Example: Standard Library Map

The standard library's Map module, which implements a dictionary data structure using balanced binary trees, is based on functors. In this section, we study how to use it. You can see the implementation of that module on GitHub as well as its interface.

The Map module defines a functor Make that creates a structure implementing a map over a particular type of keys. That type is the input structure to Make. The type of that input structure is Map.OrderedType, which are types that support a compare operation:

module type OrderedType = sig
  type t
  val compare : t -> t -> int

The Map module needs ordering because balanced binary trees need to be able to compare keys to determine whether one is greater than another. According to the library's documentation, compare must satisfy this specification:

(* This is a two-argument function [f] such that
 * [f e1 e2] is zero if the keys [e1] and [e2] are equal,
 * [f e1 e2] is strictly negative if [e1] is smaller than [e2],
 * and [f e1 e2] is strictly positive if [e1] is greater than [e2].
 * Example: a suitable ordering function is the generic structural
 * comparison function [Stdlib.compare]. *)
val compare : t -> t -> int

Arguably this specification is a missed opportunity for good design: the library designers could instead have defined a variant:

type order = LT | EQ | GT

and required the output type of compare to be order. But historically many languages have used comparison functions with similar specifications, such as the C standard library's strcmp function.

The output of Map.Make is a structure whose type is (almost) Map.S and supports all the usual operations we would expect from a dictionary:

module type S =
    type key
    type 'a t

    val empty: 'a t
    val mem:   key -> 'a t -> bool
    val add:   key -> 'a -> 'a t -> 'a t
    val find:  key -> 'a t -> 'a

There are two reasons why we say that the output is "almost" that type:

  1. The Map module actually specifies a sharing constraint (which we covered in the previous notes): type key = Ord.t. That is, the output of Map.Make shares its key type with the type Ord.t. That enables keys to be compared with Ord.compare. The way that sharing constraint is specified is in the type of Make (which can be found in map.mli, the interface file for the map compilation unit):

    module Make : functor (Ord : OrderedType) -> (S with type key = Ord.t)
  2. The Map module actually specifies something called a variance on the representation type, writing +'a t instead of 'a t as we did above. We won't concern ourselves with what this means; it's related to subtyping and polymorphic variants.

The functor Map.Make itself (which can be found in map.ml, the implementation file for the map compilation unit) is currently defined as follows, though of course the library is free to change its internals in the future:

module Make(Ord: OrderedType) = struct
  type key = Ord.t

  type 'a t =
    | Empty
    | Node of 'a t * key * 'a * 'a t * int
      (* left subtree * key * value * right subtree * height of node *)

  let empty = Empty

  let rec mem x = function
    | Empty -> false
    | Node(l, v, _, r, _) ->
        let c = Ord.compare x v in
        c = 0 || mem x (if c < 0 then l else r)

The key type is defined to be a synonym for the type t inside Ord, so key values are comparable using Ord.compare. The mem function uses that to compare keys and decide whether to recurse on the left subtree or right subtree.

Using the Map Module

A map for integer keys. To create a map, we have to pass a structure into Map.Make, and that structure has to define a type t and compare function. The simplest way to do that is to pass an anonymous structure into the functor:

# module IntMap = Map.Make(struct type t = int let compare = Stdlib.compare end);;
module IntMap : 
    type key = int                                                              
    type 'a t
    val empty : 'a t

# open IntMap;;

# let m1 = add 1 "one" empty;;
val m1 : string t = <abstr>

# find 1 m1;;
- : string = "one"

# mem 42 m1;;
- : bool = false

# find 42 m1;;
Exception: Not_found.

# bindings m1;;
- : (int * string) list = [(1, "one")]

# let m2 = add 1 1. empty;;
val m2 : float t = <abstr>

# bindings m2;;
- : (int * float) list = [(1, 1.)]

Here are some things to note about the utop transcript above:

  • We can write a structure on one line, even though until now we've always used line breaks to keep them readable. When writing a structure on on line (which we'll only do for really short structures) it can be useful to use the double semicolon between definitions to enhance readability:

    # module IntMap = Map.Make(struct type t = int;; let compare = Stdlib.compare end);;

    This is an exception to the general style rule of avoiding double semicolon inside source code.

    If we didn't want to pass an anonymous structure, we could instead define a module and pass it:

    module Int = struct
      type t = int
      let compare = Stdlib.compare
    module IntMap = Map.Make(Int)
  • The signature of the structure returned by Map.Make records the fact that keys are of type int. The type 'a t is the name of the representation type of an IntMap. The 'a type variable in it is the type of values in the map. Although in general the map could have any value type, once we add a single value to a map, that "pins down" the value type of that particular map. When we add the binding from key 1 to string "one above, notice that the map value returned is of type string t.

  • The bindings function of a map returns an association list of all the bindings in the map. Association lists are, of course, another data structure that implements a dictionary. But they are less efficient than the balanced binary search tree implementation used by Map.

  • The mem function tests whether a key is a member of a map. The find function finds the value associated with a key, and raises the Not_found exception if the key is not bound in the map. That's the same exception that List.assoc raises if a key is not bound in an association list.

A map for string keys. If a module already provides a type t that can be compared, we can immediately use that module as an argument to Map.Make. Several standard library modules are designed to be used in that way. For example, the String module defines a type t and a compare function that meet the specification of Map.OrderedType. So we can easily create maps whose key type is string:

# module StringMap = Map.Make(String);;
module StringMap :
    type key = string                                                           

Now we could use the string map like we used the int map. This time, for sake of example, let's not open the StringMap module:

# let m = StringMap.(add "one" 1 empty);;
# let m' = StringMap.(add "two" 2 m);;
# StringMap.bindings m';;
- : (string * int) list = [("one", 1); ("two", 2)] 
# StringMap.bindings m;;
- : (string * int) list = [("one", 1)] 

Note that maps are a functional data structure: adding a mapping to m did not mutate m; rather, it produced a new map that we bound to m', and both the new map and old map remain available for use.

A map for record keys. When the type of a key becomes more complicated than a built-in primitive type, we might want to write our own custom comparison function. For example, suppose we want a map in which keys are records representing names, and in which names are sorted alphabetically by last name then by first name. In the code below, we provide a module Name that can compare records that way:

type name = {first:string; last:string}

module Name = struct
  type t = name
  let compare {first=first1;last=last1}
              {first=first2;last=last2} =
    match Stdlib.compare last1 last2 with
    | 0 -> Stdlib.compare first1 first2
    | c -> c

The Name module can be used as input to Map.Make because it matches the Map.OrderedType signature:

module NameMap = Map.Make(Name)

And now we could add some names to a map. Below, for sake of example, we map some names to birth years, and we use the pipeline operator to easily add multiple bindings one after another:

let k1 = {last="Kardashian"; first="Kourtney"}
let k2 = {last="Kardashian"; first="Kimberly"}
let k3 = {last="Kardashian"; first="Khloe"}
let k4 = {last="West"; first="Kanye"}

let nm = NameMap.(
  empty |> add k1 1979 |> add k2 1980 
        |> add k3 1984 |> add k4 1977)

let lst = NameMap.bindings nm

The value of lst will be

[({first = "Khloe"; last = "Kardashian"}, 1984);
 ({first = "Kimberly"; last = "Kardashian"}, 1980);
 ({first = "Kourtney"; last = "Kardashian"}, 1979);
 ({first = "Kanye"; last = "West"}, 1977)]

Note how the order of keys in that list is not the same as the order in which we added them. The list is sorted according to the Name.compare function we wrote. Several of the other functions in the Map.S signature will also process map bindings in that sorted order—for example, map, fold, and iter.

Code Reuse with Map

Stepping back from the mechanics of how to use Map, let's think about how it achieves code reuse. The implementor of Map had a tricky problem to solve: balanced binary search trees require a way to compare keys, but the implementor can't know in advance all the different types of keys that a client of the data structure will want to use. And each type of key might need its own comparison function. Although the standard library's Stdlib.compare can be used to compare any two values of the same type, the result it returns isn't necessarily what a client will want. For example, it's not guaranteed to sort names in the way we wanted above.

So the implementor of Map parameterized it on a structure that bundles together the type of keys with a function that can be used to compare them. It's the client's responsibility to implement that structure. Given it, all the code in Map can be re-used by the client.

The Java Collections Framework solves a similar problem in the TreeMap class, which has a constructor that takes a Comparator. There, the client has the responsibility of implementing a class for comparisons, rather than a structure. Though the language features are different, the idea is the same.

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