What's the reason of marking a recursive function as rec in F#?

Question

I am not sure if this is a stupid question but I was going through the tutorial that comes with VS 2010 and there is a function like this:

let rec factorial          


        

Answer 1

It's necessary so that the function can be recursive. A non-rec function only knows about bindings at the place where it's defined, not after (so it doesn't know about itself).