Allow required named parameters

[DartBot]

This issue was originally filed by giris...@gmail.com

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I really like the named parameters feature in dart, but was disappointed that it can only be used if the parameter is optional. Is there any reason why it is not allowed in general?

It is really usefull to write

  flipFlags(on: true, up: false, hidden: true);,

even if the parameters were not optional. I some cases, i dont want to use optional parameters but still like to call a function with named parameters.

[dgrove]

Set owner to @gbracha.
Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

We've considered this quite recently and decided against it. In principle, making named parameters and optionality orthogonal is elegant. However, the interactions between the various situations (named, optional) add complexity to the runtime, and this particular situation doesn't strike us as very compelling. We are looking at allowing optional positional parameters, which does seem to have more justification.

I'm renaming the bug so it is clear what it is about.

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Added NotPlanned label.
Changed the title to: "Allow required named parameters".

[DartBot]

This comment was originally written by giri...@gmail.com

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optional positional parameters seems good enough. The advantage of this is mainly that it makes the code more readable, rather than "you dont need to guess the position of params". It should be easily implementable by just ignoring the names.

[DartBot]

This comment was originally written by dford...@smart-soft.com

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I have (in Java) preferred constructors (over setters) for initializing "required" fields. The problem is that when the list of constructor args gets really long, the code becomes less readable:

ProductCatalog pc = new ProductCatalog(dep1,dep2,dep3,dep4);

The whole reason i prefer constructor initialization is so that the compiler/ide can make sure i didn't forget a required param.

Does Dart address this is some other way?

[gbracha]

Regarding comment 4: The forums are better palce for this question, but here goes:

Optional parameters may be helpful here, as you may be able to provide reasonable defaults in some cases and reduce the need for of long lists of actual parameters.

Also, because dart constructors do not undermine encapsulation the way traditional constructors do, using them is not an antipattern as it is in Java (regardless of how popular it may be).

The use of getters can be helpful as well, as you can initialize such fields lazily with much less syntactic overhead than in Java.

[DartBot]

This comment was originally written by @seaneagan

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One thing this would help with is initially making args required, and then once you find a good default value, changing them to optional.

[filiph]

It is worth noting that the meta package (maintained by the Dart Team) has, among other things, the @required annotation.

To get to the #4188 (comment) use case, one would simply write.

import 'package:meta/meta.dart';

void flipFlags({@required bool on, @required bool up, @required bool hidden}) {
  // ...
}

The analyzer understands these annotations and will provide warnings at call site. You can even give reason, like @required('The callback needs to be set').

We might want to give this package a bit more visibility in our docs. The package is version 1.0.2 as of August 16.