Registers

ErgoScript (as well as ErgoTree) is typed, so accessing a register is an operation that involves some expected type given in brackets.

Thus, the SELF.R4[Int] expression should evaluate a valid value of the Option[Int] type.

For example, val x = SELF.R4[Int] expects the register - if it is present - to have type Int.

There are three cases:

1. If the register doesn't exist. Then val x = SELF.R4[Int] succeeds and returns the None value, which conforms to any value of type Option[T] for any T. (In the example above, T is equal to Int). Calling x.get fails when x is equal to None, but x.isDefined succeeds and returns false.
2. If the register contains a value v of type Int. Then val x = SELF.R4[Int] succeeds and returns Some(v), which is a valid value of type Option[Int]. In this case, calling x.get succeeds and returns the value v of type Int. Calling x.isDefined returns true.
3. If the register contains a value v of type T other than Int. Then val x = SELF.R4[Int] fails because there is no way to return a valid value of type Option[Int]. The register value is present, so returning it as None would break the typed semantics of registers collection.

Within some use cases, a register may have values of different types. To access such a register, we can use an additional register as a tag.

val tagOpt = SELF.R5[Int]
val res = if (tagOpt.isDefined) {
  val tag = tagOpt.get
  if (tag == 1) {
    val x = SELF.R4[Int].get
    // compute res using value x is of type Int
  } else if (tag == 2) {
    val x = SELF.R4[GroupElement].get
    // compute res using value x is of type GroupElement
  } else if (tag == 3) {
    val x = SELF.R4[ Array[Byte] ].get
    // compute res using value x of type Array[Byte]
  } else {
    // compute `res` when `tag` is not 1, 2 or 3
  }
} else {
  // compute value of res when register is not present
}