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=begin pod
=TITLE Debugging
=SUBTITLE Debug Perl 6 programs
There are at least two useful debuggers available for Rakudo, the Perl 6 compiler:
=item L<Debugger::UI::CommandLine|>
A command-line debugger frontend for Rakudo. This module installs the
C<perl6-debug-m> command-line utility, and is bundled with the Rakudo
Star distributions. Please check
L<its repository|>
for instructions and a tutorial.
=item L<Grammar::Debugger|> (and C<Grammar::Tracer> in the same distribution)
Simple tracing and debugging support for Perl 6 grammars.
Please see the documentation for these programs for further information.
Historically others have existed and others are likely to be written in
the future, check the L<Perl 6 Modules|>
There are also environment variables that can be set to aid debugging
various aspects of your program. See L<Perl 6 Environment
Variables|> and
L<Running rakudo from the command
for more information.
=head2 C<trace> pragma
The C<trace> pragma causes the program to print out step-by-step which
lines get executed:
use trace;
sub foo { say "hi" }
# 2 (/tmp/script.p6 line 2)
# sub foo { say "hi" }
# 5 (/tmp/script.p6 line 3)
# foo
# 3 (/tmp/script.p6 line 2)
# say "hi"
# hi
=head2 Dumper function C<dd>
X<|dd> X<|dumper>
B<Note:> this routine is a Rakudo-specific debugging feature and not
standard Perl 6.
The Tiny Data Dumper: This function takes the input list of variables
and C<note>s them (on C<$*ERR>) in an easy to read format, along with
the C<name> of the variable. Thus,
=begin code :ok-test<dd>
my $a = 42;
my %hash = "a" => 1, "b" => 2, "c" => 3;
dd %hash, $a;
Hash %hash = {:a(1), :b(2), :c(3)}
Int $a = 42
=end code
to the standard error stream.
Please note that C<dd> will ignore named parameters. You can use a
C<Capture> or C<Array> to force it to dump everything passed to it.
=begin code :ok-test<dd>
dd \((:a(1), :b(2)), :c(3));
dd [(:a(1), :b(2)), :c(3)];
=end code
If you don't specify any parameters at all, it will just print the type
and name of the current subroutine / method to the standard error
=begin code :ok-test<dd>
sub a { dd }; a # OUTPUT: «sub a()␤»
=end code
This can be handy as a cheap trace function.
=head2 Using backtraces
The L<Backtrace> class gets the current call stack, and can return it as
a string:
my $trace =;
sub inner { say; }
sub outer { inner; }
# perl6 /tmp/script.p6
# in sub inner at /tmp/script.p6 line 2
# in sub outer at /tmp/script.p6 line 3
# in block <unit> at /tmp/script.p6 line 4
=end pod
# vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6