Prework

• I understand and agree to the code of conduct and the contributing guidelines.
• I have already submitted a discussion topic or issue to discuss my idea with the maintainer.

Related GitHub issues and pull requests

• Ref: Expose function arguments for targets script and data store #477

Summary

With efforts like Target Markdown and https://github.com/wlandau/targets-shiny, targets is becoming more of a backend tool. That means it needs to be more configurable. As of this PR, targets now has functionality to configure the target script file (default: _targets.R), the data store (default: _targets/), and the YAML file with configuration settings (default: _targets.yaml). You can override the settings for a function call using the script and store arguments (and the config argument of tar_watch() and tar_watch_server()).

With the exception of tar_config_set(), the arguments script, store, and config operate by temporarily modifying in-memory global config settings. This technique avoids a lot of painful refactoring, but it also means these arguments will cause bugs if R ever becomes arbitrarily multithreaded. Since arbitrary multithreading is almost surely never going to happen, I am willing to make this tradeoff. Knowing what I know now, if I were to design targets from the beginning, I would delegate the file paths to various internal classes. But doing that at this stage would be a staggering amount of effort for something that is never going to impact usage or maintainability.