Golang函数和函数变量语义

Question

Why can't I define a recursive function as a variable? I seem to be able to define arbitrary functions except when they recurse.

This is legal:

func f(i int) int {
    if i == 0 {
        return 1
    }
    return i * f(i-1)
}

func main() {
    fmt.Println(f(2))
}

This is illegal:

var f func(int) int = func(i int) int {
    if i == 0 {
        return 1
    }
    return i * f(i-1)
}

func main() {
    fmt.Println(f(2))
}

This is legal and I'm guessing it is just because you can figure out f after initialization:

func main() {
    var f *func(int) int;
    t := func(i int) int {
        if i == 0 {
            return 1
        }
        return i * (*f)(i-1)
    }
    f = &t
    fmt.Println((*f)(2))
}

So it looks like it comes down to function and variable declarations of a function type are treated differently though from the reading the documentation I wouldn't expect that to be the case. Did I miss the part of the documentation detailing this?

I would expect the illegal case to work just because it works in other languages. Like in JavaScript:

(function () {
  var f = function (i) {
    if (i == 0) {
      return 1;
    }
    return i * f(i - 1);
  };

  console.log(f(2));
})();

Answer 1

The below code would be the preferred way of doing what you describe. Note that you do not have to create an additional variable, nor do you have a pointer to a function:

package main

import "fmt"

func main() {
    var f func(int) int
    f = func(i int) int {
        if i == 0 {
            return 1
        }
        return i * f(i-1)
    }
    fmt.Println(f(2))
}