Final answer:
It is true that in OCaml, you must explicitly state that a function is recursive by using the 'let rec' keyword. If the 'rec' keyword is not included, the function will not be able to call itself, which is typically required in recursive function definitions.
Step-by-step explanation:
True: In OCaml, you must explicitly state that a function is recursive. This is done by using the let rec keyword. Without the rec keyword, the OCaml compiler will not understand that the function should be able to call itself, and consequently, it will not allow the function to do so.
For example, if you're creating a function to calculate the factorial of a number, you need to define it recursively to handle the calculation correctly. Here's how you would define such a function:
let rec factorial n =
if n <= 1 then
1
else
n * factorial (n - 1)
In this case, the factorial function calls itself, and hence, the use of let rec is imperative to inform the compiler of its recursive nature.