Something I don't want to go into for this PR, but maybe in the future: The semantics of pathIsDirectory and pathIsRegularFile are not obvious:

• They return false when the path doesn't exist, which is often not what you want.
• They always return false for symlinks without checking where the symlink points (it could point to a directory or a regular file)

We should consider having alternate functions with cleaner semantics in the future:

• isDirectory :: Path -> Bool: Returns true iff the path is a directory, following symlinks. An error is thrown if the path doesn't exist.
• isFile :: Path -> Bool: Returns true iff the path is a file, following symlinks. An error is thrown if the path doesn't exist.

These laws would then hold:

• If isDirectory path succeeds and is true, then builtins.readDir path succeeds
• If isFile path succeeds and is true, then builtins.readFile path succeeds
• isDirectory path != isFile path

Unfortunately the implementation of these functions would have to be a bit hacky (throwing for non-existent paths not implemented here):

{
  isDirectory = path: builtins.pathExists "${toString path}/";
  isFile = path: ! builtins.pathExists "${toString path}/";
}

This uses the fact that:

• toString can turn paths to strings, which would not be the case with lazy trees, and would violate the minimal assumptions of the path library
• builtins.pathExists works not only on paths, but also on strings, with the important difference that strings aren't normalised, therefore keeping the trailing /

To implement these I'd propose adding more builtins, either:

• builtins.readSymlink: return the target of a symlink
• builtins.isDirectory: A builtin version of the above isDirectory

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-47/27387/1

We should consider having alternate functions with cleaner semantics in the future:

• isDirectory :: Path -> Bool: Returns true iff the path is a directory, following symlinks. An error is thrown if the path doesn't exist.
• isFile :: Path -> Bool: Returns true iff the path is a file, following symlinks. An error is thrown if the path doesn't exist.

That's good. How about this?

• isDirectory :: Path -> Bool: Returns true iff the path is a directory, following symlinks. An error is thrown if the path doesn't exist.
• existsDirectory :: Path -> Bool: Returns pathExists f && isDirectory f
• same for *File

existsDirectory is just a rename of pathIsDirectory, but less ambiguous.

@roberth I'd probably go for directoryExists instead of existsDirectory, this then also mimics pathExists

Successfully created backport PR for release-23.05:

• [Backport release-23.05] Improvements to pathType, pathIsDirectory and pathIsRegularFile #233560

Git push to origin failed for release-23.05 with exitcode 1