uiop:directory-exists-p

(uiop:directory-exists-p pathname)

Answers whether a directory exists: the pathname (with a trailing /) when it does, nil when it does not. The directory twin of uiop:file-exists-p, and libraries use it to validate a directory root before walking it.

It is also what tells an EMPTY directory from a missing one: directory answers nil for both.

Backend support

All four backends, over the same one primitive directory uses -- so a host without a filesystem (the browser playground) answers nil rather than failing, and a WASM module answers nil for everything unless it was run with --dir.