Defining a sequential task with Def.sequential 

sbt 0.13.8 added Def.sequential function to run tasks under semi-sequential semantics. To demonstrate the sequential task, let’s create a custom task called compilecheck that runs compile in Compile and then scalastyle in Compile task added by scalastyle-sbt-plugin.

Here’s how to set it up

project/build.properties 

sbt.version=0.13.11

project/style.sbt 

addSbtPlugin("org.scalastyle" %% "scalastyle-sbt-plugin" % "0.8.0")

build.sbt 

lazy val compilecheck = taskKey[Unit]("compile and then scalastyle")

lazy val root = (project in file(".")).
  settings(
    compilecheck in Compile := Def.sequential(
      compile in Compile,
      (scalastyle in Compile).toTask("")
    ).value
  )

To call this task type in compilecheck from the shell. If the compilation fails, compilecheck would stop the execution.

root> compilecheck
[info] Compiling 1 Scala source to /Users/x/proj/target/scala-2.10/classes...
[error] /Users/x/proj/src/main/scala/Foo.scala:3: Unmatched closing brace '}' ignored here
[error] }
[error] ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed

Looks like we were able to sequence these tasks.