[Feature Request] Fork-Join

[juzerali]

Sometimes based on some condition, a pipeline needs to be forked, meaning there is a need to follow one of several disparate paths which have nothing in common. Optionally, later the disjoint paths may join again.

$pipeline = (new Pipeline)
    ->pipe(new TimeTwoStage)
    ->pipe(new AddOneStage)
    ->fork(function($payload) {
            if($payload == 0) return "zero";
            if($payload < 0) return "-";
            if($payload > 0) return "+";
            return false; // for short-circuit
        })
        ->disjoin("zero", $zeroProcessingPipeline)
        ->disjoin("-", $negativeNumberProcessingPipeline)
        ->disjoin("+", $positiveNumberProcessingPipeline)
        ->join()
    ->pipe(new DivideByFiveStage);

[juzerali]

Also it will be better if the fluent API to build the pipeline is shifted to the builder. As per Single Responsibility Principle, responsibility to build an object with complex graph should be outside the object itself, i.e. in a builder.

So building the pipeline will look like this:

$pipeline = (new PipelineBuilder)
    ->pipe(new TimeTwoStage)
    ->fork(function($payload) {
            if($payload < 0) return "-";
            if($payload >= 0) return "+";
        })
        ->disjoin("-", $negativeNumberProcessingPipeline)
        ->disjoin("+", $positiveNumberOrZeroProcessingPipeline)
        ->join()
    ->pipe(new DivideByFiveStage)
    ->build();

[shadowhand]

I think this would be better solved by using something like php-fp-either, where every stage returns a Right or Left:

use PhpFp\Either\Either;

$pipeline = new Pipeline();
$pipeline = $pipeline->pipe(function (Either $a) {
    return $a->chain(function ($x) {
        return $x * 2;
    };
});

$output = $pipeline->process(Either::of(2))->either(
    function ($left) {
        return 'failed';
    },
    function ($right) {
        return $right;
    }
);

assert($output === 4);