[swift-evolution] multi-line string literals.

L. Mihalkovic laurent.mihalkovic at gmail.com
Wed Apr 27 13:58:10 CDT 2016



Regards
(From mobile)

> On Apr 27, 2016, at 6:51 PM, Shawn Erickson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Can this concept be dealt with my allowing a string file to be imported and assigned to a string constant? All happening at compile time and using #include type concept from C? It would tackle at least a reasonable subset of the issue in a way that I think works better for source control, etc.
> 
> let xml = @stringfile("Foo/Bar/baz.xml");

If memory serves me, this is not unlike what D does, with the added restriction of binding the contents to a single variable.

> 
>> On Wed, Apr 27, 2016 at 9:35 AM John Siracusa via swift-evolution <swift-evolution at swift.org> wrote:
>>> On Wed, Apr 27, 2016 at 9:08 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
>>> 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.
>> 
>> I also see this as one of the most important goals of any multi-line string-handling proposal. The "no escapes required" syntax doesn't have to be the only way to handle multi-line strings, but it should be one of the available ways.
>> 
>> -John
>> 
>> P.S. - Secondarily (and broader than just multi-line strings) I think there should also be a way to avoid backslash-itis in strings of all kinds by providing some flexibility in string delimiters. But that's for another thread…
>> 
>>  
>>> 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>
>>>> 
>>>> 
>>>>   Multiline string literals
>>>> 
>>>>   * Proposal: SE-NNNN
>>>>     <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>
>>>>   * Author(s): Brent Royal-Gordon <https://github.com/brentdax>
>>>>   * Status: First Draft
>>>>   * Review manager: TBD
>>>> 
>>>> 
>>>>     <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://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>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>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>:
>>>> 
>>>>     “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>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>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>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>Alternatives
>>>>     considered
>>>> 
>>>> 
>>>>       <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>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>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
>>>> 
>>>> 
>>>> 
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