Trying to create an empty array with space for at least 1 element, add an element at position 0, then retrieve the element

array-test.pony

actor Main
  new create(env: Env) =>
    let arr: Array[Bool] = Array[Bool](1)

    try
      arr(0) = true
      env.out.print(arr(0).string())
    else
      env.out.print("fail")
    end

What you are looking for in this case is...

actor Main
  new create(env: Env) =>
    let arr = Array[(Bool|None)].init(None, 10)

    try
      arr.insert(8, true)
      env.out.print(arr(8).string())
    else
      env.out.print("fail")
    end

actor Main
  new create(env: Env) =>
    let arr: Array[Bool] = Array[Bool](1)

    try
      arr.insert(0, true)
      env.out.print(arr(0).string())
    else
      env.out.print("fail")
    end

If you note from the Array constructor...

    """
    Create an array with zero elements, but space for len elements.
    """

And from update that you were trying to use

    """
    Change the i-th element, raising an error if the index is out of bounds.
    """

If you were using numbers, you could use undefined but that isn't.

From a type perspective, the Bool or None that I use is what you are trying to define here.