[hopia]

In functional programming classes at the uni you probably wouldn't hear this because if they teach Haskell which I assume most do, every function always takes just one argument. That's because functions are curried by default.

I personally haven't really heard this mentioned anywhere outside of the Erlang/Elixir sphere.

[cthalupa]

This thread really surprised me - I learned about arity when learning about ternary operators, which are pretty universal - you can find ?: in all sorts of languages. I think I was learning about C when I first got a detailed explanation on the subject.

[pmarreck]

Even if every function is curried by default (and "currying" is yet another concept that may be alien to users of some languages), it is probably still useful to know with what number of arguments a function can evaluate to a value that is not just another function