Module Map

Map creates a "mapping". For instance, lets say I have some data that is users and their associated passwords. I could with the Map module create a mapping from user names to their passwords. The mapping module not only does this but it does it fairly efficiently. It also does this in a functional way. In the example below I am going to do a mapping from strings to strings. However it is possible to do mappings with all different types of data.

To create a Map I can do:

# module MyUsers = Map.Make(String);;
module MyUsers : sig type key = String.t type 'a t = 'a Map.Make(String).t val empty : 'a t val is_empty : 'a t -> bool val mem : key -> 'a t -> bool val add : key -> 'a -> 'a t -> 'a t val singleton : key -> 'a -> 'a t val remove : key -> 'a t -> 'a t val merge : (key -> 'a option -> 'b option -> 'c option) -> 'a t -> 'b t -> 'c t val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool val iter : (key -> 'a -> unit) -> 'a t -> unit val fold : (key -> 'a -> 'b -> 'b) -> 'a t -> 'b -> 'b val for_all : (key -> 'a -> bool) -> 'a t -> bool val exists : (key -> 'a -> bool) -> 'a t -> bool val filter : (key -> 'a -> bool) -> 'a t -> 'a t val partition : (key -> 'a -> bool) -> 'a t -> 'a t * 'a t val cardinal : 'a t -> int val bindings : 'a t -> (key * 'a) list val min_binding : 'a t -> key * 'a val max_binding : 'a t -> key * 'a val choose : 'a t -> key * 'a val split : key -> 'a t -> 'a t * 'a option * 'a t val find : key -> 'a t -> 'a val map : ('a -> 'b) -> 'a t -> 'b t val mapi : (key -> 'a -> 'b) -> 'a t -> 'b t end

OK, we have created the module MyUsers. Now, lets start putting something into it. Where do we start? Well, let's create an empty map to begin with:

# let m = MyUsers.empty;;
val m : 'a MyUsers.t = <abstr>

Hummm. An empty map is kind of boring, so lets add some data.

# let m = MyUsers.add "fred" "sugarplums" m;;
val m : string MyUsers.t = <abstr>

We have now created a new map—again called m, thus masking the previous one—by adding "fred" and his password "sugarplums" to our previous empty map. There is a fairly important point to make here. Once we have added the string "sugarplums" we have fixed the types of mappings that we can do. This means our mapping in our module MyUsers is from strings to strings. If we want a mapping from strings to integers or a mapping from integers to whatever we will have to create a different mapping.

Lets add in some additional data just for kicks.

# let m = MyUsers.add "tom" "ilovelucy" m;;
val m : string MyUsers.t = <abstr> # let m = MyUsers.add "mark" "ocamlrules" m;;
val m : string MyUsers.t = <abstr> # let m = MyUsers.add "pete" "linux" m;;
val m : string MyUsers.t = <abstr>

Now that we have some data inside of our map, wouldn't it be nice to be able to view that data at some point? Lets begin by creating a simple print function.

# let print_users key password =
    print_string(key ^ " " ^ password ^ "\n");;
val print_users : string -> string -> unit = <fun>

We have here a function that will take two strings, a key and a pasword, and print them out nicely, including a new line character at the end. All we need to do is to have this function applied to our mapping. Here is what that would look like.

# MyUsers.iter print_users m;;
fred sugarplums mark ocamlrules pete linux tom ilovelucy - : unit = ()

The reason we put our data into a mapping however is probably so we can quickly find the data. Lets actually show how to do a find.

# MyUsers.find "fred" m;;
- : string = "sugarplums"

This should quickly and efficienty return Fred's password: "sugarplums".