单值上下文错误中的多值返回变量和多返回函数

Question

I'm trying to return a variable along with a function that returns multiple values.

Consider this contrived example:

func twoInts() (int, int) {
    return 2, 3
}
func threeInts() (int, int, int) {
    return 1, twoInts()
}

Calling threeInts() returns "multiple-value twoInts() in single-value context".

I understand I can do the following

func twoInts() (int, int) {
    return 2, 3
}
func threeInts() (int, int, int) {
    num1, num2 := twoInts()
    return 1, num1, num2
}

but I am trying to understand why the return is a single-value context.

Does Go consider the return to be int, (int, int)? Is there a way to expand the first function return so that it does return int, int, int?

Answer 1

https://golang.org/ref/spec#Return_statements

There are three ways to return values from a function with a result type:

1. The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and assignable to the corresponding element of the function's result type.

2. The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.

3. The expression list may be empty if the function's result type specifies names for its result parameters. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.

None of the 3 ways allow mixing single-value expressions with a call to a multi-valued function.

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"Is there a way to expand the first function return so that it does return int, int, int?" No, there isn't.