relaxed return type

Author: Arogbonlo

lib_eio/process.ml

@@ -143,7 +143,10 @@ let run t ?cwd ?stdin ?stdout ?stderr ?(is_success = Int.equal 0) ?env ?executab
 
 let pipe (type tag) ~sw ((Resource.T (v, ops)) : [> tag mgr_ty] r) =
   let module X = (val (Resource.get ops Pi.Mgr)) in
-  X.pipe v ~sw
+  let r, w = X.pipe v ~sw in
+  let r = (r : [Flow.source_ty | Resource.close_ty] r :> [< Flow.source_ty | Resource.close_ty] r) in
+  let w = (w : [Flow.sink_ty   | Resource.close_ty] r :> [< Flow.sink_ty   | Resource.close_ty] r) in
+  r, w
 
 let parse_out (type tag) (t : [> tag mgr_ty] r) parse ?cwd ?stdin ?stderr ?is_success ?env ?executable args =
   Switch.run ~name:"Process.parse_out" @@ fun sw ->

lib_eio/process.mli

@@ -140,7 +140,7 @@ val parse_out :
 
 (** {2 Pipes} *)
 
-val pipe : sw:Switch.t -> _ mgr -> [Flow.source_ty | Resource.close_ty] r * [Flow.sink_ty | Resource.close_ty] r
+val pipe : sw:Switch.t -> _ mgr -> [< Flow.source_ty | Resource.close_ty] r * [< Flow.sink_ty | Resource.close_ty] r
 (** [pipe ~sw mgr] creates a pipe backed by the OS.
 
     The flows can be used by {!spawn} without the need for extra fibers to copy the data.