gistfile1.rb

base_dir = Dir.pwd
build_dir = "#{base_dir}/build"
src_dir = "#{base_dir}/src"
lib_dir = "#{base_dir}/lib"

def classpath(location)
  places = File.join(location, "*.jar")
  return Dir.glob(places).join(":")
end

task :prepare do
  sh("mkdir -p #{build_dir}")
end

task :compile => [:prepare] do
  libs = classpath(lib_dir)
  cmd = "scalac -cp #{libs} -make:changed -d #{build_dir} #{src_dir}/*"
  sh(cmd)
end

task :clean do
  sh("rm -rf #{build_dir}")
end

task :run => [:compile] do
  libs = classpath(lib_dir) + ":#{build_dir}"
  cmd = "scala -cp #{libs} com.zentrope.Kernel"
  sh cmd do | ok, res |
    # Ignore what happens
  end
end

I don't know anything about Rake. Here's what I came up with by googling around for sample code. I use this to build my Scala app. The one problem I had is that the "classpath" function couldn't "see" the variables declared at the top. This is probably because my knowledge of Ruby is pretty thin.