uniqueness typing

I found the following text when visiting the Clean (a functional language) site: "Clean is the only functional language in the world which has a special type system, uniqueness typing. It enables to update function arguments destructively retaining the purity of the language." Then I have some questions: 1. Can anyone explain what does "update function arguments destructively" mean? 2. If this is a desirable thing, why does Haskell not implement it? Thanks, -- Andre