[swift-evolution] Get rid of #endif

[David Hart]

But in that case, you are placing #if at the same syntactic level as standard blocks and you loose a fair amount of expressive power.

> On 10 Mar 2016, at 20:03, Jean-Daniel Dupas <mailing at xenonium.com> wrote:
> 
> 
>> Le 9 mars 2016 à 12:40, David Hart via swift-evolution <swift-evolution at swift.org> a écrit :
>> 
>> You would at least need #} to disambiguate. With your proposal:
>> 
>> #if FALSE {
>> if false {
>> }
>> print("hello")
>> #if FALSE {
>> }
>> }
>> 
>> This either:
>> • is invalid syntax
>> • prints hello
>> • does nothing
> 
> Swift required the block in compiler directive to be syntactically correct. So, you can’t have a dandling "{" and so that sample should not introduced ambiguity.
> 
>> Sent from my iPhone
>>> On 09 Mar 2016, at 02:22, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 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> 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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>>>> 
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