[swift-evolution] [Accepted] SE-0168: Multi-Line String Literals

Thu Apr 20 11:48:52 CDT 2017

The multi-line string literal as it’s accepted right now only allows pretty code generation with smaller lines. The literal itself is not reserved for JSON, XML and similar syntaxes only, which automatically implies the existence of conventions with longer lines. For whatever reasons a developer might have, it’s essential to allow manual line wrapping without injecting a new line into the resulting string. Not everyone uses the same editor width nor the same editor with exact the same settings. You simply cannot and really should not rely on any editor or linter for that matter, nor do I vision it as a strong argument against having the ability to escape the new line injection. I don’t think we should ever expect the average Swift developer sitting in-front of an ultra wide monitor.

Consider this example:

// Currently it would look like this:

let myLongString = "Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.\n\nNam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.\n\nDuis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis."

// With the accepted version of the proposal it becomes a little bit better, but still to long,
// because we can only replace `\n` characters with lines and that's it.

let myLongString = """
   Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.  

   Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.  

   Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore eu feugiat nulla facilisis.  
   """

// This is how it should ideally look like and be editor/IDE/linter independent.  
// The string produces the same result as above and does not rely on any  
// soft-wrapping functionality and is written within some smaller line width.
// The trailing precision is a really good tradeoff at this point.

let myLongString = """
   Ut wisi enim ad minim veniam, quis nostrud exerci tation ullamcorper suscipit \
   lobortis nisl ut aliquip ex ea commodo consequat. Duis autem vel eum iriure \
   dolor in hendrerit in vulputate velit esse molestie consequat, vel illum dolore \
   eu feugiat nulla facilisis at vero eros et accumsan et iusto odio dignissim qui \
   blandit praesent luptatum zzril delenit augue duis dolore te feugait nulla facilisi.  

   Nam liber tempor cum soluta nobis eleifend option congue nihil imperdiet doming \
   id quod mazim placerat facer possim assum. Lorem ipsum dolor sit amet, consectetuer \
   adipiscing elit, sed diam nonummy nibh euismod tincidunt ut laoreet dolore magna \
   aliquam erat volutpat. Ut wisi enim ad minim veniam, quis nostrud exerci tation \
   ullamcorper suscipit lobortis nisl ut aliquip ex ea commodo consequat.  

   Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse molestie \
   consequat, vel illum dolore eu feugiat nulla facilisis.
   """
The string concatenation uses optimization magic behind the scenes which is not obvious for everyone. I personally think that every operation involved in concatenation or any operation in-general adds a performance overhead and theoretically needs more time to resolve the expression at runtime, which is the natural way of thinking without any knowledge about the optimization the compiler is able to do for you. A string literal is able to solve that issue during compile time is simply the perfect place for that.

Some words about the trailing precision. Joe said that we could use \("") as workaround, but if I recall correctly literals are banned from the interpolation itself, which will result in us doing something like this:

let end = ""

let myString = """
   <space><space>foo<space><space>\(end)
   """
This is a very dirty and tedious solution for that problem.

As accepted right now, no one should ever expect the result string to include any whitespace characters at the end of each line unless there is a visible annotation provided for precision. Providing a warning for trailing whitespace characters would be ideal solution right now and the trailing backslash becomes additive but not impossible to add later.

A few people already argued that the core team decided not to include a new line at the end of each multi-line string, where you yourself said that the absence of a trailing backslash will produce a string which always ends with a new line. That behavior would be really strange and painful to prevent if there is no backslash for escaping it.

The trailing backslash does not add any complexity but instead it adds more flexibility to the literal model, which results in better readability if the precision is desired for code formatting!

-- 
Adrian Zubarev
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Am 20. April 2017 um 07:30:29, Xiaodi Wu (xiaodi.wu at gmail.com) schrieb:

You can use a plain text editor and no linter, or a plain text editor and a linter, or an IDE and no linter, etc., and in any of these scenarios you can already choose whether or not you want trailing newlines stripped. Why should the compiler try to enforce any rules here?

Since Unicode is supported, it is never possible to look at a string literal and be 100% sure of what glyphs are involved. We should be clear that such a criterion cannot and should not be a design goal. If it supports Unicode and is really literal, then confusables and invisibles will make it impossible to be sure of what you see; you would have to either stop supporting Unicode or stop being literal.

I'm not sure this "coding style" you describe can properly be thought of as a multiline string literal. It sounds like what you want isn't multiline (in fact, you want a new way to write a very long single-line string) and it isn't literal (you want to use newlines in your code that do not represent a literal newline). If there is something extremely critical about a particular string, where you simply must start half of it on a separate line to help the readers of your code understand what you are doing, you can already do this by writing "foo" + [newline] "bar". Or you could just let your editor soft-wrap your long string. Making your single-line string wrap the same way in every IDE just doesn't seem like it's related to or worth complicating the syntax for multiline string literals. I would be strongly opposed to such a feature.

On Wed, Apr 19, 2017 at 23:42 Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
True, but this is not about IDEs or editors. The feature itself doesn’t know what an editor is and what it capable of, nor should be ever rely on that. Not everyone uses the same settings and you cannot be 100% sure to expect the same string from looking at it, which was written in a different editor if we don’t warn about trailing whitespaces now.

The trailing whitespaces might not do any harm for the currently accepted version, but we’ll have to warn about them if we decide to add the trailing backspace. As currently accepted we still have a hole to fill for coding styles, we do not support multi-lined string literals for code formatting only, nor do we have trailing precision for the same matter. (That’s what the backslash was meant for.) That said, I cannot break up a really long hardcoded string, which in my IDE is softly wrapped, into a multi-line string literal so that it looks in every editor the same and still expect the same result and be precise about the trailing whitespace characters.

-- 
Adrian Zubarev
Sent with Airmail

Am 20. April 2017 um 00:27:48, Brent Royal-Gordon via swift-evolution (swift-evolution at swift.org) schrieb:

On Apr 19, 2017, at 3:18 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:

Other common tools like Git already flag trailing whitespace by default, so even if Swift doesn't warn about it, you might still need to satisfy other tools in your pipeline.

Isn't that an equally good argument for Swift *not* warning you about it? If it's harmful, you'll have other tools in the pipeline to flag it for you.

Cosigned. We already have an Xcode setting to strip trailing whitespace, a Git setting to flag it, and linter settings to remove it. (For instance, SwiftFormat has a --trimwhitespace flag.) Not every tool needs to handle every case of questionable style.

-- 
Brent Royal-Gordon
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