为什么单元素记录汇总需要命名关联？ [英] Why is named association required for a one-element record aggregate?

问题描述

当一个聚合仅包含一个元素时，如下所示，位置表示法会导致编译错误，我们只能使用命名的表示法。为什么？

When an aggregate contains only one element, as below, positional notation results in a compilation error and we have to use named notation only. Why?

type singleton is record
   v : integer;
end record;

v1 : singleton := (0);

产生编译器消息

check.adb:6:23: positional aggregate cannot have one component
check.adb:6:23: write instead "V => ..."
gnatmake: "check.adb" compilation error

这是可以的：

whereas this is OK:

v2 : singleton := (v => 0);

推荐答案

围绕表达式的括号是多余的，因此（0）= 0，它是一个整数，而不是数组集合

Parentheses round an expression are redundant, so (0) = 0 and it's an integer not an array aggregate.

因此，对于单元素聚合的特殊情况，需要命名关联才能将聚合与简单值区分开。

So, for the special case of a one-element aggregate, named association is required to distinguish an aggregate from a simple value.

将此值与（0,0）进行比较，后者只能是一个总计；因此没有歧义。

Contrast this with (0,0) which can only be an aggregate; therefore there is no ambiguity.

即使在上下文中问题，这对人类程序员来说是显而易见的，并非总是如此。

Even though in the context of the question, it's obvious to a human programmer which is intended, that will not always be the case.

Consi在多维数组中将一个元素汇总为记录的一个字段；可能存在一些歧义，编译器无法解决（至少在读取更多的源文件之前！），并使阅读该程序的人很难过。

Consider a one-element aggregate in a multi-dimensional array which is one field of a record; there can be ambiguities that the compiler cannot resolve (at least, before reading a whole lot more of the source file!) and would make life pretty difficult for anyone reading the program.

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