Module expr 

Expr — a closed, serializable expression vocabulary over session state.

Where Guard is the closed vocabulary for boolean questions about state and marking, Expr is the closed vocabulary for values computed from state: the language of computed (derived) state variables authored as data. Every atom is a named, parameterized operation; there are no closures, so an Expr round-trips through JSON, can be edited in a UI, validated at load time (Expr::keys_read feeds the state-key diff), and evaluated identically in the live runtime, the offline simulator, and generated code.

use gemini_adk_rs::expr::Expr;
use gemini_adk_rs::state::State;

// risk = 0.6 * overdue_ratio + 0.4 * missed_payments / 10
let expr: Expr = serde_json::from_value(serde_json::json!({
    "add": [
        {"mul": [{"const": 0.6}, {"key": "overdue_ratio"}]},
        {"mul": [{"const": 0.04}, {"key": "missed_payments"}]}
    ]
})).unwrap();

let state = State::new();
state.set("overdue_ratio", 0.5).unwrap();
state.set("missed_payments", 3).unwrap();
assert_eq!(expr.eval(&state), Some(serde_json::json!(0.42)));

Evaluation semantics

• Reads go through State::get, so the derived: fallback applies — one computed variable can read another by its bare key.
• Arithmetic and comparison atoms are strict: a missing key or non-numeric operand makes the whole expression None (no write).
• Logic atoms (all/any/not) are total: a missing or non-boolean operand counts as false, mirroring Guard::is_true on an unset key.
• coalesce returns the first operand that evaluates to a value; if selects a branch on the truthiness of its condition.

Enums

Expr

A serializable expression over session state. See the module docs.