Module 1: Your First Legal Rule

Prerequisites: Module 0

In this module, you'll write your first legal rule in L4—a simple legal obligation with conditions, deadlines, and consequences.

Learning Objectives

By the end of this module, you will be able to:

Write a basic legal obligation using PARTY and MUST
Add conditions using IF
Set deadlines using WITHIN
Define consequences using HENCE and LEST
Test your rule using #EVAL

A Simple Legal Obligation

Let's start with something every lawyer understands: a simple legal obligation. Find the complete working example later in this document.

The annual return obligation: "A registered charity must file an annual return."

In L4, we write this as:

GIVEN charity IS A `Registered Charity`
`the annual return obligation` MEANS
    PARTY `the charity`
    MUST `file the annual return`

Let's break this down:

Code	Meaning
``GIVEN charity IS A `Registered Charity```	This rule applies to registered charities
`the annual return obligation` MEANS	The name of this rule
PARTY `the charity`	The charity is the one with the obligation
`MUST`	This creates a legal obligation
`file the annual return`	This is what they must do

Backtick Names

Notice the backticks around file the annual return. In L4, backticks let you use spaces and special characters in names. These are called quoted identifiers.

-- Without backticks, names cannot contain spaces:
return              -- a single word
-- With backticks, names read like natural language:
`file the return`         -- quoted identifier (with spaces)
`file the annual return`  -- more descriptive

Use backticks liberally, so your names read like natural language.

Try It Yourself

Write a rule that says "A solicitor must maintain client confidentiality."

Every obligation needs to know who can act and what they can do. Copy this scaffold for now—DECLARE is taught fully in Module 2, and DEONTIC (the type of obligations) in Module 6:

DECLARE Party IS ONE OF
    `the solicitor`
    `the client`

DECLARE Action IS ONE OF
    `maintain client confidentiality`

GIVETH A DEONTIC Party Action
`the confidentiality obligation` MEANS
    -- your PARTY ... MUST ... goes here
    PARTY `the solicitor`
    MUST `maintain client confidentiality`

Your job: read the scaffold and make sure the PARTY ... MUST ... part matches the rule in plain English. Then try changing the action name or adding a second action to the Action list.

Adding Conditions

Real legal rules have conditions. Let's add one:

IF charity's `the status` EQUALS Active
THEN PARTY `the charity`
     MUST `file the annual return`
ELSE FULFILLED

The IF keyword adds a condition that must be true for the obligation to apply. The ELSE FULFILLED says: if the condition doesn't hold, there is nothing to do.

Multiple Conditions

Use AND and OR for multiple conditions:

IF charity's `the status` EQUALS Active
   AND charity's `the annual income` > 10000

Accessing Fields

The 's syntax accesses fields of a record:

charity's `the status`         -- the status field of charity
charity's `the annual income`  -- the annual income field of charity

Setting Deadlines

Legal obligations usually have deadlines. Use WITHIN:

PARTY `the charity`
MUST `file the annual return`
WITHIN 60

WITHIN 60 means "within 60 days." L4 uses days as the default time unit.

Consequences: HENCE and LEST

What happens when someone complies or doesn't comply? Use HENCE and LEST:

PARTY `the charity`
MUST `file the annual return`
WITHIN 60
HENCE FULFILLED
LEST BREACH

Keyword	Meaning
`HENCE`	What happens if they comply
`LEST`	What happens if they don't comply

Chaining Obligations

HENCE can trigger another obligation:

PARTY `the seller`
MUST `deliver the goods`
WITHIN 14
HENCE
    PARTY `the buyer`
    MUST `pay the invoice` 1000
    WITHIN 30
    HENCE FULFILLED
    LEST BREACH
LEST BREACH

This creates a chain: if the seller delivers, the buyer must pay.

Complete Example

-- Module 1: Your First Legal Rule - Complete Examples
-- All examples are validated and use natural language identifiers

-- =============================================================================
-- SECTION 1: Basic Obligation (simplified for teaching)
-- =============================================================================

-- Define types needed for obligations
DECLARE `Charity Status` IS ONE OF
    Active
    Suspended
    Dissolved

DECLARE `Registered Charity`
    HAS `the name` IS A STRING
        `the status` IS A `Charity Status`
        `the annual income` IS A NUMBER

-- Define actors and actions
DECLARE Party IS ONE OF
    `the charity`
    `the buyer`
    `the seller`
    `the Commissioner`

DECLARE Action IS ONE OF
    `file the annual return`
    `deliver the goods`
    `pay the invoice` HAS amount IS A NUMBER
    `cancel the order`
    `issue a Required Steps Notice`

-- =============================================================================
-- SECTION 2: The Annual Return Obligation
-- =============================================================================

-- A charity's obligation to file an annual return
GIVEN charity IS A `Registered Charity`
GIVETH A DEONTIC Party Action
`the annual return obligation` MEANS
    IF charity's `the status` EQUALS Active
    THEN PARTY `the charity`
         MUST `file the annual return`
         WITHIN 60
         HENCE FULFILLED
         LEST PARTY `the Commissioner`
              MAY `issue a Required Steps Notice`
              HENCE FULFILLED
    ELSE FULFILLED

-- =============================================================================
-- SECTION 3: Chained Obligations - Sale Contract
-- =============================================================================

-- The seller must deliver, then the buyer must pay
GIVETH A DEONTIC Party Action
`the sale contract` MEANS
    PARTY `the seller`
    MUST `deliver the goods`
    WITHIN 14
    HENCE
        PARTY `the buyer`
        MUST `pay the invoice` 1000
        WITHIN 30
        HENCE FULFILLED
        LEST BREACH
    LEST
        PARTY `the seller`
        MAY `cancel the order`
        HENCE FULFILLED

-- =============================================================================
-- SECTION 4: Test Data
-- =============================================================================

`Animal Welfare Society` MEANS `Registered Charity` "Animal Welfare Society" Active 50000

-- =============================================================================
-- SECTION 5: Tests
-- =============================================================================

-- Test field access
#EVAL `Animal Welfare Society`'s `the name`
#EVAL `Animal Welfare Society`'s `the status` EQUALS Active
#EVAL `Animal Welfare Society`'s `the annual income` > 10000

-- Test obligation - charity files on time
#TRACE `the annual return obligation` `Animal Welfare Society` AT 0 WITH
    PARTY `the charity` DOES `file the annual return` AT 30

-- Test sale contract - happy path
#TRACE `the sale contract` AT 0 WITH
    PARTY `the seller` DOES `deliver the goods` AT 10
    PARTY `the buyer` DOES `pay the invoice` 1000 AT 25

Included are:

Type definitions for charities, actors, and actions
The annual return obligation with conditions and deadlines
A chained sale contract
Test data and #TRACE simulations

Understanding GIVETH A DEONTIC

When a function returns an obligation (not just a value), we use GIVETH A DEONTIC:

GIVETH A BOOLEAN - returns true/false
GIVETH A NUMBER - returns a number
GIVETH A DEONTIC Actor Action - returns an obligation (specifying actor and action types)

Don't worry about mastering DEONTIC yet—copy the pattern for now; it is taught fully in Module 6.

Testing with #EVAL

Use #EVAL to test expressions. These examples use the Animal Welfare Society test charity defined in

-- Module 1: Your First Legal Rule - Complete Examples
-- All examples are validated and use natural language identifiers

-- =============================================================================
-- SECTION 1: Basic Obligation (simplified for teaching)
-- =============================================================================

-- Define types needed for obligations
DECLARE `Charity Status` IS ONE OF
    Active
    Suspended
    Dissolved

DECLARE `Registered Charity`
    HAS `the name` IS A STRING
        `the status` IS A `Charity Status`
        `the annual income` IS A NUMBER

-- Define actors and actions
DECLARE Party IS ONE OF
    `the charity`
    `the buyer`
    `the seller`
    `the Commissioner`

DECLARE Action IS ONE OF
    `file the annual return`
    `deliver the goods`
    `pay the invoice` HAS amount IS A NUMBER
    `cancel the order`
    `issue a Required Steps Notice`

-- =============================================================================
-- SECTION 2: The Annual Return Obligation
-- =============================================================================

-- A charity's obligation to file an annual return
GIVEN charity IS A `Registered Charity`
GIVETH A DEONTIC Party Action
`the annual return obligation` MEANS
    IF charity's `the status` EQUALS Active
    THEN PARTY `the charity`
         MUST `file the annual return`
         WITHIN 60
         HENCE FULFILLED
         LEST PARTY `the Commissioner`
              MAY `issue a Required Steps Notice`
              HENCE FULFILLED
    ELSE FULFILLED

-- =============================================================================
-- SECTION 3: Chained Obligations - Sale Contract
-- =============================================================================

-- The seller must deliver, then the buyer must pay
GIVETH A DEONTIC Party Action
`the sale contract` MEANS
    PARTY `the seller`
    MUST `deliver the goods`
    WITHIN 14
    HENCE
        PARTY `the buyer`
        MUST `pay the invoice` 1000
        WITHIN 30
        HENCE FULFILLED
        LEST BREACH
    LEST
        PARTY `the seller`
        MAY `cancel the order`
        HENCE FULFILLED

-- =============================================================================
-- SECTION 4: Test Data
-- =============================================================================

`Animal Welfare Society` MEANS `Registered Charity` "Animal Welfare Society" Active 50000

-- =============================================================================
-- SECTION 5: Tests
-- =============================================================================

-- Test field access
#EVAL `Animal Welfare Society`'s `the name`
#EVAL `Animal Welfare Society`'s `the status` EQUALS Active
#EVAL `Animal Welfare Society`'s `the annual income` > 10000

-- Test obligation - charity files on time
#TRACE `the annual return obligation` `Animal Welfare Society` AT 0 WITH
    PARTY `the charity` DOES `file the annual return` AT 30

-- Test sale contract - happy path
#TRACE `the sale contract` AT 0 WITH
    PARTY `the seller` DOES `deliver the goods` AT 10
    PARTY `the buyer` DOES `pay the invoice` 1000 AT 25

#EVAL `Animal Welfare Society`'s `the name`
-- Result: "Animal Welfare Society"

#EVAL `Animal Welfare Society`'s `the status` EQUALS Active
-- Result: TRUE

In VS Code with the L4 extension, hover over #EVAL to see the result.

Testing with #TRACE

For regulative rules (obligations), use #TRACE to simulate scenarios:

#TRACE `the annual return obligation` `Animal Welfare Society` AT 0 WITH
    PARTY `the charity` DOES `file the annual return` AT 30

This simulates:

Starting at day 0
The charity filing their return at day 30

The result shows whether the obligation was FULFILLED or BREACH.

Common Mistakes

1. Missing Type Declaration

-- ❌ Wrong: Type not declared
GIVEN charity IS A `Registered Charity`

-- ✅ Right: Declare the type first
DECLARE `Registered Charity`
    HAS `the name` IS A STRING

2. Wrong Field Access

-- ❌ Wrong: Missing 's
IF charity `the status` EQUALS Active

-- ✅ Right: Use 's for field access
IF charity's `the status` EQUALS Active

3. Missing Backticks for Multi-Word Names

-- ❌ Wrong: Spaces without backticks
MUST file the annual return

-- ✅ Right: Use backticks
MUST `file the annual return`

Exercises

Exercise 1: Simple Obligation

Write an L4 rule for: "An employee must submit a timesheet every week."

Solution

DECLARE `Employment Party` IS ONE OF
    `the employee`
    `the employer`

DECLARE `Employment Action` IS ONE OF
    `submit the timesheet`

GIVETH A DEONTIC `Employment Party` `Employment Action`
`the timesheet obligation` MEANS
    PARTY `the employee`
    MUST `submit the timesheet`
    WITHIN 7
    HENCE FULFILLED
    LEST BREACH

#TRACE `the timesheet obligation` AT 0 WITH
    PARTY `the employee` DOES `submit the timesheet` AT 5

Exercise 2: Conditional Obligation

Write an L4 rule for: "If a tenant is more than 14 days late on rent, the landlord may issue an eviction notice."

Solution

DECLARE `Tenancy Party` IS ONE OF
    `the landlord`
    `the tenant`

DECLARE `Tenancy Action` IS ONE OF
    `issue an eviction notice`

GIVEN `the days the rent is late` IS A NUMBER
GIVETH A DEONTIC `Tenancy Party` `Tenancy Action`
`the eviction permission` MEANS
    IF `the days the rent is late` > 14
    THEN PARTY `the landlord`
         MAY `issue an eviction notice`
         HENCE FULFILLED
    ELSE FULFILLED

#TRACE `the eviction permission` 20 AT 0 WITH
    PARTY `the landlord` DOES `issue an eviction notice` AT 1

Note the MAY: the landlord is permitted to act, but not obliged. You'll meet MAY again in Module 6.

Exercise 3: Chained Obligations

Write L4 rules for: "The seller must deliver goods within 14 days. If delivered, the buyer must pay within 30 days."

Solution

DECLARE `Sale Party` IS ONE OF
    `the seller`
    `the buyer`

DECLARE `Sale Action` IS ONE OF
    `deliver the goods`
    `pay the invoice`

GIVETH A DEONTIC `Sale Party` `Sale Action`
`the sale agreement` MEANS
    PARTY `the seller`
    MUST `deliver the goods`
    WITHIN 14
    HENCE
        PARTY `the buyer`
        MUST `pay the invoice`
        WITHIN 30
        HENCE FULFILLED
        LEST BREACH
    LEST BREACH

#TRACE `the sale agreement` AT 0 WITH
    PARTY `the seller` DOES `deliver the goods` AT 10
    PARTY `the buyer` DOES `pay the invoice` AT 25

Summary

In this module, you learned:

Concept	Syntax
Declare parameters	`GIVEN name IS A Type`
Create obligation	`PARTY actor MUST action`
Add condition	`IF condition`
Set deadline	`WITHIN days`
Compliance result	`HENCE consequence`
Non-compliance result	`LEST consequence`
Test expression	`#EVAL expression`
Simulate scenario	`#TRACE rule AT time WITH events`

What's Next?

In Module 2: Legal Entities and Relationships, you'll learn how to model complex legal entities with proper types, including records, enums, and relationships between entities.