Elixir, no circular reference possible?

Question

I am learning Elixir, up to chapter 7 PragProg book, and after thinking about the immutability and other items, I was thinking it was not generally possible to create a circular reference in the Elixir Maps/Tuples/Lists, etc.. Where A -> B -> C -> A.

Without going into really trying to trick the system, is this true?

I found this in a post by Robert Virding [in the mailing list](http://erlang.org/pipermail/erlang-questions/2008-July/036446.html) "Erlang is suited for reference counting as it has no circular structures, well it has a few circular references but they are only in well-known places so no real problems." I guess you could request him for details (or for an answer here). :) — Dogbert, Dec 13 '16 at 08:20
You may want to modify your question a bit; I'm not sure that Maps, Tuples and Lists can be considered "equal" in the context of circular references. They're certainly not equivalent in other senses. You may want to narrow your question. — Onorio Catenacci, Dec 14 '16 at 18:54

Aleksei Matiushkin · Accepted Answer · 2016-12-14T17:58:18.350

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Due to the immutability, there is a chicken-egg problem creating circulars. In fact, there is nothing done by Elixir to prevent it; it just comes out of the box within the immutability.

Proof: Since C in your chain links A on creation, A must exist in advance; A in turn links B hence B must exist even before; B links C, requiring C to exist, but it is not yet created. QED.

One can not simply refer an unexisting Term^*, and can not modify the existing one, therefore, it is impossible in Elixir.

*_{please read the discussion in comments on wording here and why Term was finally chosen.}

edited Dec 14 '16 at 17:58

answered Dec 13 '16 at 06:49

Aleksei Matiushkin

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Well reasoned argument. – Onorio Catenacci Dec 13 '16 at 13:40
I think you might be able to create circular reference with process links. – Fred the Magic Wonder Dog Dec 13 '16 at 23:04
@FredtheMagicWonderDog you mean an agent, holding a reference to another agent? You can, but this won’t be an object referencing object. – Aleksei Matiushkin Dec 14 '16 at 07:04
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Yes, however using the word "object" gets very fuzzy. I think when discussing elixir/erlang using term to reference data is much clearer. Terms are immutable, thus you can't have a circular reference of Terms. – Fred the Magic Wonder Dog Dec 14 '16 at 17:56
@FredtheMagicWonderDog Agreed. I modified my answer accordingly. – Aleksei Matiushkin Dec 14 '16 at 17:58
Yeah that question of exact terminology had crossed my mind as well. – Onorio Catenacci Dec 14 '16 at 18:52

Elixir, no circular reference possible?

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