英文:
Can I type assert a slice of interface values?
问题
我正在尝试从[]Node断言为[]Symbol。在我的代码中，Symbol实现了Node接口。
以下是一些相关的代码：
43 func applyLambda(args []Node, env Env) Node {
44 if len(args) > 2 {
45 panic("invalid argument count")
46 }
47 fixed, rest := parseFormals(args.([]Symbol))
48 return Func{
49 Body: args[1],
50 FixedVarNames: fixed,
51 RestVarName: rest,
52 }
53 }
这是我得到的错误：
./builtins.go:47: invalid type assertion: args.([]Symbol) (non-interface type []Node on left)
我确定这有一个很好的原因。继续进行的最佳方式是什么？
英文:
I am trying to type assert from a []Node, to []Symbol. In my code, Symbol implements the Node interface.
Here is some surrounding code:
43 func applyLambda(args []Node, env Env) Node {
44 if len(args) > 2 {
45 panic("invalid argument count")
46 }
47 fixed, rest := parseFormals(args.([]Symbol))
48 return Func{
49 Body: args[1],
50 FixedVarNames: fixed,
51 RestVarName: rest,
52 }
53 }
Here's the error I get:
./builtins.go:47: invalid type assertion: args.([]Symbol) (non-interface type []Node on left)
I'm sure there's a good reason for this. What's the best way to proceed?
答案1
得分: 25
在说x.(T)时，变量x应该是接口类型，因为只有对于接口类型的变量，动态类型才不固定。而Node是一个接口，[]Node不是。切片是一个独立的、非接口类型。所以假设一个切片的接口值也是一个接口是没有意义的。
类型Node在你的代码中有一个明确的定义，因此是一个接口。你已经为它指定了方法列表。而[]Node不是这样的。它定义了哪些方法呢？
我理解你的意思。这可能是一个有用的快捷方式，但是没有意义。这有点像期望当syms的类型是[]Symbol而Method是为Symbol时，syms.Method()能够工作。
用以下代码替换第47行可以实现你想要的效果：
symbols := make([]Symbol, len(args))
for i, arg := range args { symbols[i] = arg.(Symbol) }
fixed, rest := parseFormals(symbols)
英文:
In saying x.(T) variable x should be of interface type, because only for variables of type interface dynamic type is not fixed. And while Node is an interface, []Node is not. A slice is a distinct, non-interface type. So it just doesn't make sense to assume a slice of interface values is an interface too.
Type Node has a clear definition in your code and thus is an interface. You have specified the list of methods for it. Type []Node isn't like that. What methods does it define?
I understand where your are coming from with this. It may be a useful shortcut, but just doesn't make sense. It's kind of like expecting syms.Method() to work when syms's type is []Symbol and Method is for Symbol.
Replacing line 47 with this code does what you want:
symbols := make([]Symbol, len(args))
for i, arg := range args { symbols[i] = arg.(Symbol) }
fixed, rest := parseFormals(symbols)
答案2
得分: 6
Go不允许这样做。您需要逐个将Node转换为Symbol。
不允许这样做的原因是[]Node和[]Symbol具有不同的表示形式，因此转换需要为[]Symbol分配内存。
英文:
Go does not allow this. You need to convert Node to Symbol individually.
The reason why it isn't allowed is that []Node and []Symbol have different representations, so the conversion would need to allocate memory for []Symbol.
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