Chapter 10. Functions to Extension Functions

Kotlin has a special kind of procedure called an extension function, that is called like a method but is in fact (usually) a top-level function. It’s easy to convert from a normal function to an extension function and back. When should we prefer one to the other?

Functions and Methods

Object-oriented programming is the art of solving problems by sending messages to objects. Want to know the length of myString? Ask it by sending it a message myString.length(). Want to print that string to the console? Put the string in a message and ask another object representing the console to print it for you: System.out.println(myString). In classic OO languages, we define how an object reacts to a message by defining methods on classes. Methods are bound to their class and have access to the members (fields and other methods) associated with a particular instance. When we invoke a method, the runtime arranges for the correct version to be called (depending on the runtime type of the object), and for it to have access to instance state.

In contrast, in functional programming, we solve problems by calling functions with values. We find the length of myString by passing it to a function: length(myString). We print to the console with println(myString), and if we wanted to print somewhere else, we would pass that to the function: println(myString, System.err). Functions are not defined on a type; function parameters and results have a type.


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