Prove $0! = 1$ from first principles
Why does 0! = 1?
All I know of factorial is that x! is equal to the product of all the numbers that come before it. The product of 0 and anything is 0, and seems like it would be reasonable to assume that 0! = 0. I'm perplexed as to why I have to account for this condition in my factorial function (Trying to learn Haskell). Thanks.