I wrote about it here, you’ll find a concrete example. See also the blog posts from Nathaniel J Smith, esp. this one for a good explanation of the pitfalls associated with traditional imperative concurrency.
Maintaining a hierarchy of running processes has several benefits :
- when a process fails it can propagate the exception to its parent
- when a parent process detects a failing child it can make an appropriate decision such as cancelling the siblings
- the lifetime of child processes are enclosed by their parent, so resources allocated by children are guaranteed to be freed when the parent terminates
Now, when a process is forked with an imperative construct (e.g with future, go), it doesn’t know which one is its parent. You have to program defensively, and implement your own error handling and cancellation support. You may end up building a supervisor hierarchy by hand.
In contrast, in functional style, logical processes are values. If a process is defined as a composition of other processes, the parent-child relationship is made explicit by construction. When the process is ultimately spawned, the process data structure is naturally materialized into a supervisor hierarchy.