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Date: Tue, 19 Sep 2017 18:32:24 -0400
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Subject: [RFC] Pre-draft for PipeOp v2
From: pollita@php.net (Sara Golemon)

I was planning to update the RFC, but wiki.php.net is having issues
atm and isn't coming back up with basic coaxing, so I'll just start
discussion informally, and the RFC can be updates later.

Background: I made an RFC some time ago to implement HackLang's Pipe
Operator https://docs.hhvm.com/hack/operators/pipe-operator which
provides fluent calling for non-object interfaces.
I circulated it to mixed reviews, many negative, with the negativity
feeling like it centered on the use of a magic placeholder token `$$`.

After discussion with Levi and others who suggested a simpler
approach, I'd like to offer
https://github.com/php/php-src/compare/master...sgolemon:pipe2 as an
alternate possibility.

This version removes the $$ token, and instead treats the RHS of the
expression as a callable obeying the same rules as the callable
typehint elsewhere in the language.  Specifically:
* Free functions as strings containing the function name. (e.g. 'funcname')
* Object methods as array($object, 'methodname')
* Static methods as array('Classname', 'methodname')
* Closure expression (e.g.  function($x) { return ...; }  )
* Object instance with an __invoke() method.

In a given pipe expression, the output of the LHS expression feeds a
single arg to the callable on the RHS.
Examples:

$x = "hello"
|> 'strtoupper'
|> function($x) { return $x . " world"; };
// $x === "HELLO world"

Non-Goal: I didn't include support for base function names (e.g.
`"hello" |> strtoupper`) because of the conflict with constants.
Using a constant to store your function name is totes legit and
consistent with language syntax.

Future Scope: Short Lambdas `$x => $x + 1` and Partial Functions
`someFunc('fixed val1', ..., 'fixed val2')`  would help make this
functionality more useful and are worth discussing as a sub-thread,
but are not required to be implemented at the same time.

-Sara