4 ms·

Not neccessarily, when your program is (tail-)recursive you can carry the "current" value along without ever really mutating it. Haskell has the State monad whi

• andrekandre 5y ago
  > when your program is (tail-)recursive you can carry the "current" value along without ever really mutating it right, what im curious though is, how one would go about making an app like say, itunes or the appstore in that way... it probably would go along way to help people understand these concepts
  • thewakalix 5y ago
    If your program interacts with the outside world, then yes, it needs to be impure "along the edges" at least. But it can still keep track of state in a pure, functional way. Here's a simple example: main :: IO () main = loop (0 :: Int) where loop n = do putStrLn $ "The counter is currently " <> show n <> ". Increase it by how much?" increase <- readLn loop $ n + increase Here, the counter is updated purely, rather than by mutation.
  • dtagames 5y ago
    You can't. No useful program is stateless. Only examples and proofs-of-concept can be purely functional. The lambda calculus is a way to define math in the functional style. Guess what? It needs state passed in to work. The names and orders of the integers must be passed in and held in a "state" in order to do anything useful. Since functional programming is centered around the concepts from the lambda calculus, this implies that all functional programs must have some "state" somewhere in order to start or do useful work.