expandFilename works only on existing files and directories.

[FedericoCeratto]

Also, the documentation does not mention this behavior.

[dom96]

So unless there is a use case for this I think it's fine to just document this behaviour.

[FedericoCeratto]

Expanding file names is very common for logging (you don't want to log out a relative path). A typical use case is to log out that you are going to create a file before doing so.

[andreaferretti]

I think one reason it work like that is symlinks (assuming this function also resolves them). You cannot expand the filename without knowing whether some intermediate part is a symlink, and for that you have to touch the filesystem and work on existing paths. If instead this is a pure function with no reference to the filesystem, it makes sense that it works at an abstract level without requiring files to exist

[Araq]

I think it's fine.

proc expandFilename*(filename: string): string {.rtl, extern: "nos$1",
  tags: [ReadDirEffect], noNimScript.} =
  ## Returns the full (`absolute`:idx:) path of an existing file `filename`.
  ##
  ## Raises `OSError` in case of an error. Follows symlinks.