英文:
Assign-and-compare in go's while-equivalent?
问题
在Go语言中，你可以这样做：
if foo := bar(); foo != nil {
...
}
在C/C++中，你可以这样做：
while ((foo = bar()) != NULL) {
...
}
然而，Go语言的语法似乎没有提供在while循环中进行赋值和比较的等效方式；Go语言用for特定的调用替代了while（例如for a等同于for ; a ;）。尝试使用if版本的语法会让解析器困惑，因为它期望有第三个语句。
我可以这样写：
for foo := bar(); foo != nil; foo = bar() {
....
}
但在这种情况下，bar()调用相当长、复杂，不容易拆分成自己的函数（虽然我可以声明一个本地的func来调用，但这仍然降低了代码的清晰度）。
目前我这样做：
for {
foo := bar()
if foo == nil {
break
}
...
}
但这看起来不够简洁，因为它将循环条件与循环语句本身分开，并且依赖于break。
那么，在Go语言中有没有一种简洁、惯用的方式来在while循环中进行赋值和比较呢？这是一个如此常见的用例，我无法想象没有办法做到这一点。
英文:
In Go you can do:
if foo := bar() ; foo != nil {
...
}
In C/C++ you can do:
while ((foo = bar()) != NULL) {
...
}
However, Go's syntax does not seem to provide any equivalent way of doing assign-and-compare in a while loop; Go has replaced while with a specific invocation of for (e.g. for a is equivalent to for ; a ;). Simply trying to use the if version syntax confuses the parser, as it's expecting there to be a third statement.
I could just say:
for foo := bar() ; foo != nil ; foo = bar() {
....
}
but in this case, the bar() call is fairly long, complex, and not easy to break out into its own function (although I could do something like declaring a local func to call, but that still reduces the clarity of the code).
For now I am doing:
for {
foo := bar();
if foo == nil { break; }
...
}
but this seems unclean, both because it separates the loop criteria from the loop statement itself, and because it relies on break.
So, is there a clean, idiomatic way of doing an assign-and-compare in a while loop in Go? This is such a common use case I can't imagine that there's no way of doing it.
答案1
得分: 16
没有。Go语言没有while语句，只有for语句的特殊形式 - 而赋值是一个语句，而不是一个表达式。你的例子在我看来是符合Go语言习惯的。
英文:
No. Go has no while statement, only the special form of the for statement - and assignment is a statement, not an expression. Your examples are IMHO idiomatic Go.
答案2
得分: 0
你可以创建一个迭代器，类似于bufio#Scanner.Scan：
package main
type bar struct { foo int }
func (b *bar) next() bool {
b.foo++
return b.foo < 10
}
func main() {
var b bar
for b.next() {
println(b.foo)
}
}
然后在Next函数中，你可以放入所有你需要的复杂逻辑。
英文:
You can create an iterator, similar to bufio#Scanner.Scan:
package main
type bar struct { foo int }
func (b *bar) next() bool {
b.foo++
return b.foo < 10
}
func main() {
var b bar
for b.next() {
println(b.foo)
}
}
Then in the Next function, you can put all the complicated logic you need.
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