DECLARE

DECLARE introduces a new type: a record, an enumeration, or a synonym for another type.

Syntax

DECLARE Name HAS field1 IS A Type1, field2 IS A Type2
DECLARE Name IS ONE OF Value1, Value2, Value3
DECLARE Name IS A Type
DECLARE Name OF a, b HAS ...

Example files:

-- DECLARE keyword examples

-- Record Types (Product Types)

-- Basic record with multiple fields
DECLARE Person
  HAS
    `full name` IS A STRING
    age         IS A NUMBER

-- Record with single line syntax
DECLARE Point HAS x IS A NUMBER, y IS A NUMBER

-- Enum Types (Sum Types)

-- Simple enum with value constructors
DECLARE Colour IS ONE OF `bright red`, `forest green`, `ocean blue`

-- Enum with constructors that have fields
DECLARE Shape IS ONE OF
  Circle HAS radius IS A NUMBER
  Rectangle HAS width IS A NUMBER, height IS A NUMBER
  Point

-- Type Synonyms

DECLARE Age IS NUMBER
DECLARE `Person name` IS STRING

-- Parameterised Types

-- The type parameter written after the name ...
DECLARE Box OF a HAS contents IS AN a

-- ... or declared with GIVEN above the DECLARE
GIVEN a IS A TYPE, b IS A TYPE
DECLARE `Pair of values` HAS first IS AN a, second IS A b

-- Usage Examples

-- Create record instances
DECIDE `the applicant` IS Person WITH `full name` IS "John", age IS 30
DECIDE `the origin` IS Point WITH x IS 0, y IS 0

-- Use enum constructors
DECIDE `my favourite colour` IS `bright red`
DECIDE `my shape` IS Circle WITH radius IS 5

-- Use parameterised types
GIVETH A Box OF NUMBER
DECIDE `box with number` IS Box WITH contents IS 42
GIVETH A Box OF STRING
DECIDE `box with text` IS Box WITH contents IS "hello"
GIVETH A `Pair of values` OF NUMBER, STRING
DECIDE `a pair` IS `Pair of values` WITH first IS 1, second IS "one"

-- Evaluate examples
#EVAL `the applicant`'s `full name`
#EVAL `the applicant`'s age
#EVAL `my favourite colour`
#EVAL `box with text`'s contents
#EVAL `a pair`'s second

-- Example: Record type declaration
-- Demonstrates product types with named fields

DECLARE Person
  HAS `full name` IS A STRING
      age         IS A NUMBER

-- Example: Creating a person record
`the applicant` MEANS Person WITH
  `full name` IS "John Doe"
  age         IS 30

#EVAL `the applicant`'s `full name`
#EVAL `the applicant`'s age

-- Example: Enum type declaration
-- Demonstrates sum types with named constructors

DECLARE Color IS ONE OF
  Red
  Green
  Blue

-- Example usage
`my favorite color` MEANS Red

`name of the color` MEANS
  CONSIDER `my favorite color`
  WHEN Red   THEN "red"
  WHEN Green THEN "green"
  WHEN Blue  THEN "blue"

#EVAL `name of the color`

Records

A record has named fields. Write them after HAS, one per line or separated by commas:

DECLARE Person
  HAS
    name IS A STRING
    age IS A NUMBER

DECLARE Point HAS x IS A NUMBER, y IS A NUMBER

A field can also be written name: STRING or name: A STRING.

Making a record. Give every field by name with WITH, or give the values in the order the fields are declared, with OF and commas or with spaces:

DECLARE Person HAS
  name IS A STRING
  age IS A NUMBER

`the buyer` MEANS Person WITH name IS "Kim", age IS 30
`the seller` MEANS Person OF "Lee", 45
`the agent` MEANS Person "Jo" 28

#EVAL `the buyer`'s age

Reading a field. Use the genitive 's: `the buyer`'s age. See Syntax and directives: Genitive.

Computed fields

A field followed by MEANS is computed from the record's other fields. It is read with 's like any other field, and it is not given when the record is made.

DECLARE Employee HAS
    `name`          IS A STRING
    `year of birth` IS A NUMBER
    `current year`  IS A NUMBER
    `age`           IS A NUMBER
        MEANS `current year` - `year of birth`
    `adult`         IS A BOOLEAN
        MEANS `age` >= 18

`the employee` MEANS Employee WITH
    `name`          IS "Lim"
    `year of birth` IS 1990
    `current year`  IS 2026

#EVAL `the employee`'s `adult`

The body can use the record's other fields by name, including other computed fields, and call functions defined elsewhere.
It can use WHERE and LET … IN.
It takes no inputs of its own: there is no GIVEN for a computed field. Its only input is the record.
Computed fields that depend on each other in a cycle are an error ("Circular dependency detected between computed fields").

Putting the stored fields first and the computed fields after them makes a declaration easier to read.

Example file:

-- Computed Fields Example
-- Demonstrates "methods" on records: fields with MEANS clauses
-- whose values are automatically derived from other fields.

IMPORT prelude

-- ─────────────────────────────────────────────────────────
-- Basic computed fields (methods)
-- ─────────────────────────────────────────────────────────
-- Style: stored fields first, then computed/derived fields.

DECLARE Employee HAS
    -- stored fields (primary attributes)
    `name`               IS A STRING
    `date of birth`      IS A NUMBER
    `date of employment` IS A NUMBER
    `monthly salary`     IS A NUMBER
    `current year`       IS A NUMBER
    -- computed fields (derived attributes / methods)
    `age`                IS A NUMBER
        MEANS `current year` - `date of birth`
    `years of service`   IS A NUMBER
        MEANS `current year` - `date of employment`
    `senior`             IS A BOOLEAN
        MEANS `years of service` >= 10
    `eligible`           IS A BOOLEAN
        MEANS `senior`
          AND `monthly salary` <= 6000

-- Construction: only stored fields are supplied.
-- Computed fields are derived automatically.
veteran MEANS Employee WITH
    `name`               IS "Lim"
    `date of birth`      IS 1960
    `date of employment` IS 1990
    `monthly salary`     IS 3000
    `current year`       IS 2026

-- Access computed fields with 's, just like stored fields.
#EVAL veteran's `age`            -- 66
#EVAL veteran's `years of service` -- 36
#EVAL veteran's `senior`         -- TRUE
#EVAL veteran's `eligible`       -- TRUE

-- ─────────────────────────────────────────────────────────
-- Calling external functions from computed fields
-- ─────────────────────────────────────────────────────────
-- Multi-argument calls can use juxtaposition, OF, or mixfix:
--   `apply rate` `balance` `rate`         -- juxtaposition
--   `apply rate` OF `balance`, `rate`     -- OF (comma-separated)
--   `apply` `balance` `at rate` `rate`    -- mixfix (natural language)
-- All are equivalent. Choose whichever reads best in context.

GIVEN `base` IS A NUMBER
      `rate` IS A NUMBER
GIVETH A NUMBER
`apply rate` MEANS `base` * `rate` / 100

DECLARE Account HAS
    `balance`   IS A NUMBER
    `rate`      IS A NUMBER
    `interest`  IS A NUMBER
        MEANS `apply rate` `balance` `rate`
    `projected` IS A NUMBER
        MEANS `balance` + `interest`

savings MEANS Account WITH
    `balance` IS 10000
    `rate`    IS 5

#EVAL savings's `interest`   -- 500.0
#EVAL savings's `projected`  -- 10500.0

-- ─────────────────────────────────────────────────────────
-- WHERE and LET/IN inside computed fields
-- ─────────────────────────────────────────────────────────

GIVEN `x` IS A NUMBER
GIVETH A NUMBER
`square` MEANS `x` * `x`

DECLARE Shape HAS
    `width`   IS A NUMBER
    `height`  IS A NUMBER
    `area`    IS A NUMBER
        MEANS `width` * `height`
    `diag sq` IS A NUMBER
        MEANS `result`
          WHERE
            `w2` MEANS `square` `width`
            `h2` MEANS `square` `height`
            `result` MEANS `w2` + `h2`
    `scaled`  IS A NUMBER
        MEANS LET `factor` MEANS 2 IN `area` * `factor`

rect MEANS Shape WITH
    `width`  IS 3
    `height` IS 4

#EVAL rect's `area`     -- 12
#EVAL rect's `diag sq`  -- 25
#EVAL rect's `scaled`   -- 24

Enumerations

IS ONE OF lists the values a type can have:

DECLARE Colour IS ONE OF Red, Green, Blue

A value can carry fields of its own:

DECLARE Shape IS ONE OF
  Circle HAS radius IS A NUMBER
  Rectangle HAS width IS A NUMBER, height IS A NUMBER
  Point

`the plot` MEANS Rectangle WITH width IS 20, height IS 30
`the pond` MEANS Circle 4

Use CONSIDER to tell the values apart.

Type synonyms

DECLARE Name IS Type gives another name to an existing type. The two names are the same type.

DECLARE Age IS A NUMBER

GIVEN age IS AN Age
GIVETH A BOOLEAN
`is an adult` age MEANS age >= 18

A synonym can take type parameters: DECLARE Optional OF a IS A MAYBE a.
A synonym cannot refer to itself, directly or through other synonyms; L4 reports a circular dependency between them.
L4 follows a chain of synonyms for at most 1000 steps. For a longer chain, declare the first synonym as the type the chain ends in.

Parameterised types

A type can take type parameters, so that the same declaration works for numbers, text or any other type. Write the parameters after the name, with OF and commas, or declare them with GIVEN … IS A TYPE above the DECLARE:

DECLARE Box OF a HAS contents IS AN a

GIVEN a IS A TYPE
      b IS A TYPE
DECLARE `Pair of values` HAS first IS AN a, second IS A b

GIVETH A Box OF NUMBER
`a box of 42` MEANS Box WITH contents IS 42

The parameter can also follow the name without OF (DECLARE Box a HAS …). A use of the type gives the parameters with OF: Box OF NUMBER, `Pair of values` OF NUMBER, STRING.

A and AN

A and AN before a type are for readability only. They are interchangeable and can be left out: IS A NUMBER, IS AN Account and IS NUMBER mean the same.

DECLARE Account HAS balance IS A NUMBER

GIVEN account IS AN Account
GIVETH NUMBER
`balance of` account MEANS account's balance

Type expressions

Wherever a type is expected (after IS A, GIVETH, in a field):

Type expression	Meaning
`NUMBER`, `STRING`, `BOOLEAN`, `DATE`	a built-in type (Types)
`LIST OF Type`	a list
`MAYBE Type`	a value that may be missing
`EITHER Type1 Type2`	one of two types
`Name OF Type1, Type2`	a parameterised type, given its parameters
`FUNCTION FROM Type1 AND Type2 TO Type3`	a function
`TYPE`	a type parameter

Types: built-in types
ASSUME: a name with a type and no definition
FOR ALL: polymorphic types for ASSUME
AKA: other names for a type