Mainly, there are 3-4 reasons for considering alternatives to using booleans:

1. Booleans are primitive types without rich domain-specific information or context. When using true/false, it can be more beneficial to employ more descriptive options such as AnsweredYes/AnsweredNo, PositiveResult/NegativeResult, LightsOn/LightsOff, or UserOnline/UserOffline. Mixing and matching booleans of different domain types can lead to errors that the compiler may not catch. For instance:

    bool isLightsOn = true;
    bool isUserOnline = false;
    bool isAdmin = true;

    if(isLightsOn && !isUserOnline && !isAdmin) {
      ...
    }

compare with Elixir:

    case {lights_status, connection_status, user_role} do
       {:lights_on, :user_offline, :unprivileged_role} -> ...
       ...
    end

The example above is basically a Decision Table [1].

While booleans can still be used in system code or libraries, it is advisable to avoid them in application code.

2. Preemptive pluralization [2] makes it easier to refactor code. Going from two options to three is simpler than transitioning from one option to two. Consider the following examples:

    bool isLightsOn;

or

    type LightsStatus = Bool

vs

    type LightsStatus = LightsOn | LightsOff

Suppose there is a new requirement to support dimmed lights. In the former case, significant code refactoring is necessary. In the latter case, adding a new option to the sum type is straightforward, and the compiler assists in handling the changes:

    type LightsStatus = LightsOn | LightsOff | LightsDimmed

Similarly, let's examine another scenario:

    boolean isOnline;

or

    type ConnectionStatus = Bool

vs

type ConnectionStatus = Online | Offline

The latter option can easily evolve into:

    type ConnectionStatus = Online | Offline | BadConnectivity

and later:

    type ConnectionStatus = 
        | OnlineMobile
        | OnlineFastInternet
        | Offline
        | BadConnectivity
        | IntermittentConnection
        | ConnectedToMeshnet

3. Booleans encourage the overuse of conditionals and branching, including nested branching. This can increase the cyclomatic complexity [3] of the code. It is important to manage cyclomatic complexity to keep the codebase maintainable and easier to reason about.

Also see "Get rid of those boolean function parameters" [4].

4. Booleans can indicate whether something occurred, but they lack both the necessary context and the timestamp indicating when the event took place. However, by transforming booleans into nullable or optional timestamps, one can achieve a rudimentary implementation of Domain Events or Event Sourcing.

   user_connected_at:   nullable(Timestamp)
   lights_turned_on_at: nullable(Timestamp)
   record_deleted_at:   nullable(Timestamp)

NOTE: I'm not advocating for the pattern above, but it's widely used because of the limitations inherent in booleans.

--

References:

[1] https://en.wikipedia.org/wiki/Decision_table

[2] https://www.swyx.io/preemptive-pluralization

[3] https://en.wikipedia.org/wiki/Cyclomatic_complexity

[4] https://mortoray.com/get-rid-of-those-boolean-function-param...