[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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