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Async / Await

Nebra supports async functions that compile to Lua coroutines.

Async Functions​

async function fetchData(url: string): string
    local response = await httpGet(url)
    return response
end

Async Methods​

class ApiClient
    async function get(url: string): string
        return await httpRequest("GET", url)
    end

    async function post(url: string, body: string): string
        return await httpRequest("POST", url, body)
    end
end

Anonymous Async Functions​

local handler = async function(x: number): number
    local result = await compute(x)
    return result * 2
end

Await​

await can only be used inside async functions. It suspends execution until the awaited operation completes.

async function multiStep(): string
    local a = await fetch("url1")
    local b = await fetch("url2")
    return a .. b
end

How It Compiles​

Async Function​

An async function wraps its body in a coroutine. It accepts an optional __done callback as the last parameter:

local function fetchData(url, __done)
    local __co = coroutine.create(function()
        local response = coroutine.yield({httpGet, url, n = 2})
        return response
    end)
    __nebra_async_drive(__co, __done)
end

Await Expression​

await expr compiles to coroutine.yield(...). The async driver resumes the coroutine when the result is ready.

Runtime Driver​

Nebra emits a __nebra_async_drive helper that:

  1. Resumes the coroutine
  2. If the yielded value is a table with a function, calls it with a callback
  3. The callback resumes the coroutine with the result
  4. When the coroutine finishes, calls __done with the final result

Callback Adaptation​

await automatically adapts callback-style APIs. If you await a function that takes a callback as its last parameter, Nebra injects the callback:

-- If httpGet(url, callback) is callback-style:
local data = await httpGet(url)
-- Nebra injects the callback automatically