[swift-evolution] multi-line string literals.

Ted F.A. van Gaalen tedvgiosdev at gmail.com
Wed Apr 27 14:20:50 CDT 2016


The solutions discussed so far have the following disadvantages:
  - They need a specific delimiter token, which cannot be used in the text itself.
  - The data cannot be used as-is, escape \ characters are needed.
  - Imho, they are somewhat complicated. 

In my emails previously submitted I came up with a "data line” concept,
a very simple solution, which tackles these disadvantages and is
very easy to use and understand!

Did you actually read it? 
I now quote the primary description hereunder again, in case you’ve missed it,
imaginable, because there is a lot of text flying around.

Additionally, the following improvements have developed in my thinking: 

In the mean time I consider using two tokens   \\ and \@:  
    \\…data…data…data…data…………..   accepts data and converts \n \t etc.
or data lines starting with: 
    \@…data…data…data…data… .…….  accepts data as-is, without conversion and 
                         respects the linefeeds of the source-file.

the // and /@ tokens do not have to start at the beginning of a line, 
are allowed further to the right as long as there are only space(s) to the
left of it.

let music =
\\when Doves Cry\nBy Prince and
\\ The Revolution\nWhat music
\\  can be\t\t\t\tIt is \”Magic\” really.
If you do or don’t like this solution 
-apart from cosmetic viewpoints-
please tell me why. so I can improve it,
or try to come back with something better if i can.

let epilogue =
\@Thank you so much for reading my
\@ “contribution"  Please tell me 
\@what you think, I \\\\\\\\\\\\\value//////////// your opinion!
\@“””””””””””””””””””””” kind Regards! “”””””””””””””””"
\@ Ted. 

(the previous text following \@ data lines are accepted as-is, including 
the source-file’s linefeeds or crlf and the double quotes) 

Of course, other tokens are possible as well. 

Can’t be any simpler than this, I guess.


===================As Previously Written:=================

This could be a simple solution: 

Starting each line with a special token.

In the example here it is the \\  double-backslash .  
when the \\ appears in the first two columns of a source line, 
this tells the compiler that it is a data line and that more might follow.
the last line starting with \\ completes the data entry.

Here is an example of a string declaration with some XML
(no escape sequences needed for “) 
Of course it could be anything other kind of textual data as well.

let  str =  
\\<!DOCTYPE html>
\\<h1>W3Schools Internal Note</h1>\n
\\<b>To:</b> <span id="to"></span><br>\n
\\<b>From:</b> <span id="from"></span><br>\n
\\<b>Message:</b> <span id="message"></span>
\\var <smb://var> txt, parser, xmlDoc;
\\txt <smb://txt> = "<note>" +
\\"<to>Tove</to>" +
\\etc <smb://etc>. this is the last data line.


- Every line starting with \\ in first and second column of the line 
  is  treated as a data line.
- All characters behind the \\ are regarded as data, thus note that:
     - the “ is not regarded as a string delimiter
     - the // chars and whatever follows it are interpreted as data on such a line, not as comment.
     -  \\  within the data itself are treated as data e.g. this line is valid:
\\There <smb://There> \\ are three backslashes (as data) in this line \\\\ today.
the above data line is empty but is allowed. 
for \\
- Leading and embedded spaces are respected.
- Tabs, Linefeeds etc. can be inserted the usual way using \t \n etc. 
- trailing spaces and line terminators cr lf are ignored, filtered out.


 Wed, 27 Apr 2016 16:08:13 +0300"Vladimir.S" <svabox at gmail.com <mailto:svabox at gmail.com>> wrote:
> On 27.04.2016 11:52, Brent Royal-Gordon via swift-evolution wrote:
>>> If you agree that these are all orthogonal pieces, then treat them as
>>> such: I’d suggest that you provide a proposal that just tackles the
>>> continuation string literals.  This seems simple, and possible to get in
>>> for Swift 3.
>> I've gone ahead and drafted this proposal, with some small extensions and
>> adjustments. See the "Draft Notes" section for details of what I've changed
>> and what concerns I have.
> I don't feel like this proposal(draft) solves the issue we want(?) to solve 
> with multi-line feature.
> I expect to be able to have multiline text exactly *"as-is"* in my source 
> file. No escaping, no interpolation, etc. I believe this should be a target 
> of the proposal.
> Otherwise, I reject to see any reason to introduce anything new at this 
> area - we already can concatenate strings and place them on next line and 
> escape special characters.
> In your proposal, you have to escape characters in your text, you need to 
> carefully modify the copy-pasted text to be allowed as correct multi-line 
> string. Also, what if I need to have spaces *at the end of string* ? Editor 
> can just trimmed them, and we can't clearly see them.
> Personally I need to be able to have this(in some way) in my code:
> (note this "\tuttorial" and "\(edition" - this is just text, I want to have 
> inside my xml)
> let xml = ... // some marker to start the multi-line str
> <?xml version="1.0"?>
> <catalog>
>     <book id="myid" empty="">
>         <author>myAuthor</author>
>         <title>myTitle \tutorial 1\(edition 2)</title>
>     </book>
> </catalog>
> ... // some marker here to stop multi-line str
> It seems like we need some markers for end-of-the-line to be able to keep 
> spaces/tabs in the end of line.
> What about something like this. Two suported variants : first when we need 
> to keep spaces in the end of line, and second when we don't need 
> spaced(will be trimmed by parser)
> #1 (parser should just take the text between first and last quote *as-is*)
> let xml = "\
> "<?xml version="1.0"?>     " // yes, *I need* these spaces at the end
> "<catalog>"
> "    <book id="myid" empty="">"
> "        <author>myAuthor</author>"
> "        <title>myTitle \tutorial 1\(edition 2)</title>"
> "    </book>"
> "</catalog>"
> "
> #2 in this case we don't need any spaces/tabs in the end of lines(don't care):
> (parser takes all that is after | as-is but trims any trailing spaces/tabs 
> in lines to be clear in behaviour)
> let xml = "\
> |<?xml version="1.0"?>
> |<catalog>
> |    <book id="myid" empty="">
> |        <author>myAuthor</author>
> |        <title>myTitle \tutorial 1\(edition 2)</title>
> |    </book>
> |</catalog>
> "
> Or these two could be combined in one(as-is between |..|) but I'm not sure:
> let xml = "\
> |<?xml version="1.0"?>     | // yes, I need these spaces
> |<catalog>| // we have to have closing symbol in this case in other lines
> |    <book id="myid" empty="">|
> |        <author>myAuthor</author>|
> |        <title>myTitle \tutorial 1\(edition 2)</title>|
> |    </book>|
> |</catalog>|
> "
>> Gist: <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f>>
>>  Multiline string literals
>>  * Proposal: SE-NNNN
>>    <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>>
>>  * Author(s): Brent Royal-Gordon <https://github.com/brentdax <https://github.com/brentdax>>
>>  * Status: First Draft
>>  * Review manager: TBD
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#introduction <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#introduction>>Introduction
>> In Swift 2.2, the only means to insert a newline into a string literal is
>> the |\n| escape. String literals specified in this way are generally ugly
>> and unreadable. We propose a multiline string feature inspired by English
>> punctuation which is a straightforward extension of our existing string
>> literals.
>> Swift-evolution thread: multi-line string literals.
>> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160418/015500.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160418/015500.html>>
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#draft-notes <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#draft-notes>>Draft
>>    Notes
>>  *
>>    This draft differs from the prototypes being thrown around on the list
>>    in that it specifies that comments should be treated as whitespace, and
>>    that whitespace-only lines in the middle of a multiline string should
>>    be ignored. I'm not sure if this is feasible from a parsing standpoint,
>>    and I'd like feedback from implementers on this point.
>>  *
>>    This draft also specifies diagnostics which should be included.
>>    Feedback on whether these are good choices would be welcome.
>>  *
>>    I am considering allowing you to put a backslash before the newline to
>>    indicate it should /not/ be included in the literal. In other words,
>>    this code:
>>    print("foo\
>>    "bar")
>>    Would print |"foobar"|. However, I think this should probably be
>>    proposed separately, because there may be a better way to do it.
>>  *
>>    I've listed only myself as an author because I don't want to put anyone
>>    else's name to a document they haven't seen, but there are others who
>>    deserve to be listed (John Holdsworth at least). Let me know if you
>>    think you should be included.
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#motivation <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#motivation>>Motivation
>> As Swift begins to move into roles beyond app development, code which needs
>> to generate text becomes a more important use case. Consider, for instance,
>> generating even a small XML string:
>> let xml = "<?xml version=\"1.0\"?>\n<catalog>\n\t<book id=\"bk101\"
>> empty=\"\">\n\t\t<author>\(author)</author>\n\t</book>\n</catalog>"
>> The string is practically unreadable, its structure drowned in escapes and
>> run-together characters; it looks like little more than line noise. We can
>> improve its readability somewhat by concatenating separate strings for each
>> line and using real tabs instead of |\t| escapes:
>> let xml = "<?xml version=\"1.0\"?>\n" +
>>          "<catalog>\n" +
>>          " <book id=\"bk101\" empty=\"\">\n" +
>>          " <author>\(author)</author>\n" +
>>          " </book>\n" +
>>          "</catalog>"
>> However, this creates a more complex expression for the type checker, and
>> there's still far more punctuation than ought to be necessary. If the most
>> important goal of Swift is making code readable, this kind of code falls
>> far short of that goal.
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#proposed-solution <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#proposed-solution>>Proposed
>>    solution
>> We propose that, when Swift is parsing a string literal, if it reaches the
>> end of the line without encountering an end quote, it should look at the
>> next line. If it sees a quote mark there (a "continuation quote"), the
>> string literal contains a newline and then continues on that line.
>> Otherwise, the string literal is unterminated and syntactically invalid.
>> Our sample above could thus be written as:
>> let xml = "<?xml version=\"1.0\"?>
>> "<catalog>
>>          " <book id=\"bk101\" empty=\"\">
>> "     <author>\(author)</author>
>>          " </book>
>> "</catalog>"
>> (Note that GitHub is applying incorrect syntax highlighting to this code
>> sample, because it's applying Swift 2 rules.)
>> This format's unbalanced quotes might strike some programmers as strange,
>> but it attempts to mimic the way multiple lines are quoted in English
>> prose. As an English Stack Exchange answer illustrates
>> <http://english.stackexchange.com/a/96613/64636 <http://english.stackexchange.com/a/96613/64636>>:
>>    “That seems like an odd way to use punctuation,” Tom said. “What harm
>>    would there be in using quotation marks at the end of every paragraph?”
>>    “Oh, that’s not all that complicated,” J.R. answered. “If you closed
>>    quotes at the end of every paragraph, then you would need to reidentify
>>    the speaker with every subsequent paragraph.
>>    “Say a narrative was describing two or three people engaged in a
>>    lengthy conversation. If you closed the quotation marks in the previous
>>    paragraph, then a reader wouldn’t be able to easily tell if the
>>    previous speaker was extending his point, or if someone else in the
>>    room had picked up the conversation. By leaving the previous
>>    paragraph’s quote unclosed, the reader knows that the previous speaker
>>    is still the one talking.”
>>    “Oh, that makes sense. Thanks!”
>> Similarly, omitting the ending quotation mark tells the code's reader (and
>> compiler) that the literal continues on the next line, while including the
>> continuation quote reminds the reader (and compiler) that this line is part
>> of a string literal.
>>      <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#benefits-of-continuation-quotes <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#benefits-of-continuation-quotes>>Benefits
>>      of continuation quotes
>> It would be simpler to not require continuation quotes, so why are they
>> required by this proposal? There are three reasons:
>> 1.
>>    *They help the compiler pinpoint errors in string literal
>>    delimiting.* If continuation quotes were not required, then a missing
>>    end quote would be interpreted as a multiline string literal. This
>>    string literal would continue until the compiler encountered either
>>    another quote mark—perhaps at the site of another string literal or in
>>    a comment—or the end of the file. In either case, the compiler could at
>>    best only indicate the start of the runaway string literal; in
>>    pathological cases (for instance, if the next string literal
>>    was |"+"|), it might not even be able to do that properly.
>>    With continuation quotes required, if you forget to include an end
>>    quote, the compiler can tell that you did not intend to create a
>>    multiline string and flag the line that actually has the problem. It
>>    can also provide immediately actionable fix-it assistance. The fact
>>    that there is a redundant indication on each line of the programmer's
>>    intent to include that line in a multiline quote allows the compiler to
>>    guess the meaning of the code.
>> 2.
>>    *They separate indentation from the string's contents.* Without
>>    continuation quotes, there would be no obvious indication of whether
>>    whitespace at the start of the line was intended to indent the string
>>    literal so it matched the surrounding code, or whether that whitespace
>>    was actually meant to be included in the resulting string. Multiline
>>    string literals would either have to put subsequent lines against the
>>    left margin, or apply error-prone heuristics to try to guess which
>>    whitespace was indentation and which was string literal content.
>> 3.
>>    *They improve the ability to quickly recognize the literal.* The |"| on
>>    each line serves as an immediately obvious indication that the line is
>>    part of a string literal, not code, and the row of |"| characters in a
>>    well-formatted file allows you to quickly scan up and down the file to
>>    see the extent of the literal.
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#detailed-design <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#detailed-design>>Detailed
>>    design
>> When Swift is parsing a string literal and reaches the end of a line
>> without finding a closing quote, it examines the next line, applying the
>> following rules:
>> 1.
>>    If the next line is all whitespace, it is ignored; Swift moves on to
>>    the line afterward, applying these rules again.
>> 2.
>>    If the next line begins with whitespace followed by a continuation
>>    quote, then the string literal contains a newline followed by the
>>    contents of the string literal starting on that line. (This line may
>>    itself have no closing quote, in which case the same rules apply to the
>>    line which follows.)
>> 3.
>>    If the next line contains anything else, Swift raises a syntax error
>>    for an unterminated string literal. This syntax error should offer two
>>    fix-its: one to close the string literal at the end of the current
>>    line, and one to include the next line in the string literal by
>>    inserting a continuation quote.
>> Rules 1 and 2 should treat comments as though they are whitespace; this
>> allows you to comment out individual lines in a multiline string literal.
>> (However, commenting out the last line of the string literal will still
>> make it unterminated, so you don't have a completely free hand in commenting.)
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#impact-on-existing-code <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#impact-on-existing-code>>Impact
>>    on existing code
>> Failing to close a string literal before the end of the line is currently a
>> syntax error, so no valid Swift code should be affected by this change.
>>    <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#alternatives-considered <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#alternatives-considered>>Alternatives
>>    considered
>>      <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#requiring-no-continuation-character <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#requiring-no-continuation-character>>Requiring
>>      no continuation character
>> The main alternative is to not require a continuation quote, and simply
>> extend the string literal from the starting quote to the ending quote,
>> including all newlines between them. For example:
>> let xml = "<?xml version=\"1.0\"?>
>> <catalog>
>> <book id=\"bk101\" empty=\"\">
>> <author>\(author)</author>
>> </book>
>> </catalog>"
>> This has several advantages:
>> 1.
>>    It is simpler.
>> 2.
>>    It is less offensive to programmers' sensibilities (since there are no
>>    unmatched |"| characters).
>> 3.
>>    It does not require that you edit the string literal to insert a
>>    continuation quote in each line.
>> Balanced against the advantages, however, is the loss of the improved
>> diagnostics, code formatting, and visual affordances mentioned in the
>> "Benefits of continuation quotes" section above.
>> In practice, we believe that editor support (such as "Paste as String
>> Literal" or "Convert to String Literal" commands) can make adding
>> continuation quotes less burdensome, while also providing other
>> conveniences like automatic escaping. We believe the other two factors are
>> outweighed by the benefits of continuation quotes.
>>      <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#use-a-different-delimiter-for-multiline-strings <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#use-a-different-delimiter-for-multiline-strings>>Use
>>      a different delimiter for multiline strings
>> The initial suggestion was that multiline strings should use a different
>> delimiter, |"""|, at the beginning and end of the string, with no
>> continuation characters between. This solution was rejected because it has
>> the same issues as the "no continuation character" solution, and because it
>> was mixing two orthogonal issues (multiline strings and alternate delimiters).
>> Another suggestion was to support a heredoc syntax, which would allow you
>> to specify a placeholder string literal on one line whose content begins on
>> the next line, running until some arbitrary delimiter. For instance, if
>> Swift adopted Perl 5's syntax, it might support code like:
>> connection.sendString(<<"END")
>> <?xml version="1.0"?>
>> <catalog>
>>    <book id="bk101" empty="">
>>        <author>\(author)</author>
>>    </book>
>> </catalog>
>> END
>> In addition to the issues with the |"""| syntax, heredocs are complicated
>> both to explain and to parse, and are not a natural extension of Swift's
>> current string syntax.
>> Both of these suggestions address interesting issues with string literals,
>> solving compelling use cases. They're just not that good at fixing the
>> specific issue at hand. We might consider them in the future to address
>> those problems to which they are better suited.
>>      <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#fixing-other-string-literal-readability-issues <https://gist.github.com/brentdax/c580bae68990b160645c030b2d0d1a8f#fixing-other-string-literal-readability-issues>>Fixing
>>      other string literal readability issues
>> This proposal is narrowly aimed at multiline strings. It intentionally
>> doesn't tackle several other problems with string literals:
>>  *
>>    Reducing the amount of double-backslashing needed when working with
>>    regular expression libraries, Windows paths, source code generation,
>>    and other tasks where backslashes are part of the data.
>>  *
>>    Alternate delimiters or other strategies for writing strings
>>    with |"| characters in them.
>>  *
>>    String literals consisting of very long pieces of text which are best
>>    represented completely verbatim.
>> These are likely to be subjects of future proposals, though not necessarily
>> during Swift 3.
>> This proposal also does not attempt to address regular expression literals.
>> The members of the core team who are interested in regular expression
>> support have ambitions for that feature which put it out of scope for Swift 3.
>> --
>> Brent Royal-Gordon
>> Architechies
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
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