Imperative Haskell

fib.hs

{-# LANGUAGE UnicodeSyntax #-}
module Main
where

import Control.Monad.ST
import Data.STRef
import Prelude.Unicode -- from base-unicode-symbols
import System.Environment
import Control.Monad.Loops

fib ∷ ℤ → ℤ
fib n = runST $ do
  a ← newSTRef 0
  b ← newSTRef 1
  counter ← newSTRef 1
  whileM_ ((< n) <$> readSTRef counter) $ do
    oldA ← readSTRef a
    writeSTRef a =<< readSTRef b
    modifySTRef' b (+ oldA)
    modifySTRef' counter (+ 1)
  readSTRef b

run ∷ [String] → IO ()
run [n] = print $ fib (read n)
run _ = error "I want one number n, and will give the nth fibonacci number"

main ∷ IO ()
main = getArgs >>= run