<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On Dec 4, 2015, at 5:51 AM, Adrian Zubarev <<a href="mailto:adrian.zubarev@devandartist.com" class="">adrian.zubarev@devandartist.com</a>> wrote:</div><div class=""><div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);"><p style="margin: 15px 0px; -webkit-margin-before: 0px;" class="">We have finally a corner where people can share their ideas on how Swift can be improved, and I’m glad my ideas might be part of this fun.</p><p style="margin: 15px 0px;" class="">Swift introduced a few<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">Convertible</code><span class="Apple-converted-space"> </span>protocols, but it still needs some more If you ask me.<span class="Apple-converted-space"> </span></p><p style="margin: 15px 0px;" class="">Something like this:</p><pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">public protocol CustomConvertible {
typealias CustomType
public init(value: Self.CustomType) // or in a better way I can't come up with
}
</code></pre><p style="margin: 15px 0px;" class="">But it should work in a way that we could create more different types like<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">enums</code><span class="Apple-converted-space"> </span>and<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">ErrorType</code>.</p><pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">protocol AConvertible: CustomConvertible { /* some stuff here*/ }
protocol BConvertible: CustomConvertible { /* some stuff here*/ }
struct A {
var foo: Int = 0
}
struct B {
var boo: String = "Hello World"
}
struct C: AConvertible, BConvertible {
typealias CustomAType = A
typealias CustomBType = B
var magical: SomeType
init(value: CustomAType) {
// implement it
}
init(value: CustomBType) {
// implement it
}
}
//===========================
let firstC: C = A() // this is what I'm missing in Swift
let secondC: C = B() // this would be a great syntax sugar
</code></pre><p style="margin: 15px 0px;" class="">What do you think?</p></div></div></blockquote><div>This is effectively a request for user-defined conversion operators. We actually did support those at one point in the history of Swift, but we removed them because they both introduced a lot of confusion and errors into common user idioms and were very problematic for the type-checker. It’s easy to look at obviously-contextually-typed examples like this and say that the type-checker should just make it work, but in the context of a general System F-sub type system, it is a massive source of added complexity.</div><div><br class=""></div><div>In other words, at present, this is not an extension we feel we can deliver a satisfactory experience for, and to be taken seriously, any proposal is going need to demonstrate a lot of familiarity with type systems.</div><div><br class=""></div><div>John.</div></div></body></html>