The only way to make it the norm is to make it the norm.

Maybe someone sees this...

  type Expr =
    | Const of int
    | CustomOneArg of (int -> int) * Expr
    | CustomTwoArgs of (int -> int -> int) * Expr * Expr

  interface Expr {
  }

  class Const extends Expr {
      public Integer val;
      public (Integer val) {
          this.val = val;
      }
  }

  class CustomOneArg extends Expr {
      public Function<Integer, Integer> fun;
      public int val;
      public (Function<Integer, Integer> fun, Integer val) {
          this.fun = fun;
          this.val = val;
      }
  }

  class CustomTwoArgs extends Expr {
      public BiFunction<Integer, Integer, Integer> fun;
      public Integer valA;
      public Integer valB;
      public (BiFunction<Integer, Integer, Integer> fun, Integer valA, Integer valB) {
          this.fun = fun;
          this.valA = valA;
          this.valA = valB;
      }
  }

That said, I don't want to put words into the author's mouth, but this particular article looks more "by FPers for FPers" than FP advocacy.