Re: upvalues and closures

[Peter Shook <pshook@...>]

Roger D. Vargas wrote:
> I know it is in the manual, but seems that I dont clearly underestand it. So 
> can somebody explain me what are upvalues and closures? 

In Damian Conway's words:

  A closure is just a function that refers to one or more lexical
  variables declared outside the function itself.

There is a Lua 5.0 example here:
http://lua-users.org/lists/lua-l/2003-06/msg00059.html

Prior to Lua 5.0, Lua didn't have lexically scoped variables.  Instead 
Lua had upvalues which are just read-only values copied from local 
variables and stored within the function.  The same example in Lua 4 
requires the extra %

$ cat closure.lua

do
  local name = 'bob'

  function print_name()
    print('name', %name)
  end
end
-- cannot access name directly from here on,
-- but print_name has a read-only copy of name's value
print_name()

$ bin/lua -v closure.lua
Lua 4.0.1  Copyright (C) 1994-2000 TeCGraf, PUC-Rio
name    bob


The only change in the C API between Lua 5 and 4 -- as far as upvalues 
are concerned -- is the upvalues for C-closures are at virtual stack 
indexes given by lua_upvalueindex(i) instead of being pushed on the 
stack after the function arguments.  Also you can now access and alter 
the upvalues with lua_getupvalue and lua_setupvalue.

- Peter