// requires java21
import java.util.Optional;

record Int(int x) implements expr {}
record Plus(expr a, expr b) implements expr {}
record Times(expr a, expr b) implements expr {}
record Div(expr a, expr b) implements expr {}
record Neg(expr a) implements expr {}

sealed interface expr permits Int, Plus, Times, Div, Neg
{
  default Optional<java.lang.Integer> eval() {
      return
        switch (this) {
        case Int(int x) -> Optional.of(x);
        case Plus(expr a, expr b) ->
            a.eval().flatMap(x -> b.eval().map(y -> x+y));
        case Times(expr a, expr b) ->
            a.eval().flatMap(x -> b.eval().map(y -> x*y));            
        case Neg(expr a) -> a.eval().map(x -> -1 * x);
        case Div(expr a, expr b) ->
            b.eval().flatMap(y -> {
                    if (y!=0) return a.eval().map(x -> x/y);
                    else return Optional.empty();
                });
        };
    } // eval

    default String to_string() {  // can't override toString in interface
        switch (this) {
        case Int(int x): return x+"";
        case Plus(expr a, Neg(expr b)):
            return "("+a.to_string() + " - " + b.to_string() + ")";
        case Plus(expr a, expr b):
            return "("+a.to_string() + " + " + b.to_string() + ")";
        case Times(expr a, expr b):
            return a.to_string() + " * " + b.to_string();
        case Div(expr a, Int(int x)) when x==1 : return a.to_string();
        case Div(expr a, expr b):
            return a.to_string() + " / " + b.to_string();
        case Neg(Neg(expr a)): return a.to_string();
        case Neg(expr a): return "-"+a.to_string();
        }
    }// to_string
}//interface - looking more like a Rust trait...

public class exprnew {
  public static void main(String[] args) {
      expr e = new Plus(new Int(3), new Neg(new Times(new Int(5),new Neg(new Int(7)))));
      e.eval()
          .ifPresent(r -> System.out.printf("%s evals to %d\n",e.to_string(),r));
  }//main
}