What is the function-as-interface pattern in Go and how does it enable flexible APIs?

Go allows you to declare a named function type and attach methods to it — making it satisfy an interface. This eliminates the need for a wrapper struct when the only state needed is the function itself. It is used extensively in net/http, testing, and plugin architectures.

// Define an interface
type Transformer interface {
    Transform(s string) string
}

// Named function type
type TransformFunc func(string) string

// Implement the interface on the function type
func (f TransformFunc) Transform(s string) string { return f(s) }

// Usage: pass a closure directly
func applyAll(s string, ts ...Transformer) string {
    for _, t := range ts {
        s = t.Transform(s)
    }
    return s
}

result := applyAll("  hello world  ",
    TransformFunc(strings.TrimSpace),
    TransformFunc(strings.ToUpper),
    TransformFunc(func(s string) string { return "[" + s + "]" }),
)
fmt.Println(result) // [HELLO WORLD]

// Struct implementing the same interface — full control over state
type PrefixTransformer struct{ Prefix string }
func (p PrefixTransformer) Transform(s string) string {
    return p.Prefix + s
}

result2 := applyAll("world",
    PrefixTransformer{Prefix: "Hello, "},
    TransformFunc(strings.ToUpper),
)
fmt.Println(result2) // HELLO, WORLD

The pattern makes APIs flexible: callers can provide a simple closure or a stateful struct — both satisfy the interface. The API never needs to know which was used. This is the basis of middleware chains, plugin hooks, and event callbacks in Go.

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