yeah no you have to implement your own

Tests are limited to within Int range, which is specified to be at least 29 bits IIRC.

That fixed test added. Solution should be invalidated now.

Q: Does this have a problem with empty input?

That's undefined behavior

Its not? There just arent big enough test cases to see a difference. Try triangular 100000000 `shouldBe` 5000000050000000 for example.

Of course, I was just commenting because for me, as a mathematician, it was on the first glance confusing that this actually works :D

To be fair, the type definition is Int -> Int, not (Num a) => a -> Int, so a Double would never be allowed into the function.

Note that this definition would not work for Double, e.g. 2.25=1.5^2 would be sent to 2^2=4, because 1.5 gets rounded to 2, although 2.25 is closer to 1 than to 4.

Not hard to do by hand, just eta conversion and infix <-> prefix notation.

Definitely not a best practice in terms of code clarity though! :D

uhm.. what is the point?