I would add WAT 2.5: https://play.golang.org/p/gwcHh4-QhHK https://play.golang.org/p/gwcHh4-QhHK func main() { x := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

I would add WAT 2.5: https://play.golang.org/p/gwcHh4-QhHK https://play.golang.org/p/gwcHh4-QhHK func main() { x := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9} fmt.Println("orig: ", x) mutate(x) fmt.Println("append: ", x) mutate(x[:4]) fmt.Println("sliced: ", x) } Appending to the end of the slice creates a new slice and therefore the caller doesn't see the mutation. However appending to a slice-of-the-slice leaves capacity in mutate's copy-of-the-slice to append without allocating a new backing array. Therefore mutate happy bumps the len of its slice copy and mutates the callers slice! This bit me once in real production code when reusing a []byte array to avoid allocations. The bug was obviously my fault, but this behavior can be easy to inadvertently trigger if you're trying to avoid allocations! Edit: fixed code formatting. It's 2018 YC, please please please implement at least a subset of markdown.