1. Don't have NULL at all 2. Have sum types 3. Have several useful kinds of them (like Option and Result) 4. Allow people to make their own to serve their sp

1. Don't have NULL at all 2. Have sum types 3. Have several useful kinds of them (like Option and Result) 4. Allow people to make their own to serve their specific scenario Basically Rust, Haskell, OCaml.