[swift-evolution] [Discussion] Here we go again: Extension Functions

[Radosław Pietruszewski]

I have chills when I hear the idea of a closure changing the `self` from underneath me. JavaScript made me this way.

Of course, this is different than JavaScript, because it’s type-safe and you have to explicitly make a specific closure behave this way.

Still:
- I fear that this can be awfully confusing because there’s no indication on call site that the closure has a different `self`.
- what if you do want to use your “outer self” inside the self-binding closure? Sure, in some cases, you could differentiate by `foo` and `self.foo`, but I suspect most self-binding closures would also be @noescape. And so, back to the JavaScript-like convention of `let this = self`? Ugh :(

I do like that this syntax is not super noisy with all the $0’s, and also has the advantage that it doesn’t break when you only have a single expression (https://github.com/devxoul/Then#trouble-shooting <https://github.com/devxoul/Then#trouble-shooting>). But maybe it’s not the way to go.

— Radek

> On 25 Feb 2016, at 23:47, Stephen Celis via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I'm reopening a topic that has already been discussed here at length[1]: setup closures and `self`-binding closures.
> 
> I've been wondering if Swift could adopt what Kotlin calls "extension function expressions"[2]. These would allow us to encode a closure's receiver type (`self`) into the closure type itself, providing a powerful, type-safe way to define DSLs.
> 
> Erica's earlier draft proposed this builder pattern:
> 
>    with let task = NSTask() {
>        launchPath = "/usr/bin/mdfind"
>        arguments = ["kMDItemDisplayName == *.playground"]
>        standardOutput = pipe
>    }
> 
> With extension functions, we could approach something similar:
> 
>    func with<T>(value: T, body: T.() -> ()) -> T {
>        value.body()
>        return value
>    }
> 
>    let task = with(NSTask()) {
>        launchPath = "/usr/bin/mdfind"
>        arguments = ["kMDItemDisplayName == *.playground"]
>        standardOutput = pipe
>    }
> 
> Or, using the `then`[3] pattern:
> 
>    protocol Builder {}
>    extension Builder {
>        func then(body: Self.() -> ()) -> Self {
>            body()
>            return self
>        }
>    }
>    extension NSTask: Builder {}
> 
>    let task = NSTask().then {
>        launchPath = "/usr/bin/mdfind"
>        arguments = ["kMDItemDisplayName == *.playground"]
>        standardOutput = pipe
>    }
> 
> How about BDD-style frameworks (like Quick/Nimble[4] and Spectre[5])?
> 
>    describe("a person") {
>        let person = Person(name: "Kyle")
> 
>        it("has a name") {
>            try expect(person.name) == "Kyle"
>        }
>    }
> 
> And HTML builders?
> 
>    html {
>        head { title("Hello, World!") }
>    }
> 
> And block-based, transactional APIs?
> 
>    db.inTransaction {
>        delete("users", "first_name = ?", ["Jake"])
>    }
> 
> The main benefits have been previously discussed:
> 
> 1. The ability to remove noise ("$0" everywhere)
> 2. The ability to avoid defining globals (in favor of a safer, scoped interface)
> 
> Would such an enhancement be feasible?
> 
> ---
> Stephen
> 
> Footnotes:
> 
> [1] A probably-incomplete list:
> - "Idea for enabling DSLs: bind to self in closures": https://lists.swift.org/pipermail/swift-evolution/2015-December/000114.html
> - "Request for Discussion: Setup closures": https://lists.swift.org/pipermail/swift-evolution/2015-December/000211.html
> - "Method cascading (was Re: Request for Discussion: Setup closures)": https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151130/000729.html
> - "Fluent syntax (replacing void with a useful default return value)": http://thread.gmane.org/gmane.comp.lang.swift.evolution/374
> - "Lexical scope statement (with .. do)": http://thread.gmane.org/gmane.comp.lang.swift.evolution/1408
> - "Scoped resources (like C# using statement)": http://thread.gmane.org/gmane.comp.lang.swift.evolution/1641
> - "Customized Inline Init Closure": http://thread.gmane.org/gmane.comp.lang.swift.evolution/1946
> - "Then Support": http://thread.gmane.org/gmane.comp.lang.swift.evolution/2054
> - "Draft proposal: multi-property assignment .= operator": http://thread.gmane.org/gmane.comp.lang.swift.evolution/2960
> - "Custom default names for arguments of closures": http://thread.gmane.org/gmane.comp.lang.swift.evolution/6969
> - https://bugs.swift.org/browse/SR-160
> 
> [2] https://kotlinlang.org/docs/reference/lambdas.html#function-literals-with-receiver
> [3] https://github.com/devxoul/Then
> [4] https://github.com/Quick/Quick
> [5] https://github.com/kylef/Spectre
> 
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