How to avoid repetition of function in luasnip in nixvim?

[amalgame21]

Hi, I am new to luasnip.
I use plugins.luasnip.fromLua = [{paths = ./snippets}] to load the snippets.
I followed this guide to configure it, it reuse some of the function like this
Avoiding repetition of get_visual functions.
How can I do the same thing in nixvim?
I tried extraFiles but I cannot see the corresponding file generated in the ./result directory after I ran nix build.
Also I don't know how exactly extraFiles works, it have little description and the source code are so complicated to me.
I am using standalone mode (build with makeNixvimWithModule )

[amalgame21]

extraFiles works