Simple Data. Rich Semantics. Adaptive Interaction.

Libertas bridges computational simplicity and the richness of human cognition through declarative interaction semantics.

A Boolean field holds true or false. Add a Default, a few words of answer text, or an optional (Nullable) type to its declaration, and every client asks a different question, each one correct for what the field means. A Union goes one step further: each answer brings its own questions. The App writes no UI code. Each example below shows the declaration, what the person sees, and why it matters.

Try the seven examples on an interactive phone: step from one declaration to the next and answer each question yourself.

Required decision

Rust

/// Accept invitation
/// Choose whether to accept the invitation.
pub accept_invitation: bool,

What the person sees: Yes and No, neither selected. The editor cannot be submitted until one is chosen. Yes stores true; No stores false.

Why it matters: an unanswered question is never quietly saved as false.

Illustrative phone region: Accept invitation, with Yes and No both unselected. — No Default: the person must choose.

Illustrative phone region: Accept invitation after choosing No; No is selected. — No selected: the value is `false`.

Answers in the person's words

Rust

/// Accept invitation
/// [TrueText]
/// I accept
/// [FalseText]
/// I decline
pub accept_invitation: bool,

What the person sees: the two answers read “I accept” and “I decline”. An answer is still required.

Why it matters: the person answers in plain words, while the App still receives true or false. Answer text never changes the stored value.

Illustrative phone region: I accept and I decline are both unselected. — I accept or I decline; neither is selected yet.

A preset preference

Rust

/// Receive reminders
#[libertas_default(false)]
pub receive_reminders: bool,

What the person sees: an on/off switch, starting off. A switch appears only when a Boolean has a Default and no answer text.

Why it matters: a setting with a sensible starting value takes one tap to change, and needs no answer to be submitted.

Illustrative phone region: Receive reminders with its switch off. — Off: the Default, `false`.

Illustrative phone region: Receive reminders after turning the switch on. — On: `true`.

A preset answer in the person's words

Rust

/// Receive reminders
/// [TrueText]
/// Send me reminders
/// [FalseText]
/// Do not send reminders
#[libertas_default(false)]
pub receive_reminders: bool,

What the person sees: two worded answers, with “Do not send reminders” already selected. Answer text keeps the two choices even with a Default.

Why it matters: the person sees exactly what the preset means and can change it, rather than guessing what “on” would do.

Illustrative phone region: Send me reminders and Do not send reminders are radio choices; Do not send reminders is selected. — Do not send reminders is preselected: `false`.

Three answers

Rust

/// Student status
/// [TrueText]
/// Yes, I am a student
/// [FalseText]
/// No, I am not a student
/// [NullText]
/// Prefer not to tell
pub student_status: Option<bool>,

What the person sees: three answers in this order, none selected. “Prefer not to tell” stores null, and all three stay visible after any choice.

Why it matters: declining to answer is a real answer, distinct from not having answered yet. Only an unanswered question blocks submission.

Illustrative phone region: Yes, I am a student; No, I am not a student; and Prefer not to tell are all unselected. — Three answers; none is selected yet.

Illustrative phone region: Prefer not to tell is selected; Yes and No remain visible. — Prefer not to tell: the value is `null`.

Null as a complete answer

Rust

/// Date of death in 2026
/// [NullText]
/// Alive at the end of 2026
/// [SetText]
/// Died in 2026 on
#[libertas_date_only]
#[libertas_number(min = 20260101, max = 20261231)]
#[libertas_default($null)]
pub death_date: Option<u32>,

This field comes from a tax return. Its optional date holds both the person's status and the date.

What the person sees: one question with two answers. “Alive at the end of 2026” starts selected, because the Default is null. Choosing “Died in 2026 on” opens a date editor directly below, limited to 2026. While the date is blank, the question is unanswered and the editor cannot be submitted.

Why it matters: null is a complete answer here, not missing data. One field replaces a separate status Boolean and a date that only sometimes applies.

Illustrative phone region: Alive at the end of 2026 is selected; no date editor is shown. — **Alive:** the value is `null`.

Illustrative phone region: Died in 2026 on is selected and an empty date editor shows Choose date. — **Died, date still blank:** unanswered; submission waits for a date.

Illustrative phone region: Died in 2026 on is selected and the date editor shows Aug 12, 2026. — **Died on August 12, 2026:** the value is `20260812`.

Answers that bring their own questions

Rust

/// Reply to the invitation
#[libertas_inline_choices]
pub reply: Reply,

pub enum Reply {
    /// I will attend
    Attending {
        /// Guests besides me
        #[libertas_number(min = 0, max = 3)]
        guests: u32,
    },
    /// Not sure yet
    Tentative {
        /// I will confirm by
        #[libertas_date_only]
        #[libertas_number(min = 20261101, max = 20261130)]
        confirm_by: u32,
    },
    /// I cannot attend
    Declined,
}

A Rust enum whose variants carry data is a Union. Each variant's comment is its answer text, and each variant's fields are the questions that answer brings.

What the person sees: three answers, none selected. Choosing one draws its own questions directly below it: a guest count under “I will attend”, a date under “Not sure yet”, nothing for “I cannot attend”. Choosing another answer replaces those questions.

Why it matters: the declaration is the whole condition, so the App writes no show/hide logic. An impossible combination, such as declining with two guests, cannot be stored. The editor can be submitted only when an answer is chosen and that answer's own questions are answered; a blank guest count is unanswered, never zero.

Illustrative phone region: I will attend, Not sure yet and I cannot attend are all unselected. — Three answers; none is selected yet.

Illustrative phone region: I will attend is selected, and Guests besides me shows 2 directly below it, above Not sure yet. — **I will attend, with 2 guests:** the value is `Attending { guests: 2 }`.

Illustrative phone region: I cannot attend is selected and no further question is shown. — **I cannot attend:** the guest count is gone; the value is `Declined`.

The same meaning in every language

Answer texts are ordinary localized documentation. Translating them changes only the words a person reads; the stored values, the validation and the way each question behaves stay the same, with no change to App code.

About these examples

Each fragment is one field of an App's Rust type, not a complete App. Every example is a fresh editable field with no value supplied by the App or copied from earlier data. The phone regions are illustrative mockups rendered from HTML, not product screenshots; each client draws the same controls in its own native style.

The exact rules live in the reference: Boolean answer controls, Nullable choices, inline choices for a Union, answer texts and other document properties, reserved Default values such as $null and validation.

More examples