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

Thorsten Seitz tseitz42 at icloud.com
Sat Apr 22 12:57:48 CDT 2017

> Am 22.04.2017 um 17:08 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
> On Sat, Apr 22, 2017 at 3:21 AM, Thorsten Seitz <tseitz42 at icloud.com <mailto:tseitz42 at icloud.com>> wrote:
>> Am 21.04.2017 um 20:48 schrieb Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>> On Fri, Apr 21, 2017 at 1:45 PM, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> wrote:
>>> On Apr 21, 2017, at 12:40 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Fri, Apr 21, 2017 at 8:48 AM, Robert Bennett via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> Xiaodi, I think one thing you're neglecting is that users may never print out a multiline literal string at all. A string might never be printed or read by a human outside of the code it resides in. In this case it seems perfectly reasonable to ask that it be possible to format the string nicely in the code and disregard how it would actually be printed.
>>> Can you give an example of such a use case, where a string is never seen by a human but one cannot insert literal newlines and would need elided ones instead?
>> The most common reason is that the code is maintained by a (non-human) developer, who wants to be able to see and update the code in a readable form, but that represents a single line that will automatically wrapped by, for example, a UITextView for (human) consumption. 
>> A different scenario from what Robert's describing, but sure. This goes to my question to David Hart. Isn't this an argument for a feature to allow breaking a single-line string literal across multiple lines? What makes this a use case for some feature for _multiline_ string literals in particular?
> I think „single-line“ and „multiline“ should foremost apply to the code representation of a string and not its result.
> Otherwise "foo\nbar“ would be a multiline string with your reasoning, wouldn’t you agree?
> I think Robert Bennett has summarized the differences between his view and my view of literals very well. Keep in mind we're talking about _string literals_, not strings. To me, a literal is something that represents its data as literally as possible. Therefore, what makes something a _multiline_ string literal is simple: it permits literal newlines.

I know that we are talking about string literals, not strings, that’s why I talked about _representations_ vs. _results_. 

A literal is a _textual representation_ of data as opposed to a calculation. Requiring for the special case of _string_ literals that the textual representation has to be as literal as possible is an artificial restriction that I do not share. 
Following your argument you would have to prohibit `\t` and `\n` from multiline strings and `\t` from single-line strings because these characters can be written literally and therefore should (to be represented "as literally as possible“).

So, to reiterate, the valuable distinction between single-line and multiline string literals is their textual representation and *not* whether the result of that has one or more lines. Multiline strings allow easier maintenance of _long_ strings (which may or may not have multiple lines). 

And just as `\n` is allowed in single-line string literals resulting in a multiline string it would make sense to allow `\` in multiline string literals to suppress literal newlines.
The result would be 
(a) single-line string literals which are always written in a single line but can represent single-line or multiline strings (just as today, using `\n`) and 
(b) multiline string literals which are always written in multiple lines but can equally represent single-line (not possible today) or multiline strings (using literal newlines or `\n`).

A notable difference between single-line string literals and multiline string literals is that whitespace is not visible anymore at the end of each line (as has been pointed out several times by Adrian). Within a single-line string literal whitespace is visible everywhere within the string. Within a multiline string literal whitespace is visible at the beginning of each line due to the position of the closing delimiter and the corresponding indentation suppression, it is visible within each line but it is *not* visible at the end of a line. That would be fixed by suppressing trailing whitespace and only allowing it before a `\`.
This is similar to the normalization of the literal newline to `\n`.

Pointing to tools or editor features to strip trailing whitespace is simply *wrong* IMHO because the idea of stripping trailing whitespace by editors or tools is only intended for *non-relevant* whitespace!
How often have you worked in teams where each developer has different settings for his editor (if they even use the same editor)? Relying on this will lead to stripping of relevant whitespace because someone has opened the file with the stripping setting on in his editor — and the problem is that this change is not even visible.

In another mail you pointed out that Unicode makes relying on visible characters difficult anyway. While that is true, I think there is a significant difference between characters that I _see_ which might not have the correct character code and characters that I _do not even see_.


> Therefore a multiline string is one which is written over several lines of *code* to make maintenance easier. 
> From that follows naturally that as soon as line breaks are introduced for hard wrapping we are talking about multiline strings.
> In addition as soon as line breaks are introduced in the code the question of indentation arises which is solved neatly with the multiline string proposal by the position of the ending delimiter which is not possible with single-line strings. 
> -Thorsten

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