Difference between type and explicitly expanded type in Swift

Question

This really puzzles me: what is the difference between an implicitly unwrapped optional and the type itself?

For instance,

var s:String!
var s2: String

Don't these two exactly match? They represent a String that cannot be nil, so why on earth do we need! thing?

Answer 1

Implicitly expanded optional parameter optional , in fact you can assign a valuenil

var s: String! = nil

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let s: String! = nil
var s1: String = s // Execution was interrupted, reason: EXC_BAD_INSTRUCTION ...

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