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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:
# class safe_account =
#
object
#
inherit account
#
method deposit x = if zero#leq x then balance <- balance#plus x
#
end;;
class safe_account :
object
method balance : Euro.c
method deposit : Euro.c -> unit
method withdraw : Euro.c -> Euro.c
val mutable balance : Euro.c
end
However, the bug might be fixed more safely by the following definition:
# class safe_account =
#
object
#
inherit account as unsafe
#
method deposit x =