Idiomatically defining dynamic tasks in SBT 0.13? - scala

Question

I'm moving an SBT plugin from 0.12 over to 0.13. At various points in my plugin I schedule a dynamic set of tasks onto the SBT build graph.
Below is my old code. Is this still the idiomatic way to express this, or is it possible to leverage the macros to make everything prettier?
import sbt._
import Keys._
object Toplevel extends Build
{
lazy val ordinals = taskKey[Seq[String]]("A list of things")
lazy val times = taskKey[Int]("Number of times to list things")
lazy val inParallel = taskKey[Seq[String]]("Strings to log in parallel")
lazy val Foo = Project( id="Foo", base=file("foo"),
settings = Defaults.defaultSettings ++ Seq(
scalaVersion := "2.10.2",
ordinals := Seq( "First", "Second", "Third", "Four", "Five" ),
times := 3,
inParallel <<= (times, ordinals, streams) flatMap
{ case (t, os, s) =>
os.map( o => toTask( () =>
{
(0 until t).map( _ => o ).mkString(",")
} ) ).join
}
)
)
}
Apologies for the entirely contrived example!
EDIT
So, taking Mark's advice into account I have the following tidier code:
import sbt._
import Keys._
object Toplevel extends Build
{
lazy val ordinals = taskKey[Seq[String]]("A list of things")
lazy val times = taskKey[Int]("Number of times to list things")
lazy val inParallel = taskKey[Seq[String]]("Strings to log in parallel")
def parTask = Def.taskDyn
{
val t = times.value
ordinals.value.map(o => ordinalTask(o, t)).join
}
def ordinalTask(o: String, t: Int) = Def.task
{
(0 until t).map(_ => o).mkString(",")
}
lazy val Foo = Project( id="Foo", base=file("foo"),
settings = Defaults.defaultSettings ++ Seq(
scalaVersion := "2.10.2",
ordinals := Seq( "First", "Second", "Third", "Four", "Five" ),
times := 3,
inParallel := parTask.value
)
)
}
This seems to be nearly there, but fails the build with:
[error] /home/alex.wilson/tmp/sbt0.13/project/build.scala:13: type mismatch;
[error] found : sbt.Def.Initialize[Seq[sbt.Task[String]]]
[error] required: sbt.Def.Initialize[sbt.Task[?]]
[error] ordinals.value.map(o => ordinalTask(o, t)).join

Answer 1

You can use Def.taskDyn, which provides the new syntax for flatMap. The difference from Def.task is that the expected return type is a task Initialize[Task[T]] instead of just T. Translating your example,
inParallel := parTask.value
def parTask = Def.taskDyn {
val t = times.value
ordinals.value.map(o => ordinalTask(o, t)).joinWith(_.join)
}
def ordinalTask(o: String, t: Int) = Def.task {
(0 until t).map(_ => o).mkString(",")
}