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@bradparker /Main.hs
Last active Oct 11, 2018

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Essence of the iterator pattern word count
module Main where
import Data.Bool (bool)
import Control.Monad ((<=<))
import Control.Monad.State (State, evalState, get, put)
import Data.Char (isSpace)
import Data.Foldable (traverse_)
import Data.Functor.Compose (Compose(Compose, getCompose))
import Data.Functor.Const (Const(Const, getConst))
import Data.Functor.Product (Product(Pair))
import Data.Maybe (listToMaybe)
import Data.Monoid (Sum(Sum, getSum))
import Prelude hiding (lines, words)
import System.Environment (getArgs)
prod :: (a -> f b) -> (a -> g b) -> a -> Product f g b
prod f g a = Pair (f a) (g a)
runProd :: (f a -> g a -> b) -> Product f g a -> b
runProd f (Pair fa ga) = f fa ga
type Count = Const (Sum Integer)
getCount :: Count a -> Integer
getCount = getSum . getConst
count :: Count a
count = Const (Sum 1)
ignore :: Count a
ignore = Const (Sum 0)
countWhen :: Bool -> Count a
countWhen = bool ignore count
chars :: Char -> Count ()
chars = const count
lines :: Char -> Count ()
lines = countWhen . ('\n' ==)
type StatefulCount s = Compose (State s) Count
runStatefulCount :: StatefulCount s a -> s -> Integer
runStatefulCount c s = getCount (evalState (getCompose c) s)
words :: Char -> StatefulCount Bool ()
words c =
Compose $
curry (countWhen . uncurry ((&&) . not))
<$> get
<* put (not (isSpace c))
<*> get
data WordCount = WordCount
{ wcChars :: Integer
, wcLines :: Integer
, wcWords :: Integer
} deriving (Show)
wordCount :: String -> WordCount
wordCount =
runProd (runProd runAll) . traverse (chars `prod` lines `prod` words)
where
runAll c l w =
WordCount (getCount c) (getCount l) (runStatefulCount w False)
-- Result:
-- $ runhaskell Main.hs ./Main.hs
-- WordCount {wcChars = 1857, wcLines = 73, wcWords = 306}
main :: IO ()
main = traverse_ (print . wordCount <=< readFile) =<< listToMaybe <$> getArgs
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