Allow user to decline to overwrite existing file, from use_template()

[boshek]

Currently use_template throws an error when one chooses not to overwrite an existing file:

usethis::use_template(template = "CONTRIBUTING.md")
Overwrite pre-existing file 'CONTRIBUTING.md'?
1: Absolutely
2: No
3: Not now
 
Selection: 3
Error: 'CONTRIBUTING.md' already exists.

Is there any appetite for a PR that would add a allow_existing argument (or whatever the name) to use_template? So that is, if a file already exists and the allow_existing argument is set to TRUE, and a user selects to not overwrite (at the prompt), use_template simply outputs a 'CONTRIBUTING.md' already exists' warning rather than an error?

The context is that we have a series of templates (CoC, README, etc) where a user, when setting up a project or package, may accept the template for some and keep existing files for others. Since use_template via write_over throws an error, this type of process where sequential asking which files need to be overwritten is currently not possible.

[jennybc]

I agree it makes sense to be able to opt-out of overwrite and not throw an error. However, my mind is very much elsewhere right now.

Before making a PR, will you give a few bullet points re: what you plan to do so @hadley can also give a quick 👍?

The objective is to make sure the change really needs to happen in usethis vs. in your package (without forcing you to do some awkward tryCatch() thing). Let's make sure the accommodations are made in the most sensible place(s).

[boshek]

awkward tryCatch()

Definitely where I am at right now

Ok here is a shot at what I would try:

• Add this into write_over here:

if (!can_overwrite(full_path)){
  ## Keep existing file AND allow_existing = FALSE (default)
  if(!allow_existing) stop(value(path), " already exists.", call. = FALSE)
  
  ## Keep existing file AND allow_existing = TRUE
  if(allow_existing) warning(value(path), " already exists.")
}

• Then add a allow_existing argument to write_over and use_template
• Add param documentation to allow_existing for use_template

[jennybc]

I don't see why this needs to stop() if interactive(), i.e. a human was presented with and made a choice. I propose you make a PR with no new argument. If !can_overwrite() and interactive() then warn or message and return FALSE from write_over().