Layering env, args, and config

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Maintainer's note

• Designing for layered configs #2763
• Allow adding "external argv"s to be parsed alongside/before command line #748

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Issue by synek317
Monday Feb 19, 2018 at 10:02 GMT
Originally opened as TeXitoi/structopt#72

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There is great config-rs crate that provides hierarchical configuration and deserialization into struct. I think it would be great if clap / structopt could handle feature like this. With this, one can handle app configuration that can be provided in multiple config files and/or command line all in one place and with single structure.

I suggest to add from_opts_and_config function that would accept Config instance (from config-rs) that would provide defaults when argument is not passed in command line.

I think that it is a best way because only structopt has full knowledge if and what arguments has been passed and which are mandatory and what should be default value if arg is not passed. Config crate requires that all values are defined, otherwise deserialization returns Err.

Of course config-rs dependency should be configurable with features.

If you think it is a good idea, I could prepare some implementation.

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Comment by TeXitoi
Monday Feb 19, 2018 at 13:40 GMT

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You might be interested in TeXitoi/structopt#32

StructOpt is just a wrapper above clap. Looking at your proposition, it seems better to include this functionality directly in clap. But you may have some arguments to do it in structopt.

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Comment by synek317
Monday Feb 19, 2018 at 13:46 GMT

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I have done a sketch implementation and it is very simple to use config-rs in StructOpt. However, that is not really a great argument.

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Comment by TeXitoi
Monday Feb 19, 2018 at 13:59 GMT

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You can point to a branch, and I'll look a it to seen what is your idea.

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Comment by TeXitoi
Wednesday Feb 21, 2018 at 14:53 GMT

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@synek317 ping

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Comment by synek317
Friday Feb 23, 2018 at 16:42 GMT

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After some tries I think that there is a better way to solve this. Clap has 'default_value' attrs. If it could be 'default_value_fn' then merging config from file(s) with command line args could be done by 3rd party lib.

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Comment by synek317
Monday Mar 05, 2018 at 04:50 GMT

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@TeXitoi Could you please take a look at my solution sketch at https://www.reddit.com/r/rust/comments/81tp16/wip_platformconfig_multisource_configuration_for/ ?

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See also #2763