Paul Evans > Sentinel > Sentinel



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Sentinel - create lightweight SCALARs with get/set callbacks


 package Some::Class;

 use Sentinel;

 sub foo :lvalue
    my $self = shift;

 sub bar :lvalue
    my $self = shift;
    sentinel value => $self->get_bar,
             set   => sub { $self->set_bar( $_[0] ) };

 sub splot :lvalue
    sentinel obj => shift, get => \&get_splot, set => \&set_splot;

 sub wibble :lvalue
    sentinel obj => shift, get => "get_wibble", set => "set_wibble";


This module provides a single lvalue function, sentinel, which yields a scalar that invoke callbacks to get or set its value. Primarily this is useful to create lvalue object accessors or other functions, to invoke actual code when a new value is set, rather than simply updating a scalar variable.


$scalar = sentinel %args

Returns (as an lvalue) a scalar with magic attached to it. This magic is used to get the value of the scalar, or to inform of a new value being set, by invoking callback functions supplied to the sentinel. Takes the following named arguments:

get => CODE

A CODE reference or obj method name to invoke when the value of the scalar is read, to obtain its value. The value returned from this code will appear as the value of the scalar.

set => CODE

A CODE reference or obj method name to invoke when a new value for the scalar is written. The code will be passed the new value as its only argument.

value => SCALAR

If no get callback is provided, this value is given as the initial value of the scalar. If the scalar manages to survive longer than a single assignment, its value on read will retain the last value set to it.

obj => SCALAR

Optional value to pass as the first argument into the get and set callbacks. If this value is provided, then the get and set callbacks may be given as direct sub references to object methods, or simply method names, rather than closures that capture the referent object. This avoids the runtime overhead of creating lots of small one-use closures around the object.


A useful behaviour of this module is generation of mutation accessor methods that automatically wrap get_/set_ accessor/mutator pairs:

 foreach (qw( name address age height )) {
    my $name = $_;

    no strict 'refs';
    *$name = sub :lvalue {
       sentinel obj => shift, get => "get_$name", set => "set_$name";

This is especially useful for methods whose values are simple strings or numbers, because they allow Perl's rich set of mutation operators to be applied to the object's values.

 $obj->name =~ s/-/_/g;

 substr( $obj->address, 100 ) = "";


 $obj->height /= 100;


If an XS compiler is available at build time, this module is implemented using XS. If not, it falls back on an implementation using a tied scalar. A pureperl installation can also be requested at build time by passing the --pp argument to Build.PL:

 $ perl Build.PL --pp
 $ ./Build


With thanks to leont, Zefram, and others from for assisting with trickier bits of XS logic. Thanks to mst for suggesting a pureperl implementation for XS-challenged systems.


Paul Evans <>

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