


encoding::warningPerl)Programmers Referenceencoding::warnings(3p)


NAME
       encoding::warnings - Warn on implicit encoding conversions

VERSION
       This document describes version 0.11 of
       encoding::warnings, released June 5, 2007.

SYNOPSIS
           use encoding::warnings; # or 'FATAL' to raise fatal exceptions

           utf8::encode($a = chr(20000));  # a byte-string (raw bytes)
           $b = chr(20000);                # a unicode-string (wide characters)

           # "Bytes implicitly upgraded into wide characters as iso-8859-1"
           $c = $a . $b;

DESCRIPTION
   Overview of the problem
       By default, there is a fundamental asymmetry in Perl's
       unicode model: implicit upgrading from byte-strings to
       unicode-strings assumes that they were encoded in ISO
       8859-1 (Latin-1), but unicode-strings are downgraded with
       UTF-8 encoding.  This happens because the first 256
       codepoints in Unicode happens to agree with Latin-1.

       However, this silent upgrading can easily cause problems,
       if you happen to mix unicode strings with non-Latin1 data
       -- i.e. byte-strings encoded in UTF-8 or other encodings.
       The error will not manifest until the combined string is
       written to output, at which time it would be impossible to
       see where did the silent upgrading occur.

   Detecting the problem
       This module simplifies the process of diagnosing such
       problems.  Just put this line on top of your main program:

           use encoding::warnings;

       Afterwards, implicit upgrading of high-bit bytes will
       raise a warning.  Ex.: "Bytes implicitly upgraded into
       wide characters as iso-8859-1 at - line 7".

       However, strings composed purely of ASCII code points
       (0x00..0x7F) will not trigger this warning.

       You can also make the warnings fatal by importing this
       module as:

           use encoding::warnings 'FATAL';

   Solving the problem
       Most of the time, this warning occurs when a byte-string
       is concatenated with a unicode-string.  There are a number
       of ways to solve it:



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encoding::warningPerl)Programmers Referenceencoding::warnings(3p)


       o   Upgrade both sides to unicode-strings

           If your program does not need compatibility for Perl
           5.6 and earlier, the recommended approach is to apply
           appropriate IO disciplines, so all data in your
           program become unicode-strings.  See encoding, open
           and "binmode" in perlfunc for how.

       o   Downgrade both sides to byte-strings

           The other way works too, especially if you are sure
           that all your data are under the same encoding, or if
           compatibility with older versions of Perl is desired.

           You may downgrade strings with "Encode::encode" and
           "utf8::encode".  See Encode and utf8 for details.

       o   Specify the encoding for implicit byte-string
           upgrading

           If you are confident that all byte-strings will be in
           a specific encoding like UTF-8, and need not support
           older versions of Perl, use the "encoding" pragma:

               use encoding 'utf8';

           Similarly, this will silence warnings from this
           module, and preserve the default behaviour:

               use encoding 'iso-8859-1';

           However, note that "use encoding" actually had three
           distinct effects:

           o   PerlIO layers for STDIN and STDOUT

               This is similar to what open pragma does.

           o   Literal conversions

               This turns all literal string in your program into
               unicode-strings (equivalent to a "use utf8"), by
               decoding them using the specified encoding.

           o   Implicit upgrading for byte-strings

               This will silence warnings from this module, as
               shown above.

           Because literal conversions also work on empty
           strings, it may surprise some people:

               use encoding 'big5';




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encoding::warningPerl)Programmers Referenceencoding::warnings(3p)


               my $byte_string = pack("C*", 0xA4, 0x40);
               print length $a;    # 2 here.
               $a .= "";           # concatenating with a unicode string...
               print length $a;    # 1 here!

           In other words, do not "use encoding" unless you are
           certain that the program will not deal with any raw,
           8-bit binary data at all.

           However, the "Filter => 1" flavor of "use encoding"
           will not affect implicit upgrading for byte-strings,
           and is thus incapable of silencing warnings from this
           module.  See encoding for more details.

CAVEATS
       For Perl 5.9.4 or later, this module's effect is lexical.

       For Perl versions prior to 5.9.4, this module affects the
       whole script, instead of inside its lexical block.

SEE ALSO
       perlunicode, perluniintro

       open, utf8, encoding, Encode

AUTHORS
       Audrey Tang

COPYRIGHT
       Copyright 2004, 2005, 2006, 2007 by Audrey Tang
       <cpan@audreyt.org>.

       This program is free software; you can redistribute it
       and/or modify it under the same terms as Perl itself.

       See <http://www.perl.com/perl/misc/Artistic.html>





















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