Allow both optional positional and optional named arguments in the same function signature.

[lrhn]

Dart functions currently can have only either optional named parameters or optional positional parameters, but not both.

While developing libraries repeatedly run into cases where this restriction gets in the way of the signature we want.
It would be preferable if you could have both kinds of parameters on the same function.
The only syntax change necessary is to the declaration of functions. The call syntax is unaffected, and so is noSuchMethod/Function.apply.

[floitschG]

This issue is related to issue dart-lang/sdk#6496, but asks for less.

[DartBot]

This comment was originally written by @seaneagan

---

Is this asking for:

• 2 separate optional parameters within the same declaration, one named, one positional
• allowing a single optional parameter to be both named and positional

The former could be solved by non-overlapping [] and {}. If overlapping [] and {} are allowed, then would have to decide whether to allow both {[ and [{ as well as }] and ]].

[floitschG]

It is asking for non-overlapping [] and {}.
Examples:
new List([length = 0], {fill: null});
Stream.subscribe([onData], {onError, onDone})
new Date(year, [month, ...., milliseconds], {isUtc: false})

[gbracha]

Added this to the Later milestone.
Added Accepted label.

[DartBot]

This comment was originally written by @seaneagan

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Sounds perfect! The only other thing I would change (also mentioned in issue dart-lang/sdk#6496) is to use = instead of : for named positional default values:

new List([length = 0], {fill = null});

The = to default positional optionals doesn't mimic call site syntax, so why does the named positional default syntax try to? I think it's more importatnt to be consistent between how to default optional parameters regardless of whether they are named or positional.

[DartBot]

This comment was originally written by @rbishop-bah

---

Related: Issue dart-lang/sdk#17101

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[gbracha]

Issue dart-lang/sdk#17101 has been merged into this issue.

[lrhn]

Marked this as blocking dart-lang/sdk#21406.

[donny-dont]

Since 2.0 is thinking of some drastic changes is there any way the more extreme do like C#, Python, of dart-lang/sdk#6496 can be revisited.

@gbracha would a DEP be required for this?

[seaneagan]

@donny-dont if this issue is fixed then we will be able to do:

greet([String salutation = 'Hello'], {String who}) {
  print('$salutation $who!');
}

Then if there is really a need to allow parameters to be both named and positional, we could further allow something like this syntax:

greet({[String salutation = 'Hello', String who]}) {
  print('$salutation $who!');
}

But personally, I think that creates unnecessarily bloated API surface.

[bobjackman]

Any guesses when this might get implemented?