eff.swift

func effIt<Target, Result>(_ keypath: KeyPath<Target, Result>) -> (Target) -> Result {
    return { $0[keyPath: keypath] }
}

struct Foo {
    var x: Int
    var y: Int
}

let f = effIt(\Foo.x)
let foo = Foo(x: 42, y: 43)
print(f(foo))