[swift-evolution] [Pre-pitch] Conditional default arguments

Tony Allevato tony.allevato at gmail.com
Tue Nov 28 11:01:41 CST 2017

On Tue, Nov 28, 2017 at 8:45 AM Matthew Johnson <matthew at anandabits.com>

> On Nov 28, 2017, at 10:06 AM, Tony Allevato <tony.allevato at gmail.com>
> wrote:
> On Mon, Nov 27, 2017 at 10:32 PM Slava Pestov <spestov at apple.com> wrote:
>> Hi Tony,
>> So if my understanding is correct, the basic proposal is the following:
>> func id<T>(t: T ?= T.defaultValue) { return t }
>> extension Int { static var defaultValue = 0 }
>> extension String { static var defaultValue = “” }
>> id() as Int // returns 0
>> id() as String // returns “”
>> id() as SomeRandomType // fails to type check — no default argument
>> I don’t understand what would happen if the caller is itself generic
>> though, for example:
>> callsID<T>(_ t: T) {
>>   _ = id() as T
>> }
>> It appears that body of callsID() itself cannot type check without
>> knowledge of the concrete T that will be used with this function.
> Thanks for bringing up this example, Slava.
> Unless I'm misunderstanding, the issue you're describing is inherent to
> the *problem* described in the original post, not to any specific
> hypothetical syntax for adding the default arguments, correct? In other
> words, if this was written using extensions as can be done today:
> ```
> struct Foo<T> {
>   func id(t: T) -> T { return t }
> }
> extension Foo where T == Void {
>   func id() -> T { return () }
> }
> extension Foo where T == Int {
>   func id() -> T { return 0 }
> }
> callsID<T>(_ t: T) {
>   _ = Foo().id() as T    // mark
> }
> ```
> The compiler would still reject the marked line because there's no
> guarantee that T is one of the types that has the necessary overload.
> But now that you've mentioned it, it does have me thinking that this
> problem might be better left to extensions. In one sense, default arguments
> are a kind of "overload synthesis”,
> They appear to callers as if they were overloads but I think it’s
> important that they actually do so *without* introducing an overload.
> Reducing the size of an overload set is good for users, library authors and
> the compiler.  The benefits that come to all parties when the size of an
> overload set is reduced is the primary reason I started this thread.
> but on the other hand, there's an expectation that the default value
> expression is of a single type (or set of related types) known at compile
> time. Even if it's generic, it still must be expressed in terms of whatever
> constraints are present on that generic type—you can't use a disjunction of
> types, but instead have to have a common protocol that would provide some
> operation.
> This should not change for any given default value expression.  This
> thread doesn’t discuss changing that.  I discusses the ability to constrain
> the presence of a default value expression.

But that's exactly the distinction that I'm driving at—the more I think
about it, the more I have difficulty expressing it cleanly and I think
that's because these aren't the same as default arguments—they really are
distinct overloads because you're talking about the presence or absence of
an argument based on specific constraints instead of something that applies
to the function uniformly for all possible types that satisfy the
constraint. So what I'm suggesting is that maybe conflating this concept
with default arguments is the wrong approach after all.

>  While it would be useful to allow multiple default value expressions for
> different constraints the most common case will be a single constrained
> default value expression for any given argument.  We could just allow a
> trailing where clause on the default value expression itself like this:
> func makeResource(
>     with configuration: Configuration = () where Configuration == Void,
>     actionHandler: @escaping (Action) -> Void = { _ in } where Action ==
> Never
> )
> That addresses the most common cases for this feature with a fairly
> obvious and direct syntax.

Allowing only one constraint seems arbitrary and I imagine that the
immediately following question would be "what if I want more than one?" I
wouldn't want to address only part of the problem. My ICU example further
up in the thread shows a scenario where I would want defaults for two
distinct types (if I also wanted to publicize the general initializer to

>  It also avoids the potential ambiguity that could arise from allowing
> multiple defaults with different (potentially overlapping) constraints.

That would be no different than the existing problem of multiple overloads
with different (potentially overlapping) constraints, right? If what you're
looking for is a way to have the compiler automatically synthesize
overloads in extensions for you based on certain constraints, I would
expect the same restrictions to apply as if you had explicitly written them
out long-form.

>> Slava
>> On Nov 27, 2017, at 4:10 PM, Tony Allevato via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> I totally agree that that's a good rule in general—I'm not 100%
>> comfortable making an exception to it for this, but I wanted to start a
>> discussion about a different approach than had been considered so far.
>> The idea of forcing the user to acknowledge the explicitness of SFINAE
>> with a strawman syntax `=?` instead of `=` was a thought experiment to
>> bridge the wild-west-C++ world of templates and Swift's stricter generics,
>> but I can definitely understand if even that kind of approach is something
>> that the core team (who are far more familiar with the C++ side of that
>> coin than I am) doesn't wish to support. As was pointed out, it's not
>> something Swift supports anywhere else today.
>> If we look at it from that point of view, where such a semantic treatment
>> of generics would not be supported, I think it becomes a lot harder to
>> rationalize treating this as "default arguments". What you really do have
>> (and what writing it as constrained extensions makes clear) is additional
>> overloads, because they only apply to certain subsets of types. If that's
>> the case, maybe it's the wrong approach to try to turn overloads into
>> "partial default values".
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