Full Programs: Counters, once, Private State and Memoise

A customer clicked Alice's checkout button twice, because the page was slow, and the shop charged the card twice. The button's code did exactly what it was told, twice. The fix is a wrapper called once: it runs the payment the first time and ignores every later click. To do that, the wrapper must remember that it has already run, and that memory is a closure. This lesson writes once and five more programs that use closures on purpose.