> On Nov 20, 2016, at 6:40 AM, Alan Cabrera <[email protected]> wrote:
>> On Nov 19, 2016, at 8:57 PM, John McCall <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>> On Nov 19, 2016, at 6:03 PM, Alan Cabrera <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>> On Nov 19, 2016, at 4:02 PM, John McCall <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>>
>>>>> On Nov 19, 2016, at 3:31 PM, Alan Cabrera <[email protected]
>>>>> <mailto:[email protected]>> wrote:
>>>>>> On Nov 19, 2016, at 1:21 PM, John McCall <[email protected]
>>>>>> <mailto:[email protected]>> wrote:
>>>>>>
>>>>>>> On Nov 19, 2016, at 10:07 AM, Alan Cabrera via swift-evolution
>>>>>>> <[email protected] <mailto:[email protected]>> wrote:
>>>>>>>> On Nov 19, 2016, at 9:27 AM, Jean-Daniel <[email protected]
>>>>>>>> <mailto:[email protected]>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>>> Le 19 nov. 2016 à 15:58, Alan Cabrera via swift-evolution
>>>>>>>>> <[email protected] <mailto:[email protected]>> a
>>>>>>>>> écrit :
>>>>>>>>>
>>>>>>>>> I’m not sure if this was proposed or not; or even if this is a
>>>>>>>>> Swift-ly way of doing things. It would be pretty handy to be able to
>>>>>>>>> declare init() functions in my module to register handlers. It’s a
>>>>>>>>> common pattern in enterprise software.
>>>>>>>>>
>>>>>>>>> Currently, I have to generate a lot of boilerplate code to emulate
>>>>>>>>> the behavior. I think it would be cleaner to have these global
>>>>>>>>> init() functions.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I’d rather like a swift attribute equivalent to :
>>>>>>>> __attribute__((constructor))
>>>>>>>>
>>>>>>>> It will not force me to call my initializer init, and moreover it will
>>>>>>>> let me declare multiple functions so I would be able to register
>>>>>>>> multiples handlers from a single module without having to group all
>>>>>>>> the register call into a single init() function.
>>>>>>>>
>>>>>>>
>>>>>>> I’m not quite following what “__attribute__((constructor))” means; it
>>>>>>> looks like an LLVM implementation bit. Do you mean defining a new
>>>>>>> Swift declaration attribute named “constructor”? If so, I really like
>>>>>>> that idea. I think that the specific attribute name “constructor” may
>>>>>>> be a bit confusing though, since it’s not really constructing anything
>>>>>>> specific. Maybe “startup” would be a more descriptive attribute name?
>>>>>>>
>>>>>>> @startup
>>>>>>> func registerHandlers() {
>>>>>>> }
>>>>>>>
>>>>>>> The attribute would also help the compiler and IDEs prevent direct
>>>>>>> calling of the startup functions, thus reinforcing/focusing the startup
>>>>>>> functions’ role as global startup functions. Maybe global teardown
>>>>>>> functions would be helpful as well.
>>>>>>>
>>>>>>> I’m going to try goofing around with the idea on my fork.
>>>>>>
>>>>>> Some sort of reflective discovery would be better, I think. Eager
>>>>>> global initialization is superficially attractive — what could be
>>>>>> simpler than just running some code at program launch? — but as a
>>>>>> program scales up and gains library dependencies, it very quickly runs
>>>>>> into problems. What if an initializer depends on another already having
>>>>>> been run? What if an initializer needs to be sensitive to the arguments
>>>>>> or environment? What if an initializer need to spawn a thread? What if
>>>>>> an initializer needs to do I/O? What if an initializer fails? Global
>>>>>> initialization also has a lot of the same engineering drawbacks as
>>>>>> global state, in that once you've introduced a dependency on it, it's
>>>>>> extremely hard to root that out because entire APIs get built around the
>>>>>> assumption that there's no need for an explicit
>>>>>> initialization/configuration/whatever step. And it's also quite bad for
>>>>>> launch performance — perhaps not important for a server, but important
>>>>>> for pretty much every other kind of program — since every subsystem
>>>>>> eagerly initializes itself whether it's going to be used or not, and
>>>>>> that initialization generally has terrible locality.
>>>>>
>>>>> Very good points. I recognize the dangers. However.
>>>>>
>>>>> Don’t these problems already exist given that user code can still execute
>>>>> at program startup? It cannot be denied that the pattern is used and is
>>>>> extremely useful though, as you point out above, it should be used
>>>>> carefully. Thinking on it, there are always pros and cons to most
>>>>> language features and one relies on best practices to avoid shooting
>>>>> oneself in the foot. For each of the specters listed above, there are
>>>>> simple accepted practices that can be adopted to avoid them; most of
>>>>> those practices are already being employed for other situations.
>>>>>
>>>>> And the pattern is not just useful in enterprise software. Complex
>>>>> applications’ app-delegate did-finish-launching methods are chucked full
>>>>> of hand stitched roll calls to framework initialization code. This
>>>>> needlessly places a brittle dependency/burden on the application
>>>>> developer in what should be a simple behind the scenes collaboration.
>>>>>
>>>>> One could argue that such a thing was never needed before. I would point
>>>>> to CocoaPods, Carthage, and the other influx of enterprise influenced
>>>>> tooling and frameworks. Today’s mobile applications are no longer simply
>>>>> todo apps.
>>>>>
>>>>> Global init() functions are a clean solution to what engineers are
>>>>> already boiler plating with static singleton code.
>>>>
>>>> No, they aren't a clean solution for the reasons I listed. They may be a
>>>> solution you're used to using, but they're not a *clean* solution, and
>>>> Swift's line against providing them is for the best.
>>>>
>>>> I'm surprised that you keep talking about enterprise / complex
>>>> applications as some sort of argument for them, because those are exactly
>>>> the applications where, in my experience, global initializers completely
>>>> break down as a reasonable approach. It's the small applications that can
>>>> get away with poor software engineering practices, because the accumulated
>>>> maintenance/complexity/performance costs are, well, small.
>>>
>>>
>>> It’s difficult to subscribe to the slippery slope arguments that contain
>>> specters that can still afflict applications without global init functions.
>>> Any feature can be abused and it seems hyperbolic to provide arguments
>>> that seems to ascribe the above problems as an inevitability solely
>>> afflicting global init functions. My and others’ experience with them has
>>> been very different from yours.
>>>
>>> With that said, I took some time to re-read your reply, after my afternoon
>>> nap. I really like the idea of some kind of reflective discovery. How
>>> would that work? Maybe having a special @tag attribute that can be
>>> searched at runtime?
>>
>> There was another thread that mentioned this idea recently, but it would be
>> reasonable to provide some way to get a P.Type for every type in the program
>> that conforms to a protocol P.
>
> Great, are there any keywords I can use to search for this thread?
>
>> This would be opt-in at the protocol level, because we wouldn't want to be
>> prevented from e.g. stripping an unused type from the program just because
>> it implemented Equatable. There are some other complexities here, but
>> that's the basic idea, and it's totally reasonable to support.
>
> Does the other thread go into detail about this? I’m not sure that I follow
> why it should be opt-in as opposed to simply searching for all
> implementations of a specific protocol.
We would want this to be opt-in on the protocol because it would inhibit
removing a conforming type from the program. The compiler can ordinarily
remove types that you never directly use, and that's an important optimization
when e.g. linking in large libraries that you only use a small part of. We
wouldn't want to lose the ability to do that just because a type implemented
Equatable or Collection, because a lot of types implement those protocols, and
the ability to iterate all those types is probably not very useful — certainly
it isn't useful enough to justify losing that optimization. In contrast, when
there's a protocol that is useful to iterate over all the conformances of, like
a Deserializable protocol that registers types with a deserialization engine,
you always certainly know that when you're defining the protocol.
John.
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