One rough edge I’ve found with open variant error types is that they don’t seem to work well with exhaustiveness checking. For example, the following compiles just fine despite the superfluous Error `bar case:  module M : sig   val foo : (unit, [> `foo ]) Result.t end = struct   let foo = Error `foo end  let () =   match M.foo with   | Ok () -> ()   | Error `foo -> ()   | Error `bar -> () ;;  We can change the type to val foo : (unit, [ `foo ]) Result.t, but only at the cost of unification with ...