FilterIntoTrueAndFalse.swift

fileprivate extension Array {
    func filterIntoTrueAndFalse(_ shouldBeIncludedInFirstResult: (Element) throws -> Bool) rethrows -> ([Element], [Element]) {
        var trueResults = [Element]()
        var falseResults = [Element]()
        
        for i in self {
            if try shouldBeIncludedInFirstResult(i) {
                trueResults.append(i)
            } else {
                falseResults.append(i)
            }
        }
        
        return (trueResults, falseResults)
    }
}

I just bounced in here by chance (from SO), and couldn't help writing another (probably slightly less performant and surely more try messy) alternative to the above, for the fun of it :)

func filterIntoTrueAndFalse(_ shouldBeIncludedInFirstResult: (Element) throws -> Bool) rethrows -> ([Element], [Element]) {
    return (try filter(shouldBeIncludedInFirstResult),
            try filter{!(try shouldBeIncludedInFirstResult($0))})
}