So the reason for this is actually because OCaml has 2 types of expressions, which is sort of confusing. The first is a top-level and the second is just a norma

So the reason for this is actually because OCaml has 2 types of expressions, which is sort of confusing. The first is a top-level and the second is just a normal expression. In top-level expressions, they are implicitly put into scope. If you've seen any of the ";;" in tutorials, this indicates that you're working in the top-level (although these are pretty much always automatically inserted assuming you're not running a REPL). Now, if you're not in the top-level, you need to use the `let x = ... in ...` syntax, where the `in` is roughly analogous to a semi-colon. So the reason the first works is because the REPL runs at the top-level, and the second would only work inside of a function.