defer statement

Nice way of co-locating cleanup code together with initialization code + a way to make sure that cleanup code gets run.

Tricky Scoping Issues

It isn’t obvious what should happen in a code block like this:

var y = -1
do {
    var x = 0
    defer { y = x }
    x = 1
}
print(y)

Naively in many functional programming languages x would be captured immutably causing the print statement to print 0; in Swift this prints 1, the defer block effectively being copied to occur at the end of the scope. This seems like the most sensible thing to do in my opinion.