"dynamic" closures

[spir <denis.spir@...>]

Hello,

Was trying to understand Lua closures better (the indications in historical docs are not enough for my limited brain), when I stepped on this:

x = 1
function f()
  n = 1
  function g(a)
    print (x + n + a) 
  end
  return g
end

g= f()
g(1)	--> 3

x = 2
g(1)	--> 4

I'm a bit surprised the code works "as unexpected" ;-) As a consequence, g becomes totally referentially "opaque", or do I misinterpret? (Maybe I miss a basic point, but for me this smells strong like naughty bugs. Even more since x is a global.) Do you have use cases for such an idiom (that could hardly be refactored into harmless expression)?

About implementation, it seems to indicate Lua embebs symbolic references (*) to outer _variables_ in reachable scopes, when creating a closure for g0. Not pointer references (**) to _values_; otherwise the replacement of x would not be seen by the closure. Before trying, I thought the outcome would be 3, meaning values are kind of frozen at closure creation time (would be the case if references were pointers).

[I imagine this behaviour may match the one of closures in functional languages (?), but anyway the whole paradigm is so different...]

Denis

(*) by name, indirect access, second table lookup
(**) by address, direct access, no second lookup
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