rules-engine-demo

rules-engine-demo

This example shows how to build a business rules engine entirely on top of the high-level Lua API.

Rust owns the order data and host capabilities. Lua owns the decision logic. The example avoids low-level LuaVM usage, raw value arrays, and manual stack handling.

What this example demonstrates

• Exposing Rust capabilities with Lua::register_function()
• Building input data with Lua::create_table() and Table::set()
• Loading the rules script through Lua::load(...).exec()
• Calling evaluate_order(order) through Lua::call_global1() and decoding a structured result

Scenario

The sample models a checkout rules engine:

• Rust builds orders, customers, and line items
• Lua decides approval, discounts, shipping tier, and manual review
• Lua can call these Rust host functions during evaluation:
  • risk_score(email, total_cents, country)
  • inventory_available(sku, quantity)
  • shipping_eta(country, tier)
  • audit(message)

Run it

Use the embedded rules script:

cargo run -p rules-engine-demo

Use your own rules file:

cargo run -p rules-engine-demo -- --script examples/rules-engine-demo/scripts/checkout_rules.lua

Main files

• src/main.rs: registers host functions, builds tables, and calls Lua
• scripts/checkout_rules.lua: the Lua rules script

What you can change

Edit scripts/checkout_rules.lua to:

• raise or lower risk thresholds
• tune VIP discounts
• add country-specific compliance rules
• change shipping behavior for product categories

You can change the Lua policy and rerun the demo without touching the Rust host code.