Merge pull request #857 from geoffliu/master

[Scala/en] Reorganize the Scala tutorial

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