[swift-evolution] [Pitch] separate syntax of class inheritance and protocol conformance

[Vladimir.S]

On 22.07.2016 19:25, Duan wrote:
> The current syntax establishes a relationship.
>
>   X: Y
>
> means "X will have interface of Y". Note there's no mention of inheritance or protocol here: they both establish this relationship. This is why we can use
>
>   z is Y
>
> To check whether such relationship holds. Again, there's no need to worry whether Y is a class or protocol.

But we see that some mature languages like Java, C# and C++, Object Pascal 
has this distinction: Java with 'implements' keyword, C# with 'I' prefix 
for interfaces(their protocols), C++ with both `implements` and 'I' prefix 
conversion, Object Pascal with 'I' prefix. I.e. I believe that in any case 
new syntax will establish a relationship, but more clearly distinct 
inheritance with conformance to help the reader to understand the 
composition of the reviewed class code.

>
> That being said, I agree that user have to look up Y's declaration to determine which one they are writing/reading. But is that a real issue? Why would you write/use "class X: Y {}" before knowing what Y is?

The problem arises not when you write/use but when you *read* someone's 
code, and sometimes you read that code not in XCode/IDE.


>
> Daniel Duan
> Sent from my iPhone
>
>> On Jul 22, 2016, at 6:14 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
>>
>> I remember that this was discussed, but can't find any decision regarding this.. So, as a last chance, don't we want in Swift 3.0, as big source breaking change, separate class inheritance and protocol conformance in syntax?
>>
>> Sorry if there was a decision about this suggestions. Please let know in this case.
>>
>> I.e. when I see the following I can't understand if the class inherits from base class and conforms to protocols or just conforms to two protocols:
>>
>> class MyClass : First, Second, Third {
>> }
>>
>> We don't have a rule to name protocols with 'Protocol'/other suffix/prefix, or classes with 'T'/'C' prefix or something like this, so I believe to improve the clarity of code we should separate in syntax inheritance and conformance.
>>
>> As I understand we should discuss changes in these areas:
>>
>> 1. class inheritance :
>> class Child: BaseClass
>>
>> 2. class conformance :
>> class Child: SomeProtocol1, SomeProtocol2
>>
>> 3. class inheritance + conformance :
>> class Child: BaseClass, SomeProtocol1, SomeProtocol2
>>
>> 4. protocol conformance for structs:
>> struct Struct: SomeProtocol1, SomeProtocol2
>>
>> 5. protocol inheritance:
>> protocol Child: BaseProtocol1, BaseProtocol2
>>
>>
>> My suggestions:
>>
>> I) separate inheritance with double colon :
>>
>> 1. class inheritance :
>> class Child:: BaseClass
>>
>> 2. class conformance :
>> class Child: SomeProtocol1, SomeProtocol2
>>
>> 3. class inheritance + conformance :
>> class Child:: BaseClass : SomeProtocol1, SomeProtocol2
>>
>> 4. protocol conformance for structs:
>> struct Struct: SomeProtocol1, SomeProtocol2
>>
>> 5. protocol inheritance:
>> protocol Child:: BaseProtocol1, BaseProtocol2
>>
>>
>> II) in class definition use parenthesis to separate inheritance and conformance :
>>
>> 1. class inheritance :
>> class Child: BaseClass
>>
>> 2. class conformance :
>> class Child: (SomeProtocol1, SomeProtocol2)
>>
>> 3. class inheritance + conformance :
>> class Child: BaseClass (SomeProtocol1, SomeProtocol2)
>>
>> 4. protocol conformance for structs:
>> struct Struct: SomeProtocol1, SomeProtocol2
>> or
>> struct Struct: (SomeProtocol1, SomeProtocol2)
>> should be discussed
>>
>> 5. protocol inheritance:
>> protocol Child: BaseProtocol1, BaseProtocol2
>>
>>
>> III) special word like 'conforms'
>>
>> 1. class inheritance :
>> class Child: BaseClass
>>
>> 2. class conformance :
>> class Child: conforms SomeProtocol1, SomeProtocol2
>> or
>> class Child conforms SomeProtocol1, SomeProtocol2
>>
>> 3. class inheritance + conformance :
>> class Child: BaseClass conforms SomeProtocol1, SomeProtocol2
>>
>> 4. protocol conformance for structs:
>> struct Struct: conforms SomeProtocol1, SomeProtocol2
>> or
>> struct Struct conforms SomeProtocol1, SomeProtocol2
>>
>> 5. protocol inheritance:
>> protocol Child: BaseProtocol1, BaseProtocol2
>>
>>
>> Thoughts?
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>
>