ensure-directories-exist

(ensure-directories-exist pathspec)

Creates the directory component of pathspec, including every missing parent, and returns pathspec. The directory component is everything up to and including the last /, so "logs/app.log" creates logs/ and leaves the file alone; a namestring that already ends in / is the directory; a namestring with no / names a file in the working directory and creates nothing. An existing directory is not an error.

Lite: Common Lisp returns (values pathspec created) and this returns the pathspec only — a second value would not survive the function boundary on the compiled backends, so promising one would be misleading.

Works on all four backends. Both WASM backends create every missing level through the path_create_directory WASI import (Preview 1 directly, --component through wasi:filesystem's create-directory-at), and signal when the directory could not be created — unlike file-write-date this operation has no "cannot be determined" answer in its contract: either the directory exists afterwards or it does not.

(ensure-directories-exist "logs/2026/app.log")
(with-open-file (out "logs/2026/app.log" :direction :output)
  (write-line "started" out))