mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-11-22 12:32:09 +03:00
23 KiB
23 KiB
| language | filename | contributors | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Scala | learnscala.scala |
|
Scala - the scalable language
/////////////////////////////////////////////////
// 0. Basics
/////////////////////////////////////////////////
/*
Setup Scala:
1) Download Scala - http://www.scala-lang.org/downloads
2) Unzip/untar to your favorite location and put the bin subdir in your `PATH` environment variable
*/
/*
Try the REPL
Scala has a tool called the REPL (Read-Eval-Print Loop) that is analogous to
commandline interpreters in many other languages. You may type any Scala
expression, and the result will be evaluated and printed.
The REPL is a very handy tool to test and verify code. Use it as you read
this tutorial to quickly explore concepts on your own.
*/
// Start a Scala REPL by running `scala`. You should see the prompt:
$ scala
scala>
// By default each expression you type is saved as a new numbered value
scala> 2 + 2
res0: Int = 4
// Default values can be reused. Note the value type displayed in the result..
scala> res0 + 2
res1: Int = 6
// Scala is a strongly typed language. You can use the REPL to check the type
// without evaluating an expression.
scala> :type (true, 2.0)
(Boolean, Double)
// REPL sessions can be saved
scala> :save /sites/repl-test.scala
// Files can be loaded into the REPL
scala> :load /sites/repl-test.scala
Loading /sites/repl-test.scala...
res2: Int = 4
res3: Int = 6
// You can search your recent history
scala> :h?
1 2 + 2
2 res0 + 2
3 :save /sites/repl-test.scala
4 :load /sites/repl-test.scala
5 :h?
// Now that you know how to play, let's learn a little scala...
/////////////////////////////////////////////////
// 1. Basics
/////////////////////////////////////////////////
// Single-line comments start with two forward slashes
/*
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)
// Hello world!
// 10
// Printing, without forcing a new line on next print
print("Hello world")
print(10)
// Hello world10
// Declaring values is done using either var or val.
// val declarations are immutable, whereas vars are mutable. Immutability is
// a good thing.
val x = 10 // x is now 10
x = 20 // error: reassignment to val
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:
*/
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
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
6 / 4 // 1
6.0 / 4 // 1.5
6 / 4.0 // 1.5
// Evaluating an expression 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
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
// Strings have the usual Java methods defined on them
"hello world".length
"hello world".substring(2, 6)
"hello world".replace("C", "3")
// They also have some extra Scala methods. See also: scala.collection.immutable.StringOps
"hello world".take(5)
"hello world".drop(5)
// String interpolation: notice the prefix "s"
val n = 45
s"We have $n apples" // => "We have 45 apples"
// 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"
// 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"
// Raw strings, ignoring special characters.
raw"New line feed: \n. Carriage return: \r." // => "New line feed: \n. Carriage return: \r."
// 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""
// 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>"""
/////////////////////////////////////////////////
// 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
}
// The { } can be omitted if the function body is a single expression:
def sumOfSquaresShort(x: Int, y: Int): Int = x * x + y * y
// Syntax for calling functions is familiar:
sumOfSquares(3, 4) // => 25
// You can use parameters names to specify them in different order
def subtract(x: Int, y: Int): Int = x - y
subtract(10, 3) // => 7
subtract(y=10, x=3) // => -7
// 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
// 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
// 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 here you may expect foo(7) should return 17 but it returns 7:
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) + 10 // This line is the return value of foo
}
foo(7) // => 7
/////////////////////////////////////////////////
// 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
// Why doesn't `println` need any parameters here?
// Stay tuned for first-class functions in the Functional Programming section below!
(5 to 1 by -1) foreach (println)
// A while loop
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, but using the combinators and comprehensions above is easier
// to understand and parallelize
// A do-while loop
i = 0
do {
println("i is still less than 10")
i += 1
} while (i < 10)
// Recursion is the idiomatic way of repeating an action in Scala (as in most
// other functional languages).
// Recursive functions need an explicit return type, the compiler can't infer it.
// Here it's Unit, which is analogous to a `void` return type in Java
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) // Int = 1
a(3) // Int = 5
a(21) // Throws an exception
val m = Map("fork" -> "tenedor", "spoon" -> "cuchara", "knife" -> "cuchillo")
m("fork") // java.lang.String = tenedor
m("spoon") // java.lang.String = cuchara
m("bottle") // Throws an exception
val safeM = m.withDefaultValue("no lo se")
safeM("bottle") // java.lang.String = no lo se
val s = Set(1, 3, 7)
s(0) // Boolean = false
s(1) // Boolean = true
/* Look up the documentation of map here -
* https://www.scala-lang.org/api/current/scala/collection/immutable/Map.html
* 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)
// The function divideInts gives you the result and the remainder
divideInts(10, 3) // (Int, Int) = (3,1)
// To access the elements of a tuple, use _._n where n is the 1-based index of
// the element
val d = divideInts(10, 3) // (Int, Int) = (3,1)
d._1 // Int = 3
d._2 // Int = 1
// Alternatively you can do multiple-variable assignment to tuple, which is more
// convenient and readable in many cases
val (div, mod) = divideInts(10, 3)
div // Int = 3
mod // Int = 1
/////////////////////////////////////////////////
// 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.
*/
// 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")
// Abstract methods are simply methods with no body. If we uncomment the
// def line below, class Dog would need to be declared abstract like so:
// abstract class Dog(...) { ... }
// def chaseAfter(what: String): String
}
val mydog = new Dog("greyhound")
println(mydog.breed) // => "greyhound"
println(mydog.bark) // => "Woof, woof!"
// 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 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")
// And many others. Case classes also get pattern matching for free, see below.
// Traits
// Similar to Java interfaces, traits define an object type and method
// signatures. Scala allows partial implementation of those methods.
// Constructor parameters are not allowed. Traits can inherit from other
// traits or classes without parameters.
trait Dog {
def breed: String
def color: String
def bark: Boolean = true
def bite: Boolean
}
class SaintBernard extends Dog {
val breed = "Saint Bernard"
val color = "brown"
def bite = false
}
scala> val b = new SaintBernard
res0: SaintBernard = SaintBernard@3e57cd70
scala> b.breed
res1: String = Saint Bernard
scala> b.bark
res2: Boolean = true
scala> b.bite
res3: Boolean = false
// A trait can also be used as Mixin. The class "extends" the first trait,
// but the keyword "with" can add additional traits.
trait Bark {
def bark: String = "Woof"
}
trait Dog {
def breed: String
def color: String
}
class SaintBernard extends Dog with Bark {
val breed = "Saint Bernard"
val color = "brown"
}
scala> val b = new SaintBernard
b: SaintBernard = SaintBernard@7b69c6ba
scala> b.bark
res0: String = Woof
/////////////////////////////////////////////////
// 6. Pattern Matching
/////////////////////////////////////////////////
// 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.
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
}
// Regular expressions are also built in.
// Create a regex with the `r` method on a string:
val email = "(.*)@(.*)".r
// 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"
// You can match by type:
case x: Double => "Got a Double: " + x
// You can specify conditions:
case x: Int if x > 10000 => "Got a pretty big number!"
// 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"
// You can match data structures:
case List(1, b, c) => s"Got a list with three elements and starts with 1: 1, $b, $c"
// You can nest patterns:
case List(List((1, 2, "YAY"))) => "Got a list of list of tuple"
// Match any case (default) if all previous haven't matched
case _ => "Got unknown object"
}
// 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"
}
/////////////////////////////////////////////////
// 7. Functional Programming
/////////////////////////////////////////////////
// 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)
// 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
// Using `s` from above:
// val s = Set(1, 3, 7)
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)
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))
// Certain collections (such as List) in Scala have a `foreach` method,
// which takes as an argument a type returning Unit - that is, a void method
val aListOfNumbers = List(1, 2, 3, 4, 10, 20, 100)
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
/////////////////////////////////////////////////
/* 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 implicits, not declare your own.
*/
// 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
// 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 => _, _}
// Java classes can also be imported. Scala syntax can be used
import java.swing.{JFrame, JWindow}
// Your program's entry point is defined in a 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.fromFile("myfile.txt").getLines())
println(line)
// To write a file use Java's PrintWriter
val writer = new PrintWriter("myfile.txt")
writer.write("Writing line for line" + util.Properties.lineSeparator)
writer.write("Another line here" + util.Properties.lineSeparator)
writer.close()