英文:
Should type redeclare order in a scope be effected by outer scope?
问题
我得到了一个奇怪的错误信息cannot use []feed literal (type []feed) as type []feed in field value，经过一些调试和简化源代码，我发现这种情况似乎会产生错误：
type user struct {
Feeds []feed
}
type feed struct{}
func fn() {
type user struct {
Feeds []feed // 似乎引用了外部的 feed 类型
}
type feed struct{}
_ = user{
// "cannot use []feed literal (type []feed) as type []feed in field value"
Feeds: []feed{},
}
}
这是预期的行为还是一个 bug？我花了一些时间阅读语言规范，但没有找到明确说明作用域中类型声明顺序应该如何工作的内容。在外部作用域中顺序无关紧要，但在内部作用域中却很不直观。
英文:
I got a strange error message cannot use []feed literal (type []feed) as type []feed in field value and after some fiddling and minimizing the source I figured out that this situation seems to produce the error:
type user struct {
Feeds []feed
}
type feed struct{}
func fn() {
type user struct {
Feeds []feed // seems to refer to the outer feed type
}
type feed struct{}
_ = user{
// "cannot use []feed literal (type []feed) as type []feed in field value"
Feeds: []feed{},
}
}
http://play.golang.org/p/gNIGhPwAgl
Is this the intended behaviour or a bug? I've spend some time reading the language specification but can't find anything explicitly stating how type declaration order in scopes should work. It's a bit unintuitive that order does not matter in the outer scope but does in the inner.
答案1
得分: 1
这是语言规范中的规定。
引用相关部分：声明和作用域：
在函数内部声明的类型标识符的作用域从标识符在TypeSpec中开始，到最内层包含块的结束。
在函数内部声明的类型只在类型标识符（正在声明的标识符）之后的作用域内有效。在此之前是无效的。
type user struct {
Feeds []feed // 这只能是外部的feed类型
}
type feed struct{} // 新的feed类型从这一行开始有效
英文:
This is so by the language spec.
Quoting the relevant part: Declarations and scope:
> The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block.
Types declared inside a function are only in scope from the type identifier (being declared). Before that they are not.
type user struct {
Feeds []feed // This can only be the outer feed type
}
type feed struct{} // new feed type is in scope from this line
答案2
得分: 0
在golang-nuts列表中找到了一篇关于这个问题的帖子（链接：https://groups.google.com/d/msg/golang-nuts/w40shSAlObk/XQvoxqbJ9v4J）：
简而言之，这是因为在函数内部声明的顺序是有关系的，但在函数外部则不是。在包范围内，符号的声明顺序是根据它们的依赖关系来确定的，与它们在页面上的出现顺序无关；而在函数内部，它们是按照词法顺序声明的。稍加思考就能明白为什么这种不一致性是有道理的，尽管它确实是不一致的。
-rob
英文:
Found thread on golang-nuts list talking about this:
> Briefly, this is because order of declaration matters inside a function but does not outside. At package scope, the symbols are declared in the order that satisfies their dependencies independent of their appearance on the page; inside a function, they are declared in lexical order. A moment's reflection will show why this inconsistency has merit, although it is, well, inconsistent.
-rob
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