Context::Preserve5.12 man page on Darwin

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Context::Preserve(3)  User Contributed Perl Documentation Context::Preserve(3)

NAME
       Context::Preserve - run code after a subroutine call, preserving the
       context the subroutine would have seen if it were the last statement in
       the caller

SYNOPSIS
       Have you ever written this?

	   my ($result, @result);

	   # run a sub in the correct context
	   if(!defined wantarray){
	       some::code();
	   }
	   elsif(wantarray){
	       @result = some::code();
	   }
	   else {
	       $result = some::code();
	   }

	   # do something after some::code
	   $_ += 42 for (@result, $result);

	   # finally return the correct value
	   if(!defined wantarray){
	       return;
	   }
	   elsif(wantarray){
	       return @result;
	   }
	   else {
	       return $result;
	   }

       Now you can just write this instead:

	 use Context::Preserve;

	 return preserve_context { some::code() }
		    after => sub { $_ += 42 for @_ };

DESCRIPTION
       Sometimes you need to call a function, get the results, act on the
       results, then return the result of the function.	 This is painful
       because of contexts; the original function can behave different if it's
       called in void, scalar, or list context.	 You can ignore the various
       cases and just pick one, but that's fragile.  To do things right, you
       need to see which case you're being called in, and then call the
       function in that context.  This results in 3 code paths, which is a
       pain to type in (and maintain).

       This module automates the process.  You provide a coderef that is the
       "original function", and another coderef to run after the original
       runs.  You can modify the return value (aliased to @_) here, and do
       whatever else you need to do.  "wantarray" is correct inside both
       coderefs; in "after", though, the return value is ignored and the value
       "wantarray" returns is related to the context that the original
       function was called in.

EXPORT
       "preserve_context"

FUNCTIONS
   preserve_context { original } [after|replace] => sub { after }
       Invokes "original" in the same context as "preserve_context" was called
       in, save the results, runs "after" in the same context, then returns
       the result of "original" (or "after" if "replace" is used).

       If the second argument is "after", then you can modify @_ to affect the
       return value.  "after"'s return value is ignored.

       If the second argument is "replace", then modifying @_ doesn't do
       anything.  The return value of "after" is returned from
       "preserve_context" instead.

       Run "preserve_context" like this:

	 sub whatever {
	     ...
	     return preserve_context { orginal_function() }
			after => sub { modify @_	  };
	 }

	 or

	 sub whatever {
	     ...
	     return preserve_context   { orginal_function() }
			replace => sub { return @new_return };
	 }

       Note that there's no comma between the first block and the "after =>"
       part.  This is how perl parses functions with the "(&@)" prototype.
       The alternative is to say:

	     preserve_context(sub { original }, after => sub { after });

       You can pick the one you like, but I think the first version is much
       prettier.

AUTHOR AND COPYRIGHT
       Jonathan Rockway "<jrockway@cpan.org>"

       Copyright (c) 2008 Infinity Interactive.	 You may redistribute this
       module under the same terms as Perl itself.

perl v5.12.5			  2008-01-15		  Context::Preserve(3)
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