Do file only once in Lua

Question

I was wondering if there's a way to do a lua file only once and have any subsequent attempts to do that lua file will result in a no-op.

I've already thought about doing something akin to C++ header's #if/else/endif trick. I'm wondering if there's a standard way to implement this.

James

• lutz: What do you mean by "do a lua file"?
• jameszhao00: I'm using dofile() at the moment from multiple places on 1 file.
• nonchip: i'd always prefer require before dofile, except you explicitely want to do the whole file on each call

Answer 1

well, require pretty much does that.

require "file" -- runs "file.lua"
require "file" -- does not run the "file" again

• jameszhao00: Awesome thanks. I kept thinking I needed a module definition for the lua files too for some reason.
• RBerteig: If the file run by require doesn't create a module table or return something, require stores true in package.loaded. Otherwise, it stores the value returned by executing the file. The module function interacts with this well. This is all explained in PiL, but the online copy is not as good as the print edition on these details.

Answer 2

The only problem with require is that it works on module names, not file names. In particular, require does not handle names with paths (although it does use package.path and package.cpath to locate modules in the file system).

If you want to handle names with paths you can write a simple wrapper to dofile as follows:

do
 local cache={}
 local olddofile=dofile
 function dofile(x)
 if cache[x]==nil then
 olddofile(x)
 cache[x]=true
 end 
 end
end

• jameszhao00: THC4k's solution above works on files too. If a file is called abc.lua, require('abc') works.
• lhf: Yes, but not if you do dofile"/home/lhf/scripts/abc.lua".
• nonchip: actually you can also use require"/path/to/file.lua"
• nonchip: ok, sorry, misremembered that. what you can do is require "relative/path/to/file" (without .lua), because the default search path includes ./?.lua.

Answer 3

based on lhf's answer, but utilising package, you can also do this once:

package.preload["something"]=dofile "/path/to/your/file.lua"

and then use:

local x=require "something"

to get the preloaded package again. but that's a bit abusive...