Ah, I now understand the if let thing better.
Optionals evaluate to a boolean so if foo is a String? then you can say if foo to test if foo is non-nil.
Now if foo is non-nil, you can say foo! to get that value (you'll get a runtime error if foo is nil).
So:
if let bar = foo {
...
}
is syntactic sugar for:
if foo {
let bar = foo!
...
}