relaxed return types

Author: Arogbonlo

lib_eio/unix/private.ml

@@ -12,7 +12,10 @@ type _ Effect.t +=
 let await_readable fd = Effect.perform (Await_readable fd)
 let await_writable fd = Effect.perform (Await_writable fd)
 
-let pipe sw = Effect.perform (Pipe sw)
+let pipe sw =
+  let (src, sink) = Effect.perform (Pipe sw) in
+  (src :> source_ty r),  
+  (sink :> sink_ty r)
 
 module Rcfd = Rcfd
 module Fork_action = Fork_action