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

[Tony Allevato]

On Mon, Nov 27, 2017 at 11:12 AM Matthew Johnson via swift-evolution <
swift-evolution at swift.org> wrote:

> On Nov 27, 2017, at 12:50 PM, Douglas Gregor <dgregor at apple.com> wrote:
>
>
>
> On Nov 24, 2017, at 3:11 PM, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> As mentioned in my prior message, I currently have a PR open to update the
> generics manifesto (https://github.com/apple/swift/pull/13012).  I
> removed one topic from that update at Doug Gregor’s request that it be
> discussed on the list first.
>
> The idea is to add the ability to make default arguments conditional (i.e.
> depend on generic constraints).  It is currently possible to emulate
> conditional default arguments using an overload set.  This is verbose,
> especially when several arguments are involved.  Here is an example use
> case using the overload method to emulate this feature:
>
> ```swift
> protocol Resource {
>   associatedtype Configuration
>   associatedtype Action
> }
> struct ResourceDescription<R: Resource> {
>   func makeResource(with configuration: R.Configuration, actionHandler:
> @escaping (R.Action) -> Void) -> R {
>     // create a resource using the provided configuration
>     // connect the action handler
>     // return the resource
>   }
> }
>
> extension ResourceDescription where R.Configuration == Void {
>   func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R {
>     return makeResource(with: (), actionHandler: actionHandler)
>   }
> }
>
> extension ResourceDescription where R.Action == Never {
>   func makeResource(with configuration: R.Configuration) -> R {
>     return makeResource(with: configuration, actionHandler: { _ in })
>   }
> }
>
> extension ResourceDescription where R.Configuration == Void, R.Action ==
> Never {
>   func makeResource() -> R {
>     return makeResource(with: (), actionHandler: { _ in })
>   }
> }
>
> ```
>
> Adding language support for defining these more directly would eliminate a
> lot of boilerplate and reduce the need for overloads.
>
>
> If one could refer to `self` in a default argument (which is not a big
> problem), you could turn the default into a requirement itself… although it
> doesn’t *quite* work with your example as written because it would always
> need to be implemented somehow:
>
> protocol Resource {
>   associatedtype Configuration
>   associatedtype Action
>
>     func defaultConfiguration() -> Configuration
>     func defaultHandler() -> ((R.Action) -> Void)
>
> }
>
>
> This won’t work on its own for this use case because there is only a valid
> default in relatively narrow (but common) cases.  For most values of
> Configuration and Action an argument must be provided by the caller.
> Xiaodi’s proposed syntax is the best fit (so far) for the use case I had in
> mind.
>

Out of curiosity, what part of my proposed syntax misses the mark for your
use case?

The parts that I think are somewhat unfortunate are losing a small bit of
reference locality and the introduction of a new operator to distinguish
between "default argument not present if no match found" vs. "compile time
error", definitely. However, one unfortunate trend I've noticed when new
features are proposed is that there's a tendency to end up with "let's
invent @yet_another_attribute" and that just doesn't seem scalable or clean
if we can strive to better integrate it into the syntax of the language.

I'm particularly interested in this because I hit a use case that's similar
to yours in my own code base. I wanted a generic type that could be
instantiated with a disjoint type set—either a RawRepresentable whose
RawValue is Int, or an Int itself. This could have been solved by having
Int retroactively conform to RawRepresentable, but doing that to a
fundamental built-in type Feels Dirty™, so I made it work by not
constraining the generic type at all and moving the public initializers to
constrained extensions so that the only way you could *instantiate* the
type is if you have a correct type argument, and those initializers pass
through to an internal one, sending along a closure that does the correct
transformation:
https://github.com/allevato/icu-swift/blob/master/Sources/ICU/RuleBasedBreakCursor.swift#L208

With your proposed addition, I could hoist those closures into default
arguments based on the constraints instead of adding extensions.

That being said, I don't find the extension-based approach *that*
restrictive, and it kind of makes sense from the point of view of "this
overload only exists for this type under a particular constraint". I wonder
how often this comes up that combinatorial explosion is truly harmful.



>
> That said, the ability to refer to self in default arguments is
> complementary as it would expand the cases where conditional default
> arguments could be provided.  For example, in the example above it would
> allow a resource to provide a nontrivial default configuration.
>
>
>  Doug mentioned that it may also help simplify associated type inference (
> https://github.com/apple/swift/pull/13012#discussion_r152124535).
>
>
> Oh, I thought this was something related to choosing a defaults for
> associated types, which might have helped with my current
> associated-type-inference quandary. The topic you actually wanted to
> discuss is disjoint (sorry).
>
>
> I was wondering how it was related and wondered if it was somehow due to
> reduction in the size of the overload set.  If you have any ideas on
> changes that might help guide the inference algorithm somehow please start
> a new thread.  Even if you’re only able to describe the challenges it might
> be worth a thread if it’s possible others might have useful ideas.
> Improving the reliability and predictability of inference is a very
> important topic!
>
>
> - Doug
>
>
>
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