this post was submitted on 12 Feb 2025
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Programming
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Simply put, because you often want to change the state of something without breaking all the references to it.
Wild off the top of my head example: you're simulating a football game. Everything is represented by objects which hold references to other objects that are relevant. The ball object is held by player object W, player object X is in collision with and holds a reference to player object Y, player Z is forming a plan to pass to player object X (and that plan object holds a reference to player object X) and so on.
You want to be able to change the state of the ball object (its position say) without creating a new object, because that would invalidate how every other existing object relates to the ball.
What you need here is not the stability in memory (i.e. of pointers, which you lose when you recreate an object) but instead just the stability of an identifier (e.g. the index into a list).
This is close, but as someone already said, an index into a list just means you are mutating the list.
Your stable "identifier" needs to be a function, ie. a reused design pattern. Compared to the list, this would be an index function which gets an element from an arbitrary list, meaning you don't have to mutate your list anymore, you just build a new one which still works with your function.
This is why languages which avoid mutation and side effects are always (to my knowledge) functional languages.