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Typing Anonymous Functions

Part of the Introduction To Luau section of Coddy's Lua journey. Lesson 24 of 80.

In Lua, functions are values: you've stored them in variables and passed them around: local f = function(x) return x * 2 end. These anonymous functions (function expressions) take type annotations exactly like named functions do:

local double = function(x: number): number
    return x * 2
end

local shout = function(text: string): string
    return string.upper(text) .. "!"
end

Parameters get name: type inside the parentheses, and the return type follows the closing parenthesis: the only difference from a named function is that the function keyword has no name and the whole expression is assigned to a variable.

If you've seen JavaScript or TypeScript, note that Luau has no arrow functions. There is no => syntax. Every function expression is written with function ... end, long or short.

Once assigned, you call it through the variable like any function: double(21) returns 42, and the checker verifies every call against the annotations you wrote.

challenge icon

Challenge

Easy

Store typed functions in local variables.

  1. Rewrite subtract as local subtract = function(a: number, b: number): number, keeping its body.
  2. Add local isPositive = function(num: number): boolean that returns num > 0.
  3. Print subtract(10, 3) and isPositive(-5).

Press Run Code: the output shows 7 and false.

Try it yourself

-- Rewrite this as an anonymous function stored in a local
function subtract(a: number, b: number): number
    return a - b
end

All lessons in Introduction To Luau

1Getting Started with Luau

What Is Luau?Why Use Luau?Your First Luau CodeType Checking & Error ModesRecap: Introduction to Luau

4Working with Functions

Typing Params & Return ValuesTyping Anonymous FunctionsFunctions Returning NothingOptional ParametersDefault Parameter ValuesDefaults for BooleansVariadic FunctionsDefining Function TypesRecap: Typed Functions

7Project: Typed Task Manager

Project: The Task ShapeFunction to Add a TaskProject lessonFunction to Change Task StatusProject lessonFunction: List Tasks by StatusProject lessonFunction: Print Task SummaryProject lessonCommand Loop: AddProject lessonCommand Loop: List and DoneProject lessonCommand Loop: SummaryProject lesson

10Project: Generic Inventory

Project: Inventory ItemFunction: Add ItemsProject lessonThe Weapons ListProject lessonFunction: Find Item By IDProject lessonSpecific Inventory TypesProject lessonFunction: Describe ItemsProject lessonAnswer Item QueriesProject lesson

2Core Types

Number, String and BooleanThe 'any' Type: Escape HatchThe 'unknown' TypeNil & Optional TypesType Inference in ActionExplicit Type AnnotationsRecap: Core Types Practice

5Aliases, Unions, Intersections

Type Aliases for PrimitivesUnion TypesWorking with Union TypesLiteral TypesIntersection TypesCombining Type AliasesRecap: Advanced Type Combos

8Enums, the Luau Way

The Enum Pattern in LuauNumeric Enums with TablesString Enums as UnionsUsing Literal Union EnumsFreezing Constant TablesRecap: Enums, the Luau Way

11Advanced Topics

Type AssertionsType Guards With typeofThe never TypeNil Safety In Strict ModeIndex SignaturesRecap: Config Value TypesRecap: Fine-Tuning Types

3Typed Tables: Arrays & Maps

Typed ArraysAdding and Reading ElementsWhat is a Map Type?Declaring and Accessing MapsIterating TablesMixed-Shape TablesReading a GridBuilding a Gridtable.unpackRecap: Arrays and Maps

6Typing Table Shapes

Inline Shape AnnotationsType Aliases for ShapesOptional PropertiesShapes vs. Loose TablesExtending ShapesAdding Methods to ShapesSelf and Colon MethodsRecap: Defining Table Shapes

9Generics: A First Look

The Problem Generics SolveA Generic Identity FunctionUsing a Generic FunctionGeneric ArraysGeneric Type AliasesRecap: Generic Functions

Practice on your own: Online Luau compiler