Swift: allow a noreturn func to match any type

Originator:kenferry
Number:rdar://18579265 Date Originated:
Status:Open Resolved:
Product: Product Version:
Classification: Reproducible:
 
For Cocoa initializers that may fail and return an NSError, it's not uncommon for a particular use to only fail on programmer error. 

For example, NSRegularExpression(pattern:options:error:) will only fail on a bad pattern, and usually patterns are static strings known at compile time. 

For this case, the desired behavior is to throw an exception or assert if the init doesn't succeed, and to log the error. If it does succeed, unwrap the optional.

I suggest that ?? ought to work for that if the right hand side is a noreturn func.

let regex = NSRegularExpression(pattern: "abc", options: nil, error: &e) ?? assertionFailure( e!.localizedDescription)

to work around, I wrote this function:

func unwrapAssert<T>(maybe : T?, message: @autoclosure () -> String) -> T {
    assert(maybe != nil, message())
    return maybe!
}

which allows 

let regex = unwrapAssert(NSRegularExpression(pattern: "abc", options: nil, error: &e), assertionFailure( e!.localizedDescription))

Comments

I tried this as an alternative, 

@noreturn func die<T>(message: @autoclosure()->String) -> T { assertionFailure(message()) }

let regex = NSRegularExpression(pattern: "abc", options: nil, error: &e) ?? die(e!.localizedDescription)

…but it doesn't work because of the overload of ?? added in b6 or so. :-/ "Ambiguous use of operator '??'"

By kenferry at Oct. 8, 2014, 9:42 a.m. (reply...)

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