Throw away matches

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I apologize if this has been discussed before, and I’m sure it probably has been, but it’s very hard to search for. I’ve been putting off asking about it for a while now, though, so I’m curious about this pattern:

_ = SomeMod.some_func("some_arg")

I’ve seen this in the docs before but the most prominent example I know of is the first two lines of the first function definition in Phoenix, here. If you aren’t a fan of clicking it looks like this:

# Warm up caches
_ = Phoenix.Template.engines()
_ = Phoenix.Template.format_encoder("index.html")

The best explanation I can come up with is that it’s signalling a side-effect. Is that it?? If it is, I find that kind of really nice, although perhaps at the same time it’s warmed me up a bit to typing (in Elixir) as it took me way to long to figure that out, lol.

But really, it is that it? If not, what’s up with this?

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It was probably to avoid so kind of generated warning. If i see function that is not assigned or matched to anything I would assume it has a side effect, so I would not see the need for _ =.

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If you ignore a return value you get a warning, so people just assign to _ when they are really not interested in what’s returned. And since a number of CI/CD pipelines include warnings_as_errors: true, you know.

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4

To focus the discussion. The warnings come from an (optional) dialyzer setting. That‘s why this approach is not present in all codebases, but just some.

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Yeah, I often use such pattern to mark that I run that piece of code for sole purpose of side effects and I do not care about return value. So it will not be perceived as unintentional omission, but deliberate choice of ignoring returned values.

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