promises.lua

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