[swift-server-dev] [HTTP] Value vs Reference Types

Chris Bailey BAILEYC at uk.ibm.com
Mon Dec 12 06:08:53 CST 2016 

Thanks Ben. This is really useful.

Its probably worth also adding some information on the approach taken by 
Foundation today for URLRequest/Response. 

For outbound requests, Foundation provides both NSURLRequest (class) and 
URLRequest (struct). In the case of URLRequest (struct), there is a mix of 
types as it contains a getter that returns a stream class for the body - 
and the with a number of warnings commented in the code:
        "The stream is returned for examination only; it is not safe for 
the caller to manipulate the stream in any way"
(
https://github.com/apple/swift-corelibs-foundation/blob/5ab46791996cb65079d02f98115cef6562d4819b/Foundation/URLRequest.swift#L187
)
This is presumably because you shouldn't need to modify the stream for an 
outbound request

For responses from outbound requests, only NSURLResponse (class) is 
provided as the body is read from the stream.

This effectively give us a model where:
        outbound        can be a struct (you shouldn't need to touch the 
stream)
        inbound class (reading from stream)
For the server case, we need the mirror opposite of this - with incoming 
requests being a class and the outgoing response potentially being a 
struct. 

This leads to two questions:
1. Do we think the Foundation model is correct?
2. Do we think it would be confusing to switch been classes and structs 
when working with inbound vs. outbound requests, or do we think it would 
enforce correct behaviour?

Chris




From:   Ben Cohen via swift-server-dev <swift-server-dev at swift.org>
To:     Dan Appel <dan.appel00 at gmail.com>
Cc:     "swift-server-dev at swift.org" <swift-server-dev at swift.org>
Date:   08/12/2016 02:04
Subject:        Re: [swift-server-dev] [HTTP] Value vs Reference Types
Sent by:        swift-server-dev-bounces at swift.org



Hi Dan,

Thanks for spelling out these questions, I think they are a great starting 
point for a discussion. A few comments inline.

On Nov 23, 2016, at 1:19 PM, Dan Appel via swift-server-dev <
swift-server-dev at swift.org> wrote:


My own responses:


>1. Do we want to use concrete types or protocols for Request/Response?


When working on Open Swift, this was a hot topic since we believed that it 
would be unsafe to have a protocol that would allow both value and 
reference types. 

Bear in mind that a protocol is more than just the methods and types it 
declares – it is also its documentation. For example, a number of 
protocols in the standard library state things like complexity 
requirements in their documenting comments. The language has no way of 
enforcing these, but your type does not truly “conform” to the protocol 
unless you adhere to them. Value semantics are similar – you can document 
that it is invalid to implement a protocol with reference semantics. So I 
don’t think this is a blocker to wanting to use protocols.

We arrived upon the `{Request|Response}Representable` pattern which worked 
but was a bit of a mess. Because of this, I would prefer concrete 
Request/Response types.


You could think of there as being 3 purposes to using protocols here, 
roughly in order of importance:

being able to write generic code
allowing different frameworks to interoperate
documenting what you need to implement

The first one is the only reason why a protocol must be included in a 
library, and the key question to ask when considering defining a protocol 
like this is “What common algorithms do you want to write across multiple 
different conforming types in your program”?  (such as generic functions, 
including protocol extensions, or functions that take an existential if 
the protocol has no associated types).

This is distinct from wanting to be able to be able to choose from 
different library implementations of Request. You might want to choose 
between the Acme Inc Web Framework’s Request type, or some Swift-Server 
“official" Request type, but you never need to use both at once and write 
code spanning them. You just want to make sure that one can serve as a 
source-compatible “drop-in” replacement for the other in your code. This 
doesn’t mean you can’t write your own extensions – but you would extend 
the concrete Request not a RequestRepresentable protocol.

Next, it’s possible that there might be a collection of 3rd-party 
frameworks out there that don’t define Request, but want to be able to 
write methods that take or extend multiple possible Request 
implementations. This seems a bit unlikely in the case of these types, 
more likely in other cases like networking, so it’s kind of a what-if 
scenario where there are both multiple popular implementations of Request, 
and various frameworks that want to interact with them. Anyone can add a 
conformance to anything, so those frameworks can define a protocol of 
their own with a subset of the functionality they need, and then just 
extend the popular implementations to conform to it. If this gets really 
common, at that point it might be worth creating an official protocol for 
everyone to share – but this can be done later, doesn’t have to be done 
up-front. 

Finally, if you do expect multiple implementations and want people to be 
able to swap them in and out when they choose, the protocol can serve to 
document what methods and properties you are expected to implement to be 
“source compatible". This can be done in documentation instead, the 
benefit of the protocol being it helps the library developer ensure 
they’ve got all the signatures right etc. But this isn’t something you 
expose to users, it’s something on the side to help implementors.

Based on all the above, it seems like there isn’t a pressing need for a 
protocol for these types right now and initial designs should focus on a 
concrete implementation until one emerges, if only to avoid premature 
generalization. Useful protocols tend to be discovered, rather than 
designed, through a desire to share common operations on different 
concrete types.

>2. If we use concrete types, do we want value or reference semantics?


What I think makes this easier is that the "big four" have each taken a 
slightly different approach that can be used as a reference.
Zewo - struct, value semantics
Vapor - closed class, reference semantics
Kitura - closed class + has-a pattern, reference semantics
Perfect - class protocol, reference semantics

Zewo is the outlier here, but I would like to note as a contributor to 
Zewo that we have not ran into situations where value semantics create an 
impassable roadblock. 

To me, it makes sense to pass them around as values since they don't have 
any logic of their own. Requests/Responses can't send themselves, they can 
only read and modified. It also gives me as a user more safety to pass 
them around since I know that they won't be modified implicitly.

Take the following pseudo-code as an example:

HTTPServer.onRequest { request in
    print(request.sourceIp)
    HTTPClient.send(request)
    print(request.sourceIp)

}

With reference semantics, there is no guarantee that sourceIp will be the 
same before and after sending off the request. After all, it could make 
sense for the HTTPClient to modify the sourceIp before sending off the 
request. This of course a contrived example, but the point stands.


Not contrived at all, this is a perfect illustration of why reference 
semantics make it harder to reason about your code and identify the cause 
of bugs.

Anyway, I think it would be great if we could have people talk about their 
own experiences.


>3. When is it more convenient to have reference semantics?



Convenience is a double-edged thing. Pointers with possibly-null values, 
or integer indexes into Unicode strings, are often considered convenient. 
But that convenience comes with a hidden cost to correctness – unexpected 
nulls, accidentally indexing into the middle of a grapheme cluster etc. 
When making a proper effort to handle these things correctly, code quickly 
becomes less convenient, and less readable, compared to the alternatives.

It’s generally the style in Swift that correctness shouldn't be sacrificed 
for convenience, but when things work out well, convenience and ergonomics 
can be mutually reinforcing – the code is nice to use correctly, awkward 
to use incorrectly. For example, optionals that force you to handle nil 
help with correctness, but they have sugar like ?? or optional chaining to 
handle common patterns clearly and idiomatically, ! as a shorthand for 
asserting something is non-nil etc.

In the middleware chain architecture that we decided on in Zewo (the other 
ones have something similar), it can be convenient to modify requests in 
the responder and have that reflect in the middleware. I think this 
problem is best solved with `inout` parameters rather than reference 
types, but that is my personal opinion.


FWIW, this design view is also strongly held by those of us working on the 
Swift Standard Library. I also brought this up with several members of the 
Core Team and they also strongly felt that inout and value types was the 
general approach we should take with such types in Swift. The consensus 
there was that reference types really should be mostly used when identity 
of the value is important.


>4. Are there problems that can't be solved with value semantics?


I haven't found any, but I'm sure others can bring something interesting 
to the table.



Shared mutable state is one. With an unavoidably-shared resource, like a 
network connection or a handle to a window on a screen, reference 
semantics are often what you want.


On Wed, Nov 23, 2016 at 1:07 PM Dan Appel <dan.appel00 at gmail.com> wrote:
Hello everyone!

I was unable to make the kick-off meeting for the HTTP sub-team, but I 
looked over the meeting notes and found some topics that I think could use 
some more on-the-record discussion.

A few questions that I wanted to raise:

1. Do we want to use concrete types or protocols for Request/Response?
2. If we use concrete types, do we want value or reference semantics?
3. When is it more convenient to have reference semantics?
4. Are there problems that can't be solved with value semantics?

I would like to avoid bike-shedding, and I think this can be done by 
providing real examples rather than just talking about the pros and cons.
-- 
Dan Appel
-- 
Dan Appel
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