Assigning values to multiple variables in Go

Question

I recently came up across this:

func main() {
    x, y := 0, 1
    x, y = y, x+y
    fmt.Println(y)
}

What I thought was that:

x, y = y, x+y

Is identical to:

x = y
y = x+y

Which would result to final values x = 1, y = 2

However the final values I get is x = 1, y = 1

Why is that?

Thanks.

Answer 1

This is how it's specified:

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

Assignment first evaluates all expressions on the right side and then assigns the results to the variables on the left side.

Your

x, y = y, x+y

is basically equivalent to this

tmp1 := y
tmp2 := x+y
x = tmp1
y = tmp2

You can even use this fact to swap 2 variables in one line, like this:

a, b = b, a