Paul Evans > Future > Future::Utils

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NAME ^

Future::Utils - utility functions for working with Future objects

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" );
       ...
 };

 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;

 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;

INVOKING A BLOCK OF CODE ^

$f = call { CODE }

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

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

$f = call_with_escape { CODE }

The 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 call function. However, if the code captures this future and completes it by calling done or fail on it, the future returned by 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 call-with-escape-continuation as found in some Scheme implementations).

REPEATING A BLOCK OF CODE ^

The repeat function provides a way to repeatedly call a block of code that returns a Future (called here a "trial future") until some ending condition is satisfied. The repeat function itself returns a Future to represent running the repeating loop until that end condition (called here the "eventual future"). The first time the code block is called, it is passed no arguments, and each subsequent invocation is passed the previous trial future.

The result of the eventual future is the result of the last trial future.

If the eventual future is cancelled, the latest trial future will be cancelled.

If some specific subclass or instance of Future is required as the return value, it can be passed as the return argument. Otherwise the return value will be constructed by cloning the first non-immediate trial Future.

$future = repeat { CODE } while => CODE

Repeatedly calls the CODE block while the while condition returns a true value. Each time the trial future completes, the while condition is passed the trial future.

 $trial_f = $code->( $previous_trial_f )
 $again = $while->( $trial_f )

If the $code block dies entirely and throws an exception, this will be caught and considered as an immediately-failed Future with the exception as the future's failure. The exception will not be propagated to the caller.

$future = repeat { CODE } until => CODE

Repeatedly calls the CODE block until the until condition returns a true value. Each time the trial future completes, the until condition is passed the trial future.

 $trial_f = $code->( $previous_trial_f )
 $accept = $until->( $trial_f )

$future = repeat { CODE } foreach => ARRAY, otherwise => CODE

Calls the CODE block once for each value obtained from the array, passing in the value as the first argument (before the previous trial future). When there are no more items left in the array, the otherwise code is invoked once and passed the last trial future, if there was one, otherwise undef if the list was originally empty. The result of the eventual future will be the result of the future returned from otherwise.

The referenced array may be modified by this operation.

 $trial_f = $code->( $item, $previous_trial_f )
 $final_f = $otherwise->( $last_trial_f )

The otherwise code is optional; if not supplied then the result of the eventual future will simply be that of the last trial.

$future = repeat { CODE } foreach => ARRAY, while => CODE, ...

$future = repeat { CODE } foreach => ARRAY, until => CODE, ...

Combines the effects of foreach with while or until. Calls the CODE block once for each value obtained from the array, until the array is exhausted or the given ending condition is satisfied.

If a while or until condition is combined with otherwise, the otherwise code will only be run if the array was entirely exhausted. If the operation is terminated early due to the while or until condition being satisfied, the eventual result will simply be that of the last trial that was executed.

$future = repeat { CODE } generate => CODE, otherwise => CODE

Calls the CODE block once for each value obtained from the generator code, passing in the value as the first argument (before the previous trial future). When the generator returns an empty list, the otherwise code is invoked and passed the last trial future, if there was one, otherwise undef if the generator never returned a value. The result of the eventual future will be the result of the future returned from otherwise.

 $trial_f = $code->( $item, $previous_trial_f )
 $final_f = $otherwise->( $last_trial_f )

 ( $item ) = $generate->()

The generator is called in list context but should return only one item per call. Subsequent values will be ignored. When it has no more items to return it should return an empty list.

For backward compatibility this function will allow a while or until condition that requests a failure be repeated, but it will print a one-time warning if it has to do that. To apply repeating behaviour that can catch and retry failures, use try_repeat instead.

$future = try_repeat { CODE } ...

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

In some later version the 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 while {} loop in Perl. Code that specifically wishes to catch failures in trial futures and retry the block should use try_repeat specifically.

$future = try_repeat_until_success { CODE } ...

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

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

APPLYING A FUNCTION TO A LIST ^

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

While this behaviour can also be implemented using repeat, the main reason to use an 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 fmap* function:

foreach => ARRAY

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

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

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.

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.

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 return argument, the result will be put into the given instance. This can be used to return subclasses, or specific instances.

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.

$future = fmap_concat { CODE } ...

This version of 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 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 ARRAY references and final flattening operation used to implement this behaviour, this function is slightly less efficient than fmap_scalar or 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 fmap to emphasise its similarity to perl's map keyword.

$future = fmap_scalar { CODE } ...

This version of fmap acts more like the 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 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 fmap1.

$future = fmap_void { CODE } ...

This version of 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 fmap0.

AUTHOR ^

Paul Evans <leonerd@leonerd.org.uk>

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