Type Checking

is asks what a value is, and produces a bool.

mew

use std;
  
let number: any = 32;
println($"{number is i32}");
println($"{number is string}");

true
false

It reads a type you declare, a union, and an interface the value's type implements.

mew

use std;
  
pub interface Describable {
    fn describe() -> string;
}
  
pub type Point {
    pub field x: i32;
    pub field y: i32;
}
  
pub type Circle {
    pub field radius: i32;
}
  
impl Describable for Point {
    pub fn describe() -> string {
        return "a point";
    }
}
  
let boxed: any = new Point { x: 32, y: 40 };
  
println($"{boxed is Point}");
println($"{boxed is Describable}");
println($"{boxed is Circle}");

true
true
false

Asking about something the compiler already knows is a warning, since the answer cannot be anything else.

mew

let text = "hello";
let known = text is string;

Warning [MEW2041]: The given expression is always of the provided ('string') type

is is how a cast is made safe, because a cast that is wrong ends the program.

mew

use std;
  
pub interface Describable {
    fn describe() -> string;
}
  
pub type Point {
    pub field x: i32;
    pub field y: i32;
}
  
pub type Circle {
    pub field radius: i32;
}
  
impl Describable for Point {
    pub fn describe() -> string {
        return "a point";
    }
}
  
let boxed: any = new Point { x: 32, y: 40 };
  
if boxed is Circle {
    println($"{(boxed as Circle).radius}");
}

Note

A union case is not a type, so is cannot ask which case a value is. match is what asks that.