[swift-evolution] Control Flow Expressions

Cameron Knight camjknight at mac.com
Mon Dec 7 13:24:39 CST 2015

Why not use a keyword? What if, the keyword 'returning' (or something like that) was used to specify the control flow behavior.

// Replaces ternary operator
let paint.color = returning if door.color == .Red { .Black } else { door.color }

// Supports additional conditions
let paint.finish = returning switch paint.color {
	case .Black:
	case .White:

// Removes ambiguity of single statement behavior
let ages: [Int] = people.map returning { $0.age }

// Perhaps overreaching a bit
let label = returning UILabel(frame: CGRect.zero) {
	.text = "Hello World"
	.color = UIColor.red

I think it adds clarity without too much syntax bloat. I haven't thought out all the corner cases though, so maybe I'm missing something obvious.

> On Dec 6, 2015, at 4:56 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
>> On Dec 6, 2015, at 12:17 PM, Per Melin <p at greendale.se <mailto:p at greendale.se>> wrote:
>> On Sat, Dec 5, 2015 at 7:15 PM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>> Further, it is important to consider whether the code written using this will actually be *better* than the code written with these things as statements.  For example, the “switch” blocks tend to be very large, and turning them into expressions encourages additional indentation.
>> If you give functions implicit return at the same time – as in Haskell, Erlang, Scala, Rust, Ruby, Lisp/Scheme/Clojure, etc –  there would be no need for additional indentation half of the time.
> This isn’t something that I’m personally interested in.  I think that it is *feature* of swift that statements an declarations start with keywords.  This greatly simplifies the grammar in various ways, and allows declmodifiers to be introduced without taking keywords space.  
> For example, relevant to this proposal, if/when we support “tail return foo()" for example, you don’t want to take “tail” as a keyword to make “tail foo()” work.
>> Not even Slava Pestov would factor Swift that aggressively.
> Underestimating Slava is not a good idea! :-)
> -Chris
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