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 $trial_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;

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

INVOKING A BLOCK OF CODE

   call
          $f = call { CODE }

       Since version 0.22.

       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).

   call_with_escape
          $f = call_with_escape { CODE }

       Since version 0.22.

       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".

   repeat+while
          $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.

   repeat+until
          $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 )

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

       Since version 0.13.

       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, or "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. If there was no trial, because the "foreach" list was already empty, then an
       immediate successful future with an empty result is returned.

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

       Since version 0.13.

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

       Since version 0.13.

       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.

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

       Since version 0.13.

       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 warning if it has to do that. To apply repeating behaviour that
       can catch and retry failures, use "try_repeat" instead. This old behaviour is now deprecated and will be
       removed in the next version.

   try_repeat
          $future = try_repeat { CODE } ...

       Since version 0.18.

       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.

   try_repeat_until_success
          $future = try_repeat_until_success { CODE } ...

       Since version 0.18.

       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, "shift"ing 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.

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

   fmap_concat
          $future = fmap_concat { CODE } ...

       Since version 0.14.

       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.

   fmap_scalar
          $future = fmap_scalar { CODE } ...

       Since version 0.14.

       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".

   fmap_void
          $future = fmap_void { CODE } ...

       Since version 0.14.

       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>