How do generics work in Go?

Question

Accepted Answer

**Generics** (added in Go 1.18) let you write functions and types that work with **multiple types** via type parameters, while keeping compile-time type safety. They eliminate the old trade-off of either duplicating code per type or losing safety with `interface{}`.

## The problem generics solve

```go
// ❌ before generics — either duplicate per type...
func MaxInt(a, b int) int { ... }
func MaxFloat(a, b float64) float64 { ... }   // copy-paste for each type

// ...or use interface{} and lose type safety + need casts/assertions
func Max(a, b interface{}) interface{} { ... }   // unsafe, clunky
```

## A generic function

```go
// T is a type parameter constrained to be "ordered" (comparable with < >)
func Max[T constraints.Ordered](a, b T) T {
    if a > b {
        return a
    }
    return b
}

Max(3, 5)         // T inferred as int → 5
Max(2.5, 1.1)     // T inferred as float64 → 2.5
Max("a", "b")     // T inferred as string → "b"
```

The `[T constraints.Ordered]` declares a type parameter `T` with a **constraint** (what types are allowed). The compiler infers `T` from the arguments — one implementation works for many types, fully type-safe.

## Type constraints

```go
// a constraint is an interface listing allowed types or required methods
type Number interface {
    ~int | ~int64 | ~float64        // the | union means "any of these"
}

func Sum[T Number](nums []T) T {
    var total T
    for _, n := range nums {
        total += n                   // safe — T is constrained to numeric types
    }
    return total
}
Sum([]int{1, 2, 3})       // 6
Sum([]float64{1.5, 2.5})  // 4.0

// built-in constraints:
any                        // any type (alias for interface{})
comparable                 // types usable with == (for map keys, etc.)
// golang.org/x/exp/constraints: Ordered, Integer, Float, etc.
```

Constraints define what operations are valid on the type parameter (the `~` means "any type whose underlying type is this").

## Generic types

```go
// a generic stack that works for any type
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 {
    item := s.items[len(s.items)-1]
    s.items = s.items[:len(s.items)-1]
    return item
}

intStack := Stack[int]{}       // a stack of ints
strStack := Stack[string]{}     // a stack of strings — same code
```

## When to use (and not) generics

```text
✓ Generic data structures (stacks, trees, sets), utility functions over collections
  (Map, Filter, Reduce), type-safe containers
✗ Don't overuse — if interfaces or a concrete type work simply, prefer them.
  Go's philosophy still favors simplicity; generics are a tool, not a default.
```

## Why it matters

Generics were a major, long-awaited addition to Go (1.18) that resolved a real pain point: before them, writing reusable code for multiple types forced you to either duplicate it per type (verbose, unmaintainable) or use `interface{}` (losing compile-time type safety and requiring runtime assertions).

Generics let you write type-safe, reusable functions and data structures (containers, collection utilities like Map/Filter/Reduce) once, with the compiler enforcing correctness and inferring types.

Understanding type parameters, constraints (including `any`, `comparable`, and the union/`~` syntax), and generic types is increasingly important as the ecosystem adopts them.

Equally important is the Go mindset that generics are a **tool for genuine reuse, not a default** — overusing them works against Go's simplicity philosophy.

This is a current, frequently-discussed topic reflecting fluency with modern Go.