Explain SBT syntax like compile:compile - scala

Question

Although SBT is called simple build tools, it's far from being simple. I still can't get this syntax in sbt session like compile:compile? What's the difference between this and just compile?

Answer 1

The main trick in here is in scopes. If you want really understand how SBT works then always use three commands:
show <setting> - Displays the value of the specified setting.
show <task> - Evaluates the specified task and display the value returned by the task.
inspect <key> - shows info about setting
inspect tree <key> - displays key and its dependencies in a tree structure.
There are many other good commands, but this will help you most to understand the basics of SBT.
As for the syntax. Each build consists of settings, tasks, projects and scopes. There are too much to tell about them, there is a good explanation given on the official site. And the syntax you gave is all about this terms, for example let's take a look at:
compile:scalaSource::sourceDirectory
1 2 3
1 - it is a Compile scope
2 - it is a dependant Setting
3 - dependency Setting
If you type inspect scalaSource you'll see that, if you type just scalaSource in the SBT session this will call scalaSource in the compile scope(compile:scalaSource), this explains the difference between compile:compile and compile, this are the same (call inspect on compile). The second thing you should take a look at in inspect scalaSource is the Dependencies: part: compile:sourceDirectory, so scalaSource depends on the sourceDirectory setting in compile:sourceDirectory and if you've seen some build on github, in *.sbt or *.scala build files it's written like:
sourceDirectory in (Compile, scalaSource) := ....
Just for the exercise, call:
show compile:scalaSource::sourceDirectory
and you'll see the output like this: <project-dir>/src/main and then call:
set sourceDirectory in (Compile, scalaSource) <<= baseDirectory(_ / "src" / "sc")
and then again:
show compile:scalaSource::sourceDirectory