Module function 

Answering the functions of a dialogue.

Where a >> command does something, a function answers something. The dialogue asks the game for a value and carries on with it, without ever suspending. The answer of a function is a Value

For example: [let $stat = get_stat(Alice)] reads a component off the entity Alice, [party_size()] counts them.

The arguments arrive already typed, as they do for a command, and the return type is written back by IntoValue:

#[derive(Resource)]
struct Gold(i64);

/// `Alice: I have [gold()] coins.`
fn gold(In(()): In<()>, gold: Res<Gold>) -> i64 {
    gold.0
}

Declare the name over the system. #[replique_function] writes down what the dialogue calls it, next to what it does, and add_dialogue_function registers it from there. This is the way to register a function: the name is written once, where it can be read and documented.

/// Coins the player has, as the dialogue reads it.
#[replique_function(name = "gold")]
fn coins(In(()): In<()>, gold: Res<Gold>) -> i64 {
    gold.0
}

app.add_dialogue_function(coins);

add_dialogue_function_named takes the name as a string instead, for a system the attribute cannot be put on.

A function may only read. The ReadOnlySystem bound makes it a compile error to register one that spawns, mutates a component or writes a message. Writing to the world is what a >> command is for.

A function cannot suspend. It answers on the spot, inside the call to DialogueVm::resume_with. Anything that has to wait for the player is a command.

A function that has no answer to give says so with a Result, which is return by the VM with an VmError::ExprEvalError.

#[derive(Component, PartialEq)]
struct Name(String);

/// `[let $stat = hp_of("Alice")]`
fn hp_of(In((who,)): In<(String,)>, names: Query<&Name>) -> Result<i64, String> {
    names
        .iter()
        .find(|name| name.0 == who)
        .map(|_| 12)
        .ok_or_else(|| format!("{who} is not on the scene"))
}

The bevy_custom_function example holds a full scene built this way.

Traits

DialogueFunctionAppExt

Registers the system to answer a function.

RepliqueFunction

A function declared with #[replique_function], name and doc included.