What is the difference between adding a dependency via libraryDependencies versus an sbt plugin?

Question

I am a Scala newbie trying to understand the nuances of the language and tooling. I'm looking through a sample at https://github.com/swagger-api/swagger-samples/tree/master/scala/scala-play2.4 that uses play and I notice that the play dependency is added like so:

addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.4.6")
https://github.com/swagger-api/swagger-samples/blob/master/scala/scala-play2.4/project/plugins.sbt

However in various other SO posts I see the dependency being added to libraryDependencies like so:

libraryDependencies ++= Seq("com.typesafe.play" %% "play" % "2.2.2")
https://stackoverflow.com/a/22215070/201657

or

libraryDependencies += "com.typesafe.play" %% "play-json" % "2.3.4"
https://stackoverflow.com/a/19438083/201657

What is the difference, and what are the implications, of these two techniques of adding a dependency? TIA.

Answer 1

As mentioned in linked SO answer, sbt-plugins are used to enhance build behaviour.

In case of addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.4.6") the pluging is modifying your build's libraryDependencies by following code. Thus the dependency management is done by the plugin.

If you choose to manage it yourself, you could use following without enabling the com.typesafe.play" % "sbt-plugin.

libraryDependencies ++= Seq("com.typesafe.play" %% "play" % "2.4.6")

If you choose to use sbt to launch your play application with hot reload functionality, you should consider using the sbt-plugin. But if you don't care about it, just add play as libraryDependencies.