[Sorry for my absence lately.]
If yield does work, then we're capturing deep stacks. A more
plausible example:
function j(x) {
(lambda (y) {
(lambda (z) {
yield z;
}
(y + " and a dyne"));
}
(x + ", a poundal"));
}
print(j("I love you").next());
This is just a desugaring of some nested lets, but we still have
yield capturing many frames. If this doesn't work, lambda is really
useless.
This issue really comes down to whether `lambda' is properly tail
calling. If so, then the `yield' in this example occurs as if it were
directly within the body of `j', because there's no stack beyond the
activation frame of `j'.
To be a little more precise: when people refer to "Tennent," the main
property they're talking about is that (modulo syntax) an expression E
is equivalent to the expression (lambda () E)(). Namely, in any
expression context (roughly, a position in a program where you can
place an expression), either expression will result in the same
program behavior.
If we don't want generators to capture deep stacks, then we can make
it a dynamic error for `yield' to occur in a non-empty sub-stack. Then
in order to preserve the property that (lambda () E)() for all E, we'd
have to mandate that (lambda () E)() is really a tail call and
guaranteed not to push any stack.
Alternatively, we could allow generators to capture deep lambda-
stacks, but not deep function-stacks. However...
For what it's worth, speaking as a long-time Scheme fan, I wouldn't
add lambda to ES. It seems too similar to function; there will be
endless blog posts explaining the differences and motivation, mostly
slightly wrong. The best ones will say, "Don't use it; just use
function." And lambda introduces an awful lot of subtlety for
something whose main claim to utility would be in allowing precise
and clear definitions of new control constructs through desugaring.
Natural language is bad, but not this bad.
I hear you. I love lambda, I love the syntactic simplicity of
Smalltalk blocks (and we could probably approximate its syntactic
simplicity with a literal syntax), and I love the idea of adding a
properly tail-calling function form. Nevertheless, I recognize that
the similarity to `function' is a usability hazard.
Dave
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