what is the purpose of using ":" in OCaml function declaration

Question

What is actually meant by the OCaml statement?

let func (v: A.a) : unit =    

#rest of the function

1. does it mean it takes v of type A.a and return unit
2. or it takes two parameters, v and A.a and return a unit?
3. or it takes a function v with parameter A.a and returns a unit?
4. or something else?

Answer 1

let func (v: A.a) : unit =

The first : means v is a parameter and its type is expected to be A.a.

The second : means func is expected to return a type of unit