encoding::warnings - Warn on implicit encoding conversions
This document describes version 0.13 of encoding::warnings, released
June 20, 2016.
As of Perl 5.26.0, this module has no effect. The internal Perl feature
that was used to implement this module has been removed. In recent years,
much work has been done on the Perl core to eliminate discrepancies in the
treatment of upgraded versus downgraded strings. In addition, the
encoding pragma, which caused many of the problems, is no longer
supported. Thus, the warnings this module produced are no longer
necessary.
Hence, if you load this module on Perl 5.26.0, you will get one warning
that the module is no longer supported; and the module will do nothing
thereafter.
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;
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.
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';
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:
- 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
perlfunc/binmode for how.
- 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.
- 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:
Because literal conversions also work on empty strings, it may surprise
some people:
use encoding 'big5';
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.
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.
perlunicode, perluniintro
open, utf8, encoding, Encode
Audrey Tang
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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