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A Generic Identity Function

Part of the Introduction To Luau section of Coddy's Lua journey. Lesson 62 of 80.

Here's the identity function done right, as a generic function:

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

The <T> after the function name declares a type parameter: a placeholder that stands for "some type, decided at each call". Inside the signature you use T exactly like a real type: the parameter is a T, and the function returns a T.

Using the same T for input and output is the whole trick. It ties them together. Call identity("hello") and the checker decides T = string for that call, so the result is a string. Call identity(42) and T = number, so the result is a number. One body, every type, nothing lost.

T is only a conventional name (short for "Type"): <Item> or <Value> work just as well. And as always in Luau, the angle brackets exist for the checker only: at runtime this is the plain Lua function function identity(value) return value end.

challenge icon

Challenge

Easy

Write one function that returns any value with its type intact.

  1. Write local function identity<T>(value: T): T that returns value.
  2. Declare local luckyNumber: number = identity(7) and local greeting: string = identity("Hello, Luau!").
  3. Print luckyNumber, then greeting.

Press Run Code: the output shows 7 and Hello, Luau!.

Try it yourself

-- 1) identity<T>(value: T): T
-- 2) luckyNumber and greeting from identity calls
-- 3) print both

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