maxguzenski

Nil > 25 returns true – is this expected behavior?

Hi everyone,

I’m using Elixir 1.18.4 with OTP 27 (I’ve tested this across multiple versions of both Elixir and Erlang), and surprisingly, the expression nil > 25 returns true. Is this correct?

Both ChatGPT and Claude stated that this should not be possible — that a comparison like this should raise an ArgumentError, as nil cannot be compared to numbers using > or <. They even suggested that something might be wrong with my machine.

However, I tested this on https://playground.functional-rewire.com/ as well, and I got the same result: nil > 25 returns true.

So my question is:

Has it always been this way, or is this a recent change/bug?

Thanks in advance!

17 comments

#language #bug #behaviour

0 894 17

2025-07-03 16:02:16 UTC

First Post!

sodapopcan

Yes, comparison operators can be used across types. As for why nil > 25 returns true, see the good ol’ docs. Of note: nil is simply syntactic sugar for the atom :nil (you can prove it with iex> nil == :nil).

Post #1

Most Liked

sabiwara

Elixir Core Team

While there is no runtime warning for the reasons explained above, it’s worth noting that the new type system might be able to catch it and warn at compile time when it is sure types are disjoint:

comparison between distinct types found:

    nil > 25

given types:

    nil > integer()

While Elixir can compare across all types, you are comparing across types which are always disjoint, and the result is either always true or always false

Post #8

sodapopcan

You sound like one of those people who actually know how to use a dynamic programming language properly Also, we’re in the BEAM world here so I believe you meant “behaviour”

As grumpy old man jumping on the opportunity to rant this has been fairly common recently, “common” meaning this is the ~7th time in a couple of months I’ve seen a question online where people said they just asked AI, and not just for Elixir questions! I’m sure it’ll all be “fixed” soon enough, but I still enjoy reading docs.

Back on track, another little gotcha is that true and false are also sugar for :true and :false, so when you throw nil into the mix and compare them it’s done so alphabetically. I know a couple of people who got tripped up by this because they assumed that since true is greater than false that false would be greater than nil, but it’s not. Though again, if you aren’t abusing dynamic languages then you’ll never run into this

k, I’m off to go yell at a cloud.

Post #3

adamu

software developers in particular are prone to being convinced by these hazards and few in the field seem to have ever had that “oh my, I can’t always trust my own judgement and reasoning” moment

Post #6

Last Post!

dimitarvp

It is, but it’s also predictable, I was a teenager when I already expected it (30 years ago). There’s just something about Homo Sapiens that makes them want to believe in magic even with a mountain of evidence that it doesn’t exist.

It was only a matter of time. And, well, the time has come.

And back on topic, not checking after the LLM is a recipe for disaster. That’s also quite obvious. And for whomever it isn’t obvious, it’ll become obvious soon enough.

Post #17