Generics and Type Parameters

Basic Type Parameters

package main

// Generic function with type parameter
func Max[T constraints.Ordered](a, b T) T {
    if a > b {
        return a
    }
    return b
}

// Multiple type parameters
func Map[T, 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
func main() {
    maxInt := Max(10, 20)           // T = int
    maxFloat := Max(3.14, 2.71)     // T = float64
    maxString := Max("abc", "xyz")  // T = string

    nums := []int{1, 2, 3}
    doubled := Map(nums, func(n int) int { return n * 2 })
    strings := Map(nums, func(n int) string { return fmt.Sprintf("%d", n) })
}

Type Constraints

import "constraints"

// Built-in constraints
type Number interface {
    constraints.Integer | constraints.Float
}

func Sum[T Number](numbers []T) T {
    var total T
    for _, n := range numbers {
        total += n
    }
    return total
}

// Custom constraints with methods
type Stringer interface {
    String() string
}

func PrintAll[T Stringer](items []T) {
    for _, item := range items {
        fmt.Println(item.String())
    }
}

// Approximate constraint using ~
type Integer interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64
}

type MyInt int

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

// Works with both int and MyInt
func main() {
    fmt.Println(Double(5))          // int
    fmt.Println(Double(MyInt(5)))   // MyInt
}

Generic Data Structures

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

func NewStack[T any]() *Stack[T] {
    return &Stack[T]{
        items: make([]T, 0),
    }
}

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
}

func (s *Stack[T]) IsEmpty() bool {
    return len(s.items) == 0
}

// Usage
intStack := NewStack[int]()
intStack.Push(1)
intStack.Push(2)

stringStack := NewStack[string]()
stringStack.Push("hello")
stringStack.Push("world")

Generic Map Operations

// Filter with generics
func Filter[T any](slice []T, predicate func(T) bool) []T {
    result := make([]T, 0, len(slice))
    for _, v := range slice {
        if predicate(v) {
            result = append(result, v)
        }
    }
    return result
}

// Reduce/Fold
func Reduce[T, U any](slice []T, initial U, fn func(U, T) U) U {
    acc := initial
    for _, v := range slice {
        acc = fn(acc, v)
    }
    return acc
}

// Keys from map
func Keys[K comparable, V any](m map[K]V) []K {
    keys := make([]K, 0, len(m))
    for k := range m {
        keys = append(keys, k)
    }
    return keys
}

// Values from map
func Values[K comparable, V any](m map[K]V) []V {
    values := make([]V, 0, len(m))
    for _, v := range m {
        values = append(values, v)
    }
    return values
}

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

sum := Reduce(numbers, 0, func(acc, n int) int { return acc + n })

m := map[string]int{"a": 1, "b": 2}
keys := Keys(m)     // []string{"a", "b"}
values := Values(m) // []int{1, 2}

Generic Pairs and Tuples

// Generic Pair
type Pair[T, U any] struct {
    First  T
    Second U
}

func NewPair[T, U any](first T, second U) Pair[T, U] {
    return Pair[T, U]{First: first, Second: second}
}

func (p Pair[T, U]) Swap() Pair[U, T] {
    return Pair[U, T]{First: p.Second, Second: p.First}
}

// Usage
pair := NewPair("name", 42)
swapped := pair.Swap() // Pair[int, string]

// Generic Result type (like Rust's Result<T, E>)
type Result[T any] struct {
    value T
    err   error
}

func Ok[T any](value T) Result[T] {
    return Result[T]{value: value}
}

func Err[T any](err error) Result[T] {
    return Result[T]{err: err}
}

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

func (r Result[T]) Unwrap() (T, error) {
    return r.value, r.err
}

func (r Result[T]) UnwrapOr(defaultValue T) T {
    if r.err != nil {
        return defaultValue
    }
    return r.value
}

Comparable Constraint

// Find using comparable
func Find[T comparable](slice []T, target T) (int, bool) {
    for i, v := range slice {
        if v == target {
            return i, true
        }
    }
    return -1, false
}

// Contains
func Contains[T comparable](slice []T, target T) bool {
    _, found := Find(slice, target)
    return found
}

// Unique elements
func Unique[T comparable](slice []T) []T {
    seen := make(map[T]struct{})
    result := make([]T, 0, len(slice))

    for _, v := range slice {
        if _, exists := seen[v]; !exists {
            seen[v] = struct{}{}
            result = append(result, v)
        }
    }

    return result
}

// Usage
nums := []int{1, 2, 2, 3, 3, 4}
unique := Unique(nums) // []int{1, 2, 3, 4}

idx, found := Find([]string{"a", "b", "c"}, "b") // 1, true

Generic Interfaces

// Generic interface
type Container[T any] interface {
    Add(item T)
    Remove() (T, bool)
    Size() int
}

// Implementation
type Queue[T any] struct {
    items []T
}

func (q *Queue[T]) Add(item T) {
    q.items = append(q.items, item)
}

func (q *Queue[T]) Remove() (T, bool) {
    if len(q.items) == 0 {
        var zero T
        return zero, false
    }
    item := q.items[0]
    q.items = q.items[1:]
    return item, true
}

func (q *Queue[T]) Size() int {
    return len(q.items)
}

// Function accepting generic interface
func ProcessContainer[T any](c Container[T], item T) {
    c.Add(item)
    fmt.Printf("Container size: %d\n", c.Size())
}

Type Inference

// Type inference works in most cases
func Identity[T any](x T) T {
    return x
}

// No need to specify type
result := Identity(42)          // T inferred as int
str := Identity("hello")        // T inferred as string

// Type inference with constraints
func Min[T constraints.Ordered](a, b T) T {
    if a < b {
        return a
    }
    return b
}

// Inferred from arguments
minVal := Min(10, 20)           // T = int
minFloat := Min(1.5, 2.5)       // T = float64

// Explicit type when needed
result := Map[int, string]([]int{1, 2}, func(n int) string {
    return fmt.Sprintf("%d", n)
})

Generic Channels

// Generic channel operations
func Merge[T any](channels ...<-chan T) <-chan T {
    out := make(chan T)
    var wg sync.WaitGroup

    for _, ch := range channels {
        wg.Add(1)
        go func(c <-chan T) {
            defer wg.Done()
            for v := range c {
                out <- v
            }
        }(ch)
    }

    go func() {
        wg.Wait()
        close(out)
    }()

    return out
}

// Generic pipeline stage
func Stage[T, U any](in <-chan T, fn func(T) U) <-chan U {
    out := make(chan U)
    go func() {
        defer close(out)
        for v := range in {
            out <- fn(v)
        }
    }()
    return out
}

// Usage
ch1 := make(chan int)
ch2 := make(chan int)

merged := Merge(ch1, ch2)

numbers := make(chan int)
doubled := Stage(numbers, func(n int) int { return n * 2 })
strings := Stage(doubled, func(n int) string { return fmt.Sprintf("%d", n) })

Union Constraints

// Union of types
type StringOrInt interface {
    string | int
}

func Process[T StringOrInt](val T) string {
    return fmt.Sprintf("%v", val)
}

// More complex unions
type Numeric interface {
    int | int8 | int16 | int32 | int64 |
    uint | uint8 | uint16 | uint32 | uint64 |
    float32 | float64
}

func Abs[T Numeric](n T) T {
    if n < 0 {
        return -n
    }
    return n
}

// Union with methods
type Serializable interface {
    string | []byte
}

func Serialize[T Serializable](data T) []byte {
    switch v := any(data).(type) {
    case string:
        return []byte(v)
    case []byte:
        return v
    default:
        panic("unreachable")
    }
}

Quick Reference

Feature	Syntax	Use Case
Basic generic	`func F[T any]()`	Any type
Constraint	`func F[T Constraint]()`	Restricted types
Multiple params	`func F[T, U any]()`	Multiple type variables
Comparable	`func F[T comparable]()`	Types supporting == and !=
Ordered	`func F[T constraints.Ordered]()`	Types supporting <, >, <=, >=
Union	`T interface{int \| string}`	Either type
Approximate	`~int`	Include type aliases

8.8 KiB Raw Blame History

Generics and Type Parameters

Basic Type Parameters

Type Constraints

Generic Data Structures

Generic Map Operations

Generic Pairs and Tuples

Comparable Constraint

Generic Interfaces

Type Inference

Generic Channels

Union Constraints

Quick Reference

8.8 KiB

Raw Blame History