fmt.Println是否必须在Go中的函数内？

Question

Does fmt.Println need to always belong to a function?

Have used Python before and it allows it but on research, it seems that Java doesn't

fmt.Println("can I do it?")

Returns:

syntax error: non-declaration statement outside function body

Answer 1

It may be outside of a function, see this example:

var n, err = fmt.Println("I can do it")

func main() {
    fmt.Println("In main(),", n, err)
}

It outputs (try it on the Go Playground):

I can do it
In main(), 12 <nil>

(The output values 12 <nil> are the values returned by the first fmt.Println() call, the number of bytes it has written and the error it returned which is nil indicating no error.)

Also note that you don't even have to store the return values of fmt.Prinln(), you can use the blank identifier like this:

var _, _ = fmt.Println("I can do it")

It cannot stand on its own at the top level "between" top-level declarations, but the above variable declaration (with blank identifier) pretty much achieves the same.

Spec: Source file organization:

Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.

SourceFile       = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .

Obviously a package clause or import declaration can't contain an fmt.Println() call, and the top level declarations:

Declaration   = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .

A constant declaration cannot contain an fmt.Println() call, that's not a constant expression. A type declaration also cannot contain function calls.

A variable declaration can, as shown in the example at the top of the answer.

Function and method declarations could also call fmt.Println(), but you were asking specifically if fmt.Println() can be called outside of them.

So the only place it is allowed outside of functions that is allowed at the top level is in variable declarations.