sbt does allow lazy dependencies

[jroper]

The README says sbt doesn't allow lazy dependencies. This is false, sbt does so allow lazy dependencies:

val flag = taskKey[Boolean]("a flag")
val taskA = taskKey[Unit]("task a")
val taskB = taskKey[Unit]("task b")
val dependentTask = taskKey[Unit]("dependent task")

taskA := println("task a")
taskB := println("task b")
flag := true

dependentTask <<= (flag, taskA.task, taskB.task).flatMap { (flag, a, b) =>
  if (flag) a else b
}

Running it shows that tasks A and B are lazily depended on:

> dependentTask
task a
> set flag := false
> dependentTask
task b
> 

[cvogt]

@jroper thanks, good to now. Corrected in #61

For reference, here is the CBT equivalent

def flag = true
def a = println("task a")
def b = println("task b")
def dependentTask = if(flag) a else b

The question about how to dynamically replace flag with falsein CBT is still open. Right now it would take creating another build that's based on the original and overrides the member. Related issue is #62