Let’s take back our multiply function to see what’s a signature:
let multiply a b =
a * b;;
Here, the signature of the function is int -> int -> int. It corresponds, from left to right, to argument types and return type, separated by arrows.
The signature is automatically determined by the compiler, depending on what the function does with arguments. In our example, we multiply both arguments using the * operator, so the only possible signature is the one we have. If we write the same function using *., we will have another signature: float -> float -> float, because *. is defined for floats.