newline.pod6

=begin pod :tag<perl6> :page-order<a38>

=TITLE Newline handling in Perl 6

=SUBTITLE How the different newline characters are handled, and how to change the behavior.

Different operating systems use different characters, or combinations of them,
to represent the transition to a new line. Every language has its own set of
rules to handle this. Perl 6 has the following ones:

=item C<\n> in a string literal means Unicode codepoint 10.

=item The default L<nl-out|/routine/nl-out> that is appended to a string by say
is also C<\n>.

=item On output, when on Windows, the encoder will by default transform a C<\n>
into a C<\r\n> when it's going to a file, process, or terminal (it won't do this
on a socket, however).

=item On input, on any platform, the decoder will by default normalize C<\r\n>
into C<\n> for input from a file, process, or terminal (again, not socket).

=item These above two points together mean that you can - socket programming
aside - expect to never see a C<\r\n> inside of your program (this is how things
work in numerous other languages too).

X<|:$translate-nl>
=item The L<C<:$translate-nl>|/type/Encoding#method_decoder> named parameter
exists in various places to control this transformation, for instance, in
L<C<Proc::Async.new>|/type/Proc::Async#method_new> and
L<C<Proc::Async.Supply>|/type/Proc::Async#method_Supply>.

=item A C<\n> in the L<regex|/language/regexes> language is logical, and will
match a C<\r\n>.

X<|:nl-out>
You can change the default behavior for a particular handle by setting the
C<:nl-out> attribute when you create that handle.

    my $crlf-out = open(IO::Special.new('<STDOUT>'), :nl-out("\\\n\r"));
    $*OUT.say: 1;     #OUTPUT: «1␤»
    $crlf-out.say: 1; #OUTPUT: «1\␤␍»

In this example, where we are replicating standard output to a new handle by
using L<IO::Special>, we are appending a C<\> to the end of the string, followed
by a newline C<␤> and a carriage return C<␍>; everything we print to that handle
will get those characters at the end of the line, as shown.

In regular expressions,
L<C<\n>|/language/regexes#index-entry-regex_\n-regex_\N-\n_and_\N> is defined in
terms of the L<Unicode definition of logical newline|http://unicode.org/reports/tr18/#Line_Boundaries>. It will match C<.>
and also C<\v>, as well as any class that includes whitespace.

=end pod

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