1 
struct BoxView<SelectionValue: Hashable>: View {

vs.

struct BoxView<SelectionValue>: View where SelectionValue: Hashable {

?? Both compile just fine in this case. But sometime I think I have to use where version. What's the different in these two? When to use which? Why Swift don't just allow only one style?

1 Like
2

There is no semantic difference.

The top one predates the bottom one, but could not express as many things, such as constrained associated types (Something<T: Collection> where T.Element: Hashable). The older syntax was likely kept around simply for backward compatibility. Or possibly because it is shorter where it does work.

3 Likes
3

Your choice. Some developers use only the second. Others use the first in every situation where it is sufficient, and only resort to the second when necessary.

2 Likes
4

Ok, I see now. The simpler form is easier to read for me. I will stick to the simpler form and use where only when needed.