Add configurable buffer_alternate_command option

[jordan-yee]

The buffers user-mode provides the a key mapping to switch to an alternate buffer, but it is currently just a pass-through to ga.

This PR adds a configurable option for setting a custom command for switching to an alternate buffer. The intended use case is to provide a command to switch to a test or spec file as the alternate file, if one exists, rather than the previous buffer.

While looking through the filetype scripts bundled with Kakoune I noticed that more than one filetype script provides an alternate file command along with an 'alt' alias. I think it would be a good idea to simply attempt the 'alt' command alias as a default for the 'a' mapping, with ga as a fallback because it would Just Work for certain file types without changing the behavior for others. However, I went the route of leaving the default behavior unchanged and up to the user, while suggesting this in the README instead.

[krobelus]

Thanks for this patch, I have some complaints ;)

• I think making features require configuration is not really appropriate
  for plugin. I like it because I can install it, add a mapping for
  : enter-user-mode buffers<ret>, and I'm done. Plus, this behavior is already
  easy to set up with a quick map global buffers a ': alt<ret>'.
• :alt toggles between test/implementation or .c/.h file. OTOH, ga
  switches to the last buffer. I use both of them, so I'd like to bind both.

I've resorted to typing :alt but maybe we can add these mappings?

map global buffers a 'ga'			-docstring 'alternate buffer ↔'
map global buffers t ': alt<ret>'		-docstring 'alternate file ↔'

[jordan-yee]

Thanks for the response!

I can appreciate not wanting to add configuration for the plugin, and recognize that I can accomplish what I'm after fairly easily in my local config. So then is there anything left to change in the plugin itself?

Bottom line: I think ba should exec : alt<ret> instead of ga since it's just as easy to use ga to accomplish ga. That way, ba and ga each do something useful and different (there are only so many 2-key combos available).

I should mention 3 primary motives for this PR:

1. The included ba keybinding provided by the buffers user mode is redundant to the built-in ga key binding. And there is another "previous" buffers mapping, bp.
2. I think that the term "alternate" is misleading and inaccurate if it really means "previous", or "last".
3. The :alt alias exists for multiple file types, and I think it makes sense for the buffers user-mode to take advantage of this.

My opinionated change would be to modify the ba mapping to use the alt command instead of ga. Adding a new mapping instead, like you suggest, would be another valid way to utilize the existence of :alt in this plugin, though I'd just as soon change ba in my own config.

In either case, there are 2 follow up questions to consider:

• What should happen if the alt is not defined?
• What should happen If alt is defined, but throws an error (no alternate file found or something)?

If in a context in which no alt command exists, then ga is a possible fallback that should always work. The reason I added a custom command for ba mapping instead of just : alt<ret> was to define this behavior. However, I could see someone wanting to just see the echo'd error message instead.

[krobelus]

I see, that all makes sense! Changing the binding to always run :alt sounds promising (I'd make it error if unset).
ga should probably still be in there, for discoverability (?). The descriptions "last" and "previous" are already taken, so maybe something like "toggle".. I'm not sure

[jordan-yee]

I updated the PR to include a simplified buffers-alternate command without any configuration options.

It now attempts the :alt command on ba. If that errors, it then uses ga instead, which would happen if :alt is not defined or if the target alt file doesn't exist). I'm not sure I think this is the ideal behavior, but it's the next step.

A few questions:

1. What should happen if the :alt command isn't defined?
2. What should happen if the :alt command errors (probably because the alternate file doesn't exist).
3. What should the mapping be for this?
   • As previously stated, my intent is to update the ba mapping to what [I think] it should be.
   • Alternatively, a new mapping could be added. I don't know that I can improve on thebt you mentioned. or A could be used, but the extra modifier isn't great.