When writing your neovim config, you sometimes might want to inspect the value of a variable or object. Using :echo is not very useful, if you use lua. Lua's print statement comes to mind, but it only works on primitive types, with tables you simply get a useless output such as table 0x0193eer56. For those cases, there is vim.pretty_print to inspect the content in human-readable format. However, typing :lua vim.pretty_print (…) all the time is quite tedious. Putting pp = vim.pretty_print into our config helps somewhat, but it's still quite a bit to type: :lua pp().
So why not create our own vim command? We can use :command to register our own command, and let it run a function of our choice, passing the argument from the vim command line to that function (even without needing to use parenthesis). In that function, we accept an object and use pretty_print, wrapped in luaeval, because :command passes the argument as a string instead of the expression. The final result is a small four-liner to put somewhere into our config:
1cmd [[:command! -nargs=1 I lua inspectFn(<f-args>)]]
2function inspectFn(obj)
3 vim.pretty_print(fn.luaeval(obj))
4end
Now we can just run :I obj instead of :lua vim.pretty_print(obj). Much more convenient.