Two
possibilities (but I’m not entirely sure how much sense they make):
-
Use a keyword that enables custom break and continue (problem: "for"
clashes with iterators):
for mycollection.each({ | x | if (x ===
0) break })
Really clashes with paren-free, which is not ready for promotion from
strawman. In a comprehension or generator _expression_ you must use
paren-free for/of syntax and only for/of. But could be confusing just on
this basis.
I still think we're talking about reifying break as an exception here.
That plus EIBTI mean we should do what Francois proposes, if we want to
support this form, and require "throw break".
- Standardize a BreakException. Then breaking isn’t the
loop’s responsibility, any more. `continue` as an early return seems
less important, because you can use break as follows:
mycoll.forEach { | x |
block: {
if (x < 0) {
break block; // "continue"
}
// do more with x here
}
}
But this is ugly, and if we have "throw break" we may as well have
"throw continue". Again I'm not on board, just noting the alternatives.
Why stop half-way *if* we want to reify as exceptions (which again, to
confirm, is what any mycoll.forEach break/continue hook-up would
require)?
/be
If I understand
your suggestion, you're proposing that non-local break and continue
should be exposed as standard exceptions, and then implementors of
loop-like abstractions could choose to catch them. E.g. you could
implement forEach as:
Array.prototype.forEach = function(f) {
for (let i = 0, n = this.length; i < n; i++) {
try {
f.call(this, this[i], i);
}
catch (e) {
if (e instanceof BreakException)
break;
else if (e instanceof
ContinueException)
continue;
else
throw e;
}
}
};
Whereas a function that does *not* want to expose whether
it's using loops would simply do nothing with BreakException and
ContinueException, and they would propagate out and you'd get the
lexical scoping semantics. Meanwhile, break/continue with an explicit
target would never be catch-able.
Did I understand your
suggestion correctly?
This *may* not violate TCP (I'm not quite
sure), but I'm not enthusiastic about the idea. The semantics is
significantly more complicated, and it requires you to understand
whether a higher-order function like forEach is catching these
exceptions or not. So it becomes an additional part of the API of a
function. If someone doesn't document what they do with BreakException
and ContinueException, then writing callbacks you won't actually be able
to predict what `break` and `continue` will do.
Dave
I think it’s a valid concern.
The idea is: If I can implement my own loops (the nice-looking
paren-free syntax feeds that illusion!) then I also want those loops to
have break and continue. You could statically determine what construct,
say, a break applies to and either throw a BreakException (if it applies
to a lambda) or TCP-break (if it applies to an enclosing non-lambda
loop). In the examples below, when I see a continue, I look for the
innermost enclosing loop braces and the ones belong to list[i].forEach
are definitely candidates.
Block
lambdas have been a hot topic, recently, but there's a point of
significant divergence between Ruby (which appears to be the
inspiration)
Not Ruby alone, and not in any chauvinist my-language-is-better sense.
Smalltalk is the original inspiration for Ruby blocks, and the
correspondence principle has deep roots.
and the
proposed solution, in the handling of continue (called 'next', in Ruby)
and 'break'.
To whit: In Ruby, 'next' will end the current run
(iteration) of the block, and 'break' will (somehow) terminate the
method lexically connected with the block. It can be claimed that this
is more intuitive than the current proposal, which aims to make 'break'
and 'continue' propagate through block lambdas in the same way 'return'
would.
"Intuitive" depends on intuition, which is not well-defined. Do you mean
a Rubyist might expect different behavior for break? That is possible
but JS ain't Ruby and break should not change to do something like what
it does in Ruby (and we aren't defining a next equivalent for JS).
Ruby does also support syntactic loops and the same keywords
therein and so directly violates Tennent's Correspondence Principle,
even though such has been touted as a core reason for the construct.
Instead, I believe it reasonable to invoke intuition in this matter. It
is intuitive for 'return' to return a value from the lexically enclosing
method and it is intuitive for 'continue' to commence the next
iteration of the current loop,
Wait, why do you think break and continue without label operands do
anything other than break from the nearest enclosing loop (or switch or
labeled statement if break), or continue the nearest enclosing loop? The
proposal specifies this.
function
find_odds_in_arrays(list, // array of arrays
skip) // if found, skip rest
{
let a = [];
for (let i = 0; i <
list.length; i++) {
list[i].forEach {
|e|
if (e === skip) {
continue; // continue the for loop
}
if (e & 1) {
a.push(e);
}
}
}
return a;
}
function
find_more_odds(list, stop) {
let a = [];
for (let i = 0; i <
list.length; i++) {
list[i].forEach {
|e|
if (e === stop) {
break; // break from the for loop
}
if (e & 1) {
a.push(e);
}
}
}
return a;
}
however that loop is constructed.
What do you mean by this? The spec talks about nearest enclosing loop or
relevant control structure in the source code. Are you talking about
internal loops in implementations (dynamically dispatched at that) of
methods that take block-lambdas as arguments? I.e.
function find_first_odd(a) {
a.forEach { |e, i|
if (e & 1)
return i; } // returns from function
return -1;
}
The Array.prototype.forEach method's internal implementation is its
business, and a break instead of the return would be a static error in
this example. It would not be a dynamic throw-like construct that is
caught by forEach's implementation.
/be
Block lambdas have been a hot
topic, recently, but there's a point of significant divergence between
Ruby (which appears to be the inspiration) and the proposed solution, in
the handling of continue (called 'next', in Ruby) and 'break'.
To whit: In Ruby, 'next' will end the current run
(iteration) of the block, and 'break' will (somehow) terminate the
method lexically connected with the block. It can be claimed that this
is more intuitive than the current proposal, which aims to make 'break'
and 'continue' propagate through block lambdas in the same way 'return'
would.
Ruby does also support syntactic loops and the same
keywords therein and so directly violates Tennent's Correspondence
Principle, even though such has been touted as a core reason for the
construct. Instead, I believe it reasonable to invoke intuition in this
matter. It is intuitive for 'return' to return a value from the
lexically enclosing method and it is intuitive for 'continue' to
commence the next iteration of the current loop, however that loop is
constructed.
Note that the label-based break/continue could still
have the desired effect, if the proposal was updated to be more like
Ruby's blocks.
I don't have a strong opinion on
the subject, but I hadn't noticed the above being discussed, elsewhere,
and thought it worth raising. If there is a better place for me to
raise this, please let me know where and accept my apologies.
Regards, Grant Husbands.
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