PHP: rfc:pipe-operator-v2

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====== PHP RFC: Pipe Operator v2 ======
  * Version: 2
  * Date: 2020-04-20
  * Author: Larry Garfield <larry@garfieldtech.com>
  * Status: Declined
  * First Published at: http://wiki.php.net/rfc/pipe-operator-v2

===== Introduction =====

Code like the following is quite common in procedural code: 

<code php>
getPromotions(mostExpensiveItem(getShoppingList(getCurrentUser(), 'wishlist'), ['exclude' => 'onSale']), $holiday);
</code>

That is ugly, error prone, and hard to read or maintain.

Generally, breaking it up as follows will improve readability:

<code php>
$user = getCurrentUser();
$shoppingList = getShoppingList($user, 'wishlist');
$mostExpensiveItem = mostExpensiveItem($shoppingList, ['exclude' => 'onSale']);
$promotions = getPromotions($mostExpensiveItem, $holiday);
</code>

That, however, is still rather verbose and requires defining intermediary variables, and thus either coming up with  names for them or using generic placeholder names like ''$x''.  The result is still error prone and it is possible to get confused by the variable names without realizing it.  It's also not intuitively obvious that what's happening is passing the output of one function to the next.

In OOP, it's common to answer "well use a fluent interface," which might look like this:

<code php>
$promotions = getCurrentUser()
    ->getShoppingList('wishlist')
    ->mostExpensiveItem(['exclude' => 'onSale'])
    ->getPromotions($holiday);
</code>

That's easier to read, but requires very specific methods on very specific objects, which are not always logical or possible to put there.

This RFC aims to improve this type of code with the introduction of a "Pipe Operator," which is a common approach in many other languages.

===== Proposal =====

This RFC introduces a new operator ''|>'', "pipe".  Pipe evaluates left to right by passing the value (or expression result) on the left as the first and only parameter to the callable on the right.  That is, the following two code fragments are exactly equivalent:

<code php>
$result = "Hello World" |> 'strlen';
</code>

<code php>
$result = strlen("Hello World");
</code>

For a single call that is not especially useful.  It becomes useful when multiple calls are chained together.  That is, the following two code fragments are also exactly equivalent:

<code php>
$result = "Hello World"
    |> 'htmlentities'
    |> 'str_split'
    |> fn($x) => array_map('strtoupper', $x)
    |> fn($x) => array_filter($x, fn($v) => $v != 'O');
</code>

<code php>
$result = array_filter(
    array_map('strtoupper', 
        str_split(htmlentities("Hello World"))
        ), fn($v) => $v != 'O'
    );
</code>

The left-hand side of the pipe may be any value or expression.  The right-hand side may be any valid PHP callable that takes a single parameter, or any expression that evaluates to such a callable.  Functions with more than one required parameter are not allowed and will fail as if the function were called normally with insufficient arguments.  If the right-hand side does not evaluate to a valid callable it will throw an Error.

===== Language theory =====

The pipe operator is a form of function composition.  Function composition is a basic, fundamental feature of functional programming and functional languages.  However, it is also comfortable within object-oriented languages as a general tool to pass data from one function (or method) to another without intermediary variables or ugly nesting.

It also cleanly enables "point-free style", an approach to programming that limits the use of unnecessary intermediary variables.  Point-free style has been gaining popularity in JavaScript circles, so will be familiar to JavaScript developers using that style.

===== Callable syntax =====

Pipes support any callable on the right hand side, using any callable syntax supported by PHP now or in the future.  As of 8.0, that is unfortunately not a particularly strong list.  However, over time that should improve, such as via the [[rfc:first_class_callable_syntax|First-class callable syntax]] RFC.

Should a version of [[rfc:partial_function_application|Partial Function Application]] be adopted in the future, that would also integrate nicely with pipes with no further effort.

The examples below largely assume that the first-class-callable RFC has passed, which as of this writing appears guaranteed.

Additionally, functions that return callables may be used to conveniently produce pipe-compatible callables.  The following example includes some obvious examples.

===== Alternate comprehension syntax ====

A prior RFC proposed a [[rfc:comprehensions|dedicated syntax for array comprehensions]].  The pipe operator would also address that use case, in combination with a few user-space functions (which the RFC author pledges to write and maintain a library for).  For example:

<code php>
// array_map() but for any iterable.
function itmap(callable $c) {
  return function(iterable $it) use ($c) {
    foreach ($it as $val) {
      yield $c($val);
    }
  };
}

// array_filter() but for any iterable.
function itfilter(callable $c) {
  return function(iterable $it) use ($c) {
    foreach ($it as $val) {
      if ($c($val)) {
        yield $val;
      }
    }
  };
}

// count(), but runs out an iterator to do so.
function itcount(iterable $it) {
  $count = 0;
  foreach ($it as $v) {
    $count++;
  }
  return $count;
}
</code>

And now comprehension-like behavior can be written using pipes, without the need for a dedicated syntax.

<code php>
$list = [1, 2, 3, 4, 5];

$new_list = $list
  |> itmap(fn($x) => $x * 2)
  |> itfilter(fn($x) => $x % 3)
  |> iterator_to_array(...);
</code>

Any combination of map, filter, reduce, or other array-oriented operation can be wrapped up this way and added to a pipe chain, allowing a similar result to comprehensions without a one-off syntax, and can be mixed-and-matched with any other callable as appropriate.

String-oriented functions would be equally easy to produce.  They would also serve to essentially eliminate the needle/haystack question (when used with an appropriate utility function), by splitting the call into two: One to capture the non-array/string arguments, and one to just take the array/string and apply it.

===== Additional semantics =====

Functions that accept their first parameter by reference are allowed, as are functions that return by reference.  They will behave semantically the same as if they were passed a variable by reference or returned a variable by reference via a "normal" call.  In practice, however, reference variables are of little use in pipes so this is more of a design artifact than design intent.

When evaluating a pipe, the left-hand side is fully evaluated first, then the right-hand side, then the right-hand side is invoked using the left-hand side.  That is, evaluation is strictly left-to-right.

The pipe operator evaluates immediately.  It does not produce a new function.  However, it is simple to produce a new function by writing an arrow function:

<code php>
$array_op = fn(iterable $list) => $list
  |> itmap(fn($x) => $x * 2)
  |> itfilter(fn($x) => $x % 3)
  |> iterator_to_array(...);
  
$result = $array_op([1, 2, 3, 4, 5]);
</code>

===== Further examples =====

Given the utilities above, the following examples would all be valid.

<code php>
// Take a string, sanitize it, 
// split it to an array, 
// upper-case everything, 
// and remove the letter O.
$result = "Hello World"
    |> htmlentities(...)
    |> str_split(...)
    |> itmap(strtoupper(...))
    |> itfilter(fn($v) => $v != 'O');
</code>

The example from the start of this RFC could be written as:

<code php>
$holiday = "Lincoln's Birthday";
$result = getCurrentUser()
   |> getShoppingList('wishlist')
   |> mostExpensiveItem(['exclude' => 'onSale'])
   |> getPromotions($holiday);
</code>

For a more robust example, the following routine would, given a directory, give a line count of all files in the directory tree that have a specific extension.  (Thanks to Levi Morrison for this example.)

<code php>
function nonEmptyLines(\SplFileInfo $file): iterable {
  try {
    $object = $file->openFile("r");
    $object->setFlags(\SplFileObject::SKIP_EMPTY);
    yield from $object;
  } catch (\Throwable $error) {
    // File system error handling irrelevant for the moment.
  }
};

function getLineCount(string $directory, string $ext): int {
  return new RecursiveDirectoryIterator('.')
    |> fn($x) => new RecursiveIteratorIterator($x)
    |> itfilter(fn ($file) => $file->getExtension() == $ext)
    |> itmap(nonEmptyLines(...))
    |> itcount(...)
  ;
}

print getLineCount('foo/bar/baz', 'php');
</code>

===== Prior art =====

A previous RFC, [[https://wiki.php.net/rfc/pipe-operator|Pipe Operator v1]] from 2016 by Sara Golemon and Marcelo Camargo, proposed similar functionality.  Its primary difference was to model on Hack, which allowed an arbitrary expression on the right-hand side and introduced a new ''$$'' magic variable as a placeholder for the left-hand side.  While promising, the v2 authors concluded that short-lambdas made a custom one-off syntax unnecessary.  The semantics proposed here are more consistent with most languages that offer a pipe operator.

Additionally, the comprehension-esque usage noted above would be infeasible with a non-callable right hand side.

Portions of this RFC are nonetheless based on the previous iteration, and the author wishes to thank the v1 authors for their inspiration.

===== Existing implementations =====

Multiple user-space libraries exist in PHP that attempt to replicate pipe-like behavior.  All are clunky and complex by necessity compared to a native solution, but demonstrate that there is desire for pipeline behavior.

  * The PHP League has a [[https://pipeline.thephpleague.com/|Pipeline]] library that encourages wrapping all functions into classes with an ''%%__invoke()%%'' method to allow them to be referenced, and using a ''->pipe()'' call for each step.
  * Laravel includes a [[https://github.com/illuminate/pipeline|Illuminate/Pipeline]] package that has an [[https://agoalofalife.medium.com/pipeline-and-php-d9bb0a6370ca|even more cumbersome syntax]].
  * The [[https://github.com/azjezz/psl|PHP Standard Library]] (PSL) library includes a [[https://github.com/azjezz/psl/blob/1.8.x/src/Psl/Fun/pipe.php|pipe function]], though it is more of a function concatenation operation.
  * [[https://github.com/sebastiaanluca/php-pipe-operator|Sebastiaan Luca]] has a pipe library that works through abuse of the ''%%__call%%'' method.  It only works for named functions, I believe, not for arbitrary callables.
  * Various blogs speak of "the Pipeline Pattern" ([[https://medium.com/@aaronweatherall/the-pipeline-pattern-for-fun-and-profit-9b5f43a98130|for example]])

Those libraries would be mostly obsoleted by this RFC, with a more compact, more universal, better-performing syntax.

===== Comparison with other languages =====

Several languages already support a pipe operator, using similar or identical syntax.  In practice, the semantics proposed here are closest to Elixir and F#.

==== Hacklang ====

Hack has [[https://docs.hhvm.com/hack/expressions-and-operators/pipe|very similar functionality]], also using the ''|>'' operator.  However, in Hack the operator's right-hand side is an arbitrary expression in which a special placeholder, ''$$'' is used to indicate where the left-hand side should be injected.  Effectively it becomes a one-off form of partial application.

That is atypical among languages with such functionality and introduces additional questions about what sigil to use and other implementation details.  The RFC authors believe that a fully-fleshed out partial function application syntax (in a separate RFC) is superior, and integrates cleanly with this RFC.

The Hack syntax was the subject of the [[https://wiki.php.net/rfc/pipe-operator|v1 Pipe Operator RFC]].

==== Haskell ====

Haskell has a [[https://wiki.haskell.org/Function_composition|function concatenation operator]], ''.''.  However, its semantics are backwards.  ''reverse . sort'' is equivalent to ''reverse(sort())'', not to ''sort(reverse())''  It also returns a new composed callable rather than invoking immediately.

The inverse ordering is more difficult to reason about, and unfamiliar for PHP developers.  The ''.'' operator itself would also cause confusion with the string concatenation operator, especially as strings can be callables.  That is:

<code php>
'hello' . 'strlen'
</code>

Could be interpreted as evaluating to "hellostrlen" or to int 5.  For that reason the ''.'' operator is not feasible.

Haskell also has a ''&'' operator, which is the "reverse application operator."  Its semantics are essentially the same as described here, including listing functions "forward" rather than backward.

==== F# ====

F# has no less than four function composition operators: Pipe forward ''|>'', Pipe back ''<|'', Compose forward ''>>'' and Compose back ''<<''.  The two pipe operators apply a value to a function, while the composer operator concatenates two functions to produce a new function that is the composition of the specified functions.  The forward and back variants allow you to put the callable on either the left or right-hand side.

The author decided that supporting both forward and back versions was too confusing.  Additionally, a concatenation operator is unnecessary since users can simply form a short-lambda closure themselves.

That is, this RFC proposes an equivalent of only the "pipe forward" operator.

==== Elixir ====

[[https://elixirschool.com/en/lessons/basics/pipe-operator/|Elixir has a pipe operator]], ''|>'', using essentially the same semantics as described here.

==== Ruby ====

[[https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/|Ruby 2.6]] added a similar syntax, although more akin to F#'s compose forward and compose back operators.

==== Javascript ====

A pipeline operator ''|>'' has been [[https://github.com/tc39/proposal-pipeline-operator/wiki|proposed for Javascript]].  As of this writing it is still in early stages and no implementations support it, but it may get accepted in the future.  The semantics are essentially the same as described here.

==== OCaml ====

OCaml includes a [[https://riptutorial.com/ocaml/example/22018/composition-operators|Composition operator]], following its common implementation in user-space.  It also is denoted ''|>'', and its semantics are essentially the same as described here.

===== Future Scope =====

This RFC suggests a number of additional improvements.  They have been left for future work so as to keep this RFC focused and non-controversial.  Should this RFC pass the authors intend to attempt these follow up improvements.  (Assistance in doing so is quite welcome.)

* Generic partial function application.  While the prior RFC was declined due to its perceived use cases being insufficient to justify its complexity, increased use of pipes will likely provide sufficient justification.  (Alternatively, a less complex implementation might be found.)

* Iterable right-hand side.  The pipe operator as presented here can only be used in a hard-coded fashion.  A possible extension is to support an iterable of callables on the right-hand side, allowing for a runtime-defined pipeline.

* A ''%%__bind%%'' method or similar on objects.  If implemented by an object on the left-hand side, the right-hand side would be passed to that method to invoke as it sees fit.  Effectively this would be operator overloading, which could be part of a second attempt at full operator overloading or a one-off magic method.  It could also be implemented as a separate operator instead, for clarity.  Such a feature would be sufficient to support arbitrary monadic behavior in PHP in a type-friendly way.

These options are mentioned here for completeness and to give an indication of what is possible, but are *not* in scope and are *not* part of this RFC at this time.

===== Proposed PHP Version(s) =====

8.1

===== Backward compatibility issues =====

None.

===== Proposed Voting Choices =====

Adopt the Pipe Operator yes/no?  Requires a 2/3 majority.

<doodle title="Pipe Operator" auth="crell" voteType="single" closed="true">
   * Yes
   * No
</doodle>


===== Patches and Tests =====

PR is available here: https://github.com/php/php-src/pull/7214

(It's my first PHP PR.  Please be gentle.)
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