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#  You may distribute under the terms of either the GNU General Public License
#  or the Artistic License (the same terms as Perl itself)
#
#  (C) Paul Evans, 2013-2016 -- leonerd@leonerd.org.uk

package Future::Utils;

use strict;
use warnings;

our $VERSION = '0.34';

use Exporter 'import';
# Can't import the one from Exporter as it relies on package inheritance
sub export_to_level
{
   my $pkg = shift; local $Exporter::ExportLevel = 1 + shift; $pkg->import(@_);
}

our @EXPORT_OK = qw(
   call
   call_with_escape

   repeat
   try_repeat try_repeat_until_success
   repeat_until_success

   fmap  fmap_concat
   fmap1 fmap_scalar
   fmap0 fmap_void
);

use Carp;
our @CARP_NOT = qw( Future );

use Future;

=head1 NAME

C<Future::Utils> - utility functions for working with C<Future> objects

=head1 SYNOPSIS

 use Future::Utils qw( call_with_escape );

 my $result_f = call_with_escape {
    my $escape_f = shift;
    my $f = ...
       $escape_f->done( "immediate result" );
       ...
 };

Z<>

 use Future::Utils qw( repeat try_repeat try_repeat_until_success );

 my $eventual_f = repeat {
    my $trial_f = ...
    return $trial_f;
 } while => sub { my $f = shift; return want_more($f) };

 my $eventual_f = repeat {
    ...
    return $trail_f;
 } until => sub { my $f = shift; return acceptable($f) };

 my $eventual_f = repeat {
    my $item = shift;
    ...
    return $trial_f;
 } foreach => \@items;

 my $eventual_f = try_repeat {
    my $trial_f = ...
    return $trial_f;
 } while => sub { ... };

 my $eventual_f = try_repeat_until_success {
    ...
    return $trial_f;
 };

 my $eventual_f = try_repeat_until_success {
    my $item = shift;
    ...
    return $trial_f;
 } foreach => \@items;

Z<>

 use Future::Utils qw( fmap_concat fmap_scalar fmap_void );

 my $result_f = fmap_concat {
    my $item = shift;
    ...
    return $item_f;
 } foreach => \@items, concurrent => 4;

 my $result_f = fmap_scalar {
    my $item = shift;
    ...
    return $item_f;
 } foreach => \@items, concurrent => 8;

 my $done_f = fmap_void {
    my $item = shift;
    ...
    return $item_f;
 } foreach => \@items, concurrent => 10;

Unless otherwise noted, the following functions require at least version
I<0.08>.

=cut

=head1 INVOKING A BLOCK OF CODE

=head2 call

   $f = call { CODE }

I<Since version 0.22.>

The C<call> function invokes a block of code that returns a future, and simply
returns the future it returned. The code is wrapped in an C<eval {}> block, so
that if it throws an exception this is turned into an immediate failed
C<Future>. If the code does not return a C<Future>, then an immediate failed
C<Future> instead.

(This is equivalent to using C<< Future->call >>, but is duplicated here for
completeness).

=cut

sub call(&)
{
   my ( $code ) = @_;
   return Future->call( $code );
}

=head2 call_with_escape

   $f = call_with_escape { CODE }

I<Since version 0.22.>

The C<call_with_escape> function invokes a block of code that returns a
future, and passes in a separate future (called here an "escape future").
Normally this is equivalent to the simple C<call> function. However, if the
code captures this future and completes it by calling C<done> or C<fail> on
it, the future returned by C<call_with_escape> immediately completes with this
result, and the future returned by the code itself is cancelled.

This can be used to implement short-circuit return from an iterating loop or
complex sequence of code, or immediate fail that bypasses failure handling
logic in the code itself, or several other code patterns.

 $f = $code->( $escape_f )

(This can be considered similar to C<call-with-escape-continuation> as found
in some Scheme implementations).

=cut

sub call_with_escape(&)

local/lib/perl5/Future/Utils.pm  view on Meta::CPAN

   return $future;
}

=head2 try_repeat

   $future = try_repeat { CODE } ...

I<Since version 0.18.>

A variant of C<repeat> that doesn't warn when the trial fails and the
condition code asks for it to be repeated.

In some later version the C<repeat> function will be changed so that if a
trial future fails, then the eventual future will immediately fail as well,
making its semantics a little closer to that of a C<while {}> loop in Perl.
Code that specifically wishes to catch failures in trial futures and retry
the block should use C<try_repeat> specifically.

=cut

sub try_repeat(&@)
{
   # defeat prototype
   &repeat( @_, try => 1 );
}

=head2 try_repeat_until_success

   $future = try_repeat_until_success { CODE } ...

I<Since version 0.18.>

A shortcut to calling C<try_repeat> with an ending condition that simply tests
for a successful result from a future. May be combined with C<foreach> or
C<generate>.

This function used to be called C<repeat_until_success>, and is currently
aliased as this name as well.

=cut

sub try_repeat_until_success(&@)
{
   my $code = shift;
   my %args = @_;

   # TODO: maybe merge while/until conditions one day...
   defined($args{while}) or defined($args{until})
      and croak "Cannot pass 'while' or 'until' to try_repeat_until_success";

   # defeat prototype
   &try_repeat( $code, while => sub { shift->failure }, %args );
}

# Legacy name
*repeat_until_success = \&try_repeat_until_success;

=head1 APPLYING A FUNCTION TO A LIST

The C<fmap> family of functions provide a way to call a block of code that
returns a L<Future> (called here an "item future") once per item in a given
list, or returned by a generator function. The C<fmap*> functions themselves
return a C<Future> to represent the ongoing operation, which completes when
every item's future has completed.

While this behaviour can also be implemented using C<repeat>, the main reason
to use an C<fmap> function is that the individual item operations are
considered as independent, and thus more than one can be outstanding
concurrently. An argument can be passed to the function to indicate how many
items to start initially, and thereafter it will keep that many of them
running concurrently until all of the items are done, or until any of them
fail. If an individual item future fails, the overall result future will be
marked as failing with the same failure, and any other pending item futures
that are outstanding at the time will be cancelled.

The following named arguments are common to each C<fmap*> function:

=over 8

=item foreach => ARRAY

Provides the list of items to iterate over, as an C<ARRAY> reference.

The referenced array will be modified by this operation, C<shift>ing one item
from it each time. The can C<push> more items to this array as it runs, and
they will be included in the iteration.

=item generate => CODE

Provides the list of items to iterate over, by calling the generator function
once for each required item. The function should return a single item, or an
empty list to indicate it has no more items.

 ( $item ) = $generate->()

This function will be invoked each time any previous item future has completed
and may be called again even after it has returned empty.

=item concurrent => INT

Gives the number of item futures to keep outstanding. By default this value
will be 1 (i.e. no concurrency); larger values indicate that multiple item
futures will be started at once.

=item return => Future

Normally, a new instance is returned by cloning the first non-immediate future
returned as an item future. By passing a new instance as the C<return>
argument, the result will be put into the given instance. This can be used to
return subclasses, or specific instances.

=back

In each case, the main code block will be called once for each item in the
list, passing in the item as the only argument:

 $item_f = $code->( $item )

The expected return value from each item's future, and the value returned from
the result future will differ in each function's case; they are documented
below.

For similarity with perl's core C<map> function, the item is also available
aliased as C<$_>.

=cut

# This function is invoked in two circumstances:
#  a) to create an item Future in a slot,
#  b) once a non-immediate item Future is complete, to check its results
# It can tell which circumstance by whether the slot itself is defined or not
sub _fmap_slot
{
   my ( $slots, undef, $code, $generator, $collect, $results, $return ) = @_;

   SLOT: while(1) {
      # Capture args each call because we mutate them
      my ( undef, $idx ) = my @args = @_;

      unless( $slots->[$idx] ) {
         # No item Future yet (case a), so create one
         my $item;
         unless( ( $item ) = $generator->() ) {
            # All out of items, so now just wait for the slots to be finished
            undef $slots->[$idx];
            defined and return $return for @$slots;

            # All the slots are done
            $return ||= Future->new;

            $return->done( @$results );
            return $return;
         }

         my $f = $slots->[$idx] = Future->call( $code, local $_ = $item );

         if( $collect eq "array" ) {
            push @$results, my $r = [];
            $f->on_done( sub { @$r = @_ });
         }
         elsif( $collect eq "scalar" ) {
            push @$results, undef;
            my $r = \$results->[-1];
            $f->on_done( sub { $$r = $_[0] });
         }
      }

      my $f = $slots->[$idx];

      # Slot is non-immediate; arrange for us to be invoked again later when it's ready
      if( !$f->is_ready ) {
         $args[-1] = ( $return ||= $f->new );
         $f->on_done( sub { _fmap_slot( @args ) } );
         $f->on_fail( $return );

         # Try looking for more that might be ready
         my $i = $idx + 1;
         while( $i != $idx ) {
            $i++;
            $i %= @$slots;
            next if defined $slots->[$i];

            $_[1] = $i;
            redo SLOT;
         }
         return $return;
      }

      # Either we've been invoked again (case b), or the immediate Future was
      # already ready.
      if( $f->failure ) {
         $return ||= $f->new;
         $return->fail( $f->failure );
         return $return;
      }

      undef $slots->[$idx];
      # next
   }
}

sub _fmap
{
   my $code = shift;
   my %args = @_;

   my $concurrent = $args{concurrent} || 1;
   my @slots;

   my $results = [];
   my $future = $args{return};

   my $generator;
   if( $generator = $args{generate} ) {
      # OK
   }
   elsif( my $array = $args{foreach} ) {
      $generator = sub { return unless @$array; shift @$array };
   }
   else {
      croak "Expected either 'generate' or 'foreach'";
   }

   # If any of these immediately fail, don't bother continuing
   foreach my $idx ( 0 .. $concurrent-1 ) {
      $future = _fmap_slot( \@slots, $idx, $code, $generator, $args{collect}, $results, $future );
      last if $future->is_ready;
   }

   $future->on_fail( sub {
      !defined $_ or $_->is_ready or $_->cancel for @slots;
   });
   $future->on_cancel( sub {
      $_->cancel for @slots;
   });

   return $future;
}

=head2 fmap_concat

   $future = fmap_concat { CODE } ...

I<Since version 0.14.>

This version of C<fmap> expects each item future to return a list of zero or
more values, and the overall result will be the concatenation of all these
results. It acts like a future-based equivalent to Perl's C<map> operator.

The results are returned in the order of the original input values, not in the
order their futures complete in. Because of the intermediate storage of
C<ARRAY> references and final flattening operation used to implement this
behaviour, this function is slightly less efficient than C<fmap_scalar> or
C<fmap_void> in cases where item futures are expected only ever to return one,
or zero values, respectively.

This function is also available under the name of simply C<fmap> to emphasise
its similarity to perl's C<map> keyword.

=cut

sub fmap_concat(&@)
{
   my $code = shift;
   my %args = @_;

   _fmap( $code, %args, collect => "array" )->then( sub {
      return Future->done( map { @$_ } @_ );
   });
}
*fmap = \&fmap_concat;

=head2 fmap_scalar

   $future = fmap_scalar { CODE } ...

I<Since version 0.14.>

This version of C<fmap> acts more like the C<map> functions found in Scheme or
Haskell; it expects that each item future returns only one value, and the
overall result will be a list containing these, in order of the original input
items. If an item future returns more than one value the others will be
discarded. If it returns no value, then C<undef> will be substituted in its
place so that the result list remains in correspondence with the input list.

This function is also available under the shorter name of C<fmap1>.

=cut

sub fmap_scalar(&@)
{
   my $code = shift;
   my %args = @_;

   _fmap( $code, %args, collect => "scalar" )
}
*fmap1 = \&fmap_scalar;

=head2 fmap_void

   $future = fmap_void { CODE } ...

I<Since version 0.14.>

This version of C<fmap> does not collect any results from its item futures, it
simply waits for them all to complete. Its result future will provide no
values.

While not a map in the strictest sense, this variant is still useful as a way
to control concurrency of a function call iterating over a list of items,
obtaining its results by some other means (such as side-effects on captured
variables, or some external system).

This function is also available under the shorter name of C<fmap0>.

=cut

sub fmap_void(&@)
{
   my $code = shift;
   my %args = @_;

   _fmap( $code, %args, collect => "void" )
}
*fmap0 = \&fmap_void;

=head1 AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

=cut

0x55AA;