Ruby Mapping for Enumerations

Ruby does not have an enumerated type, so a Slice enumeration is emulated using a Ruby class: the name of the Slice enumeration becomes the name of the Ruby class; for each enumerator, the class contains a constant with the same name as the enumerator. For example:

Slice

enum Fruit { Apple, Pear, Orange };

The generated Ruby class looks as follows:

Ruby

class Fruit
    include Comparable

    Apple = # ...
    Pear = # ...
    Orange = # ...

    def Fruit.from_int(val)

    def to_i

    def to_s

    def <=>(other)

    def hash

    # ...
end

The compiler generates a class constant for each enumerator that holds a corresponding instance of Fruit. The from_int class method returns an instance given its Slice value, while to_i returns the Slice value of an enumerator and to_s returns its Slice identifier.

Given the above definitions, we can use enumerated values as follows:

Ruby

f1 = Fruit::Apple
f2 = Fruit::Orange

if f1 == Fruit::Apple   # Compare for equality
    # ...

if f1 < f2              # Compare two enums
    # ...

case f2
when Fruit::Orange
    puts "found Orange"
else
    puts "found #{f2.to_s}"
end

Comparison operators are available as a result of including Comparable, which means a program can compare enumerators according to their Slice values. Note that, when using custom enumerator values, the order of enumerators by their Slice values may not match their order of declaration.

Suppose we modify the Slice definition to include a custom enumerator value:

Slice

enum Fruit { Apple, Pear = 3, Orange };

We can use from_int to examine the Slice values of the enumerators:

Ruby

Fruit::from_int(0)  # Apple
Fruit::from_int(1)  # nil
Fruit::from_int(3)  # Pear
Fruit::from_int(4)  # Orange

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