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.