Module 4: Decision logic

In this module, you'll learn how to encode constitutive rules - legal rules that determine facts, classifications, and eligibility.

Learning Objectives

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

Distinguish between constitutive rules (decision logic) and regulative rules (obligations)
Express eligibility determinations using DECIDE
Define computations and definitions using MEANS
Write function signatures with GIVEN and GIVETH
Break down complex calculations with WHERE
Write decision logic that reads like legal text

What is Decision Logic?

Constitutive vs Regulative Rules

Legal systems contain two types of rules:

Regulative rules (Modules 1 and 6) specify what parties must, may, or must not do:

"The applicant must submit documentation within 30 days"
"The landlord may terminate the lease if rent is unpaid"
"The seller shall not disclose confidential information"

Constitutive rules (this module) determine facts, classifications, and eligibility:

"An applicant is eligible if they are a citizen and over 18"
"The tax owed is 20% of gross income minus deductions"
"A person is a resident if they have lived here for 183 days or more"

In L4, we express constitutive rules as decision logic using functions. Definitions and duties explains why L4 keeps the two kinds apart.

Every input is declared

A decision in L4 can only use what its GIVEN declares, so the signature lists every fact the decision depends on. When someone says "it depends", L4 makes them say on what. Functional purity explains why this matters for legal rules.

Working Examples

All examples in this module are in:

-- Module 4: Decision Logic - Complete Examples
-- All examples are validated and use natural language identifiers

IMPORT prelude
IMPORT daydate

-- =======================================
-- SECTION 1: Basic Eligibility Decisions
-- =======================================

-- Simple age-based eligibility
GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the person is an adult` IF `the person's age` >= 18

-- Voting eligibility
GIVEN `the person's age` IS A NUMBER
      `is citizen` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is eligible to vote` IF
    `the person's age` >= 18
    AND `is citizen`

-- Senior discount qualification
GIVEN `the customer's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    `the customer's age` >= 65

-- Student status determination
GIVEN `the person's age` IS A NUMBER
      `is enrolled` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is a student` IF
    `the person's age` >= 16
    AND `the person's age` <= 25
    AND `is enrolled`

-- =======================================
-- SECTION 2: Using DECIDE IF vs DECIDE IS vs MEANS
-- =======================================

-- DECIDE IF: Best for yes/no questions with conditions
GIVEN `the income` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for exemption` IF `the income` < 10000

-- DECIDE IS: Best for classification or value assignment
GIVEN `the income` IS A NUMBER
GIVETH A STRING
DECIDE `the tax bracket` IS
    BRANCH
        IF `the income` < 10000 THEN "exempt"
        IF `the income` < 50000 THEN "standard"
        OTHERWISE "higher rate"

-- MEANS: Best for computations and definitions
GIVEN `the income` IS A NUMBER
GIVETH A STRING
`the income category` MEANS
    BRANCH
        IF `the income` < 20000 THEN "low"
        IF `the income` < 80000 THEN "medium"
        OTHERWISE "high"

-- =======================================
-- SECTION 3: Tax and Financial Calculations with WHERE
-- =======================================

-- Simple tax calculation
GIVEN `the gross income` IS A NUMBER
      `the tax rate` IS A NUMBER
GIVETH A NUMBER
`the tax owed` MEANS
    `the taxable income` * `the tax rate`
    WHERE
        `the standard deduction` MEANS 12000
        `the taxable income` MEANS
            IF `the gross income` > `the standard deduction`
            THEN `the gross income` - `the standard deduction`
            ELSE 0

-- Progressive tax with multiple brackets
GIVEN `the income` IS A NUMBER
GIVETH A NUMBER
`the income tax owed` MEANS
    `tax on first bracket` + `tax on second bracket` + `tax on third bracket`
    WHERE
        `tax on first bracket` MEANS
            `amount in first bracket` * 0.10

        `tax on second bracket` MEANS
            `amount in second bracket` * 0.20

        `tax on third bracket` MEANS
            `amount in third bracket` * 0.30

        `amount in first bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN `the income`
                OTHERWISE 10000

        `amount in second bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN 0
                IF `the income` <= 50000 THEN `the income` - 10000
                OTHERWISE 40000

        `amount in third bracket` MEANS
            BRANCH
                IF `the income` <= 50000 THEN 0
                OTHERWISE `the income` - 50000

-- Net income calculation
GIVEN `the gross income` IS A NUMBER
      `the expenses` IS A NUMBER
GIVETH A NUMBER
`the net income` MEANS
    `the gross income` - `the total deductions`
    WHERE
        `the standard deduction` MEANS
            IF `calculated deduction` >= 1000
            THEN `calculated deduction`
            ELSE 1000

        `calculated deduction` MEANS `the gross income` * 0.10

        `the total deductions` MEANS
            `the standard deduction` + `the expenses`

-- Compound interest with a named helper
GIVEN `the principal` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the years` IS A NUMBER
GIVETH A NUMBER
`the compound interest` MEANS
    `the principal` * EXPONENT `the growth factor` `the years`
    WHERE
        `the growth factor` MEANS 1 + `the annual rate`

-- Loan payment with several helpers
GIVEN `the loan amount` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the term in months` IS A NUMBER
GIVETH A NUMBER
`the monthly payment` MEANS
    `the loan amount` *
    (`the monthly rate` * `the compound factor`) /
    (`the compound factor` - 1)
    WHERE
        `the monthly rate` MEANS `the annual rate` / 12
        `the compound factor` MEANS EXPONENT (1 + `the monthly rate`) `the term in months`

-- =======================================
-- SECTION 4: Structured Entity Examples
-- =======================================

DECLARE Person HAS
    name IS A STRING
    age IS A NUMBER
    citizenship IS A STRING
    `annual income` IS A NUMBER
    `criminal record` IS A BOOLEAN

-- Comprehensive benefit eligibility
GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefit` IF
    `the applicant is a qualifying resident`
    AND `the applicant is of age`
    AND `the applicant meets income threshold`
    AND NOT `the applicant is disqualified`
    WHERE
        `the applicant is a qualifying resident` MEANS
            `the applicant`'s citizenship = "citizen"

        `the applicant is of age` MEANS
            `the applicant`'s age >= 21
            AND `the applicant`'s age < 65

        `the applicant meets income threshold` MEANS
            `the applicant`'s `annual income` < 30000

        `the applicant is disqualified` MEANS
            `the applicant`'s `criminal record`

-- Tax bracket determination with record
GIVEN `the taxpayer` IS A Person
GIVETH A STRING
DECIDE `the taxpayer's bracket` IS
    BRANCH
        IF `the taxpayer`'s `annual income` < 10000 THEN "exempt"
        IF `the taxpayer`'s `annual income` < 50000 THEN "standard"
        IF `the taxpayer`'s `annual income` < 100000 THEN "higher"
        OTHERWISE "top rate"

-- =======================================
-- SECTION 5: Multi-Factor Decisions
-- =======================================

-- Loan approval decision
GIVEN `the income` IS A NUMBER
      `the credit score` IS A NUMBER
      `the loan amount` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the loan is approved` IF
    `income requirement is met`
    AND `credit score is sufficient`
    AND `debt to income ratio is acceptable`
    WHERE
        `income requirement is met` MEANS
            `the income` >= `the loan amount` * 0.3

        `credit score is sufficient` MEANS
            `the credit score` >= 650

        `debt to income ratio is acceptable` MEANS
            `the loan amount` <= `the income` * 4

-- Insurance premium calculation
GIVEN `the insured age` IS A NUMBER
      `the coverage amount` IS A NUMBER
      `is smoker` IS A BOOLEAN
GIVETH A NUMBER
`the annual premium` MEANS
    `the base premium` * `the age factor` * `the smoking factor`
    WHERE
        `the base premium` MEANS
            `the coverage amount` * 0.001

        `the age factor` MEANS
            BRANCH
                IF `the insured age` < 30 THEN 1.0
                IF `the insured age` < 50 THEN 1.5
                OTHERWISE 2.0

        `the smoking factor` MEANS
            IF `is smoker` THEN 1.5 ELSE 1.0

-- =======================================
-- SECTION 6: Real-World Contract Scenarios
-- =======================================

-- Late payment penalty calculation
GIVEN `the principal amount` IS A NUMBER
      `days overdue` IS A NUMBER
GIVETH A NUMBER
`the late payment penalty` MEANS
    BRANCH
        IF `days overdue` <= 0 THEN 0
        IF `days overdue` <= 30 THEN `the principal amount` * 0.02
        OTHERWISE `the principal amount` * 0.05

-- Lease renewal qualification
GIVEN `years as tenant` IS A NUMBER
      `late payments` IS A NUMBER
      `current rent` IS A NUMBER
      `market rent` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for lease renewal` IF
    `tenant is in good standing`
    AND `rent is at market rate`
    WHERE
        `tenant is in good standing` MEANS
            `years as tenant` >= 1
            AND `late payments` = 0

        `rent is at market rate` MEANS
            `current rent` >= `market rent` * 0.9

-- =======================================
-- SECTION 7: Optional Values and Dates
-- =======================================

-- A contract with an optional termination date
DECLARE Contract HAS
    `the effective date` IS A DATE
    `the termination date` IS A MAYBE DATE
    `the notice period in days` IS A NUMBER

-- A party who may or may not have given notice
DECLARE `Contract Party` HAS
    `the party's name` IS A STRING
    `notice was given on` IS A MAYBE DATE

-- Days between two dates, using serial day numbers
GIVEN `the first date` IS A DATE
      `the second date` IS A DATE
GIVETH A NUMBER
`the days between` MEANS
    DATE_SERIAL `the second date` - DATE_SERIAL `the first date`

-- Contract clause interpretation with MAYBE and dates
GIVEN `the contract` IS A Contract
      `the party` IS A `Contract Party`
      `today` IS A DATE
GIVETH A BOOLEAN
DECIDE `the party may terminate` IF
    `the contract is active`
    AND `the party has given sufficient notice`
    WHERE
        `the contract is active` MEANS
            `today` >= `the contract`'s `the effective date`
            AND CONSIDER `the contract`'s `the termination date`
                WHEN NOTHING THEN TRUE
                WHEN JUST `the end date` THEN `today` < `the end date`

        `the party has given sufficient notice` MEANS
            CONSIDER `the party`'s `notice was given on`
            WHEN NOTHING THEN FALSE
            WHEN JUST `the notice date` THEN
                `the days between` `the notice date` `today` >= `the contract`'s `the notice period in days`

-- =======================================
-- SECTION 8: Test Data
-- =======================================

`test person eligible` MEANS
    Person "Alice" 25 "citizen" 25000 FALSE

`test person ineligible age` MEANS
    Person "Bob" 17 "citizen" 20000 FALSE

`test person ineligible citizenship` MEANS
    Person "Carol" 30 "permanent resident" 20000 FALSE

`test person ineligible income` MEANS
    Person "David" 30 "citizen" 50000 FALSE

`test person disqualified` MEANS
    Person "Eve" 30 "citizen" 20000 TRUE

`the open-ended contract` MEANS Contract (Date 1 1 2024) NOTHING 30
`the party with notice` MEANS `Contract Party` "Acme Ltd" (JUST (Date 1 3 2024))
`the party without notice` MEANS `Contract Party` "Beta Ltd" NOTHING

-- =======================================
-- SECTION 9: Tests
-- =======================================

-- Basic eligibility tests
#EVAL `the person is an adult` 21
#EVAL `the person is an adult` 16
#EVAL `the person is eligible to vote` 25 TRUE
#EVAL `the person is eligible to vote` 16 TRUE
#EVAL `qualifies for senior discount` 70
#EVAL `qualifies for senior discount` 50

-- Tax calculation tests
#EVAL `the tax owed` 50000 0.20
#EVAL `the income tax owed` 75000
#EVAL `the net income` 100000 15000
#EVAL `the compound interest` 1000 0.05 10
#EVAL `the monthly payment` 300000 0.06 360

-- Structured entity tests
#EVAL `the applicant is eligible for benefit` `test person eligible`
#EVAL `the applicant is eligible for benefit` `test person ineligible age`
#EVAL `the applicant is eligible for benefit` `test person ineligible citizenship`
#EVAL `the applicant is eligible for benefit` `test person ineligible income`
#EVAL `the applicant is eligible for benefit` `test person disqualified`
#EVAL `the taxpayer's bracket` `test person eligible`

-- Multi-factor decision tests
#EVAL `the loan is approved` 60000 700 200000
#EVAL `the loan is approved` 40000 700 200000
#EVAL `the annual premium` 35 100000 FALSE
#EVAL `the annual premium` 35 100000 TRUE

-- Contract scenario tests
#EVAL `the late payment penalty` 1000 15
#EVAL `the late payment penalty` 1000 45
#EVAL `qualifies for lease renewal` 2 0 1800 2000
#EVAL `qualifies for lease renewal` 2 3 1800 2000

-- Optional values and dates tests
#EVAL `the days between` (Date 1 3 2024) (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party with notice` (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party without notice` (Date 15 4 2024)

Basic Decisions with DECIDE

Eligibility Determinations

The most common legal decision: "Does X qualify?"

GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefits` IF
    `the applicant is a citizen`
    AND `the applicant's age` >= 18
    AND NOT `the applicant is disqualified`

Use DECIDE IF for yes/no questions, with the conditions after IF joined by AND, OR and NOT.

Classification Rules

"What category does X fall into?"

Use BRANCH for multi-way classification decisions:

GIVEN `the income` IS A NUMBER
GIVETH A STRING
DECIDE `the tax bracket` IS
    BRANCH
        IF `the income` < 10000 THEN "low"
        IF `the income` < 50000 THEN "medium"
        OTHERWISE "high"

BRANCH reads like a legal test with several conditions, and avoids the indentation problems of nested IFs (Module 3).

DECIDE IS vs DECIDE IF

DECIDE `the person is an adult` IF age >= 18 and DECIDE `the person is an adult` IS age >= 18 mean the same. Use IF when the rule reads like a legal condition, and IS with a BRANCH for a classification, as above.

Computations with MEANS

Defining Calculations

Use MEANS to define computed values and formulas:

GIVEN `the gross income` IS A NUMBER
      `the deductions` IS A NUMBER
GIVETH A NUMBER
`the taxable income` MEANS
    `the gross income` - `the deductions`

DECIDE vs MEANS

Both define things. Use whichever reads better:

Use DECIDE when...	Use MEANS when...
Asking a yes/no question	Defining a value or computation
Determining classification/status	Stating what something equals
Condition reads naturally with IF	Definition reads naturally with =

DECIDE `the person is an adult` IF age >= 18 reads more naturally than `the person is an adult` MEANS age >= 18, and `the net income` MEANS `gross income` - `expenses` more naturally than the same with DECIDE … IS. All four are valid.

Function Signatures

Every decision or computation has a signature that declares:

Inputs (GIVEN): what information is needed, one name and type per input
Output (GIVETH): what type of result is produced

The rule is a...	Typical signature
Yes/no decision	GIVEN `the person` IS A Person and `GIVETH A BOOLEAN`
Classification	GIVEN `the entity` IS AN Entity and `GIVETH A STRING`, or an enum
Computation	GIVEN `the income` IS A NUMBER and `GIVETH A NUMBER`

A rule can have several inputs. List each on its own line, aligned under the first:

GIVEN `the days since the decision` IS A NUMBER
      `the appeal period in days` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the appeal is in time` IF
    `the days since the decision` AT MOST `the appeal period in days`

Callers pass the arguments in the order of the GIVEN lines: `the appeal is in time` 10 21 is TRUE. Other result types include GIVETH A Person (a record), GIVETH A LIST OF NUMBER and GIVETH A MAYBE NUMBER.

L4 can usually work out the result type on its own, so GIVETH may be left out. Write it anyway: it documents the rule, and it gives clearer error messages when the body does not match.

Breaking Down Complex Logic with WHERE

Local Helper Calculations

Legal formulas often involve intermediate calculations. Use WHERE to break them down:

GIVEN `the principal` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the years` IS A NUMBER
GIVETH A NUMBER
`the compound interest` MEANS
    `the principal` * EXPONENT `the growth factor` `the years`
    WHERE
        `the growth factor` MEANS 1 + `the annual rate`

(EXPONENT base power is L4's built-in exponentiation function.)

Benefits:

Makes complex formulas readable
Gives meaningful names to intermediate values
Matches how legal documents explain calculations

Multiple WHERE Definitions

GIVEN `the loan amount` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the term in months` IS A NUMBER
GIVETH A NUMBER
`the monthly payment` MEANS
    `the loan amount` *
    (`the monthly rate` * `the compound factor`) /
    (`the compound factor` - 1)
    WHERE
        `the monthly rate` MEANS `the annual rate` / 12
        `the compound factor` MEANS EXPONENT (1 + `the monthly rate`) `the term in months`

Realistic Legal Scenarios

Example 1: Benefit Eligibility

From

-- Module 4: Decision Logic - Complete Examples
-- All examples are validated and use natural language identifiers

IMPORT prelude
IMPORT daydate

-- =======================================
-- SECTION 1: Basic Eligibility Decisions
-- =======================================

-- Simple age-based eligibility
GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the person is an adult` IF `the person's age` >= 18

-- Voting eligibility
GIVEN `the person's age` IS A NUMBER
      `is citizen` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is eligible to vote` IF
    `the person's age` >= 18
    AND `is citizen`

-- Senior discount qualification
GIVEN `the customer's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    `the customer's age` >= 65

-- Student status determination
GIVEN `the person's age` IS A NUMBER
      `is enrolled` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is a student` IF
    `the person's age` >= 16
    AND `the person's age` <= 25
    AND `is enrolled`

-- =======================================
-- SECTION 2: Using DECIDE IF vs DECIDE IS vs MEANS
-- =======================================

-- DECIDE IF: Best for yes/no questions with conditions
GIVEN `the income` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for exemption` IF `the income` < 10000

-- DECIDE IS: Best for classification or value assignment
GIVEN `the income` IS A NUMBER
GIVETH A STRING
DECIDE `the tax bracket` IS
    BRANCH
        IF `the income` < 10000 THEN "exempt"
        IF `the income` < 50000 THEN "standard"
        OTHERWISE "higher rate"

-- MEANS: Best for computations and definitions
GIVEN `the income` IS A NUMBER
GIVETH A STRING
`the income category` MEANS
    BRANCH
        IF `the income` < 20000 THEN "low"
        IF `the income` < 80000 THEN "medium"
        OTHERWISE "high"

-- =======================================
-- SECTION 3: Tax and Financial Calculations with WHERE
-- =======================================

-- Simple tax calculation
GIVEN `the gross income` IS A NUMBER
      `the tax rate` IS A NUMBER
GIVETH A NUMBER
`the tax owed` MEANS
    `the taxable income` * `the tax rate`
    WHERE
        `the standard deduction` MEANS 12000
        `the taxable income` MEANS
            IF `the gross income` > `the standard deduction`
            THEN `the gross income` - `the standard deduction`
            ELSE 0

-- Progressive tax with multiple brackets
GIVEN `the income` IS A NUMBER
GIVETH A NUMBER
`the income tax owed` MEANS
    `tax on first bracket` + `tax on second bracket` + `tax on third bracket`
    WHERE
        `tax on first bracket` MEANS
            `amount in first bracket` * 0.10

        `tax on second bracket` MEANS
            `amount in second bracket` * 0.20

        `tax on third bracket` MEANS
            `amount in third bracket` * 0.30

        `amount in first bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN `the income`
                OTHERWISE 10000

        `amount in second bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN 0
                IF `the income` <= 50000 THEN `the income` - 10000
                OTHERWISE 40000

        `amount in third bracket` MEANS
            BRANCH
                IF `the income` <= 50000 THEN 0
                OTHERWISE `the income` - 50000

-- Net income calculation
GIVEN `the gross income` IS A NUMBER
      `the expenses` IS A NUMBER
GIVETH A NUMBER
`the net income` MEANS
    `the gross income` - `the total deductions`
    WHERE
        `the standard deduction` MEANS
            IF `calculated deduction` >= 1000
            THEN `calculated deduction`
            ELSE 1000

        `calculated deduction` MEANS `the gross income` * 0.10

        `the total deductions` MEANS
            `the standard deduction` + `the expenses`

-- Compound interest with a named helper
GIVEN `the principal` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the years` IS A NUMBER
GIVETH A NUMBER
`the compound interest` MEANS
    `the principal` * EXPONENT `the growth factor` `the years`
    WHERE
        `the growth factor` MEANS 1 + `the annual rate`

-- Loan payment with several helpers
GIVEN `the loan amount` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the term in months` IS A NUMBER
GIVETH A NUMBER
`the monthly payment` MEANS
    `the loan amount` *
    (`the monthly rate` * `the compound factor`) /
    (`the compound factor` - 1)
    WHERE
        `the monthly rate` MEANS `the annual rate` / 12
        `the compound factor` MEANS EXPONENT (1 + `the monthly rate`) `the term in months`

-- =======================================
-- SECTION 4: Structured Entity Examples
-- =======================================

DECLARE Person HAS
    name IS A STRING
    age IS A NUMBER
    citizenship IS A STRING
    `annual income` IS A NUMBER
    `criminal record` IS A BOOLEAN

-- Comprehensive benefit eligibility
GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefit` IF
    `the applicant is a qualifying resident`
    AND `the applicant is of age`
    AND `the applicant meets income threshold`
    AND NOT `the applicant is disqualified`
    WHERE
        `the applicant is a qualifying resident` MEANS
            `the applicant`'s citizenship = "citizen"

        `the applicant is of age` MEANS
            `the applicant`'s age >= 21
            AND `the applicant`'s age < 65

        `the applicant meets income threshold` MEANS
            `the applicant`'s `annual income` < 30000

        `the applicant is disqualified` MEANS
            `the applicant`'s `criminal record`

-- Tax bracket determination with record
GIVEN `the taxpayer` IS A Person
GIVETH A STRING
DECIDE `the taxpayer's bracket` IS
    BRANCH
        IF `the taxpayer`'s `annual income` < 10000 THEN "exempt"
        IF `the taxpayer`'s `annual income` < 50000 THEN "standard"
        IF `the taxpayer`'s `annual income` < 100000 THEN "higher"
        OTHERWISE "top rate"

-- =======================================
-- SECTION 5: Multi-Factor Decisions
-- =======================================

-- Loan approval decision
GIVEN `the income` IS A NUMBER
      `the credit score` IS A NUMBER
      `the loan amount` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the loan is approved` IF
    `income requirement is met`
    AND `credit score is sufficient`
    AND `debt to income ratio is acceptable`
    WHERE
        `income requirement is met` MEANS
            `the income` >= `the loan amount` * 0.3

        `credit score is sufficient` MEANS
            `the credit score` >= 650

        `debt to income ratio is acceptable` MEANS
            `the loan amount` <= `the income` * 4

-- Insurance premium calculation
GIVEN `the insured age` IS A NUMBER
      `the coverage amount` IS A NUMBER
      `is smoker` IS A BOOLEAN
GIVETH A NUMBER
`the annual premium` MEANS
    `the base premium` * `the age factor` * `the smoking factor`
    WHERE
        `the base premium` MEANS
            `the coverage amount` * 0.001

        `the age factor` MEANS
            BRANCH
                IF `the insured age` < 30 THEN 1.0
                IF `the insured age` < 50 THEN 1.5
                OTHERWISE 2.0

        `the smoking factor` MEANS
            IF `is smoker` THEN 1.5 ELSE 1.0

-- =======================================
-- SECTION 6: Real-World Contract Scenarios
-- =======================================

-- Late payment penalty calculation
GIVEN `the principal amount` IS A NUMBER
      `days overdue` IS A NUMBER
GIVETH A NUMBER
`the late payment penalty` MEANS
    BRANCH
        IF `days overdue` <= 0 THEN 0
        IF `days overdue` <= 30 THEN `the principal amount` * 0.02
        OTHERWISE `the principal amount` * 0.05

-- Lease renewal qualification
GIVEN `years as tenant` IS A NUMBER
      `late payments` IS A NUMBER
      `current rent` IS A NUMBER
      `market rent` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for lease renewal` IF
    `tenant is in good standing`
    AND `rent is at market rate`
    WHERE
        `tenant is in good standing` MEANS
            `years as tenant` >= 1
            AND `late payments` = 0

        `rent is at market rate` MEANS
            `current rent` >= `market rent` * 0.9

-- =======================================
-- SECTION 7: Optional Values and Dates
-- =======================================

-- A contract with an optional termination date
DECLARE Contract HAS
    `the effective date` IS A DATE
    `the termination date` IS A MAYBE DATE
    `the notice period in days` IS A NUMBER

-- A party who may or may not have given notice
DECLARE `Contract Party` HAS
    `the party's name` IS A STRING
    `notice was given on` IS A MAYBE DATE

-- Days between two dates, using serial day numbers
GIVEN `the first date` IS A DATE
      `the second date` IS A DATE
GIVETH A NUMBER
`the days between` MEANS
    DATE_SERIAL `the second date` - DATE_SERIAL `the first date`

-- Contract clause interpretation with MAYBE and dates
GIVEN `the contract` IS A Contract
      `the party` IS A `Contract Party`
      `today` IS A DATE
GIVETH A BOOLEAN
DECIDE `the party may terminate` IF
    `the contract is active`
    AND `the party has given sufficient notice`
    WHERE
        `the contract is active` MEANS
            `today` >= `the contract`'s `the effective date`
            AND CONSIDER `the contract`'s `the termination date`
                WHEN NOTHING THEN TRUE
                WHEN JUST `the end date` THEN `today` < `the end date`

        `the party has given sufficient notice` MEANS
            CONSIDER `the party`'s `notice was given on`
            WHEN NOTHING THEN FALSE
            WHEN JUST `the notice date` THEN
                `the days between` `the notice date` `today` >= `the contract`'s `the notice period in days`

-- =======================================
-- SECTION 8: Test Data
-- =======================================

`test person eligible` MEANS
    Person "Alice" 25 "citizen" 25000 FALSE

`test person ineligible age` MEANS
    Person "Bob" 17 "citizen" 20000 FALSE

`test person ineligible citizenship` MEANS
    Person "Carol" 30 "permanent resident" 20000 FALSE

`test person ineligible income` MEANS
    Person "David" 30 "citizen" 50000 FALSE

`test person disqualified` MEANS
    Person "Eve" 30 "citizen" 20000 TRUE

`the open-ended contract` MEANS Contract (Date 1 1 2024) NOTHING 30
`the party with notice` MEANS `Contract Party` "Acme Ltd" (JUST (Date 1 3 2024))
`the party without notice` MEANS `Contract Party` "Beta Ltd" NOTHING

-- =======================================
-- SECTION 9: Tests
-- =======================================

-- Basic eligibility tests
#EVAL `the person is an adult` 21
#EVAL `the person is an adult` 16
#EVAL `the person is eligible to vote` 25 TRUE
#EVAL `the person is eligible to vote` 16 TRUE
#EVAL `qualifies for senior discount` 70
#EVAL `qualifies for senior discount` 50

-- Tax calculation tests
#EVAL `the tax owed` 50000 0.20
#EVAL `the income tax owed` 75000
#EVAL `the net income` 100000 15000
#EVAL `the compound interest` 1000 0.05 10
#EVAL `the monthly payment` 300000 0.06 360

-- Structured entity tests
#EVAL `the applicant is eligible for benefit` `test person eligible`
#EVAL `the applicant is eligible for benefit` `test person ineligible age`
#EVAL `the applicant is eligible for benefit` `test person ineligible citizenship`
#EVAL `the applicant is eligible for benefit` `test person ineligible income`
#EVAL `the applicant is eligible for benefit` `test person disqualified`
#EVAL `the taxpayer's bracket` `test person eligible`

-- Multi-factor decision tests
#EVAL `the loan is approved` 60000 700 200000
#EVAL `the loan is approved` 40000 700 200000
#EVAL `the annual premium` 35 100000 FALSE
#EVAL `the annual premium` 35 100000 TRUE

-- Contract scenario tests
#EVAL `the late payment penalty` 1000 15
#EVAL `the late payment penalty` 1000 45
#EVAL `qualifies for lease renewal` 2 0 1800 2000
#EVAL `qualifies for lease renewal` 2 3 1800 2000

-- Optional values and dates tests
#EVAL `the days between` (Date 1 3 2024) (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party with notice` (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party without notice` (Date 15 4 2024)

DECLARE Person HAS
    name IS A STRING
    age IS A NUMBER
    citizenship IS A STRING
    `annual income` IS A NUMBER
    `criminal record` IS A BOOLEAN

GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefit` IF
    `the applicant is a qualifying resident`
    AND `the applicant is of age`
    AND `the applicant meets income threshold`
    AND NOT `the applicant is disqualified`
    WHERE
        `the applicant is a qualifying resident` MEANS
            `the applicant`'s citizenship = "citizen"

        `the applicant is of age` MEANS
            `the applicant`'s age >= 21
            AND `the applicant`'s age < 65

        `the applicant meets income threshold` MEANS
            `the applicant`'s `annual income` < 30000

        `the applicant is disqualified` MEANS
            `the applicant`'s `criminal record`

Each condition has a name, and WHERE spells out what each one means, much as legislation defines its terms.

Example 2: Progressive Tax Calculation

`the income tax owed` in

-- Module 4: Decision Logic - Complete Examples
-- All examples are validated and use natural language identifiers

IMPORT prelude
IMPORT daydate

-- =======================================
-- SECTION 1: Basic Eligibility Decisions
-- =======================================

-- Simple age-based eligibility
GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the person is an adult` IF `the person's age` >= 18

-- Voting eligibility
GIVEN `the person's age` IS A NUMBER
      `is citizen` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is eligible to vote` IF
    `the person's age` >= 18
    AND `is citizen`

-- Senior discount qualification
GIVEN `the customer's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    `the customer's age` >= 65

-- Student status determination
GIVEN `the person's age` IS A NUMBER
      `is enrolled` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is a student` IF
    `the person's age` >= 16
    AND `the person's age` <= 25
    AND `is enrolled`

-- =======================================
-- SECTION 2: Using DECIDE IF vs DECIDE IS vs MEANS
-- =======================================

-- DECIDE IF: Best for yes/no questions with conditions
GIVEN `the income` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for exemption` IF `the income` < 10000

-- DECIDE IS: Best for classification or value assignment
GIVEN `the income` IS A NUMBER
GIVETH A STRING
DECIDE `the tax bracket` IS
    BRANCH
        IF `the income` < 10000 THEN "exempt"
        IF `the income` < 50000 THEN "standard"
        OTHERWISE "higher rate"

-- MEANS: Best for computations and definitions
GIVEN `the income` IS A NUMBER
GIVETH A STRING
`the income category` MEANS
    BRANCH
        IF `the income` < 20000 THEN "low"
        IF `the income` < 80000 THEN "medium"
        OTHERWISE "high"

-- =======================================
-- SECTION 3: Tax and Financial Calculations with WHERE
-- =======================================

-- Simple tax calculation
GIVEN `the gross income` IS A NUMBER
      `the tax rate` IS A NUMBER
GIVETH A NUMBER
`the tax owed` MEANS
    `the taxable income` * `the tax rate`
    WHERE
        `the standard deduction` MEANS 12000
        `the taxable income` MEANS
            IF `the gross income` > `the standard deduction`
            THEN `the gross income` - `the standard deduction`
            ELSE 0

-- Progressive tax with multiple brackets
GIVEN `the income` IS A NUMBER
GIVETH A NUMBER
`the income tax owed` MEANS
    `tax on first bracket` + `tax on second bracket` + `tax on third bracket`
    WHERE
        `tax on first bracket` MEANS
            `amount in first bracket` * 0.10

        `tax on second bracket` MEANS
            `amount in second bracket` * 0.20

        `tax on third bracket` MEANS
            `amount in third bracket` * 0.30

        `amount in first bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN `the income`
                OTHERWISE 10000

        `amount in second bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN 0
                IF `the income` <= 50000 THEN `the income` - 10000
                OTHERWISE 40000

        `amount in third bracket` MEANS
            BRANCH
                IF `the income` <= 50000 THEN 0
                OTHERWISE `the income` - 50000

-- Net income calculation
GIVEN `the gross income` IS A NUMBER
      `the expenses` IS A NUMBER
GIVETH A NUMBER
`the net income` MEANS
    `the gross income` - `the total deductions`
    WHERE
        `the standard deduction` MEANS
            IF `calculated deduction` >= 1000
            THEN `calculated deduction`
            ELSE 1000

        `calculated deduction` MEANS `the gross income` * 0.10

        `the total deductions` MEANS
            `the standard deduction` + `the expenses`

-- Compound interest with a named helper
GIVEN `the principal` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the years` IS A NUMBER
GIVETH A NUMBER
`the compound interest` MEANS
    `the principal` * EXPONENT `the growth factor` `the years`
    WHERE
        `the growth factor` MEANS 1 + `the annual rate`

-- Loan payment with several helpers
GIVEN `the loan amount` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the term in months` IS A NUMBER
GIVETH A NUMBER
`the monthly payment` MEANS
    `the loan amount` *
    (`the monthly rate` * `the compound factor`) /
    (`the compound factor` - 1)
    WHERE
        `the monthly rate` MEANS `the annual rate` / 12
        `the compound factor` MEANS EXPONENT (1 + `the monthly rate`) `the term in months`

-- =======================================
-- SECTION 4: Structured Entity Examples
-- =======================================

DECLARE Person HAS
    name IS A STRING
    age IS A NUMBER
    citizenship IS A STRING
    `annual income` IS A NUMBER
    `criminal record` IS A BOOLEAN

-- Comprehensive benefit eligibility
GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefit` IF
    `the applicant is a qualifying resident`
    AND `the applicant is of age`
    AND `the applicant meets income threshold`
    AND NOT `the applicant is disqualified`
    WHERE
        `the applicant is a qualifying resident` MEANS
            `the applicant`'s citizenship = "citizen"

        `the applicant is of age` MEANS
            `the applicant`'s age >= 21
            AND `the applicant`'s age < 65

        `the applicant meets income threshold` MEANS
            `the applicant`'s `annual income` < 30000

        `the applicant is disqualified` MEANS
            `the applicant`'s `criminal record`

-- Tax bracket determination with record
GIVEN `the taxpayer` IS A Person
GIVETH A STRING
DECIDE `the taxpayer's bracket` IS
    BRANCH
        IF `the taxpayer`'s `annual income` < 10000 THEN "exempt"
        IF `the taxpayer`'s `annual income` < 50000 THEN "standard"
        IF `the taxpayer`'s `annual income` < 100000 THEN "higher"
        OTHERWISE "top rate"

-- =======================================
-- SECTION 5: Multi-Factor Decisions
-- =======================================

-- Loan approval decision
GIVEN `the income` IS A NUMBER
      `the credit score` IS A NUMBER
      `the loan amount` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the loan is approved` IF
    `income requirement is met`
    AND `credit score is sufficient`
    AND `debt to income ratio is acceptable`
    WHERE
        `income requirement is met` MEANS
            `the income` >= `the loan amount` * 0.3

        `credit score is sufficient` MEANS
            `the credit score` >= 650

        `debt to income ratio is acceptable` MEANS
            `the loan amount` <= `the income` * 4

-- Insurance premium calculation
GIVEN `the insured age` IS A NUMBER
      `the coverage amount` IS A NUMBER
      `is smoker` IS A BOOLEAN
GIVETH A NUMBER
`the annual premium` MEANS
    `the base premium` * `the age factor` * `the smoking factor`
    WHERE
        `the base premium` MEANS
            `the coverage amount` * 0.001

        `the age factor` MEANS
            BRANCH
                IF `the insured age` < 30 THEN 1.0
                IF `the insured age` < 50 THEN 1.5
                OTHERWISE 2.0

        `the smoking factor` MEANS
            IF `is smoker` THEN 1.5 ELSE 1.0

-- =======================================
-- SECTION 6: Real-World Contract Scenarios
-- =======================================

-- Late payment penalty calculation
GIVEN `the principal amount` IS A NUMBER
      `days overdue` IS A NUMBER
GIVETH A NUMBER
`the late payment penalty` MEANS
    BRANCH
        IF `days overdue` <= 0 THEN 0
        IF `days overdue` <= 30 THEN `the principal amount` * 0.02
        OTHERWISE `the principal amount` * 0.05

-- Lease renewal qualification
GIVEN `years as tenant` IS A NUMBER
      `late payments` IS A NUMBER
      `current rent` IS A NUMBER
      `market rent` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for lease renewal` IF
    `tenant is in good standing`
    AND `rent is at market rate`
    WHERE
        `tenant is in good standing` MEANS
            `years as tenant` >= 1
            AND `late payments` = 0

        `rent is at market rate` MEANS
            `current rent` >= `market rent` * 0.9

-- =======================================
-- SECTION 7: Optional Values and Dates
-- =======================================

-- A contract with an optional termination date
DECLARE Contract HAS
    `the effective date` IS A DATE
    `the termination date` IS A MAYBE DATE
    `the notice period in days` IS A NUMBER

-- A party who may or may not have given notice
DECLARE `Contract Party` HAS
    `the party's name` IS A STRING
    `notice was given on` IS A MAYBE DATE

-- Days between two dates, using serial day numbers
GIVEN `the first date` IS A DATE
      `the second date` IS A DATE
GIVETH A NUMBER
`the days between` MEANS
    DATE_SERIAL `the second date` - DATE_SERIAL `the first date`

-- Contract clause interpretation with MAYBE and dates
GIVEN `the contract` IS A Contract
      `the party` IS A `Contract Party`
      `today` IS A DATE
GIVETH A BOOLEAN
DECIDE `the party may terminate` IF
    `the contract is active`
    AND `the party has given sufficient notice`
    WHERE
        `the contract is active` MEANS
            `today` >= `the contract`'s `the effective date`
            AND CONSIDER `the contract`'s `the termination date`
                WHEN NOTHING THEN TRUE
                WHEN JUST `the end date` THEN `today` < `the end date`

        `the party has given sufficient notice` MEANS
            CONSIDER `the party`'s `notice was given on`
            WHEN NOTHING THEN FALSE
            WHEN JUST `the notice date` THEN
                `the days between` `the notice date` `today` >= `the contract`'s `the notice period in days`

-- =======================================
-- SECTION 8: Test Data
-- =======================================

`test person eligible` MEANS
    Person "Alice" 25 "citizen" 25000 FALSE

`test person ineligible age` MEANS
    Person "Bob" 17 "citizen" 20000 FALSE

`test person ineligible citizenship` MEANS
    Person "Carol" 30 "permanent resident" 20000 FALSE

`test person ineligible income` MEANS
    Person "David" 30 "citizen" 50000 FALSE

`test person disqualified` MEANS
    Person "Eve" 30 "citizen" 20000 TRUE

`the open-ended contract` MEANS Contract (Date 1 1 2024) NOTHING 30
`the party with notice` MEANS `Contract Party` "Acme Ltd" (JUST (Date 1 3 2024))
`the party without notice` MEANS `Contract Party` "Beta Ltd" NOTHING

-- =======================================
-- SECTION 9: Tests
-- =======================================

-- Basic eligibility tests
#EVAL `the person is an adult` 21
#EVAL `the person is an adult` 16
#EVAL `the person is eligible to vote` 25 TRUE
#EVAL `the person is eligible to vote` 16 TRUE
#EVAL `qualifies for senior discount` 70
#EVAL `qualifies for senior discount` 50

-- Tax calculation tests
#EVAL `the tax owed` 50000 0.20
#EVAL `the income tax owed` 75000
#EVAL `the net income` 100000 15000
#EVAL `the compound interest` 1000 0.05 10
#EVAL `the monthly payment` 300000 0.06 360

-- Structured entity tests
#EVAL `the applicant is eligible for benefit` `test person eligible`
#EVAL `the applicant is eligible for benefit` `test person ineligible age`
#EVAL `the applicant is eligible for benefit` `test person ineligible citizenship`
#EVAL `the applicant is eligible for benefit` `test person ineligible income`
#EVAL `the applicant is eligible for benefit` `test person disqualified`
#EVAL `the taxpayer's bracket` `test person eligible`

-- Multi-factor decision tests
#EVAL `the loan is approved` 60000 700 200000
#EVAL `the loan is approved` 40000 700 200000
#EVAL `the annual premium` 35 100000 FALSE
#EVAL `the annual premium` 35 100000 TRUE

-- Contract scenario tests
#EVAL `the late payment penalty` 1000 15
#EVAL `the late payment penalty` 1000 45
#EVAL `qualifies for lease renewal` 2 0 1800 2000
#EVAL `qualifies for lease renewal` 2 3 1800 2000

-- Optional values and dates tests
#EVAL `the days between` (Date 1 3 2024) (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party with notice` (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party without notice` (Date 15 4 2024)

taxes each income bracket at its own rate. Each bracket's amount and its tax are named helpers under one WHERE, so the formula at the top reads `tax on first bracket` + `tax on second bracket` + `tax on third bracket`, just as a statute's schedule would.

Optional Values with MAYBE

Legal data is often incomplete: a contract may or may not have a termination date; notice may or may not have been given. L4 models "a value that might be absent" with the MAYBE type:

MAYBE DATE means "a date, or nothing"
NOTHING is the value when it's absent
JUST value wraps a value when it's present

You inspect a MAYBE with CONSIDER, handling both cases:

CONSIDER `the contract`'s `the termination date`
WHEN NOTHING THEN TRUE                 -- no termination date: contract is open-ended
WHEN JUST `the end date` THEN ...      -- there is one: use `the end date`

The type system forces you to say what happens in both cases—no "null pointer" surprises, and no silently ignored edge case.

Working with Dates

L4 has a built-in DATE type. With IMPORT daydate, you can construct dates as Date day month year (for example, Date 15 4 2024) and compare them directly with <, >=, and so on.

To count the days between two dates, convert them to serial day numbers with DATE_SERIAL:

GIVEN `the first date` IS A DATE
      `the second date` IS A DATE
GIVETH A NUMBER
`the days between` MEANS
    DATE_SERIAL `the second date` - DATE_SERIAL `the first date`

Example 3: Contract Clause Interpretation

Putting MAYBE and dates together (from

-- Module 4: Decision Logic - Complete Examples
-- All examples are validated and use natural language identifiers

IMPORT prelude
IMPORT daydate

-- =======================================
-- SECTION 1: Basic Eligibility Decisions
-- =======================================

-- Simple age-based eligibility
GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the person is an adult` IF `the person's age` >= 18

-- Voting eligibility
GIVEN `the person's age` IS A NUMBER
      `is citizen` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is eligible to vote` IF
    `the person's age` >= 18
    AND `is citizen`

-- Senior discount qualification
GIVEN `the customer's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    `the customer's age` >= 65

-- Student status determination
GIVEN `the person's age` IS A NUMBER
      `is enrolled` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the person is a student` IF
    `the person's age` >= 16
    AND `the person's age` <= 25
    AND `is enrolled`

-- =======================================
-- SECTION 2: Using DECIDE IF vs DECIDE IS vs MEANS
-- =======================================

-- DECIDE IF: Best for yes/no questions with conditions
GIVEN `the income` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for exemption` IF `the income` < 10000

-- DECIDE IS: Best for classification or value assignment
GIVEN `the income` IS A NUMBER
GIVETH A STRING
DECIDE `the tax bracket` IS
    BRANCH
        IF `the income` < 10000 THEN "exempt"
        IF `the income` < 50000 THEN "standard"
        OTHERWISE "higher rate"

-- MEANS: Best for computations and definitions
GIVEN `the income` IS A NUMBER
GIVETH A STRING
`the income category` MEANS
    BRANCH
        IF `the income` < 20000 THEN "low"
        IF `the income` < 80000 THEN "medium"
        OTHERWISE "high"

-- =======================================
-- SECTION 3: Tax and Financial Calculations with WHERE
-- =======================================

-- Simple tax calculation
GIVEN `the gross income` IS A NUMBER
      `the tax rate` IS A NUMBER
GIVETH A NUMBER
`the tax owed` MEANS
    `the taxable income` * `the tax rate`
    WHERE
        `the standard deduction` MEANS 12000
        `the taxable income` MEANS
            IF `the gross income` > `the standard deduction`
            THEN `the gross income` - `the standard deduction`
            ELSE 0

-- Progressive tax with multiple brackets
GIVEN `the income` IS A NUMBER
GIVETH A NUMBER
`the income tax owed` MEANS
    `tax on first bracket` + `tax on second bracket` + `tax on third bracket`
    WHERE
        `tax on first bracket` MEANS
            `amount in first bracket` * 0.10

        `tax on second bracket` MEANS
            `amount in second bracket` * 0.20

        `tax on third bracket` MEANS
            `amount in third bracket` * 0.30

        `amount in first bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN `the income`
                OTHERWISE 10000

        `amount in second bracket` MEANS
            BRANCH
                IF `the income` <= 10000 THEN 0
                IF `the income` <= 50000 THEN `the income` - 10000
                OTHERWISE 40000

        `amount in third bracket` MEANS
            BRANCH
                IF `the income` <= 50000 THEN 0
                OTHERWISE `the income` - 50000

-- Net income calculation
GIVEN `the gross income` IS A NUMBER
      `the expenses` IS A NUMBER
GIVETH A NUMBER
`the net income` MEANS
    `the gross income` - `the total deductions`
    WHERE
        `the standard deduction` MEANS
            IF `calculated deduction` >= 1000
            THEN `calculated deduction`
            ELSE 1000

        `calculated deduction` MEANS `the gross income` * 0.10

        `the total deductions` MEANS
            `the standard deduction` + `the expenses`

-- Compound interest with a named helper
GIVEN `the principal` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the years` IS A NUMBER
GIVETH A NUMBER
`the compound interest` MEANS
    `the principal` * EXPONENT `the growth factor` `the years`
    WHERE
        `the growth factor` MEANS 1 + `the annual rate`

-- Loan payment with several helpers
GIVEN `the loan amount` IS A NUMBER
      `the annual rate` IS A NUMBER
      `the term in months` IS A NUMBER
GIVETH A NUMBER
`the monthly payment` MEANS
    `the loan amount` *
    (`the monthly rate` * `the compound factor`) /
    (`the compound factor` - 1)
    WHERE
        `the monthly rate` MEANS `the annual rate` / 12
        `the compound factor` MEANS EXPONENT (1 + `the monthly rate`) `the term in months`

-- =======================================
-- SECTION 4: Structured Entity Examples
-- =======================================

DECLARE Person HAS
    name IS A STRING
    age IS A NUMBER
    citizenship IS A STRING
    `annual income` IS A NUMBER
    `criminal record` IS A BOOLEAN

-- Comprehensive benefit eligibility
GIVEN `the applicant` IS A Person
GIVETH A BOOLEAN
DECIDE `the applicant is eligible for benefit` IF
    `the applicant is a qualifying resident`
    AND `the applicant is of age`
    AND `the applicant meets income threshold`
    AND NOT `the applicant is disqualified`
    WHERE
        `the applicant is a qualifying resident` MEANS
            `the applicant`'s citizenship = "citizen"

        `the applicant is of age` MEANS
            `the applicant`'s age >= 21
            AND `the applicant`'s age < 65

        `the applicant meets income threshold` MEANS
            `the applicant`'s `annual income` < 30000

        `the applicant is disqualified` MEANS
            `the applicant`'s `criminal record`

-- Tax bracket determination with record
GIVEN `the taxpayer` IS A Person
GIVETH A STRING
DECIDE `the taxpayer's bracket` IS
    BRANCH
        IF `the taxpayer`'s `annual income` < 10000 THEN "exempt"
        IF `the taxpayer`'s `annual income` < 50000 THEN "standard"
        IF `the taxpayer`'s `annual income` < 100000 THEN "higher"
        OTHERWISE "top rate"

-- =======================================
-- SECTION 5: Multi-Factor Decisions
-- =======================================

-- Loan approval decision
GIVEN `the income` IS A NUMBER
      `the credit score` IS A NUMBER
      `the loan amount` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `the loan is approved` IF
    `income requirement is met`
    AND `credit score is sufficient`
    AND `debt to income ratio is acceptable`
    WHERE
        `income requirement is met` MEANS
            `the income` >= `the loan amount` * 0.3

        `credit score is sufficient` MEANS
            `the credit score` >= 650

        `debt to income ratio is acceptable` MEANS
            `the loan amount` <= `the income` * 4

-- Insurance premium calculation
GIVEN `the insured age` IS A NUMBER
      `the coverage amount` IS A NUMBER
      `is smoker` IS A BOOLEAN
GIVETH A NUMBER
`the annual premium` MEANS
    `the base premium` * `the age factor` * `the smoking factor`
    WHERE
        `the base premium` MEANS
            `the coverage amount` * 0.001

        `the age factor` MEANS
            BRANCH
                IF `the insured age` < 30 THEN 1.0
                IF `the insured age` < 50 THEN 1.5
                OTHERWISE 2.0

        `the smoking factor` MEANS
            IF `is smoker` THEN 1.5 ELSE 1.0

-- =======================================
-- SECTION 6: Real-World Contract Scenarios
-- =======================================

-- Late payment penalty calculation
GIVEN `the principal amount` IS A NUMBER
      `days overdue` IS A NUMBER
GIVETH A NUMBER
`the late payment penalty` MEANS
    BRANCH
        IF `days overdue` <= 0 THEN 0
        IF `days overdue` <= 30 THEN `the principal amount` * 0.02
        OTHERWISE `the principal amount` * 0.05

-- Lease renewal qualification
GIVEN `years as tenant` IS A NUMBER
      `late payments` IS A NUMBER
      `current rent` IS A NUMBER
      `market rent` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for lease renewal` IF
    `tenant is in good standing`
    AND `rent is at market rate`
    WHERE
        `tenant is in good standing` MEANS
            `years as tenant` >= 1
            AND `late payments` = 0

        `rent is at market rate` MEANS
            `current rent` >= `market rent` * 0.9

-- =======================================
-- SECTION 7: Optional Values and Dates
-- =======================================

-- A contract with an optional termination date
DECLARE Contract HAS
    `the effective date` IS A DATE
    `the termination date` IS A MAYBE DATE
    `the notice period in days` IS A NUMBER

-- A party who may or may not have given notice
DECLARE `Contract Party` HAS
    `the party's name` IS A STRING
    `notice was given on` IS A MAYBE DATE

-- Days between two dates, using serial day numbers
GIVEN `the first date` IS A DATE
      `the second date` IS A DATE
GIVETH A NUMBER
`the days between` MEANS
    DATE_SERIAL `the second date` - DATE_SERIAL `the first date`

-- Contract clause interpretation with MAYBE and dates
GIVEN `the contract` IS A Contract
      `the party` IS A `Contract Party`
      `today` IS A DATE
GIVETH A BOOLEAN
DECIDE `the party may terminate` IF
    `the contract is active`
    AND `the party has given sufficient notice`
    WHERE
        `the contract is active` MEANS
            `today` >= `the contract`'s `the effective date`
            AND CONSIDER `the contract`'s `the termination date`
                WHEN NOTHING THEN TRUE
                WHEN JUST `the end date` THEN `today` < `the end date`

        `the party has given sufficient notice` MEANS
            CONSIDER `the party`'s `notice was given on`
            WHEN NOTHING THEN FALSE
            WHEN JUST `the notice date` THEN
                `the days between` `the notice date` `today` >= `the contract`'s `the notice period in days`

-- =======================================
-- SECTION 8: Test Data
-- =======================================

`test person eligible` MEANS
    Person "Alice" 25 "citizen" 25000 FALSE

`test person ineligible age` MEANS
    Person "Bob" 17 "citizen" 20000 FALSE

`test person ineligible citizenship` MEANS
    Person "Carol" 30 "permanent resident" 20000 FALSE

`test person ineligible income` MEANS
    Person "David" 30 "citizen" 50000 FALSE

`test person disqualified` MEANS
    Person "Eve" 30 "citizen" 20000 TRUE

`the open-ended contract` MEANS Contract (Date 1 1 2024) NOTHING 30
`the party with notice` MEANS `Contract Party` "Acme Ltd" (JUST (Date 1 3 2024))
`the party without notice` MEANS `Contract Party` "Beta Ltd" NOTHING

-- =======================================
-- SECTION 9: Tests
-- =======================================

-- Basic eligibility tests
#EVAL `the person is an adult` 21
#EVAL `the person is an adult` 16
#EVAL `the person is eligible to vote` 25 TRUE
#EVAL `the person is eligible to vote` 16 TRUE
#EVAL `qualifies for senior discount` 70
#EVAL `qualifies for senior discount` 50

-- Tax calculation tests
#EVAL `the tax owed` 50000 0.20
#EVAL `the income tax owed` 75000
#EVAL `the net income` 100000 15000
#EVAL `the compound interest` 1000 0.05 10
#EVAL `the monthly payment` 300000 0.06 360

-- Structured entity tests
#EVAL `the applicant is eligible for benefit` `test person eligible`
#EVAL `the applicant is eligible for benefit` `test person ineligible age`
#EVAL `the applicant is eligible for benefit` `test person ineligible citizenship`
#EVAL `the applicant is eligible for benefit` `test person ineligible income`
#EVAL `the applicant is eligible for benefit` `test person disqualified`
#EVAL `the taxpayer's bracket` `test person eligible`

-- Multi-factor decision tests
#EVAL `the loan is approved` 60000 700 200000
#EVAL `the loan is approved` 40000 700 200000
#EVAL `the annual premium` 35 100000 FALSE
#EVAL `the annual premium` 35 100000 TRUE

-- Contract scenario tests
#EVAL `the late payment penalty` 1000 15
#EVAL `the late payment penalty` 1000 45
#EVAL `qualifies for lease renewal` 2 0 1800 2000
#EVAL `qualifies for lease renewal` 2 3 1800 2000

-- Optional values and dates tests
#EVAL `the days between` (Date 1 3 2024) (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party with notice` (Date 15 4 2024)
#EVAL `the party may terminate` `the open-ended contract` `the party without notice` (Date 15 4 2024)

DECLARE Contract HAS
    `the effective date` IS A DATE
    `the termination date` IS A MAYBE DATE
    `the notice period in days` IS A NUMBER

DECLARE `Contract Party` HAS
    `the party's name` IS A STRING
    `notice was given on` IS A MAYBE DATE

GIVEN `the contract` IS A Contract
      `the party` IS A `Contract Party`
      `today` IS A DATE
GIVETH A BOOLEAN
DECIDE `the party may terminate` IF
    `the contract is active`
    AND `the party has given sufficient notice`
    WHERE
        `the contract is active` MEANS
            `today` >= `the contract`'s `the effective date`
            AND CONSIDER `the contract`'s `the termination date`
                WHEN NOTHING THEN TRUE
                WHEN JUST `the end date` THEN `today` < `the end date`

        `the party has given sufficient notice` MEANS
            CONSIDER `the party`'s `notice was given on`
            WHEN NOTHING THEN FALSE
            WHEN JUST `the notice date` THEN
                `the days between` `the notice date` `today` >= `the contract`'s `the notice period in days`

Try it with the test data from the examples file:

`the open-ended contract` MEANS Contract (Date 1 1 2024) NOTHING 30
`the party with notice` MEANS `Contract Party` "Acme Ltd" (JUST (Date 1 3 2024))

#EVAL `the party may terminate` `the open-ended contract` `the party with notice` (Date 15 4 2024)
-- Result: TRUE

Exercises

Exercise 1: Simple Eligibility

Write a decision that determines if a person qualifies for a senior discount (age 65 or older):

GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    -- Your code here

Solution

GIVEN `the person's age` IS A NUMBER
GIVETH A BOOLEAN
DECIDE `qualifies for senior discount` IF
    `the person's age` >= 65

#EVAL `qualifies for senior discount` 70
#EVAL `qualifies for senior discount` 50

Exercise 2: Multi-Condition Eligibility

A person qualifies for a student loan if they:

Are between 18 and 35 years old
Are enrolled in an accredited institution
Have no prior loan defaults

Write the decision logic.

Solution

GIVEN `the applicant's age` IS A NUMBER
      `the applicant is enrolled in an accredited institution` IS A BOOLEAN
      `the applicant has prior loan defaults` IS A BOOLEAN
GIVETH A BOOLEAN
DECIDE `the applicant qualifies for a student loan` IF
    `the applicant is of eligible age`
    AND `the applicant is enrolled in an accredited institution`
    AND NOT `the applicant has prior loan defaults`
    WHERE
        `the applicant is of eligible age` MEANS
            `the applicant's age` >= 18
            AND `the applicant's age` <= 35

#EVAL `the applicant qualifies for a student loan` 25 TRUE FALSE
#EVAL `the applicant qualifies for a student loan` 40 TRUE FALSE
#EVAL `the applicant qualifies for a student loan` 25 TRUE TRUE

Exercise 3: Calculation with WHERE

Write a computation for net income that:

Starts with gross income
Subtracts standard deduction (calculated as 10% of gross, minimum $1000)
Subtracts itemized deductions

Use WHERE to break down the calculation clearly.

Solution

GIVEN `the gross income` IS A NUMBER
      `the itemized deductions` IS A NUMBER
GIVETH A NUMBER
`the net income` MEANS
    `the gross income` - `the standard deduction` - `the itemized deductions`
    WHERE
        `the standard deduction` MEANS
            IF `the calculated deduction` >= 1000
            THEN `the calculated deduction`
            ELSE 1000

        `the calculated deduction` MEANS
            `the gross income` * 0.10

#EVAL `the net income` 100000 5000
#EVAL `the net income` 8000 500

Common Mistakes

Using Programmer Names

-- Wrong: Programmer style
GIVEN p IS A Person
GIVETH A BOOLEAN
isEligible p MEANS p.age >= 18 && !p.disqualified

-- Right: Natural language
GIVEN `the person` IS A Person
GIVETH A BOOLEAN
DECIDE `the person is eligible` IF
    `the person`'s age >= 18
    AND NOT `the person`'s disqualified

Summary

Concept	Use for	Syntax
DECIDE IF	Yes/no questions, eligibility	`DECIDE name IF condition`
DECIDE IS	Classification, value assignment	`DECIDE name IS expression`
MEANS	Definitions, computations	`name MEANS expression`
BRANCH	Multi-way classification	`BRANCH IF cond1 THEN val1 ... OTHERWISE`
WHERE	Breaking down complex logic	`expression WHERE helpers`
GIVEN ... GIVETH ...	Signature: inputs and result type	`GIVEN inputs GIVETH OutputType name...`

Next, Module 5: Functions goes deeper into the machinery behind decision logic: recursion, and higher-order functions such as map and filter that apply a decision across a whole list of cases.