How to find the difference between two slices of strings

Assuming Go maps are ~O(1), here is an ~O(n) difference function that works on unsorted slices.

// difference returns the elements in `a` that aren't in `b`.
func difference(a, b []string) []string {
    mb := make(map[string]struct{}, len(b))
    for _, x := range b {
        mb[x] = struct{}{}
    }
    var diff []string
    for _, x := range a {
        if _, found := mb[x]; !found {
            diff = append(diff, x)
        }
    }
    return diff
}