Sawyer X > Dancer > Dancer::Exception

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NAME ^

Dancer::Exception - class for throwing and catching exceptions

SYNOPSIS ^

    use Dancer::Exception qw(:all);

    register_exception('DataProblem',
                        message_pattern => "test message : %s"
                      );

    sub do_stuff {
      raise DataProblem => "we've lost data!";
    }

    try {
      do_stuff()
    } catch {
      # an exception was thrown
      my ($exception) = @_;
      if ($exception->does('DataProblem')) {
        # handle the data problem
        my $message = $exception->message();
      } else {
        $exception->rethrow
      }
    };

DESCRIPTION ^

Dancer::Exception is based on Try::Tiny. You can try and catch exceptions, like in Try::Tiny.

Exceptions are objects, from subclasses of Dancer::Exception::Base.

However, for internal Dancer usage, we introduce a special class of exceptions, called Dancer::Continuation. Exceptions that are from this class are not caught with a catch block, but only with a continuation. That's a cheap way to implement a workflow interruption. Dancer users should ignore this feature.

What it means for Dancer users

Users can throw and catch exceptions, using try and catch. They can reuse some Dancer core exceptions (Dancer::Exception::Base::*), but they can also create new exception classes, and use them for their own means. That way it's easy to use custom exceptions in a Dancer application. Have a look at register_exception, raise, and the methods in Dancer::Exception::Base.

METHODS ^

try

Same as in Try::Tiny

catch

Same as in Try::Tiny. The exception can be retrieved as the first parameter:

    try { ... } catch { my ($exception) = @_; };

continuation

To be used by Dancer developers only, in Dancer core code.

raise

  # raise Dancer::Exception::Base::Custom
  raise Custom => "user $username is unknown";

  # raise Dancer::Exception::Base::Custom::Frontend
  raise 'Custom::Frontend' => "user $username is unknown";

  # same, raise Dancer::Exception::Base::Custom::Frontend
  raise custom_frontend => "user $username is unknown";

  # raise My::Own::ExceptionSystem::Invalid::Login
  raise '+My::Own::ExceptionSystem::Invalid::Login' => "user $username is unknown";

raise provides an easy way to throw an exception. First parameter is the name of the exception class, without the Dancer::Exception:: prefix. other parameters are stored as raising arguments in the exception. Usually the parameters is an exception message, but it's left to the exception class implementation.

If the exception class name starts with a +, then the Dancer::Exception:: won't be added. This allows one to build their own exception class hierarchy, but you should first look at register_exception before implementing your own class hierarchy. If you really wish to build your own exception class hierarchy, we recommend that all exceptions inherit of Dancer::Exception::. Or at least it should implement its methods.

The exception class can also be written as words separated by underscores, it'll be camelized automatically. So 'Exception::Foo' and 'exception_foo' are equivalent. Be careful, 'MyException' can't be written 'myexception', as it would be camelized into 'Myexception'.

register_exception

This method allows one to register custom exceptions, usable by Dancer users in their route code (actually pretty much everywhere).

  # simple exception
  register_exception ('InvalidCredentials',
                      message_pattern => "invalid credentials : %s",
                     );

This registers a new custom exception. To use it, do:

  raise InvalidCredentials => "user Herbert not found";

The exception message can be retrieved with the $exception->message method, and we'll be "invalid credentials : user Herbert not found" (see methods in Dancer::Exception::Base)

  # complex exception
  register_exception ('InvalidLogin',
                      composed_from => [qw(Fatal InvalidCredentials)],
                      message_pattern => "wrong login or password",
                   );

In this example, the InvalidLogin is built as a composition of the Fatal and InvalidCredentials exceptions. See the does method in Dancer::Exception::Base.

registered_exceptions

  my @exception_classes = registered_exceptions;

Returns the list of exception class names. It will list core exceptions and custom exceptions (except the one you've registered with a leading +, see register_exception). The list is sorted.

GLOBAL VARIABLE ^

$Dancer::Exception::Verbose

When set to 1, exceptions will stringify with a long stack trace. This variable is similar to $Carp::Verbose. I recommend you use it like that:

  local $Dancer::Exception::Verbose;
  $Dancer::Exception::Verbose = 1;

All the Carp global variables can also be used to alter the stacktrace generation.

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