If it's necessary, program it as implicit state in a recursive loop, or as an FRP signal, or as one of several varieties of mutable cells which have automatic n

If it's necessary, program it as implicit state in a recursive loop, or as an FRP signal, or as one of several varieties of mutable cells which have automatic nice multithreading stories. FP has mutable state. People keep saying it doesn't because it's the first chance they've had to play with a keyboard that doesn't have a caps lock and they've realized that the number of times they accidentally TYPED A WHOLE PARAGRAPH IN UPCAPS has gone to zero and that tiny mental burden of tracking the state of the keyboard has vanished. Removing mutable state improves trust.