Merge branch 'master' of https://github.com/torgeek/learnxinyminutes-docs

1 files changed, 64 insertions, 13 deletions

diff --git a/scala.html.markdown b/scala.html.markdown
index d33b6234..08fd37e4 100644
--- a/scala.html.markdown
+++ b/scala.html.markdown
@@ -25,7 +25,7 @@ Scala - the scalable language
 /*
   Try the REPL
 
-  Scala has a tool called the REPL (Read-Eval-Print Loop) that is anologus to
+  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.  
 
@@ -252,7 +252,7 @@ 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:
+// 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)
@@ -260,9 +260,10 @@ def foo(x: Int): Int = {
     else
       z + 2    // This line is the return value of anonFunc
   }
-  anonFunc(x)  // This line is the return value of foo
+  anonFunc(x) + 10  // This line is the return value of foo
 }
 
+foo(7) // => 7
 
 /////////////////////////////////////////////////
 // 3. Flow Control
@@ -276,6 +277,8 @@ 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
@@ -299,7 +302,7 @@ do {
 // 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.
+// Here it's Unit, which is analagous to a `void` return type in Java
 def showNumbersInRange(a: Int, b: Int): Unit = {
   print(a)
   if (a < b)
@@ -343,7 +346,7 @@ s(0)      // Boolean = false
 s(1)      // Boolean = true
 
 /* Look up the documentation of map here -
- * http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map
+ * https://www.scala-lang.org/api/current/scala/collection/immutable/Map.html
  * and make sure you can read it
  */
 
@@ -412,8 +415,8 @@ class Dog(br: String) {
   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 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
 }
@@ -455,13 +458,57 @@ george.phoneNumber  // => "1234"
 Person("George", "1234") == Person("Kate", "1236")  // => false
 
 // Easy way to copy
-// otherGeorge == Person("george", "9876")
+// 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> b  
+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"
+}
 
-// Traits coming soon!
+scala> val b = new SaintBernard
+b: SaintBernard = SaintBernard@7b69c6ba
+scala> b.bark
+res0: String = Woof
 
 
 /////////////////////////////////////////////////
@@ -479,7 +526,9 @@ def matchPerson(person: Person): String = person match {
   case Person(name, number)     => "We matched someone : " + name + ", phone : " + number
 }
 
-val email = "(.*)@(.*)".r  // Define a regex for the next example.
+// 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
@@ -545,6 +594,8 @@ List("Dom", "Bob", "Natalia") foreach println
 
 
 // Combinators
+// Using `s` from above:
+// val s = Set(1, 3, 7)
 
 s.map(sq)
 
@@ -564,8 +615,8 @@ List(
 ).filter(_.age > 25) // List(Person("Bob", 30))
 
 
-// Scala a foreach method defined on certain collections that takes a type
-// returning Unit (a void method)
+// 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
@@ -666,7 +717,7 @@ import scala.collection.immutable.{Map => _, Set => _, _}
 // Java classes can also be imported. Scala syntax can be used
 import java.swing.{JFrame, JWindow}
 
-// Your programs entry point is defined in an scala file using an object, with a
+// Your programs entry point is defined in a scala file using an object, with a
 // single method, main:
 object Application {
   def main(args: Array[String]): Unit = {