Minimal-ish JS Promise implementation in lua

promise.lua

--[[
Promises fully contained in a function.
This function returns a promise where you can resolve/reject just like in JS.
Example use case:
---
local promise = make_promise()
http.get(url, function(data) promise:resolve(data) end)
promise
  :next(function(data) return json.parse(data) end)
  :catch(function(err) print(err) end)
---
]]
local function make_promise(status, queue, err, rejector)
  local COMPACT_THRESHOLD = 50
  local STATUS = {
    PENDING = 0,
    RESOLVED = 1,
    REJECTED = 2
  }

  local obj = { status = status or STATUS.PENDING, queue = queue or { n = 1 }, err = err, rejector = rejector }
  function obj:try_compact_queue()
    if #self.queue > COMPACT_THRESHOLD then -- configurable
      local n, j = #self.queue, 1
      for i = self.queue.n, n do
        if i ~= j then
          self.queue[j] = self.queue[i]
          self.queue[i] = nil
        end
        j = j + 1
      end
    end
  end
  function obj:exec()
    local proc = self.queue[self.queue.n]
    self.queue.n = self.queue.n + 1
    self:try_compact_queue()  -- this is added for fun, can be omitted but may be useful

    local errdata
    local function on_error(msg)
      errdata = msg .. "\n" .. debug.traceback(nil, 2)
    end
    local result = { xpcall(proc, on_error, table.unpack(self.resdata)) }

    if not result[1] then -- call rejector
      self.status = STATUS.REJECTED
      self.rejdata = { errdata }
    else
      self.status = STATUS.RESOLVED
      self.resdata = {table.unpack(result, 2)}
    end
  end
  function obj:run_next()
    if self.queue.n <= #self.queue and self.status == STATUS.RESOLVED then
      self:exec()
      return self:run_next()
    elseif self.status == STATUS.REJECTED and self.rejector then
      self.rejector(table.unpack(self.rejdata))
    end
  end
  function obj:next(fn)
    table.insert(self.queue, fn)
    local new = make_promise(self.status, self.queue, self.err, self.rejector)
    new:run_next()
    return new
  end
  function obj:catch(fn)
    self.rejector = fn
    local new = make_promise(self.status, self.queue, self.err, self.rejector)
    new:run_next()
    return new
  end
  function obj:resolve(...)
    if self.status == STATUS.PENDING then
      self.status = STATUS.RESOLVED
      self.resdata = {...}
      self:run_next()
    end
    return self
  end
  function obj:reject(...)
    if self.status == STATUS.PENDING then
      self.status = STATUS.REJECTED
      self.rejdata = {...}
      self:run_next()
    end
    return self
  end
  return obj
end