Pipeline operator proposal for PHP 7

This document aims to propose the implementation of the pipeline operator (|>) for PHP 7.

Description

• Associativity: Left
• Precedence: Lowest
• Token: T_PIPELINE

The pipeline operator applies the function on its right side to the value on the left side. This makes easy to write complex expressions that are evaluated from left-to-right/top-to-bottom. Many people find the left-to-right order more readable, especially in cases like this where the filters are higher-order functions that take parameters. When called with functions that receive more than 2 parameters, the left operand is applied as the last argument.

Example

Old style

function even(int $x): int { return $x % 2 === 0; }
function println($x) { echo $x, PHP_EOL; }

Seq::each("println",
  Seq::map("sqrt",
    Seq::filter("even",
      range(1, 10)
  )
);

Pipeline operator

function even(int $x): int { return $x % 2 === 0; }
function println($x) { echo $x, PHP_EOL; }

range(1, 10) |> Seq::filter ("even")
             |> Seq::map ("sqrt")
             |> Seq::each ("println");

Implemented in

• LiveScript
• F#
• Shellscript
• Elixir
• OCaml

Benefits

• Readability
• No need to nest functions

Problems

• 1. Parameterization of built-in functions that differ, such as array_map and array_filter.

Solutions

• 1. Use a wildcard $ for parameter allocation:

range(1, 10) |> array_filter($, "even")
             |> array_map("sqrt", $)
             |> array_walk($, "println");

Equivalence

This feature can actually be emulated by using objects and self-return, with fluent-interfaces:

Implementation

class Seq
{
  private $value;
  
  function __construct(array $xs)
  {
    $this->value = $xs;
  }
  
  function map(callable $fn)
  {
    $this->value = array_map($fn, $this->value);
    return $this;
  }
  
  function filter(callable $fn)
  {
    $this->value = array_filter($this->value, $fn);
    return $this;
  }
  
  function each(callable $fn)
  {
    array_walk($this->value, $fn);
    return $this;
  }
}

Usage

(new Seq(range(1, 10))) -> filter("even")
                        -> map("sqrt")
                        -> each("println");

Practical example