从函数返回函数，为什么省略括号？

Question

I am currently in UCI Go Lang Course and Came across this function which the Syntax I did not understand. The function is directly from the example code, but has a syntax error I do not understand line 2 bracket. Also I come from JS, and C and Why in go are some functions declared without outer brackets?

 func MakeDistOrigin(o_x, o_y float64)
            func (float64, float64) float64 {
   fn := func (x, y float64) float64 {
            return math.Sqrt(math.Pow(x - o_x, 2) + 
                      math.Pow(y - o_y, 2))
         }
   return fn
    }

I expected this function to have outer brackets for each function and to return a function that then takes in another variable. Thanks!

Answer 1

If you got a syntax error, probably you have a newline after func MakeDistOrigin(o_x, o_y float64). Move the func (float64, float64) float64 { to the same line as the MakeDistOrigin declaration, and it should be good.

The second line in the code you pasted is not a function declaration. It is the return type of MakeDistOrigin. Look at it this way:

func MakeDistOrigin(o_x, o_y float64) T {
}

In the above declaration, T is the following type:

func (float64, float64) float64 

So in fact you can simplify this declaration by:

type T func (float64, float64) float64 

func MakeDistOrigin(o_x, o_y float64) T {
}

So MakeDistOrigin is a function that returns a function of type T, which is a function that gets two float64 values and returns a float64 value.

Inside MakeDistOrigin, a variable fn is declared. This variable is of type T as well, that is a function that takes two float64s and returns one.

The variable fn is initialized with the function definition given next to it, which is, again, a function of type T.