# DECLARE Declares a new type in L4. Types can be records (product types), enums (sum types), or type synonyms. ## Syntax ```l4 DECLARE TypeName IS ... DECLARE TypeName HAS ... DECLARE TypeName IS ONE OF ... ``` ## Forms ### Record Types (Product Types) Records are types with named fields: ```l4 DECLARE Person HAS name IS A STRING age IS A NUMBER ``` ### Enum Types (Sum Types) Enums define a type with multiple alternatives: ```l4 DECLARE Colour IS ONE OF red, green, blue ``` Enum constructors can have fields: ```l4 DECLARE Shape IS ONE OF Circle HAS radius IS A NUMBER Rectangle HAS width IS A NUMBER, height IS A NUMBER ``` ### Computed Fields (Methods) Record fields can have a `MEANS` clause that defines a derived value — computed automatically from the record's other fields. These are analogous to **methods**, **calculated properties**, or **derived attributes** in other languages. ```l4 DECLARE Employee HAS -- stored fields (primary attributes) `name` IS A STRING `date of birth` IS A NUMBER `current year` IS A NUMBER -- computed fields (derived attributes) `age` IS A NUMBER MEANS `current year` - `date of birth` `adult` IS A BOOLEAN MEANS `age` >= 18 ``` **Key points:** - Computed fields are accessed with `'s` just like stored fields: `employee's `age`` - Computed fields may depend on other computed fields (chaining) - Computed fields may call external functions using OF syntax: `MEANS `f`OF`x`, `y`` - Computed fields may use WHERE and LET/IN for local bindings - When constructing a record with WITH, only stored fields are supplied — computed fields are derived automatically **Referential transparency:** Computed fields are _pure_ — they may only reference sibling fields of the same record. There is no way to add a GIVEN parameter to a MEANS clause inside a DECLARE; the only input is the record itself. This is a deliberate design choice rooted in functional programming: a computed field is a total function from the record's stored state to a derived value, with no hidden dependencies on external state or arguments. In the language of the lambda calculus, each computed field is a closed term over the record's own bindings. This purity guarantee means that computed fields are referentially transparent — evaluating `employee's \`age\`` will always yield the same result for the same record, regardless of when or where it is called. It also makes cycle detection decidable: the compiler builds a dependency graph over a finite set of sibling fields and rejects any strongly connected component, ensuring termination. **Style guide:** Group stored fields first, then computed fields, so readers see the primary data before the derived logic. Depart from this convention when expository clarity calls for a different ordering — for instance, placing a computed field immediately after the stored fields it depends on. **Example file:** ```l4-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 ``` ### Type Synonyms Create an alias for an existing type: ```l4 DECLARE Age IS NUMBER DECLARE PersonName IS STRING ``` ## Examples **Example file:** ```l4-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 ```l4 DECLARE Customer HAS name IS A STRING email IS A STRING balance IS A NUMBER ``` ### Parameterized Types Types can have type parameters: ```l4 GIVEN a IS A TYPE DECLARE Box HAS contents IS AN a ``` ### Field Syntax Variations L4 supports multiple syntaxes for fields: ```l4 -- 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 ``` ## Related Keywords - **[TYPE-KEYWORDS](/l4/reference/types/keywords.md)** - Type-related keywords (IS, HAS, ONE OF) ## See Also - **[Types Reference](/l4/reference/types.md)** - Complete type system documentation