Re: "dynamic" closures

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

Mark Hamburg <mark@grubmah.com> dixit:

> > 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  
> 
> This is exactly how true closures work in all languages with closures. The creation of g closes over the references to n and x (which as you've written them are global but might as well be local -- you'll get the same results). If you use local variables, a new instance of n is created whenever x is executed, but there is presumably only one point where x is created hence all references lead back to the same value.
> 
> Perhaps you have experience with a poor man's closure implementation in which the values are copied into the closure at the time the closure is created? That meets many of the needs of a closure, but it is just a weak copy. Consider for example wanting to write:
> 
> 	local sum = 0
> 	table.foreachi( array, function( i, v ) sum = sum + v end )
> 	print( "Sum = ", sum )
> 
> In order for this to work, the reference to sum in the function have to refer to the mutable variable declared outside the function.
> 
> Mark

Thank you, your explaination makes things much easier. I like the expression "mutable variable", as it clear highlights that what is referred to by the closure is indeed a variable (while the _value(s)_ may even be immutable, like in your example).
All this was difficult for me, I guess, because usual references are (masked) pointer; Ids are adresses. If upvalue where identified by address, the code above wouldn't work, I guess.
May I suggest that upvalues are analog to parameters passed by name? (which is indeed not Lua's paradigm)

Denis
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