Type tests ask whether a value has a type. Casts state that a value should be treated as a type.

let value := 42;
let isInt := value :? Int;
let same := value :?> Int;
same;

Test with `:?`

let value := 42;
let isInt := value :? Int;

The result is Bool. Use it before a branch when runtime shape matters.

Cast with `:?>`

let value := 42;
let number := value :?> Int;

A cast is stronger than a test. Use it only when the program has already proved the shape.

The standard library uses casts for bit-width helpers.

let Bits := import "@std/bits";
let byte := Bits.fromNat8(10 :?> Nat);

Keep conversions close to the boundary that needs them.