[swift-evolution] ternary operator ?: suggestion
Charles Constant
charles at charlesism.com
Tue Dec 29 08:54:46 CST 2015
I'm with Matthew/Craig.
We discussed a couple very ternary-like versions earlier in the thread,
which I increasingly think are the best options.
The major objection to this came from Lattner, and his objection, if I have
it right, is "this proposal doesn't add enough functionality to justify the
additional complexity/confusion"
The sticking point, at the moment, is formulating a really persuasive
argument for "why we need this." If we can't do that, this proposal is dead.
On Tue, Dec 29, 2015 at 5:38 AM, Matthew Johnson via swift-evolution <
swift-evolution at swift.org> wrote:
>
>
> 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:
>
>
> 1. Is documented right in the IDE including code completion,
> statements aren’t (you don’t see quick help for `for`!)
> 2. 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?
> 3. It is simpler to implement, maintain, and change a library function
> that a built-in.
> 4. 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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