https://photino.gitbooks.io/rust-notes/content/programming-paradigms.html
rutst学习网站
https://kaisery.gitbooks.io/rust-book-chinese/content/content/Closures%20%E9%97%AD%E5%8C%85.html
https://rustwiki.org/rust-by-example/flow_control/if_let.html if let
https://kaisery.gitbooks.io/rust-book-chinese/content/content/if%20let.html