Perl Module to easily declare, throw and match exception objects
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    Err - Easily declare, throw and match exception objects

      use Err qw(declare_err throw_err is_err);

      ### create a bunch of exception classes ###

      # this makes Err::Exception::Starship, subclass of Err::Exception
      # (which itself is a subclass of Exception::Class::Basee)
      declare_err ".Starship"
        description => "The space ship is broken.";

      # this makes Err::Exception::Starship::WarpDrive
      # subclass of Err::Exception::Starship
      declare_err ".Starship.WarpDrive"
        description => "The warp drive is broken.  We can't go FTL.";

      ### throw and catch errors ###

      use Try::Tiny;
      try {
        throw_err ".Starship.WarpDrive", "Have ejected warp core!"
          if $have_ejected_the_warp_core;
      } catch {
        if (is_err ".Starship") {
          call_scotty(); return
        die $_;

    WARNING: This is an alpha release and the interface and functionailty
    may change without notice in future releases. A non-alpha 1.0 release
    will be released to the CPAN on or before August 1st, 2012.

    The module allows you to easily declare, throw and match exceptions.
    It's further syntatic sugar for Exception::Class. It doesn't provide a
    try/catch syntax but instead is designed to work well with plain old
    evals, Try::Tiny, TryCatch, etc.

    These module exports functions on demand, or you can call them fully

    declare_err EXCEPTION_CODE, @optinal_args
        Easy declarative syntax for defining exception class. EXCEPTION_CODE
        must be a literal quoted string. See "Declaring Exceptions" below
        for more details.

    throw_err EXCEPTION_CODE, $message, @optional_args
        Throws the exception with the attached message. EXCEPTION_CODE must
        be a literal quoted string. See "Throwing Exceptions" below for more

    ex_is_err EXCEPTION_CODE
        Functions to examine $_ and $@ respectively for exception objects.
        EXCEPTION_CODE must be a literal quoted string. See "Matching
        Exceptions" below for more details.

  Understanding Exception Codes
    An exception code is a brief form of the exception classname. They're
    provided as a way to visually distinguish exception classes from your
    normal class hierarchy.

    The rules to compute a class name from an exception code are blindingly

    Replace any leading dot with "Err::Exception::"
    Replace any other dot with "::"

    For example, the code ".Starship" refers to the
    "Err::Exception::Starship" class. The "Err::Exception::Aliens::Klingons"
    class has the code "Aliens.Klingons". The "Something.Else" code (no
    leading dot) refers to the "Something::Else".

    As a special case the code "" refers to anything that is a subclass of
    "Exception::Class::Base" itself, and "." refers to anything that is a
    subclass of "Err::Exception".

    If you don't like the exception code syntax you should note that under
    the above rules any valid class name will function as a valid exception
    code. (so for example the exception code "Foo::Bar" is the identical
    class name "Foo::Bar".)

  Declaring Exceptions
    Exceptions are declared with the "declare_err" syntax.

    In the simpliest form

    isa => $exception_code
        Explicitly set the superclass of the exception by passing in the
        exception code of the parent class. This isn't often needed, since
        by default "declare_err" will simply set the class to be the natural
        parent of the class (the class that's derived from removing the last
        ::Whatever from the classname or, if the exception's classname is
        singular, Err::Exception.)

    description => $description
        Set the description of this class to give a human readable
        description of this error that is applicable to all exceptions
        thrown of this class.

    anything_else => $some_value
        This becomes a new field in your exception object and provides a
        *default value* that "throw_err" will populate the exception with
        when exceptions are thrown.

    So, writing:

       declare_err ".Foo.Bar.Baz";

    Is the same as writing

       use Exception::Class {
          "Err::Exception::Foo::Bar::Baz" => {
             isa => "Err::Exception::Foo::Bar",


       declare_err ".Foo.Bar.Buzz";
          isa => ".Onomatopoeia",
          description => "*Bzzz* that's wrong",
          volume => 11;

    Is almost the same as writing

       use Exception::Class {
          "Err::Exception::Foo::Bar::Buzz" => {
             isa => "Err::Exception::Onomatopoeia",
             fields => ["volume"]

    And then remembering always to do

       Err::Exception::Foo::Bar::Buzz->throw( volume => 11 );

    When you throw it if you don't have an alternative volume to pass in.

  Throwing Exceptions
    Exceptions can be thrown with the "throw_err" keyword. This keyword
    takes the code for the exception followed by an error message as it's
    arguments, and essentially constructs a new instance of the exception
    and then throws it. In other words:

      throw_err ".Starship", "Self destruct sequence activated!";

    Is the the same as:

        message => "Self destruct sequence activated "

    You can also pass other name value pairs after the message, that will be
    passed through to the

      throw_err ".Foo.Bar.Buzz", "Time's up", volume => 4;

    These will be passed to Exception::Class::Base's throw method, meaning
    the above is the same as:

        message => "Time's up",
        volume => 4,

  Match Exceptions
    The "is_err" and "ex_is_err" can be used to check if $_ or $@ contain

    With plain old eval:

      eval {
        throw ".Starship.Holodeck", "Morarity has become sentient!";
      if (ex_is_err(".Starship.Holodeck")) {
      } elsif ($@) { die }

    With Try::Tiny

      try {
        throw ".Starship.Holodeck", "Morarity has become sentient!";
      } catch {
          if (is_err(".Starship.Holodeck")) {
          die $_;

    With TryCatch

      try {
        throw ".Starship.Holodeck", "Morarity has become sentient!";
      } catch ($e where { is_err(".StarShip.Holodeck") }) {

  Enforced Declaring of Exceptions Before Use
    If you have the B::CallChecker module installed (and I highly recommend
    you do) this module will check at compile time that the exceptions you
    throw and match with "throw_err", "is_err" and "ex_is_err" have been
    declared first. If you have not declared your exception class in at the
    point in your code where it's used Perl will not compile your class.

    For those of you that are interested, this is achieved by hooking the
    CHECK routine for these functions (and, for that matter "declare_err"
    too) and interspecing the OP tree to extract the first argument to them
    so we can check if it's been declared first. But you don't need to know
    that to use this module - it's all magic.

    If you don't have B::CallChecker installed you lose this checking
    functionality but your exception classes will otherwise remain fully

    Written by Mark Fowler <>

    Copyright OmniTI 2012. All Rights Rerserved.

    Copyright Mark Fowler 2012. All Rights Rerserved.

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

    None known. This module has a 100% test coverage (branch, statement and

    Bugs (or feature requests) should be reported via this distribution's
    CPAN RT queue. This can be found at

    You can also address issues by forking this distribution on github and
    sending pull requests. It can be found at

    Exception::Class - syntax for declaring, throwing and matching
    Err::Exception objects.

    Try::Tiny - simple improved try/catch syntax with little dependancies

    TryCatch - powerful dependancy heavy improved try/catch syntax