The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


autobox - use builtin datatypes as first-class objects


    use autobox;

    # call methods on builtin values and literals

    # integers

        my $range = 10->to(1); # [ 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 ]

    # floats

        my $error = 3.1415927->minus(22/7)->abs();

    # strings

        my $uri = ''->f($domain, $arg->escape());
        my $links = 'autobox'->google();

        my $word = 'rubicund';
        my $definition = $word->lookup_on_dictionary_dot_com();

        my $greeting = "Hello, World"->upper(); # "HELLO, WORLD"

        $greeting->to_lower(); # greeting is now "hello, world"

    # ARRAY refs

        my $schwartzian = [ @_ ]->map(...)->sort(...)->map(...);
        my $sd = [ 1, 8, 3, 3, 2, 9 ]->standard_deviation();

    # HASH refs

        { alpha => 'beta', gamma => 'vlissides' }->for_each(...);

    # CODE refs

        my $plus_five = (\&add)->curry()->(5);
        my $minus_three = sub { $_[0] - $_[1] }->reverse->curry->(3);

    # can() and isa() work as expected

        if ("Hello, World"->can('foo')) ...
        if (3.1415927->isa('SCALAR')) ...


The autobox pragma endows Perl's core datatypes with the capabilities of first-class objects. This allows methods to be called on ARRAY refs, HASH refs, CODE refs and raw scalars in exactly the same manner as blessed references. The autoboxing is transparent: boxed values are not blessed into their (user-defined) implementation class (unless the method elects to bestow such a blessing) - they simply use its methods as though they are.

autobox is lexically scoped, and handlers (see below) for an outer scope can be overridden or countermanded in a nested scope:

        use autobox; # default handlers
            use autobox SCALAR => 'MyScalar';
        # back to the default

Autoboxing can be turned off entirely by using the no syntax:

        use autobox;
        no autobox;

- as well as by specifying a sole default value of undef (see below):

    use autobox DEFAULT => undef;

Autoboxing is not performed for barewords i.e.

    my $foo = Foo->new();


    my $foo = new Foo;

behave as expected.

In addition, it only covers named methods, so while this works:

    my $foobar = { foo => 'bar' }->some_method();

These don't:

    my $method1 = 'some_method';
    my $method2 = \&HASH::some_method;

    my $error1 = { foo => 'bar' }->$method1();
    my $error2 = { foo => 'bar' }->$method2();

The classes into which the core types are boxed are fully configurable. By default, a method invoked on a non-object value is assumed to be defined in a package whose name corresponds to the ref() type of that value - or 'SCALAR' if the value is a non-reference.

Thus a vanilla:

    use autobox;

registers the following default handlers (for the current lexical scope):

        SCALAR  => 'SCALAR',
        ARRAY   => 'ARRAY',
        HASH    => 'HASH',
        CODE    => 'CODE'


    "hello, world"->upper()

would be invoked as:

    SCALAR::upper("hello, world")


    [ 1 .. 10 ]->for_each(sub { ... })

resolves to:

    ARRAY::for_each([ 1 .. 10 ], sub { ... })

A mapping from the builtin type to the user-defined class can be specified by passing a list of key/value bindings to the use autobox statement.

The following example shows the range of valid arguments:

    use autobox SCALAR  => 'MyScalar'       # package name
                ARRAY   => 'MyNamespace::', # package prefix (ending in '::')
                HASH    => '',              # use the default i.e. HASH 
                CODE    => undef,           # don't autobox this type
                UNDEF   => ...,             # can take any of the 4 types above
                DEFAULT => ...,             # can take any of the 4 types above
                DEBUG   => ...;             # boolean or coderef

SCALAR, ARRAY, HASH, CODE, UNDEF and DEFAULT can take four different types of value:

  • A package name e.g.

        use autobox SCALAR => 'MyScalar';

    This overrides the default package - in this case SCALAR. All methods invoked on literals or values of builtin type 'key' will be dispatched as methods of the package specified in the corresponding 'value'.

    If a package name is supplied for DEFAULT, it becomes the default package for all unhandled cases. Thus:

        use autobox ARRAY   => 'MyArray',
                    DEFAULT => 'MyDefault';

    will invoke ARRAY methods on MyArray and all other methods on MyDefault.

  • A namespace: this is a package prefix (up to and including the final '::') to which the name of the default handler for this type will be appended:


        use autobox ARRAY => 'Prelude::';

    binds ARRAY types to the Prelude::ARRAY package.

    As with the package name form, specifying a default namespace e.g.

        use autobox SCALAR  => 'MyScalar',
                    DEFAULT => 'MyNamespace::';

    binds MyNamespace::ARRAY, MyNamespace::HASH &c. to the corresponding builtin types.

  • An empty string: this is shorthand for the builtin type name. e.g.

        use autobox SCALAR  => 'MyScalar',
                    ARRAY   => '',
                    DEFAULT => 'MyDefault::';

    is equivalent to:

        use autobox SCALAR  => 'MyScalar'
                    ARRAY   => 'ARRAY',
                    DEFAULT => 'MyDefault::';

    which in turn is equivalent to:

        use autobox SCALAR  => 'MyScalar'
                    ARRAY   => 'ARRAY',
                    HASH    => 'MyDefault::HASH',
                    CODE    => 'MyDefault::CODE';

    If DEFAULT is set to an empty string (as it is by default), it fills in the default type for all the unhandled cases e.g.

        use autobox SCALAR  => 'MyScalar',
                    CODE    => 'MyCode',
                    DEFAULT => '';

    is equivalent to:

        use autobox SCALAR  => 'MyScalar',
                    CODE    => 'MyCode',
                    ARRAY   => 'ARRAY',
                    HASH    => 'HASH';
  • undef: this disables autoboxing for the specified type, or all unhandled types in the case of DEFAULT.

In addition to the SCALAR, ARRAY, HASH, CODE and DEFAULT options above, there are two additional options: UNDEF and DEBUG.


The pseudotype, UNDEF, can be used to autobox undefined values. These are not autoboxed by default (i.e. the default value is undef):

This doesn't work:

    use autobox;

    undef->foo() # runtime error

This works:

    use autobox UNDEF => 'MyPackage'; 

    undef->foo(); # ok

So does this:

    use autobox UNDEF => 'MyNamespace::'; 

    undef->foo(); # ok


DEBUG exposes the current handlers by means of a callback, or a static debugging function.

This can be useful if one wishes to see the computed bindings in 'longhand'.

Debugging is ignored if the value corresponding to the DEBUG key is false.

If the value is a CODE ref, then this sub is called with a reference to the HASH containing the computed handlers for the current scope.

Finally, if DEBUG is true but not a CODE ref, the handlers are dumped to STDERR.


    use autobox DEBUG => 1, ...


    use autobox DEBUG => sub { ... }, ...


    sub my_callback ($) {
        my $hashref = shift;

    use autobox DEBUG => \&my_callback, ...


Due to Perl's precedence rules some autoboxed literals may need to be parenthesized:

For instance, while this works:

    my $curried = sub { ... }->curry();

this doesn't:

    my $curried = \&foo->curry();

The solution is to wrap the reference in parentheses:

    my $curried = (\&foo)->curry();

The same applies for signed integer and float literals:

    # this works
    my $range = 10->to(1);

    # this doesn't work
    my $range = -10->to(10);

    # this works
    my $range = (-10)->to(10);

Perl's special-casing for the print BLOCK ... syntax (see perlsub) means that print { expression() } ... (where the curly brackets denote an anonymous HASH ref) may require some further disambiguation:

    # this works (
    print { foo => 'bar' }->foo();

    # and this
    print { 'foo', 'bar' }->foo();

    # and even this
    print { 'foo', 'bar', @_ }->foo();

    # but this doesn't
    print { @_ }->foo() ? 1 : 0 

In the latter case, the solution is to supply something other than a HASH ref literal as the first argument to print():

    # e.g.
    print STDOUT { @_ }->foo() ? 1 : 0;

    # or
    my $hashref = { @_ };
    print $hashref->foo() ? 1 : 0; 

    # or
    print '', { @_ }->foo() ? 1 : 0; 

    # or
    print '' . { @_ }->foo() ? 1 : 0; 

    # or even
    { @_ }->print_if_foo(1, 0); 

Although can and isa are "overloaded" for autoboxed values, the VERSION method isn't. Thus, while these work:

        [ ... ]->can('pop')


This doesn't:

        use MyScalar 1.23;

        use autobox SCALAR => MyScalar;

        print "Hello, World"->VERSION(), $/;

Though, of course:

        print MyScalar->VERSION(), $/;


        print $MyScalar::VERSION, $/;

continue to work.

This is due to a limitation in perl's implementation of use and no. Likewise, import and unimport are unaffected by the autobox pragma:

        'Foo'->import() # equivalent to Foo->import() rather than MyScalar->import('Foo')

        []->import()  # error: Can't call method "import" on unblessed reference





chocolateboy: <>


Copyright (c) 2003-2007, chocolateboy.

This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.