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- Subject: Re: Flush the term upvalue from 5.1 manual
- From: Rici Lake <lua@...>
- Date: Fri, 25 Nov 2005 15:59:01 -0500
On 25-Nov-05, at 2:46 PM, Philippe Lhoste wrote:
I believe upvalues has a sense in Lua 4.0 and before, where you needed
a special syntax to access local variables of upper scope.
There was/is a thing with "captured (or frozen) value" with which I
was never comfortable with, coming from a procedural world...
I don't even know if this trick is still possible in the lexically
I think that's true. "upvalues" in Lua 4.0 really were values (as are
the residual upvalues in C closures in Lua 5).
If you really need to capture the current value of a variable (or an
expression) in Lua 5, you need to do it explicitly, although you can
reuse the variable name if you want to confuse your friends.
If I recall Lua 4 syntax correctly, it looked like this:
local a = 3
local function foo3()
a = 4
--> would have printed 3, the value of a when captured.
Removing the % and running that in Lua 5, it will print 4, the value of
a at the time foo3 is called.
You could achieve the old behaviour like this:
local a = 3
-- Here's the confuse your friends bit
local a = a
a = 4 -- this is not the same a as in foo3
I have to say that although once in a blue moon I find myself doing
that, it's pretty rare, and I don't miss Lua 4's upvalues nearly as
much as I thought I might when they were about to disappear.