Its currently possible to use the following dsl below when checking out code in a declarative pipeline:

checkout(
  git(
    branches: [[name: '*/master']],
    extensions: [[
      $class: 'RelativeTargetDirectory', 
      relativeTargetDir: 'some-dir'
    ]]
  )
)

It would be nice if it could support a richer dsl rather than the current $class syntax for extensions. e.g. something like:

checkout(
  git(
    branches: [[name: '*/master']],
    extensions: [[
      relativeTargetDir('some-dir')
    ]]
  )
)

This could be something i could help contribute if its acceptable?!

Implementation Process

• List existing symbol definitions from configuration as code matched with $Class samples for that symbol (create a checklist of symbols to create)
• Select a symbol and add a test for the symbol as a $Class form
• Add the symbol
• Add a test that uses the symbol instead of the $Class form
• Confirm symbol appears in Pipeline syntax generator
• Confirm symbol appears in Configuration as Code
• Confirm configuration as code can still read previous configuration files
• Write help for the symbol, assure it is clear, correct, complete, and visible in the online Pipeline syntax
• Write documentation for the symbol in the plugin README so that it is available outside of Jenkins

Existing Symbol Definitions

See the Karl Shultz pull request for progress.