Growable list constructor

[DartBot]

This issue was originally filed by sugashi.s...@gmail.com

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What steps will clearly show the issue / need for enhancement?

1. add 'growable' parameter to new List()
   2.
   3.

What is the current output?

  var list = new List()..length = 10;

What would you like to see instead?

  var list = new List(10, growable:true);

Please provide any additional information below.

  growable should be true in default.

[lrhn]

The current List constructor is designed the way it is in order to allow compilers to statically detect whether the resulting list is growable or not.

That is why new List(null) is not the same as new List(), even if the constructor appears to be declared as factory List([int n]).
If we add another parameter, it would not be possible to detect the type of list created in all cases (if the growable parameter isn't a compile-time constant).

That said, I'm not unsympathetic to the suggestion, even if it will add overhead to the simple case.

The growable parameter has to default to null, so that the constructor retains its current behavior - growable if no arguments and not growable if one argument. With two arguments, it will then depend on the value of growable.

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Added Library-Core, Triaged labels.

[lrhn]

One complication, which is probably fatal for this idea, is that we can't add growable as a named parameter, because the function already has an optional positional parameter, and Dart doesn't allow both optional positional and optional named parameters on the same function.

[DartBot]

This comment was originally written by sugashi.swi...@gmail.com

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Thanks.I understand why cannot add growable as a named parameter.

But I want to know a possibility if growable are the second positional paramerter.

that means,

new List(); //growable and empty
new List(10); //fixed
new List(10, true); //growable

[lrhn]

It's possible, but I think it would not be desirable.
All other places where a "growable" parameter is used, it is named. Having it positional here, just because it can't be named, will be confusing and error-prone.

Since an easy workaround is to create a factory method:

    List makeList(int length, {bool growable: true}) =>
        growable ? []..length = length : new List(length);

and use that instead of new List, it's not an urgent need.

(It's slightly harder if you want type parameters:
    class ListMaker<T> { create(int length, {bool growable: true}) =>
                          growable ? <T>[]..length = length : new List<T>(length); }
then replace new List<T>(..) with new ListMaker<T>.create(length, growable: false).

I'd rather wait until Dart allows both optional named and positional arguments on the same function (if that ever happens, but I'll keep crossing my fingers, and starring http://dartbug.com/7056).

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Added label.

[lrhn]

Added Triaged label.
Marked this as being blocked by #7056.

[floitschG]

Fwiw it should be easy to just write: []..length=XY;
The VM and dart2js could recognize that pattern.