Difference between if as an expression and if as a statement

So I was watching this video on the Go language - https://www.youtube.com/watch?v=p9VUCp98ay4 , and at around 6:50 a guy asks a question about why they implemented if's as statements and not expressions. What is the difference between those two implementations? As far as I know, I've never had to change the way I use a conditional based on the language.

Edit: and what does he mean that "you need values rather than variables" in his question?

The difference between expressions and statements is that expressions produce a value and thus can be used in places where values are required. So expressions can be used as values for variables, arguments to functions or operands to operators. Statements can't.

> and what does he mean that "you need values rather than variables" in his question?

I assume that by vals he means constants (which are called vals in Scala for example).

If if were an expression, you could do this:

const myValue = if condition { value1 } else { value2 }

Since if is not an expression, you have to do this:

var myValue
if condition {
    myValue = value1
} else {
    myValue = value2
}

So you needed to make your variable mutable (use var instead of const), which is what the person asking the question likely meant.

You can get the same code elegance that you would get from if expression by using an instantly invoked function (IIF) combined with if statements in GO. I'm not a GO programmer (mainly typescript) so please let me know if this is bad for performance for any reason.

func main() {
	amIHungry := true
	didMyPaycheckComeInYet := false

	choice := func() string {
		if(!amIHungry && !didMyPaycheckComeInYet){
			return "stay-home"
		}
		if(!amIHungry && didMyPaycheckComeInYet){
			return "buy-new-keyboard"
		}
		if(amIHungry && !didMyPaycheckComeInYet){
			return "make-ramen"
		}
		return "taco-bell-time"
	}()

	println(choice)

}

and then later on in your program, rather than having a bunch of out of context states, you can have a simplified "choice" to choose your app logic.

if(choice == "taco-bell-time"){
   println("Order a bean burrito, but with black beans,")
}

is a little easier to reason about than

if(amIHungry && didMyPaycheckComeInYet){
     println("Order a bean burrito, but with black beans,")
}