Closures and captured state

The idea

A closure is a value that can be passed to another function or stored for later use. Its parameter and return types describe the behavior a caller expects. A function accepting (Int) -> Bool can reuse one traversal with different selection rules. Short closure syntax is convenient, but named parameters often make a complex transformation easier to read and reason about.

Closures can capture values from the surrounding scope. A factory that declares a local counter and returns a closure creates independent retained state on each factory call. Two references to one returned closure share its state. Captures can also keep class instances alive, so delayed or stored callbacks need a deliberate ownership policy. Treat side effects as part of the closure’s contract instead of assuming every map or filter operation is pure.

func makeCounter(_ start:Int)->()->Int {var n=start; return {n += 1; return n}}
let a=makeCounter(0)
let b=makeCounter(10)
print(a())
print(a())
print(b())

1
2
11

Each factory call captures its own local variable, while repeated calls through a retain its updated state.