Scala in intellij cannot find my function definition? - scala

Question

I implemented a toy worksheet in intellij.
abstract class Nat:
def isZero: Boolean
def predecessor: Nat
def successor: Nat
def + (that: Nat): Nat
def - (that: Nat): Nat
end Nat
object Zero extends Nat:
def isZero: Boolean = true
def predecessor: Nat = ???
def successor: Nat = Succ(this)
def + (that: Nat): Nat = that
def - (that: Nat): Nat = if that.isZero then this else ???
override def toString = "Zero"
end Zero
class Succ(n: Nat) extends Nat:
def isZero: Boolean = false
def predecessor: Nat = n
def successor: Nat = Succ(this)
def + (that: Nat): Nat = Succ(n + that)
def - (that: Nat): Nat = if that.isZero then this else n - that.predecessor
override def toString = s"Succ($n)"
end Succ
val two = Succ(Succ(Zero)) // : Succ =
val one = Succ(Zero)
two + one
two - one
//one - two
Other worksheet has been evaluated fine so far, but when I evaluated this one, I encountered some errors show below:
// defined class Nat
4 | def successor: Nat = Succ(this)
| ^^^^
| Not found: Succ
// defined class Succ
1 |val two = Succ(Succ(Zero))
| ^^^^
| Not found: Zero
1 |val one = Succ(Zero)
| ^^^^
| Not found: Zero
1 |two + one
|^^^
|Not found: two
1 |two - one
|^^^
|Not found: two
How to resolve the not found error?

Answer 1

The specific problem is that worksheets are executed in order and Succ has not been defined when Zero is being defined.
More generally I suggest switching to a full App rather than a worksheet. Writing and running an App is pretty east. Create a subclass of App with your code:
object Worksheet extends App {
println("Hello world")
}
Then just click the green arrow to the left of the object and IntelliJ will compile and run the program.
Or right click and select Debug so that you can put breakpoints on key points the code and then step through line by line.