9. Enumerations

Enumerations define a type whose values come from a fixed set of named cases. They are useful when you want to model a small closed set of states or options.

An enumeration is defined like:

enum State:
    ON
    OFF
end

And used as follows:

s1 := State.ON
s2 := State.OFF
assert(s1 != s2)
assert(s1 == State.ON)

Enumeration values are qualified with the enum name, so State.ON and State.OFF are distinct values of the type State.

Enums work naturally with control flow:

func is_on(s):
    if s == State.ON:
        return true
    end
    return false
end

When code needs to handle every possible enum value, use an exhaustive match. Every enum member must be handled exactly once, so adding a new member makes affected matches fail at compile time until they are updated.

func signal_name(s: State, r: Region) -> String:
    match s:
        case State.ON:
            return "on"[r]
        case State.OFF:
            return "off"[r]
    end
end

Enum cases must be qualified, including for imported enums. case State.ON: is valid; bare case ON: is not.

Use enums when the possible values are known in advance and do not need to carry additional data. If each case needs associated values, variants are the more general construct.

A table can associate one stored value with every member of an enum.

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