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);;

Ok we have created the module now lets start putting something into it. Where do we start? Well lets try an empty module to begin with

  let m = MyUsers.empty;;

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

  let m = MyUsers.add "fred" "sugarplums" m;;

We have now added "fred" and his password of "sugarplums" to are module. 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 are mapping in are module MyUsers is from strings to strings. If we want a mapping from strings to integes or a mappings 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;;
  let m = MyUsers.add "mark" "ocamlrules" m;;
  let m = MyUsers.add "pete" "linux" m;;

Now that we have some data inside are Map module 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");;

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 have this function applied to are mapping. Here is what that would look like.

 MyUsers.iter print_users m;;

The reason we put are 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;;

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