Registers and types

kushti · May 19, 2020, 9:05pm

By @morphic with very minor edits of mine:

ErgoScript (as well as ErgoTree) is typed, so accessing a register is an operation which involves some expected type given in brackets. Thus SELF.R4[Int] expression should evaluate to 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:

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.
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.
If the register contains a value v of type T other then Int. Then val x = SELF.R4[Int] fails, because there is no way to return a valid value of type Option[Int]. The value of register is present, so returning it as None would break the typed semantics of registers collection.

In some use cases one register may have values of different types. To access such register an additional register can be used 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
   }