Add new show command

[julen]

When configurations use inheritance, it is handy to see the expanded
configuration to get a better feel of what is going to be run. While
Config::Neat already provides a command for this, having a show
command helps with the feature being discoverable.

Fixes #40.

[julen]

I realize there are probably many ways to do this better, but I decided to give it a shot in order to get more acquainted with the codebase.

[iafan]

Here we need to add need_config => 1, (see below)

[iafan]

On a separate note, I think the command needs a new name. 'show' (and I guess git show was an inspiration for the name here) could be used to show various props from the config file (to facilitate some scripting) serge show destination_languages <config.serge> would gather and report all destination languages from a config or multiple config files.

What we have here is an explanation of how config works. It's not an original config, but a transformed one. So I'm more inclined towards serge explain <config.serge> (inspired by SQL EXPLAIN) that would explain how the system would understand the config (which not only includes expanding inherited properties, but would also resolve environment variable-based macros).

[julen]

Yes, git show was definitely the inspiration.

On a first read I said, ok, I'm fine making this explain, but after leaving this PR rest for a bit, part of me now thinks that showing props from the config file is what you described as expansion plus a filter on the output to only display what users specified on the CLI. So I'm not quite sure I agree this has to be two separate commands.

Getting back to the original inspiration, git show requires an <sha1> (or an alias to an sha1 such as HEAD, which is the default if no sha1 is provided), which will show the entire diff with respect to the previous changeset. The output can selectively be filtered, e.g. based on file path git show <sha1> -- path/or/file/to/display/. This is a pattern that other git commands support too, and it becomes very intuitive as users get involved with more commands.

Along those lines, I think for Serge it would be beneficial to keep the CLI surface as small as possible, and allow showing with a single command entire configuration files, as well as specific parts of them, because in the end they display an interpreted version of configuration files. For instance:

$ serge show foo.serge
<expanded contents of the config file>

$ serge show foo.serge destination_languages
<list of destination languages>

The option to filter on properties would require more thought (is the output displayed per-job? or does it merge properties across jobs? etc.). While it is very good we are already thinking about this use case, I'd say let's address its implementation separately.

[julen]

I wasn't sure if you would prefer to use $#config_files == 0 or rather scalar @config_files == 1. Please let me know if you have any preferences.

[prat0088]

I find scalar(@config_files) == 1 easier to read. $# is less common, requires extra Perl knowledge, and it looks like a check for a zero-length list if you're just skimming code.

You could further simplify the predicate to @config_files == 1.

[prat0088]

Just checking in on the status of this PR. It looks really helpful. We're in the process of rolling out Serge and I was looking forward to using this feature to debug config files.

[iafan]

@prat0088 note that for debugging config files you can already use dump-nconf <yourconfig.serge>. This is a tool that comes with the underlying Config::Neat parser library.

This PR introduces the same functionality under serge show <yourconfig.serge> alias.

[codecov-io]

Codecov Report

Merging #43 into master will decrease coverage by 0.04%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   54.51%   54.47%   -0.05%     
==========================================
  Files          93       94       +1     
  Lines        6954     6967      +13     
  Branches     1738     1739       +1     
==========================================
+ Hits         3791     3795       +4     
- Misses       2491     2500       +9     
  Partials      672      672

Impacted Files	Coverage Δ
lib/Serge/Command/show.pm	30.76% <30.76%> (ø)

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