I've got a library which implements a set (interface with documentation available here: http://pastebin.com/j9QUyN1G). I understand everything apart from this fragment:
val iter : ('a -> unit) -> 'a t -> unit
(** [iter f s] applies [f] to all elements in set [s]. The elements
are passed to [f] in increasing order with respect to the ordering
used to create the set. *)
So iter takes a function as one of the arguements and applies it to all elements of set. So I would expect something like ('a -> 'a) which takes an element of the set and changes it to element of the same type with other value or ('a -> 'b) which takes 'a t and transforms it to 'b t. But instead iter takes a function of type ('a -> unit) and also returns unit, not an 'a t nor 'b t.
So how should an example function passed to iter look like?
iter doesn't change the elements of the set. It's executed purely for its side effects. You might use it to print the elements, for example:
module StringSet = Set.Make(String)
…
StringSet.iter print_endline ss
The set data structure is immutable, so you can't change the elements of the set. You can build a new set whose elements are derived from an existing set. For a list, there's the function map which takes a list [x1; …; xn] and returns a new list [f x1; …; f xn]. There is no similar function in the Set module because elements in a set are not stored in the order chosen by the caller: there's no such thing as a set with its elements in an order derived from another set. If you want to build a set from the images of the elements of a set, insert the new elements one by one.
module Int = struct
type t = int
let compare = Pervasives.compare
end
module IntSet = Set.Make(Int)
module StringSet = Set.Make(String)
let int_to_string_set is =
IntSet.fold (fun i ss -> StringSet.add (string_of_int i) ss) is StringSet.empty