如何区分Clojure中值为零的密钥中缺失的密钥？(How to distinguish a missing key from a key with value nil in Clojure?)

编程入门行业动态更新时间:2024-10-22 12:31:38

我们经常通过测试散列映射来查找某个键的存在，例如:data ，通过测试该键对nil的值的试验检索，如

(defn test-my-hash-map [my-hash-map] (if (:data my-hash-map) 42 "plugh!"))

对于任何包含key :data hash值的散列映射，其值都为42 ，除了值为nil 。

(map test-my-hash-map [{:data "Hello!"} ; ~~> 42 {:no-data "Yikes!"} ; ~~> "plugh!" {:data nil} ; ~~> "plugh!", but I need it to say 42 :( ])

我根本没有办法做到这一点，而这正在搞乱一些数据处理场景，我们从非Clojure源获取数据，其中缺少的密钥意味着“我在该密钥处没有数据”以及键值nil意味着“我在那里有数据，但无论出于何种原因，我都无法给你。” 我没有看到用Clojure代码区分这些情况的方法。我非常谦虚的解决方法是插入一个Java Shim，检测差异并在交给Clojure之前为特殊情况插入额外的列（键）。

We are often testing hash-maps for the presence of a certain key, say :data, by testing a trial retrieval of that key's value against nil, as in

(defn test-my-hash-map [my-hash-map] (if (:data my-hash-map) 42 "plugh!"))

This produces 42 for any hash-map that contains the key :data with any value, except the value nil.

(map test-my-hash-map [{:data "Hello!"} ; ~~> 42 {:no-data "Yikes!"} ; ~~> "plugh!" {:data nil} ; ~~> "plugh!", but I need it to say 42 :( ])

I don't see any way to do it at all, and this is messing up some data processing scenarios where we're getting data from non-Clojure sources in which a missing key means "I have no data at that key" and a key with a nil value means "I have data there, but I can't give it to you for whatever reason." I don't see a way to distinguish these cases in Clojure code. My extremely yucchy work-around is to insert a Java shim that detects the differences and inserts extra columns (keys) for the special cases before handing off to Clojure.