Vim Tabpanel: Can You Use Script-Local Functions?

Hey guys! Today, we're diving deep into a fascinating corner of Vimscript: Can we use script-local functions with the :set tpl command, specifically for tabpanels? This question arises from a user's curiosity about following the :h tabpanel documentation, which seems to lean towards using global functions as recommended by Bram himself. Let's unravel this mystery together!

Understanding the Tabpanel Option in Vim

To kick things off, let's get a solid grasp on what the tabpanel option in Vim actually does. This option, as detailed in :h tabpanel, allows you to customize the appearance and behavior of the tabline, that strip at the top of your Vim window showing your open tabs. By setting the tabpanel option, you can specify a command that Vim will execute to generate the content of the tabline. This opens up a world of possibilities for creating dynamic and informative tablines that suit your workflow perfectly.

Now, the official documentation suggests using a global function for this purpose, like so: set tabpanel=%!TabPanel(). This means Vim will call the global function TabPanel() to get the tabline content. But, our inquisitive user wondered if script-local functions could be used instead. This is where things get interesting! Script-local functions, defined within a Vimscript file, offer a way to keep your function namespace clean and avoid naming conflicts. The user attempted something like set tabpanel=%!<SID>TabPanel(), where <SID> represents the script ID, a way to reference script-local entities. The question is, does this work? And if not, why?

To really understand this, we need to delve into how Vim handles function execution in this context. When Vim executes the command specified in tabpanel, it does so in a particular scope. The scope determines which functions and variables are accessible. Global functions, as the name suggests, are always accessible. Script-local functions, however, are tied to the script in which they are defined. This scoping difference is crucial to understanding why using script-local functions with tabpanel might present a challenge. We'll explore this challenge and its potential solutions in the following sections, making sure you're equipped with the knowledge to create awesome custom tablines!

The Challenge: Script Scope and tabpanel

So, what's the snag with using script-local functions for tabpanel? It boils down to how Vim manages scope when it executes the command specified in the tabpanel option. When Vim needs to update the tabline, it essentially runs the command you've set in tabpanel in a global context. Think of it like this: Vim is saying,