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  • Fundamentals
    Functional Programming
    Description

    a haskell code

     Code 
    module NumberPrint where

import Control.Monad         (when)
import Data.IORef            (newIORef)
import Data.StateVar         (($=), ($~), get)
import Foreign.Ptr           (Ptr (..))
import Foreign.Marshal.Alloc (alloca)
import System.IO.Unsafe      (unsafePerformIO)

whileM :: Monad m => m Bool -> m () -> m ()
whileM mb a = do
  b <- mb
  when b $ a >> whileM mb a

numberprint :: Int -> Integer
numberprint n = read . unsafePerformIO $ do 
  alloca $ \(i :: Ptr Int)-> do
    i $= 1
    s <- newIORef ""
    whileM ((<n) <$> get i) $ do
      c <- get i
      s $~ (show c++)
      i $~ (1+)
    whileM ((>= 1) <$> get i) $ do
      c <- get i
      s $~ (show c++)
      i $~ (-1+)
    get s
    Test Cases 
    module NumberPrintSpec where

import Test.Hspec
import NumberPrint

spec :: Spec
spec = do
    describe "Tests" $ do
        it "Fixed Tests" $ do
            numberprint 1 `shouldBe` 1
            numberprint 2 `shouldBe` 121
            numberprint 10 `shouldBe` 12345678910987654321

main = hspec spec
    View Run Output
  • Code
    • module NumberPrint where
    • import Control.Monad (when)
    • import Data.IORef (newIORef)
    • import Data.StateVar (($=), ($~), get)
    • import Foreign.Ptr (Ptr (..))
    • import Foreign.Marshal.Alloc (alloca)
    • import System.IO.Unsafe (unsafePerformIO)
    • whileM :: Monad m => m Bool -> m () -> m ()
    • whileM mb a = do
    • b <- mb
    • when b $ a >> whileM mb a
    • numberprint :: Int -> Integer
    • numberprint = read . (>>= id) . ((++) <*> (reverse . init)) . (\x -> show <$> [1..x])
    • numberprint n = read . unsafePerformIO $ do
    • alloca $ \(i :: Ptr Int)-> do
    • i $= 1
    • s <- newIORef ""
    • whileM ((<n) <$> get i) $ do
    • c <- get i
    • s $~ (show c++)
    • i $~ (1+)
    • whileM ((>= 1) <$> get i) $ do
    • c <- get i
    • s $~ (show c++)
    • i $~ (-1+)
    • get s