existential.ml

module Runtime : sig
  type 'a t
  type t1
  type t2
  val foo : t1 t -> unit
  val bar : _ t -> unit
  val create1 : unit -> t1 t
  val create2 : unit -> t2 t
end = struct
  type 'a t = int
  type t1 = [ `A ]
  type t2 = [ `B ]
  let create1 () = 42
  let create2 () = 100500
  let foo _ = ()
  let bar _ = ()
end 
  
module type Arg = sig
  type runtime
  val baz : runtime -> unit
end

module F (A:Arg) = struct let zzz rt = Runtime.bar rt; () end

module Arg1 = struct
  type runtime = Runtime.t1 Runtime.t
  let baz rt = Runtime.foo rt
end
         
module Arg1_applied = F (Arg1)

module Arg2 = struct
  type runtime = Runtime.t2 Runtime.t
  let baz rt = Runtime.bar rt
end

module Arg2_applied = F (Arg2)

module Arg_any = struct
  type runtime = R : _ Runtime.t -> runtime [@@unboxed]
  let baz (R rt) = Runtime.bar rt
end
         
module Arg_any_applied = F (Arg_any)