layout: post
title: Threads
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7 November 2011

A thread object is a kind of function references; makethread turns a function reference into a thread function reference;

What if the function reference has captures / non local references ? One can either

• disallow use of such a function reference as thread
• moved the captured references from stack to heap, and capture variables are adjusted to the stack.

When the function is called via this reference, then a new interpreter thread is created, the new thread has its own stack. Function parameters are pushed onto the new stack. The new thread stack has a reference to the function object; so that both threads (invoking and created thread) hold a reference to the function object.

When the thread calls the yield function for the first time (or returns a value) , then the invocation of the function reference from calling thread returns the yield/return value.

The calling thread can later call resume on the function object; this will pass controll back to the thread function; resume can receive an argument, a message from calling thread to new thread; this value will be returned by the yield function, which has been called previously by the new thread.
A thread terminates, when it calls return ; The calling thread can check if the thread is running by calling

The function object that represents the thread will need to have a reference to the thread (in order to pass values around and resume and stuff)
the thread has a reference to the parent thread + calling function object (in order to pass values around).

Passing control to the thread

Controll passes back to thre thread if

• Implicitly if generator function is called to produce a value.
• By means of thread object; (in this case the thread can get a message from the calling thread)

Passing value back to creator of the thread

When a thread Yields a value, or exits, then control is passed back to the invoking thread.

• On thread exit the calling function gets the Nil value; on Yield it gets a copy of the expression (if pointer to heap then reference to heap is copied).
• The FOR loop will use the yielded value and assign it to loop invariant for use in next iteration.
• The thread object can return the yielded value by special accessor function.

Yield control to arbitrary thread

we don’t have that here.