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Comment by colanderman | Hacker News Reader

This isn't a problem; you can get around it by indirecting such "pointers" through an (immutable) array. When you want to reference something, make sure it's in the array and reference its location. When you want to "mutate" something, update the array with that thing's old entry pointing to its new value instead.

(Side note: the union-find algorithm is another interesting algorithm with the same "problem".)

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I came across a similar solution when creating graphs with immutable data structures. Just store your nodes in an array, and describe the links between them as pairs of indexes. It does require that extra level of indirection, as hinted at in the "Dual Index Problem" article he links to. It is a bit odd, since you're basically reimplementing mutable pointers, where each operation returns a new set of memory.

I will add as a disclaimer that I'm somewhat new to the world of immutable data, and am still experimenting with this graph structure -- seeing how it scales with complicated data and relations.

"This isn't a problem"? You're so sure?

How many nodes are there? Let's presume there are a lot, otherwise the problem is trivial and speed doesn't matter anyway. So now you have an N-long immutable array that you are copying every time you want to change a node pointer? So you have changed the operation of pointer-changing from O(1) to O(N)? What does that do to the run time of the algorithm?

Also, your garbage velocity just went WAY up. What does this do for the runtime of your program generally?

(The language implementation can of course engage in strategies to avoid a full array copy, that you as the user have little insight into, but these are often questionable as they slow down run time in other cases, and anyway, they can only mitigate this problem, which is not going to go away.)

Uh, most good implementations of languages with immutable arrays perform O(1) or O(log n) array updates. The two I use most, Mercury and Erlang, both do (O(1) and O(log n) respectively).

If you, the user, has "little insight" into the runtime of data structures you're using, you have bigger issues. Every standard library I've ever seen guarantees the asymptotic behavior of hash tables, sets, linked lists, etc. Immutable arrays are no different.

1. Yes, I'm sure. See my reply below.

2. Excessive generation of garbage is a problem for functional languages in general; that is nothing specific to immutable arrays. Good compilers analyze object lifetimes and perform updates in-place when possible (e.g. Mercury does this). Even if your compiler doesn't do this, the garbage generated is constant or logarithmic with respect to the size of the array for most common immutable-array implementations.

So yes, I'm sure.

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