[swift-evolution] Get rid of #endif

[David Hart]

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

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
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
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>> 
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