15-chained-comparisons

15 — Chained comparisons

Borrowed from Python.

Write a range test the way maths does. a < b < c means a < b && b < c — not (a < b) < c — with each operand evaluated once and the chain short-circuiting:

if (0 <= $i < count($xs)) …        // 0 <= $i && $i < count($xs)
if ($lo <= $temp <= $hi) …
$ok = 1 < 2 < 3;                    // true

Any run of the relational operators < <= > >= chains, and they can be mixed:

5 > 3 < 4;        // 5 > 3 && 3 < 4   => true
10 >= $n > 0;     // 10 >= $n && $n > 0

Semantics

• Pairwise. a op1 b op2 c is (a op1 b) && (b op2 c), extended to any length.
• Each operand once. The shared middle operands are evaluated a single time, left to right — 0 < mid() < 10 calls mid() exactly once.
• Short-circuit. As soon as a link is false the rest is skipped, so later operands may not be evaluated at all: in 5 < 2 < rhs(), rhs() never runs.
• The result is always a boolean.

What does not change (no BC break)

In stock PHP the relational operators are non-associative, so a < b < c was a syntax error — this feature only gives previously-illegal code a meaning, so nothing that compiles today changes:

• The equality tier (== != === !== <=>) is untouched and stays non-associative: 1 == 1 == 1 is still a parse error.
• Mixed-precedence expressions keep their meaning: 1 < 2 == true is still (1 < 2) == true, because < binds tighter than ==.

How it works

Two small changes, no new tokens or opcodes:

1. Grammar: < <= > >= become left-associative (they were non-associative), so a < b < c parses as ((a < b) < c) instead of erroring.

2. Compiler: when a relational node's left operand is itself relational, the whole left-nested run is lowered to the chained && form, stashing each middle operand in a hidden write-once temporary so it is evaluated once and reused:

   a < b <= c   ==>   a < ($t1 = b) && $t1 <= c
   

   The temporaries are NUL-prefixed, interned, and unreachable from userland. Reusing ordinary &&, comparison and assignment nodes gives the short-circuit and single-evaluation behaviour for free. See feature.patch and the RFC.

Try it

scripts/setup.sh 15-chained-comparisons && scripts/build.sh
features/15-chained-comparisons/smoke.sh