[swift-evolution] ternary operator ?: suggestion
Craig Cruden
ccruden at novafore.com
Tue Dec 29 07:28:21 CST 2015
That looks pretty ugly.
I think the best we can hope for at this point is maybe another keyword that mirrors switch but is expression based (aka match) — leaving the ternary ? : expression as is - which is not all that bad since any if else that becomes a compound expression or more than two resultant values (chaining) quickly becomes a mess.
I am not sure that even a “match” expression would be accepted at this point because there seems to be general resistance to anything more than the existing paradigm with a few functional decorations — and the way of doing things is good enough.
Concurrency is also currently off the table at this point -- the fact that immutable pure functional code can theoretically be parsed into a dependance graph which would allow for out of order [within scope] parallel execution on different threads [not sure if the overhead of doing so would outweigh the benefits]…. would also not be of sufficient benefit.
The primary focus of Swift is a language for UI development, not server development….
> On 2015-12-29, at 15:07:57, James Campbell via swift-evolution <swift-evolution at swift.org> wrote:
>
> What if you could wrap the existing switch statement in a closure and return a value from that closure like so
>
> Let value = { switch (other) {
> Case .Some(let value):
> Return value // because this is in a closure the closure will return the value not the function this is in
> Case .None:
> Return "hello"
> }}
>
>
> Sent from my iPhone
>
> On 29 Dec 2015, at 07:53, Howard Lovatt via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>> You can replace the proposed statement `which` (another thread), the existing statement `?:` (this thread), and the global function `??` (which is an odd ball) with matching library methods.
>>
>> A library method is likely slower than a built in at this stage until the optimiser improves, but a library function:
>>
>> Is documented right in the IDE including code completion, statements aren’t (you don’t see quick help for `for`!)
>> Having a library function allows the use case to be throughly investigated. Is worth while as a language statement? What exact features are useful? EG should `which` support pattern matching, general boolean expressions, or simply be `Equatable` as shown below?
>> It is simpler to implement, maintain, and change a library function that a built-in.
>> There is no need for a keyword.
>>
>> First `which`:
>>
>> // Alternative to introducing `which` statement
>>
>> final
>> class Which<I: Equatable, R> {
>> private
>> var result: R?
>>
>> private
>> let which: I
>>
>> init(_ which: I) {
>> self.which = which
>> }
>>
>> func match(value: I, @noescape matchResult: () throws -> R) rethrows -> Self {
>> if self.result == nil && self.which == value {
>> self.result = try matchResult()
>> }
>> return self
>> }
>>
>> func matchDefault(@noescape defaultResult: () throws -> R) rethrows -> R {
>> switch self.result {
>> case .None:
>> return try defaultResult()
>> case .Some(let value):
>> return value
>> }
>> }
>> }
>>
>>
>> // Demo
>> enum Color {
>> case Red, Blue, Green
>> }
>>
>> // Which with a default value
>> let i1 = Which(Color.Red) // i = 16711680 <tel:16711680>
>> .match(.Red) { 0xFF0000 }
>> .match(.Green) { 0x00FF00 }
>> .match(.Blue) { 0x00000FF }
>> .matchDefault { 0 }
>>
>> // Which that throws an error if it defaults
>> let i2: Int! = Which(Color.Green) // i = 16711680 <tel:16711680>
>> .match(.Red) { 0xFF0000 }
>> .match(.Green) { 0x00FF00 }
>> .match(.Blue) { 0x00000FF }
>> .matchDefault { nil } // Cant type call to fatalError as no return, hence nil and type Int! (note !)
>>
>> Note runtime check for default rather than static check via compiler, not as good but not a big deal most of the time. The vast majority of languages don't do a compiler check on `switch`.
>>
>> Similarly the `?:` statement can be replaced:
>>
>> // Replacement for `?:` operator
>>
>> struct IfFalse<R> {
>> private
>> let result: R?
>>
>> func ifFalse(@noescape falseResult: () throws -> R) rethrows -> R {
>> switch self.result {
>> case .None:
>> return try falseResult()
>> case .Some(let value):
>> return value
>> }
>> }
>> }
>>
>> extension Bool {
>> func ifTrue<R>(@noescape trueResult: () throws -> R) rethrows -> IfFalse<R> {
>> switch self {
>> case true:
>> return IfFalse(result: try trueResult())
>> case false:
>> return IfFalse(result: nil)
>> }
>> }
>> }
>>
>>
>> // Demo
>> let sB = true.ifTrue{"True"}.ifFalse{"False"} // "True" - for some reason needs {} and not () thinks () form throws
>>
>> Whilst the `??` operator is already a library function it is difficult to see in an expression, it gets buried, and is inconsistent in style because it is a non-mathematical operator and a symbol rather than a keyword or keyword followed by a symbol. The space either side of the `??` operator also makes it look like both arguments are of equal importance, whereas it is the left hand side that is important and the right hand side is just a catch.
>>
>> // Replacement for `??` operator
>>
>> extension Optional {
>> func ifNil(@noescape nilResult: () throws -> Wrapped) rethrows -> Wrapped {
>> switch self {
>> case .None:
>> return try nilResult()
>> case .Some(let value):
>> return value
>> }
>> }
>> }
>>
>>
>> // Demo
>> let o: String? = nil
>> let sO = o.ifNil{"Nil"} // "Nil" - for some reason needs {} and not () thinks () form throws
>>
>>
>> Sent from my iPad
>>
>> On 29 Dec 2015, at 4:00 AM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>> No exhaustiveness checking is a serious deficiency :-(
>>>
>>> -Thorsten
>>>
>>>> Am 17.12.2015 um 08:09 schrieb Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>>>>
>>>> Actually, this *almost* does what you want. No @autoclosure for the values and no exhaustiveness checking, but otherwise...
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