Implicits

scala.html.markdown

@@ -507,7 +507,60 @@ for { n <- s; nSquared = n * n if nSquared < 10} yield nSquared
 // 8. Implicits
 /////////////////////////////////////////////////
 
-// Coming soon!
+/* 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.
+ */
+
+// 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 elligible 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.
 
 
 /////////////////////////////////////////////////