Re: one more little thing about upvalues

["Peter Shook" <pshook@...>]

jason zhang wrote:
> I am one of those who don't know upvalues and closure:-)
> However, I'd like to find a tutorial about that. The manual
> seems to be not clear on that. Thanks for any help.

Rici uses closures a lot so look for his stuff in the list archive.

there is stuff on the wiki:  http://lua-users.org/wiki/MinimisingClosures

Any good book on Perl should will talk about closures.
http://www.perl.com/pub/a/2002/05/29/closure.html

And here is a simple example.

$ cat closure.lua

-- To quote Damian Conway:
--   A closure is just a function that refers to one or more lexical
--   variables declared outside the function itself.

do
 local name = 'bob'

 function print_name()
   print('name', name)
 end
end

-- cannot access name directly from here on, but print_name can

print_name()

-- Here is a simple example:

function make_counter(count)
 local function counter()
   local c = count
   count = count + 1
   return c
 end
 return counter
end

counter1 = make_counter(10)
counter2 = make_counter(100)

print( 'c1', counter1(), 'c2', counter2() )
print( 'c2', counter2() )
print( 'c2', counter2() )
print( 'c1', counter1() )
print( 'c1', counter1(), 'c2', counter2() )

$ lua closure.lua
name    bob
c1      10      c2      100
c2      101
c2      102
c1      11
c1      12      c2      103

- Peter Shook

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