PHP: rfc:chaining_comparison

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====== PHP RFC: Chaining Comparison ======
  * Version: 0.2
  * Date: 2016-12-13
  * Author: David Walker (dave@mudsite.com)
  * Author: Richard Fussenegger (php@fleshgrinder.com)
  * Status: Inactive
  * First Published at: http://wiki.php.net/rfc/chaining_comparison

===== Introduction =====
This RFC proposes a syntax change to allow arbitrary chaining together of comparison and equality operations ''[==, !=, !==, ===, <, <=, >, >=]''.  The initial request that spawned this RFC[1] was initially only for interval checking.  Discussion on the thread expanded the scope of the request to go from strictly interval checking to allowing more arbitrary number of comparisons.  It evolved from there to expand to a majority of the comparison operations.  The primary benefit to this proposal would be to make for more readable code when doing numerous comparisons between a single variable.

<file php>
<?php
$a = 10;

/*
 * The initial request of this proposal was to change the following syntax
 */
if (0 < $a && $a < 100) {
    echo "Value is between 0 and 100\n";
}

/*
 * To be functionally equivalent to this syntax
 */
if (0 < $a < 100) {
    echo "Value is between 0 and 100\n";
}
</file>

===== Proposal =====
Proposals herein will contain a dump of relevant AST (php-ast) nodes and OPCodes (vld) to better visualize the compilation, and execution.

==== Comparison Chaining ====
The proposal creates a new AST operation type ''ZEND_AST_COMPARE_OP'' which will be compiled in a left-recursive manor.  

<file php>
<?php
$a = 1;
$b = 10;

var_dump($a < 5 < $b++); // bool(true)

/*
 * AST Dump
 *
 * 2:  AST_CALL
 *      expr: AST_NAME
 *          flags: NAME_NOT_FQ (1)
 *        name: "var_dump"
 *      args: AST_ARG_LIST
 *         0: AST_COMPARE_OP
 *             flags: COMPARE_IS_SMALLER (19)
 *             left: AST_COMPARE_OP
 *                 flags: COMPARE_IS_SMALLER (19)
 *                 left: AST_VAR
 *                     name: "a"
 *                 right: 5
 *             right: AST_POST_INC
 *                 var: AST_VAR
 *                     name: "b"
 */

/*
 * OPCodes
 *
 *    2        INIT_FCALL                                               'var_dump'
 *    3        IS_SMALLER                                       ~4      !0, 5
 *    4      > JMPZ_EX                                          ~4      ~4, ->7
 *    5    >   POST_INC                                         ~5      !1
 *    6    >   IS_SMALLER                                       ~4      ~4, ~5
 *    7    >   SEND_VAL                                                 ~4
 *    8        DO_ICALL                                                 
 */
</file>

So we can see what this feature will do internally.  Speaking directly at the OPCodes we see how our ''JMPZ_EX'' code injection works.  Since the first evaluated ''IS_SMALLER'' op is the left side-recursive of the expression, we determine this expressions result.  If the result evaluates to true (and if you look at the code it checks to see if there's an extended_value flag) we continue to the ''POST_INC'' otherwise we skip to the sending of the value which would be false.

==== Equality Chaining ====
The proposal is also extended to allow chaining of equality operators.  However, equality operators are at a higher (or not as significant) precedence as the comparison operations.  This enforces equality operators operate on either booleans, or values that will be compared to a boolean value.  Example:

<file php>
<?php
$a = 1;
$b = 10;

var_dump($a == 1 === true); // bool(true)

/*
 * AST Dump
 *
 *   2: AST_CALL
 *       expr: AST_NAME
 *           flags: NAME_NOT_FQ (1)
 *           name: "var_dump"
 *       args: AST_ARG_LIST
 *           0: AST_COMPARE_OP
 *               flags: COMPARE_IS_IDENTICAL (15)
 *               left: AST_COMPARE_OP
 *                   flags: COMPARE_IS_EQUAL (17)
 *                   left: AST_VAR
 *                       name: "a"
 *                   right: 1
 *               right: AST_CONST
 *                   name: AST_NAME
 *                       flags: NAME_NOT_FQ (1)
 *                       name: "true"
 */

/*
 * OPCodes
 *
 *   2        INIT_FCALL                                               'var_dump'
 *   3        IS_EQUAL                                         ~2      !0, 1
 *   4      > JMPZ_EX                                          ~2      ~2, ->6
 *   5    >   IS_IDENTICAL                                     ~2      ~2, <true>
 *   6    >   SEND_VAL                                                 ~2
 */
</file>

==== False Short Circuiting ====
In doing this compilation we can ensure short cutting of righter operations if the left sides have evaluated to false.  To accomplish this we introduce a new means of emitting an operation, by noting where a ''JMPZ_EX'' may need to exist (see implementations for ''zend_emit_op_at'').  This will shift operations that may have been emitted by compiling the right side of this AST compare to allow jumping over them if the left side of the operation is evaluated to false.  I believe this means is necessary because we can't just shortcut if the left operation is false, ''false < $a++'' should still evaluate the right part of the expression.  We should only inject the JMPZ_EX ops, IF, the left child is a chained ''ZEND_AST_COMPARE_OP''.  The proposal also changes the associativity of the equality, and comparison, operations to being left associative.

===== Backward Incompatible Changes =====
BC Breaking changes expected depending on open-issue answers

===== Proposed PHP Version(s) =====
Next PHP (currently 7.2)

===== RFC Impact =====
==== To Opcache ====
I'm unsure; we're adding new op-codes and/or order of opcodes, but are not introducing any new codes

===== Open Issues =====
====Should equality and comparison expressions be treated as same precedence?====
This is harder of a question that it seems.  What we are asking is how should we parse a seemingly simple expression:
''1 < 2 == 3 < 4''

Why is this even a question, much less a challenging one?  Well, a seemingly majority of languages ''[C[2], C++[3], Java[4], Ruby[5], Perl[6]]'' all would tell you that the expression would evaluate to true.  However some, like ''Python[7]'', would evaluate that expression to false.  Some, like ''[Numbers, LibreOffice]'' will raise a syntax error, or give awkward answers.  The question we have is which way should PHP go with the evaluation of this expression?  Clearly we can ascertain that the true-evaluating languages have the precedence of the less-than operator more imporatant than that of the equality, so they check if true == true.  Whereas the false-evaluating languages treat comparisons and equality with the same precedence.  As such they compare 1 less than 2, 2 is-equal 3.  The latter group are apparently more strictly typed and won't compare bools to numbers, but even there we can see the precedence is equal, as it's comparing the result of the first expression into the next ''(1 < 2) == 3''

It is important to point out that the example syntax is currently valid in PHP 7.1.  PHP 7.1 currently has a C-like precedence where ''[<, <=, >, >=]'' are a higher precedence than ''[==, !=, ===, !==]''[8].  Below are expressions and their return values in PHP 7.1, and with the two potential methods of evaluating that expression.

<file php>
<?php

/*
 * PHP <= 7.1
 */
var_dump(1 < 2 == 3 < 4); // bool(true)
var_dump(1 < 2 == 3 < 4 == 5 < 6) // Syntax Error

/*
 * Proposed Chaining, comparators evaluated first; equality second [See: Implementation #1]
 */
var_dump(1 < 2 == 3 < 4); // bool(true)
var_dump(1 < 2 == 3 < 4 == 5 < 6) // bool(true)

/*
 * Proposed Strict Chaining [See: Implementation #2]
 */
var_dump(1 < 2 == 3 < 4); // bool(false)
var_dump(1 < 2 == 3 < 4 == 5 < 6) // bool(false)
var_dump((1 < 2) == (3 < 4) == (5 < 6)) // bool(true)
</file>

==== Right Recursion ====
Another syntax difference that could be BC problematic is with right-recursion of the chained expression.  Currently PHP will evaluate right recursive single expression comparisons.  The proposed feature would raise a compile time error doing this.  The question is should it, or should we permit right-recursive chaining? The test case we can look at:

<file php>
<?php
var_dump(1 < (2 < 3));
var_dump(1 < 2 == 3);
var_dump(1 < 2 == 3 == 4);
var_dump(1 < 2 == (3 == 4));
</file>

We will go over how PHP 7.1 currently would evaluate each, and then how a right-recursive chain would pan out.
<file php>
<?php
var_dump(1 < (2 < 3));
/*
 * 1 < (2 < 3) := 1 < true := false
 */

var_dump(1 < 2 == 3);
/*
 * (1 < 2) == 3 := true == 3 := true
 */

var_dump(1 < 2 == 3 == 4);
/*
 * Parse Error, unexpected ==
 */

var_dump(1 < 2 == (3 == 4));
/*
 * (1 < 2) == (3 == 4) := true == false := false
 */
</file>

The current proposal (implemented) evaluation method.  You'll notice that we do permit right-recursion for equality operations.  This is due to the fact that equality operations will evaluate against boolean, or boolean-converted values.  Since you don't really care what the left-node of the right-recursive side is, you only care if the right side evaluates to true or not.
<file php>
<?php
var_dump(1 < (2 < 3));
/*
 * Parse Error: No right recursion
 */

var_dump(1 < 2 == 3);
/*
 * (1 < 2) == 3 := true == 3 := true
 */

var_dump(1 < 2 == 3 == 4);
/*
 * ((1 < 2) == 3) == 4 := (true == 3) == 4 := true == 4 := true
 */

var_dump(1 < 2 == (3 == 4));
/*
 * (1 < 2) == (3 == 4) := true == false := false
 */
</file>

If however we permitted right recursive comparison operations we would evaluate as such:
<file php>
<?php
var_dump(1 < (2 < 3));
/*
 * 1 < (2 < 3) := 1 && (2 < 3) && (1 < 2) := true && true && true := true
 */

var_dump(1 < 2 == 3);
/*
 * (1 < 2) == 3 := true == 3 := true
 */

var_dump(1 < 2 == 3 == 4);
/*
 * ((1 < 2) == 3) == 4 := (true == 3) == 4 := true == 4 := true
 */

var_dump(1 < 2 == (3 == 4));
/*
 * (1 < 2) == (3 == 4) := true == false := false
 */
</file>

If the first example in this last one looks a little odd, it's because it is.  We do design for short-cutting of a long expression when a fault is found to prevent further execution much like you have in ''if()'' statements.  However, we do process in a left-to-right manor.  So the first thing would require us to ensure the left most side evaluates to true, and if it wasn't ''1'' but rather ''$a++'', we'd want to ensure to get that left nodes potential opcodes to execute before comparing the right hand side.  Since we are chaining, we'd want to evaluate the right, then return the left node of it to be evaluated against the top's left node.  This, odd syntax is why I didn't implement a right-recursive chaining of comparison operations.

Although allow right-recursion of equality operations does itself introduce some slightly odd syntax like:
<file php>
<?php
/*
 * Right chained comparison syntax
 */
var_dump(1 < (2 == 2)); // bool(false)

/*
 * Is Functionally identical to PHP 7.1's allowed syntax
 */
var_dump(1 < (2 <= 2)); // bool(false)
</file>
Since we don't chain together the right/left node of an equality operator, this is functionally identical to PHP 7.1's allowed syntax.  We could, for equality operations denote if they were in-fact a right node-continuation of a chain, thus would allow them to evaluate to either the left node, or false.


As we can see right-recursive comparison operations do have numerous caveats and oddities.  For these reasons we didn't implement it, and generally are on the side of forbidding right-recursive comparison operations.

===== Unaffected PHP Functionality =====
Does not alter the operation of the comparison Spaceship [<=>] operator.

===== Proposed Voting Choices =====
Requires 2/3 vote

===== Patches and Tests =====
Working Implementation: comparisons evaluated before equality: https://github.com/php/php-src/compare/master...bp1222:multi-compare

Will need eyes of those more familiar with AST/VM to review.

===== Implementation =====


===== References =====
  * [1] - [[http://marc.info/?l=php-internals&m=147846422102802&w=2|Initial idea on Internals]]
  * [2] - [[https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html#Operator-Precedence|Precedence in C]]
  * [3] - [[https://msdn.microsoft.com/en-us/library/126fe14k.aspx|Precedence in C++]]
  * [4] - [[http://introcs.cs.princeton.edu/java/11precedence/|Precedence in Java]]
  * [5] - [[https://ruby-doc.org/core-2.2.0/doc/syntax/precedence_rdoc.html|Precedence in Ruby]]
  * [6] - [[http://perldoc.perl.org/perlop.html#Operator-Precedence-and-Associativity|Precedence in Perl]]
  * [7] - [[https://docs.python.org/2/reference/expressions.html#operator-precedence|Precedence in Python]]
  * [8] - [[http://php.net/manual/fa/language.operators.precedence.php|Precedence in PHP]]


===== Rejected Features =====
Keep this updated with features that were discussed on the mail lists.
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