use file.path instead of paste0 in cache_rds

[avila]

Very simple change, so that is not necessary to pass a trailing slash (/) to the dir argument of cache_rds. This is specially useful when the using file.path() itself for the dir argument, such as:

path <- list(checkpoints = file.path("checkpoints"))

long_calcs <- cache_rds(
  expr = {
    Sys.sleep(10)
  },
  dir = file.path(path$checkpoints)
)

Maybe it is a better practice to always use a trailing slash for folders and no slash for files, but it is not always practical. The output of here::here(), for example, doesn't differ between folder and files in this way.

Please let me know if I am doing something wrong, or just disregard the PR. Thank you for these great miscellaneous functions :)

[CLAassistant]

All committers have signed the CLA.

[yihui]

Thanks! I was intentionally using paste0() here mainly because I was considering the case of using this function inside knitr code chunks. The chunk option cache.path is not necessarily a dir but just a path prefix.

I totally understand that file.path() would make more sense without considering this special case, and just suggested two changes that would make the special case still work. Do they make sense to you?

BTW, xfun::mark_dirs() can add the trailing slash to dirs (but only if a dir exists).

[yihui]

Suggested change

	dir = d
	dir = I(d)

[yihui]

Suggested change

	path = file.path(dir, file)
	path = (if (inherits(dir, 'AsIs')) paste0 else file.path)(dir, file)

[yihui]

Also the example is failing on Windows: https://github.com/yihui/xfun/actions/runs/3215127507/jobs/5256006471