What do the % and %% operators do when setting up SBT dependencies?

Question

In Lift Web Framework, dependencies for Simple Build Tool (SBT) are specified in LiftProject.scala. That file includes this code:

  override def libraryDependencies = Set(
    "net.liftweb"             %% "lift-webkit"   % liftVersion % "compile->default",
    "net.liftweb"             %% "lift-mapper"   % liftVersion % "compile->default",
    "org.mortbay.jetty"       % "jetty"          % "6.1.22"    % "test->default",
    "junit"                   % "junit"          % "4.5"       % "test->default",
    "org.scala-tools.testing" %% "specs"         % "1.6.6"     % "test->default",
    "org.scala-lang"          % "scala-compiler" % "2.8.1"     % "test->default",
    "org.apache.tomcat"       % "tomcat-juli"    % "7.0.0"     % "test->default",
    "com.h2database"          % "h2"             % "1.2.138"
  ) ++ super.libraryDependencies

What do the % and %% operators do here? If I paste this code into the scala interpreter, it errors out, and neither % nor %% is defined for String or RichString. What's going on here?

Answer 1

The difference between these functions is that %% considers Scala version when SBT resolve dependency, so for example net/liftweb/lift-webkit_2.8.1/2.3/lift-webkit_2.8.1-2.3.jar will be downloaded from repo.

Regarding compile error - these methods should be called when some implicit methods defined in SBT class hierarchy that make actual conversion are in scope.

Best regards, Vladimir