This is a concept that I have thought about before, and even something that I thought might already be possible. I'd be interested to discuss this more, especially since this not only brings some exciting features to Jekyll but some possible headaches as well (cue the WordPress child theme flashbacks). This is issue thread the best place to discuss? I'd hate to spam people on a topic that could get really long

@daviddarnes Go ahead and list down all that you have to say regarding this feature..

Alrighty, I'll just list up the positive and possible negative implications of this:

Positive

1. Bundling of code for multiple sites; I've worked on WordPress multi-sites that used a parent and child theme system to share code, this would be a similar feature
2. Faster development of theme ecosystem; More themes can be made at a more rapid pace
3. Reduced complexity of theme creation; Anyone who's new to theming can get involved without starting from scratch
4. Boilerplate standards can be produced causing better theme code quality

Negative

1. Increased complexity when using multiple themes. Tracking down a bug, style or value could prove annoying
2. Inheritance, how will it work and will it be consistent?
3. Conflicts with the simple nature of Jekyll. There has been a lot of powerful features added to Jekyll, including theming, but is this going too far

I've numbered these so they can be easily referenced. I'll be honest, those negatives are more me being devils advocate to myself. I've put them incase people have similar thoughts or are able to justify them better than I can

@daviddarnes Thanks for laying things out, it's best if we think things through together 👍

Here's how I envision this feature:

• This should not change anything for end-users, they can use child themes as they use themes today. You would still be using a single theme (a child theme, a parent theme or a regular theme),
• We should log the theme name (path?) in debug mode, e.g.:
  Liquid Exception: Liquid syntax error (line 6): Unknown tag 'this_will_not_work' in <theme_name>/_layouts/home.html
• Inheritance will work like it's already working today when you override a theme with local folders, except we add a new level: parent theme > child theme > local files
• same for config files: parent theme config > child theme config > local config
• local files always win on theme files
• It's up to theme developers to declare the theme they're building upon, (through child theme config and gemspec )

@DirtyF it just dawned on me why I thought this was possible, because I thought you could already import a theme gem just like a Jekyll gem plugin anyway. This is the convention we're talking about yes? What currently prevents us importing a theme into another theme as a gem?

What currently prevents us importing a theme into another theme as a gem?

I just tried it. I overode minima and publish the theme gem: https://rubygems.org/gems/jekyll-theme-minima

Now when I create a new jekyll site and specify the child theme, it won't build:

Dependency Error: Yikes! It looks like you don't have minima or one of its dependencies installed. In order to use Jekyll as currently configured, you'll need to install this gem. The full error message from Ruby is: 'cannot load such file -- minima'
bundler: failed to load command: jekyll (/Users/frank/gems/bin/jekyll)
Jekyll::Errors::MissingDependencyException: minima
  /Users/frank/code/jekyll/jekyll/lib/jekyll/external.rb:70:in `rescue in block in require_with_graceful_fail'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/external.rb:58:in `block in require_with_graceful_fail'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/external.rb:57:in `each'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/external.rb:57:in `require_with_graceful_fail'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/plugin_manager.rb:44:in `block in require_theme_deps'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/plugin_manager.rb:41:in `each'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/plugin_manager.rb:41:in `require_theme_deps'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/plugin_manager.rb:20:in `conscientious_require'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/site.rb:117:in `setup'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/site.rb:32:in `initialize'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/commands/build.rb:30:in `new'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/commands/build.rb:30:in `process'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/command.rb:89:in `block in process_with_graceful_fail'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/command.rb:89:in `each'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/command.rb:89:in `process_with_graceful_fail'
  /Users/frank/code/jekyll/jekyll/lib/jekyll/commands/build.rb:18:in `block (2 levels) in init_with_program'
  /Users/frank/gems/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
  /Users/frank/gems/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
  /Users/frank/gems/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
  /Users/frank/gems/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
  /Users/frank/gems/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
  /Users/frank/code/jekyll/jekyll/exe/jekyll:15:in `<top (required)>'
  /Users/frank/gems/bin/jekyll:23:in `load'
  /Users/frank/gems/bin/jekyll:23:in `<top (required)>'

I guess this is what needs to be fixed in order for it to work. Do we need to establish behaviour of child/parent themes too?

Any more progress on this topic? Clearly not a huge demand, but certainly had potential

No, Jekyll core team is very limited at the moment. Any help is warmly welcome if someone is interested in contributing.

@issuehuntfest has funded $60.00 to this issue. See it on IssueHunt

@0maxxam0 has funded $2.00 to this issue.

• Submit pull request via IssueHunt to receive this reward.
• Want to contribute? Chip in to this issue via IssueHunt.
• Checkout the IssueHunt Issue Explorer to see more funded issues.
• Need help from developers? Add your repository on IssueHunt to raise funds.