Top level is a chunk
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- Subject: Top level is a chunk
- From: Gavin Wraith <gavin@...>
- Date: 2011年10月31日 22:59:46 GMT
It had somehow never hit home to me that the top level of a Lua program
is a chunk. Because of this I had been enclosing my programs within
do ... end structures so that I could use local variables at the top level
( sledgehammers for nuts, as the programs were tiny, but that is not
the point ). Then I experimented, writing ( in Lua 5.2beta )
local x = 8
print(x)
This ran fine, so all those do ... ends had been unnecessary. Disassembly
gives
local x = 8
--[[
1 LOADK 0 -1 ; 8
--]]
print(x)
--[[
2 GETTABUP 1 0 -2 ; _ENV "print"
3 MOVE 2 0
4 CALL 1 2 1
5 RETURN 0 1
--]]
whereas disassembly of
x = 8
print(x)
gives
x = 8
--[[
1 SETTABUP 0 -1 -2 ; _ENV "x" 8
--]]
print(x)
--[[
2 GETTABUP 0 0 -3 ; _ENV "print"
3 GETTABUP 1 0 -1 ; _ENV "x"
4 CALL 0 2 1
5 RETURN 0 1
--]]
I do not have to worry about all those extra do ... end structures because
disassembly of
do
local x = 8
print(x)
end
gives
do
local x = 8
--[[
1 LOADK 0 -1 ; 8
--]]
print(x)
--[[
2 GETTABUP 1 0 -2 ; _ENV "print"
3 MOVE 2 0
4 CALL 1 2 1
--]]
end
--[[
5 RETURN 0 1
--]]
- exactly the same bytecode as if the do ... end were not there.
I expect all this is not news to seasoned Lua users, but I thought the
disassembly listings might entertain.
--
Gavin Wraith (gavin@wra1th.plus.com)
Home page: http://www.wra1th.plus.com/