NAME

Data::Flow - Perl extension for simple-minded recipe-controlled build of data.

SYNOPSIS

  use Data::Flow;
  $recipes = { path  => { default => './MANIFEST'},
	       contents => { prerequisites => ['path', 'x'] ,
			     process => 
			     sub {
			       my $data = shift; 
			       $data->{ shift() } = `cat $data->get('path')`
				 x $data->get('x');
			     }
			   },
	     };

  $request = new Data::Flow $recipes;
  $request->set( x => 1);
  print $request->get('contents');

  tie %request, Data::Flow, $recipes;
  $request{x} = 1;
  print $request{contents};

DESCRIPTION

The module Data::Flow provides its services via objects. The objects may be obtained by the usual

$request = new Data::Flow $recipes;

paradigm. The argument $recipes is a hash reference, which provides the rules for request processing. The objects support two methods, set() and get(). The first one is used to provide input data for processing, the second one to obtain the output.

The unit of requested information is a field. The method set() takes a pair field => value, the method get() takes one argument: the field.

Every object is created without any fields filled, but it knows how to construct fields basing on other fields or some global into. This knowledge is provided in the argument $recipe of the new() function. This is a reference to a hash, keyed by fields. The values of this hash are hash references themselves, which describe how to acquire the field which is the corresponding key of the initial hash.

The internal hashes may have the following keys:

default

describes the default value for the key, if none is provided by set(). The value becomes the value of the field of the object. No additional processing is performed. Example:

default => $Config{installdir}

prerequisites

gives the fields which are needed for the construction of the given field. The corresponding value is an array references. The array contains the required fields.

If defaults did not satisfy the request for a field, but $recipe->{field}{prerequisites} exists, the required fields are build before any further processing is done. Example:

prerequisites => [ qw(prefix arch) ]

process

contains the rule to build the field. The value is a reference to a subroutine taking 2 arguments: the object $request, and the name of the required field. It is up to the subroutine to actually fill the corresponding field of $data, an error condition is raised if it did not. Example:

  process => sub { my $data = shift;
		   $data->set( time => localtime(time) ) }

output

the corresponing value has the same meaning as for process, but the return value of the subroutine is used as the value of the field. Example:

output => sub { localtime(time) }

filter

contains the rule to build the field basing on other fields. The value is a reference to an array. The first element of the array is a reference to a subroutine, the rest contains names of the fields. When the subroutine is called, the arguments are the values of fields of the object $request which appear in the array (in the same order). The return value of the subroutine is used as the value of the field. Example:

  filter => [ sub { shift + shift }, 
	      'first_half', 'second_half' ]

Note that the mentioned field will be automatically marked as prerequisites.

class_filter

is similar to filter, but the first argument is the name of the method to call, second one is the name of the package to use for the method invocation. The rest contains names of field to provide as method arguments. Example:

class_filter => [ 'new', 'FileHandle', 'filename' ]

method_filter

is similar to class_filter, but the second argument is the name of the field which is used to call the method upon. Example:

method_filter => [ 'show', 'widget_name', 'current_display' ]

Tied interface

The access to the same functionality is available via tied hash interface.

AUTHOR

Ilya Zakharevich, ilya@math.ohio-state.edu

SEE ALSO

perl(1), make(1).