autobox(3) User Contributed Perl Documentation autobox(3)
NAME
autobox - call methods on native types
SYNOPSIS
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!"
$greeting->for_each(&character_handler);
# 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(...);
%hash->keys();
# code refs
my $plus_five = (&add)->curry()->(5);
my $minus_three = sub { $_[0] - $_[1] }->reverse->curry->(3);
# can, isa, VERSION, import and unimport can be accessed via autobox_class
42->autobox_class->isa('MyNumber')
say []->autobox_class->VERSION
DESCRIPTION
The autobox pragma allows methods to be called on integers, floats, strings, arrays, hashes, and code references in exactly the same manner
as blessed references.
The autoboxing is transparent: boxed values are not blessed into their (user-defined) implementation class (unless the method elects to
bestow such a blessing) - they simply use its methods as though they are.
The classes (packages) into which the native types are boxed are fully configurable. By default, a method invoked on a non-object is
assumed to be defined in a class whose name corresponds to the "ref()" type of that value - or SCALAR if the value is a non-reference.
This mapping can be overridden by passing key/value pairs to the "use autobox" statement, in which the keys represent native types, and the
values their associated classes.
As with regular objects, autoboxed values are passed as the first argument of the specified method. Consequently, given a vanilla "use
autobox":
"Hello, world!"->upper()
is invoked as:
SCALAR::upper("hello, world!")
while:
[ 1 .. 10 ]->for_each(sub { ... })
resolves to:
ARRAY::for_each([ 1 .. 10 ], sub { ... })
Values beginning with the array "@" and hash "%" sigils are passed by reference, i.e. under the default bindings:
@array->join(', ')
@{ ... }->length()
%hash->keys()
%$hash->values()
are equivalent to:
ARRAY::join(@array, ', ')
ARRAY::length(@{ ... })
HASH::keys(\%hash)
HASH::values(\%$hash)
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).
Thus:
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).
Likewise:
use autobox SCALAR => 'Foo';
use autobox SCALAR => 'Bar';
use autobox ARRAY => 'Baz';
and
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();
and:
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 asociated 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';
@array->some_array_method();
- 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.
Thus:
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.
OPTIONS
A mapping from native types to their user-defined classes can be specified by passing 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:
o 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".
o 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:
Thus:
use autobox ARRAY => 'Prelude::';
is equivalent to:
use autobox ARRAY => 'Prelude::ARRAY';
o A reference to an array of class names and/or namespaces. This associates multiple classes with the specified type.
DEFAULT
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.
Thus:
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.
Thus:
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
UNDEF
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
NUMBER, SCALAR and UNIVERSAL
The virtual types NUMBER, SCALAR and UNIVERSAL function as macros or shortcuts which create bindings for their subtypes. The type hierarchy
is as follows:
UNIVERSAL -+
|
+- SCALAR -+
| |
| +- NUMBER -+
| | |
| | +- INTEGER
| | |
| | +- FLOAT
| |
| +- STRING
|
+- ARRAY
|
+- HASH
|
+- CODE
Thus:
use autobox NUMBER => 'MyNumber';
is equivalent to:
use autobox
INTEGER => 'MyNumber',
FLOAT => 'MyNumber';
And:
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
"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.
Thus:
use autobox DEBUG => 1, ...
or
use autobox DEBUG => sub { ... }, ...
or
sub my_callback ($) {
my $hashref = shift;
...
}
use autobox DEBUG => &my_callback, ...
METHODS
import
On its own, "autobox" doesn't implement any methods that can be called on native types. However, its static method, "import", 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;
$class->SUPER::import(STRING => 'String::Trim::Scalar');
}
package String::Trim::Scalar;
sub trim {
my $string = shift;
$string =~ s/^s+//;
$string =~ s/s+$//;
$string;
}
1;
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 SCALAR 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();
EXPORTS
autobox_class
"autobox" makes a single method available to autoboxed types: "autobox_class". This can be used to call "isa", "can", "VERSION", "import"
and "unimport". e.g.
if (42->autobox_class->isa('SCALAR')) ...
if (sub { ... }->autobox_class->can('curry')) ...
type
"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
CAVEATS
Performance
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!")
Gotchas
Precedence
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);
print BLOCK
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 "eval"s 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;
42->foo();
EOS
Note that the "eval BLOCK" form works as expected:
use autobox;
eval { 42->foo() }; # OK
VERSION
2.75
SEE ALSO
o autobox::Core
o Moose::Autobox
o perl5i
o Scalar::Properties
AUTHOR
chocolateboy <chocolate@cpan.org>
COPYRIGHT
Copyright (c) 2003-2011, chocolateboy.
This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.
perl v5.16.2 2011-07-21 autobox(3)