Literal Ruby types

In Ruby, a type is any object that responds to === (triple-equals case equality) with a truthy or falsy value. Pretty much every object in Ruby is already a type.

For example, Ruby classes respond to === by checking if the given object is an instance of the class or an instance of a subclass of the class.

String === "Hello" # => true
String === 42 # => false

Objects typically respond to === by checking if the given object is an equivalent object, it’s essentially an alias of == (double-equals):

"Hello" === "Hello" # => true
"Hello" === "World" # => false

The === method on a Range will check if the given object is within the range:

(1..10) === 5 # => true
(1..10) === 15 # => false

Procs alias === to call:

is_even = proc { |it| it % 2 == 0 }
is_even === 42 # => true
is_even === 21 # => false

How Ruby uses types

Ruby uses these types in a few different ways already. For example, in a case statement, Ruby will use the === method of each case to determine if it matches the case.

case 42
when String
  puts "It's a string!"
when Integer
  puts "It's an integer!"
end

The above case statement is essentially equivalent to the following:

object = 42

if String === object
  puts "It's a string!"
elsif Integer === object
  puts "It's an integer!"
end

Ruby also uses types for pattern matching, both in and out of case statements.

case [42]
in Array[String]
  puts "It's a string in an array!"
in Array[Integer]
  puts "It's an integer in an array!"
end

The above pattern matching is essentially equivalent to the following:

object = [42]

if Array === object && String === object.deconstruct[0]
  puts "It's a string in an array!"
elsif Array === object && Integer === object.deconstruct[0]
  puts "It's an integer in an array!"
end

Ruby also uses types for methods on Arrays:

[1, 2, 3].all?(String) # false
[1, 2, 3].all?(Integer) # true

[1, "2", 3].any?(String) # true
[1, "2", 3].any?(Integer) # true

The methods all? and any? determine if all or any of the elements in the array match the given type by calling its === method. This is more efficient than passing a block and (depending on the type) usually avoids object allocations.

What about generics?

If a type is any object that responds to ===, then we can create generic functions that generate parameterized types.

Let’s make a generic function that generates a type that checks that an object is an array and all of its elements are of a certain type.

For this example, we can just make a method that generates a proc, since as we saw above, procs respond to ===. It would make sense to call this function Array, but since Ruby already has a method called Array, we’ll prefix it with an underscore _Array.

def _Array(type)
  proc { |it| Array === it && it.all?(type) }
end

Now we can use this generic function, passing it a type:

_Array(String) === [1, 2, 3] # false
_Array(Integer) === [1, 2, 3] # true

Literal has a number of built-in generics just like this.