Generics in Go

Overview

Go 1.18 introduced Generics, allowing functions and types to work with any set of types while maintaining type safety. This eliminates the need for code duplication and unsafe type assertions.

Before generics, you had to either duplicate code for each type or use interface{} and lose type safety.

The Problem Generics Solve

Without Generics
With Generics

// Need separate functions for each type
func PrintInt(v int) {
    fmt.Println(v)
}

func PrintString(v string) {
    fmt.Println(v)
}

func PrintUser(v User) {
    fmt.Println(v)
}

// Or lose type safety with interface{}
func Print(v interface{}) {
    fmt.Println(v)  // No compile-time type checking
}

task1/main.go

package main

import "fmt"

func Print[T any](v T) {
    fmt.Println(v)
}

func main() {
    Print(10)           // T is int
    Print("golang")     // T is string
    Print(true)         // T is bool

    type User struct {
        name string
    }

    u := User{name: "Ravi"}
    Print(u)            // T is User
}

One function works for all types!

Basic Syntax: Type Parameters

The syntax [T any] defines a type parameter:

func FunctionName[T TypeConstraint](param T) T {
    // T can be used like any other type
    return param
}

Square Brackets [T any]

Declares a type parameter named T with the constraint any.

The any Constraint

any is an alias for interface{}, meaning “any type is allowed.”

Use T Like a Type

Inside the function, T acts like a normal type.

Generic Functions

Simple Generic Function

task3/main.go

package main

func Identity[T any](a T) T {
	return a
}

func main() {
   Identity(10)        // Returns 10 (int)
   Identity("Hello")   // Returns "Hello" (string)
   Identity(2.67)      // Returns 2.67 (float64)
}

The compiler infers the type parameter automatically from the arguments.

Generic Swap Function

task4/main.go

package main

import "fmt"

func Swap[T any](a, b *T) {
	*a, *b = *b, *a
}

func main() {
	x := 10
	y := 20
	Swap(&x, &y)
	fmt.Println(x, y)  // 20 10

	s1 := "hello"
	s2 := "world"
	Swap(&s1, &s2)
	fmt.Println(s1, s2)  // world hello
}

Without generics, you’d need SwapInt, SwapString, SwapFloat, etc. Or use reflection, which is slow and loses type safety.

Generic Types

You can also create generic structs:

task2/main.go

package main

import "fmt"

type Box[T any] struct {
	value T
}

func main() {
	intBox := Box[int]{value: 10}
	strBox := Box[string]{value: "golang"}

	fmt.Println(intBox.value)  // 10
	fmt.Println(strBox.value)  // golang
}

Real-World Use Case:

type Stack[T any] struct {
    items []T
}

func (s *Stack[T]) Push(item T) {
    s.items = append(s.items, item)
}

func (s *Stack[T]) Pop() (T, bool) {
    if len(s.items) == 0 {
        var zero T
        return zero, false
    }
    item := s.items[len(s.items)-1]
    s.items = s.items[:len(s.items)-1]
    return item, true
}

// Usage
intStack := Stack[int]{}
intStack.Push(1)
intStack.Push(2)
val, _ := intStack.Pop()  // val is int, guaranteed

Type Constraints

any is too permissive sometimes. You might need to restrict which types are allowed.

Built-in Constraints

import "golang.org/x/exp/constraints"

// Only numeric types
func Sum[T constraints.Ordered](a, b T) T {
    return a + b
}

Sum(10, 20)       // ✅ Works
Sum(1.5, 2.3)     // ✅ Works
Sum("a", "b")     // ❌ Compile error: strings don't support +

Common Constraints

any
comparable
constraints.Ordered
constraints.Integer

func Print[T any](v T) {
    fmt.Println(v)
}

Allows any type.

func Contains[T comparable](slice []T, item T) bool {
    for _, v := range slice {
        if v == item {  // Requires ==
            return true
        }
    }
    return false
}

Types that support == and !=.

import "golang.org/x/exp/constraints"

func Max[T constraints.Ordered](a, b T) T {
    if a > b {  // Requires >
        return a
    }
    return b
}

Types that support <, >, <=, >=.

func Double[T constraints.Integer](n T) T {
    return n * 2
}

Only integer types (int, int8, int16, uint, etc.).

Custom Constraints

You can define your own constraints using interfaces:

// Only types with a String() method
type Stringer interface {
    String() string
}

func PrintString[T Stringer](v T) {
    fmt.Println(v.String())
}

// Usage
type User struct {
    Name string
}

func (u User) String() string {
    return u.Name
}

u := User{Name: "Alice"}
PrintString(u)  // ✅ User has String() method

Union Constraints

Allow specific types using the | operator:

type Number interface {
    int | int64 | float64
}

func Add[T Number](a, b T) T {
    return a + b
}

Add(10, 20)          // ✅ int
Add(1.5, 2.5)        // ✅ float64
Add("a", "b")        // ❌ Compile error: string not in union

Generic Data Structures

Generic Linked List

type Node[T any] struct {
    Value T
    Next  *Node[T]
}

type LinkedList[T any] struct {
    Head *Node[T]
}

func (l *LinkedList[T]) Add(value T) {
    node := &Node[T]{Value: value, Next: l.Head}
    l.Head = node
}

func (l *LinkedList[T]) Print() {
    current := l.Head
    for current != nil {
        fmt.Println(current.Value)
        current = current.Next
    }
}

// Usage
list := LinkedList[int]{}
list.Add(1)
list.Add(2)
list.Add(3)
list.Print()  // 3, 2, 1

Generic Map Functions

func Map[T any, U any](slice []T, fn func(T) U) []U {
    result := make([]U, len(slice))
    for i, v := range slice {
        result[i] = fn(v)
    }
    return result
}

// Usage
numbers := []int{1, 2, 3, 4}
doubled := Map(numbers, func(n int) int {
    return n * 2
})  // [2, 4, 6, 8]

strings := Map(numbers, func(n int) string {
    return fmt.Sprintf("#%d", n)
})  // ["#1", "#2", "#3", "#4"]

Generic Filter

func Filter[T any](slice []T, predicate func(T) bool) []T {
    result := []T{}
    for _, v := range slice {
        if predicate(v) {
            result = append(result, v)
        }
    }
    return result
}

// Usage
numbers := []int{1, 2, 3, 4, 5, 6}
evens := Filter(numbers, func(n int) bool {
    return n%2 == 0
})  // [2, 4, 6]

Type Inference

Go can infer type parameters in most cases:

result := Max[int](10, 20)

You only need explicit type parameters when:

The compiler can’t infer the type
You want to be explicit for clarity
You’re creating a value of a generic type

// Explicit needed here
stack := Stack[int]{}  // What would T be without [int]?

// Inferred here
stack.Push(42)  // Go knows T is int from the stack type

When to Use Generics

Good Use Cases
When NOT to Use

1. Data Structures

type Stack[T any] struct { /* ... */ }
type Queue[T any] struct { /* ... */ }
type Tree[T comparable] struct { /* ... */ }

2. Algorithms

func Sort[T constraints.Ordered](slice []T) { /* ... */ }
func BinarySearch[T comparable](slice []T, target T) int { /* ... */ }

3. Utility Functions

func Map[T, U any](slice []T, fn func(T) U) []U { /* ... */ }
func Filter[T any](slice []T, fn func(T) bool) []T { /* ... */ }
func Reduce[T, U any](slice []T, init U, fn func(U, T) U) U { /* ... */ }

4. Type-Safe Wrappers

type Result[T any] struct {
    Value T
    Err   error
}

type Option[T any] struct {
    value   T
    present bool
}

1. Simple cases

// ❌ Overkill
func Add[T int](a, b T) T { return a + b }

// ✅ Better
func Add(a, b int) int { return a + b }

2. When interfaces are clearer

// ❌ Worse
func Process[T Processor](p T) { p.Process() }

// ✅ Better
func Process(p Processor) { p.Process() }

3. Over-abstraction

// ❌ Too complex
func DoThing[T any, U comparable, V constraints.Ordered](
    a T, b U, c V) (T, U, V) { /* ... */ }

Don’t add generics just because you can. Use them when they eliminate real duplication or improve type safety.

Generics vs Interfaces

When to Use Which

Use Generics When:

You need the exact same logic for multiple types
You want compile-time type safety
You’re building data structures or algorithms

// Generic: Type is known at compile time
func First[T any](slice []T) T {
    return slice[0]
}

Use Interfaces When:

You need runtime polymorphism
Different types have different implementations
You want dependency injection

// Interface: Implementation varies by type
type Shape interface {
    Area() float64
}

func TotalArea(shapes []Shape) float64 {
    total := 0.0
    for _, s := range shapes {
        total += s.Area()  // Different for each shape
    }
    return total
}

Best Practices

Use descriptive type parameter names

// ❌ Unclear
func Process[A any, B any](a A, b B) {}

// ✅ Clear
func Convert[Input any, Output any](in Input) Output {}

// ✅ Standard conventions
func Map[T any, U any](slice []T, fn func(T) U) []U {}

Keep constraints minimal

// ❌ Too restrictive
func Print[T fmt.Stringer](v T) {
    fmt.Println(v.String())
}

// ✅ More flexible
func Print[T any](v T) {
    fmt.Println(v)
}

Prefer type inference

// ❌ Explicit (unnecessary)
result := Max[int](10, 20)

// ✅ Inferred (cleaner)
result := Max(10, 20)

Don't over-generalize

Only add generics when you have at least 2-3 use cases.

“A little copying is better than a little dependency” - Rob Pike

Real-World Example: Result Type

type Result[T any] struct {
    Value T
    Err   error
}

func (r Result[T]) IsOk() bool {
    return r.Err == nil
}

func (r Result[T]) Unwrap() (T, error) {
    return r.Value, r.Err
}

// Usage
func FetchUser(id int) Result[User] {
    user, err := db.GetUser(id)
    return Result[User]{Value: user, Err: err}
}

result := FetchUser(123)
if result.IsOk() {
    fmt.Println(result.Value.Name)
} else {
    fmt.Println("Error:", result.Err)
}

Key Takeaways

Generics provide type-safe code reuse without duplication
Use [T any] for type parameters
Constraints limit which types are allowed (any, comparable, custom interfaces)
Type inference works in most cases—explicit types rarely needed
Use generics for data structures and algorithms, not everything
Interfaces and generics solve different problems—use the right tool

Next Steps

Error Handling - Custom error types and patterns
Concurrency - Goroutines, channels, and patterns

Get Started

Fundamentals

Advanced Concepts

Web Development

Projects

Overview

The Problem Generics Solve

Basic Syntax: Type Parameters

Generic Functions

Simple Generic Function

Generic Swap Function

Generic Types

Type Constraints

Built-in Constraints

Common Constraints

Custom Constraints

Union Constraints

Generic Data Structures

Generic Linked List

Generic Map Functions

Generic Filter

Type Inference

When to Use Generics

Generics vs Interfaces

Best Practices

Real-World Example: Result Type

Key Takeaways

Next Steps

Build docs developers (and LLMs) love

Get Started

Fundamentals

Advanced Concepts

Web Development

Projects

​Overview

​The Problem Generics Solve

​Basic Syntax: Type Parameters

​Generic Functions

​Simple Generic Function

​Generic Swap Function

​Generic Types

​Type Constraints

​Built-in Constraints

​Common Constraints

​Custom Constraints

​Union Constraints

​Generic Data Structures

​Generic Linked List

​Generic Map Functions

​Generic Filter

​Type Inference

​When to Use Generics

​Generics vs Interfaces

​Best Practices

​Real-World Example: Result Type

​Key Takeaways

​Next Steps

Build docs developers (and LLMs) love

Overview

The Problem Generics Solve

Basic Syntax: Type Parameters

Generic Functions

Simple Generic Function

Generic Swap Function

Generic Types

Type Constraints

Built-in Constraints

Common Constraints

Custom Constraints

Union Constraints

Generic Data Structures

Generic Linked List

Generic Map Functions

Generic Filter

Type Inference

When to Use Generics

Generics vs Interfaces

Best Practices

Real-World Example: Result Type

Key Takeaways

Next Steps