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Expression

Tiny safe rule formulas live in serenade-expression (#251).

Symfony ExpressionLanguage shaped: boolean / arithmetic / dotted property paths only. Not a scripting language. No function calls. No arbitrary Rust eval.

Pieces

Type Role
Value Bool, Int, Str
ExpressionContext Flat key → value map (user.role is one key)
evaluate / evaluate_bool Parse + run
ExpressionError Parse, unknown variable, type, division by zero
ExpressionGuard (in serenade-workflow) Workflow guard from an expression string

Operators

== != < <= > >= && || ! + - * / and parentheses. Literals: true / false, integers, "strings" (escapes \" \\ \n \t).

Example (security-style check)

use serenade_expression::{ExpressionContext, Value, evaluate_bool};

let mut ctx = ExpressionContext::new();
ctx.insert("user.role", Value::string("admin"));
ctx.insert("count", Value::Int(2));
assert!(evaluate_bool(r#"user.role == "admin" && count > 0"#, &ctx)?);

Example (workflow guard)

use std::sync::Arc;
use serenade_workflow::ExpressionGuard;

workflow.add_guard(
    "publish",
    Arc::new(ExpressionGuard::new(
        r#"!(subject_id == "guest") && place.draft"#,
    )),
);

Built-in guard variables: subject_id, workflow, transition, and place.<name> for each place in the current marking.

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