1. let 和 let* 区别

// 这是对的(let* ([x (random 4)]       [o (random 4)]       [diff (number->string (abs (- x o)))])  (cond [(> x o) (string-append "X wins by " diff)]        [(> o x) (string-append "O wins by " diff)]        [else "cat's game"])    )// 这是错的(let ([x (random 4)]       [o (random 4)]       [diff (number->string (abs (- x o)))])  (cond [(> x o) (string-append "X wins by " diff)]        [(> o x) (string-append "O wins by " diff)]        [else "cat's game"])    )

lets 变量定义后可以直接使用 let 不可以.