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->{'path'}`
                                        x $data->{'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 three methods, set(), get(), aget(), and already_set().
       The first one is used to provide input data for processing, the second one to obtain the
       output. The third one to obtain a reference to an array with results of repeated get(),
       and the last one to query whether a field is already known.

       The unit of requested information is a field. The method set() takes a pair "field =>
       value", the methods get() and already_set() take one argument: the "field", and the method
       aget() takes multiple fields.

       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 reference to a hash with all the fields which have been
               set, and the name of the required field. It is up to the subroutine to actually
               fill the corresponding field of the hash, an error condition is raised if it did
               not. Example:

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

       "oo_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 $request, an
               error condition is raised if it did not. Example:

                 oo_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) }

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

                 output => sub { my $self = shift; $self->get('r') . 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.

       "self_filter"
               is similar to "filter", but an extra argument, the object itself, is put in front
               of the list of arguments.  Example:

                 self_filter => [ sub { my ($self, $first_half = (shift, shift);
                                        $first_half *= -$self->get('total')*100
                                          if $first_half < 0;  # negative means percentage
                                        $first_half + shift },
                             'first_half', 'second_half' ]

       "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, cpan@ilyaz.org, with multiple additions from Terrence Monroe Brannon and
       Radoslav Nedyalkov.

SEE ALSO

       perl(1), make(1).