Custom error classes and respond_to_missing?

[eitoball]

Summary

Replace string raises with structured error classes and add respond_to_missing? to the extractors for better debugging and extension.

Problem

Opaque string errors

Errors today are plain strings:

raise "No comma format for class #{self.class} defined for style #{style}"
raise "Unknown data symbol #{arg.inspect}"
raise "Unknown header symbol #{arg.inspect}"

Callers cannot rescue Comma::UnknownStyle programmatically. Messages are fine for humans but fragile for apps that want to handle missing formats gracefully.

Locations:

• lib/comma/object.rb
• lib/comma/data_extractor.rb
• lib/comma/header_extractor.rb

Missing respond_to_missing?

Both extractors use method_missing without respond_to_missing?. RuboCop excludes this via .rubocop_todo.yml (Style/MissingRespondToMissing).

That breaks expected Ruby semantics for introspection (respond_to?, IRB, some tooling).

Proposed approach

Define error hierarchy under Comma:

module Comma
  class Error < StandardError; end
  class UnknownStyle < Error; end
  class UnknownColumnArgument < Error; end
  class CircularStyleReference < Error; end  # pairs with #1
end

Raise these with messages matching or improving current strings (backward compatible for string-matching rescues).

Add respond_to_missing? on both extractors delegating to the same rules as method_missing (always true for DSL method names inside a comma block context, or mirror method_missing behavior).

Files likely involved

• New: lib/comma/errors.rb
• lib/comma/object.rb
• lib/comma/data_extractor.rb
• lib/comma/header_extractor.rb
• lib/comma/extractor.rb
• Specs asserting error type (can keep message assertions too)

Acceptance criteria

• Missing style raises Comma::UnknownStyle (message still mentions class and style)
• Invalid column args raise Comma::UnknownColumnArgument
• respond_to_missing? implemented on both extractors
• Existing specs that expect RuntimeError or message strings updated intentionally (document in PR if message-only rescues exist in the wild)
• RuboCop todo entry for MissingRespondToMissing can be removed for extractor files

Labels (suggested)

refactor, enhancement

Depends on

Optional: #1 (CircularStyleReference error class)

Notes

Consider documenting rescuable errors in README/wiki in a follow-up; not required for this issue.