[Rd] source(), parse(), and foreign UTF-8 characters

[Duncan Murdoch]

On 09/05/2017 3:42 AM, Kirill Müller wrote:
> Hi
>
>
> I'm having trouble sourcing or parsing a UTF-8 file that contains
> characters that are not representable in the current locale ("foreign
> characters") on Windows. The source() function stops with an error, the
> parse() function reencodes all foreign characters using the <U+xxxx>
> notation. I have added a reproducible example below the message.
>
> This seems well within the bounds of documented behavior, although the
> documentation to source() could mention that the file can't contain
> foreign characters. Still, I'd prefer if UTF-8 "just worked" in R, and
> I'm willing to invest substantial time to help with that. Before
> starting to write a detailed proposal, I feel that I need a better
> understanding of the problem, and I'm grateful for any feedback you
> might have.
>
> I have looked into character encodings in the context of the dplyr
> package, and I have observed the following behavior:
>
> - Strings are treated preferentially in the native encoding
> - Only upon specific request (via translateCharUTF8() or enc2utf8() or
> ...), they are translated to UTF-8 and marked as such
> - On UTF-8 systems, strings are never marked as UTF-8
> - ASCII strings are marked as ASCII internally, but this information
> doesn't seem to be available, e.g., Encoding() returns "unknown" for
> such strings
> - Most functions in R are encoding-agnostic: they work the same
> regardless if they receive a native or UTF-8 encoded string if they are
> properly tagged
> - One important difference are symbols, which must be in the native
> encoding (and are always converted to native encoding, using <U+xxxx>
> escapes)
> - I/O is centered around the native encoding, e.g., writeLines() always
> reencodes to the native encoding
> - There is the "bytes" encoding which avoids reencoding.
>
> I haven't looked into serialization or plot devices yet.
>
> The conclusion to the "UTF-8 manifesto" [1] suggests "... to use UTF-8
> narrow strings everywhere and convert them back and forth when using
> platform APIs that don’t support UTF-8 ...". (It is written in the
> context of the UTF-16 encoding used internally on Windows, but seems to
> apply just the same here for the native encoding.) I think that Unicode
> support in R could be greatly improved if we follow these guidelines.
> This seems to mean:
>
> - Convert strings to UTF-8 as soon as possible, and mark them as such
> (also on systems where UTF-8 is the native encoding)
> - Translate to native only upon specific request, e.g., in calls to API
> functions or perhaps for .C()
> - Use UTF-8 for symbols
> - Avoid the forced round-trip to the native encoding in I/O functions
> and for parsing (but still read/write native by default)
> - Carefully look into serialization and plot devices
> - Add helper functions that simplify mundane tasks such as
> reading/writing a UTF-8 encoded file

Those are good long term goals, though I think the effort is easier than 
you think.  Rather than attempting to do it all at once, you should look 
for ways to do it gradually and submit self-contained patches.  In many 
cases it doesn't matter if strings are left in the local encoding, 
because the encoding doesn't matter.  The problems arise when UTF-8 
strings are converted to the local encoding before it's necessary, 
because that's a lossy conversion.  So a simple way to proceed is to 
identify where these conversions occur, and remove them one-by-one.

Currently I'm working on bug 16098, "Windows doesn't handle high Unicode 
code points".  It doesn't require many changes at all to handle input of 
those characters; all the remaining issues are avoiding the problems you 
identify above.  The origin of the issue is the fact that in Windows 
wchar_t is only 16 bits (not big enough to hold all Unicode code 
points).  As far as I know, Windows has no standard type to hold a 
Unicode code point, most of the run-time functions still use the 16 bit 
wchar_t.

I think once that bug is dealt with, 90+% of the remaining issues could 
be solved by avoiding translateChar on Windows.  This could be done by 
avoiding it everywhere, or by acting as though Windows is running in a 
UTF-8 locale until you actually need to write to a file.  Other systems 
tend to have UTF-8 locales in common use, so they're already fine.

You offered to spend time on this.  I'd appreciate some checks of the 
patch I'm developing for 16098, and also some research into how certain 
things (e.g. the iswprint function) are handled on Windows.

Duncan Murdoch
>
> I'm sure I've missed many potential pitfalls, your input is greatly
> appreciated. Thanks for your attention.
>
> Further ressources: A write-up by Prof. Ripley [2], a section in R-ints
> [3], a blog post by Ista Zahn [4], a StackOverflow search [5].
>
>
> Best regards
>
> Kirill
>
>
>
> [1] http://utf8everywhere.org/#conclusions
>
> [2] https://developer.r-project.org/Encodings_and_R.html
>
> [3]
> https://cran.r-project.org/doc/manuals/r-devel/R-ints.html#Encodings-for-CHARSXPs
>
> [3]
> http://people.fas.harvard.edu/~izahn/posts/reading-data-with-non-native-encoding-in-r/
>
> [4]
> http://stackoverflow.com/search?tab=votes&q=%5br%5d%20encoding%20windows%20is%3aquestion
>
>
>
> # Use one of the following:
> id <- "Gl\u00fcck"
> id <- "\u5e78\u798f"
> id <- "\u0441\u0447\u0430\u0441\u0442\u044c\u0435"
> id <- "\ud589\ubcf5"
>
> file_contents <- paste0('"', id, '"')
> Encoding(file_contents)
> raw_file_contents <- charToRaw(file_contents)
>
> path <- tempfile(fileext = ".R")
> writeBin(raw_file_contents, path)
> file.size(path)
> length(raw_file_contents)
>
> # Escapes the string
> parse(text = file_contents)
>
> # Throws an error
> print(source(path, encoding = "UTF-8"))
>
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