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When people talk about threads, they are usually referring to a mechanism provided by the operating system. That's why we prefer to use the term "multitasking". Threads are generally "preemptive", whereas Euphoria multitasking is "cooperative". With preemptive threads, the operating system can force a switch from one thread to another at virtually any time. With cooperative multitasking, each task decides when to give up the CPU and let another task get control. If a task were "greedy" it could keep the CPU for itself for long intervals. However since a program is written by one person or group that wants the program to behave well, it would be silly for them to favor one task like that. They will try to balance things in a way that works well for the user. An operating system might be running many threads, and many programs, that were written by different people, and it would be useful to enforce a reasonable degree of sharing on these programs. Preemption makes sense across the whole operating system. It makes far less sense within one program.

Furthermore, threading is notorious for causing subtle bugs. Nasty things can happen when a task loses control at just the wrong moment. It may have been updating a global variable when it loses control and leaves that variable in an inconsistent state. Something as trivial as incrementing a variable can go awry if a thread-switch happens at the wrong moment. e.g. consider two threads. One has:

x = x + 1 

and the other also has:

x = x + 1 

At the machine level, the first task loads the value of x into a register, then loses control to the second task which increments x and stores the result back into x in memory. Eventually control goes back to the first task which also increments x *using the value of x in the register*, and then stores it into x in memory. So x has only been incremented once instead of twice as was intended. To avoid this problem, each thread would need something like:

lock x 
x = x + 1 
unlock x 

where lock and unlock would be special primitives that are safe for threading. It's often the case that programmers forget to lock data, but their program seems to run ok. Then one day, many months after they've written the code, the program crashes mysteriously.

Cooperative multitasking is much safer, and requires far fewer expensive locking operations. Tasks relinquish control at safe points once they have completed a logical operation.

Summary

For a complete function reference, refer to the Library Documentation Multitasking.