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SOOO confused about module varargs
Posted: Thu Aug 27, 2015 6:41 pm
by parallax7d
So the Quickie library has an init module. When I print ... I get the path to the library:
But when I print out the arguments in arg I get 3 different values:
Code: Select all
arg[-2] = '/Applications/love.app/Contents/MacOS/love'
arg[-1] = 'embedded boot.lua'
arg[1] = '[path to love zip file]'
The question: Why are ... and arg different? When I use varargs as a function parameter I can access ... or arg and get the same exact values back. I always assumed they were 2 interfaces into the same data store? Is this a Lua thing or is Love doing something explicit with module varargs?
Re: SOOO confused about module varargs
Posted: Thu Aug 27, 2015 6:50 pm
by slime
'arg' being a table containing the vararg statement was only a feature of Lua 5.0 and older. Lua 5.1 (released in 2006)
changed it to just be '...'. LÖVE uses LuaJIT 2, which is based on Lua 5.1, so 'arg' doesn't contain varargs.
Unfortunately the online version of the Programming in Lua book is the first edition, which was written for Lua 5.0, so it will tell you incorrect things about features that have changed in 5.1.
The standalone Lua interpreter executable, as well as the LÖVE executable, populate a global 'arg' table with the command-line arguments passed to the program.
Re: SOOO confused about module varargs
Posted: Thu Aug 27, 2015 7:14 pm
by parallax7d
That clarifies things, thanks. By the way, which version is Love using 5.1.4 or 5.1.5? When I print out the version it just says 5.1
Re: SOOO confused about module varargs
Posted: Thu Aug 27, 2015 7:35 pm
by parallax7d
After some experimentation, it seems like both arg and ... will work in functions with a ... param. But they seem to conflict with each other.
works
Code: Select all
function argTable (...)
print(arg[1])
end
argTable(3)
> 3
works
Code: Select all
function vararg (...)
print(...)
end
vararg(3)
> 3
huh?
Code: Select all
function combined (...)
print(...)
print(arg[1])
end
combined(3)
> 3
> stdin:3: attempt to index local 'arg' (a nil value)
Since there seems to be support for either syntax, and tables are sometimes nicer to work with than functions - #arg vs. select('#', ...) for example, I'm confused on what to do. Is it the case that Lua only 'half' dropped access to arg, and it should really be considered deprecated and unsafe? Or is it that you should just choose one style or the other? Things are starting to make more sense, but still kinda confused on what would be best practice.
Re: SOOO confused about module varargs
Posted: Thu Aug 27, 2015 7:40 pm
by slime
LÖVE 0.9.2 uses
LuaJIT 2.0.3, which is based on Lua 5.1.x. The changes between Lua 5.1.4 and 5.1.5 were just
bugfixes in Lua's own code, but LuaJIT doesn't use Lua's codebase so it's not really based on any specific 5.1.x release of Lua.
parallax7d wrote:Is it the case that Lua only 'half' dropped access to arg, and it should really be considered deprecated and unsafe?
Lua 5.1 can be built with a "5.0 compatibility" mode enabled, which will let you use 'arg' like that even though it's deprecated. LuaJIT (which LÖVE uses) is not built like that, and other builds of Lua 5.1 might not have that backwards-compatibility feature enabled either.
You can easily replicate the behaviour of Lua 5.0's 'arg' by doing this:
Code: Select all
function foo(...)
local args = {...}
print(#args)
end
So there's no reason to use 'arg' even when it's available.
Re: SOOO confused about module varargs
Posted: Thu Aug 27, 2015 10:01 pm
by davisdude
Maybe arg in this case is just an accidental global?