We can define a
become_older method that accepts a number indicating the years to grow:
class Person getter :age def initialize(@name : String, @age : Int = 0) end def become_older @age += 1 end def become_older(years) @age += years end end john = Person.new "John" john.age # => 0 john.become_older john.age # => 1 john.become_older 5 john.age # => 6
That is, you can have different methods with the same name and different number of parameters and they will be considered as separate methods. This is called method overloading.
Methods overload by several criteria:
- The number of parameters
- The type restrictions applied to parameters
- The names of required named parameters
- Whether the method accepts a block or not
For example, we can define four different
class Person @age = 0 # Increases age by one def become_older @age += 1 end # Increases age by the given number of years def become_older(years : Int32) @age += years end # Increases age by the given number of years, as a String def become_older(years : String) @age += years.to_i end # Yields the current age of this person and increases # its age by the value returned by the block def become_older(&) @age += yield @age end end person = Person.new "John" person.become_older person.age # => 1 person.become_older 5 person.age # => 6 person.become_older "12" person.age # => 18 person.become_older do |current_age| current_age < 20 ? 10 : 30 end person.age # => 28
Note that in the case of the method that yields, the compiler figured this out because there's a
yield expression. To make this more explicit, you can add a dummy
&block parameter at the end:
class Person @age = 0 def become_older(&block) @age += yield @age end end
In generated documentation the dummy
&block method will always appear, regardless of you writing it or not.
Given the same number of parameters, the compiler will try to sort them by leaving the less restrictive ones to the end:
class Person @age = 0 # First, this method is defined def become_older(age) @age += age end # Since "String" is more restrictive than no restriction # at all, the compiler puts this method before the previous # one when considering which overload matches. def become_older(age : String) @age += age.to_i end end person = Person.new "John" # Invokes the first definition person.become_older 20 # Invokes the second definition person.become_older "12"
However, the compiler cannot always figure out the order because there isn't always a total ordering, so it's always better to put less restrictive methods at the end.