[swift-evolution] [Pitch] Simpler interpretation of a reference to a generic type with no arguments

[Dave Abrahams]

on Tue Oct 11 2016, Slava Pestov <swift-evolution at swift.org> wrote:

> I could if there’s interest. Since we intend on maintaining source
> compatibility, it will not result in a simpler implementation, though,
> since we’ll need to keep the old code path around for Swift 3
> mode. Still worth it?

IIUC it especially makes sense in light of
https://github.com/apple/swift-evolution/blob/master/proposals/0068-universal-self.md
Doesn't that make Self a perfect substitute for the use-case your
proposal eliminates?

> Slava
>
>> On Oct 11, 2016, at 1:58 PM, Pyry Jahkola <pyry.jahkola at iki.fi> wrote:
>> 
>> I was reminded of this proposal which seems like an obvious win in clarity. Still planning to
> submit it, Slava?
>> 
>> — Pyry
>> 
>>> On 28 Jun 2016, at 21:13, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> on Thu Jun 23 2016, Slava Pestov <swift-evolution at swift.org> wrote:
>>> 
>>>> Simpler interpretation of a reference to a generic type with no
>>>> arguments
>>>> 
>>>> Proposal: SE-9999
>>>> <https://github.com/slavapestov/swift-evolution/blob/silly-proposals/proposals/9999-simplify-unbound-generic-type.md>
>>>> Author: Slava Pestov <https://github.com/slavapestov>
>>>> Status: Awaiting review
>>>> Review manager: TBD
>>>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#introduction>Introduction
>>>> 
>>>> This proposal cleans up the semantics of a reference to a generic type
>>>> when no generic arguments are applied.
>>>> 
>>>> Swift-evolution thread: Discussion thread topic for that proposal
>>>> <http://news.gmane.org/gmane.comp.lang.swift.evolution>
>>>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#motivation>Motivation
>>>> 
>>>> Right now, we allow a generic type to be referenced with no generic
>>>> arguments applied in a handful of special cases. The two primary rules
>>>> here are the following:
>>>> 
>>>> If the scope from which the reference is made is nested inside the
>>>> definition of the type or an extension thereof, omitting generic
>>>> arguments just means to implicitly apply the arguments from context.
>>>> 
>>>> For example,
>>>> 
>>>> struct GenericBox<Contents> {
>>>> let contents: Contents
>>>> 
>>>> // Equivalent to: func clone() -> GenericBox<Contents>
>>>> func clone() -> GenericBox {
>>>>   return GenericBox(contents: contents)
>>>> }
>>>> }
>>>> 
>>>> extension GenericBox {
>>>> func print() {
>>>>   // Equivalent to: let cloned: GenericBox<Contents>
>>>>   let cloned: GenericBox = clone()
>>>>   print(cloned.contents)
>>>> }
>>>> }
>>>> If the type is referenced from an unrelated scope, we attempt to infer
>>>> the generic parameters.
>>>> 
>>>> For example,
>>>> 
>>>> func makeABox() -> GenericBox<Int> {
>>>> // Equivalent to: GenericBox<Int>(contents: 123)
>>>> return GenericBox(contents: 123)
>>>> }
>>>> The problem appears when the user expects the second behavior, but
>>>> instead encounters the first. For example, the following does not type
>>>> check:
>>>> 
>>>> extension GenericBox {
>>>> 
>>>> func transform<T>(f: Contents -> T) -> GenericBox<T> {
>>>>   // We resolve 'GenericBox' as 'GenericBox<Contents>', rather than
>>>>   // inferring the type parameter
>>>>   return GenericBox(contents: f(contents))
>>>> }
>>>> }
>>>> <https://github.com/slavapestov/swift-evolution/tree/silly-proposals/proposals#proposed-solution>Proposed
>>>> solution
>>>> 
>>>> The proposed solution is to remove the first rule altogether. If the
>>>> generic parameters cannot be inferred from context, they must be
>>>> specified explicitly with the usual Type<Args...> syntax.
>>> 
>>> SGTM.  I've always found this shorthand to be somewhat surprising,
>>> including in C++ where (IIUC) it originated.
>>> 
>>> 
>>> -- 
>>> Dave
>> 
>
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-- 
-Dave