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| -rw-r--r-- | scala.html.markdown | 383 |
1 files changed, 212 insertions, 171 deletions
diff --git a/scala.html.markdown b/scala.html.markdown index 775956cb..dc039f0c 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 --- @@ -20,34 +21,47 @@ 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 +val b: Double = 10 // Notice automatic conversion from Int to Double, result is 10.0, not 10 // Boolean values true @@ -64,9 +78,11 @@ 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 @@ -78,23 +94,63 @@ 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 + +// Strings have the usual Java methods defined on them +"hello world".length +"hello world".substring(2, 6) +"hello world".replace("C", "3") -// Everything is an object, including a function. Type these in the REPL: +// They also have some extra Scala methods. See also: scala.collection.immutable.StringOps +"hello world".take(5) +"hello world".drop(5) -7 // results in res30: Int = 7 (res30 is just a generated var name for the result) +// 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 +///////////////////////////////////////////////// // The next line gives you a function that takes an Int and returns it squared -(x:Int) => x * x +(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> + + 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. @@ -105,28 +161,68 @@ val sq = (x:Int) => x * x 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 +// 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 +///////////////////////////////////////////////// +// 3. Flow Control +///////////////////////////////////////////////// -// Anonymous functions can be used instead of named functions: -List(1, 2, 3) map (x => x + 10) +1 to 5 +val r = 1 to 5 +r.foreach( println ) -// 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) +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 -// 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 +(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? -// Data structures +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 +241,8 @@ 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 */ @@ -175,110 +271,25 @@ 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) - -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 a foreach method defined on certain collections that takes a type -// returning Unit (a void method) -val aListOfNumbers = Set(1,2,3,45,234) -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) -} -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" - - - -// Object oriented features +/* + 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. +*/ -// Classname is Dog class Dog(br: String) { var breed: String = br //A method called bark, returning a String @@ -296,8 +307,6 @@ println(mydog.bark) // => "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) @@ -305,8 +314,12 @@ case class Person(name:String, phoneNumber:String) Person("George", "1234") == Person("Kate", "1236") +// Objects and traits coming soon! + -// Pattern matching +///////////////////////////////////////////////// +// 6. Pattern Matching +///////////////////////////////////////////////// val me = Person("George", "1234") @@ -345,49 +358,77 @@ matcher("52917") // => "No match on '52917'" matcher("52752-16432-22178-47917") // => "Serial key: 52752, 16432, 22178, 47917" -// Strings +///////////////////////////////////////////////// +// 7. Functional Programming +///////////////////////////////////////////////// -"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 +// Scala allows methods and functions to return, or take as parameters, other +// functions or methods. -// Seealso: scala.collection.immutable.StringOps +List(1, 2, 3) map add10 // List(11, 12, 13) - add10 is applied to each element -println("ABCDEF".length) -println("ABCDEF".substring(2, 6)) -println("ABCDEF".replace("C", "3")) +// Anonymous functions can be used instead of named functions: +List(1, 2, 3) map (x => x + 10) -// String interpolation -val n = 45 -println(s"We have $n apples") // => "We have 45 apples" +// 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) -// 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" +// 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 -// 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" -// Ignoring special characters. -println(raw"New line feed: \n. Carriage return: \r.") // => "New line feed: \n. Carriage return: \r." +// Combinators -// 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"" +s.map(sq) -// 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>""" +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, phoneNumber:String) +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) +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 +///////////////////////////////////////////////// +Coming soon! -// Application structure and organization +///////////////////////////////////////////////// +// 9. Misc +///////////////////////////////////////////////// // Importing things import scala.collection.immutable.List |