Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

Compile-Time Constants

Earlier we told you that vars can have their values changed and vals cannot. That … was a white lie. In fact, there are special cases where a val can return different values, which we will discuss in Chapter 13. If you have a piece of data that you want to be absolutely, positively immutable – to never change – consider a compile-time constant.

A compile-time constant must be defined outside any function, including main, because its value must be assigned at compile time (that is, when the program compiles) – hence the name. main and your other functions are called during runtime (when the program is executed), and the variables within them are assigned their values then. A compile-time constant exists before ...