Lua: Avoiding Circular Requirements

Question

Problem

How can I avoid the following Lua 5.1 error when trying to make a circular request?

$ lua main.lua lua: ./bar.lua:1: loop or previous error loading module 'foo' stack traceback: [C]: in function 'require' ./bar.lua:1: in main chunk [C]: in function 'require' ./foo.lua:1: in main chunk [C]: in function 'require' main.lua:1: in main chunk [C]: ? 

File structure

main.lua

 require "foo" require "bar" print (Foo.getName()) print (Bar.getName()) 

foo.lua

 require 'bar' Foo = {} Foo.name = 'foo' function Foo:getName() return Foo.name .. Bar.name end 

bar.lua

 require 'foo' Bar = {} Bar.name = 'bar' function Bar:getName() return Bar.name .. Foo.name end 

Expected Result

 $ lua main.lua foobar barfoo 

Answer 1

Another way to solve this problem would be to change the structure of the code and extract the “mutual” functionality into the third module, which will require both Foo and Bar .

Answer 2

Decision

main.lua

 Foo = Foo or require "foo" Bar = Bar or require "bar" print (Foo.getName()) print (Bar.getName()) 

foo.lua

 Foo = {} Bar = Bar or require "bar" Foo.name = 'foo' function Foo:getName() return Foo.name .. Bar.name end return Foo 

bar.lua

 Bar = {} Foo = Foo or require "foo" Bar.name = 'bar' function Bar:getName() return Bar.name .. Foo.name end return Bar 

Explanation

Since you are setting global variables, you can check if a file (aka; global already defined) is required before trying another:

 Bar = Bar or require "bar" 

Then your bar.lua will have to return the definition of Bar;

 Bar = {} -- ... return Bar 

This will not completely solve the problem, since bar.lua expects Foo to determine. To solve this problem, you can define a dummy variable with the same name:

 Foo = {} Bar = Bar or require "bar" 

This is only possible because you delay the use of Foo until the function is called. If you want to call Foo.name from the bar.lua , you will have the same circular dependency problem.