Spread Collections

[munificent]

Solution for #46.

Feature specification.

Most other languages have a "spread" or "splat" syntax that interpolates the elements of an existing collection into a new collection. In JS, it's a prefix .... In Ruby, Python, and a couple of others, it's a prefix *.

I propose we follow JS and use ....

This proposal is now accepted. Implementation work is being tracked in #164.

[zoechi]

Do you think there is place in this proposal to also consider
flutter/flutter#17862
where null values are dropped when spread?

[munificent]

I don't think it's a good idea to silently discard null values. That might be the right thing for some APIs, but others may consider null values to be useful and meaningful. If the spread syntax always discarded them, it means you could never use a spread with those APIs.

[zoechi]

I meant perhaps something like null-aware operators combined with spread to ignore null values, not ignoring them in general.
Just thought it might make sense to consider this use case when designing the spread feature.

[zoechi]

I guess an extension method/getter like someList.nonNullValues would do anyway

[cbazza]

Just keep in mind that when designing a spread operator it goes hand in hand with the syntactically identical rest operator which goes hand in hand with destructuring syntax ;)

[munificent]

Yes, the proposal doesn't mention that, but it was designed with destructuring and rest parameters in mind.

[munificent]

The proposal for this has now landed: https://github.com/dart-lang/language/blob/master/working/spread-collections/feature-specification.md

[alorenzen]

AngularDart makes heavy use of const lists as part of our configuration for the template compiler. In order to support items stored in another const list, we allow a "recursive" union type:
List<T | List>

According to your proposal, Spread elements are not allowed in const lists or maps.

Could this be adjusted to allow const elements to be spread in const lists?

For example,

const foo = [Foo];

static bar = [Bar];

const containingFoo = [
  ...foo, // Allowed
  ...bar, // Not allowed
];

[xster]

I don't know if it's the right protocol but what @zoechi pointed out is a common Flutter issue (though it may need an orthogonal category of solution). Branching that thread to #62

[nshahan]

It seems to me that there is a missing option in the discussion about the null aware spread operator.

In the the postfix version could the ? appear before the ...?

[foo?.bar?...]
// ^ |  ^   |
// '-'  '---'

In my eyes this seems more consistent with the existing null aware operator since the question mark appears directly after the value being checked for null.

[dnfield]

1. One potential source of confusion for JavaScript developers is that this syntax will apply to lists and maps, but not objects.
2. Can it apply to Iterables? If so, reading more closely, it looks like any iterable should work, so the points raised by @zoechi @xster become:

...listWithNullValues.where((entry) => entry != null),

I don't see this as being so burdensome that we should be trying to get a operator that does it automatically - and something doing it automatically could be very confusing to people who do want to preserve nulls in there.