stags: Scala tags generator


Using graalvm: Recommended

Requires native-image.

Clone this repository and call sbt cli/graalvm-native-image:packageBin. This will create a native image in cli/target/graalvm-native-image/stags. Copy this file somewhere in your $PATH.

Using Coursier:

coursier bootstrap co.pjrt:stags-cli_2.12:0.5.0 -o stags

If you want to use stags tag generation as a library, you can add it to sbt with:

libraryDependencies += "co.pjrt" % "stags_2.12" % "0.5.0"

Using Nailgun:

You can use Coursier to create a standalone cli for starting Stags with Nailgun like this:

coursier bootstrap --standalone co.pjrt:stags-cli_2.12:0.5.0 \
  -o stags_ng -f --main com.martiansoftware.nailgun.NGServer
stags_ng & // start nailgun in background
ng ng-alias stags co.pjrt.stags.cli.Main
ng stags --version

You can then create an alias for ng stags if that's still too much typing.

Caveats and tips:

  • You must call ng ng-alias after every restart of the nailgun server. You could create a script to do this
    • You could also simply make an alias in your terminal (ie: alias stags=ng co.pjrt.stags.cli.Main).
  • If you are running multiple Nailgun instances (for example, one for stags and one for scalafmt) you must run one of them in a different port.
    • In the above example, simply call stags_ng $new_port to run the stags ng instance in a different port. Then all ng calls need to have the flag --nailgun-port $new_port in them.
    • You could in theory simply feed both jars (ie: stags and scalafmt) into the same ng instance but beware this could cause classpath conflicts between the two (or more) jars.


stags ./

This will fetch all Scala files under the current directory. The tags file will be generated in ./tags. To place the tags file somewhere else, do:

stags ./ -o path/to/tags


The two main differences between stags and a general ctags generator like Universal CTags is its ability to understand Scala code (with all its intricacies) and the ability to produce qualified tags.

Understanding Scala intricacies and static tagging them

What are static tags? Static tags are tags for "static functions". In the C world this means functions that can only be used in the file where they are defined; you could think of them as "private". Vim understand static tags and will match them first before anything else.

Static tags lend themselves nicely to private field and functions, so stags marks private statements and fields as static, while taking care of some Scala intricacies.

If a def/val/class/ect is private within its file, then it is static. If it is private for some large scope, then it isn't static. This means that if it is private[X] then we check if X is an enclosing object within the file. However, if X isn't an enclosing object in this file, then we mark it as non-static. For example

package org.example.somepackage.test

object X {
  object Y {
    private[X] def f = …

object K {
  private[somepackage] def g = …

In this example, f would be static, but g isn't because g might be accessed from outside the file.

Other cases that are marked as static are:

  • constructor fields in classes (ie: in class X(a: Int, b: String, c: Boolean), a, b and c will all be static)
    • But non-static for the first parameter group of case classes (since those are accessible by default)
      • case class X(a: Int)(b: Int) <- a will be non-static, but b will be static
    • Any that are marked as "private" are static
  • the single field in an implicit class/case class
    • implicit class X(val x: Int) <- x is static
    • this is done because chances are that x will never be accessed anywhere but this file
  • all implicit things (val, defs, class, etc)
    • these things are rarely, if ever, accessed via their tokens

Qualified tags

A common pattern found when importing conflicting fields is to use them in a qualified form. For example:

import org.example.SomeObject
import org.example.OtherObject

In order to differentiate between the two, stags generates tags for all fields along with an extra tag that combines their parent with the tag itself. Note that stags never generates qualified tags for fields/methods in trait and class (only objects and package objects) since said fields/methods cannot be qualifiedly referenced.

Following code, by default, would produce three tags: Example, foo and

package object test {
  object Example {
    def foo(...)

The depth of the qualified tags is controlled by --qualified-depth. Setting it to three (3) would produce a third tag

Vim support for qualified tags

Vim won't understand such a tag right off the bat. The following modification is required:

function! QualifiedTagJump() abort
  let l:plain_tag = expand("<cword>")
  let l:orig_keyword = &iskeyword
  set iskeyword+=\.
  let l:word = expand("<cword>")
  let &iskeyword = l:orig_keyword

  let l:splitted = split(l:word, '\.')
  let l:acc = []
  for wo in l:splitted
    let l:acc = add(l:acc, wo)
    if wo ==# l:plain_tag

  let l:combined = join(l:acc, ".")
    execute "ta " . l:combined
  catch /.*E426.*/ " Tag not found
    execute "ta " . l:plain_tag

nnoremap <silent> <C-]> :<C-u>call QualifiedTagJump()<CR>

Tips for tagging jars

Vim support for jars

Vim supports the ability to look at the contents of a zip file by default in the following way:

vim zipfile:/path/to/zip::/path/inside/zip

This will open contents of the path inside of the zip in a new buffer. Since jars are just zip files, we can use that to create tags for jar files. This is specially useful when wanting to tag external sources.

Before we can use that though, the following settings in vim need to be disabled:

set notagrelative " Required due to the way it tries to search for the tag relative to the tags file

au BufEnter zipfile:/*.scala set nomodifiable " This isn't required, but nice if you want it to behave like an IDE

You could get away with keeping tagrelative on IFF your set tags is set to to ./tags and nothing else. If you search for the tags file outsise of the current directly, then you MUST disable tagrelative.

With that set, we can now call stags, but we must pass --absolute-files. Since we disabled tagrelative we must now create tags as absolute tags (path to files are absolute, not relative).

stags --absolute-files -o /path/to/output ./

Tagging source files: Global cache

Since the tags setting in vim allows for multiple tag files (in order of preference), we can have a local tags files and a global one for all sources like so:

set tags=./tags,/home/user/globalTags
$ cd /home/user
$ stags --absolute-files -o globalTags /home/user/.cache/coursier/ /home/user/.ivy2/cache/

Some caveats:

  • This will create a massive tags file.
  • Since you can have different derisions of libraries in the cache, this will create a lot of duplicate tags (one for each version). This can lead to confusion in the future.

Tagging source files: Downloading source files to local project

Another strategy is to instead use sbt to copy over the source jar that the project requires, and tagging only those files. For this to happen you will need a small plugin:

import sbt.Keys._
import sbt._
import java.nio.file.{FileAlreadyExistsException, Files, Path, Paths}
import scala.util.{Failure, Success, Try}

object DownloadSourcesPlugin {
  val downloadSources = taskKey[Unit]("Download sources")
  val downloadSourcesLocation = settingKey[File]("Download sounds location")
  val downloadSourcesTypes = settingKey[List[String]](
    "Types of sources to download (default: javadoc, source)"

  def downloadSettings =
      // We store all sources in one directory for all projects.
      // This allows us to remove duplicates below, which are common in multi-projects.
      // Change this to `target.value / "externalSources"` if you want to have one per project.
      downloadSourcesLocation := file(".") / "target" / "externalSources",
      cleanFiles += baseDirectory.value / downloadSourcesLocation.value.toString,
      downloadSourcesTypes := List("sources"),
      downloadSources := {
        val report = updateClassifiers.value
        val log = streams.value.log
        val dir = downloadSourcesLocation.value
        val types = downloadSourcesTypes.value
        def matchesTypes(f: File) =
          types.exists(t => f.getName().endsWith(t + ".jar"))
        dir.mkdirs() {
          case target if matchesTypes(target) =>
            val newLink = (dir / target.getName).toPath
            Try(Files.createLink(newLink, target.toPath)) match {
              case Success(_)                             => ()
              case Failure(e: FileAlreadyExistsException) => ()
              case Failure(e)                             => log.error(e.getMessage)
          case _ => ()

After adding this plugin, we can call sbt downloadSources.

This plugin will copy the source files from the cache into target/externalSources for a project. stags can then pick them up from there (alongside the source files). We can then tag sources and jars separately:

stags --file-types jar -o tags-external target/externalSources
stags --file-types scala -o tags ./

For this to work we need to also change our tags setting like so:

set tags=./tags,tags-external

One way to add this plugin to all projects without committing anything into your repos is to:

  • Place the file above in ~/.sbt/1.0/plugins/
  • Create a GlobalPlugin in ~/.sbt/1.0/plugins/ with the following contents
import sbt._

object GlobalPlugin extends AutoPlugin {
  override def requires = sbt.plugins.JvmPlugin
  override def trigger = allRequirements

  override def projectSettings = DownloadSourcesPlugin.downloadSettings