Question: what does mean in f#?
Answer: Up vote 4. (a -> b) means "function from a to b". In type annotation, it denotes a function type. For example, f : (int -> String) means that f refers to a function that takes an integer and returns a string. It is also used as a contstructor of such values, as in val f : (int -> int) = fun n -> n * 2.