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.\" ========================================================================
.\"
.IX Title "custom::failures 3pm"
.TH custom::failures 3pm "2022-12-07" "perl v5.36.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
custom::failures \- Minimalist, customized exception hierarchy generator
.SH "VERSION"
.IX Header "VERSION"
version 0.004
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    package MyApp::failure;
\&
\&    use custom::failures qw/io::file io::network/;
\&
\&    # customize failure methods…
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This module works like failures but lets you define a customized exception
hierarchy if you need a custom namespace, additional attributes, or customized
object behaviors.
.PP
Because failure classes have an \f(CW@ISA\fR chain and Perl by default uses
depth-first-search to resolve method calls, you can override behavior anywhere
in the custom hierarchy and it will take precedence over default \f(CW\*(C`failure\*(C'\fR
behaviors.
.PP
There are two methods that might be useful to override:
.IP "\(bu" 4
message
.IP "\(bu" 4
throw
.PP
Both are described further, below.
.SH "USAGE"
.IX Header "USAGE"
.SS "Defining a custom failure hierarchy"
.IX Subsection "Defining a custom failure hierarchy"
.Vb 1
\&    package MyApp::failure;
\&
\&    use custom::failures qw/foo::bar/;
.Ve
.PP
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):
.PP
.Vb 7
\&    MyApp::failure::foo::bar \-\-> failure::foo::bar
\&           |                        |
\&           V                        V
\&    MyApp::failure::foo      \-\-> failure::foo
\&           |                        |
\&           V                        V
\&    MyApp::failure           \-\-> failure
.Ve
.PP
Alternatively, if you want a different namespace for the hierarchy, do it this way:
.PP
.Vb 1
\&    use custom::failures \*(AqMyApp::Error\*(Aq => [ \*(Aqfoo::bar\*(Aq ];
.Ve
.PP
That will create the following classes and relationships:
.PP
.Vb 7
\&    MyApp::Error::foo::bar \-\-> failure::foo::bar
\&           |                        |
\&           V                        V
\&    MyApp::Error::foo      \-\-> failure::foo
\&           |                        |
\&           V                        V
\&    MyApp::Error           \-\-> failure
.Ve
.PP
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:
.PP
.Vb 9
\&    use Safe::Isa; # for $_isa
\&    try {
\&        MyApp::failure::foo::bar\->throw;
\&    }
\&    catch {
\&        if ( $_\->$_isa( "failure::foo" ) ) {
\&            # handle it here
\&        }
\&    };
.Ve
.SS "Adding custom attributes"
.IX Subsection "Adding custom attributes"
Failure classes are implemented with Class::Tiny, so adding
attributes is trivially easy:
.PP
.Vb 1
\&    package MyApp::failure;
\&
\&    use custom::failures qw/foo::bar/;
\&
\&    use Class::Tiny qw/user/;
.Ve
.PP
This adds a \f(CW\*(C`user\*(C'\fR attribute to \f(CW\*(C`MyApp::failure\*(C'\fR and
all its subclasses so it can be set in the argument to \f(CW\*(C`throw\*(C'\fR:
.PP
.Vb 1
\&    MyApp::failure::foo\->throw( { msg => "Ouch!", user => "me" } );
.Ve
.PP
Be sure to load \f(CW\*(C`Class::Tiny\*(C'\fR \fBafter\fR you load \f(CW\*(C`custom::failures\*(C'\fR
so that your \f(CW@ISA\fR is already set up.
.ie n .SS "Overriding the ""message"" method"
.el .SS "Overriding the \f(CWmessage\fP method"
.IX Subsection "Overriding the message method"
Overriding \f(CW\*(C`message\*(C'\fR lets you modify how the error string is produced.
The \f(CW\*(C`message\*(C'\fR method takes a string (typically just the \f(CW\*(C`msg\*(C'\fR field) and
returns a string.  It should not produce or append stack trace information.
That is done during object stringification.
.PP
Call \f(CW\*(C`SUPER::message\*(C'\fR if you want the standard error text prepended (\f(CW"Caught
$class: ..."\fR).
.PP
For example, if you want to use String::Flogger to render messages:
.PP
.Vb 1
\&    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) );
\&    }
.Ve
.PP
Then you can pass strings or array references or code references as the \f(CW\*(C`msg\*(C'\fR
for \f(CW\*(C`throw\*(C'\fR:
.PP
.Vb 3
\&    MyApp::failure\->throw( "just a string"               );
\&    MyApp::failure\->throw( [ "show some data %s", $ref ] );
\&    MyApp::failure\->throw( sub { call_expensive_sub() }  );
.Ve
.PP
Because the \f(CW\*(C`message\*(C'\fR method is only called during stringification (unless you
call it yourself), the failure class type can be checked before any expensive
rendering is done.
.ie n .SS "Overriding the ""throw"" method"
.el .SS "Overriding the \f(CWthrow\fP method"
.IX Subsection "Overriding the throw method"
Overriding \f(CW\*(C`throw\*(C'\fR 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 \f(CW\*(C`SUPER::throw\*(C'\fR with a hash reference to actually throw the error.
.PP
For example, to capture the filename associated with file errors:
.PP
.Vb 1
\&    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 \*(Aqmsg\*(Aq and \*(Aqfilename\*(Aq
\&    }
.Ve
.PP
Later you could use it like this:
.PP
.Vb 1
\&    MyApp::failure::file\->throw( opening => $some_file );
.Ve
.SS "Using \s-1BUILD\s0"
.IX Subsection "Using BUILD"
\&\f(CW\*(C`Class::Tiny\*(C'\fR supports \f(CW\*(C`BUILD\*(C'\fR, so you can also use that to
do things with failure objects when thrown.  This example
logs exceptions as they are built:
.PP
.Vb 1
\&    use Log::Any qw/$log/;
\&
\&    sub BUILD {
\&        my ($self) = @_;
\&        $log\->error( $self\->message );
\&    }
.Ve
.PP
By using \f(CW\*(C`message\*(C'\fR instead of stringifying \f(CW$self\fR, we
log the message but not the trace (if any).
.SH "AUTHOR"
.IX Header "AUTHOR"
David Golden <dagolden@cpan.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is Copyright (c) 2013 by David Golden.
.PP
This is free software, licensed under:
.PP
.Vb 1
\&  The Apache License, Version 2.0, January 2004
.Ve