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FScript Supported Types Specification

Purpose

This document specifies the value and type system used by the interpreter.

Primitive types

  • unit
  • int (int64 at runtime)
  • float
  • bool
  • string

Composite/container types

  • List: 'a list
  • Tuple: (t1 * t2 * ...)
  • Option: 'a option
  • Map (string-keyed alias): 'a map
  • Record: structural record types
  • Discriminated union: named union types with cases

Map literals

  • Native map literal syntax:
    • empty: {}
    • populated: { ["a"] = 1; ["b"] = 2 }
    • spread update: { ["a"] = 1; ..tail }
    • multiline entries are supported in an indented block.
  • Keys are bracketed expressions and must have type string (for example { [keyExpr] = value }).
  • Record literals and map literals share { ... } braces:
    • map entries use [expr] = value
    • record entries use Field = value
    • {} denotes an empty map.
  • Values are inferred and unified to a single value type.
  • See ./map-matching-reference.md for focused map-pattern examples.

Function types

  • Functions use curried arrow types:
    • t1 -> t2
    • t1 -> t2 -> t3 is right-associative
  • Function type syntax is available in parameter annotations.
  • Parameter annotations support two inline record type forms:
    • structural: let f (x: {| A: int; B: string |}) = ...
    • declared-type-by-shape: let f (x: { A: int; B: string }) = ...
  • { ... } resolves only if exactly one declared record type matches that shape.
  • Inline record annotation fields are semicolon-separated in single-line form.

Named record declarations

  • Top-level record declarations:
    • type Name = { ... }
    • type rec Name = { ... }
  • Recursive record references use type rec.
  • Named record types and matching record shapes unify structurally.

Discriminated union declarations

  • Top-level union declarations are supported:
    • type Shape = | Point | Circle of int
    • type rec Node = | Empty | Branch of (int * Node list)
  • Union case names start with an uppercase identifier.
  • A case payload is optional:
    • no payload: | Point
    • payload: | Circle of int
  • Payloads are single-type payloads; tuple payloads are used for multi-value payloads.

Union constructors and matching

  • Union cases are constructors in expression position:
    • let x = Circle 3
    • let p = Point
  • match supports union-case patterns:
    • | Circle r -> ...
    • | Point -> ...
  • option values (Some/None) are also represented as union-style constructors and patterns.

Type inference model

  • Hindley–Milner style inference.
  • Let-polymorphism.
  • Structural typing for record values.
  • Field access is typed when the target expression is inferred as a record.

Runtime value representation (high level)

  • VUnit, VInt, VFloat, VBool, VString
  • VList, VTuple
  • VRecord
  • VMap
  • VOption
  • VUnionCase, VUnionCtor
  • VClosure
  • VExternal
  • VTypeToken

Reflection

  • typeof Name yields a type token.
  • nameof identifier yields the bound identifier name as string.
  • Type tokens are consumed by host externs (for example JSON/XML decoding).