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       # xdge - XDG Base Directory Specification for Eio

     

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       This library implements the [XDG Base Directory

     

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       Specification](https://specifications.freedesktop.org/basedir-spec/latest/) for

     

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       OCaml applications using the [Eio](https://github.com/ocaml-multicore/eio)

     

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       effects-based I/O library.

     

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       ## What is XDG?

     

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

     

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       - Config (`~/.config/app`): User preferences and settings

     

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       - Data (`~/.local/share/app`): Persistent application data

     

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       - Cache (`~/.cache/app`): Non-essential cached data (safe to delete)

     

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       - State (`~/.local/state/app`): Logs, history, and runtime state

     

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       - Runtime (`$XDG_RUNTIME_DIR/app`): Sockets, pipes, and session-bound files

     

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       The specification also defines system-wide search paths (`/etc/xdg`,

     

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       `/usr/share`) and a precedence system using environment variables

     

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       (`XDG_CONFIG_HOME`, `XDG_DATA_HOME`, and so on).

     

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       ## Why Eio?

     

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       Eio uses a **capability-based** approach to I/O where filesystem access must be

     

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       explicitly passed to functions. This design aligns naturally with XDG directory

     

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       management. For example:

     

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       ```ocaml

     

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       (* Filesystem access is an explicit capability *)

     

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       let xdg = Xdge.create env#fs "myapp"

     

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       ```

     

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       The capability model provides the benefit that code that needs filesystem

     

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       access must receive the `fs` capability, with no hidden global state or ambient

     

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       authority. The `Eio.Path.t` type returned by xdge encapsulates both the

     

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       filesystem capability and the path, preventing path traversal outside the

     

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       granted capability. Applications can restrict filesystem access by passing a

     

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       sandboxed `fs` capability, and xdge respects those boundaries.

     

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       ## Usage

     

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       ```ocaml

     

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       Eio_main.run @@ fun env ->

     

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         let xdg = Xdge.create env#fs "myapp" in

     

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         (* Access XDG directories as Eio paths *)

     

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         let config = Xdge.config_dir xdg in

     

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         let data = Xdge.data_dir xdg in

     

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         (* Search for files following XDG precedence *)

     

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         match Xdge.find_config_file xdg "settings.json" with

     

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         | Some path -> (* use path *)

     

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         | None -> (* use defaults *)

     

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       ```

     

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       For CLI applications, xdge provides Cmdliner terms that handle environment

     

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       variable precedence and command-line overrides:

     

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       ```ocaml

     

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       let () =

     

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         Eio_main.run @@ fun env ->

     

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         let term = Xdge.Cmd.term "myapp" env#fs () in

     

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         (* Generates --config-dir, --data-dir, etc. flags *)

     

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         (* Respects MYAPP_CONFIG_DIR > XDG_CONFIG_HOME > default *)

     

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       ```

     

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       ## Installation

     

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       ```

     

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       opam install xdge

     

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       ```

     

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       ## License

     

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       ISC