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Module 2: Legal Entities and Relationships

Prerequisites: Modules 0–1

In this module, you'll learn how to model structured legal entities using L4's type system.

Learning Objectives

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

  • Define record types with DECLARE and HAS
  • Create enumeration types with IS ONE OF
  • Model relationships between entities
  • Access fields using the possessive syntax
  • Construct record values

From Strings to Structured Types

The Problem with Strings

Using simple text leads to errors and ambiguity. You can't distinguish a charity from a person, access parts (name, address, etc.), or detect typos.

Structured Types: The Solution

The complete working example:

-- Module 2: Legal Entities and Relationships - Complete Examples
-- All examples are validated and use natural language identifiers

-- =============================================================================
-- SECTION 1: Basic Record Types
-- =============================================================================

-- A person with natural language field names
DECLARE Person
    HAS `the person's name` IS A STRING
        `the person's address` IS A STRING
        `the person is a governor` IS A BOOLEAN

-- A registered charity
DECLARE `Registered Charity`
    HAS `the charity's name` IS A STRING
        `the registration number` IS A STRING
        `the registration date` IS A NUMBER
        `the address` IS A STRING

-- =============================================================================
-- SECTION 2: Enumeration Types
-- =============================================================================

-- Charitable purposes from legislation
DECLARE Purpose IS ONE OF
    `prevention or relief of poverty`
    `advancement of education`
    `advancement of religion`
    `advancement of health`
    `advancement of animal welfare`
    `other purpose` HAS `the description` IS A STRING

-- Status of a charity
DECLARE Status IS ONE OF
    Active
    Suspended HAS `the reason` IS A STRING
    Deregistered

-- =============================================================================
-- SECTION 3: Nested Types
-- =============================================================================

DECLARE `Bank Account`
    HAS `the bank name` IS A STRING
        `the account number` IS A STRING
        `the SWIFT code` IS A STRING

DECLARE Company
    HAS `the company name` IS A STRING
        `the jurisdiction` IS A STRING
        `the bank account` IS A `Bank Account`

-- =============================================================================
-- SECTION 4: Union Types (Legal Entity)
-- =============================================================================

DECLARE `Legal Entity` IS ONE OF
    Individual HAS `the person` IS A Person
    Corporation HAS `the company` IS A Company

-- =============================================================================
-- SECTION 5: Pattern Matching
-- =============================================================================

-- Check if a purpose is charitable
GIVEN purpose IS A Purpose
GIVETH A BOOLEAN
DECIDE `the purpose is charitable` IS
    CONSIDER purpose
    WHEN `advancement of education` THEN TRUE
    WHEN `advancement of animal welfare` THEN TRUE
    WHEN `prevention or relief of poverty` THEN TRUE
    WHEN `advancement of health` THEN TRUE
    WHEN `advancement of religion` THEN TRUE
    WHEN `other purpose` description THEN FALSE
    OTHERWISE FALSE

-- Get the name of a legal entity
GIVEN entity IS A `Legal Entity`
GIVETH A STRING
`the entity's name` MEANS
    CONSIDER entity
    WHEN Individual person THEN person's `the person's name`
    WHEN Corporation company THEN company's `the company name`

-- =============================================================================
-- SECTION 6: Test Data
-- =============================================================================

-- Create test persons
`Alice Smith` MEANS Person "Alice Smith" "123 Main Street, Jersey" TRUE
`Bob Jones` MEANS Person "Bob Jones" "456 High Street, Jersey" FALSE

-- Create test purposes
`education purpose` MEANS `advancement of education`
`custom purpose` MEANS `other purpose` "local community support"

-- Create test company
`Acme Bank Account` MEANS `Bank Account` "DBS Bank" "123-456789-0" "DBSSSGSG"
`Acme Corporation` MEANS Company "Acme Corp" "Singapore" `Acme Bank Account`

-- Create test legal entities
`Alice as individual` MEANS Individual `Alice Smith`
`Acme as corporation` MEANS Corporation `Acme Corporation`

-- =============================================================================
-- SECTION 7: Tests
-- =============================================================================

-- Field access tests
#EVAL `Alice Smith`'s `the person's name`
#EVAL `Alice Smith`'s `the person is a governor`

-- Nested field access
#EVAL `Acme Corporation`'s `the bank account`'s `the bank name`

-- Pattern matching tests
#EVAL `the purpose is charitable` `education purpose`
#EVAL `the purpose is charitable` `custom purpose`

-- Legal entity name extraction
#EVAL `the entity's name` `Alice as individual`
#EVAL `the entity's name` `Acme as corporation`

DECLARE `Registered Charity`
    HAS `the charity's name` IS A STRING
        `the registration number` IS A STRING
        `the registration date` IS A DATE
        `the charity's address` IS A STRING
        `the charity's purposes` IS A LIST OF Purpose

Benefits of structured types:

  • Prevents errors: Can't use a person where you need a charity
  • Captures relationships: Links charities to their purposes, addresses
  • Enables validation: L4 checks if all required information is present

Declaring Record Types

Use DECLARE with HAS to define record types:

DECLARE Person
    HAS `the person's name` IS A STRING
        `the person's age` IS A NUMBER
        `the person is an adult` IS A BOOLEAN

Syntax Rules

  1. Indentation matters: All fields must be indented consistently under HAS
  2. Each field on its own line (or separated by commas)
  3. Field names must be unique within the type
  4. Use natural language field names with backticks: `the person's name`

Creating Record Values

Use the type name followed by values in field order:

alice MEANS Person "Alice" 30 TRUE

Enumeration Types

The Problem with Free Text

Using strings for categories allows mistakes—typos and inconsistencies go undetected.

Enumerations: The Solution

Use IS ONE OF to define exact legal categories:

DECLARE Purpose IS ONE OF
    `prevention or relief of poverty`
    `advancement of education`
    `advancement of religion`
    `advancement of health`
    `advancement of animal welfare`

Now only these exact values are allowed—typos are compile-time errors!

Enumerations with Data

Some enum variants can carry additional data:

DECLARE Purpose IS ONE OF
    `advancement of education`
    `advancement of health`
    `other purpose` HAS `the description` IS A STRING

The other purpose variant includes a description for edge cases.

Pattern Matching on Enumerations

Use CONSIDER to handle different variants:

GIVEN purpose IS A Purpose
GIVETH A BOOLEAN
DECIDE `the purpose is charitable` IS
    CONSIDER purpose
    WHEN `advancement of education` THEN TRUE
    WHEN `advancement of health` THEN TRUE
    WHEN `other purpose` description THEN FALSE

CONSIDER Syntax

The general pattern is:

  • CONSIDER expression
  • WHEN pattern THEN result (one for each variant)
  • OTHERWISE default result (optional catch-all)

Connecting Multiple Entities

Legal systems involve relationships between entities. A field can reference another type, creating relationships.

See the Company type in

-- Module 2: Legal Entities and Relationships - Complete Examples
-- All examples are validated and use natural language identifiers

-- =============================================================================
-- SECTION 1: Basic Record Types
-- =============================================================================

-- A person with natural language field names
DECLARE Person
    HAS `the person's name` IS A STRING
        `the person's address` IS A STRING
        `the person is a governor` IS A BOOLEAN

-- A registered charity
DECLARE `Registered Charity`
    HAS `the charity's name` IS A STRING
        `the registration number` IS A STRING
        `the registration date` IS A NUMBER
        `the address` IS A STRING

-- =============================================================================
-- SECTION 2: Enumeration Types
-- =============================================================================

-- Charitable purposes from legislation
DECLARE Purpose IS ONE OF
    `prevention or relief of poverty`
    `advancement of education`
    `advancement of religion`
    `advancement of health`
    `advancement of animal welfare`
    `other purpose` HAS `the description` IS A STRING

-- Status of a charity
DECLARE Status IS ONE OF
    Active
    Suspended HAS `the reason` IS A STRING
    Deregistered

-- =============================================================================
-- SECTION 3: Nested Types
-- =============================================================================

DECLARE `Bank Account`
    HAS `the bank name` IS A STRING
        `the account number` IS A STRING
        `the SWIFT code` IS A STRING

DECLARE Company
    HAS `the company name` IS A STRING
        `the jurisdiction` IS A STRING
        `the bank account` IS A `Bank Account`

-- =============================================================================
-- SECTION 4: Union Types (Legal Entity)
-- =============================================================================

DECLARE `Legal Entity` IS ONE OF
    Individual HAS `the person` IS A Person
    Corporation HAS `the company` IS A Company

-- =============================================================================
-- SECTION 5: Pattern Matching
-- =============================================================================

-- Check if a purpose is charitable
GIVEN purpose IS A Purpose
GIVETH A BOOLEAN
DECIDE `the purpose is charitable` IS
    CONSIDER purpose
    WHEN `advancement of education` THEN TRUE
    WHEN `advancement of animal welfare` THEN TRUE
    WHEN `prevention or relief of poverty` THEN TRUE
    WHEN `advancement of health` THEN TRUE
    WHEN `advancement of religion` THEN TRUE
    WHEN `other purpose` description THEN FALSE
    OTHERWISE FALSE

-- Get the name of a legal entity
GIVEN entity IS A `Legal Entity`
GIVETH A STRING
`the entity's name` MEANS
    CONSIDER entity
    WHEN Individual person THEN person's `the person's name`
    WHEN Corporation company THEN company's `the company name`

-- =============================================================================
-- SECTION 6: Test Data
-- =============================================================================

-- Create test persons
`Alice Smith` MEANS Person "Alice Smith" "123 Main Street, Jersey" TRUE
`Bob Jones` MEANS Person "Bob Jones" "456 High Street, Jersey" FALSE

-- Create test purposes
`education purpose` MEANS `advancement of education`
`custom purpose` MEANS `other purpose` "local community support"

-- Create test company
`Acme Bank Account` MEANS `Bank Account` "DBS Bank" "123-456789-0" "DBSSSGSG"
`Acme Corporation` MEANS Company "Acme Corp" "Singapore" `Acme Bank Account`

-- Create test legal entities
`Alice as individual` MEANS Individual `Alice Smith`
`Acme as corporation` MEANS Corporation `Acme Corporation`

-- =============================================================================
-- SECTION 7: Tests
-- =============================================================================

-- Field access tests
#EVAL `Alice Smith`'s `the person's name`
#EVAL `Alice Smith`'s `the person is a governor`

-- Nested field access
#EVAL `Acme Corporation`'s `the bank account`'s `the bank name`

-- Pattern matching tests
#EVAL `the purpose is charitable` `education purpose`
#EVAL `the purpose is charitable` `custom purpose`

-- Legal entity name extraction
#EVAL `the entity's name` `Alice as individual`
#EVAL `the entity's name` `Acme as corporation`

which has a Bank Account field.

The elem function (from prelude) checks if an item is in a list.

Field Access

Possessive Syntax

Use 's to access record fields. Using the Registered Charity type declared above:

charity's `the charity's name`        -- the name field
charity's `the registration number`   -- the registration number field
charity's `the charity's purposes`    -- the list of purposes

Chained Access

Access nested fields by chaining:

company's `the bank account`'s `the bank name`
-- Gets the bank name field of the bank account field of company

Function Application Alternative

You can also write field access as function application:

`the charity's name` charity    -- same as: charity's `the charity's name`

This is useful in complex expressions.

Nested Types

Types can contain other types:

DECLARE `Bank Account`
    HAS `the bank name` IS A STRING
        `the account number` IS A STRING

DECLARE Company
    HAS `the company name` IS A STRING
        `the bank account` IS A `Bank Account`

Union Types for Alternatives

When something can be one of several different types:

DECLARE `Legal Entity` IS ONE OF
    Individual HAS `the person` IS A Person
    Corporation HAS `the company` IS A Company

A Legal Entity can be an individual or corporation—each with different data.

Using Union Types

GIVEN entity IS A `Legal Entity`
GIVETH A STRING
`the entity's name` MEANS
    CONSIDER entity
    WHEN Individual person THEN person's `the person's name`
    WHEN Corporation company THEN company's `the company name`

Real-World Example: Charity Registration

For a complete charity registration system with types, see

-- Module 7: Putting It Together - Capstone Example
-- A complete charity registration system
-- All examples are validated and use natural language identifiers

IMPORT prelude

-- =============================================================================
-- SECTION 1: Type Definitions
-- =============================================================================

-- Charitable purposes (from legislation)
DECLARE Purpose IS ONE OF
    `prevention or relief of poverty`
    `advancement of education`
    `advancement of religion`
    `advancement of health`
    `advancement of animal welfare`
    `other purpose` HAS `the description` IS A STRING

-- Legal status of a charity
DECLARE Status IS ONE OF
    Active
    Suspended HAS `the reason` IS A STRING
    Deregistered

-- Criminal conviction record
DECLARE Conviction
    HAS `the description` IS A STRING
        `the conviction is spent` IS A BOOLEAN

-- Governor of a charity
DECLARE Governor
    HAS `the governor's name` IS A STRING
        `the governor's age` IS A NUMBER
        `the governor is bankrupt` IS A BOOLEAN
        `the governor's convictions` IS A LIST OF Conviction

-- The main charity record
DECLARE `Registered Charity`
    HAS `the charity's name` IS A STRING
        `the registration number` IS A STRING
        `the charity's status` IS A Status
        `the charity's purposes` IS A LIST OF Purpose
        `the charity's governors` IS A LIST OF Governor

-- =============================================================================
-- SECTION 2: Eligibility Rules
-- =============================================================================

-- A governor must be an adult (at least 18)
GIVEN governor IS A Governor
GIVETH A BOOLEAN
DECIDE `the governor is an adult` IF
    governor's `the governor's age` >= 18

-- Check for disqualifying convictions (unspent convictions)
GIVEN governor IS A Governor
GIVETH A BOOLEAN
DECIDE `the governor has a disqualifying conviction` IF
    any (GIVEN c YIELD c's `the conviction is spent` EQUALS FALSE) (governor's `the governor's convictions`)

-- A person can be a governor if they meet all criteria
GIVEN governor IS A Governor
GIVETH A BOOLEAN
DECIDE `the person can be a governor` IF
    `the governor is an adult` governor
    AND NOT governor's `the governor is bankrupt`
    AND NOT `the governor has a disqualifying conviction` governor

-- A purpose is charitable if it's from the approved list
GIVEN purpose IS A Purpose
GIVETH A BOOLEAN
DECIDE `the purpose is charitable` IS
    CONSIDER purpose
    WHEN `prevention or relief of poverty` THEN TRUE
    WHEN `advancement of education` THEN TRUE
    WHEN `advancement of religion` THEN TRUE
    WHEN `advancement of health` THEN TRUE
    WHEN `advancement of animal welfare` THEN TRUE
    WHEN `other purpose` description THEN FALSE

-- A charity is valid if it has valid purposes and valid governors
GIVEN charity IS A `Registered Charity`
GIVETH A BOOLEAN
DECIDE `the charity is valid` IF
    charity's `the charity's status` EQUALS Active
    AND NOT null (charity's `the charity's purposes`)
    AND all (GIVEN p YIELD `the purpose is charitable` p) (charity's `the charity's purposes`)
    AND NOT null (charity's `the charity's governors`)
    AND all (GIVEN g YIELD `the person can be a governor` g) (charity's `the charity's governors`)

-- =============================================================================
-- SECTION 3: Classification and Fees
-- =============================================================================

-- Classify a charity by its annual income (multi-way decision with BRANCH)
GIVEN `the annual income` IS A NUMBER
GIVETH A STRING
DECIDE `the charity's size band` IS
    BRANCH
        IF `the annual income` < 10000 THEN "small"
        IF `the annual income` < 500000 THEN "medium"
        OTHERWISE "large"

-- The annual filing fee, decomposed with WHERE
GIVEN charity IS A `Registered Charity`
      `the annual income` IS A NUMBER
GIVETH A NUMBER
`the annual filing fee` MEANS
    `the base fee` + `the governor levy`
    WHERE
        `the base fee` MEANS
            BRANCH
                IF `the charity's size band` `the annual income` EQUALS "small" THEN 50
                IF `the charity's size band` `the annual income` EQUALS "medium" THEN 200
                OTHERWISE 500

        `the governor levy` MEANS
            10 * count (charity's `the charity's governors`)

-- =============================================================================
-- SECTION 4: Filing Obligations
-- =============================================================================

-- Actors and actions for the regulatory system
DECLARE Actor IS ONE OF
    `the Charity`
    `the Commissioner`

DECLARE Action IS ONE OF
    `file the annual return`
    `issue a Required Steps Notice`
    `correct the deficiencies`
    `distribute profits to the governors`

-- Annual return filing obligation
GIVEN charity IS A `Registered Charity`
GIVETH A DEONTIC Actor Action
`the annual return obligation` MEANS
    IF charity's `the charity's status` EQUALS Active
    THEN
        PARTY `the Charity`
        MUST `file the annual return`
        WITHIN 60
        HENCE FULFILLED
        LEST
            PARTY `the Commissioner`
            MUST `issue a Required Steps Notice`
            WITHIN 14
            HENCE `the correction period`
            LEST BREACH BY `the Commissioner` BECAUSE "failed to issue notice"
    ELSE FULFILLED

-- After notice is issued, charity has time to correct
GIVETH A DEONTIC Actor Action
`the correction period` MEANS
    PARTY `the Charity`
    MUST `correct the deficiencies`
    WITHIN 30
    HENCE FULFILLED
    LEST BREACH BY `the Charity` BECAUSE "failed to correct after notice"

-- A prohibition: charities must not distribute profits to their governors
GIVETH A DEONTIC Actor Action
`the profit distribution prohibition` MEANS
    PARTY `the Charity`
    SHANT `distribute profits to the governors`
    HENCE FULFILLED
    LEST BREACH BY `the Charity` BECAUSE "distributed profits to governors"

-- =============================================================================
-- SECTION 5: Test Data
-- =============================================================================

-- Valid governor
`Jane Smith` MEANS Governor "Jane Smith" 45 FALSE (LIST)

-- Governor with issues
`John Doe (bankrupt)` MEANS Governor "John Doe" 50 TRUE (LIST)

`unspent conviction` MEANS Conviction "Fraud conviction 2020" FALSE
`Bob Jones (with conviction)` MEANS Governor "Bob Jones" 40 FALSE (LIST `unspent conviction`)

`Young Person` MEANS Governor "Young Person" 16 FALSE (LIST)

-- Valid charity
`Jersey Animal Welfare` MEANS `Registered Charity` "Jersey Animal Welfare" "CH001" Active (LIST `advancement of animal welfare`, `advancement of education`) (LIST `Jane Smith`)

-- Charity with invalid governor
`Problem Charity` MEANS `Registered Charity` "Problem Charity" "CH002" Active (LIST `advancement of education`) (LIST `John Doe (bankrupt)`)

-- Suspended charity
`Suspended Charity` MEANS `Registered Charity` "Suspended Charity" "CH003" (Suspended "Financial irregularities") (LIST `advancement of health`) (LIST `Jane Smith`)

-- =============================================================================
-- SECTION 6: Tests
-- =============================================================================

-- Governor eligibility tests
#EVAL `the governor is an adult` `Jane Smith`
#EVAL `the governor is an adult` `Young Person`
#EVAL `the person can be a governor` `Jane Smith`
#EVAL `the person can be a governor` `John Doe (bankrupt)`
#EVAL `the person can be a governor` `Bob Jones (with conviction)`
#EVAL `the person can be a governor` `Young Person`

-- Charity validity tests
#EVAL `the charity is valid` `Jersey Animal Welfare`
#EVAL `the charity is valid` `Problem Charity`
#EVAL `the charity is valid` `Suspended Charity`

-- Classification and fee tests
#EVAL `the charity's size band` 5000
#EVAL `the charity's size band` 60000
#EVAL `the charity's size band` 750000
#EVAL `the annual filing fee` `Jersey Animal Welfare` 60000

-- Test filing obligation scenarios

-- Happy path: charity files on time
#TRACE `the annual return obligation` `Jersey Animal Welfare` AT 0 WITH
    PARTY `the Charity` DOES `file the annual return` AT 30

-- Suspended charity: no filing required
#TRACE `the annual return obligation` `Suspended Charity` AT 0 WITH

-- Late filing: the charity misses the 60-day deadline (its day-65 filing
-- arrives too late), the Commissioner issues a notice, then the charity
-- corrects within the correction period
#TRACE `the annual return obligation` `Jersey Animal Welfare` AT 0 WITH
    PARTY `the Charity` DOES `file the annual return` AT 65
    PARTY `the Commissioner` DOES `issue a Required Steps Notice` AT 70
    PARTY `the Charity` DOES `correct the deficiencies` AT 90

-- Prohibition: distributing profits is a breach
#TRACE `the profit distribution prohibition` AT 0 WITH
    PARTY `the Charity` DOES `distribute profits to the governors` AT 10

in the capstone module.

Common Mistakes

1. Inconsistent Indentation

-- ❌ Wrong: Fields not aligned
DECLARE Person
    HAS name IS A STRING
      age IS A NUMBER

-- ✅ Right: All fields aligned
DECLARE Person
    HAS name IS A STRING
        age IS A NUMBER

2. Circular Type References

-- ❌ Problematic: A refers to B before B is defined
DECLARE A HAS b IS A B
DECLARE B HAS a IS A A

3. Missing Field Values

-- ❌ Wrong: Missing age field
alice MEANS Person "Alice" TRUE

-- ✅ Right: Provide all fields
alice MEANS Person "Alice" 30 TRUE

Exercises

Exercise 1: Define a Contract Type

Define types for a simple contract with parties and an amount.

Solution 
DECLARE Contract
    HAS `the first party` IS A STRING
        `the second party` IS A STRING
        `the contract amount` IS A NUMBER

`the consulting contract` MEANS Contract "Acme Corp" "Jane Smith" 5000

#EVAL `the consulting contract`'s `the contract amount`

Exercise 2: Employment Status Enumeration

Define an enumeration for employment status (full-time, part-time, contractor, terminated).

Solution 
DECLARE `Employment Status` IS ONE OF
    `full time`
    `part time`
    contractor
    terminated

Exercise 3: Pattern Matching

Write a function that returns a person's employment description based on their status.

Solution 
GIVEN status IS A `Employment Status`
GIVETH A STRING
`the employment description` MEANS
    CONSIDER status
    WHEN `full time` THEN "employed full time"
    WHEN `part time` THEN "employed part time"
    WHEN contractor THEN "engaged as a contractor"
    WHEN terminated THEN "no longer employed"

#EVAL `the employment description` `full time`
#EVAL `the employment description` contractor

Summary

Concept Syntax
Record type DECLARE Type HAS field IS A FieldType
Enumeration DECLARE Type IS ONE OF Variant1, Variant2
Enum with data Variant HAS field IS A Type
Create record Type value1 value2 or Type WITH field IS value
Field access record's field
Pattern match CONSIDER expr WHEN pattern THEN result
Check list membership elem item list

What's Next?

In Module 3: Control Flow, you'll learn how to handle conditional logic, work with lists, and use boolean operators effectively.