Racket 哈希表
创建不可修改的哈希表：
> (define ht (hash "key1" "value1" 'key2 1234 3 (list 1 2) (list 'key4) 'value4))
取值：
> (hash-ref ht "key1")
"value1"
> (hash-ref ht 'key2)
1234
> (hash-ref ht 3)
'(1 2)
> (hash-ref ht (list 'key4))
'value4
创建可修改的哈希表：
> (define ht (make-hash))
> (hash-set! ht "key1" 'v1)
> (hash-set! ht (list 1 2) #hash(("k" . "v")))
> (hash-ref (hash-ref ht (list 1 2)) "k")
"v"