Add Eio_unix.Stdenv.with_env for updating environments

[patricoferris]

An increasingly popular style of using Eio is the following.

Instead of passing individual capabilities around, programs now define a environment that is a subtype of Eio_unix.Stdenv.t. For example, in Shelter I have:

type 'a env =
  < clock : [> float Eio.Time.clock_ty ] Eio.Resource.t
  ; fs : Eio.Fs.dir_ty Eio.Path.t
  ; net : [> [> `Generic | `Unix ] Eio.Net.ty ] Eio.Resource.t
  ; process_mgr : [> [> `Generic ] Eio.Process.mgr_ty ] Eio.Resource.t
  ; stdout : [> Eio.Flow.sink_ty ] Eio.Resource.t
  ; stdin : [> Eio.Flow.source_ty ] Eio.Resource.t
  ; .. >
  as
  'a

which is passed around internally carving out only the pieces I need in the right places. I ran into some bugs yesterday and I wanted to make full use of this capability-style and modify the process manager to print the args before spawning a child process.

let debug_process_mgr (mgr : 'a Eio_unix.Process.mgr) : 'a Eio_unix.Process.mgr
    =
  let module D = struct
    type t = unit

    let spawn_unix () ~sw ?cwd ?pgid ?uid ?gid ~env ~fds ~executable args =
      Eio.traceln "Spawning subprocess... %a" Fmt.(list ~sep:(Fmt.any " ") string) args;
      Eio_unix.Process.spawn_unix ~sw ?cwd ?pgid ?uid ?gid mgr ~env ~fds ~executable args
  end in
  let module V = Eio_unix.Process.Make_mgr (D) in
  Eio.Resource.T ((), Eio_unix.Process.Pi.mgr_unix (module V))

At this point I just needed a way to update my env to use this debugger process manager, which with this function would be simple!

    Eventloop.run @@ fun env ->
    let cmd_file = Option.map (Eio.Path.( / ) env#fs) cmd_file in
    let dir = state_dir env#fs "shelter" in
    let env =
      Eio_unix.Stdenv.with_env ~process_mgr:(debug_process_mgr env#process_mgr) env
    in
    Shelter.main config (env :> _ Shelter.env) dir cmd_file

---

As an aside, I do wonder if it might be time to reconsider going back to objects for Eio's resources? The ergonomics, I think, are far superior to the Eio.Resource.T API.

[avsm]

As an aside, I do wonder if it might be time to reconsider going back to objects for Eio's resources? The ergonomics, I think, are far superior to the Eio.Resource.T API.

I, too, miss the objects.