<html><head><style>body{font-family:Helvetica,Arial;font-size:13px}</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">I kinda feel my proposal was a little waste of time now. Or am I wrong here? ^^</div> <br> <div id="bloop_sign_1463557524728443136" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 18. Mai 2016 bei 09:35:56, Austin Zheng via swift-evolution (<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
<div dir="ltr">I've put together a considerably more detailed draft
proposal, taking into account as much of Matthew's feedback as I
could. You can find it below:
<div><br></div>
<div><a href="https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md">
https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md</a><br>
</div>
<div><br></div>
<div>Since there is no chance this will come up for review anytime
soon, I expect to make significant revisions to it over the next
month or so. Any feedback would be greatly appreciated.</div>
<div><br></div>
<div>Austin</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, May 17, 2016 at 9:52 PM, Austin
Zheng <span dir="ltr"><<a href="mailto:austinzheng@gmail.com" target="_blank">austinzheng@gmail.com</a>></span> wrote:<br>
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<div class="gmail_quote"><span class="">On Tue, May 17, 2016 at
1:25 PM, Matthew Johnson <span dir="ltr"><<a href="mailto:matthew@anandabits.com" target="_blank">matthew@anandabits.com</a>></span> wrote:</span>
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<div><span>Within the angle brackets are zero or more 'clauses'.
Clauses are separated by semicolons. (This is so commas can be used
in where constraints, below. Better ideas are welcome. Maybe it's
not necessary; we can use commas exclusively.)</span></div>
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<div>I’m not a fan of the semicolon idea. I don’t see any
reason for this. The `where` keyword separates the protocol
list from the constraints just fine. The list on either side
should be able to use commas with no problem (or line breaks if
that proposal goes through).</div>
<span><br></span></div>
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<div><br></div>
<div>I'm leaning towards getting rid of the commas, but would like
to write out a few 'dummy' examples to see if there are any
readability issues that arise. <br></div>
</div>
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<div><br></div>
<div>Replaced with what? Whitespace separation? I
suppose that might work for the protocol list but it feels
inconsistent with the rest of Swift. Commas plus (hopefully)
the alternative of newline seem like the right direction to
me.</div>
</div>
</div>
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<div><br></div>
<div>Sorry, I completely misspoke (mistyped?). I meant I want to
get rid of the semicolons and use commas. I've come to the
conclusion that there are no readability issues, protocol<>
already uses commas, and semicolons used in this manner don't have
a precedent anywhere else in the language.</div>
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<div><span class=""><br></span></div>
<div><span class="">There are five different possible
clauses:</span></div>
<div><span class=""><br></span></div>
<div>
<ul>
<li><span class="">'class'. Must be the first clause, if present.
Places a constraint on the existential to be any class type.
(Implies: Only one can exist. Mutually exclusive with class name
clause.)<br></span></li>
</ul>
</div>
<div><span class=""><br></span></div>
<div><span class="">(In the future a follow-up proposal should add
in 'struct' or 'value' as a counterpart.)</span></div>
</div>
</div>
</div>
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<div><span class=""><br></span></div>
<div>If we’re going to allow `struct` we should also allow `enum`.
`value` would allow either of those.</div>
<span><br></span></div>
</div>
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<div><br></div>
<div>Of course. A future proposal can allow list members to discuss
the exact details as to how struct, value, or enum specifiers
should work. <br></div>
</div>
</div>
</div>
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<div><br></div>
<div>Yep, agree. Just mentioning that if we’re going to
reference it we should not leave obvious holes in what would be
considered. :)</div>
</div>
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<div><br></div>
<div>Absolutely.</div>
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<div>
<ul>
<li><span class="">Class name. Must be the first clause, if
present. (Implies: Only one can exist. Mutually exclusive with
'class'.) Places a constraint on the existential (not really an
existential anymore) to be an instance of the class, or one of its
subclasses.<br></span></li>
</ul>
</div>
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<div>It is still be an existential if it includes protocol
requirements that the class does not fulfill. For example,
you might have Any<UIView, SomeProtocol> where UIView does
not conform to SomeProtocol, but various subclasses do.</div>
<div><br></div>
</div>
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<div><br></div>
<div>Fair enough. (I don't think the way things work would be
affected.)</div>
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<div>Your proposal doesn’t discuss composing Any in the way
that Adrian’s did like this:</div>
<div><br></div>
<div>typealias Foo = Any<SomeClass, SomeProtocol,
OtherProtocol></div>
<div>Any<AnotherProtocol, Foo></div>
</div>
</div>
</blockquote>
<div><br></div>
<div>I didn't think it needed to be discussed. An Any<...>
existential type is a type 'expression' just like any other, and
should be allowed to participate in other Any<...>s.</div>
</div>
</div>
</div>
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<div>I like the idea of composition as it allows us to factor out
constraints. If we are going to do that we should allow a
class to be specified in the composition as long is it is a
subclass of all class requirements of Any types it composes.
For example, this should be allowed:</div>
<div><br></div>
<div>typealias Bar = Any<SubclassOfSomeClass, Foo,
AnotherProtocol></div>
<div><br></div>
<div>This is still one class requirement for Bar, it just refines
the class requirement of Foo to be SubclassOfSomeClass rather than
just SomeClass.</div>
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<div>This is a good point. There should be clarification as to how
special cases of Any<...> used in another Any<...>
behave. For example, like you said Any<MyClass,
Any<SomeSubclassOfMyClass, Protocol>> should be valid.
This will go into any proposal that emerges from the
discussion.</div>
</div>
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<div>Yes, this is why we need to discuss Any composition.
There are also cases of incompatible associated type constraints
which need to be rejected (such as composing two Any’s where one
has Element == String and another has Element == Int).</div>
</div>
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<div>
<div><span>Example: Any<UIViewController; UITableViewDataSource;
UITableViewDelegate></span></div>
<div><span>"Any UIViewController or subclass which also satisfies
the table view data source and delegate protocols"</span></div>
<div>
<ul>
<li><span>Dynamic protocol. This is entirely composed of the name
of a protocol which has no associated types or Self
requirement.</span></li>
</ul>
</div>
</div>
</div>
</div>
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<div>
<div>
<div><span>Example: Any<CustomStringConvertible;
BooleanType></span></div>
<div><span>"Any type which conforms to both the
CustomStringConvertible and BooleanType protocols"</span></div>
<div><span><br></span></div>
<div><span>I'm going to use 'static protocol' to refer to a
protocol with associated types or self requirements. Feel free to
propose a more sound name.</span></div>
<div><span><br></span></div>
<div>
<ul>
<li><span>Self-contained static protocol, simple. This is composed
of the name of a static protocol, optionally followed by a 'where'
clause in which the associated types can be constrained (with any
of the three basic conformance types: subclassing, protocol
conformance, or type equality). Associated types are referred to
with a leading dot.<br></span></li>
</ul>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<div>Please do not introduce terms “dynamic protocol” and “static
protocol”. We want to support existentials of protocols that
have self or associated type requirements. The dynamic vs
static distinction is a limitation of the current implementation of
Swift and doesn’t make sense for the long term vision.</div>
</div>
</div>
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<div><br></div>
<div>I'm not trying to introduce new terms, these are just
placeholders. At the same time "protocols with self or associated
type requirements" is cumbersome to work with and it would be nice
for someone to come up with a descriptive term of art for referring
to them.</div>
</div>
</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>I agree that a better term would be useful. In the
meantime, I would prefer something like “trivial” and “nontrivial”
protocols.</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>I've decided to just use the full name until the community
comes up with better names. Clarity is preferable to brevity in
this case.</div>
<div><span class=""> </span></div>
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<div>
<div><span class=""><br></span></div>
<div><span class="">Example: Any<Collection where
.Generator.Element : NSObject, .Generator.Element :
SomeProtocol></span></div>
<div>"Any type that is a Collection, whose elements are NSObjects
or their subclasses conforming to SomeProtocol.”</div>
</div>
</div>
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<div><br></div>
<div>Swift does not allow disjunction of requirements. Only
conjunctions are supported. That means the correct reading
is:</div>
<div><br></div>
<div>"Any type that is a Collection, whose elements are
NSObjects<span> </span><b>and</b><span> </span>their
subclasses conforming to SomeProtocol.”</div>
<br></div>
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<div><br></div>
<div>Yes, that is what I meant. "whose elements are (NSObjects or
their subclasses) conforming to SomeProtocol”.</div>
</div>
</div>
</div>
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<div><br></div>
<div>Ok, good. Wasn’t quite clear to me.</div>
</div>
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<div><br></div>
<div>Yes, the verbiage will need to be clearer in the future. That
sentence could be ambiguously parsed.</div>
<div><span class=""> </span></div>
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<div>
<div><span class=""><br></span></div>
<div>
<ul>
<li><span class="">Bound static protocol. This is the same as a
self-contained static protocol, but with a leading "<name> as
" which binds the protocol to a generic typealias. The name can be
then be used in subsequent clauses to build
constraints.<br></span></li>
</ul>
</div>
<div><span class=""><br></span></div>
<div><span class="">Example: Any<T as Collection;
IntegerLiteralConvertible where .IntegerLiteralType ==
T.Element>.</span></div>
<div>"Any type that is a Collection, and also can be built from an
integer literal, in which the collection elements are the same type
as the type of the integer used for the integer literal
conformance.”</div>
</div>
</div>
</div>
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<div><br></div>
<div>I’m not sure about this, but if we’re going to do it it should
be the other way around: `Collection as T` with the
alias<span> </span><i>after</i> the name of the protocol.
</div>
<div><br></div>
</div>
</div>
</blockquote>
<div><br></div>
<div>I like this, it flows better. "Protocol as T where
Protocol.Foo == Int, Protocol.Bar : Baz”.</div>
</div>
</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>Why did you introduce an alias here and then not use it?
Did you mean "Protocol as T where T.Foo == Int, T.Bar : Baz"</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>Another result of rushing to compose an email. Sorry!</div>
<div><span class=""> </span></div>
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<div><span class="">You are also using “dot shorthand” here to
refer to an associated type of IntegerLiteralConvertible. I
think “dot shorthand” should be limited to cases where there is
only one protocol that is getting constrained. In other
cases, we need to be clear about which protocol we are referring
to.</span></div>
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<div><span class=""><br></span></div>
<div><span class="">I borrowed dot shorthand from the generics
manifesto. But you are right, it should only be allowed if there is
one protocol with associated types or self requirements clause in
the Any<...> construction.</span></div>
</div>
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<div>I would actually go further and limit it to one protocol
period, and possibly even to one protocol and no type names (as
types can have nested types and typealiases). When we allow
shorthand it should be immediately unambiguous what the shorthand
references with no need to look at type or protocol
declarations.</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>It might be desirable to propose the proposal with no
allowance for shorthand, and have the dot shorthand be a smaller
follow-up proposal.</div>
<div><span class=""> </span></div>
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<div><span class=""><br></span></div>
<div><span class="">There will be rules to prevent recursive
nesting. For example, if generic typealiases are allowed, they
cannot refer to each other in a circular manner (like how structs
can't contain themeselves, and you can't create a cyclic graph of
enums containing themselves).</span></div>
<div><span class=""><br></span></div>
<div><span class="">How an existential can be used depends on what
guarantees are provided by the clauses. For example,
'Any<Equatable>' can't be used for much; if there were any
methods on Equatable that did not use the associated types at all
you'd be able to call them, but that's about it. However,
'Any<Equatable where .Self == String>' would allow for == to
be called on instances. (This is a stupid example, since
Any<Equatable where .Self == String> is equivalent to
'String', but there are almost certainly useful examples one could
come up with.)</span></div>
</div>
<div><span class=""><br></span></div>
<div><span class="">In order of increasing 'power':</span></div>
<div>
<ul>
<li><span class="">Don't constrain any associated types. You can
pass around Any<Equatable>s, but that's about it.</span></li>
<li><span class="">Constrain associated types to conform to
protocols.</span></li>
<li><span class="">Fully constrain associated types.</span></li>
</ul>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<div>I think we need to spell out pretty clearly what members we
expect to be available or not available. This section
probably needs the most design and elaboration. </div>
<div><br></div>
<div>For example, we probably can’t access a member who uses an
associated type as an input unless it is constrained to a specific
type. On the other hand output types probably don’t need to
limit access to a member. However, if the output type is Self
or an associated type the visible signature would have an output
type which has the relevant constraints of the existential applied,
but no more. In some cases this means the output type would
simply be Any.</div>
</div>
</blockquote>
<div><br></div>
<div>Absolutely. This is vaguely what I had in mind but I wanted to
get something down first. Thanks for thinking through some of the
implications :).</div>
</div>
</div>
</div>
</div>
</blockquote>
<div><br></div>
<div>That’s what I thought. Just wanted to start the process
of elaborating expectations.</div>
<br>
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<div><span>Where this really gets tricky is for compound types like
functions, generic types, etc. Working out the details in
these cases is pretty complex. I will defer to Doug on
whether it is best to just defer those cases to the future, leave
them up to the implementer, or try to work out all of the relevant
details in the proposal (in which case we probably need a type
system expert to help!).</span></div>
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<div>Yes, exactly! For example, can Any<...> existentials
involving protocols with associated types or self requirements be
used within generic function or type definitions? Maybe there's an
argument that existential types of this nature are redundant if you
have access to generics (e.g. defining a property on a generic type
that is a Collection containing Ints; you should be able to do that
today). On the other hand, maybe there are use cases I haven't
thought of…</div>
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<div>I see no reason they shouldn’t be. They are not
redundant at all. For example, you may want to store
instances in a heterogeneous collection. You need
existentials to do that.</div>
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<div>A simple example of what I was referring to there is something
like this:</div>
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<div>protocol P {</div>
<div> associatedtype Foo</div>
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<div> func bar(callback: (Foo) -> ())</div>
<div>}</div>
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<div>In other words, types in the signature of a protocol member
are complex types that reference Self or associated types. I
think you really need a formal understanding of the type system to
understand how to expose these members through a constrained
existential. We can probably understand the expected behavior
in some of the simpler cases on a case by case basis, but that
approach doesn’t scale at all and is arbitrary. If they’re
going to be supported an expert is going to need to be involved in
the design.</div>
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<div>Yes. I have some ideas regarding this topic.</div>
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<div><span class="">One area you didn’t touch on is “opening” the
existential? Is that out of scope for this proposal?
That would be fine with me as this proposal is already taking on a
lot. But if so, you should mention something about future
directions as it is pretty closely related to this
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<div><span class="">Yes, existential opening is explicitly separate
from this (although I wanted to mention it in the section where I
talk about how Any<Equatable> is not very useful). But you
are absolutely right, this proposal should discuss how it wants to
interact with possible future directions.</span></div>
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<div><span class="">Another area you didn’t touch on is whether Any
constructs (and typealiases referring to them) should be usable as
generic constraints. I would expect this to be possible but I
think we need to spell it out.</span></div>
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<div><span class="">I'm hoping for community input. This is a
tricky subject, and at some point we'll bump into implementation
limitations.</span></div>
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<div>I don’t think it’s too tricky. You can just unpack the
constraints of the Any into the list of generic constraints.
Maybe I’m missing something, but I don’t think so.</div>
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<div><span><font color="#888888">-Matthew</font></span></div>
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