[swift-evolution] Get rid of #endif

[Félix Cloutier]

I don't think that this change can be implemented at all. `#if swift` doesn't parse inactive branches, so you wouldn't be able to disambiguate an `#if swift` end brace from a normal end brace.

Félix

> Le 8 mars 2016 à 19:33:49, Richard Ross via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> -1. Keeping it the current way makes it significantly more obvious where the conditional compilation ends, rather than having to scour for yet another pesky curly brace.
> 
> Additionally, some of us are logical and prefer not to use conditional compilation as an extra indentation level :)
> --
> Richard
> 
>> On Mar 8, 2016, at 4:23 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Swift inherited an odd preprocessor-related inconsistency from C, and I'm wondering if we should change it.
>> 
>> Swift and C both use curly-bracket blocks to delimit if statements and other runtime control flow:
>> 
>> 	if foo {
>> 		blah
>> 	}
>> 	else {
>> 		yadda
>> 	}
>> 
>> However, the preprocessor/build configuration equivalent uses keywords in the style of languages like BASIC:
>> 
>> 	#if FOO
>> 		blah
>> 	#else
>> 		blah
>> 	#endif
>> 
>> In addition to the inconsistency, I consider this to have several concrete disadvantages:
>> 
>> - It leaves the preferred indentation of a conditional block ambiguous. Some people indent, others don't.
>> - It gives us no syntax to build on for other things that should be "scoped". For instance, clang's `#pragma clang diagnostic push/pop` is as strange as it is because there's no sensible way to delimit a block structure.
>> 
>> C justifies this because the preprocessor is a separate pass with its own rules, but Swift does not have this limitation.
>> 
>> Therefore, I suggest we switch to this syntax:
>> 
>> 	#if FOO {
>> 		blah
>> 	}
>> 	#else {
>> 		blah
>> 	}
>> 
>> That gives us a basis to cleanly extend compiler directives to add new features. For instance, the #suppress directive discussed in the "[Idea] #suppress(warning-identifier)" could be given a syntax like this:
>> 
>> 	#suppress self-in-closure {
>> 		blah
>> 	}
>> 
>> If their design ends up being purely compile-time with no runtime impact, we might even consider using this syntax for behaviors:
>> 
>> 	#behavior var lazy<Value>: Value {
>> 		private var value: Value?
>> 		
>> 		get {
>> 			if let value = value {
>> 				return value
>> 			}
>> 			let newValue = initialValue
>> 			value = newValue
>> 			return newValue
>> 		}
>> 		set {
>> 			value = newValue
>> 		}
>> 	}
>> 
>> There are two disadvantages I can identify:
>> 
>> - The fact that a particular } is associated with a compiler directive may not be immediately obvious when reading code.
>> 
>> - `#if swift(...)` may not be able to correctly parse the close of the block if unrecognized new language features inside the block appear to unbalance the curly brackets. (For instance, if a new literal syntax is used to quote a closing curly and older Swift compilers don't recognize it.)
>> 
>> If these problems are considered serious enough, an alternative would be to use `#}` to indicate the close of the compiler directive's scope. This is obviously not an ordinary curly bracket and is unlikely to appear in source for any other reason.
>> 
>> -- 
>> Brent Royal-Gordon
>> Architechies
>> 
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