encoding::warnings - phpMan

encoding::warnings(3pm) Perl Programmers Reference Guide encoding::warnings(3pm)

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:

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';

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.