On Feb 23, 2015, at 1:52 PM, Fritz Anderson <fri...@manoverboard.org> wrote:
>
> This is written in haste, and few people are experts…
>
> theURL.host may yield nil (host is declared String!, implicitly
> unwrapped, but optional).
>
> theURL.host?.lowercaseString will short-circuit and be evaluated as nil
> if .host is nil.
>
> anyString.lowercaseString may also yield nil; the var is declared
> String!.
>
> anyString.lowercaseString! attempts to unwrap a String that the compiler
> has implicitly unwrapped already.
>
> Therefore, because of the short-circuit “?”, the expression
> theUrl.host?.lowercaseString is of type String?, which may be .None (nil), or
> .Some(String) (and therefore have to be unwrapped).
>
> You are passing that optional String as the argument for String(string: {that
> optional expression}). The argument may be nil; or it may need unwrapping.
> The initializer _requires_ a non-nil argument. It’s a righteous error.
>
> The expression
>
> NSString(string: foo!)
>
> is different; by adding the bang you’ve taken responsibility for the
> parameter’s not being nil.
>
> Because .lowercaseString returns an implicitly-unwrapped String (String!),
>
> theURL?.host?.lowercaseString!
>
> tries to unwrap something that is already, syntactically, an unwrapped String.
>
> Quincey — my instinct is that the conditional-unwrap chain necessarily forces
> left-to-right evaluation, tighter than the . operator. The ! would mean, “if
> you got this far without short-circuiting, and b returns an optional, then
> unwrap it.”
>
> As of today’s Swift 1.2b2, the following in a playground:
>
> let aURL = NSURL(string: "http://wt9t.com/ <http://wt9t.com/>")!
> NSString(string: aURL.host?.lowercaseString)
>
> complains as you suggest: .host? is potentially nil — that would force the
> whole expression to be nil, and thus forbidden as a parameter.
> lowercaseString still gets treated as implicitly unwrapped — so a bang at the
> end of the whole expression is an attempt to unwrap something the compiler
> was going to unwrap anyway.
It’s sure great that we have Swift these days to make our coding so much
easier, isn’t it?
Charles
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