What is the empty interface (any / interface{}) and when should you use it?

An interface with zero methods is satisfied by every type in Go. Written as interface{} or its alias any (Go 1.18+), it lets a variable or function parameter hold a value of any type. It is Go's mechanism for truly generic containers — at the cost of compile-time type safety.

// any is an alias for interface{} (Go 1.18+)
func describe(v any) {
    fmt.Printf("Type: %-10T  Value: %v\n", v, v)
}

describe(42)              // Type: int         Value: 42
describe("hello")         // Type: string      Value: hello
describe([]int{1, 2, 3})  // Type: []int       Value: [1 2 3]

// Heterogeneous collection
row := []any{1, "Alice", true, 3.14}

// Must type-assert to get the concrete value back
var v any = "hello"
s, ok := v.(string)  // safe — ok=false if not a string
// s2 := v.(int)     // panics if v is not an int

// Type switch — handle multiple possible types
for _, item := range row {
    switch x := item.(type) {
    case int:    fmt.Println("int:", x)
    case string: fmt.Println("string:", x)
    case bool:   fmt.Println("bool:", x)
    default:     fmt.Println("other:", x)
    }
}

// PREFER: narrow interfaces, specific types, or generics over 'any'
// 'any' loses compile-time type safety — errors become runtime panics

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