PHP and immutability - part two - Simon Holywell

[Planet PHP]

In the last article we learnt how to create an immutable data structure in PHP. There were a few issues to work through, but we got there in the end. Now onto making the immutable class more useful and easier to create modified copies. Note that these are copies and not modifications, in-place, to the original objects.

Simple parameter mutations


When you want to modify a property in an immutable object you must, by definition, create a new object and insert the modified value into that new location. You could simply get the values from the current instance and pass them into a new instance as you create it.

$a = new Immutable('Test');
echo $a->getX(); // Test
$b = new Immutable($a->getX() . ' again');
echo $b->getX(); // Test again

So simple. Too simple!

This technique works reasonably well for this small dataset, but what if we had five or ten parameters that would have to be replayed every time? An exaggerated example to illustrate my point follows.

$a = new Immutable('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K');
echo $a->getK(); // K
$b = new Immutable(
    $a->getA(), $a->getB(), $a->getC(), $a->getD(), $a->getE(), $a->getF(),
    $a->getG(), $a->getH(), $a->getI(), $a->getJ(), $a->getK() . ' some change'
);
echo $b->getK(); // K some change

It is certainly doable, but I, for one, am not going to be executing that every time I need to work with an immutable instance. Certainly, not if I can avoid it.

Mutation at clone time


There is a very handy quirk in PHP that we can exploit. It will allow us t

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