ArgsResult should have a way to determine if the user actual set the value

[DartBot]

Originally opened as dart-lang/sdk#16227

This issue was originally filed by [email protected]

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Currently there is no way to differentiate between the user specifying the default value and the user not specifying anything, both result in the default value being in ArgResults. This causes a problem when you have default values and multiple sources of configuration information that you'd like to merge. For example if you would like to have a default configuration that you to allow be overridden by environment variables and command line arguments, with command line arguments having the highest priority.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added Area-Library, Library-Args, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Area-Library label.
Added Area-Pkg label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="48" height="48"hspace="10"> Comment by seaneagan

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Interesting. Dart used to have an equivalent feature for function arguments, until it was removed by popular demand (issue dart-lang/sdk#9097).

If the default value is calculated like that, then does it make sense to have an explicit default value? Seems like it might make more sense to just explain the default calculation in the help:, and leave null as the default value, in which case you can tell whether a value was specified since there is no way to pass null from the command line. However for flags, defaultsTo defaults to false, not null, so would need to do defaultsTo: null for that, haven't tested if that works.

[DartBot]

This comment was originally written by [email protected]

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I like having default values for a nice out of the box experience. I ended creating a settings object that takes a map as a source and provides the default values if they're not in the map. Then I merge Platform.environment and ArgResults[] into the map provided to the settings object. There's a bit of friction as ArgResults doesn't implement Map or provide an accessor to a Map view, so I have to loop through the options rather than just using Map.addAll. Any thoughts on why ArgResults doesn't implement map or provide an accessor to a Map view? I think having ArgResults implement Map or provide an accessor to a Map view might make merging configuration sources cleaner.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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Removed Library-Args label.
Added Pkg-Args label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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I like having default values for a nice out of the box experience.

Agreed. In particular, default values are shown in the usage information.

Any thoughts on why ArgResults doesn't implement map or provide an accessor to a Map view?

Args is super old so I think the map type was still in flux when I first wrote it. I'll follow up on the other bug for this.

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Set owner to @munificent.
Removed Type-Defect label.
Added Type-Enhancement, Started labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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I just pushed arg 0.12.0, which adds a .wasParsed() method to ArgResults.

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Added Fixed label.

[DartBot]

This comment was originally written by [email protected]

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Excellent. Thank you!