[Date Prev][Date Next][Thread Prev][Thread Next]
- Subject: Re: Visibility and upvalues (and macros)
- From: Luiz Henrique de Figueiredo <lhf@...>
- Date: Thu, 29 Jul 1999 00:09:24 -0300 (EST)
>From email@example.com Wed Jul 28 23:16:17 1999
>From: Dave Bollinger <DBollinger@compuserve.com>
>I wonder why can't a local function access an
>enclosing function's local variables?
>I understand the explanation that the
>variable might no longer exist - and I *assume* that this is because Lua
>needs to be able to make a compile-time decision on this issue, yes?
No. The whole point is that the "enclosing" scope might no longer exist
when the function is executed. Remember that functions are first-class values
and may be returned from functions:
return function (x) print(x+a) end -- ERROR: cannot access 'a'
If you expect g(2) to print 3, then that's exactly what upvalues are for:
you should write
return function (x) print(x+%a) end -- ok now
Otherwise, what would x+a mean inside g?
The whole point of the '%' notation is to signal that the *value* of 'a'
is being used, not the variable itself.
> How is it different from this (which does work):
fl = nil
local gl = fl -- no problem referencing a global, even if nil
print(gl) -- gl is of course itself still nil
This does work because g gets the value of the global fl when it runs.
If you do fl=12 after defining g but before calling it, 'g' will use 12,
not nil. To use nil, you should 'freeze' the value of 'fl', by using an upvalue.
So, upvalues have less to do with scope and visibility than with freezing
Upvalues take some getting used to, but they are powerful.