SYNOPSIS

    use Nice::Try;

    print( "Hello, I want to try\n" );
    # Try out {
    print( "this piece of code\n" );
    try 
    {
        # Not so sure }
        print( "I am trying!\n" );
        die( "Bye cruel world..." );
        # Never going to reach this
        return( 1 );
    }
    # Some comment
    catch( Exception $e ) {
        return( "Caught an exception $e" );
    }
    # More comment with space too

    catch( $e ) {
        print( "Got an error: $e\n" );
    }
    finally
    {
        print( "Cleaning up\n" );
    }
    print( "Ok, then\n" );

When run, this would produce, as one would expect:

    Hello, I want to try
    this piece of code
    I am trying!
    Got an error: Bye cruel world... at ./some/script.pl line 18.
    Cleaning up
    Ok, then

VERSION

    v0.2.0

DESCRIPTION

Nice::Try{.perl-module} is a lightweight implementation of Try-Catch exception trapping block using perl filter{.perl-module}. It behaves like you would expect.

Here is a list of its distinctive features:

• No routine to import like Nice::Try qw( try catch ). Just add use Nice::Try in your script

• Properly report the right line number for the original error message

• Allows embedded try-catch block within try-catch block, such as:

      use Nice::Try;
  
      print( "Wow, something went awry: ", &gotcha, "\n" );
  
      sub gotcha
      {
          print( "Hello, I want to try\n" );
          # Try out {
          CORE::say( 'this piece' );
          try 
          {
              # Not so sure }
              print( "I am trying!\n" );
              try
              {
                  die( "Bye cruel world..." );
                  return( 1 );
              }
              catch( $err )
              {
                  die( "Dying again with embedded error: '$err'" );
              }
          }
          catch( Exception $e ) {
              return( "Caught an exception \$e" );
          }
          catch( $e ) {
              try
              {
                  print( "Got an error: $e\n" );
                  print( "Trying something else.\n" );
                  die( "No really, dying out... with error: $e\n" );
              }
              catch( $err2 )
              {
                  return( "Returning from catch L2 with error '$err2'" );
              }
          }
          CORE::say( "Ok, then" );
      }
  

• No need for semicolon on the last closing brace

• It does not rely on perl regular expression, but instead uses PPI{.perl-module} (short for "Perl Parsing Interface").

• Variable assignment in the catch block works. For example:

      try
      {
          # Something or
          die( "Oops\n" );
      }
      catch( $funky_variable_name )
      {
          return( "Oh no: $funky_variable_name" );
      }
  

• $@ is always available too

• You can return a value from try-catch blocks, even with embedded try-catch blocks

• It recognises @_ inside try-catch blocks, so you can do something like:

      print( &gotme( 'Jacques' ), "\n" );
  
      sub gotme
      {
          try
          {
              print( "I am trying my best $_[0]!\n" );
              die( "But I failed\n" );
          }
          catch( $some_reason )
          {
              return( "Failed: $some_reason" );
          }
      }
  

  Would produce:

      I am trying my best Jacques!
      Failed: But I failed
  

• try or catch blocks can contain flow control keywords such as next, last and redo

      while( defined( my $product = $items->[++$i] ) )
      {
          try
          {
              # Do something
              last if( !$product->active );
          }
          catch( $oops )
          {
              log->( "Error: $oops" );
              last;
          }
      }
      continue
      {
          try
          {
              if( $product->region eq 'Asia' )
              {
                  push( @asia, $product );
              }
              else
              {
                  next;
              }
          }
          catch( $e )
          {
              $log->( "An unexpected error has occurred. Is $product an object? $e" );
              last;
          }
      }
  

• Can be used with or without a catch block

• Supports a finally block called in void context for cleanup for example

WHY USE IT?

There are quite a few implementations of try-catch blocks in perl, and they can be grouped in 4 categories:

1 Try-Catch as subroutines

: For example Try::Tiny{.perl-module}

2 Using Perl Filter

: For example Nice::Try{.perl-module}, Try::Harder{.perl-module}

3 Using Devel::Declare{.perl-module}

: For example TryCatch{.perl-module}

4 Others

: For example Syntax::Keyword::Try{.perl-module} and now perl with version 5.33 using experimental feature{.perl-module}.

Group 1 requires the use of semi-colons like:

    try
    {
        # Something
    }
    catch
    {
        # More code
    };

It also imports the subroutines try and catch in your namespace.

And you cannot do exception variable assignment like catch( $err )

In group 2, Try::Harder{.perl-module} does a very nice work, but relies on perl regular expression with Text::Balanced{.perl-module} and that makes it susceptible to failure if the try-catch block is not written as it expects it to be. For example if you put comments between try and catch, it would not work anymore. This is because parsing perl is famously difficult. Also, it does not do exception variable assignment, or catch filtered based on exception class like:

    try
    {
        # Something
        die( Exception->new( "Failed!" ) );
    }
    catch( Exception $e )
    {
        # Do something if exception is an Exception class
    }

See "die" in perlfunc{.perl-module} for more information on dying with an object.

Also Try::Harder{.perl-module} will die if you use only try with no catch, such as:

    use Try::Harder;
    try
    {
        die( "Oops\n" );
    }
    # Will never reach this
    print( "Got here with $@\n" );

In this example, the print line will never get executed. With Nice::Try{.perl-module} you can use try alone as an equivalent of "eval" in perlfunc{.perl-module} and the $@ will be available too. So:

    use Nice::Try;
    try
    {
        die( "Oops\n" );
    }
    print( "Got here with $@\n" );

will produces:

    Got here with Oops

In group 3, TryCatch{.perl-module} was working wonderfully, but was relying on Devel::Declare{.perl-module} which was doing some esoteric stuff and eventually the version 0.006020 broke TryCatch{.perl-module} and there seems to be no intention of correcting this breaking change.

In group 4, there is Syntax::Keyword::Try{.perl-module}, which is a great alternative if you do not care about exception variable assignment or exception class filter. You can only use $@

Since perl version 5.33.7{.perl-module} you can use the try-catch block using an experimental feature which may be removed in future versions, by writing:

    use feature 'try'; # will emit a warning this is experimental

This new feature supports try-catch block and variable assignment, but no exception class, nor support for finally block, so you can do:

    try
    {
        # Oh no!
        die( "Argh...\n" );
    }
    catch( $oh_well )
    {
        return( $self->error( "Something went awry: $oh_well" ) );
    }

But you cannot do:

    try
    {
        # Oh no!
        die( MyException->new( "Argh..." ) );
    }
    catch( MyException $oh_well )
    {
        return( $self->error( "Something went awry with MyException: $oh_well" ) );
    }
    # No support for 'finally' yet in perl version 5.33.7
    finally
    {
        # do some cleanup here
    }

It is probably a matter of time until this is fully implemented in perl as a regular non-experimental feature.

So, Nice::Try{.perl-module} is quite unique and fill the missing features, but because it is purely in perl and not an XS module, it is slower than XS module like Syntax::Keyword::Try{.perl-module}. I am not sure the difference would be that noticeable, since the parsing by PPI{.perl-module} is now done using an XS module, which makes things very fast.

FINALLY

Like with other language such as Java or JavaScript, the finally block will be executed even if the try or catch block contains a return statement.

This is useful to do some clean-up. For example:

    try
    {
        # Something worth dying
    }
    catch( $e )
    {
        return( "I failed: $e" );
    }
    finally
    {
        # Do some mop up
        # This would be reached even if catch already returned
        # Putting return statement here does not actually return anything.
        # This is only for clean-up
    }

However, because this is designed for clean-up, it is called in void context, so any return statement there will not actually return anything back to the caller.

CATCHING OR NOT CATCHING?

Nice::Try{.perl-module} can be used with a single try block which will, in effect, behaves like an eval and the special variable $@ will be available as always.

    try
    {
        die( "Oh no, something went wrong!\n" );
    }
    print( "Got here with $@\n" );

or even:

    try
    {
        die( "Oh no, something went wrong!\n" );
    }
    catch( $e ); # Not very meaningful, but it will work
    print( "Got here with $@\n" );

However, if you decide to catch class exceptions, make sure to add a default catch( $e ). For example:

    try
    {
        die( MyException->new( "Oh no" ) );
    }
    print( "Got here with $@\n" );

will work and print will display "Got here with Oh no". However:

    try
    {
        die( MyException->new( "Oh no" ) );
    }
    catch( Some::Exception $e )
    {
        # won't reach here
    }

will make your process die because of the exception not being caught, thus you might want to do instead:

    try
    {
        die( MyException->new( "Oh no" ) );
    }
    catch( Some::Exception $e )
    {
        # won't reach here
    }
    catch( $default )
    {
        print( "Got you! Error was: $default\n" );
    }

And the last catch will catch the exception.

Since, try-catch block can be nested, the following would work too:

    try
    {
        try
        {
            die( MyException->new( "Oh no" ) );
        }
        catch( Some::Exception $e )
        {
            # won't reach here
        }
    }
    catch( MyException $e )
    {
        print( "Got you! MyException was: $e\n" );
    }
    # to play it safe
    catch( $e )
    {
        # do something about it
    }

LOOPS

Since version v0.2.0 Nice::Try{.perl-module} supports the use of flow control keywords such as next, last and redo inside try-catch blocks. For example:

    my @names = qw( John Jack Peter Paul Mark );
    for( $i..$#names )
    {
        try
        {
            next if( $i == 2 );
            # some more code...
        }
        catch( $e )
        {
            print( "Got exception: $e\n" );
        }
    }

It also works inside the catch block or inside the continue block:

    while( defined( my $product = $items->[++$i] ) )
    {
        # Do something
    }
    continue
    {
        try
        {
            if( $product->region eq 'Asia' )
            {
                push( @asia, $product );
            }
            else
            {
                next;
            }
        }
        catch( $e )
        {
            $log->( "An unexpected error has occurred. Is $product an object? $e" );
            last;
        }
    }

Control flow with labels also work

    ELEM: foreach my $n ( @names )
    {
        try
        {
            $n->moveAfter( $this );
            next ELEM if( $n->value == 1234567 );
        }
        catch( $oops )
        {
            last ELEM;
        }
    }

However, if you enclose a try-catch block inside another block, use of next, last or redo will silently not work. This is due to perl control flow. See perlsyn{.perl-module} for more information on this. For example, the following would not yield the desired outcome:

    ELEM: foreach my $n ( @names )
    {
        { # <--- Here is the culprit
            try
            {
                $n->moveAfter( $this );
                # This next statement will not do anything.
                next ELEM if( $n->value == 1234567 );
            }
            catch( $oops )
            {
                # Neither would this one.
                last ELEM;
            }
        }
    }

DEBUGGING

And to have Nice::Try{.perl-module} save the filtered code to a file, pass it the debug_file parameter like this:

    use Nice::Try debug_file => './updated_script.pl';

You can also call your script using Filter::ExtractSource{.perl-module} like this:

    perl -MFilter::ExtractSource script.pl > updated_script.pl

or add use Filter::ExtractSource inside it.

In the updated script produced, you can add the line calling Nice::Try{.perl-module} to:

    use Nice::Try no_filter => 1;

to avoid Nice::Try{.perl-module} from filtering your script

If you want Nice::Try{.perl-module} to produce human readable code, pass it the debug_code parameter like this:

    use Nice::Try debug_code => 1;

CREDITS

Credits to Stephen R. Scaffidi for his implementation of Try::Harder{.perl-module} from which I borrowed some code.

AUTHOR

Jacques Deguest <jack@deguest.jp{classes="ARRAY(0x557b26b54a58)"}>

SEE ALSO

PPI{.perl-module}, Filter::Util::Call{.perl-module}, Try::Harder{.perl-module}, Syntax::Keyword::Try{.perl-module}, Exception::Class{.perl-module}

COPYRIGHT & LICENSE

Copyright (c) 2020-2021 DEGUEST Pte. Ltd.

You can use, copy, modify and redistribute this package and associated files under the same terms as Perl itself.