How to redefine a key in SBT?

Question

How can you redefine a key in SBT (as opposed to extend or define)?

I currently have the following in my build script (project/build.scala):

fullClasspath in Runtime <<= (fullClasspath in Runtime, classDirectory in Compile) map { (cp, classes) => (cp.files map {
  f: File =>
    if (f.getName == classes.getName) {
      val result = new File(f.getParent + File.separator + "transformed-" + f.getName)
      if (result.exists) result else f
    } else f
}).classpath }

It extends the classpath in Runtime by adding, for each directory in Compile, a new directory with the same name but with transformed- prepended to the front.

(If you are wondering why, I have a plugin which performs program transformation on the bytecode after compilation but before packaging, and selective recompilation gets very confused if you overwrite the original files.)

My problem is the following: This extends the original key, and therefore the classpath contains the original directories from Compile, plus the renamed copies, but I only want the renamed ones from Compile.

I tried to do something along the lines of

fullClasspath in Runtime := ...

but I don't know what to put on the right-hand side.

---

I've marked the answer since it lead me directly to the solution, but my final solution was to modify the above code snippet to the following

fullClasspath in Runtime := (fullClasspath in Runtime).value.files.map {
  f: File =>
    if (f.getName == (classDirectory in Compile).value.getName) {
      val result = new File(f.getParent + File.separator + "transformed-" + f.getName)
      if (result.exists) result else f
    } else f
}.classpath

which does exactly what I wanted, and is slightly better style.

Answer 1

Here's a little experiment I did just now at the sbt command line showing that it's definitely possible to remove something unwanted from fullClasspath:

% sbt
> show runtime:fullClasspath
[info] List(Attributed(.../target/classes),
            Attributed(.../jars/scala-library-2.10.4.jar),
            Attributed(.../jars/asm-all-3.3.1.jar))
> set fullClasspath in Runtime := (fullClasspath in Runtime).value.files.filterNot(_.getName.containsSlice("asm")).classpath
> show runtime:fullClasspath
[info] List(Attributed(.../target/classes),
            Attributed(.../jars/scala-library-2.10.4.jar))

Voila — the "asm-all-3.3.1" entry is gone.

Note that this isn't about <<= vs :=. The latter is just macro-based sugar for the former. The experiment turns out the same if I avoid := and do this instead:

set fullClasspath in Runtime <<= (fullClasspath in Runtime) map
  {_.files.filterNot(_.getName.containsSlice("asm")).classpath}