Course of Raku / Advanced / Debugging / Debugging with dd

Dumping variables

Call dd to see what a variable holds, much as you would do with say:

my $var = 42;
dd $var;

Together with the value, dd also shows the name of the variable:

$var = 42

If the variable has a declared type, the type appears too. Compare the output of the following program with the previous one:

my Int $var = 42;
dd $var;

Int $var = 42

This is the main difference from say: a single dd called on a scalar tells you the name, and, when known, the type of the data — so you can drop several dd calls into a program and still recognise which output belongs to which variable.

You can also dump more complex data structures, such as arrays or hashes. Here, dd prints a code-like representation of the value:

my @arr = 10, 20, [1, 2, 3], 30;
dd @arr;

[10, 20, [1, 2, 3], 30]

Notice that the nested array stays clearly visible (no variable name printed though). The same works for hashes, whose keys dd prints in sorted order:

my %hash =
    gamma => 'g',
    alpha => 'a',
    beta  => 'b';
dd %hash;

{:alpha("a"), :beta("b"), :gamma("g")}

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