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package encoding::warnings; $encoding::warnings::VERSION = '0.14'; use strict; use 5.007; =head1 NAME encoding::warnings - Warn on implicit encoding conversions =head1 VERSION This document describes version 0.13 of encoding::warnings, released June 20, 2016. =head1 NOTICE As of Perl 5.26.0, this module has no effect. The internal Perl feature that was used to implement this module has been removed. In recent years, much work has been done on the Perl core to eliminate discrepancies in the treatment of upgraded versus downgraded strings. In addition, the L<encoding> pragma, which caused many of the problems, is no longer supported. Thus, the warnings this module produced are no longer necessary. Hence, if you load this module on Perl 5.26.0, you will get one warning that the module is no longer supported; and the module will do nothing thereafter. =head1 SYNOPSIS use encoding::warnings; # or 'FATAL' to raise fatal exceptions utf8::encode($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; =head1 DESCRIPTION =head2 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 I<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. =head2 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.: C<Bytes implicitly upgraded into wide characters as iso-8859-1 at - line 7>. However, strings composed purely of ASCII code points (C<0x00>..C<0x7F>) will I<not> trigger this warning. You can also make the warnings fatal by importing this module as: use encoding::warnings 'FATAL'; =head2 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: =over 4 =item * 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 L<encoding>, L<open> and L<perlfunc/binmode> for how. =item * 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 C<Encode::encode> and C<utf8::encode>. See L<Encode> and L<utf8> for details. =item * 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, I<and> need not support older versions of Perl, use the C<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 C<use encoding> actually had three distinct effects: =over 4 =item * PerlIO layers for B<STDIN> and B<STDOUT> This is similar to what L<open> pragma does. =item * Literal conversions This turns I<all> literal string in your program into unicode-strings (equivalent to a C<use utf8>), by decoding them using the specified encoding. =item * Implicit upgrading for byte-strings This will silence warnings from this module, as shown above. =back 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 $a; # 1 here! In other words, do not C<use encoding> unless you are certain that the program will not deal with any raw, 8-bit binary data at all. However, the C<Filter =E<gt> 1> flavor of C<use encoding> will I<not> affect implicit upgrading for byte-strings, and is thus incapable of silencing warnings from this module. See L<encoding> for more details. =back =head1 CAVEATS For Perl 5.9.4 or later, this module's effect is lexical. For Perl versions prior to 5.9.4, this module affects the whole script, instead of inside its lexical block. =cut # Constants. sub ASCII () { 0 } sub LATIN1 () { 1 } sub FATAL () { 2 } sub import { if ($] >= 5.025003) { require Carp; Carp::cluck( "encoding::warnings is not supported on Perl 5.26.0 and later" ); return; } # Install a ${^ENCODING} handler if no other one are already in place. my $class = shift; my $fatal = shift || ''; local $@; return if ${^ENCODING} and ref(${^ENCODING}) ne $class; return unless eval { require Encode; 1 }; my $ascii = Encode::find_encoding('us-ascii') or return; my $latin1 = Encode::find_encoding('iso-8859-1') or return; # Have to undef explicitly here undef ${^ENCODING}; # Install a warning handler for decode() my $decoder = bless( [ $ascii, $latin1, (($fatal eq 'FATAL') ? 'Carp::croak' : 'Carp::carp'), ], $class, ); no warnings 'deprecated'; ${^ENCODING} = $decoder; use warnings 'deprecated'; $^H{$class} = 1; } sub unimport { my $class = shift; $^H{$class} = undef; undef ${^ENCODING}; } # Don't worry about source code literals. sub cat_decode { my $self = shift; return $self->[LATIN1]->cat_decode(@_); } # Warn if the data is not purely US-ASCII. sub decode { my $self = shift; DO_WARN: { if ($] >= 5.009004) { my $hints = (caller(0))[10]; $hints->{ref($self)} or last DO_WARN; } local $@; my $rv = eval { $self->[ASCII]->decode($_[0], Encode::FB_CROAK()) }; return $rv unless $@; require Carp; no strict 'refs'; $self->[FATAL]->( "Bytes implicitly upgraded into wide characters as iso-8859-1" ); } return $self->[LATIN1]->decode(@_); } sub name { 'iso-8859-1' } 1; __END__ =head1 SEE ALSO L<perlunicode>, L<perluniintro> L<open>, L<utf8>, L<encoding>, L<Encode> =head1 AUTHORS Audrey Tang =head1 COPYRIGHT Copyright 2004, 2005, 2006, 2007 by Audrey Tang E<lt>cpan@audreyt.orgE<gt>. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See L<http://www.perl.com/perl/misc/Artistic.html> =cut