Question

I’ve always liked the way that in Javascript, you can set the value of the this pointer by doing f.call(newThisPtrValue). I wrote something to do this in lua, which works:

_G.call = function(f, self, ...)
    local env = getfenv(f)
    setfenv(f, setmetatable({self = self}, {__index = env}))
    local result = {f(...)}
    setfenv(f, env)
    return unpack(result)
end

There are a couple of things I’m unsure about:

1. I expect there’s a performance overhead for unpack({...}). Is there a way around this?
2. Is this likely to horribly break the environment of the function in any way?
3. Is this a Really Bad Idea™?

Answer 1

One of the excellent benefits of Lua’s pseudo-OOP is that it’s super easy to do this already:

local Person = {}
function Person:create( firstName, lastName )
  local person = { firstName=firstName, lastName=lastName }
  setmetatable(person,{__index=self})
  return person
end
function Person:getFullName()
  return self.firstName .. " " .. self.lastName
end
local me  = Person:create( "Gavin", "Kistner" )
local you = Person:create( "Eric", "Someone" )
print( me:getFullName() )
--> "Gavin Kistner"
print( me.getFullName( you ) )
--> "Eric Someone"

I have written an article that discusses this (among other things):
Learning Lua: Pseudo-OOP Syntax and Scope.

Edit: Here’s a continuing example like jQuery’s each:

local Array = {}
function Array:new(...)
  local a = {...}
  setmetatable(a,{__index=self})
  return a
end
function Array:each(callback)
  for i=1,#self do
    callback(self[i],i,self[i])
  end
end
function Array:map(callback)
  local result = Array:new()
  for i=1,#self do
    result[i] = callback(self[i],i,self[i])
  end
  return result
end
function Array:join(str)
  return table.concat(self,str)
end

local people = Array:new( me, you )

people:each( function(self,i)
  print(self:getFullName())
end )
--> "Gavin Kistner"
--> "Eric Someone"

print( people:map(Person.getFullName):join(":") )
--> "Gavin Kistner:Eric Someone"