Enforcing arity
What is the recommended way of enforcing arity under ECMAScript 6 (while
avoiding arguments.length)?
Two ideas:
1. Parameter default value is a function call that throws an exception. Not
sure if that works.
function foo(required = throwAnException(), optional) { ... }
2. Introduce a postfix `!` matching operator, the inverse of postfix `?`:
function foo(required!, optional) { ... }
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Re: Enforcing arity
If you need to do arity analysis of parameter but also what to apply default
values, destructuring, etc I would do the following:
Instead of
function ([a,b], c,d=5, ...rest) {...}
do
function (...args) {
if (args.length 3) throw Error(too few arguments);
let [[a,b], c,d=5, ...rest] = args; //probably need some ?'s in there,
but I haven't internalized the details of the new pattern matching yet.
}
Allen
On Mar 16, 2013, at 5:02 PM, Axel Rauschmayer wrote:
What is the recommended way of enforcing arity under ECMAScript 6 (while
avoiding arguments.length)?
Two ideas:
1. Parameter default value is a function call that throws an exception. Not
sure if that works.
function foo(required = throwAnException(), optional) { ... }
2. Introduce a postfix `!` matching operator, the inverse of postfix `?`:
function foo(required!, optional) { ... }
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
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Re: Enforcing arity
On Mar 17, 2013, at 1:17 , Allen Wirfs-Brock al...@wirfs-brock.com wrote:
If you need to do arity analysis of parameter but also what to apply default
values, destructuring, etc I would do the following:
Instead of
function ([a,b], c,d=5, ...rest) {...}
do
function (...args) {
if (args.length 3) throw Error(too few arguments); // (*)
let [[a,b], c,d=5, ...rest] = args; //probably need some ?'s in
there, but I haven't internalized the details of the new pattern matching yet.
}
Nice, I forgot about rest parameters. You’d still get an exception from
matching if you omitted line (*), right?
Shame that we can’t have the destructuring semantics for parameters.
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Re: Enforcing arity
Would using (...args) incur a performance penalty and impair
optimization since the argument list has to be an array now? Or is
that still better than using 'arguments.length'? Enforcing arity is a
common enough (and important, IMO) pattern that I'd be wary of doing
it using a pattern that is going to slow everything down. Something
that makes every call create garbage seems like a likely candidate to
do that.
-kg
On Sat, Mar 16, 2013 at 5:24 PM, Axel Rauschmayer a...@rauschma.de wrote:
On Mar 17, 2013, at 1:17 , Allen Wirfs-Brock al...@wirfs-brock.com wrote:
If you need to do arity analysis of parameter but also what to apply default
values, destructuring, etc I would do the following:
Instead of
function ([a,b], c,d=5, ...rest) {...}
do
function (...args) {
if (args.length 3) throw Error(too few arguments); // (*)
let [[a,b], c,d=5, ...rest] = args; //probably need some ?'s in
there, but I haven't internalized the details of the new pattern matching
yet.
}
Nice, I forgot about rest parameters. You’d still get an exception from
matching if you omitted line (*), right?
Shame that we can’t have the destructuring semantics for parameters.
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
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Re: Enforcing arity
It's certainly plausible that an implementation could do the analysis to
recognize this pattern and implement it so there isn't an extra allocation or
any extra copying.
It it became an common perf. bottleneck you might expect the better
implementation to do so. If it isn't a common perf. issue, then it really
doesn't matter.
Allen
On Mar 16, 2013, at 5:27 PM, Kevin Gadd wrote:
Would using (...args) incur a performance penalty and impair
optimization since the argument list has to be an array now? Or is
that still better than using 'arguments.length'? Enforcing arity is a
common enough (and important, IMO) pattern that I'd be wary of doing
it using a pattern that is going to slow everything down. Something
that makes every call create garbage seems like a likely candidate to
do that.
-kg
On Sat, Mar 16, 2013 at 5:24 PM, Axel Rauschmayer a...@rauschma.de wrote:
On Mar 17, 2013, at 1:17 , Allen Wirfs-Brock al...@wirfs-brock.com wrote:
If you need to do arity analysis of parameter but also what to apply default
values, destructuring, etc I would do the following:
Instead of
function ([a,b], c,d=5, ...rest) {...}
do
function (...args) {
if (args.length 3) throw Error(too few arguments); // (*)
let [[a,b], c,d=5, ...rest] = args; //probably need some ?'s in
there, but I haven't internalized the details of the new pattern matching
yet.
}
Nice, I forgot about rest parameters. You’d still get an exception from
matching if you omitted line (*), right?
Shame that we can’t have the destructuring semantics for parameters.
--
Dr. Axel Rauschmayer
a...@rauschma.de
home: rauschma.de
twitter: twitter.com/rauschma
blog: 2ality.com
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Re: Enforcing arity?
On Sun, Jan 6, 2013 at 6:31 PM, Axel Rauschmayer a...@rauschma.de wrote:
What is the simplest way of enforcing an arity in ES6? Doesn’t it involve
arguments?
Hmm. Can you do this?
function f(x, y, ...[]) {}
-j
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Re: Enforcing arity?
Jason Orendorff wrote:
On Sun, Jan 6, 2013 at 6:31 PM, Axel Rauschmayer a...@rauschma.de
mailto:a...@rauschma.de wrote:
What is the simplest way of enforcing an arity in ES6? Doesn’t it
involve arguments?
Hmm. Can you do this?
function f(x, y, ...[]) {}
The question is, with refutable-by-default, does the empty array pattern
fail on any non-empty arraylike match target? If yes, then we need a way
to match prefixes of arraylikes (including empty prefixes!):
let [] = must_be_empty();
let [...many] = however_many();
let [first?, second?] = two();
I buy it!
/be
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Re: Enforcing arity?
I’m torn: On one hand, I love the cleverness of it and it is certainly a good
solution (and one that doesn’t necessitate introducing new language features).
On the other hand, I’m wondering if it wouldn’t be cleaner to have a
per-function flag that tells JavaScript to enforce the specified arity (which
can be done quite precisely, thanks to ES6’s advanced parameter handling). One
possibility for setting such a flag are Python-style decorators:
http://www.python.org/dev/peps/pep-0318/
Axel
On Jan 7, 2013, at 20:33 , Brendan Eich bren...@mozilla.com wrote:
Jason Orendorff wrote:
On Sun, Jan 6, 2013 at 6:31 PM, Axel Rauschmayer a...@rauschma.de
mailto:a...@rauschma.de wrote:
What is the simplest way of enforcing an arity in ES6? Doesn’t it
involve arguments?
Hmm. Can you do this?
function f(x, y, ...[]) {}
The question is, with refutable-by-default, does the empty array pattern fail
on any non-empty arraylike match target? If yes, then we need a way to match
prefixes of arraylikes (including empty prefixes!):
let [] = must_be_empty();
let [...many] = however_many();
let [first?, second?] = two();
I buy it!
/be
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Re: Enforcing arity?
Axel Rauschmayer wrote:
What is the simplest way of enforcing an arity in ES6? Doesn’t it
involve arguments?
function add(x, y) {
if (arguments.length !== 2) throw ...
}
To avoid `argument`, one could:
- ensure a maximum arity by adding a ...rest parameter and checking that
its length is 0.
- ensure a minimum arity, by giving y a default value and checking for it.
Would fail-fast destructuring work?
function add(...args) {
let [x,y] = args;
}
And it this would be possible, it could be in the signature:
function add(...[x,y,opt?]) {
}
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Herby
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Re: Enforcing arity?
On Sunday, January 6, 2013, Axel Rauschmayer wrote:
What is the simplest way of enforcing an arity in ES6? Doesn’t it involve
arguments?
function add(x, y) {
if (arguments.length !== 2) throw ...
}
To avoid `argument`, one could:
- ensure a maximum arity by adding a ...rest parameter and checking that
its length is 0.
I can't find the thread, but a similar question was asked sometime in the
last 6 months (I think??) and this was the answer that was ultimately
agreed on as the best way to check arity length.
- ensure a minimum arity, by giving y a default value and checking for it.
Explicit undefined arguments, intentional or not, would trigger the default
value.
Rick
Would fail-fast destructuring work?
function add(...args) {
let [x,y] = args;
}
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