Future::Phrasebook(3) User Contributed Perl DocumentationFuture::Phrasebook(3)NAME
"Future::Phrasebook" - coding examples for "Future" and "Future::Utils"
This documentation-only module provides a phrasebook-like approach to
giving examples on how to use Future and 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 "ALL_CAPITALS()" to make them stand out.
In the examples showing use of Futures, any function that is expected
to return a "Future" instance is named with a leading "F_" prefix. Each
example is also constructed so as to yield an overall future in a
variable called "$f", which represents the entire operation.
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
"Future" before calling the second.
my $f = F_FIRST()
->then( sub { F_SECOND(); } );
Here, the anonymous closure is invoked once the "Future" returned by
"F_FIRST()" succeeds. Because "then" invokes the code block only if the
first Future succeeds, it shortcircuits around failures similar to the
way that "die()" shortcircuits around thrown exceptions. A "Future"
representing the entire combination is returned by the method.
Because the "then" method itself returns a "Future" representing the
overall operation, it can itself be further chained.
FIRST();
SECOND();
THIRD();
my $f = F_FIRST()
->then( sub { F_SECOND(); } )
->then( sub { F_THIRD(); } );
See below for examples of ways to handle exceptions.
Passing Results
Often the result of one function can be passed as an argument to
another function.
OUTER( INNER() );
The result of the first "Future" is passed into the code block given to
the "then" method.
my $f = F_INNER()
->then( sub { F_OUTER( @_ ) } );
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 "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;
}
});
EXCEPTION HANDLING
In regular call/return style code, if any function throws an exception,
the remainder of the block is not executed, the containing "try" or
"eval" is aborted, and control is passed to the corresponding "catch"
or line after the "eval".
try {
FIRST();
}
catch {
my $e = $_;
ERROR( $e );
};
The "else" method on a "Future" can be used here. It behaves similar to
"then", but is only invoked if the initial "Future" fails; not if it
succeeds.
my $f = F_FIRST()
->else( sub { F_ERROR( @_ ); } );
Alternatively, the second argument to the "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 "Future". In that case, the "else" code
block can return an immediate "Future" instance.
my $f = F_FIRST()
->else( sub {
ERROR( @_ );
return Future->new->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 "else" block could return a new "Future" failed
with the same exception, the "else_with_f" block is passed the failed
"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 "followed_by" method is similar again, though it invokes the code
block regardless of the success or failure of the initial "Future". It
can be used to create "finally" semantics. By returning the "Future"
instance that it was passed, the "followed_by" code ensures it doesn't
affect the result of the operation.
try {
FIRST();
}
catch {
ERROR( $_ );
}
finally {
CLEANUP();
};
my $f = F_FIRST()
->else( sub {
ERROR( @_ );
return Future->new->done;
})
->followed_by( sub {
CLEANUP();
return shift;
});
ITERATION
To repeat a single block of code multiple times, a "while" block is
often used.
while( COND() ) {
FUNC();
}
The "Future::Utils::repeat" function can be used to repeatedly iterate
a given "Future"-returning block of code until its ending condition is
satisfied.
use Future::Utils qw( repeat );
my $f = repeat {
F_FUNC();
} while => sub { COND() };
Unlike the statement nature of perl's "while" block, this "repeat"
"Future" can yield a value; the value returned by "$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
"Future", it can be combined along with the loop body. The trial
"Future" returned by the code block is passed to the "while" condition
function.
my $f = repeat {
F_FUNC()
->followed_by( sub { F_COND(); } );
} while => sub { shift->get };
The condition can be negated by using "until" instead
until( HALTING_COND() ) {
FUNC();
}
my $f = repeat {
F_FUNC();
} until => sub { HALTING_COND() };
Iterating with Exceptions
Technically, this loop isn't quite the same as the equivalent "while"
loop in plain Perl, because the "while" loop will also stop executing
if the code within it throws an exception. This can be handled in
"repeat" by testing for a failed "Future" in the "until" condition.
while(1) {
TRIAL();
}
my $f = repeat {
F_TRIAL();
} until => sub { shift->failure };
When a repeat loop is required to retry a failure, the "try_repeat"
function should be used. Currently this function behaves equivalently
to "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 "repeat" function so that yields a warning.
my $f = try_repeat {
F_TRIAL();
} while => sub { shift->failure };
Another variation is the "try_repeat_until_success" function, which
provides a convenient shortcut to calling "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;
}
my $f = try_repeat_until_success {
F_TRIAL();
};
Iterating over a List
A variation on the idea of the "while" loop is the "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 "Future" using the "foreach" parameter to
the "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;
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 "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 "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 "unshift" more into it, while also
being used as the actual "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 "unshift" or "push" more items onto this list,
the tree can be walked in either depth-first or breadth-first order.
SHORT-CIRCUITING
Sometimes a result is determined that should be returned through
several levels of control structure. Regular Perl code has such
keywords as "return" to return a value from a function immediately, or
"last" for immediately stopping execution of a loop.
sub func {
foreach my $item ( @LIST ) {
if( COND($item) ) {
return $item;
}
}
return MAKE_NEW_ITEM();
}
The "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
"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 "done" or "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->new->done;
})
} foreach => \@ITEMS )->then( sub {
MAKE_NEW_ITEM();
});
};
Here, if $escape_f is completed by the condition test, the future chain
returned by the code (that is, the "then" chain of the "repeat" block
followed by "MAKE_NEW_ITEM()") will be cancelled, and $f itself will
receive this result.
CONCURRENCY
This final section of the phrasebook demonstrates a number of abilities
that are simple to do with "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 "Future"-using version to behave concurrently.
Waiting on Multiple Functions
The "Future->wait_all" constructor creates a "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();
my $f = Future->wait_all( FIRST_A(), FIRST_B() )
->then( sub { SECOND() } );
Unlike in the call/return case, this can perform the work of
"FIRST_A()" and "FIRST_B()" concurrently, only proceeding to "SECOND()"
when both are ready.
The result of the "wait_all" "Future" is the list of its component
"Future"s. This can be used to obtain the results.
SECOND( FIRST_A(), FIRST_B() );
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 "get" method will re-raise an exception caused by a failure
of either of the "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
"Future->needs_all" instead.
my $f = Future->needs_all( FIRST_A(), FIRST_B() )
->then( sub { SECOND( @_ ) } );
The "get" method of a "needs_all" dependent 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.
Waiting on Multiple Calls of One Function
Because the "wait_all" and "needs_all" constructors take an entire list
of "Future" instances, they can be conveniently used with "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 "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 @ITEMS is potentially large, this
may cause a problem due to too many items running at once. Instead, the
"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 "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 "fmap" and "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 "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 "fmap_void" function variant of the "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;
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.18.2 2014-05-14 Future::Phrasebook(3)
