Advanced examples with classes and modules

Advanced examples with classes and modules

(Chapter written by Didier Rémy)

In this chapter, we show some larger examples using objects, classes and modules. We review many of the object features simultaneously on the example of a bank account. We show how modules taken from the standard library can be expressed as classes. Lastly, we describe a programming pattern know of as virtual types through the example of window managers.

5.1

Extended example: bank accounts

In this section, we illustrate most aspects of Object and inheritance by refining, debugging, and specializing the following initial naive definition of a simple bank account. (We reuse the module Euro defined at the end of chapter 3.)

#let euro = new Euro.c;;
val euro : float -> Euro.c = <fun>
 
#let zero = euro 0.;;
val zero : Euro.c = <obj>
 
#let neg x = x#times (-1.);;
val neg : < times : float -> 'a; .. > -> 'a = <fun>
 
#class account =
   object 
     val mutable balance = zero
     method balance = balance
     method deposit x = balance <- balance # plus x
     method withdraw x =
       if x#leq balance then (balance <- balance # plus (neg x); x) else zero
   end;;
class account :
  object
    method balance : Euro.c
    method deposit : Euro.c -> unit
    method withdraw : Euro.c -> Euro.c
    val mutable balance : Euro.c
  end
 
#let c = new account in c # deposit (euro 100.); c # withdraw (euro 50.);;
- : Euro.c = <obj>

We now refine this definition with a method to compute interest.

#class account_with_interests =
   object (self)
     inherit account
     method private interest = self # deposit (self # balance # times 0.03)
   end;;
class account_with_interests :
  object
    method balance : Euro.c
    method deposit : Euro.c -> unit
    method private interest : unit
    method withdraw : Euro.c -> Euro.c
    val mutable balance : Euro.c
  end

We make the method interest private, since clearly it should not be called freely from the outside. Here, it is only made accessible to subclasses that will manage monthly or yearly updates of the account.

We should soon fix a bug in the current definition: the deposit method can be used for withdrawing money by depositing negative amounts. We can fix this directly: