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autobox - call methods on native types


    use autobox;

    # 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 @list = 'SELECT * FROM foo'->list();
        my $greeting = "Hello, world!"->upper(); # "HELLO, WORLD!"


    # arrays and array refs

        my $schwartzian = @_->map(...)->sort(...)->map(...);
        my $hash = [ 'SELECT * FROM foo WHERE id IN (?, ?)', 1, 2 ]->hash();

    # hashes and 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, isa, VERSION, import and unimport can be accessed via autobox_class

        say []->autobox_class->VERSION


The autobox pragma allows methods to be called on integers, floats, strings, arrays, hashes, and code references 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.

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

This mapping can be overridden by passing key/value pairs to the use autobox statement, in which the keys represent native types, and the values their associated classes.

As with regular objects, autoboxed values are passed as the first argument of the specified method. Consequently, given a vanilla use autobox:

    "Hello, world!"->upper()

is invoked as:

    SCALAR::upper("hello, world!")


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

resolves to:

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

Values beginning with the array @ and hash % sigils are passed by reference, i.e. under the default bindings:

    @array->join(', ')
    @{ ... }->length()

are equivalent to:

    ARRAY::join(\@array, ', ')
    ARRAY::length(\@{ ... })

Multiple use autobox statements can appear in the same scope. These are merged both "horizontally" (i.e. multiple classes can be associated with a particular type) and "vertically" (i.e. multiple classes can be associated with multiple types).


    use autobox SCALAR => 'Foo';
    use autobox SCALAR => 'Bar';

- associates SCALAR types with a synthetic class whose @ISA includes both Foo and Bar (in that order).


    use autobox SCALAR => 'Foo';
    use autobox SCALAR => 'Bar';
    use autobox ARRAY  => 'Baz';


    use autobox SCALAR => [ 'Foo', 'Bar' ];
    use autobox ARRAY  => 'Baz';

- bind SCALAR types to the Foo and Bar classes and ARRAY types to Baz.

autobox is lexically scoped, and bindings for an outer scope can be extended or countermanded in a nested scope:

        use autobox; # default bindings: autobox all native types

            # appends 'MyScalar' to the @ISA associated with SCALAR types
            use autobox SCALAR => 'MyScalar';

        # back to the default (no MyScalar)

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

        use autobox;
        no autobox;

- or can be selectively disabled by passing arguments to the no autobox statement:

    use autobox; # default bindings

    no autobox qw(SCALAR);

    []->foo(); # OK: ARRAY::foo([])

    "Hello, world!"->bar(); # runtime error

Autoboxing is not performed for barewords i.e.

    my $foo = Foo->new();


    my $foo = new Foo;

behave as expected.

Methods are called on native types by means of the arrow operator. As with regular objects, the right hand side of the operator can either be a bare method name or a variable containing a method name or subroutine reference. Thus the following are all valid:

    sub method1 { ... }
    my $method2 = 'some_method';
    my $method3 = sub { ... };
    my $method4 = \&some_method;

    " ... "->method1();
    [ ... ]->$method2();
    { ... }->$method3();
    sub { ... }->$method4();

A native type is only associated with a class if the type => class mapping is supplied in the use autobox statement. Thus the following will not work:

    use autobox SCALAR => 'MyScalar';


- as no class is specified for the ARRAY type. Note: the result of calling a method on a native type that is not associated with a class is the usual runtime error message:

    Can't call method "some_array_method" on unblessed reference at ...

As a convenience, there is one exception to this rule. If use autobox is invoked with no arguments (ignoring the DEBUG option) the four main native types are associated with classes of the same name.


    use autobox;

- is equivalent to:

    use autobox
        SCALAR => 'SCALAR',
        ARRAY  => 'ARRAY',
        HASH   => 'HASH',
        CODE   => 'CODE';

This facilitates one-liners and prototypes:

    use autobox;

    sub SCALAR::split { [ split '', $_[0] ] }
    sub ARRAY::length { scalar @{$_[0]} }

    print "Hello, world!"->split->length();

However, using these default bindings is not recommended as there's no guarantee that another piece of code won't trample over the same namespace/methods.


A mapping from native types to their user-defined classes can be specified by passing a hashref or a list of key/value pairs to the use autobox statement.

The following example shows the range of valid arguments:

    use autobox {
        SCALAR    => 'MyScalar'                     # class name
        ARRAY     => 'MyNamespace::',               # class prefix (ending in '::')
        HASH      => [ 'MyHash', 'MyNamespace::' ], # one or more class names and/or prefixes
        CODE      => ...,                           # any of the 3 value types above
        INTEGER   => ...,                           # any of the 3 value types above
        FLOAT     => ...,                           # any of the 3 value types above
        NUMBER    => ...,                           # any of the 3 value types above
        STRING    => ...,                           # any of the 3 value types above
        UNDEF     => ...,                           # any of the 3 value types above
        UNIVERSAL => ...,                           # any of the 3 value types above
        DEFAULT   => ...,                           # any of the 3 value types above
        DEBUG     => ...                            # boolean or coderef

The INTEGER, FLOAT, NUMBER, STRING, SCALAR, ARRAY, HASH, CODE, UNDEF, DEFAULT and UNIVERSAL options can take three different types of value:

  • A class name e.g.

        use autobox INTEGER => 'MyInt';

    This binds the specified native type to the specified class. All methods invoked on literals or values of type key will be dispatched as methods of the class specified in the corresponding value.

  • A namespace: this is a class prefix (up to and including the final '::') to which the specified type name (INTEGER, FLOAT, STRING &c.) will be appended:


        use autobox ARRAY => 'Prelude::';

    is equivalent to:

        use autobox ARRAY => 'Prelude::ARRAY';
  • A reference to an array of class names and/or namespaces. This associates multiple classes with the specified type.


The DEFAULT option specifies bindings for any of the four default types (SCALAR, ARRAY, HASH and CODE) not supplied in the use autobox statement. As with the other options, the value corresponding to the DEFAULT key can be a class name, a namespace, or a reference to an array containing one or more class names and/or namespaces.


    use autobox
        STRING  => 'MyString',
        DEFAULT => 'MyDefault';

is equivalent to:

    use autobox
        STRING  => 'MyString',
        SCALAR  => 'MyDefault',
        ARRAY   => 'MyDefault',
        HASH    => 'MyDefault',
        CODE    => 'MyDefault';

Which in turn is equivalent to:

    use autobox
        INTEGER => 'MyDefault',
        FLOAT   => 'MyDefault',
        STRING  => [ 'MyString', 'MyDefault' ],
        ARRAY   => 'MyDefault',
        HASH    => 'MyDefault',
        CODE    => 'MyDefault';

Namespaces in DEFAULT values have the default type name appended, which, in the case of defaulted SCALAR types, is SCALAR rather than INTEGER, FLOAT &c.


    use autobox
        ARRAY   => 'MyArray',
        HASH    => 'MyHash',
        CODE    => 'MyCode',
        DEFAULT => 'MyNamespace::';

is equivalent to:

    use autobox
        INTEGER => 'MyNamespace::SCALAR',
        FLOAT   => 'MyNamespace::SCALAR',
        STRING  => 'MyNamespace::SCALAR',
        ARRAY   => 'MyArray',
        HASH    => 'MyArray',
        CODE    => 'MyCode';

Any of the four default types can be exempted from defaulting to the DEFAULT value by supplying a value of undef:

    use autobox
        HASH    => undef,
        DEFAULT => 'MyDefault';

    42->foo # ok: MyDefault::foo
    []->bar # ok: MyDefault::bar

    %INC->baz # not ok: runtime error


The pseudotype, UNDEF, can be used to autobox undefined values. These are not autoboxed by default.

This doesn't work:

    use autobox;

    undef->foo() # runtime error

This works:

    use autobox UNDEF => 'MyUndef';

    undef->foo(); # ok

So does this:

    use autobox UNDEF => 'MyNamespace::';

    undef->foo(); # ok


The virtual types NUMBER, SCALAR and UNIVERSAL function as macros or shortcuts which create bindings for their subtypes. The type hierarchy is as follows:

             +- SCALAR -+
             |          |
             |          +- NUMBER -+
             |          |          |
             |          |          +- INTEGER
             |          |          |
             |          |          +- FLOAT
             |          |
             |          +- STRING
             +- ARRAY
             +- HASH
             +- CODE


    use autobox NUMBER => 'MyNumber';

is equivalent to:

    use autobox
        INTEGER => 'MyNumber',
        FLOAT   => 'MyNumber';


    use autobox SCALAR => 'MyScalar';

is equivalent to:

    use autobox
        INTEGER => 'MyScalar',
        FLOAT   => 'MyScalar',
        STRING  => 'MyScalar';

Virtual types can also be passed to unimport via the no autobox syntax. This disables autoboxing for the corresponding subtypes e.g.

    no autobox qw(NUMBER);

is equivalent to:

    no autobox qw(INTEGER FLOAT);

Virtual type bindings can be mixed with ordinary bindings to provide fine-grained control over inheritance and delegation. For instance:

    use autobox
        INTEGER => 'MyInteger',
        NUMBER  => 'MyNumber',
        SCALAR  => 'MyScalar';

would result in the following bindings:

    42->foo             -> [ MyInteger, MyNumber, MyScalar ]
    3.1415927->bar      -> [ MyNumber, MyScalar ]
    "Hello, world!->baz -> [ MyScalar ]

Note that DEFAULT bindings take precedence over virtual type bindings i.e.

    use autobox
        UNIVERSAL => 'MyUniversal',
        DEFAULT   => 'MyDefault'; # default SCALAR, ARRAY, HASH and CODE before UNIVERSAL

is equivalent to:

  use autobox
      INTEGER => [ 'MyDefault', 'MyUniversal' ],
      FLOAT   => [ 'MyDefault', 'MyUniversal' ], # ... &c.


DEBUG exposes the current bindings for the scope in which use autobox is called by means of a callback, or a static debugging function.

This allows the computed bindings to be seen in "longhand".

The option 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 bindings for the current scope.

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


    use autobox DEBUG => 1, ...


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


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

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



This method sets up autobox bindings for the current lexical scope. It can be used to implement autobox extensions i.e. lexically-scoped modules that provide autobox bindings for one or more native types without requiring calling code to use autobox.

This is done by subclassing autobox and overriding import. This allows extensions to effectively translate use MyModule into a bespoke use autobox call. e.g.:

    package String::Trim;

    use base qw(autobox);

    sub import {
        my $class = shift;
            STRING => 'String::Trim::String'

    package String::Trim::String;

    sub trim {
        my $string = shift;
        $string =~ s/^\s+//;
        $string =~ s/\s+$//;


Note that trim is defined in an auxiliary class rather than in String::Trim itself to prevent String::Trim's own methods (i.e. the methods it inherits from autobox) being exposed to STRING types.

This module can now be used without a use autobox statement to enable the trim method in the current lexical scope. e.g.:

    #!/usr/bin/env perl

    use String::Trim;

    print "  Hello, world!  "->trim();



autobox adds a single method to all autoboxed types: autobox_class. This can be used to call can, isa, VERSION, import and unimport. e.g.

    if (sub { ... }->autobox_class->can('curry')) ...
    if (42->autobox_class->isa('SCALAR')) ...

Note: autobox_class should always be used when calling these methods. The behaviour when these methods are called directly on the native type e.g.:


- is undefined.



autobox includes an additional module, autobox::universal, which exports a single subroutine, type.

This sub returns the type of its argument within autobox (which is essentially longhand for the type names used within perl). This value is used by autobox to associate a method invocant with its designated classes. e.g.

    use autobox::universal qw(type);

    type("Hello, world!") # STRING
    type(42)              # INTEGER
    type([])              # ARRAY
    type(sub { })         # CODE

autobox::universal is loaded automatically by autobox, and, as its name suggests, can be used to install a universal method (i.e. a method for all autobox types) e.g.

    use autobox UNIVERSAL => 'autobox::universal';

    42->type        # INTEGER
    3.1415927->type # FLOAT
    %ENV->type      # HASH



Autoboxing comes at a price. Calling

    "Hello, world!"->length()

is slightly slower than the equivalent method call on a string-like object, and significantly slower than

    length("Hello, world!")



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);

eval EXPR

Like most pragmas, autobox performs operations at compile time, and, as a result, runtime string evals are not executed within its scope i.e. this doesn't work:

    use autobox;

    eval "42->foo";

The workaround is to use autobox within the eval e.g.

    eval <<'EOS';
        use autobox;

Note that the eval BLOCK form works as expected:

    use autobox;

    eval { 42->foo() }; # OK





chocolateboy <chocolate@cpan.org>


Copyright (c) 2003-2013, chocolateboy.

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

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