Swift allows you to declare a (non-optional) variable in a local scope without setting its value if the compiler can deduce that the value _will_ be set before it's used.

nonoptionals.swift

// This will work:

var url: String

if condition {
    url = "http://example.com"
} else {
    url = "http://example.org"
}

doSomethingWithUrl(NSURL(string: url))

// If you tried to use `url` before its value was set, or
// _any_ of the branches didn't initialize it (e.g. if you omitted `else`),
// it would fail in compile time.
// Well done, Swift compiler.