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Diffstat (limited to 'scala.html.markdown')
| -rw-r--r-- | scala.html.markdown | 588 |
1 files changed, 395 insertions, 193 deletions
diff --git a/scala.html.markdown b/scala.html.markdown index 6b398b4b..e6638121 100644 --- a/scala.html.markdown +++ b/scala.html.markdown @@ -4,12 +4,13 @@ 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 --- Scala - the scalable language -```cpp +```scala /* Set yourself up: @@ -20,34 +21,50 @@ 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! +///////////////////////////////////////////////// +// 1. Basics +///////////////////////////////////////////////// +// Single line comments start with two forward slashes -// Some basics +/* + Multi line comments, as you can already see from above, look like this. +*/ // Printing, and forcing a new line on the next print println("Hello world!") +println(10) + // 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. Immutability is +// Declaring values is done using either var or val. +// 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 -x = 20 // x is now 20 +var y = 10 +y = 20 // y is now 20 -// Single line comments start with two forward slashes -/* -Multi line comments look like this. +/* + 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 + +// Notice automatic conversion from Int to Double, result is 10.0, not 10 +val b: Double = 10 // Boolean values true @@ -64,9 +81,11 @@ true == false // false 2 - 1 // 1 5 * 3 // 15 6 / 2 // 3 +6 / 4 // 1 +6.0 / 4 // 1.5 -// Evaluating a command in the REPL gives you the type and value of the result +// Evaluating an expression in the REPL gives you the type and value of the result 1 + 7 @@ -78,55 +97,179 @@ true == false // false 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 + 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" +'a' // A Scala Char +// 'Single quote strings don't exist' <= This causes an error -// Everything is an object, including a function. Type these in the REPL: +// Strings have the usual Java methods defined on them +"hello world".length +"hello world".substring(2, 6) +"hello world".replace("C", "3") -7 // results in res30: Int = 7 (res30 is just a generated var name for the result) +// They also have some extra Scala methods. See also: scala.collection.immutable.StringOps +"hello world".take(5) +"hello world".drop(5) -// The next line gives you a function that takes an Int and returns it squared -(x:Int) => x * x +// String interpolation: notice the prefix "s" +val n = 45 +s"We have $n apples" // => "We have 45 apples" -// You can assign this function to an identifier, like this: -val sq = (x:Int) => x * x +// Expressions inside interpolated strings are also possible +val a = Array(11, 9, 6) +s"My second daughter is ${a(0) - a(2)} years old." // => "My second daughter is 5 years old." +s"We have double the amount of ${n / 2.0} in apples." // => "We have double the amount of 22.5 in apples." +s"Power of 2: ${math.pow(2, 2)}" // => "Power of 2: 4" -/* The above says this - - sq: Int => Int = <function1> +// Formatting with interpolated strings with the prefix "f" +f"Power of 5: ${math.pow(5, 2)}%1.0f" // "Power of 5: 25" +f"Square root of 122: ${math.sqrt(122)}%1.4f" // "Square root of 122: 11.0454" - Which means that this time we gave an explicit name to the value - sq is a - function that take an Int and returns Int. +// Raw strings, ignoring special characters. +raw"New line feed: \n. Carriage return: \r." // => "New line feed: \n. Carriage return: \r." - sq can be executed as follows: -*/ +// Some characters need to be "escaped", e.g. a double quote inside a string: +"They stood outside the \"Rose and Crown\"" // => "They stood outside the "Rose and Crown"" -sq(10) // Gives you this: res33: Int = 100. +// Triple double-quotes let strings span multiple rows and contain quotes +val html = """<form id="daform"> + <p>Press belo', Joe</p> + <input type="submit"> + </form>""" -// 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. +///////////////////////////////////////////////// +// 2. Functions +///////////////////////////////////////////////// + +// 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 +} -List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element +// The { } can be omitted if the function body is a single expression: +def sumOfSquaresShort(x: Int, y: Int): Int = x * x + y * y -// Anonymous functions can be used instead of named functions: -List(1, 2, 3) map (x => x + 10) +// Syntax for calling functions is familiar: +sumOfSquares(3, 4) // => 25 -// 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) +// 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 -// 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 +// Functions can have default parameters: +def addWithDefault(x: Int, y: Int = 5) = x + y +addWithDefault(1, 2) // => 3 +addWithDefault(1) // => 6 +// Anonymous functions look like this: +(x:Int) => x * x -// Data structures +// 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 each argument in your anonymous function 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. +// WARNING: Using return in Scala is error-prone and should be avoided. +// It has no effect on anonymous functions. For example: +def foo(x: Int): 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 +} + + +///////////////////////////////////////////////// +// 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) +} +showNumbersInRange(1,14) + + +// 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" + + +///////////////////////////////////////////////// +// 4. Data Structures +///////////////////////////////////////////////// val a = Array(1, 2, 3, 5, 8, 13) a(0) @@ -145,8 +288,8 @@ 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 +/* 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 */ @@ -175,212 +318,271 @@ d._1 d._2 +///////////////////////////////////////////////// +// 5. Object Oriented Programming +///////////////////////////////////////////////// -// 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 +/* + 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. +*/ +// 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 + // 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") -// For comprehensions + // 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 +} -for { n <- s } yield sq(n) +val mydog = new Dog("greyhound") +println(mydog.breed) // => "greyhound" +println(mydog.bark) // => "Woof, woof!" -val nSquared2 = for { n <- s } yield sq(n) -for { n <- nSquared2 if n < 10 } yield n +// 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) +} -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. */ +// 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" -// Loops and iteration +// Per field equality (no need to override .equals) +Person("George", "1234") == Person("Kate", "1236") // => false -1 to 5 -val r = 1 to 5 -r.foreach( println ) +// Easy way to copy +// otherGeorge == Person("george", "9876") +val otherGeorge = george.copy(phoneNumber = "9876") -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 +// And many others. Case classes also get pattern matching for free, see below. -(5 to 1 by -1) foreach ( println ) -// A while loops -var i = 0 -while (i < 10) { println("i " + i); i+=1 } +// Traits coming soon! -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 +///////////////////////////////////////////////// +// 6. Pattern Matching +///////////////////////////////////////////////// -// A do while loop -do { - println("x is still less then 10"); - x += 1 -} while (x < 10) +// Pattern matching is a powerful and commonly used feature in Scala. Here's how +// you pattern match a case class. NB: Unlike other languages, Scala cases do +// not need breaks, fall-through does not happen. -// 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) +def matchPerson(person: Person): String = person match { + // Then you specify the patterns: + case Person("George", number) => "We found George! His number is " + number + case Person("Kate", number) => "We found Kate! Her number is " + number + case Person(name, number) => "We matched someone : " + name + ", phone : " + number } +val email = "(.*)@(.*)".r // Define a regex for the next example. +// Pattern matching might look familiar to the switch statements in the C family +// of languages, but this is much more powerful. In Scala, you can match much +// more: +def matchEverything(obj: Any): String = obj match { + // You can match values: + case "Hello world" => "Got the string Hello world" -// Conditionals + // You can match by type: + case x: Double => "Got a Double: " + x -val x = 10 + // You can specify conditions: + case x: Int if x > 10000 => "Got a pretty big number!" -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 } + // You can match case classes as before: + case Person(name, number) => s"Got contact info for $name!" + // You can match regular expressions: + case email(name, domain) => s"Got email address $name@$domain" + // You can match tuples: + case (a: Int, b: Double, c: String) => s"Got a tuple: $a, $b, $c" -// Object oriented features + // You can match data structures: + case List(1, b, c) => s"Got a list with three elements and starts with 1: 1, $b, $c" -// Classname is Dog -class Dog { - //A method called bark, returning a String - def bark: String = { - // the body of the method - "Woof, woof!" - } + // You can nest patterns: + case List(List((1, 2,"YAY"))) => "Got a list of list of tuple" } -// 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") +// In fact, you can pattern match any object with an "unapply" method. This +// feature is so powerful that Scala lets you define whole functions as +// patterns: +val patternFunc: Person => String = { + case Person("George", number) => s"George's number: $number" + case Person(name, number) => s"Random person's number: $number" +} -me match { case Person(name, number) => { - "We matched someone : " + name + ", phone : " + number }} -me match { case Person(name, number) => "Match : " + name; case _ => "Hm..." } +///////////////////////////////////////////////// +// 7. Functional Programming +///////////////////////////////////////////////// -me match { case Person("George", number) => "Match"; case _ => "Hm..." } +// Scala allows methods and functions to return, or take as parameters, other +// functions or methods. -me match { case Person("Kate", number) => "Match"; case _ => "Hm..." } +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 -me match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" } +// Anonymous functions can be used instead of named functions: +List(1, 2, 3) map (x => x + 10) -val kate = Person("Kate", "1234") +// 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) -kate match { case Person("Kate", _) => "Girl"; case Person("George", _) => "Boy" } +// 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 -// 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 +s.map(sq) -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 - }) -} +val sSquared = s. map(sq) -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" +sSquared.filter(_ < 10) +sSquared.reduce (_+_) -// Strings +// 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) +case class Person(name:String, age:Int) +List( + Person(name = "Dom", age = 23), + Person(name = "Bob", age = 30) +).filter(_.age > 25) // List(Person("Bob", 30)) -"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 -// Seealso: scala.collection.immutable.StringOps +// Scala a foreach method defined on certain collections that takes a type +// returning Unit (a void method) +val aListOfNumbers = List(1, 2, 3, 4, 10, 20, 100) +aListOfNumbers foreach (x => println(x)) +aListOfNumbers foreach println -println("ABCDEF".length) -println("ABCDEF".substring(2, 6)) -println("ABCDEF".replace("C", "3")) +// For comprehensions -// String interpolation -val n = 45 -println(s"We have $n apples") // => "We have 45 apples" +for { n <- s } yield sq(n) -// Expressions inside interpolated strings are also possible -val a = Array(11, 9, 6) -println(s"My second daughter is ${a(0) - a(2)} years old.") // => "My second daughter is 5 years old." -println(s"We have double the amount of ${n / 2.0} in apples.") // => "We have double the amount of 22.5 in apples." -println(s"Power of 2: ${math.pow(2, 2)}") // => "Power of 2: 4" +val nSquared2 = for { n <- s } yield sq(n) -// Formatting with interpolated strings (note the prefixed f) -println(f"Power of 5: ${math.pow(5, 2)}%1.0f") // "Power of 5: 25" -println(f"Square root of 122: ${math.sqrt(122)}%1.4f") // "Square root of 122" +for { n <- nSquared2 if n < 10 } yield n -// Ignoring special characters. -println(raw"New line feed: \n. Carriage return: \r.") // => "New line feed: \n. Carriage return: \r." +for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared -// Some characters need to be 'escaped', e.g. a double quote inside a string: -val a = "They stood outside the \"Rose and Crown\"" // => "They stood outside the "Rose and Crown"" +/* 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. */ -// Triple double-quotes let strings span multiple rows and contain quotes -val html = """<form id="daform"> - <p>Press belo', Joe</p> - | <input type="submit"> - </form>""" +///////////////////////////////////////////////// +// 8. Implicits +///////////////////////////////////////////////// +/* WARNING WARNING: Implicits are a set of powerful features of Scala, and + * therefore it is easy to abuse them. Beginners to Scala should resist the + * temptation to use them until they understand not only how they work, but also + * best practices around them. We only include this section in the tutorial + * because they are so commonplace in Scala libraries that it is impossible to + * do anything meaningful without using a library that has implicits. This is + * meant for you to understand and work with implicts, not declare your own. + */ -// Application structure and organization +// Any value (vals, functions, objects, etc) can be declared to be implicit by +// using the, you guessed it, "implicit" keyword. Note we are using the Dog +// class from section 5 in these examples. +implicit val myImplicitInt = 100 +implicit def myImplicitFunction(breed: String) = new Dog("Golden " + breed) + +// By itself, implicit keyword doesn't change the behavior of the value, so +// above values can be used as usual. +myImplicitInt + 2 // => 102 +myImplicitFunction("Pitbull").breed // => "Golden Pitbull" + +// The difference is that these values are now eligible to be used when another +// piece of code "needs" an implicit value. One such situation is implicit +// function arguments: +def sendGreetings(toWhom: String)(implicit howMany: Int) = + s"Hello $toWhom, $howMany blessings to you and yours!" + +// If we supply a value for "howMany", the function behaves as usual +sendGreetings("John")(1000) // => "Hello John, 1000 blessings to you and yours!" + +// But if we omit the implicit parameter, an implicit value of the same type is +// used, in this case, "myImplicitInt": +sendGreetings("Jane") // => "Hello Jane, 100 blessings to you and yours!" + +// Implicit function parameters enable us to simulate type classes in other +// functional languages. It is so often used that it gets its own shorthand. The +// following two lines mean the same thing: +def foo[T](implicit c: C[T]) = ... +def foo[T : C] = ... + + +// Another situation in which the compiler looks for an implicit is if you have +// obj.method(...) +// but "obj" doesn't have "method" as a method. In this case, if there is an +// implicit conversion of type A => B, where A is the type of obj, and B has a +// method called "method", that conversion is applied. So having +// myImplicitFunction above in scope, we can say: +"Retriever".breed // => "Golden Retriever" +"Sheperd".bark // => "Woof, woof!" + +// Here the String is first converted to Dog using our function above, and then +// the appropriate method is called. This is an extremely powerful feature, but +// again, it is not to be used lightly. In fact, when you defined the implicit +// function above, your compiler should have given you a warning, that you +// shouldn't do this unless you really know what you're doing. + + +///////////////////////////////////////////////// +// 9. Misc +///////////////////////////////////////////////// // Importing things import scala.collection.immutable.List |