是否可以在Clojure中使一个被memoized函数的值无效？

Question

我有一个函数，它对一个令牌发出HTTP请求，这个令牌将在以后的请求中使用。此令牌在未指定的时间内有效，可能是几个小时左右。

  (defn request-app-token
    "Request an app token from FB. Useful for app-global actions, such as creating test users."
    [client-credentials]
    (-> {"client_id" (:client-id client-credentials)
          "client_secret" (:client-secret client-credentials)
          "grant_type" "client_credentials"}
         (form-encode)
         ((partial str fb-graph-api "/oauth/access_token?"))
         (client/get {:throw-entire-message? true})
         :body
         (json/read-str)
         (get "access_token")))

对我来说，这看起来像是memoize的工作：保留令牌的副本并重用它，而不是每次我需要新的令牌时都请求新的令牌。

  (def get-app-token (memoize request-app-token)) ; So beautiful :D

  (defn with-app-token [client-credentials f]
    (try (f (get-app-token client-credentials))
         (catch Exception e ; I know I should be more specific here, I only want to catch HTTP 400 responses
           (f (request-app-token client-credentials)))))

这可能会起作用，但在第一个令牌过期后，随后对with-app-token的所有调用都将请求新的令牌。我需要一些方法来清除或使get-app-token的memoized返回值无效。

我可以编写自己的memoize函数，使用一个invalidate函数来清除特定的结果，但是我想知道语言中是否已经有一些东西可以处理这个问题？

Answer 1

clojure.core.memoize有我需要的东西：一个memo-clear！函数。在需要[clojure.core.memoize:refer[memo-clear！]]后，解决方案如下所示：

  (defn request-app-token
    "Request an app token from FB. Useful for app-global actions, such as creating test users."
    [client-credentials]
    (-> {"client_id" (:client-id client-credentials)
          "client_secret" (:client-secret client-credentials)
          "grant_type" "client_credentials"}
         (form-encode)
         ((partial str fb-graph-api "/oauth/access_token?"))
         (client/get {:throw-entire-message? true})
         :body
         (json/read-str)
         (get "access_token")))

  (def get-app-token (memo request-app-token))

  (defn with-app-token [client-credentials f]
    (try (f (get-app-token client-credentials))
         (catch Exception e ; I know I should be more specific here, I only want to catch HTTP 400 responses
           (memo-clear! get-app-token client-credentials)
           (f (get-app-token client-credentials)))))