[swift-evolution] ternary operator ?: suggestion

[Matthew Johnson]

Sent from my iPad

> On Dec 29, 2015, at 7:28 AM, Craig Cruden via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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 agree that this is probably the best path forward at the moment.  There was a post early on showing a ternary-like switch expression.  I don't remember whether there were any specific problems with that idea or not, but if there aren't that might best route forward.

> 
> 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> 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
>>>     .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
>>>     .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> 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>:
>>>>> 
>>>>> Actually, this *almost* does what you want. No @autoclosure for the values and no exhaustiveness checking, but otherwise...
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