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There's a general rule to name any unused variables with an _ in Elixir. Doing so stops anything being bound to that variable.

However I have noticed a widely used pattern of prefixing with an underscore to denote an ignored argument, in the form of _tail (with the intention being to provide a hint as to what the variable would be).

This is encouraged by the language via a warning in the shell if you try to then access _tail:

warning: the underscored variable "_tail" is used after being set. A leading underscore indicates that the value of the variable should be ignored. If this is intended please rename the variable to remove the underscore

But here's the catch; _tail has the variable bound to it, whereas when using just _ it does not.

Does this mean that there's a performance penalty when naming ignored variables with anything other than an _? Or does Elixir still bind _ behind the scenes, and just error on any attempt to access?

Edit: It looks like the Erlang compiler specifically optimizes this case to treat _* as _ and thus there is no overhead, source: http://erlang.org/doc/efficiency_guide/myths.html

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    "The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming." --Donald Knuth. Worry about a performance penalty when you find a problem with performance in your code and not before. Commented Dec 28, 2015 at 14:03
  • @OnorioCatenacci I'm more interested in how it works rather than using it for improving performance at the moment. Generally I agree with the quote you used, however not worrying about something until it becomes a problem is quite an ignorant way to work. For example; I would probably never learn the answer to this question, as I doubt anything would warrant looking at performance this closely. Commented Dec 28, 2015 at 18:15
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    @OnorioCatenacci totally agree if your code isn't working that it's not worth it. The process you described, I'm basically at the end stage of making it fast - but the customer is myself (side project). That's what lead me to ask the question above - but only because I got to wondering what it different it actually makes (if any), rather than suspecting it of any particular issue. Commented Dec 28, 2015 at 18:35
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    BTW there is some discussion of _var vs. _ in this thread from the Elixir Core mailing list: groups.google.com/forum/#!searchin/elixir-lang-core/underscore/… I'm not sure it's what you're looking for but FWIW. Commented Dec 28, 2015 at 18:35
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    Ah, ok. I did misunderstand the nature of your question then @zackehh. My apologies for the misunderstanding. Commented Dec 29, 2015 at 13:35

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Given everyone already gave the disclaimer to not worry about this kind of performance behaviour, the answer is: if a variable is not used, the compiler will notice it and the compiled bytecode will simply ignore it, as if you used _. That's the same reason why if you do x = 1 and never x, you get a compiler warning.

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Thanks for weighing in. I hoped someone with more ability to speak to the underlying mechanics would add a comment or an answer.
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Indeed there's special behaviour in Erlang (and hence Elixir) for the _ variable. But unless you measured that this is a performance problem for your application, I wouldn’t worry too much about this. I imagine overhead of binding variables would be completely insignificant if you're doing anything interesting inside the function.

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I'm not really asking for personal use, I'm more interested in the behaviour pattern - e.g. is "tail" treated the same as ""?
Just noticed my comment was butchered by markdown :) _tail treated the same as _ :)
No, it is not. _tail is treated exactly the same as any other variable (besides _). You could say that the leading underscore only controls compiler warnings.

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