[swift-evolution] [Proposal] Allow protocols to require functions with default parameter values.

[Haravikk]

I think that the point of allowing defaults in protocols is so that you can assume a default for all types conforming to that protocol. To use your example, if you receive an instance of Name, you can only call printSomething() without arguments if you test that it is an instance of type Name. If instead you test its conformance to the Good protocol (which you might do if there are a lot of different types conforming to Good) then you have to provide a value, because you can’t infer that every possible implementation will have a default.

Regarding this proposal however I think it might be useful to have a distinction between a protocol function that specifies a default value for all implementations (that they must all conform to) versus one that specifies that implementations must have a default value, but not what that value must be.

For example, to have a fixed and altered default we currently we have to do things like this:

protocol Protocol {
	func functionWithSpecificDefault(argument:String)
	func functionWithAnyDefault(argument:String)
}

extension Protocol {
	func functionWithSpecificDefault() { self.functionWithSpecificDefault(“Foo”) }
	func functionWithAnyDefault() { self.functionWithAnyDefault(“Foo”) }
}

class MyClass : Protocol {
	func functionWithSpecificDefault(argument:String) { /* Implementation here */ }
	func functionWithAnyDefault(argument:String) { /* Implementation here */ }

	func functionWithAnyDefault() { self.functionWithAnyDefault(“Bar”) } // Override default
}

Which could be replaced by:

protocol Protocol {
	func functionWithSpecificDefault(argument:String = “Foo")
	func functionWithAnyDefault(argument:String = default)
}

class MyClass : Protocol {
	func functionWithSpecificDefault(argument:String = “Foo") { /* Implementation here */ }
	func functionWithAnyDefault(argument:String = “Bar") { /* Implementation here */ }
}

However, this has the added advantage that implementing functionWithSpecificDefault with a default other than “Foo” would cause a compiler error, while doing so for functionWithAnyDefault would not (but specifying no default at all would, as one is required).

> On 16 Jan 2016, at 10:15, 肇鑫 via swift-evolution <swift-evolution at swift.org> wrote:
> 
> No. Although you protocol's function doesn't has a default parameter value. Your implementation does. So you don't need to define another func function() in your protocol.
> 
> protocol Good {
>     func printSomething(something:String)
> }
> 
> struct Name:Good {
>     func printSomething(something: String = "John") {
>         print(something)
>     }
> }
> 
> Name().printSomething()
> 
> above code works.
> 
> zhaoxin
> 
> On Sat, Jan 16, 2016 at 6:05 PM, Vatsal Manot <vatsal.manot at yahoo.com <mailto:vatsal.manot at yahoo.com>> wrote:
> It serves as a better (if not simpler) substitute for the following pattern:
>  
> protocol Protocol
> {
>     typealias Argument
>     
>     func function()
>     func function(_: Argument)
> }
>  
>> On 16-Jan-2016, at 3:29 PM, 肇鑫 <owenzx at gmail.com <mailto:owenzx at gmail.com>> wrote:
>> 
>> I wonder where is the good for a protocol designer on this?
>> 
>> zhaoxin
>> 
>> On Sat, Jan 16, 2016 at 5:23 PM, Vatsal Manot via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Currently, the following the code fails with multiple errors:
>> 
>> protocol Protocol
>> {
>>     typealias Argument
>> 
>>     func function(argument: Argument = default)
>> }
>> 
>> I propose that we allow protocols to require functions with default parameter values. I can’t see any disadvantages to this, and the change would only be additive.
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>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
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>> 
>> 
>> 
>> -- 
>> 
>> Owen Zhao
> 
> 
> 
> 
> -- 
> 
> Owen Zhao
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> swift-evolution at swift.org
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