Defining a custom failure hierarchy

    package MyApp::failure;
    use custom::failures qw/foo::bar/;

This will define a failure class hierarchy under the calling package's namespace. The following diagram show the classes that will be created (arrows denote 'is-a' relationships):

    MyApp::failure::foo::bar --> failure::foo::bar
           |                        |
           V                        V
    MyApp::failure::foo      --> failure::foo
           |                        |
           V                        V
    MyApp::failure           --> failure

Alternatively, if you want a different namespace for the hierarchy, do it this way:

    use custom::failures 'MyApp::Error' => [ 'foo::bar' ];

That will create the following classes and relationships:

    MyApp::Error::foo::bar --> failure::foo::bar
           |                        |
           V                        V
    MyApp::Error::foo      --> failure::foo
           |                        |
           V                        V
    MyApp::Error           --> failure

By having custom classes also inherit from a standard namespace, you can throw a custom error class that will still be caught in the standard namespace:

    use Safe::Isa; # for $_isa
    try {
        MyApp::failure::foo::bar->throw;
    }
    catch {
        if ( $_->$_isa( "failure::foo" ) ) {
            # handle it here
        }
    };

Adding custom attributes

Failure classes are implemented with Class::Tiny, so adding attributes is trivially easy:

    package MyApp::failure;
    use custom::failures qw/foo::bar/;
    use Class::Tiny qw/user/;

This adds a "user" attribute to "MyApp::failure" and all its subclasses so it can be set in the argument to "throw":

    MyApp::failure::foo->throw( { msg => "Ouch!", user => "me" } );

Be sure to load "Class::Tiny" after you load "custom::failures" so that your @ISA is already set up.

Overriding the "message" method

Overriding "message" lets you modify how the error string is produced. The "message" method takes a string (typically just the "msg" field) and returns a string. It should not produce or append stack trace information. That is done during object stringification.

Call "SUPER::message" if you want the standard error text prepended ("Caught $class: ...").

For example, if you want to use String::Flogger to render messages:

    package MyApp::failure;
    use custom::failures qw/foo::bar/;
    use String::Flogger qw/flog/;
    sub message {
        my ( $self, $msg ) = @_;
        return $self->SUPER::message( flog($msg) );
    }

Then you can pass strings or array references or code references as the "msg" for "throw":

    MyApp::failure->throw( "just a string"               );
    MyApp::failure->throw( [ "show some data %s", $ref ] );
    MyApp::failure->throw( sub { call_expensive_sub() }  );

Because the "message" method is only called during stringification (unless you call it yourself), the failure class type can be checked before any expensive rendering is done.

Overriding the "throw" method

Overriding "throw" lets you modify the arguments you can provide or ensure that a trace is included. It can take whatever arguments you want and should call "SUPER::throw" with a hash reference to actually throw the error.

For example, to capture the filename associated with file errors:

    package MyApp::failure;
    use custom::failures qw/file/;
    use Class::Tiny qw/filename/;
    sub throw {
        my ( $class, $msg, $file ) = @_;
        my $args = {
            msg => $msg,
            filename => $file,
            trace => failures->croak_trace,
        };
        $self->SUPER::throw( $args );
    }
    sub message {
        # do something with 'msg' and 'filename'
    }

Later you could use it like this:

    MyApp::failure::file->throw( opening => $some_file );

Using BUILD

"Class::Tiny" supports "BUILD", so you can also use that to do things with failure objects when thrown. This example logs exceptions as they are built:

    use Log::Any qw/$log/;
    sub BUILD {
        my ($self) = @_;
        $log->error( $self->message );
    }

By using "message" instead of stringifying $self, we log the message but not the trace (if any).