Section 4 of 11

Closures

Functions are values, they capture variables, and they replace most small classes.

A closure captures the variable itself, not a copy of it, and keeps it alive after the enclosing function returns. Java forces captured locals to be effectively final; Go does not. This is why Go uses closures where Java would use a small stateful class.

The closure is the object; captured vars are the fields

go.dev playground

Try it: Call counter() twice into two variables — each closure gets its own independent `count`.

Stack three of these and you have the HTTP middleware pattern: an outer function captures config, the middle captures the next handler, and the inner one runs per request.

three layers, each returning the next

func WithTimeout(d time.Duration) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {          // captures d
        return http.HandlerFunc(func(w, r) {               // captures d AND next
            ...                                            // runs per request
        })
    }
}