NAME

   Syntax::Keyword::Defer - add defer block syntax to perl

SYNOPSIS

      use Syntax::Keyword::Defer;

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

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

         ...
      }

DESCRIPTION

   This module provides a syntax plugin that implements a block which
   executes when the containing scope has finished.

   It similar to features provided by other languages; Swift, Zig, Jai,
   Nim and Odin all provide this. Note that while Go also provides a defer
   keyword, the semantics here are not the same. Go's version defers until
   the end of the entire function, rather than the closest enclosing scope
   as is common to most other languages, and this module.

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

     * A defer 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 defer 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 defer 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.

   defer 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
   defer block was never encountered.

KEYWORDS

defer

      defer {
         STATEMENTS...
      }

   The defer keyword introduces a block which runs its code body at the
   time that its immediately surrounding code block finishes.

   When the defer 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
      {
         defer { say "The function has now returned"; }
         return 123;
      }

   If multiple defer 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.

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

   A defer block 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 defer statement, the code body
   does not need to run.

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

   Unlike as would happen with e.g. a DESTROY method on a guard object,
   any exceptions thrown from a defer block are still propagated up to the
   caller in the usual way.

      use Syntax::Keyword::Defer;

      sub f
      {
         my $count = 0;
         defer { $count or die "Failed to increment count"; }

         # some code here
      }

      f();


      $ perl example.pl
      Failed to increment count at examples.pl line 6.

   Because a defer block is a true block (e.g. in the same way something
   like an if () {...} block is), rather than an anonymous sub, it does
   not appear to caller() or other stack-inspection tricks. This is useful
   for calling croak(), for example.

      sub g
      {
         my $count = 0;
         defer { $count or croak "Expected some items"; }

         $count++ for @_;
      }

   Here, croak() will correctly report the caller of the g() function,
   rather than appearing to be called from an __ANON__ sub invoked at the
   end of the function itself.

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:

     * Try to fix the double-exception test failure on Perl versions
     before v5.20. (Test currently skipped on those versions)

     * Permit the use of goto or next/last/redo within defer blocks,
     provided it does not jump to a target outside.

     E.g. the following ought to be permitted, but currently is not:

        defer {
           foreach my $item (@items) {
              $item > 5 or next;
              ...
           }
        }

     * Try to detect and forbid nonlocal flow control (goto,
     next/last/redo) from leaving the defer block.

     E.g. currently the following will crash the interpreter:

        sub func { last ITEM }

        ITEM: foreach(1..10) {
           say;
           defer { func() }
        }

AUTHOR

   Paul Evans <[email protected]>

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