encoding::warnings.3

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.\" ========================================================================
.\"
.IX Title "encoding::warnings 3"
.TH encoding::warnings 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
encoding::warnings \- Warn on implicit encoding conversions
.SH "VERSION"
.IX Header "VERSION"
This document describes version 0.11 of encoding::warnings, released
June 5, 2007.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use encoding::warnings; # or \*(AqFATAL\*(Aq 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;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
.Sh "Overview of the problem"
.IX Subsection "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 \fI\s-1ISO\s0 8859\-1 (Latin\-1)\fR, but unicode-strings are
downgraded with \s-1UTF\-8\s0 encoding.  This happens because the first 256
codepoints in Unicode happens to agree with Latin\-1.
.PP
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 \s-1UTF\-8\s0 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.
.Sh "Detecting the problem"
.IX Subsection "Detecting the problem"
This module simplifies the process of diagnosing such problems.  Just put
this line on top of your main program:
.PP
.Vb 1
\&    use encoding::warnings;
.Ve
.PP
Afterwards, implicit upgrading of high-bit bytes will raise a warning.
Ex.: \f(CW\*(C`Bytes implicitly upgraded into wide characters as iso\-8859\-1 at
\&\- line 7\*(C'\fR.
.PP
However, strings composed purely of \s-1ASCII\s0 code points (\f(CW0x00\fR..\f(CW0x7F\fR)
will \fInot\fR trigger this warning.
.PP
You can also make the warnings fatal by importing this module as:
.PP
.Vb 1
\&    use encoding::warnings \*(AqFATAL\*(Aq;
.Ve
.Sh "Solving the problem"
.IX Subsection "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:
.IP "\(bu" 4
Upgrade both sides to unicode-strings
.Sp
If your program does not need compatibility for Perl 5.6 and earlier,
the recommended approach is to apply appropriate \s-1IO\s0 disciplines, so all
data in your program become unicode-strings.  See encoding, open and
\&\*(L"binmode\*(R" in perlfunc for how.
.IP "\(bu" 4
Downgrade both sides to byte-strings
.Sp
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.
.Sp
You may downgrade strings with \f(CW\*(C`Encode::encode\*(C'\fR and \f(CW\*(C`utf8::encode\*(C'\fR.
See Encode and utf8 for details.
.IP "\(bu" 4
Specify the encoding for implicit byte-string upgrading
.Sp
If you are confident that all byte-strings will be in a specific
encoding like \s-1UTF\-8\s0, \fIand\fR need not support older versions of Perl,
use the \f(CW\*(C`encoding\*(C'\fR pragma:
.Sp
.Vb 1
\&    use encoding \*(Aqutf8\*(Aq;
.Ve
.Sp
Similarly, this will silence warnings from this module, and preserve the
default behaviour:
.Sp
.Vb 1
\&    use encoding \*(Aqiso\-8859\-1\*(Aq;
.Ve
.Sp
However, note that \f(CW\*(C`use encoding\*(C'\fR actually had three distinct effects:
.RS 4
.IP "\(bu" 4
PerlIO layers for \fB\s-1STDIN\s0\fR and \fB\s-1STDOUT\s0\fR
.Sp
This is similar to what open pragma does.
.IP "\(bu" 4
Literal conversions
.Sp
This turns \fIall\fR literal string in your program into unicode-strings
(equivalent to a \f(CW\*(C`use utf8\*(C'\fR), by decoding them using the specified
encoding.
.IP "\(bu" 4
Implicit upgrading for byte-strings
.Sp
This will silence warnings from this module, as shown above.
.RE
.RS 4
.Sp
Because literal conversions also work on empty strings, it may surprise
some people:
.Sp
.Vb 1
\&    use encoding \*(Aqbig5\*(Aq;
\&
\&    my $byte_string = pack("C*", 0xA4, 0x40);
\&    print length $a;    # 2 here.
\&    $a .= "";           # concatenating with a unicode string...
\&    print length $a;    # 1 here!
.Ve
.Sp
In other words, do not \f(CW\*(C`use encoding\*(C'\fR unless you are certain that the
program will not deal with any raw, 8\-bit binary data at all.
.Sp
However, the \f(CW\*(C`Filter => 1\*(C'\fR flavor of \f(CW\*(C`use encoding\*(C'\fR will \fInot\fR
affect implicit upgrading for byte-strings, and is thus incapable of
silencing warnings from this module.  See encoding for more details.
.RE
.SH "CAVEATS"
.IX Header "CAVEATS"
For Perl 5.9.4 or later, this module's effect is lexical.
.PP
For Perl versions prior to 5.9.4, this module affects the whole script,
instead of inside its lexical block.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
perlunicode, perluniintro
.PP
open, utf8, encoding, Encode
.SH "AUTHORS"
.IX Header "AUTHORS"
Audrey Tang
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2004, 2005, 2006, 2007 by Audrey Tang <[e-mail]>.
.PP
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
.PP
See <http://www.perl.com/perl/misc/Artistic.html>