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

[Duan]

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.

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?

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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