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Registers#

A box can have up to six additional registers with typed data. A script can access its registers (as well as registers of input and output boxes of the spending transaction).

Rather than storing a single amount (like BTC), a eutxo box has some registers to store arbitrary values, like its native tokens.

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

For example, 

// Assign the value of the R4 register of the current box to the variable x
val x = SELF.R4[Int]

expects the register (if it is present) to have the type Int. Thus, the SELF.R4[Int] expression should evaluate a valid Option[Int] type value.

There are three possible cases when attempting to get the value of the register SELF.R4[Int]:

  1. The register does not exist, so SELF.R4[Int].isDefined is false.
  2. The register contains a value of type Int, in which SELF.R4[Int].get returns that value, and SELF.R4[Int].isDefined` is true.
  3. The register contains a value of a type other than Int, in which case SELF.R4[Int] fails and cannot return a valid value of type Option[Int].

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

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