You're right; OCaml would have to supply two different functions, one that takes an object that exposes < close : (); write : string -> (); .. > and another tha

You're right; OCaml would have to supply two different functions, one that takes an object that exposes < close : (); write : string -> (); .. > and another that takes < write : string -> (); .. >. Reproducing the Go system would require the ability to, at runtime, see whether an object of type < write : string -> (); .. > can be downcasted to < close : (); write : string -> (); .. >, which would require more runtime type information than OCaml currently supplies. Some people have made proposals to implement such downcasting (throwing an exception if the downcast fails), which would permit the Go-like implementation, but as far as I know, such proposals haven't been met with much enthusiasm.