make 函数从何而来? [英] Where does a make- function come from?

问题描述

此代码有效:

(define list-of-events 
  (for/list ([(date code)
              (in-query odc "select date, code from attendance
                             where student_id = ? and term_code = ?"
                        "12345" "654321")])
    (make-attendance-event date code)))

但是，当我尝试复制另一个表的行为时，make-attendance-event 的并行项会抱怨它是未绑定的标识符".

However, when I try to duplicate the behavior for another table, the parallel item to make-attendance-event complains about it being an "unbound identifier".

现在，make-attendance-event 从何而来?

Now, where does make-attendance-event come from?

推荐答案

标识符 make-attendance-event 来自一个 (define-struct duty-event (...)).

The identifier make-attendance-event came from a (define-struct attendance-event (...)).

结构定义如

(define-struct foo (a b))

将扩展为多个定义.

1. make-foo 将构建 foo 结构
2. foo-a、foo-b 字段访问器
3. foo?一个可以判断一个值是否为 foo 的谓词

在高级语言中，您还可以获得:

In the Advanced language you also get:

1. set-foo-a!, set-foo-b!改变相应的字段.

在此处查看更多信息:http://docs.racket-lang.org/htdp-langs/advanced.html?q=define-struct#%28form._%28%28lib._lang%2Fhtdp-advanced..rkt%29._define-struct%29%29

See more here: http://docs.racket-lang.org/htdp-langs/advanced.html?q=define-struct#%28form._%28%28lib._lang%2Fhtdp-advanced..rkt%29._define-struct%29%29

请注意，您可以将鼠标悬停在 DrRacket 中的标识符 make-attendance-event 上，右键单击并选择跳转到绑定事件"以查看标识符的定义位置.

Note that you can hover over the identifier make-attendance-event in DrRacket, right click and choose "Jump to Binding Occurrence" to see where an identifier is defined.

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