UnaryExpression ::= ... | "shift" UnaryExpression

Every function containing the shift operator in its body (not contained in any nested functions) implicitly creates a continuation object every time it is called. The continuation object encapsulates the function call’s activation object.

Let the current stack consist of S followed by activation object A. Let k be the continuation object of A. Let o be the binding of this in A.

1. Evaluate the argument expression to get a value v.
2. Capture the activation object A and set the internal [[Value]] property of k to A.
3. Remove A from the stack, leaving S as the current stack.
4. Call v with k as its single argument, with this bound to o.

A continuation object k has the following methods:

• send(x):
  1. Let A be the activation object in k‘s [[Value]] property.
  2. Push A onto the current stack.
  3. Use the value of x as the result of the suspended shift expression and continue evaluating the function activation.
• throw(x):
  1. Let A be the activation object in k‘s [[Value]] property.
  2. Push A onto the current stack.
  3. Use the value of x as an exception value to throw in place of the suspended shift expression and continue evaluating the function activation.
• close():
  1. Let A be the activation object in k‘s [[Value]] property.
  2. Push A onto the current stack.
  3. Replace the suspended shift expression with let () { return }.

Note that suspending the same activation object multiple times leads to the same (in the sense of ===) continuation object.

Note that every subsequent evaluation of shift for the same activation A produces the same continuation object k. This means that as execution of A proceeds, the continuation object k reflects the changing state of the computation.

function f() {
    try {
        for (let i = 0; i < K; i++) {
            farble(i);
 
            // suspend and return the activation
            let received = shift function(k) { return k };
 
            print(received); // this will be 42
        }
    }
    catch (e) { /* ... */ }
}
 
let k = f(); // starts running and return the suspended activation
// ...
k.send(42); // resume suspended activation

• allow the argument to be optional, defaulting to function (k) { return k }

Generators are a convenience form for creating custom iterators.

PrimaryExpression ::= ... | "generator" Identifier? FunctionArguments FunctionBody

GeneratorDeclaration ::= ... | "generator" Identifier FunctionArguments FunctionBody

Within a generator body, it is a syntax error for return to take an argument expression.

The expression generator(x1,...,xn) { body ... } is equivalent to:

function(x1,...,xn) {
    shift function(k) {
        return Object.freeze({
            send:  function(x) { return k.send(x); },
            next:  function()  { return k.send(void 0); },
            throw: function(x) { return k.throw(x); },
            close: function()  { k.close(); }
        });
    };
    body ...
    throw StopIteration;
}

for the original definitions of StopIteration and Object.freeze.

Inside a function body, a return statement (which may not have an argument) is equivalent to:

throw StopIteration;

for the original definition of StopIteration.

The expression yield e is equivalent to:

let (result = e) { => (shift function(k) { return result; }) }

A couple of quick notes before the TC39 meeting:

• yield is a low-precedence operator, at the same precedence as assignment in Python and JS1.7+. This means in an argument list or comma expression, you must parenthesize on the outside: foo(a, (yield b), c). Python requires parenthesization even if there’s only one argument: foo1), but JS1.7+ do not.
• generator instead of function (in JS1.7+, after Python which reuses its def function-declaring keyword) has benefit in terms of explicitness (you don’t have to look for yield usage in the body to know it’s a generator, not a function), but breaks the ability to use const instead of function (see const functions). const generator is a bit much, and not parallel (no const function).

— Brendan Eich 2010/05/24 04:47