克洛朱尔:从映射动态创建函数 - 是时候使用宏了?
-
11-12-2019 - |
题
我有一个这样开始的函数:
(defn data-one [suser]
(def suser-first-name
(select db/firstNames
(fields :firstname)
(where {:username suser})))
(def suser-middle-name
(select db/middleNames
(fields :middlename)
(where {:username suser})))
(def suser-last-name
(select db/middleNames
(fields :lastname)
(where {:username suser})))
;; And it just continues on and on...
)
当然,我一点也不喜欢这个。我的代码库中的许多区域都重复这种模式,我想概括这一点。
因此,我想出了以下几点来开始:
(def data-input {:one '[suser-first-name db/firstNames :firstname]
'[suser-middle-name db/middleNames :middlename]
'[suser-last-name db/lastNames :lastname]})
(defpartial data-build [data-item suser]
;; data-item takes the arg :one in this case
`(def (data-input data-item)
(select (data-input data-item)
(fields (data-input data-item))
(where {:username suser}))))
这里确实有几个问题:
-- 我如何解构数据输入,以便在 x 未知时创建 x 函数,即。:one 的值未知,并且数据输入中键的数量未知。
-- 我认为现在是创建宏的时候了,但我以前从未构建过宏,所以我对这个想法很犹豫。
为了提供一些上下文,这些函数必须返回要解构的值,但我认为一旦我解决了这个问题,概括所有这些都是可行的:
(defpage "/page-one" []
(let [suser (sesh/get :username)]
(data-one suser)
[:p "Firat Name: "
[:i (let [[{fname :firstname}] suser-first-name]
(format "%s" fname))]
[:p "Middle Name: "
[:i (let [[{mname :emptype}] suser-middle-name]
(format "%s" mname))]
[:p "Last Name: "
[:i (let [[{lname :months}] suser-last-name]
(format "%s" lname))]]))
解决方案
一些建议:
def
函数内部是 真的很讨厌 - 您正在改变全局环境,这可能会导致各种并发问题。我建议将结果存储在地图中。- 你 不需要宏 这里 - 所有数据获取都可以在函数内相对轻松地完成
因此我建议:
(def data-input [[:suser-first-name db/firstNames :firstname]
[:suser-middle-name db/middleNames :middlename]
[:suser-last-name db/lastNames :lastname]])
(def data-build [data-input suser]
(loop [output {}
items (seq data-input)]
(if items
(recur
(let [[kw db fieldname] (first items)]
(assoc output kw (select db (fields fieldname) (where {:username suser}))))
(next items))
output)))
未经测试,因为我没有您的数据库设置 - 但希望这能让您了解如何在没有宏或可变全局变量的情况下执行此操作!
其他提示
很好的问题。首先是您要求的宏:
(defmacro defquery [fname table fields ]
(let [arg-name (symbol 'user-name)
fname (symbol fname)]
`(defn ~fname [~arg-name]
(print ~arg-name (str ~@ fields)))))
.
你可以像那样调用:
(defquery suser-first-name db/firstNames [:firstname])
.
或者如果您愿意在地图中保留所有配置,那么它将接受字符串作为第一个参数而不是符号:
(defquery "suser-first-name" db/firstNames [:firstname])
.
现在,如果您不介意我推荐另一个解决方案,我可能选择使用围绕配置关闭的单个函数。类似的东西:
(defn make-reader [query-configurations]
(fn [query-type user-name]
(let [{table :table field-names :fields}
(get query-configurations query-type)]
(select table
(apply fields field-names)
(where {:username suser})))))
(def data-input {:firstname {:table db/firstNames :fields :firstname}
:middlename {:table db/middleNames :fields :middlename}
:lastname {:table db/lastNames :fields :lastname}})
(def query-function (make-reader data-input))
;; Example of executing a query
(query-function :firstname "tom")
.
通过另一种使用Korma的方法:
;; This creates a template select from the table
(def table-select (select* db/firstNames))
;; This creates new select query for a specific field
(def first-name-select (fields table-select :firstname))
;; Creating yet another query that filters results by :username
(defn mkselect-for-user [suser query]
(where query {:username suser}))
;; Running the query for username "tom"
;; I fully specified exec function name only to show where it comes from.
(korma.core/exec (mkselect-for-user "tom" first-name-select))
.
了解更多信息,我强烈推荐查看 Korma消息来源
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