Type System

Experimental. The type system is still evolving, and some limitations are expected.

Wippy's gradual type system supports incremental annotations and flow-sensitive checking. Types are non-nullable by default.

This page is a language reference, not a complete program. Each code block is an isolated type-checking example, and alternatives within a block are not necessarily meant to be combined. Names such as get_data, get_user, call, and User represent application code; lines marked ERROR intentionally demonstrate diagnostics. These examples use language syntax and built-in type values, so they do not require runtime modules.

Primitives

local n: number = 3.14
local i: integer = 42         -- integer is subtype of number
local s: string = "hello"
local b: boolean = true
local a: any = "anything"     -- dynamic member and method access
local u: unknown = { source = "example" }  -- must narrow before use

any and unknown

-- any: dynamic member and method access
local a: any = get_data()
a.foo.bar.baz()              -- no error, may crash at runtime
local s: string = a          -- ERROR: any is not assignable to string

-- unknown: safe unknown, must narrow before use as a concrete type
local u: unknown = get_data()
u.foo                        -- no error: member access on unknown behaves like any
local n: number = u          -- ERROR: unknown not assignable to number, narrow first
if type(u) == "table" then
    -- u narrowed to table here
end

Nil Safety

Types are non-nullable by default. Use ? for optional values:

local x: number = nil         -- ERROR: nil not assignable to number
local y: number? = nil        -- OK: number? means "number or nil"
local z: number? = 42         -- OK

Control Flow Narrowing

The type checker tracks control flow:

local function process(x: number?): number
    if x ~= nil then
        return x              -- x is number here
    end
    return 0
end

-- Early return pattern
local user, err = get_user(123)
if err then return nil, err end
-- user narrowed to non-nil here

-- Or default
local val = get_value() or 0  -- val: number

Union Types

local val: number | string = get_value()

if type(val) == "number" then
    print(val + 1)            -- val: number
else
    print(val:upper())        -- val: string
end

Literal Types

type Status = "pending" | "active" | "done"

local s: Status = "pending"   -- OK
local s: Status = "invalid"   -- ERROR

Function Types

local function add(a: number, b: number): number
    return a + b
end

-- Multiple returns
local function div_mod(a: number, b: number): (number, number)
    return math.floor(a / b), a % b
end

-- Error returns (Lua idiom)
local function fetch(url: string): (string?, error?)
    -- returns (data, nil) or (nil, error)
end

-- First-class function types
local double: (number) -> number = function(x: number): number
    return x * 2
end

Variadic Functions

local function sum(...: number): number
    local total: number = 0
    for _, v in ipairs({...}) do
        total = total + v
    end
    return total
end

Record Types

type User = {name: string, age: number}

local u: User = {name = "alice", age = 25}

Optional Fields

type Config = {
    host: string,
    port: number,
    timeout?: number,
    debug?: boolean
}

local cfg: Config = {host = "localhost", port = 8080}  -- OK

Generics

local function identity<T>(x: T): T
    return x
end

local n: number = identity(42)
local s: string = identity("hello")

Constrained Generics

type HasName = {name: string}

local function greet<T: HasName>(obj: T): string
    return "Hello, " .. obj.name
end

greet({name = "Alice"})       -- OK
greet({age = 30})             -- ERROR: missing 'name'

Intersection Types

Combine multiple types:

type Named = {name: string}
type Aged = {age: number}
type Person = Named & Aged

local p: Person = {name = "Alice", age = 30}

Tagged Unions

type Result<T, E> =
    {ok: true, value: T}
    | {ok: false, error: E}

type LoadState =
    {status: "loading"}
    | {status: "loaded", data: User}
    | {status: "error", message: string}

local function render(state: LoadState): string
    if state.status == "loading" then
        return "Loading..."
    elseif state.status == "loaded" then
        return "Hello, " .. state.data.name
    elseif state.status == "error" then
        return "Error: " .. state.message
    end
end

The never Type

never is the bottom type: it has no possible values.

function fail(msg: string): never
    error(msg)
end

Error Handling Pattern

The checker understands the common Lua value, error return pattern:

local value, err = call()
if err then
    -- value is nil here
    return nil, err
end
-- value is non-nil here, err is nil
print(value)

Non-Nil Assertion

Use ! to assert an expression is non-nil:

local user: User? = get_user()
local name = (user!).name            -- assert user is non-nil

! is a type-checker assertion only - it narrows the type to non-nil but emits no runtime check. If the value is actually nil, the following operation fails with the usual error (e.g. indexing nil). Use when you know a value cannot be nil but the type checker cannot prove it.

Type Casts

Runtime Validation

Call a type as a function to validate a value. Validation returns the original value with the requested static type; it does not convert or coerce the value:

local data: any = get_json()
local user = User(data)              -- validates and returns User
local name = user.name               -- safe field access

This works with primitives and custom types:

local x: any = get_value()
local s = string(x)                  -- requires an existing string
local n = integer(x)                 -- requires an existing integer
local b = boolean(x)                 -- requires an existing boolean

type Point = {x: number, y: number}
local p = Point(data)                -- validates record structure

For example, string(42) raises a validation error; use tostring(42) when conversion is intended.

Type:is() Method

Type:is validates without throwing and returns either (value, nil) or (nil, error):

type Point = {x: number, y: number}
local data: any = get_input()

local p, err = Point:is(data)
if p then
    local sum = p.x + p.y            -- p is valid Point
else
    return nil, err                  -- validation failed
end

The result narrows in conditionals:

if Point:is(data) then
    local p: Point = data            -- data narrowed to Point
end

Unsafe Cast

Use :: or as for unchecked casts:

local data: any = get_data()
local user = data :: User            -- no runtime check
local user = data as User            -- same as ::

Use sparingly. Unsafe casts bypass validation and can cause runtime errors if the value doesn't match the type.

Type Reflection

Types are first-class values that provide introspection methods.

Kind and Name

type Num = number

print(Num:kind())                    -- "number"
print(Point:kind())                  -- "record"
print(Point:name())                  -- "Point"

Record Fields

Iterate over record fields:

type User = {name: string, age: number}

for name, typ in User:fields() do
    print(name, typ:kind())
end
-- name    string
-- age     number

Access individual field types:

local nameType = User.name           -- type of 'name' field
print(nameType:kind())               -- "string"

Collection Types

type NumberList = {number}
print(NumberList:elem():kind())      -- "number"

type ScoreMap = {[string]: number}
print(ScoreMap:key():kind())         -- "string"
print(ScoreMap:val():kind())         -- "number"

Optional Types

type MaybeNumber = number?
print(MaybeNumber:kind())            -- "optional"
print(MaybeNumber:inner():kind())    -- "number"

Union Types

type Status = "pending" | "active" | "done"

for variant in Status:variants() do
    print(variant)
end

Function Types

type Predicate = (number, string) -> boolean

for param in Predicate:params() do
    print(param:kind())
end
print(Predicate:ret():kind())        -- "boolean"

typeof(expression) is type syntax, not a runtime reflection function. Use it in an alias such as type Config = typeof(default_config); the resulting alias is the runtime type value.

Type Comparison

type Num = number
type Int = integer

print(Num == Num)                    -- true
print(Int <= Num)                    -- true (subtype)
print(Int < Num)                     -- true (strict subtype)

Types as Table Keys

type Point = {x: number, y: number}
type Line = {from: Point, to: Point}

local handlers = {}
handlers[Point] = function() return "point handler" end
handlers[Line] = function() return "line handler" end

local h = handlers[Point]
if h then h() end

Type Annotations

Add types to function signatures:

-- Parameter and return types
local function process(input: string): number
    return #input
end

-- Local variable types
local count: number = 0

-- Type aliases
type StringArray = {string}
type StringMap = {[string]: number}

Type Validators

Attach validation constraints to type aliases with annotations, then call the type or use Type:is() to enforce them at runtime:

type NonNegative = number @min(0)
type Percentage = number @min(0) @max(100)
type Email = string @pattern("^.+@.+$")

local x = NonNegative(1)
local percent, err = Percentage:is(50)
local email = Email("test@example.com")

An annotation on a local variable is checked statically by the linter. It does not insert an automatic runtime check at assignment; runtime enforcement occurs when a type value validates a value.

Built-in Validators

Record Field Validators

type User = {
    age: number @min(0) @max(150),
    name: string @min_len(1) @max_len(100)
}

Array Element Validators

local scores: {number @min(0) @max(100)} = {85, 90}

Union Member Validators

local id: number @min(1) | string @min_len(1) = 1

Variance Rules

Subtyping

• integer is a subtype of number
• never is a subtype of all types
• All types are subtypes of any
• Union subtyping: A is subtype of A | B

Gradual Adoption

Types can be added incrementally; untyped code continues to work:

-- Existing code works unchanged
function old_function(x)
    return x + 1
end

-- New code gets types
function new_function(x: number): number
    return x + 1
end

Useful starting points include:

1. Function signatures at API boundaries
2. HTTP handlers and queue consumers
3. Critical business logic

Type Checking

Run the type checker with:

wippy lint

The command reports type errors without executing the code.