chained_comparison_operators.md

Chained comparison operators

General idea

Currently in PHP operators if you want to check a number is in a range you have to reference it twice in an if statement:

if (0 <= x && x <= 1) {
    echo "x is between zero and one";
} 

Other languages allow operators to be chained:

if (0 <= x <= 1) {
    echo "x is between zero and one";
} 

The situation is even worse if you want to check one number is less than another, and that they are both in a range:

if (0 <= x <= y <= 1) {
    echo "Both x and y are between zero and one, and x is less than or equal to y.";
}

is the equivalent of:

if ((0 <= x <= 1) && (0 <= y <= 1) && (x <= y) ) {
    echo "Both x and y are between zero and one, and x is less than or equal to y.";
}

Lessons from Python

The implementation of chained operators in Python allows arbitrary chaining of different types of operators. This is a choice that people have come to regret.

Careful consideration of the allowed chaining would be better than just allowing everything.

Good

These are good examples that show how using chained comparision is better than not using it.

def chained_comparisons_good_uses(x, y, z):
    if 0 < x < 1:
        print("x in range (0,1)")

    if 0 <= x < 1:
        print("x in range [0,1)")

    if x <= y <= z:
        print("y in range [x,z]")

    if x >= y >= 1:
        print("y in range [1,x]")

    if x == y == z:
        print("x,y,z all equal")

Meh

These aren't terrible, but few people would call them easier to read than the unchained equivalents:

def chained_comparisons_ok_to_questionable_uses(x, y, z):
    if 0 < x < y == z < 1:
        print("x,y in (0,1) with x<y and z==y")

    if x <= y <= z != 1:
        print("y in range(x,z) with z != 1")

    if x == y == z != 1:
        print("x,y,z all equal something that isn't 1")

    if x is y is z:
        print("x,y,z all identical")

    if x is y is z in [1, 2, 3]:
        print("x,y,z all identical and in [1,2,3]")

Bad

These should be forbidden:

def chained_comparisons_bad_uses(x, y, z):
    if x < y > z:
        print("y > max(x,z)")

    if x != y != z:
        print("kinda looks like x,y,z all distinct, but may have x==z")

    if 0 > x < y != z > 1:
        print("WHY??")

    if 0 < x > 1 >> y << 1 < z > 1:
        print("please remove this from the language")

Note, the x != y != z is similar to the 'ungood' variadic empty rfc, which people did not understand imho.

Hurdles to overcome

Someone needs to have the time and inclination to implement it.

More restrictions than Python

Forecast

A lot of people will have a gut instinct against this, so the vote might be close even with a good implementation.

Notes

Some details of how they work in Python

Did you know that you can actually chain an arbitrary number of comparison operators? For example, a == b == c == d == e checks if all five variables are the same, while a < b < c < d < e checks if you have a strictly increasing sequence.

A nice video that explains some good uses, questionable uses, and bad uses of chained operators from about the 4 minute mark onwards.

Spare words

Python allows this:

if (0 <= x <= 10) {...}

i.e. compare that x is greater or equal than zero, and also that x is less than or equal to 10. It's nice as it reads nice, but also avoids a problem. That code is equivalent to:

if (0 <= x && x <= 10) {...}

But imagine we had:

if (0 <= foo() <= 10) {...}

and foo() had side-effects then the equivalent code:

if (0 <= foo() && foo() <= 10) {...}

is calling foo() twice, which is bad as the side-effect is produced twice. The workaround of:

y = foo()

if (0 <= y && y<= 10) {...}

is jankier, as it makes the code harder to read.

PHP would be better if we just stole this feature. Did I say steal? I meant be inspired by. I apologies for the confusion.