在Common Lisp中,如何定义一个“元宏”,它将宏(和其他参数)列表作为参数,并组合这些宏以生成所需的代码。
问题相当于编写一个“高阶宏”,它从其他宏的变量列表中定义一个宏。
提示问题的具体情况是我使用CLSQL进行实验,我想从CLSQL-testsuite重新表达员工类
(clsql:def-view-class employee ()
((employee-id
:db-kind :key
:db-constraints (:not-null)
:type integer)
(first-name
:accessor employee-first-name
:type (string 30)
:initarg :first-name)
(last-name
:accessor employee-last-name
:type (string 30)
:initarg :last-name)
(email
:accessor employee-email
:type (string 100)
:initarg :email)
(company-id
:type integer
:initarg :company-id)
(company
:accessor employee-company
:db-kind :join
:db-info (:join-class company
:home-key companyid
:foreign-key companyid
:set nil))
(manager-id
:type integer
:nulls-ok t
:initarg :manager-id)
(manager
:accessor employee-manager
:db-kind :join
:db-info (:join-class employee
:home-key managerid
:foreign-key emplid
:set nil))))
作为
(def-view-class-with-traits employee ()
(trait-mapsto-company trait-mapsto-manager)
((employee-id
:db-kind :key
:db-constraints (:not-null)
:type integer)
(first-name
:accessor employee-first-name
:type (string 30)
:initarg :first-name)
(last-name
:accessor employee-last-name
:type (string 30)
:initarg :last-name)
(email
:accessor employee-email
:type (string 100)
:initarg :email)))
在定义复杂的数据库模式时,掌握这种技术将有利于一致性和简洁性。
我将我需要的两个特征定义为
(defmacro trait-mapsto-company (class super slots &rest cl-options)
(declare (ignore super slots cl-options))
(let ((company-accessor-name
(intern (concatenate 'string (symbol-name class) "-COMPANY"))))
`((company-id
:type integer
:initarg :company-id)
(company
:accessor ,company-accessor-name
:db-kind :join
:db-info (:join-class company
:home-key companyid
:foreign-key companyid
:set nil)))))
(defmacro trait-mapsto-manager (class super slots &rest cl-options)
(declare (ignore super slots cl-options))
(let ((manager-accessor-name
(intern (concatenate 'string (symbol-name class) "-MANAGER"))))
`((manager-id
:type integer
:initarg :manager-id)
(manager
:accessor ,manager-accessor-name
:db-kind :join
:db-info (:join-class manager
:home-key managerid
:foreign-key emplid
:set nil)))))
然而,我写def-view-class-with-traits
的尝试被挫败了。
(defmacro def-view-class-with-traits (class super traits slots &rest cl-options)
(let ((actual-slots
(reduce (lambda (trait ax) (append (apply trait class super slots cl-options) ax))
traits
:initial-value slots)))
`(clsql:def-view-class ,class ,super ,actual-slots ,@cl-options)))
在用于缩减的lambda中,trait
代表一个宏,而我对apply的使用对Lisp没有任何意义 - 这是正确的! - 但希望将我的意图传达给其他程序员。
如何让def-view-class-with-traits
以适当的方式处理宏traits
列表?
答案 0 :(得分:6)
如果您将特征定义为类本身并使用正常继承,我会发现它不那么令人惊讶:
(def-view-class trait-mapsto-company ()
((company-id
:type integer
:initarg :company-id)
(company
:accessor company
:db-kind :join
:db-info (:join-class company
:home-key company-id
:foreign-key company-id
:set nil))))
(def-view-class trait-mapsto-manager ()
((manager-id
:type integer
:initarg :manager-id)
(manager
:accessor manager
:db-kind :join
:db-info (:join-class manager
:home-key managerid
:foreign-key emplid
:set nil)))
(def-view-class employee (trait-mapsto-company trait-mapsto-manager)
((employee-id
:db-kind :key
:db-constraints (:not-null)
:type integer)
(first-name
:accessor employee-first-name
:type (string 30)
:initarg :first-name)
(last-name
:accessor employee-last-name
:type (string 30)
:initarg :last-name)
(email
:accessor employee-email
:type (string 100)
:initarg :email)))
这肯定不会使访问者名称依赖于继承类的名称,但你真的想要吗?我的观点是,这种写它的方式表明,这实际上会破坏解耦原则。
答案 1 :(得分:4)
“调用”宏的方法是使用macroexpand-1
:
(defmacro def-view-class-with-traits (class super traits slots
&rest cl-options
&environment env)
(let ((tslots
(loop for m in traits
append (macroexpand-1 (list* m class super slots options)
env))))
`(def-view-class ,class ,super (,@tslots ,@slots) ,@cl-options)))