Futures

Futures represent asynchronous operation results. They are returned by funcs.async() and asynchronous contract calls. This page is an API reference; the target IDs and arguments in its patterns are application-defined.

Loading

Futures are not loaded as a module; asynchronous operations create them:

local funcs = require("funcs")
local future, err = funcs.async("app.compute:task", data)
if err then
    return nil, err
end

Response Channel

Use the response channel to wait for completion, then read the cached result from the future:

local ch = future:response()
local _, open = ch:receive()
if not open then
    return nil, errors.new("future response channel closed")
end

local payload, err = future:result()
if err then
    return nil, err
end
local result, data_err = payload:data()
if data_err then return nil, data_err end

channel() is an alias for response().

The channel value is the operation's payload, payload table, or error. Calling result() after the channel becomes ready provides one consistent success/error interface and returns the cached value even after the channel is drained.

Completion Check

Check whether the future has completed without blocking:

if future:is_complete() then
    local result, err = future:result()
end

Cancellation Check

Check whether the future has been marked canceled by its provider:

if future:is_canceled() then
    print("Operation was canceled")
end

Getting Result

Read the cached result without blocking:

local val, err = future:result()

Returns:

• Not complete: nil, nil
• Canceled: nil, error (kind CANCELED)
• Error: nil, error
• Success: Payload, nil or table, nil (multiple payloads)

Getting Error

Read the error when the future has failed:

local err, has_error = future:error()
if has_error then
    print("Failed:", err:message())
end

Returns: error, boolean

When an operation fails, error() returns a non-retryable INTERNAL wrapper. Use result() when the called function's original error kind and retryability must be preserved.

Canceling

Request cancellation of the asynchronous operation on a best-effort basis:

local canceled, err = future:cancel()

Returns: boolean, error

Operation may still complete if already in progress.

Timeout Pattern

local time = require("time")

local future, err = funcs.async("app.compute:slow", data)
if err then
    return nil, err
end

local timeout, err = time.after("5s")
if err then
    return nil, err
end

local r = channel.select {
    future:channel():case_receive(),
    timeout:case_receive()
}

if r.channel == timeout then
    future:cancel()
    return nil, errors.new({ kind = errors.TIMEOUT, message = "Operation timed out" })
end

local payload, result_err = future:result()
if result_err then
    return nil, result_err
end
local value, data_err = payload:data()
if data_err then return nil, data_err end
return value

First-to-Complete

local f1, err = funcs.async("app.cache:get", key)
if err then
    return nil, err
end
local f2, err = funcs.async("app.db:get", key)
if err then
    return nil, err
end

local ch1 = f1:channel()
local ch2 = f2:channel()

local r = channel.select {
    ch1:case_receive(),
    ch2:case_receive()
}

-- The slower operation may still complete; this caller ignores its result.
local winner
if r.channel == ch1 then
    winner = f1
else
    winner = f2
end

local payload, result_err = winner:result()
if result_err then
    return nil, result_err
end
local value, data_err = payload:data()
if data_err then return nil, data_err end
return value

Errors