Early reply provides concurrent processing. A running method returns control, and possibly a result, to the point from which it was called; meanwhile it continues running. The following figure illustrates this concept.

Method A includes a call to Method B. Method B contains a REPLY instruction. This returns control and a result to method A, which continues processing with the line after the call to Method B. Meanwhile, Method B also continues running.

The chains of execution represented by method A and method B are called activities. An activity is a thread of execution that can run methods concurrently with methods on other activities.

An activity contains a stack of invocations that represent the Rexx programs running on the activity. An invocation can be a main program invocation, an internal function or subroutine call, an external function or subroutine call, an INTERPRET instruction, or a message invocation. An invocation is activated when an executable unit is invoked and removed (popped) when execution completes. In Figure 12.1, “Early Reply”, the programs begins with a single activity. The activity contains a single invocation, method A. When method A invokes method B, a second invocation is added to the activity.

When method B issues a REPLY, a new activity is created (activity 2). Method B's invocation is removed from activity 1, and pushed on to activity 2. Because activities can execute concurrently, both method A and method B continue processing. The following figures illustrate this concept.

Here is an example of using early reply to run methods concurrently.

/* Example of early reply */

object1 = .example~new
object2 = .example~new

say object1~repeat(10, "Object 1 running")
say object2~repeat(10, "Object 2 running")
say "Main ended."
exit

::class example
::method repeat
use arg reps,msg
reply "Repeating" msg"," reps "times."
do reps
  say msg
end