The wikipedia definition could use a little explication. So a closure is a procedure and an environment in which to execute that procedure (the environment of the program at the time the closure is created). This is irrespective of whether or not the closure is bound to a name or is anonymous, whether the environment is empty, or contains bindings.
So what's an environment? The 'lexical environment' is what the procedure uses to look up variable references. In other words, a symbol table, a mapping between symbols and values. Naturally, variable declarations add to/pollute the environment. Going back to your case above, the addone closure's environment could be represented as a hash of 'i' => 1. The environment could be affected by many things, import declarations in Java/Scheme, using namespace in C++, #include <foo> in C, use Food::Bare, declaring more variables, etc. Now, let's say addone didn't have a reference to $i - then it's local environment would just happen to be empty.
Using this definition of closure, every procedure or sub you write is a closure. In the addone example, there are two bindings for $i, one in the closure's local environment (which is checked first), and another in the top level environment, when you set $i=10. When you invoke addone, we first check the environment packaged up with the addone closure for an entry for $i. We find that there is an entry for $i=>1, we increment that local entry for $i, (which doesn't affect the outer lexical environment's entry for $i=>10), and then return $i + 2.
If this is the definition of closure you're more interested in, you should definitely learn more about free and bound variables and scoping and its effects on the environment. Maybe learn Scheme, even ;-).
Hope that helps!
Allen
In reply to Re: Nailing down closures!
by oylee
in thread Nailing down closures!
by mattford63
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