[swift-evolution] Make generics covariant and add generics to protocols

Howard Lovatt howard.lovatt at gmail.com
Wed Jan 13 19:05:42 CST 2016


The behaviour you show for Java arrays is what I am proposing, but that
isn't type unsafe. The type error is detected at runtime and is flagged
with an ArrayStoreException. This is unlike C for example that would allow
the store, at least with a cast,  and would put into the array the address
of the string. Therefore neither the proposal or Java is type unsafe, both
are type safe and both detect type errors at runtime.

The question is whether protecting against this is worthwhile, not whether
it can be protected against or not (it can be). Arrays are a good example,
Swift takes the approach (like most languages) of checking the array size
at runtime. But languages with dependent typing check the array size at
compile time, i.e. a dependently typed language would detect this error:

let array = [1]

array[0] // 1, OK

array[1] // Error not detected by compiler but detected at runtime

because in such a language array would be typed as int array of size one,
not int array of any size.

So the real question is in a language like Java, how many times do you get
ArrayStoreException? In my own code I have never seen an array store
exception! Why? Because contravariant writes are very rare. If you were to
protect against contravariant writes you would be complicating the language
for no practical gain, which I think is a bad trade off.

Hope this explains the reasoning,

 -- Howard.

On 14 January 2016 at 09:21, Developer <devteam.codafi at gmail.com> wrote:

> It does indeed make the type system unsound in some cases.  You have
> chosen one particular variance direction because it is convenient. A
> classic counterexample is Java making Arrays covariant by default.  So this
> works
> Integer[] arr = [2, 4, 6, 8, 10];
> Object[] orr = arr;
> orr[0] = "crash bang";
> And crashes at runtime.  For that, I must give this part of the proposal a
> strong -1.  Any amount of type safety I have to give up in the name of
> convenience is far too much.
> I am, however, a fan of generic protocols.  They seem like an orthogonal
> concept given the first part here though.
> ~Robert Widmann
> 2016/01/13 17:03、Howard Lovatt via swift-evolution <
> swift-evolution at swift.org> のメッセージ:
> @Thorsten,
> It doesn't make the type system unsound; the types are mostly checked at
> compile time but some are runtime checked, either way the types are checked
> and therefore the type system is sound. I have posted an example of array
> runtime type checking in a previous response.
> You can annotate variance, but this generally adds a lot of clutter to the
> code (see some typical Java code that uses generics you will see stuff like
> ? extends Type everywhere). In other languages like Scala the annotation is
> less visually intrusive, because they use shorter syntax and because it is
> annotated at declaration site rather than use site, but it is still there.
> I think Swift arrays are a good example of partially static and partially
> runtime checked. The content is statically typed but the size is runtime
> typed. Other languages do type both the content and the size (see Dependent
> Typing on Wiki), however at some considerable burden on the programmer.
> Hope this explains the thought process,
>  -- Howard.
> On 13 January 2016 at 16:50, Thorsten Seitz <tseitz42 at icloud.com> wrote:
>> Strong -1 from me as well for making the type system unsound.
>> > Am 13.01.2016 um 02:47 schrieb Howard Lovatt via swift-evolution <
>> swift-evolution at swift.org>:
>> >
>> > Yes you can annotate for covariance, invariance, and contravariance,
>> both Java and Scala, allow all three. The problem is that the code becomes
>> splattered with variance annotations
>> Ceylon uses a different approach which is variance annotations at the
>> definition site.
>> This restricts the possible usage of the type parameters to appropriately
>> variant positions.
>> This migt be a better way to deal with variance.
>> -Thorsten
> --
>   -- Howard.
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  -- Howard.
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