[chowells]

Reduce cannot calculate literally anything, in the sense you mean. It corresponds in computational power with primitive recursion. And quite famously, there are problems primitive recursion cannot solve that general recursion can.

On the other hand, I don't think I've ever seen something as recursive as Ackermann's function in real life. So it can probably solve any problem you actually mean to solve.

[epolanski]

What the previous user means is that reduce is not a function that returns a list (albeit it can).

It just accumulates over some value, and that value can be anything.

[chuckadams]

Naw, GP is right, I'd forgotten about the limits of primitive recursion. But for almost any given real-world problem, it's something you can get away with forgetting.