You can search for identifiers within the package.
in-package search v0.2.0
type t
You only need this module, if you intend to write C stub code that get's executed in libuv's threadprool.
examples/glob.ml gives an example.
examples/glob.ml
type 'a u
val call : ('a -> 'b u -> t) -> 'a -> 'b Lwt.t