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.

[talex5]

This function seems reasonable to me, though not sure about the name. Maybe sometime like override or like?

let env =
  Eio_unix.Stdenv.override env
    ~process_mgr:(debug_process_mgr env#process_mgr)
in

[talex5]

As an aside, I do wonder if it might be time to reconsider going back to objects for Eio's resources?

I don't mind (someone else doing it), but it's a big change to do after releasing 1.0 and might restart a load of arguments. Though I think perhaps the people saying they wouldn't use Eio if it used objects didn't use it anyway. Would need to survey Eio users to see what they think about it (normally the only people making a noise are the ones who don't like the current state, and the happy users keep quiet).