Debugging

There are at least two useful debuggers available for Rakudo, the Raku compiler:

Debugger::UI::CommandLine

A command-line debugger frontend for Rakudo. This module installs the perl6-debug-m command-line utility, and is bundled with the Rakudo Star distributions. Please check its repository for instructions and a tutorial.

Grammar::Debugger (and Grammar::Tracer in the same distribution)

Simple tracing and debugging support for Raku 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 Raku Modules website.

There are also environment variables that can be set to aid debugging various aspects of your program. See Raku Environment Variables and Running rakudo from the command line for more information.

trace pragma

The trace pragma causes the program to print out step-by-step which lines get executed:

use trace;
sub foo { say "hi" }
foo;
# OUTPUT: 
# 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

Dumper function dd

[1]

The Tiny Data Dumper: This function takes the input list of variables and notes them (on $*ERR) in an easy to read format, along with the name of the variable. Thus,

my $a = 42;
my %hash = "a" => 1, "b" => 2, "c" => 3;
 
dd %hash, $a;
#`( OUTPUT:
    Hash %hash = {:a(1), :b(2), :c(3)}
    Int $a = 42
)

prints to the standard error stream the variables passed as argument.

Please note that dd will ignore named parameters. You can use a Capture or Array to force it to dump everything passed to it.

dd \((:a(1), :b(2)), :c(3));
dd [(:a(1), :b(2)), :c(3)];

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 stream:

sub a { dd }; a   # OUTPUT: «sub a()␤» 

This can be handy as a cheap trace function.

Using backtraces

The Backtrace class gets the current call stack, and can return it as a string:

my $trace = Backtrace.new;
sub inner { say ~Backtrace.new; }
sub outer { inner; }
outer;
 
# OUTPUT: 
# 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