Badger::Exception - structured exception for error handling

Badger documentation Contained in the Badger distribution.

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NAME

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Badger::Exception - structured exception for error handling

SYNOPSIS

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    use Badger::Exception;

    # create exception object
    my $exception = Badger::Exception->new({
        type => $type,
        info => $info,
    });

    # query exception type and info fields
    $type = $exception->type();
    $info = $exception->info();
    ($type, $info) = $exception->type_info();

    # print string summarising exception
    print $exception->text();

    # use automagic stringification 
    print $exception;

    # throw exception
    $exception->throw;

DESCRIPTION

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This module defines an object class for representing exceptions. These are simple objects that store various bits of information about an error condition.

The type denotes what kind of error occurred (e.g. 'file', 'parser', 'database', etc.). The info field provides further information about the error (e.g. 'foo/bar.html not found', 'parser error at line 42', 'server is on fire', etc). Other optional fields include file and line for specifying the location of the error.

In most cases you wouldn't generate and/or throw an exception object directly from your code. A better approach is to define a throw() method in a base class which does this for you.

The Badger::Base module is an example of just such a module. You can use this as a base class for your modules to inherit the throw() method.

Here's an example of a module that implements a method which expects an argument. If if doesn't get the argument it's looking for then it throws an exception via the inherited throw() method. The exception type is example and the additional information is No argument specified.

    package Your::Module;
    use base 'Badger::Base';

    sub example_method {
        my $self = shift;
        my $arg  = shift
            || self->throw( example => 'No argument specified' );

        # ...do something with ...
    }

The error() provides a higher level of abstraction. You provide the error message (which becomes the exception info) and it will generate an exception type based on the package name of your module.

    package Your::Module;
    use base 'Badger::Base';

    sub example_method {
        my $self = shift;
        my $arg  = shift
            || self->error('No argument specified' );

        # ...do something with ...
    }

In the example above, an exception will be thrown with a type defined as your.module. The module name is converted to lower case and the package delimiters are replaced with dots. There are configuration options that allow you to define other exceptions types. Consult the Badger::Base documentation for further information.

You can choose any values you like for type and info. The type is used to identify what kind of error occurred and should be a short word like "example", or a dot-separated sequence of words like "example.file.missing". In the latter case, dotted exception types are assumed to represent a hierarchy where example.file.missing error is a more specialised kind of example.file error, which in turn is a more specialised kind of example error. The match_type() method takes this into account when matching exception types.

    eval {
        # some code that throws an exception 
    };
    if ($@) {
        if ($@->match_type('example')) {
            # caught 'example' or 'example.*' error
            # ...now do something
        }
        else {
            # re-throw any other exception types
            $@->throw;
        }
    }

The info field should provide a more detailed error message in a format suitable for human consumption.

STACK TRACING

The Badger::Exception module also has a tracing mode which will automatically save the caller stack at the point at which the error is thrown. This allows you to inspect the full code path which led to the error from the comfort of you exception catching code, rather than having to deal with it at the point where the error is throw.

    # deep in your code somewhere.... in a class derived from Badger::Base
    $self->throw(
        database => 'The database is made of cheese',
        trace    => 1,
    );

The text() method (which is called whenever the object is stringified) will then append a stack track to the end of the generated message.

    # high up in your calling code:
    eval { $object->do_something_gnarly };

    if ($err = $@) {
        print $err;
        exit;
    }

You can also call the stack() method to return the stored call stack information, or the stack_trace() method to see a textual summary.

You can enable the tracing behaviour for all exception objects by setting the $TRACE package variable.

    use Badger::Exception;
    $Badger::Exception::TRACE = 1;

The trace import hook is provided as a short-cut for this.

    use Badger::Exception trace => 1;

IMPORT HOOKS

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trace

This import hook can be used to set the $TRACE package variable to enable stack tracing for the Badger::Exception module.

    use Badger::Exception trace => 1

When stack tracing is enabled, the exception will store information about the calling stack at the point at which it is thrown. This information will be displayed by the text() method. It is also available in raw form via the stack() method.

METHODS

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new()

Constructor method for creating a new exception. 

    my $exception = Badger::Exception->new(
        type => 'database',
        info => 'could not connect',
        file => '/path/to/file.pm',
        line => 420,
    );

type()

When called without arguments, this method returns the exception type, as defined by the first argument passed to the new() constructor method.

    my $type = $exception->type();

It can also be called with an argument to set a new type for the exception.

    $exception->type('database');

info()

When called without arguments, this method returns the information field for the exception.

    my $info = $exception->info();

It can also be called with an argument to define new information for the exception.

    $exception->info('could not connect');

file()

Method to get or set the name of the file in which the exception was raised.

    $exception->file('path/to/file.pm');
    print $exception->file;                 # /path/to/file.pm

line()

Method to get or set the line number at which the exception was raised.

    $exception->line(420);
    print $exception->line;                 # 420

text()

This method returns a text representation of the exception object. The string returned is formatted as $type error - $info.

    print $exception->text();   # database error - could not connect

This method is also bound to the stringification operator, allowing you to simple print the exception object to get the same result as calling text() explicitly.

    print $exception;   # database error - could not connect

trace()

Method to get or set the flag which determines if the exception captures a stack backtrace at the point at which it is thrown. It can be called as an object method to affect an individual exception object, or as a class method to get or set the $TRACE package variable which provides the default value for any exceptions created from then on.

    $exception->trace(1);               # object method
    print $exception->trace;            # 1

    Badger::Exception->trace(1);        # class method - sets $TRACE
    print Badger::Exception->trace;     # 1

match_type()

This method selects and returns a type string from the arguments passed that is the nearest correct match for the current exception type. This is used to select the most appropriate handler for the exception.

    my $match = $exception->match_type('file', 'parser', 'database')
        || die "no match for exception\n";

In this example, the exception will return one of the values file, parser or database, if and only if its type is one of those values. Otherwise it will return undef;

Exception types can be organised into a hierarchical structure by delimiting each part of the type with a period. For example, the database exception type might be further divided into the more specific database.connection, database.query and database.server_on_fire exception types.

An exception of type database.connection will match a handler type of database.connection or more generally, database. The longer (more specific) handler name will always match in preference to a shorter (more general) handler as shown in the next example:

    $exception->type('database.connection');

    my $match = $exception->match_type('database', 'database.connection')
        || die "no match for exception\n";

    print $match;    # database.connection

When there is no exact match, the match_type() method will return something more general that matches. In the following example, there is no specific handler type for database.exploded, but the more general database type still matches.

    $exception->type('database.exploded');

    my $match = $exception->match_type('database', 'database.connection')
        || die "no match for exception\n";

    print $match;    # database

You can also specify multiple exception types using a reference to a list.

    if ($exception->match_type(['warp.drive', 'shields'])) {
        ...
    }

Or using a single string of whitespace delimited exception types.

    if ($exception->match_type('warp.drive shields')) {
        ...
    }

You can also pass a reference to a hash array in which the keys are exception types. The corresponding value for a matching type will be returned.

    my $type_map = {
        'warp.drive'    => 'propulsion',
        'impulse.drive' => 'propulsion',
        'shields'       => 'defence',
        'phasers'       => 'defence'
    };

    if ($exception->match_type($type_map)) {
        ...
    }

throw()

This method throws the exception by calling die() with the exception object as an argument. If the $TRACE flag is set to a true value then the method will first save the pertinent details from a stack backtrace into the exception object before throwing it.

stack()

If stack tracing is enabled then this method will return a reference to a list of information from the caller stack at the point at which the exception was thrown. Each item in the list is a reference to a list containing the information returned by the inbuilt caller() method. See perldoc -f caller for further information. 

    use Badger::Exception trace => 1;

    eval {
        # some code that throws an exception object
        $exception->throw();
    };

    my $catch = $@;                 # exception object
    my $stack = $catch->stack;

    foreach my $caller (@$stack) {
        my ($pkg, $file, $line, @other_stuff) = @$caller;
        # do something
    }

The first set of information relates to the immediate caller of the throw() method. The next item is the caller of that method, and so on.

stack_trace()

If stack tracing is enabled then this method returns a text string summarising the caller stack at the point at which the exception was thrown.

    use Badger::Exception trace => 1;

    eval {
        # some code that throws an exception object
        $exception->throw();
    };
    if ($@) {
        print $@->stack_trace;
    }

AUTHOR

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Andy Wardley http://wardley.org/

COPYRIGHT

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Copyright (C) 1996-2009 Andy Wardley. All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

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Badger::Base

Badger documentation Contained in the Badger distribution. 
#========================================================================
#
# Badger::Exception
#
# DESCRIPTION
#   Module implementing an exception class for reporting structured
#   errors.
# 
# AUTHOR
#   Andy Wardley <abw@wardley.org>
#
#========================================================================

package Badger::Exception;

use Badger::Class
    base        => 'Badger::Base',
    version     => 0.01,
    debug       => 0,
    mutators    => 'type',
    accessors   => 'stack',
    constants   => 'TRUE ARRAY HASH DELIMITER',
    import      => 'class',
    as_text     => 'text',      # overloaded text stringification
    is_true     => 1,           # always evaluates to a true value
    exports     => {
        hooks => {
            trace  => [ 
                # args are ($self, $target, $symbol, $value)
                sub { $TRACE = $_[3] }, 
                # expects one value argument
                1 
            ],
        },
    },
    messages => {
        caller => "<4> called from <1>\n  in <2> at line <3>",
    };

our $FORMAT  = '<type> error - <info>'  unless defined $FORMAT;
our $TYPE    = 'undef'                  unless defined $TYPE;
our $INFO    = 'no information'         unless defined $INFO;
our $ANON    = 'unknown'                unless defined $ANON;
our $TRACE   = 0                        unless defined $TRACE;


sub init {
    my ($self, $config) = @_;
    $self->{ type  } = $config->{ type  } || $self->class->any_var('TYPE');
    $self->{ info  } = $config->{ info  } || '';
    $self->{ file  } = $config->{ file  };
    $self->{ line  } = $config->{ line  };
    # watch out for the case where 'trace' is set explicitly to 0
    $self->{ trace } = 
        exists $config->{ trace } 
             ? $config->{ trace } 
             : $TRACE;

    return $self;
}


sub info {
    my $self = shift;
    return @_ 
        ? ($self->{ info }  = shift) 
        : ($self->{ info } || $INFO);
}


sub file {
    my $self = shift;
    return @_ 
        ? ($self->{ file }  = shift) 
        : ($self->{ file } || $ANON);
}


sub line {
    my $self = shift;
    return @_ 
        ? ($self->{ line }  = shift) 
        : ($self->{ line } || $ANON);
}


sub text {
    my $self = shift;
    my $text = shift || $self->class->any_var('FORMAT');

    # TODO: extend Badger::Utils::xprintf to handle this
    $text  =~ s/<(\w+)>/defined $self->{ $1 } ? $self->{ $1 } : "(no $1)"/eg;
    $text .= " in $self->{ file }"      if $self->{ file };
    $text .= " at line $self->{ line }" if $self->{ line };
    
    if ($self->{ trace } && (my $trace = $self->stack_trace)) {
        $text .= "\n" . $trace;
    }

    return $text;
}


sub stack_trace {
    my $self = shift;
    my @lines;

    if (my $stack = $self->{ stack }) {
        foreach my $caller (@$stack) {
            push(@lines, $self->message( caller => @$caller ));
        }
    }
    
    return join("\n", @lines);
}


sub trace {
    my $self = shift;
    if (ref $self) {
        return @_ 
            ? ($self->{ trace } = shift )
            :  $self->{ trace };
    }
    else {
        return @_
            ? $self->class->var( TRACE => shift )
            : $self->class->var('TRACE');
    }
}

sub throw {
    my $self = shift;

    # save relevant information from caller stack for enhanced debugging,
    # but only the first time the exception is thrown
    if ($self->{ trace } && ! $self->{ stack }) {
        my @stack;
        my $i = 1;
        while (1) {
            my @info = caller($i++);
            last unless @info;
            push(@stack, \@info);
        }
        $self->{ stack } = \@stack;
    }

    die $self;
}

    


#------------------------------------------------------------------------
# match_type(@types)
# 
# Selects the most appropriate handler for the current exception type, 
# from the list of types passed in as arguments.  The method returns the
# item which is an exact match for type or the closest, more 
# generic handler (e.g. foo being more generic than foo.bar, etc.)
#------------------------------------------------------------------------

sub match_type {
    my $self  = shift;
    my $types = @_ == 1 ? shift :  [@_];
    my $type  = $self->{ type };
    
    $types = [ split(DELIMITER, $types) ]
        unless ref $types;
        
    $types = { map { $_ => $_ } @$types }
        if ref $types eq ARRAY;
    
    return $self->error( invalid => 'type match' => $types )
        unless ref $types eq HASH;
        
    while ($type) {
        return $types->{ $type }
            if $types->{ $type };

        # strip .element from the end of the exception type to find a 
        # more generic handler
        $type =~ s/\.?[^\.]*$//;
    }
    
    return undef;
}



1;
__END__


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