How to read the modified setting defined in my own sbt plugin - sbt

Question

I tried to create my own sbt plugin by following this guide, and had the following code:
build.sbt in myplugin
sbtPlugin := true
lazy val plugin = (project in file(".")).
settings(
name := "myplugin",
version := "0.1-SNAPSHOT",
scalaVersion := "2.10.4"
)
HelloPlugin.scala in myplugin
package sbthello
import sbt._
import Keys._
object HelloPlugin extends AutoPlugin {
object autoImport {
val greeting = settingKey[String]("greeting")
val obfuscate = taskKey[String]("Obfuscates files.")
}
import autoImport._
lazy val baseSettings: Seq[Def.Setting[_]] = Seq(
greeting := "Hi!",
obfuscate := {
println(greeting.value)
greeting.value + " value"
}
)
override val projectSettings =
inConfig(Compile)(baseSettings)
}
the build.sbt in test project is
lazy val usage = (project in file("."))
.enablePlugins(HelloPlugin).
settings(
name := "sbt-test",
version := "0.1",
scalaVersion := "2.10.4")
.settings(
greeting := "Hello"
)
with the files above, the execution result is:
> show obfuscate
Hi!
[info] Hi! value
> show greeting
[info] Hello
the obfuscate task cannot read the "Hello" value
if modify greeting := "Hi!" in HelloPlugin.scala to greeting in obfuscate := "Hi!",
the obfuscate task can read the "Hello" value now
> show greeting
[info] Hello
> show obfuscate
Hello
[info] Hello value
but now I cannot remove greeting := "Hello" in buildInDemo.sbt, or else it will have such errors:
[error] References to undefined settings:
[error]
[error] compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error] Did you mean compile:obfuscate::greeting ?
[error]
[error] compile:greeting from compile:obfuscate ((sbthello.HelloPlugin) HelloPlugin.scala:40)
[error] Did you mean compile:obfuscate::greeting ?
[error]

Answer 1

configuration scoping
override val projectSettings =
inConfig(Compile)(baseSettings)
This scopes all the settings in baseSettings into Compile configuration. So it's the same as saying:
Define greeting in Compile setting.
Refer to the greeting in Compile setting from obfuscate in Compile task.
So in build.sbt you should customize as
greeting in Compile := "Hello"
You can access this setting from the shell as:
> compile:greeting
If you don't want the behavior, don't put baseSettings in inConfig(...).
task and configuration scoping
greeting in obfuscate is task scoping. So combined with inConfig(...) that'll require
greeting in (Compile, obfuscate) := "Hello"
to customize.
For more details see Scopes.