App::Easer new defaults handling


An update to App::Easer for smarter handling of default values.

It’s no mistery that there’s some ongoing Feature creeping in App::Easer but whatever, it’s a module to do what I mean and I can mean a lot of things.

Sometimes of mean things, I mean.

The legacy way in which App::Easer was handling defaults in the collection of options was to put the associated handler +Default at the end of the list of sources:

+CmdLine +Environment +Parent +Default

This basically meant that command line options take the precedence over everything, then come environment variables, then a configuration that might be inherited from some parent command (i.e. a partial sub-command that appeared previously in the hierarchy) and, finally, the default values.

This meant that the merging of the resulting hashes was as simple as this:

sub hash_merge {
   return {map { $_->%* } reverse @_};

Alas, this served me well but… not perfectly.

One common option I like to add is a --config|-c that takes the path to a file (usually a JSON file) with additional configurations. This is so handy for me that it’s possible to set the sources configuration to have this out of the box:

sources => '+SourcesWitFiles', # includes +JsonFileFromConfig

As you are probably guessing, this is a pre-arranged collection that includes +JsonFileFromConfig which does exactly what I was describing above. The exact sequence from +SourcesWithFiles was the following:

+CmdLine +Environment +Parent +JsonFileFromConfig +JsonFiles +Default

One drawback with this setup is that it’s not possible to set a default configuration file, because the +Default is considered only after having attempted to loade the JSON file from option config. For this to work, we would need to somehow move the defaults before in the pipeline, but this would then make them less… defaulty.

In a first solution I decided to add another class of higher precedence defaults (so-called “hi-defaults”) to insert stuff in the right place. I eventually landed on a different approach though: make defaults appear at the beginning of the options collection, while marking them as overridable. This would make their values available since the very beginning, while still allowing sources further down the road to influence the specific values of all options.

To do this, three changes were needed:

  • the bundles of sources were changed to move +Default at the beginning:
# +DefaultSources
+Default +CmdLine +Environment +Parent 

# +SourcesWithFiles
+Default +CmdLine +Environment +Parent
  • the default handler for +Default was changed to mark all collected values, by setting their respective key with a fixed prefix //=- So, for example, key foo is turned into //=foo;

  • function hash_merge was implemented like this:

sub hash_merge {
   my (%retval, %is_overridable);
   for my $href (@_) {
      for my $src_key (keys $href->%*) {
         my $dst_key = $src_key;
         my $this_overridable;
         if ($dst_key =~ m{\A //= (.*) \z}mxs) { # overridable
            $dst_key = $1;
            $is_overridable{$dst_key} = 1 unless exists $retval{$dst_key};
            $this_overridable = 1;
         $retval{$dst_key} = $href->{$src_key}
            if $is_overridable{$dst_key} || ! exists($retval{$dst_key});
         $is_overridable{$dst_key} = 0 unless $this_overridable;
   return \%retval;
   # was a simple: return {map { $_->%* } reverse @_};

In practice, a default value foo is collected as //=foo and hash_merge takes its value until some other value sets it. This makes it possible to move the group at the beginning while still considering its items… defaults.

I think it’s everything for today, stay safe!

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