Fetch.Promise
Fetching URL is not always done synchronously: they are usually embedded within a promise.
type _ t
A promise is a monad.
val return : 'a -> 'a t
val bind : ('a -> 'b t) -> 'a t -> 'b t
val run : unit t -> unit
Waits for the promise to terminate.
val parallel : 'a t list -> 'a list t
Run all the promises in parallel, waiting for all of them to terminate.