Writing an addition visitor function in Kotlin - kotlin

Question

I'm trying to write a visitor function in Kotlin that adds two integers together. I've been working off of some sample code and I can't figure out what these .value or .visit functions are. It doesn't seem to be declared in the sample code, so I'm unsure how to declare it in my code. Whenever I compile the code, I get an error saying that value is an unresolved reference.
Relevant Kotlin code:
package backend
import org.antlr.v4.runtime.*
import grammar.*
abstract class Data
class IntData(val value: Int): Data() {
override fun toString(): String
= "Int($value)"
}
class Context(): HashMap<String, Data>() {
constructor(parent: Context): this() {
this.putAll(parent)
}
}
abstract class Expr {
abstract fun eval(scope: Context): Data
fun run(program: Expr) {
try {
val data = program.eval(Context())
println("=> ${data}")
} catch(e: Exception) {
println("[err] ${e}")
}
}
}
class IntLiteral(val value: Int): Expr() {
override fun eval(scope:Context): Data
= IntData(value)
}
enum class Op {
Add,
Sub,
Mul,
Div
}
class Arithmetic(
val op: Op,
val left: Expr,
val right: Expr): Expr() {
override fun eval(scope: Context): Data {
val x = (left.eval(scope) as IntData).value
val y = (right.eval(scope) as IntData).value
return IntData(
when(op) {
Op.Add -> x + y
Op.Mul -> x * y
Op.Sub -> x - y
Op.Div -> x / y
}
)
}
}
}
class Compiler: PLBaseVisitor<Expr>() {
val scope = mutableMapOf<String, Expr>()
override fun visitAddExpr(ctx: PLParser.AddExprContext): Expr {
val xValue = this.visit(ctx.x)
val yValue = this.visit(ctx.y)
val result = xValue.value + yValue.value
return IntLiteral(result)
}
}
Relevant Antlr Grammar:
expr : x=expr '+' y=expr # addExpr
| x=expr '-' y=expr # subExpr
| x=expr '*' y=expr # mulExpr
| x=expr '/' y=expr # divExpr
;
Code I'm trying to execute:
val test = """
x=1+2
print(x)
"""
fun parse(source: String): PLParser.ProgramContext {
val input = CharStreams.fromString(source)
val lexer = PLLexer(input)
val tokens = CommonTokenStream(lexer)
val parser = PLParser(tokens)
}
val testTree = parse(source1)
val testTree = parse(source1)
fun execute(program: Expr?) {
if(program == null) {
println("Program is null.")
return
}
try {
val data = program.eval(Context())
println("> ${data}")
} catch(e: Exception) {
println("[err] ${e}")
}
}
execute(testProgram)
Code from sample:
data class NodeValue(val value: Int)
val visitor = object: CalcBaseVisitor<NodeValue>() {
override fun visitAddition(ctx: CalcParser.AdditionContext): NodeValue {
val xValue = this.visit(ctx.x)
val yValue = this.visit(ctx.y)
return NodeValue(xValue.value + yValue.value)
}
override fun visitValue(ctx: CalcParser.ValueContext): NodeValue {
val lexeme = ctx.Number().getText()
return NodeValue(lexeme.toInt())
}
}

Answer 1

You don’t show the code for your program.eval() method.
The eval function would need to create an instance of your Visitor. (You’ve done that and called it visitor).
You also have the root expr node in you program variable.
Now you would have your visitor “visit” that node and save the return value:
val nodeVal = visitor.visit(program)
At that point nodeVal.value will have the result of visiting that expression.
note: since you’re doing the evaluation in your visitor, there’s not really any use for your Arithmetic class (unless you refactor your visitor to use it instead of just doing the math, but I don’t see much value in that as the visitor is already pretty easy to read).