path: root/lib/Future/Phrasebook.pod

#  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, 2013-2014 -- leonerd@leonerd.org.uk

=head1 NAME

C<Future::Phrasebook> - coding examples for C<Future> and C<Future::Utils>

This documentation-only module provides a phrasebook-like approach to giving
examples on how to use L<Future> and L<Future::Utils> to structure
Future-driven asynchronous or concurrent logic. As with any inter-dialect
phrasebook it is structured into pairs of examples; each given first in a
traditional call/return Perl style, and second in a style using Futures. In
each case, the generic function or functions in the example are named in
C<ALL_CAPITALS()> to make them stand out.

In the examples showing use of Futures, any function that is expected to
return a C<Future> instance is named with a leading C<F_> prefix. Each example
is also constructed so as to yield an overall future in a variable called
C<$f>, which represents the entire operation.

=head1 SEQUENCING

The simplest example of a sequencing operation is simply running one piece of
code, then immediately running a second. In call/return code we can just place
one after the other.

 FIRST();
 SECOND();

Using a Future it is necessary to await the result of the first C<Future>
before calling the second.

 my $f = F_FIRST()
    ->then( sub { F_SECOND(); } );

Here, the anonymous closure is invoked once the C<Future> returned by
C<F_FIRST()> succeeds. Because C<then> invokes the code block only if the
first Future succeeds, it shortcircuits around failures similar to the way that
C<die()> shortcircuits around thrown exceptions. A C<Future> representing the
entire combination is returned by the method.

Because the C<then> method itself returns a C<Future> representing the
overall operation, it can itself be further chained.

 FIRST();
 SECOND();
 THIRD();

Z<>

 my $f = F_FIRST()
    ->then( sub { F_SECOND(); } )
    ->then( sub { F_THIRD(); } );

See below for examples of ways to handle exceptions.

=head2 Passing Results

Often the result of one function can be passed as an argument to another
function.

 OUTER( INNER() );

The result of the first C<Future> is passed into the code block given to the
C<then> method.

 my $f = F_INNER()
    ->then( sub { F_OUTER( @_ ) } );

=head1 CONDITIONALS

It may be that the result of one function call is used to determine whether or
not another operation is taken.

 if( COND() == $value ) {
    ACTION();
 }

Because the C<then_with_f> code block is given the first future in addition to
its results it can decide whether to call the second function to return a new
future, or simply return the one it was given.

 my $f = F_COND()
    ->then_with_f( sub {
       my ( $f_cond, $result ) = @_;
       if( $result == $value ) {
          return F_ACTION();
       }
       else {
          return $f_cond;
       }
    });

=head1 EXCEPTION HANDLING

In regular call/return style code, if any function throws an exception, the
remainder of the block is not executed, the containing C<try> or C<eval> is
aborted, and control is passed to the corresponding C<catch> or line after the
C<eval>.

 try {
    FIRST();
 }
 catch {
    my $e = $_;
    ERROR( $e );
 };

The C<else> method on a C<Future> can be used here. It behaves similar to
C<then>, but is only invoked if the initial C<Future> fails; not if it
succeeds.

 my $f = F_FIRST()
    ->else( sub { F_ERROR( @_ ); } );

Alternatively, the second argument to the C<then> method can be applied, which
is invoked only on case of failure.

 my $f = F_FIRST()
    ->then( undef, sub { F_ERROR( @_ ); } );

Often it may be the case that the failure-handling code is in fact immediate,
and doesn't return a C<Future>. In that case, the C<else> code block can
return an immediate C<Future> instance.

 my $f = F_FIRST()
    ->else( sub {
       ERROR( @_ );
       return Future->done;
    });

Sometimes the failure handling code simply needs to be aware of the failure,
but rethrow it further up.

 try {
    FIRST();
 }
 catch {
    my $e = $_;
    ERROR( $e );
    die $e;
 };

In this case, while the C<else> block could return a new C<Future> failed with
the same exception, the C<else_with_f> block is passed the failed C<Future>
itself in addition to the failure details so it can just return that.

 my $f = F_FIRST()
    ->else_with_f( sub {
       my ( $f1, @failure ) = @_;
       ERROR( @failure );
       return $f1;
    });

The C<followed_by> method is similar again, though it invokes the code block
regardless of the success or failure of the initial C<Future>. It can be used
to create C<finally> semantics. By returning the C<Future> instance that it
was passed, the C<followed_by> code ensures it doesn't affect the result of
the operation.

 try {
    FIRST();
 }
 catch {
    ERROR( $_ );
 }
 finally {
    CLEANUP();
 };

Z<>

 my $f = F_FIRST()
    ->else( sub {
       ERROR( @_ );
       return Future->done;
    })
    ->followed_by( sub {
       CLEANUP();
       return shift;
    });

=head1 ITERATION

To repeat a single block of code multiple times, a C<while> block is often
used.

 while( COND() ) {
    FUNC();
 }

The C<Future::Utils::repeat> function can be used to repeatedly iterate a
given C<Future>-returning block of code until its ending condition is
satisfied.

 use Future::Utils qw( repeat );
 my $f = repeat {
    F_FUNC();
 } while => sub { COND() };

Unlike the statement nature of perl's C<while> block, this C<repeat> C<Future>
can yield a value; the value returned by C<< $f->get >> is the result of the
final trial of the code block.

Here, the condition function it expected to return its result immediately. If
the repeat condition function itself returns a C<Future>, it can be combined
along with the loop body. The trial C<Future> returned by the code block is
passed to the C<while> condition function.

 my $f = repeat {
    F_FUNC()
       ->followed_by( sub { F_COND(); } );
 } while => sub { shift->get };

The condition can be negated by using C<until> instead

 until( HALTING_COND() ) {
    FUNC();
 }

Z<>

 my $f = repeat {
    F_FUNC();
 } until => sub { HALTING_COND() };

=head2 Iterating with Exceptions

Technically, this loop isn't quite the same as the equivalent C<while> loop in
plain Perl, because the C<while> loop will also stop executing if the code
within it throws an exception. This can be handled in C<repeat> by testing for
a failed C<Future> in the C<until> condition.

 while(1) {
    TRIAL();
 }

Z<>

 my $f = repeat {
    F_TRIAL();
 } until => sub { shift->failure };

When a repeat loop is required to retry a failure, the C<try_repeat> function
should be used. Currently this function behaves equivalently to C<repeat>,
except that it will not print a warning if it is asked to retry after a
failure, whereas this behaviour is now deprecated for the regular C<repeat>
function so that yields a warning.

 my $f = try_repeat {
    F_TRIAL();
 } while => sub { shift->failure };

Another variation is the C<try_repeat_until_success> function, which provides
a convenient shortcut to calling C<try_repeat> with a condition that makes
another attempt each time the previous one fails; stopping once it achieves a
successful result.

 while(1) {
    eval { TRIAL(); 1 } and last;
 }

Z<>

 my $f = try_repeat_until_success {
    F_TRIAL();
 };

=head2 Iterating over a List

A variation on the idea of the C<while> loop is the C<foreach> loop; a loop
that executes once for each item in a given list, with a variable set to one
value from that list each time.

 foreach my $thing ( @THINGS ) {
    INSPECT( $thing );
 }

This can be performed with C<Future> using the C<foreach> parameter to the
C<repeat> function. When this is in effect, the block of code is passed each
item of the given list as the first parameter.

 my $f = repeat {
    my $thing = shift;
    F_INSPECT( $thing );
 } foreach => \@THINGS;

=head2 Recursing over a Tree

A regular call/return function can use recursion to walk over a tree-shaped
structure, where each item yields a list of child items.

 sub WALK
 {
    my ( $item ) = @_;
    ...
    WALK($_) foreach CHILDREN($item);
 }

This recursive structure can be turned into a C<while()>-based repeat loop by
using an array to store the remaining items to walk into, instead of using the
perl stack directly:

 sub WALK
 {
    my @more = ( $root );
    while( @more ) {
       my $item = shift @more;
       ...
       unshift @more, CHILDREN($item)
    }
 }

This arrangement then allows us to use C<fmap_void> to walk this structure
using Futures, possibly concurrently. A lexical array variable is captured
that holds the stack of remaining items, which is captured by the item code so
it can C<unshift> more into it, while also being used as the actual C<fmap>
control array.

 my @more = ( $root );

 my $f = fmap_void {
    my $item = shift;
    ...->on_done( sub {
       unshift @more, @CHILDREN;
    })
 } foreach => \@more;

By choosing to either C<unshift> or C<push> more items onto this list, the
tree can be walked in either depth-first or breadth-first order.

=head1 SHORT-CIRCUITING

Sometimes a result is determined that should be returned through several
levels of control structure. Regular Perl code has such keywords as C<return>
to return a value from a function immediately, or C<last> for immediately
stopping execution of a loop.

 sub func {
    foreach my $item ( @LIST ) {
       if( COND($item) ) {
          return $item;
       }
    }
    return MAKE_NEW_ITEM();
 }

The C<Future::Utils::call_with_escape> function allows this general form of
control flow, by calling a block of code that is expected to return a future,
and itself returning a future. Under normal circumstances the result of this
future propagates through to the one returned by C<call_with_escape>.

However, the code is also passed in a future value, called here the "escape
future". If the code captures this future and completes it (either by calling
C<done> or C<fail>), then the overall returned future immediately completes
with that result instead, and the future returned by the code block is
cancelled.

 my $f = call_with_escape {
    my $escape_f = shift;

    ( repeat {
       my $item = shift;
       COND($item)->then( sub {
          my ( $result ) = @_;
          if( $result ) {
             $escape_f->done( $item );
          }
          return Future->done;
       })
    } foreach => \@ITEMS )->then( sub {
       MAKE_NEW_ITEM();
    });
 };

Here, if C<$escape_f> is completed by the condition test, the future chain
returned by the code (that is, the C<then> chain of the C<repeat> block
followed by C<MAKE_NEW_ITEM()>) will be cancelled, and C<$f> itself will
receive this result.

=head1 CONCURRENCY

This final section of the phrasebook demonstrates a number of abilities that
are simple to do with C<Future> but can't easily be done with regular
call/return style programming, because they all involve an element of
concurrency. In these examples the comparison with regular call/return code
will be somewhat less accurate because of the inherent ability for the
C<Future>-using version to behave concurrently.

=head2 Waiting on Multiple Functions

The C<< Future->wait_all >> constructor creates a C<Future> that waits for all
of the component futures to complete. This can be used to form a sequence with
concurrency.

 { FIRST_A(); FIRST_B() }
 SECOND();

Z<>

 my $f = Future->wait_all( FIRST_A(), FIRST_B() )
    ->then( sub { SECOND() } );

Unlike in the call/return case, this can perform the work of C<FIRST_A()> and
C<FIRST_B()> concurrently, only proceeding to C<SECOND()> when both are ready.

The result of the C<wait_all> C<Future> is the list of its component
C<Future>s. This can be used to obtain the results.

 SECOND( FIRST_A(), FIRST_B() );

Z<>

 my $f = Future->wait_all( FIRST_A(), FIRST_B() )
    ->then( sub {
       my ( $f_a, $f_b ) = @_
       SECOND( $f_a->get, $f_b->get );
    } );

Because the C<get> method will re-raise an exception caused by a failure of
either of the C<FIRST> functions, the second stage will fail if any of the
initial Futures failed.

As this is likely to be the desired behaviour most of the time, this kind of
control flow can be written slightly neater using C<< Future->needs_all >>
instead.

 my $f = Future->needs_all( FIRST_A(), FIRST_B() )
    ->then( sub { SECOND( @_ ) } );

The C<get> method of a C<needs_all> convergent Future returns a concatenated
list of the results of all its component Futures, as the only way it will
succeed is if all the components do.

=head2 Waiting on Multiple Calls of One Function

Because the C<wait_all> and C<needs_all> constructors take an entire list of
C<Future> instances, they can be conveniently used with C<map> to wait on the
result of calling a function concurrently once per item in a list.

 my @RESULT = map { FUNC( $_ ) } @ITEMS;
 PROCESS( @RESULT );

Again, the C<needs_all> version allows more convenient access to the list of
results.

 my $f = Future->needs_all( map { F_FUNC( $_ ) } @ITEMS )
    ->then( sub {
       my @RESULT = @_;
       F_PROCESS( @RESULT )
    } );

This form of the code starts every item's future concurrently, then waits for
all of them. If the list of C<@ITEMS> is potentially large, this may cause a
problem due to too many items running at once. Instead, the
C<Future::Utils::fmap> family of functions can be used to bound the
concurrency, keeping at most some given number of items running, starting new
ones as existing ones complete.

 my $f = fmap {
    my $item = shift;
    F_FUNC( $item )
 } foreach => \@ITEMS;

By itself, this will not actually act concurrently as it will only keep one
Future outstanding at a time. The C<concurrent> flag lets it keep a larger
number "in flight" at any one time:

 my $f = fmap {
    my $item = shift;
    F_FUNC( $item )
 } foreach => \@ITEMS, concurrent => 10;

The C<fmap> and C<fmap_scalar> functions return a Future that will eventually
give the collected results of the individual item futures, thus making them
similar to perl's C<map> operator.

Sometimes, no result is required, and the items are run in a loop simply for
some side-effect of the body.

 foreach my $item ( @ITEMS ) {
    FUNC( $item );
 }

To avoid having to collect a potentially-large set of results only to throw
them away, the C<fmap_void> function variant of the C<fmap> family yields a
Future that completes with no result after all the items are complete.

 my $f = fmap_void {
    my $item = shift;
    F_FIRST( $item )
 } foreach => \@ITEMS, concurrent => 10;

=head1 AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

=cut