Merge pull request #847 from smucclaw/mengwong/methods

Author: mengwong

doc/reference/GLOSSARY.md

@@ -19,7 +19,7 @@ Keywords are reserved words that form the structure of L4 programs.
 | **GIVETH** / **GIVES** | Specifies function return type             | [GIVETH](functions/GIVETH.md)      |
 | **IN**                 | Used with LET for scoped bindings          | [LET](functions/LET.md)            |
 | **LET**                | Introduces a local binding                 | [LET](functions/LET.md)            |
-| **MEANS**              | Defines the body of a function or decision | [MEANS](functions/MEANS.md)        |
+| **MEANS**              | Defines the body of a function, decision, or computed field (method) | [MEANS](functions/MEANS.md)        |
 | **WHERE**              | Introduces local declarations              | [WHERE](functions/WHERE.md)        |
 | **YIELD**              | Creates anonymous functions (lambdas)      | [YIELD](functions/YIELD.md)        |
 
@@ -66,7 +66,7 @@ Keywords are reserved words that form the structure of L4 programs.
 | **ASSUME**     | Declares a variable of assumed type     | [ASSUME](types/ASSUME.md)          |
 | **DECLARE**    | Defines a type                          | [DECLARE](types/DECLARE.md)        |
 | **IS**         | Type assertion or definition            | [TYPE-KEYWORDS](types/keywords.md) |
-| **HAS**        | Record field declaration                | [TYPE-KEYWORDS](types/keywords.md) |
+| **HAS**        | Record field declaration (supports computed fields / methods via MEANS) | [TYPE-KEYWORDS](types/keywords.md) |
 | **LIST**       | List type or list literal               | [TYPE-KEYWORDS](types/keywords.md) |
 | **ONE OF**     | Used for enum types                     | [TYPE-KEYWORDS](types/keywords.md) |
 | **OF**         | Type application or constructor pattern | [TYPE-KEYWORDS](types/keywords.md) |
@@ -208,6 +208,7 @@ For complete documentation, see **[Syntax Reference](syntax/README.md)**.
 | **Directives**      | `#EVAL`, `#TRACE`, `#CHECK`, `#ASSERT`        |
 | **Ditto**           | `^` copy from previous line                   |
 | **Asyndetic**       | `...` (AND) and `..` (OR) implicit operators  |
+| **Computed Fields** | Derived attributes / methods via MEANS in HAS |
 | **Genitive**        | `'s` for record field access                  |
 | **Section Markers** | `§` for document sections                     |
 

doc/reference/functions/MEANS.md

@@ -54,6 +54,24 @@ circumference radius MEANS
     pi MEANS 3.14159
 ```
 
+### In Record Fields (Computed Fields / Methods)
+
+MEANS can also appear inside DECLARE HAS to define computed fields — derived attributes that are calculated automatically from other fields in the record:
+
+```l4
+DECLARE Person HAS
+    `birth year` IS A NUMBER
+    `as at year` IS A NUMBER
+    `age`        IS A NUMBER
+        MEANS `as at year` - `birth year`
+    `adult`      IS A BOOLEAN
+        MEANS `age` >= 18
+```
+
+The computed field's MEANS body can reference any sibling field by name. It can also call external functions (using OF for multi-argument calls), and use WHERE or LET/IN for local bindings.
+
+See **[DECLARE - Computed Fields](../types/DECLARE.md#computed-fields-methods)** for full documentation.
+
 ## Alternative Forms
 
 ### Using IS

doc/reference/types/DECLARE.md

@@ -39,6 +39,37 @@ DECLARE Shape IS ONE OF
   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:** [computed-fields-example.l4](computed-fields-example.l4)
+
 ### Type Synonyms
 
 Create an alias for an existing type:

doc/reference/types/computed-fields-example.l4

@@ -0,0 +1,102 @@
+-- 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

jl4-actus-analyzer/src/L4/ACTUS/FeatureExtractor.hs

@@ -516,7 +516,7 @@ conName :: ConDecl Resolved -> Text
 conName (MkConDecl _ name _) = nameText name
 
 fieldName :: TypedName Resolved -> Text
-fieldName (MkTypedName _ name _) = nameText name
+fieldName (MkTypedName _ name _ _) = nameText name
 
 isCurrencyName :: Text -> Bool
 isCurrencyName name =