Note: This is a crosspost of an entry I wrote in this year's vimways.org advent calendar. If you're interested in Vim, I recommend you pop on over there and read the other articles too.
I love you; you complete me.
- Dr. Evil
I first came to Vim by recommendation. I was looking for a good Python IDE (at the time I was new to the language) and one recommendation was to use Vim with a variety of plugins added on top. That Vim could do a lot of the things I thought only an IDE could do came as a bit of a shock. I spent a summer as an intern using Emacs at a Unix terminal, but didn't have enough curiosity at the time to use it any differently from notepad.exe. I spent that summer wishing I had automatic features for completion, indentation, and all the things that made me appreciate the IDE's I used in college. How naive I was!
So how was I directed to achieve powerful programming completions in Vim? By the use of a plugin called YouCompleteMe. My experience with it was okay, at least to start with. It took a while to install and get working, but that didn't bother me at the time since I was just playing around with it at home and the stakes were low if it suddenly broke. I did notice it slowed Vim down. Like a lot. But that was mainly when it was starting up and I didn't know enough to find it frustrating. Probably the first thing that really bothered me about the plugin was that the embedded Jedi language server used more memory than Vim itself. The other recommended plugins were similarly laggy, and I eventually went in search of something better.
What I found was Vim itself.
Did you know that Vim has built-in facilities for completions? It works admirably well out of the box too, but with a little bit of additional setup it can be great. Let's take a look at what Vim has on offer regarding completion and see what it takes to fully leverage it.
Completion in Vim
Completion in Vim is powerful, but not necessarily straightforward. Read:h ins-completion and you'll see what I mean:
Completion can be done for:
1. Whole lines |i\_CTRL-X\_CTRL-L|
2. keywords in the current file |i\_CTRL-X\_CTRL-N|
3. keywords in 'dictionary' |i\_CTRL-X\_CTRL-K|
4. keywords in 'thesaurus', thesaurus-style |i\_CTRL-X\_CTRL-T|
5. keywords in the current and included files |i\_CTRL-X\_CTRL-I|
6. tags |i\_CTRL-X\_CTRL-]|
7. file names |i\_CTRL-X\_CTRL-F|
8. definitions or macros |i\_CTRL-X\_CTRL-D|
9. Vim command-line |i\_CTRL-X\_CTRL-V|
10. User defined completion |i\_CTRL-X\_CTRL-U|
11. omni completion |i\_CTRL-X\_CTRL-O|
12. Spelling suggestions |i\_CTRL-X\_s|
13. keywords in 'complete' |i\_CTRL-N| |i\_CTRL-P|
Vim is smart enough to pull completion data from a variety of sources, but in turn expects users to know which source will provide the best answer and to invoke the correct keymap to draw the desired completions. It's not a huge hurdle in terms of a learning curve, but it's not as simple as hitting tab either.
The first thing one should do when trying to learn Vim's completion system is to disable any completion plugins and learn these keymaps. Getting comfortable with them will also help you learn and remember where Vim can pull completion information from. You should also read :h 'completefunc' and :h 'complete' for more information on user-defined completion and the complete option.
Now that we have a cursory understanding of completion in Vim, let's take a deeper look at tags and how they figure into completion.
Introduction to tags in Vim
One source of completion in Vim is tag completion, which pulls from a special file called–appropriately—a tags file. Tags files are collections of identifiers (e.g., function names) that are compiled into a single file along with references to their location in a source tree. Vim is capable of using a (properly formatted) tags file for a variety of use cases, among them navigating your source code à la Visual Studio and completion.
By default Vim doesn't do anything with a tags file except read it. See:h 'tags' to learn how to configure where Vim looks for tags files. Vimdoc also contains a very good introduction to tags more generally, so I won't spend any more time here introducing them. Let's move on and take a look at how we generate tags files.
Introduction to ctags
Tags files solve the problem of navigating and completing code in a given project, but they also create a problem: how do we create the tags file, and how do we keep it up to date? It would be a pain to manually maintain the tags file even for a small project; it would be all but impossible to do it for a large project like the Linux kernel. Luckily no one has to maintain a tags file. There are plenty of utilities to do that for you, usually bearing the name ctags, or some variant. One very popular choice is called Exuberant Ctags, which has the virtue of being extendable via regular expressions placed into a .ctags file, but the drawback of not having been updated since 2009. Another increasingly popular option is Universal Ctags, which functions as a drop-in replacement for Exuberant Ctags and is actively maintained. I've had good luck with both.
Tags files and the tools that generate them have a long history alongside advanced text editors. The history-of-computing nerd in me likes knowing that I'm using the same tool programmers have used since the early days of BSD Unix. It's also a testament to how strong of a solution they provide that folks are still using them 40 years later.
Generating tags Files
Manually
When we speak of manually generating tags files, we're talking about using any one of the aforementioned tags utilities to generate the tags file. If you're the type of person who takes pleasure in at least understanding how to do things from the command line, you should consult the manual page for your selected tags utility. Take special note of the flags necessary to recurse through all of the subdirectories if you want to generate a tags file for an entire project in one shot.
Automatically
You can always use your ctags utility to generate your tags files from the command line, but that's a heck of a lot of back and forth between your text editor and your shell, and I doubt anyone who tries to do that will enjoy the experience for long. So let's look at ways to generate them automatically.
If you only ever see yourself using tags files with Vim, then maybe a plugin will interest you. I used Gutentags for a long time, and found it "just works" as advertised. It has sane defaults, but lots of opportunities to customize its behavior, which you'll see if you visit the above link.
In spite of that, I ended up moving in a different direction with managing my tags files. There were several reasons, but the main one is that I like to think of tags files as separate from Vim, something the text editor consumes without having to manage. It's an opinionated view of things, but I increasingly didn't like to configure my text editor to manage my tags files. So I went in search of another method, and what I found was the Tim Pope method, which I've since implemented myself. Rather than using Vim itself to manage tags files, this method uses local Git hooks to rebuild the tags whenever any of a handful of common Git operations are performed. The result is a system that also just works, but does so in a half-a-dozen lines of shell script rather than a few hundred lines of Vimscript. Gotta keep that Vim config tight.
As a bonus, if you already use Tim Pope's Fugitive Git plugin (and you should), this method handily places your tags file where that plugin tells Vim to look for it—in the .git folder. Of course the shell-script approach is infinitely configurable, so you can ultimately place the tags file wherever you want. One could also tailor this for other distributed SCM tools (e.g., Mercurial).
Generically speaking, there are other options as well. You could set a filesystem watcher to watch your project tree and run ctags any time a file changes. A task runner like Grunt might be a viable option too, especially for web developers. The goal is to automate the task of (re)generating your tags file, so there is likely to be no shortage of options.
Tying It All Together
That brings us back to where we started, to the issue of code completion in Vim. Yes, Vim does offer native code completion (completing from tags is done with C-x, C-] in insert mode). No, it's probably not as powerful as what you could get with something like a Jedi plugin à la YouCompleteMe, but I've found it satisfies my needs more often than not, with :grep (or my own :GrepJob filling the gap nicely in a more native fashion.
There's more you can do here too. For instance, if you find yourself instinctively reaching for the tab key in order to complete a word, there is VimCompletesMe, which takes advantage of all of Vim's built-in completions through the clever use of an omni completion function. It works, but users do give up some control over selecting what data source Vim uses for a particular completion. I used this plugin for a while after I gave up on YouCompleteMe, but ultimately removed it because it effectively made the tab key ambiguous in Insert mode. Sometimes I wanted to insert an actual tab character, but got a completion instead.
With all of this in place, it's natural to ask whether a language server is even necessary with Vim. I don't intend here to suggest an answer to that question, but I will say that many of the solutions to date for language server integration in Vim have seemed like more trouble than they're worth. That said, with the advent of Vim 8 and its asynchronous capabilities, there is headroom for these solutions to improve, and I expect the best among them to become more compelling in the near future.
I do not recommend coming to Vim with a mindset of creating an IDE in your terminal. That said, Vim is a very powerful tool and if you invest the time to learn how it works it will take you very far. In other words, use Vim for all it's worth before looking for a plugin to help you out. Anyone who (like me) jumps right to installing a bunch of plugins—whether in a spree of grabbing anything that looks interesting or just to copy someone else's configuration—will likely end up with an unmaintainable mess of a tool that doesn't work consistently, may not work at all, or works about as slow as the IDE you wanted to break free of.
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