[swift-evolution] Simplifying Default Access Modifiers

[Matthew Johnson]

Sent from my iPad

> On Feb 13, 2017, at 3:02 AM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I would like to propose a change to the default access modifier within an enclosing scope.  The default for top level definitions would stay internal, but anything within a scope would by default have the same visibility as it’s enclosing scope.
> 
> The main reason for this is readability/maintainability, and having the intention clearly stand out.  It would also reduce a great amount of boilerplate.  It also matches the mental model of how scopes normally work regarding inheritance of visibility/properties (which means less to teach newbies).
> 
> Right now if I want to make a type and all of it’s vars/methods public, I have to mark each individual var/method public, which leads to a lot of boilerplate/noise and makes everything harder to read:
> 
>    public struct MyStruct {
>        public var a:Int
>        public var b:Int
>        private var c:Int
>        public var d:Int
>    }
> 
> Notice that the private var doesn’t really stand out as such very well.  Also, it is exceedingly rare (at least in my own coding style) that I actually want an internal variable unless the type itself is internal, and in those cases, I would like that choice to stand out as deliberate the same way I want ‘private' to stand out.  As it stands, I wait until I think I am done modifying a type to mark it public because of the extra noise generated.  I also make a point to write ‘internal' for things that I explicitly want to restrict to internal.
> 
> Consider the alternative:
> 
>    public struct MyStruct {
>        var a:Int
>        var b:Int
>        private var c:Int
>        var d:Int
>    }

There was a very intentional decision made to *not* do this.  The reason is that we don't want to allow a library to accidentally commit to a public API contract through an "error of omission".  For example, suppose `d` wasn't really intended to be visible outside the module but the author forgot to add an `internal` annotation.  This is a problem as soon as the library is published.

I think this was the right decision.  It's also worth noting the fact that most Swift users have a couple years of experience where omitting the access modifier means `internal`.  This means an "error of omission" in this regard is more likely for experienced Swift users than it might otherwise be if things worked the way you suggest from the beginning.

> 
> Now the fact that I have chosen to make ‘c’ private really stands out much better.  When revisiting the code in 6 months, the struct is much more “glance-able” (as a friend of mine likes to say).
> 
> Note also the nuance that I didn’t say that those vars were marked public (or had the same modifier), I said that they had the SAME VISIBILITY as the enclosing scope (which in this case happens to be public).  This is a concept which is hard to express currently, and IIRC this is what we had to do to make the edge cases of swift 3’s private modifier work properly.  
> 
> Basically, it already works this way for ‘private’, ‘fileprivate’, & ‘internal’, just not for ‘public’ or ‘open’… which can be surprising, especially since you don’t discover these differences until you are working across modules.  We should just extend that mental model up to include public and open.  Migration would just take internal variables of public/open types and mark them explicitly with the word ‘internal'.
> 
> Thanks,
> Jon
> 
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