You should be familiar with lazy values from sequential programming in Scala. Lazy values are value declarations that are initialized with their right-hand side expression when the lazy value is read for the first time. This is unlike regular values, which are initialized the moment they are created. If a lazy value is never read inside the program, it is never initialized and it is not necessary to pay the cost of its initialization. Lazy values allow you to implement data structures such as lazy streams; they improve complexities of persistent data structures, can boost the program's performance, and help avoid initialization order problems in Scala's mixin composition.

Lazy values are extremely useful in practice, and you will often deal with them in Scala. However, using them in concurrent programs can have some unexpected interactions, and this is the topic of this section. Note that lazy values must retain the same semantics in a multithreaded program; a lazy...