pythagoreans.hs

module Main
  ( main
  ) where

import Control.Arrow ((&&&))
import Control.Monad.Trans.State (StateT(StateT), evalStateT)
import Data.List (uncons, unfoldr)
import Data.Maybe (fromMaybe, listToMaybe)
import System.Environment (getArgs)

unconses :: [a] -> [(a, [a])]
unconses = unfoldr (((id &&& snd) <$>) . uncons)

triples :: Int -> [(Int, Int, Int)]
triples =
  evalStateT ((,,) <$> StateT unconses <*> StateT unconses <*> StateT unconses) .
  enumFromTo 1

pythagoreans :: Int -> [(Int, Int, Int)]
pythagoreans = filter (\(a, b, c) -> a ^ 2 + b ^ 2 == c ^ 2) . triples

main :: IO ()
main = do
  n <- maybe 100 read . listToMaybe <$> getArgs
  mapM_ print $ pythagoreans n