I have a simple grammar
Expr -> Byte | Sum Expr Expr
and the code below is supposed to generate the random trees for it
object randomTree extends App {
// Expr -> Byte | Sum Expr Expr
def intRnd(len: Int, start: Int = 0): Int = (math.random * len toInt) + start
def byteRnd = intRnd(256, -128)
case class Value[A](value: A, parent: Type) extends Type {
def gen = this
override def toString = value + ":" + parent.getClass.getSimpleName
}
trait Type {
def gen: Type //def gen[A]: Value[A]
override def toString = getClass.getSimpleName
}
class OR/*Enum*/(alternatives: Type*) extends Type {
def gen = alternatives(intRnd(alternatives.length)).gen
}
class AND/*Sequence*/(alternatives: Type*) extends Type {
def gen = {
println("Sum " + alternatives)// prints: Sum WrappedArray(null, null)
Value(alternatives.map(_.gen), this)
}
}
object Expr extends OR(Sum, Byte) {
override def gen = Value(super.gen, this)
}
//object Sum extends Type { // everything is fine if this Sum is used
//def gen = Value(Expr.gen -> Expr.gen, this) }
println("Expr = " + Expr) // prints: Expr = Expr$
object Sum extends AND(Expr, Expr) // this Sum causes NPE
object Byte extends Type {
def gen = Value(byteRnd, this)
}
(1 to 10) foreach { i=> println(Expr.gen) }
}
I wonder why object Sum extends AND(Expr, Expr)
expands into AND(WrappedArray(null, null))
since Expr is a non-null object and how can I initialize the Expr such that Sum comes out right?
Since there is a circular reference between
Expr
andSum
that's caused null values, By Name Parameter can be used to solve this circular reference issue to delay the object's initialization. like:In the above code:
alternatives: => Array[Type]
as the By Name parameter to delay the circular object initialization time to avoid the null values.