声明和不使用逻辑[重复]

Question

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Take this incredibly simple example. which shows variable assignment in and out of a block.

On compilation this results in: u declared and not used

var u string
{
    u, err := url.Parse("http://bing.com/search?q=dotnet")
    if err != nil {
        log.Fatal(err)
    }
}
log.Debug(u)

This simulates a logic block during which we might assess several things and set a var to the value we like depending on logic evaluation. How is this possible?

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Answer 1

Please pay attention that:

u, err := url.Parse("http://bing.com/search?q=dotnet")

in this line of code you have block scoped variable u which is differ from those one which declared in line: var u string, hence you have this error.
Moreover here you have shadowing for variable u from outer block scope which may lead to bugs, see this:

var u string
u = "blank"
{
    u, err := url.Parse("http://bing.com/search?q=dotnet")
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("%#v", u)
}
log.Printf("%#v", u)

Result will be:

2018/11/21 19:12:33 &url.URL{Scheme:"http", Opaque:"", User:(*url.Userinfo)(nil), Host:"bing.com", Path:"/search", RawPath:"", ForceQuery:false, RawQuery:"q=dotnet", Fragment:""}
2018/11/21 19:12:33 "blank"

As you can see here you have even different data types, and variable u not changed after this block.