Golang中没有返回值函数

While I was reading "the little go" book, I found that it suggests to write a function without any return value. So I proceed to test that function but the program won't compile and give me this "... used as value"error. Anyone knows what is going on here?

package main

import (
    "fmt"
)

func log(message string) {
    fmt.Println(message)
}

func main() {
    msg := log("just a message")
    fmt.Println(msg)
}

I know that this function is trivial (maybe the question is stupid also). But I am just curious to know if this type of function legal in Go?