[swift-evolution] Compiler should generate code base on the functions that have default values

[Félix Cloutier]

I agree that the compiler should be able to use a function with default values for protocol conformance.

Félix

> Le 17 janv. 2016 à 11:52:06, J. Cheyo Jimenez via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> This is very interesting. The first case seems like a bug because the compiler should not let you define that function(). 
> 
> Do you have any actual examples when you would need to conform to the same function name but with different signatures? 
> 
> On Sunday, January 17, 2016, 肇鑫 via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> This proposal is the summary of the previous protocol function default value proposal and some of my new thinkings all together.
> 
> Currently the compiler doesn't stop you defining two functions like:
> 
> func printSomething(something:String = "some thing") {
>     print(something)
> }
> 
> func printSomething() {
>     print("some thing!")
> }
> 
> However, when you call it, an error arises.
> 
> printSomething() // Ambiguous use of 'printSomething'
> 
> You may say just remove the function that has no argument. But protocol needs it.
> 
> protocol A {
>     func printSomething(something:String)
>     func printSomething()
> }
> 
> struct Foo:A {
>     func printSomething(something:String) {
>         print(something)
>     }
>     
>     func printSomething() {
>         self.printSomething("some thing")
>     }
> }
> 
> If you do't define the no-argument function in protocol A. You can not use the function by (Foo() as A).printSomething().
> 
> 
> As we all know, a function with default values, can rewrite to two or more equivalent functions. For example,
> 
> struct Bar {
>     func add(int1:Int = 1, int2:Int = 2, int3:Int = 3) {
>         print(int1 + int2 + int3)
>     }
> }
> 
> is equivalent to 
> 
> struct Bar {
>     func add(int1:Int, int2:Int, int3:Int) {
>         print(int1 + int2 + int3)
>     }
>     
>     func add() {
>         self.add(1, int2: 2, int3: 3)
>     }
>     
>     func add(int1:Int) {
>         self.add(int1, int2: 2, int3: 3)
>     }
>     
>     func add(int1:Int, int2:Int) {
>         self.add(int1, int2: int2, int3: 3)
>     }
> }
> 
> So my proposal is let compiler or pre-compiler to generate the code internally, without changing the original code,  base on the functions that have default values.
> 
> Then we need not to define multiple functions in a protocol when we need function with default values.
> 
> new code:
> 
> protocol A {
>     func printSomething(something:String)
> }
> 
> struct Foo:A {
>     func printSomething(something:String = "some thing") {
>         print(something)
>     }
> }
> 
> 
> If we don't want to change our previous code, we may also need to introduce another keyword defaultValue. This keyword is used only in a protocol to restrict if a function's argument can have a default value. If a function use it, but the implementation doesn't give a default value, or vice versa, an error arises.
> 
> new code:
> 
> protocol A {
>     func printSomething(something:String = defaultValue)
> }
> 
> struct Foo:A {
>     func printSomething(something:String = "some thing") {
>         print(something)
>     }
> }
> 
> This keyword is useful. With it, you can still use Swift in old way. Which means you need not to change code like below if you don't want to.
> 
> old code:
> 
> protocol A {
>     func printSomething(something:String)
>     func printSomething()
> }
> 
> struct Foo:A {
>     func printSomething(something:String) {
>         print(something)
>     }
>     
>     func printSomething() {
>         self.printSomething("some thing")
>     }
> }
> 
> But if you want to write new code. You can just write it in the new way, enjoining the simpler and clearer.
> 
> zhaoxin
> 
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> 
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