There's no special keyword, just a "generic" type `IO<T>` defined in standard library which has a similar "tainting" property like `async` function coloring.

Any side effect has to be performed inside `IO<T>` type, which means impure functions need to be marked as `IO<T>` return. And any function that tries to "execute" `IO<T>` side effect has to mark itself as returning `IO<T>` as well.