Hook::LexWrap(3)					User Contributed Perl Documentation					  Hook::LexWrap(3)

NAME

       Hook::LexWrap - Lexically scoped subroutine wrappers

VERSION

       This document describes version 0.23 of Hook::LexWrap.

SYNOPSIS

	       use Hook::LexWrap;

	       sub doit { print "[doit:", caller, "]"; return {my=>"data"} }

	       SCOPED: {
		       wrap doit,
			       pre  => sub { print "[pre1: @_]
" },
			       post => sub { print "[post1:@_]
"; $_[1]=9; };

		       my $temporarily = wrap doit,
			       post => sub { print "[post2:@_]
" },
			       pre  => sub { print "[pre2: @_]
  "};

		       @args = (1,2,3);
		       doit(@args);    # pre2->pre1->doit->post1->post2
	       }

	       @args = (4,5,6);
	       doit(@args);	       # pre1->doit->post1

DESCRIPTION

       Hook::LexWrap allows you to install a pre- or post-wrapper (or both) around an existing subroutine. Unlike other modules that provide this
       capacity (e.g. Hook::PreAndPost and Hook::WrapSub), Hook::LexWrap implements wrappers in such a way that the standard "caller" function
       works correctly within the wrapped subroutine.

       To install a prewrappers, you write:

	       use Hook::LexWrap;

	       wrap 'subroutine_name', pre => &some_other_sub;

	  #or: wrap *subroutine_name,  pre => &some_other_sub;

       The first argument to "wrap" is a string containing the name of the subroutine to be wrapped (or the typeglob containing it, or a reference
       to it). The subroutine name may be qualified, and the subroutine must already be defined. The second argument indicates the type of wrapper
       being applied and must be either 'pre' or 'post'. The third argument must be a reference to a subroutine that implements the wrapper.

       To install a post-wrapper, you write:

	       wrap 'subroutine_name', post => &yet_another_sub;

	  #or: wrap *subroutine_name,  post => &yet_another_sub;

       To install both at once:

	       wrap 'subroutine_name',
		    pre  => &some_other_sub,
		    post => &yet_another_sub;

       or:

	       wrap *subroutine_name,
		    post => &yet_another_sub,	# order in which wrappers are
		    pre  => &some_other_sub;	# specified doesn't matter

       Once they are installed, the pre- and post-wrappers will be called before and after the subroutine itself, and will be passed the same
       argument list.

       The pre- and post-wrappers and the original subroutine also all see the same (correct!) values from "caller" and "wantarray".

   Short-circuiting and long-circuiting return values
       The pre- and post-wrappers both receive an extra argument in their @_ arrays. That extra argument is appended to the original argument list
       (i.e. is can always be accessed as $_[-1]) and acts as a place-holder for the original subroutine's return value.

       In a pre-wrapper, $_[-1] is -- for obvious reasons -- "undef". However, $_[-1] may be assigned to in a pre-wrapper, in which case
       Hook::LexWrap assumes that the original subroutine has been "pre-empted", and that neither it, nor the corresponding post-wrapper, nor any
       wrappers that were applied before the pre-empting pre-wrapper was installed, need be run. Note that any post-wrappers that were installed
       after the pre-empting pre-wrapper was installed will still be called before the original subroutine call returns.

       In a post-wrapper, $_[-1] contains the return value produced by the wrapped subroutine. In a scalar return context, this value is the
       scalar return value. In an list return context, this value is a reference to the array of return values. $_[-1] may be assigned to in a
       post-wrapper, and this changes the return value accordingly.

       Access to the arguments and return value is useful for implementing techniques such as memoization:

	       my %cache;
	       wrap fibonacci,
		       pre  => sub { $_[-1] = $cache{$_[0]} if $cache{$_[0]} },
		       post => sub { $cache{$_[0]} = $_[-1] };

       or for converting arguments and return values in a consistent manner:

	       # set_temp expects and returns degrees Fahrenheit,
	       # but we want to use Celsius
	       wrap set_temp,
		       pre   => sub { splice @_, 0, 1, $_[0] * 1.8 + 32 },
		       post  => sub { $_[-1] = ($_[0] - 32) / 1.8 };

   Lexically scoped wrappers
       Normally, any wrappers installed by "wrap" remain attached to the subroutine until it is undefined. However, it is possible to make
       specific wrappers lexically bound, so that they operate only until the end of the scope in which they're created (or until some other
       specific point in the code).

       If "wrap" is called in a non-void context:

	       my $lexical = wrap 'sub_name', pre => &wrapper;

       it returns a special object corresponding to the particular wrapper being placed around the original subroutine. When that object is
       destroyed -- when its container variable goes out of scope, or when its reference count otherwise falls to zero (e.g. "undef $lexical"), or
       when it is explicitly destroyed ("$lexical->DESTROY") -- the corresponding wrapper is removed from around the original subroutine. Note,
       however, that all other wrappers around the subroutine are preserved.

   Anonymous wrappers
       If the subroutine to be wrapped is passed as a reference (rather than by name or by typeglob), "wrap" does not install the wrappers around
       the original subroutine. Instead it generates a new subroutine which acts as if it were the original with those wrappers around it.  It
       then returns a reference to that new subroutine. Only calls to the original through that wrapped reference invoke the wrappers. Direct by-
       name calls to the original, or calls through another reference, do not.

       If the original is subsequently wrapped by name, the anonymously wrapped subroutine reference does not see those wrappers. In other words,
       wrappers installed via a subroutine reference are completely independent of those installed via the subroutine's name (or typeglob).

       For example:

	       sub original { print "ray" }

	       # Wrap anonymously...
	       my $anon_wrapped = wrap &original, pre => sub { print "do..." };

	       # Show effects...
	       original();	       # prints "ray"
	       $anon_wrapped->();      # prints "do..ray"

	       # Wrap nonymously...
	       wrap *original,
		       pre  => sub { print "fa.." },
		       post => sub { print "..mi" };

	       # Show effects...
	       original();	       #   now prints "fa..ray..mi"
	       $anon_wrapped->();      # still prints "do...ray"

DIAGNOSTICS

       "Can't wrap non-existent subroutine %s"
	   An attempt was made to wrap a subroutine that was not defined at the point of wrapping.

       "'pre' value is not a subroutine reference"
	   The value passed to "wrap" after the 'pre' flag was not a subroutine reference. Typically, someone forgot the "sub" on the anonymous
	   subroutine:

		   wrap 'subname', pre => { your_code_here() };

	   and Perl interpreted the last argument as a hash constructor.

       "'post' value is not a subroutine reference"
	   The value passed to "wrap" after the 'post' flag was not a subroutine reference.

       "Uselessly wrapped subroutine reference in void context" (warning only)
	   When the subroutine to be wrapped is passed as a subroutine reference, "wrap" does not install the wrapper around the original, but
	   instead returns a reference to a subroutine which wraps the original (see "Anonymous wrappers").

	   However, there's no point in doing this if you don't catch the resulting subroutine reference.

AUTHOR

       Damian Conway (damian@conway.org)

BLAME

       Schwern made me do this (by implying it wasn't possible ;-)

BUGS

       There are undoubtedly serious bugs lurking somewhere in code this funky :-)

       Bug reports and other feedback are most welcome.

SEE ALSO

       Sub::Prepend

COPYRIGHT

	     Copyright (c) 2001, Damian Conway. All Rights Reserved.
	   This module is free software. It may be used, redistributed
	       and/or modified under the same terms as Perl itself.

perl v5.16.3							    2010-09-24							  Hook::LexWrap(3)