This won't work: https://github.com/NoahTheDuke/vim-just/blob/19a7f07c16f010f6c0a3eb74041787bb6e460fbb/ftdetect/just.lua#L7 That function returns 1 or 0 depending on the result, both of which evaluate as truthy in the context of a Lua expression.