Set provides the functor Set.Make. You must start by passing Set.Make a module. It specifies the element type for your set. In return, you get another module with those elements' set operations.

Disclaimer: The examples in this tutorial require OCaml 5.1. If you're running a previous version of OCaml , you can either use elements instead of to_list, which is a new function in OCaml 5.1, or upgrade OCaml by running opam update, then opam upgrade ocaml. Check your current version with ocaml --version.

If you need to work with string sets, you must invoke Set.Make(String). That returns a new module.

# module StringSet = Set.Make(String);;
module StringSet :
    type elt = string
    type t = Set.Make(String).t
    val empty : t
    val add : elt -> t -> t
    val singleton : elt -> t
    val remove : elt -> t -> t
    val union : t -> t -> t
    val inter : t -> t -> t

After naming the newly-created module StringSet, OCaml's toplevel displays the module's signature. Since it contains a large number of functions, the output copied here is shortened for brevity (...).

This module also defines two types:

  • type elt = string for the elements, and
  • type t = Set.Make(String).t for the sets.

Creating a Set

  1. We can create an empty set using StringSet.empty:
# StringSet.empty ;;
- : StringSet.t = <abstr>

# StringSet.empty |> StringSet.to_list;;
- : string list = []

For StringSet.empty, you can see that the OCaml toplevel displays the placeholder <abstr> instead of the actual value. However, converting the string set to a list using StringSet.to_list results in an empty list.

(Remember, for OCaml versions before 5.1, it will be StringeSet.empty |> StringSet.elements;;)

  1. A set with a single element is created using StringSet.singleton:
# StringSet.singleton "hello";;
- : StringSet.t = <abstr>

# StringSet.(singleton "hello" |> to_list);;
- : string list = ["hello"]
  1. Converting a list into a set using StringSet.of_list:
# StringSet.of_list ["hello"; "hi"];;
- : StringSet.t = <abstr>

# StringSet.(of_list ["hello"; "hi"] |> to_list);;
- : string list = ["hello"; "hi"]

There's another relevant function StringSet.of_seq: string Seq.t -> StringSet.t that creates a set from a sequence.

Working With Sets

Let's look at a few functions for working with sets using these two sets.

# let first_set = ["hello"; "hi"] |> StringSet.of_list;;
- : val first_set : StringSet.t = <abstr>

# let second_set = ["good morning"; "hi"] |> StringSet.of_list;;
- : val second_set : StringSet.t = <abstr>

Adding an Element to a Set

# StringSet.(first_set |> add "good morning" |> to_list);;
- : string list = ["good morning"; "hello"; "hi"]

The function StringSet.add with type string -> StringSet.t -> StringSet.t takes both a string and a string set. It returns a new string set. Sets created with the Set.Make functor in OCaml are immutable, so every time you add or remove an element from a set, a new set is created. The old value is unchanged.

Removing an Element from a Set

# StringSet.(first_set |> remove "hello" |> to_list);;
- : string list = ["hi"]

The function StringSet.remove with type string -> StringSet.t -> StringSet.t takes both a string and a string set. It returns a new string set without the given string.

Union of Two Sets

# StringSet.(union first_set second_set |> to_list);;
- : string list = ["good morning"; "hello"; "hi"]

With the function StringSet.union, we can compute the union of two sets.

Intersection of Two Sets

# StringSet.(inter first_set second_set |> to_list);;
- : string list = ["hi"]

With the function StringSet.inter, we can compute the intersection of two sets.

Subtracting a Set from Another

# StringSet.(diff first_set second_set |> to_list);;
- : string list = ["hello"]

With the function StringSet.diff, we can remove the elements of the second set from the first set.

Filtering a Set

# ["good morning"; "hello"; "hi"]
  |> StringSet.of_list
  |> StringSet.filter (fun str -> String.length str <= 5)
  |> StringSet.to_list;;
- : string list = ["hello"; "hi"]

The function StringSet.filter of type (string -> bool) -> StringSet.t -> StringSet.t creates a new set by keeping the elements that satisfy a predicate from an existing set.

Checking if an Element is Contained in a Set

# ["good morning"; "hello"; "hi"]
  |> StringSet.of_list
  |> StringSet.mem "hello";;
- : bool = true

To check if an element is contained in a set, use the StringSet.mem function.

Sets With Custom Comparators

The Set.Make functor expects a module with two definitions: a type t that represents the element type and the function compare, whose signature is t -> t -> int. The String module matches that structure, so we could directly pass String as an argument to Set.Make. Incidentally, many other modules also have that structure, including Int and Float, so they too can be directly passed into Set.Make to construct a corresponding set module.

The StringSet module we created uses the built-in compare function provided by the String module.

Let's say we want to create a set of strings that performs a case-insensitive comparison instead of the case-sensitive comparison provided by

We can accomplish this by passing an ad-hoc module to the Set.Make function:

# module CISS = Set.Make(struct
  type t = string
  let compare a b = compare (String.lowercase_ascii a) (String.lowercase_ascii b)
- : sig
    type elt = string
    type t
    val empty : t
    val is_empty : t -> bool
    val mem : elt -> t -> bool
    val add : elt -> t -> t

We name the resulting module CISS (short for "Case Insensitive String Set").

You can see that this module has the intended behavior:

# CISS.singleton "hello" |> CISS.add "HELLO" |> CISS.to_list;;
- : string list = ["hello"]

The value "HELLO" is not added to the set because it is considered equal to the value "hello", which is already contained in the set.

You can use any type for elements, as long as you define a meaningful compare operation.

# type color = Red | Green | Blue;;
type color = Red | Green | Blue

# module SC = Set.Make(struct
  type t = color
  let compare a b =
    match a, b with
    | (Red, Red) -> 0
    | (Red, Green) -> 1
    | (Red, Blue) -> 1
    | (Green, Red) -> -1
    | (Green, Green) -> 0
    | (Green, Blue) -> 1
    | (Blue, Red) -> -1
    | (Blue, Green) -> -1
    | (Blue, Blue) -> 0


We gave an overview of OCaml's Set module by creating a StringSet module using the Set.Make functor. Further, we looked at how to create sets based on a custom comparison function. For more information, refer to Set in the Standard Library documentation.

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