Promises Package

A Promise represents a value that will be available at a later time. Promises can either be fulfilled with a value or rejected. Any number of function handlers can be added to the Promise, to be called when the Promise is fulfilled or rejected. These handlers themselves are also wrapped in Promises so that they can be chained together in order for the fulfilled value of one Promise to be used to compute a value which will be used to fulfill the next Promise in the chain, or so that if the Promise is rejected then the subsequent reject functions will also be called. The input and output types of a fulfill handler do not have to be the same, so a chain of fulfill handlers can transform the original value into something new.

Fulfill and reject handlers can either be specified as classes that implment the Fulfill and Reject interfaces, or as functions with the same signatures as the apply methods in Fulfill and Reject.

In the following code, the fulfillment of the Promise causes the execution of several fulfillment functions. The output is:

fulfilled + foo
fulfilled + bar
fulfilled + baz

use "promises"

class PrintFulfill is Fulfill[String, String]
  let _env: Env
  let _msg: String
  new create(env: Env, msg: String) =>
    _env = env
    _msg = msg
  fun apply(s: String): String =>
    _env.out.print(" + ".join([s; _msg].values()))
    s

actor Main
  new create(env: Env) =>
     let promise = Promise[String]
     promise.next[String](recover PrintFulfill(env, "foo") end)
     promise.next[String](recover PrintFulfill(env, "bar") end)
     promise.next[String](recover PrintFulfill(env, "baz") end)
     promise("fulfilled")

In the following code, the fulfill functions are chained together so that the fulfilled value of the first one is used to generate a value which fulfills the second one, which in turn is used to compute a value which fulfills the third one, which in turn is used to compute a value which fulfills the fourth one. The output is the average length of the words passed on the command line or 0 if there are no command line arguments.

use "promises"

primitive Computation
  fun tag string_to_strings(s: String): Array[String] val =>
    recover s.split() end
  fun tag strings_to_sizes(sa: Array[String] val): Array[USize] val =>
    recover
      let len = Array[USize]
      for s in sa.values() do
        len.push(s.size())
      end
      len
    end
  fun tag sizes_to_avg(sza: Array[USize] val): USize =>
    var acc = USize(0)
    for sz in sza.values() do
      acc = acc + sz
    end
    acc / sza.size()
  fun tag output(env: Env, sz: USize): None =>
    env.out.print(sz.string())

actor Main
  new create(env: Env) =>
     let promise = Promise[String]
     promise.next[Array[String] val](recover Computation~string_to_strings() end)
            .next[Array[USize] val](recover Computation~strings_to_sizes() end)
            .next[USize](recover Computation~sizes_to_avg() end)
            .next[None](recover Computation~output(env) end)
     promise(" ".join(env.args.slice(1).values()))

Public Types

• actor Promise
• primitive Promises
• interface Fulfill
• interface Reject
• class FulfillIdentity
• class RejectAlways