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DECLARE

Declares a new type in L4. Types can be records (product types), enums (sum types), or type synonyms.

Syntax

DECLARE TypeName IS ...
DECLARE TypeName HAS ...
DECLARE TypeName IS ONE OF ...

Forms

Record Types (Product Types)

Records are types with named fields:

DECLARE Person
  HAS
    name IS A STRING
    age IS A NUMBER

Enum Types (Sum Types)

Enums define a type with multiple alternatives:

DECLARE Colour IS ONE OF red, green, blue

Enum constructors can have fields:

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

Type Synonyms

Create an alias for an existing type:

DECLARE Age IS NUMBER
DECLARE PersonName IS STRING

Examples

Example file:

-- 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 PersonName IS STRING

-- Parameterized Types

GIVEN a IS A TYPE
DECLARE Box HAS contents IS AN a

GIVEN a IS A TYPE, b IS A TYPE
DECLARE MyPair 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 parameterized types
DECIDE `box with number` IS Box WITH contents IS 42
DECIDE `box with text` IS Box WITH contents IS "hello"

-- Evaluate examples
#EVAL `the applicant`'s `full name`
#EVAL `the applicant`'s age
#EVAL `my favourite colour`

Basic Record

DECLARE Customer
  HAS
    name IS A STRING
    email IS A STRING
    balance IS A NUMBER

Parameterized Types

Types can have type parameters:

GIVEN a IS A TYPE
DECLARE Box
  HAS
    contents IS AN a

Field Syntax Variations

L4 supports multiple syntaxes for fields:

-- Using IS A
DECLARE Person1 HAS name IS A STRING

-- Using colon
DECLARE Person2 HAS name: STRING

-- Using colon with article
DECLARE Person3 HAS name: A STRING

See Also