NAME

Syntax::Keyword::Finally - add FINALLY phaser block syntax to perl

SYNOPSIS

   use Syntax::Keyword::Finally;

   {
      my $dbh = DBI->connect( ... ) or die "Cannot connect";
      FINALLY { $dbh->disconnect; }

      my $sth = $dbh->prepare( ... ) or die "Cannot prepare";
      FINALLY { $sth->finish; }

      ...
   }

DESCRIPTION

This module provides a syntax plugin that implements a phaser block that executes its block when the containing scope has finished. The syntax of the FINALLY block looks similar to other phasers in perl (such as BEGIN), but the semantics of its execution are different.

The operation can be considered a little similar to an END block, but with the following key differences:

  • A FINALLY block runs at the time that execution leaves the block it is declared inside, whereas an END block runs at the end time of the entire program regardless of its location.

  • A FINALLY block is invoked at the time its containing scope has finished, which means it might run again if the block is entered again later in the program. An END block will only ever run once.

  • A FINALLY block will only take effect if execution reaches the line it is declared on; if the line is not reached then nothing happens. An END block will always be invoked once declared, regardless of the dynamic extent of execution at runtime.

FINALLY blocks are primarily intended for cases such as resource finalisation tasks that may be conditionally required.

For example in the synopsis code, after normal execution the statement handle will be finished using the $sth->finish method, then the database will be disconnected with $dbh->disconnect. If instead the prepare method failed then the database will still be disconnected, but there is no need to finish with the statement handle as the second FINALLY block was never encountered.

KEYWORDS

FINALLY

   FINALLY {
      STATEMENTS...
   }

The FINALLY keyword introduces a phaser block (similar to e.g. BEGIN and END), which runs its code body at the time that its immediately surrounding code block finishes.

When the FINALLY statement is encountered, the body of the code block is pushed to a queue of pending operations, which is then flushed when the surrounding block finishes for any reason - either by implicit fallthrough, or explicit termination by return, die or any of the loop control statements next, last or redo.

   sub f
   {
      FINALLY { say "The function has now returned"; }
      return 123;
   }

If multiple FINALLY statements appear within the same block, they are pushed to the queue in LIFO order; the last one encountered is the first one to be executed.

   {
      FINALLY { say "This happens second"; }
      FINALLY { say "This happens first"; }
   }

A FINALLY phaser will only take effect if the statement itself is actually encountered during normal execution. This is in direct contrast to an END phaser which always occurs. This makes it ideal for handling finalisation of a resource which was created on a nearby previous line, where the code to create it might have thrown an exception instead. Because the exception skipped over the FINALLY statement, the code body does not need to run.

   my $resource = Resource->open( ... );
   FINALLY { $resource->close; }

TODO

This module contains a unit test file copied and edited from my core perl branch to provide the same syntax. Several test cases are currently commented out because this implementation does not yet handle them:

  • Ensure that FINALLY blocks can throw exceptions.

  • Complain on attempts to return, goto, or next/last/redo out of a FINALLY block.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>