promises.lua

   1 local PromisePrototype = {}
   2
   3 function PromisePrototype:__run_handler(func, ...)
   4         local values = {pcall(func, ...)}
   5
   6         if table.remove(values, 1) then
   7                 self:__resolve_raw(unpack(values))
   8         else
   9                 self:__reject_raw(values[1])
  10         end
  11 end
  12
  13 function PromisePrototype:__add_child(promise)
  14         if self.state == "resolved" then
  15                 promise:__resolve(unpack(self.values))
  16         elseif self.state == "rejected" then
  17                 promise:__reject(self.reason)
  18         else
  19                 table.insert(self.__children, promise)
  20         end
  21 end
  22
  23 function PromisePrototype:__resolve_raw(...)
  24         self.state = "resolved"
  25         self.values = {...}
  26         self.reason = nil
  27
  28         for _, child in ipairs(self.__children) do
  29                 child:resolve(...)
  30         end
  31 end
  32
  33 function PromisePrototype:__reject_raw(reason)
  34         self.state = "rejected"
  35         self.values = nil
  36         self.reason = reason
  37
  38         local any_child = false
  39
  40         for _, child in ipairs(self.__children) do
  41                 child:reject(reason)
  42         end
  43
  44         assert(any_child, "Uncaught (in promise): " .. reason)
  45 end
  46
  47 function PromisePrototype:then_(on_resolve, on_reject)
  48         local promise = Promise()
  49         promise.__on_resolve = on_resolve
  50         promise.__on_reject = on_reject
  51
  52         self:__add_child(promise)
  53
  54         return promise
  55 end
  56
  57 function PromisePrototype:catch(func)
  58         local promise = Promise(function() end)
  59         promise.__on_reject = func
  60
  61         self:__add_child(promise)
  62
  63         return promise
  64 end
  65
  66 function PromisePrototype:resolve(...)
  67         assert(self.state == "pending")
  68
  69         if self.__on_resolve then
  70                 self:__run_handler(self.__on_resolve, ...)
  71         else
  72                 self:__resolve_raw(...)
  73         end
  74 end
  75
  76 function PromisePrototype:reject(reason)
  77         assert(self.state == "pending")
  78
  79         if self.__on_reject then
  80                 self:__run_handler(self.__on_reject, reason)
  81         else
  82                 self:__reject_raw(reason)
  83         end
  84 end
  85
  86 Promise = setmetatable({}, {
  87         __call = function(_, resolver)
  88                 local promise = {
  89                         state = "pending",
  90                         __children = {},
  91                 }
  92
  93                 setmetatable(promise, {__index = PromisePrototype})
  94
  95                 if resolver then
  96                         resolver(
  97                                 function(...)
  98                                         promise:resolve(...)
  99                                 end,
 100                                 function(...)
 101                                         promise:reject(...)
 102                                 end
 103                         )
 104                 end
 105
 106                 return promise
 107         end
 108 })
 109
 110 function Promise.resolve(...)
 111         local args = {...}
 112         return Promise(function(resolve)
 113                 resolve(unpack(args))
 114         end)
 115 end