=== Brendan Eich wrote ===
This doesn't address Herby's TCP-violating wish for a non-return that
completes the block-lambda's control flow normally with a value (the message
to which I was replying). But you're right that it wouldn't violate TCP
either if we support it the same in a block statement as in a block-lambda
downward funarg.
===
No. it wasn't my primary wish to have a local return. I wanted to make break
and/or continue _semantics_ for lambda-block-loop-like-constructs.
So since local return is already used for 'continue' in forEach, I just
generalized the syntax continue |expr|; (note the | bars, they are part of
the syntax) to do the local return, thereby functioning as a continue
statement. (It would be convenient to have local return, but not the local
return itself was the goal).
You are true it breaks TCP. (It could be done so that it does not by
generalizing the syntax so it works in syntax-loop construct as well with
"end loop body with expr as the completion value" semantics; it's only btw,
it's too crazy to be considered, probably) So it cannot be used. :-/
By "this is de-facto continue" I was thinking more in higher semantic
level - continue as in "continue past this block", which in loops means "to
the next iteration" but beyond loops it may mean anything that is going to
happen after block completes.
Also, break is hard to do similar way, because I count out (automatically
set up) exceptions (I still live in the impression they are,
performance-wise, expensive). It seems to be possible to have "break |expr|
label;" syntax working: if the function calling the lambda-block is labeled,
it should be possible to just make it return the expr, but it is clumsy (and
there is no label-less version). (This "break |expr| label" may, too, be
generalized if TCP is of concern; again just BTW).
Overall, I am a bit optimistic, since (if I am not mistaken) lambda-blocks
only work inside its scope (as Self blocks, not as Smalltalk blocks), which
saves a lot of complications.
Herby
-----Pôvodná správa-----
From: Brendan Eich
Sent: Saturday, January 14, 2012 10:16 PM
To: François REMY
Cc: Herby Vojčík ; es-discuss@mozilla.org
Subject: Re: Block Lambdas: break and continue
François REMY
January 14, 2012 1:01 PM
If we want to avoid to break TCP, we can go with “throw break;” and “throw
continue;”.
This doesn't address Herby's TCP-violating wish for a non-return that
completes the block-lambda's control flow normally with a value (the message
to which I was replying). But you're right that it wouldn't violate TCP
either if we support it the same in a block statement as in a block-lambda
downward funarg.
It would throw a new BreakException or a new ContinueException, from the
place where they are executed. If it’s outside a block lambda, it’s outside
a block lambda. It doesn’t matter.
Yes, this would avoid TCP violations but not carry a return value -- Herby's
wish.
But it would set a “standard” for breaking throug ‘function loops’.
I considered this in drafting the block-lambda revival strawman. Other
languages have gone here. Nevertheless, I would like to leave it out
(remember N. Wirth on language design). It adds more complexity for a
use-case that I bet is rare (in any case it needs credible demonstration of
being quite common).
The complexity in the semantics is one issue Dave raised. This corresponds
to complexity for optimizing engines, compared to the purely static
break/continue semantics in the strawman.
Finally, the Array extras ship sailed. People already have to use some or
every in lieu of a break-from-forEach. Using a function callback with
forEach, one needs only to return to simulate continue. Now if we do
standardize block-lambdas and throw break or throw continue, we certainly
can elaborate the extras to catch these exceptions.
Such a more complex design seems workable with the costs noted above. But
will the benefits really outweigh those costs? I doubt it. First, Array
forEach and other uses will continue to use functions for quite a while, or
else a compiler from new standard JS to old. In the compiler case, throw and
try/catch will be required, and the compiler will have to monkey-patch the
extras to deal with the new exceptions. This will be a performance killer,
and no fun to debug.
So my thinking remains that we are better off, when in doubt, leaving
reified break and continue exceptions "out".
/be
François
From: Brendan Eich
Sent: Saturday, January 14, 2012 9:51 PM
To: Herby Vojčík
Cc: es-discuss@mozilla.org
Subject: Re: Block Lambdas: break and continue
Herby Vojčík
January 14, 2012 10:42 AM
=== David Herman wrote ===
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.
===
What about the exception-less suggestion I put in? It should work in any
loop construct with lambda-block, even if you must know a little about the
loop implementation itself. That is, to be able to put:
continue |expression|;
Who says the block-lambda is being called from a loop at all? Why should
use-cases that want an early result and completion have to use continue,
which is for loops?
Worse, this violates TCP. Now you copy and paste this block-lambda code back
into a block statement to refactor the other direction, and no such "here is
the completion value, do not flow past this point in the block" semantics
obtain.
as a statement in lambda block which instructs the lambda-block itself (not
the outer function) to return the expression? This is the de-facto continue
semantics (lambda-block, do return a value and the enclosing loop will
continue to the next iteration (possibly stopping the loop if it chooses not
to have more iterations)).
No it's not. There is no de-facto continue semantics for block-lambdas
because they haven't been prototyped. For block statements, no such continue
semantics exists.
It is not possible to enforce break in the same manner, but for continue, it
is possible.
It's possible to abuse any existing keyword, but first: why must there be a
new TCP violation? Block-lambda bodies are often expressions, or if
statements, then short/functional-style statements, not large bodies
demonstrating early-normal-completion opportunities.
We should not eliminate TCP violations only to add new ones, especially
without any evidence they're needed and pay their way. Otherwise we'll get
an infinite regress of TCP-pure-then-add-new-exceptions-and-repeat
additions.
/be
Herby
-----Pôvodná správa----- From: David Herman
Sent: Saturday, January 14, 2012 6:12 PM
To: Axel Rauschmayer
Cc: Brendan Eich ; es-discuss@mozilla.org
Subject: Re: Block Lambdas: break and continue
On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote:
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
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
David Herman
January 14, 2012 9:12 AM
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
Axel Rauschmayer
January 13, 2012 9:04 PM
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.
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
Brendan Eich
January 13, 2012 8:54 PM
Grant Husbands
January 13, 2012 7:29 PM
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
Grant Husbands
January 13, 2012 7:29 PM
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.
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
_______________________________________________ es-discuss mailing list
es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss
Brendan Eich
January 14, 2012 12:51 PM
Herby Vojčík
January 14, 2012 10:42 AM
=== David Herman wrote ===
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.
===
What about the exception-less suggestion I put in? It should work in any
loop construct with lambda-block, even if you must know a little about the
loop implementation itself. That is, to be able to put:
continue |expression|;
Who says the block-lambda is being called from a loop at all? Why should
use-cases that want an early result and completion have to use continue,
which is for loops?
Worse, this violates TCP. Now you copy and paste this block-lambda code back
into a block statement to refactor the other direction, and no such "here is
the completion value, do not flow past this point in the block" semantics
obtain.
as a statement in lambda block which instructs the lambda-block itself (not
the outer function) to return the expression? This is the de-facto continue
semantics (lambda-block, do return a value and the enclosing loop will
continue to the next iteration (possibly stopping the loop if it chooses not
to have more iterations)).
No it's not. There is no de-facto continue semantics for block-lambdas
because they haven't been prototyped. For block statements, no such continue
semantics exists.
It is not possible to enforce break in the same manner, but for continue, it
is possible.
It's possible to abuse any existing keyword, but first: why must there be a
new TCP violation? Block-lambda bodies are often expressions, or if
statements, then short/functional-style statements, not large bodies
demonstrating early-normal-completion opportunities.
We should not eliminate TCP violations only to add new ones, especially
without any evidence they're needed and pay their way. Otherwise we'll get
an infinite regress of TCP-pure-then-add-new-exceptions-and-repeat
additions.
/be
Herby
-----Pôvodná správa----- From: David Herman
Sent: Saturday, January 14, 2012 6:12 PM
To: Axel Rauschmayer
Cc: Brendan Eich ; es-discuss@mozilla.org
Subject: Re: Block Lambdas: break and continue
On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote:
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
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
David Herman
January 14, 2012 9:12 AM
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
Axel Rauschmayer
January 13, 2012 9:04 PM
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.
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
Brendan Eich
January 13, 2012 8:54 PM
Grant Husbands
January 13, 2012 7:29 PM
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
Grant Husbands
January 13, 2012 7:29 PM
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.
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
Herby Vojčík
January 14, 2012 10:42 AM
=== David Herman wrote ===
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.
===
What about the exception-less suggestion I put in? It should work in any
loop construct with lambda-block, even if you must know a little about the
loop implementation itself. That is, to be able to put:
continue |expression|;
as a statement in lambda block which instructs the lambda-block itself (not
the outer function) to return the expression? This is the de-facto continue
semantics (lambda-block, do return a value and the enclosing loop will
continue to the next iteration (possibly stopping the loop if it chooses not
to have more iterations)). It is not possible to enforce break in the same
manner, but for continue, it is possible.
Herby
-----Pôvodná správa----- From: David Herman
Sent: Saturday, January 14, 2012 6:12 PM
To: Axel Rauschmayer
Cc: Brendan Eich ; es-discuss@mozilla.org
Subject: Re: Block Lambdas: break and continue
On Jan 13, 2012, at 9:04 PM, Axel Rauschmayer wrote:
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
_______________________________________________
es-discuss mailing list
es-discuss@mozilla.org
https://mail.mozilla.org/listinfo/es-discuss
David Herman
January 14, 2012 9:12 AM
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
Axel Rauschmayer
January 13, 2012 9:04 PM
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.
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
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