how to increment a variable in functional programming

Question

How do you increment a variable in a functional programming language?

For example, I want to do:

main :: IO ()
main = do
    let i = 0
    i = i + 1
         


        

Answer 1

There are several solutions to translate imperative i=i+1 programming to functional programming. Recursive function solution is the recommended way in functional programming, creating a state is almost never what you want to do.

After a while you will learn that you can use [1..] if you need a index for example, but it takes a lot of time and practice to think functionally instead of imperatively.

Here's a other way to do something similar as i=i+1 not identical because there aren't any destructive updates. Note that the State monad example is just for illustration, you probably want [1..] instead:

module Count where
import Control.Monad.State

count :: Int -> Int
count c = c+1

count' :: State Int Int
count' = do
    c <- get
    put (c+1)
    return (c+1)

main :: IO ()
main = do
            -- purely functional, value-modifying (state-passing) way:
    print $ count . count . count . count . count . count $ 0
            -- purely functional, State Monad way
    print $ (`evalState` 0) $ do { 
            count' ; count' ; count' ; count' ; count' ; count' }