////////////////////////////// -- cgit v1.2.3