Enumerations

An enumeration defines a common type for a group of related values and enables you to work with those values in a type-safe way within your code.

Enumeration Syntax#

You introduce enumerations with the enum keyword and place their entire definition within a pair of braces:

public enum Option<A> {
    case Some(A)
    case None
}

Matching Enumeration Values with a Match Expression#

You can match individual enumeration values with a match expression:

let a = Some(1);
match a {
    case Some(i) => {
        print("Some")
    }
    case None => {
        print("None")
    }
}

Method#

Methods are functions that belong to instances of a particular enumeration. You write an instance method within the opening and closing braces of the enumeration it belongs to.

Here’s an example that defines a simple Option class, which is defined in the standard library:

public enum Option<A> {
    case Some(A)
    case None

    public isSome(): boolean {
        match this {
            case Some(_) => true
            case _ => false
        }
    }

    public isNone(): boolean {
        match this {
            case None => true
            case Some(_) => false
        }
    }

}

The Option class defines two instance methods:

• isSome() judges if the enum is in the case of Some
• isNone() judges if the enum is in the case of None

You call instance methods with the same dot syntax as properties:

let a = Some(1);
print(a.isSome());  // => true
print(a.isNone());  // => false