§

Common L4 Patterns

Reference of frequently used basic L4 patterns with working examples: type declarations, predicates, list operations, conditionals, and simple regulative rules. For more advanced structures (progressive brackets, mixfix, statutory isomorphism, nested scoping), see Advanced Patterns.

Type Patterns

Simple Record

DECLARE Person
    HAS name IS A STRING
        age IS A NUMBER
        email IS A STRING

Record with Nested Type

DECLARE Address
    HAS street IS A STRING
        city IS A STRING
        country IS A STRING

DECLARE Person
    HAS name IS A STRING
        address IS AN Address

Enumeration (Fixed Choices)

DECLARE Status IS ONE OF
    Active
    Suspended
    Terminated

Enumeration with Data

DECLARE PaymentStatus IS ONE OF
    Pending
    Paid HAS date IS A DATE
             amount IS A NUMBER
    Rejected HAS reason IS A STRING

Union Type (Either/Or)

DECLARE LegalEntity IS ONE OF
    Individual HAS person IS A Person
    Corporation HAS company IS A Company

Function Patterns

Simple Boolean Predicate

GIVEN person IS A Person
GIVETH A BOOLEAN
DECIDE `is adult` IF person's age >= 18

Boolean with Multiple Conditions

GIVEN person IS A Person
GIVETH A BOOLEAN
DECIDE `is eligible` IF
    person's age >= 18
    AND person's hasValidID
    AND NOT person's isBanned

Computation Function

GIVEN principal IS A NUMBER
      rate IS A NUMBER
      years IS A NUMBER
GIVETH A NUMBER
`compound interest` MEANS principal * (EXPONENT (1 + rate) years)

Function with WHERE

GIVEN loan IS A Loan
GIVETH A NUMBER
`monthly payment` MEANS
    payment
    WHERE
        p MEANS loan's principal
        r MEANS loan's rate / 12
        n MEANS loan's months
        payment MEANS p * (r * factor) / (factor - 1)
        factor MEANS EXPONENT (1 + r) n

Pattern Matching Function

GIVEN status IS A Status
GIVETH A STRING
`describe status` MEANS
    CONSIDER status
    WHEN Active THEN "Currently active"
    WHEN Suspended THEN "Temporarily suspended"
    WHEN Terminated THEN "No longer active"

Pattern Matching with Data Extraction

GIVEN payment IS A PaymentStatus
GIVETH A STRING
`payment description` MEANS
    CONSIDER payment
    WHEN Pending THEN "Awaiting payment"
    WHEN Paid d a THEN CONCAT "Paid ", TOSTRING a, " on ", TOSTRING d
    WHEN Rejected r THEN CONCAT "Rejected: ", r

List Patterns

Create a List

numbers MEANS LIST 1, 2, 3, 4, 5
names MEANS LIST "Alice", "Bob", "Charlie"
`no numbers` MEANS EMPTY

Check if List is Empty

GIVEN items IS A LIST OF Item
GIVETH A BOOLEAN
`has items` MEANS NOT null items

Get First Element

GIVEN items IS A LIST OF Item
GIVETH A MAYBE Item
`first item` MEANS
    CONSIDER items
    WHEN EMPTY THEN NOTHING
    WHEN x FOLLOWED BY rest THEN JUST x

Check if Element is in List

GIVEN item IS AN Item
      items IS A LIST OF Item
GIVETH A BOOLEAN
`contains item` MEANS elem item items

Check All Elements

GIVEN people IS A LIST OF Person
GIVETH A BOOLEAN
`all adults` MEANS all (GIVEN p YIELD p's age >= 18) people

Check Any Element

GIVEN people IS A LIST OF Person
GIVETH A BOOLEAN
`any minor` MEANS any (GIVEN p YIELD p's age < 18) people

Transform All Elements

GIVEN numbers IS A LIST OF NUMBER
GIVETH A LIST OF NUMBER
`double all` MEANS map (GIVEN n YIELD n * 2) numbers

Filter Elements

GIVEN people IS A LIST OF Person
GIVETH A LIST OF Person
`only adults` MEANS filter (GIVEN p YIELD p's age >= 18) people

Conditional Patterns

Simple If-Then-Else

GIVEN age IS A NUMBER
GIVETH A STRING
`age category` MEANS
    IF age < 18 THEN "minor"
    ELSE "adult"

Multi-Way Conditions

Prefer BRANCH over nested IF/ELSE chains:

GIVEN score IS A NUMBER
GIVETH A STRING
`grade` MEANS
    BRANCH
        IF score >= 90 THEN "A"
        IF score >= 80 THEN "B"
        IF score >= 70 THEN "C"
        IF score >= 60 THEN "D"
        OTHERWISE "F"

Conditional in Context

DECIDE amount IS
    IF isVIP THEN baseAmount * 0.9
    ELSE baseAmount

Regulative Rule Patterns

Simple Obligation

GIVETH A DEONTIC Party Action
`delivery obligation` MEANS
    PARTY Seller
    MUST `deliver goods`
    WITHIN 14
    HENCE FULFILLED
    LEST BREACH

Permission

GIVETH A DEONTIC Party Action
`inspection right` MEANS
    PARTY Buyer
    MAY `inspect goods`
    HENCE FULFILLED

Prohibition

GIVETH A DEONTIC Party Action
`confidentiality clause` MEANS
    PARTY Employee
    SHANT `disclose confidential information`
    HENCE FULFILLED
    LEST BREACH

Chained Obligations

GIVETH A DEONTIC Party Action
`sale contract` MEANS
    PARTY Seller
    MUST `deliver goods`
    WITHIN 14
    HENCE
        PARTY Buyer
        MUST `pay invoice`
        WITHIN 30
        HENCE FULFILLED
        LEST BREACH BY Buyer
    LEST BREACH BY Seller

Conditional Obligation

GIVETH A DEONTIC Party Action
`conditional payment` MEANS
    PARTY Buyer
    MUST `pay` amount PROVIDED amount > 0
    WITHIN 30
    HENCE FULFILLED
    LEST BREACH

Parallel Obligations (RAND)

GIVETH A DEONTIC Party Action
`mutual obligations` MEANS
    (PARTY Seller MUST `deliver` WITHIN 14 HENCE FULFILLED LEST BREACH)
    RAND
    (PARTY Seller MUST `provide documentation` WITHIN 14 HENCE FULFILLED LEST BREACH)

Alternative Paths (ROR)

GIVETH A DEONTIC Party Action
`delivery options` MEANS
    (PARTY Seller MUST `ship goods` WITHIN 14 HENCE FULFILLED)
    ROR
    (PARTY Seller MUST `arrange pickup` WITHIN 7 HENCE FULFILLED)
    LEST BREACH

Money and Currency Patterns

Money Type

DECLARE Money
    HAS amount IS A NUMBER
        currency IS A STRING

Currency Constructors

GIVEN n IS A NUMBER
GIVETH A Money
USD MEANS Money n "USD"

GIVEN n IS A NUMBER
GIVETH A Money
EUR MEANS Money n "EUR"

Money Comparison

GIVEN m1 IS A Money
      m2 IS A Money
GIVETH A BOOLEAN
`money equals` MEANS
    m1's amount = m2's amount
    AND m1's currency EQUALS m2's currency

See also the currency library for ISO 4217 currency handling.

Date Patterns

Using the daydate Library

IMPORT daydate

-- Create a date (day, month, year)
expiryDate MEANS Date 31 12 2025

-- Compare dates
GIVEN d1 IS A DATE
      d2 IS A DATE
GIVETH A BOOLEAN
`comes first` MEANS d1 `is before` d2

Days Between Dates

GIVEN start IS A DATE
      end IS A DATE
GIVETH A NUMBER
`days between` MEANS Day end MINUS Day start

See the daydate library for the full date toolkit.

Testing Patterns

Unit Test with #EVAL

#EVAL `is adult` (Person "Test" 18 "test@example.com")  -- TRUE
#EVAL `is adult` (Person "Test" 17 "test@example.com")  -- FALSE

Integration Test with #TRACE

#TRACE `sale contract` AT 0 WITH
    PARTY Seller DOES `deliver goods` AT 10
    PARTY Buyer DOES `pay invoice` AT 25
-- Expected: FULFILLED

Boundary Testing

-- Test boundaries
#EVAL `is adult` (Person "Edge" 18 "e@example.com")   -- TRUE (exactly at boundary)
#EVAL `is adult` (Person "Edge" 17 "e@example.com")   -- FALSE (just below)
#EVAL `is adult` (Person "Edge" 19 "e@example.com")   -- TRUE (just above)

Anti-Patterns (What NOT to Do)

❌ Missing Parentheses with Field Access

-- ❌ Wrong
length charity's governors > 0

-- ✅ Right
length (charity's governors) > 0

❌ Forgetting ELSE Branch

-- ❌ Wrong (missing ELSE)
DECIDE result IS IF condition THEN "yes"

-- ✅ Right
DECIDE result IS IF condition THEN "yes" ELSE "no"

❌ Missing GIVETH

-- ❌ Wrong
GIVEN n IS A NUMBER
square MEANS n * n

-- ✅ Right
GIVEN n IS A NUMBER
GIVETH A NUMBER
square MEANS n * n

❌ Inconsistent Indentation

-- ❌ Wrong (misaligned fields)
DECLARE Person
    HAS name IS A STRING
      age IS A NUMBER
        email IS A STRING

-- ✅ Right (all fields aligned)
DECLARE Person
    HAS name IS A STRING
        age IS A NUMBER
        email IS A STRING

Quick Reference

Pattern When to Use
DECLARE ... HAS Define a record type
DECLARE ... IS ONE OF Define an enumeration
GIVEN ... GIVETH ... DECIDE Define a function
CONSIDER ... WHEN Pattern matching
IF ... THEN ... ELSE Conditional expression
BRANCH Multi-way conditions
PARTY ... MUST Create obligation
PARTY ... MAY Create permission
PARTY ... SHANT Create prohibition
RAND Parallel obligations
ROR Alternative obligations
#EVAL Test expression
#TRACE Test regulative rule

See Also