xdge - XDG Base Directory Specification for Eio#

This library implements the XDG Base Directory Specification for OCaml applications using the Eio effects-based I/O library.

What is XDG?#

The XDG Base Directory Specification defines standard locations for user-specific files on Unix-like systems, keeping home directories clean and organized:

• Config (~/.config/app): User preferences and settings
• Data (~/.local/share/app): Persistent application data
• Cache (~/.cache/app): Non-essential cached data (safe to delete)
• State (~/.local/state/app): Logs, history, and runtime state
• Runtime ($XDG_RUNTIME_DIR/app): Sockets, pipes, and session-bound files

The specification also defines system-wide search paths (/etc/xdg, /usr/share) and a precedence system using environment variables (XDG_CONFIG_HOME, XDG_DATA_HOME, and so on).

Why Eio?#

Eio uses a capability-based approach to I/O where filesystem access must be explicitly passed to functions. This design aligns naturally with XDG directory management. For example:

(* Filesystem access is an explicit capability *)
let xdg = Xdge.create env#fs "myapp"

The capability model provides the benefit that code that needs filesystem access must receive the fs capability, with no hidden global state or ambient authority. The Eio.Path.t type returned by xdge encapsulates both the filesystem capability and the path, preventing path traversal outside the granted capability. Applications can restrict filesystem access by passing a sandboxed fs capability, and xdge respects those boundaries.

Usage#

Eio_main.run @@ fun env ->
  let xdg = Xdge.create env#fs "myapp" in

  (* Access XDG directories as Eio paths *)
  let config = Xdge.config_dir xdg in
  let data = Xdge.data_dir xdg in

  (* Search for files following XDG precedence *)
  match Xdge.find_config_file xdg "settings.json" with
  | Some path -> (* use path *)
  | None -> (* use defaults *)

For CLI applications, xdge provides Cmdliner terms that handle environment variable precedence and command-line overrides:

let () =
  Eio_main.run @@ fun env ->
  let term = Xdge.Cmd.term "myapp" env#fs () in
  (* Generates --config-dir, --data-dir, etc. flags *)
  (* Respects MYAPP_CONFIG_DIR > XDG_CONFIG_HOME > default *)

Installation#

opam install xdge

License#

ISC