{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE BangPatterns #-}
 
import Data.Foldable
 
data Stream a = forall s. Stream (s -> Maybe (a, s)) s
 
instance Foldable Stream where
  foldr k z (Stream sf s) = go (sf s)
    where
      go Nothing = z
      go (Just (e, ns)) = e `k` go (sf ns)
 
mysum = foldl' (+) 0
 
n :: Int
n = 100000000
 
x1 = [1..n]
x2 = Stream (\s -> if (s == n + 1) then Nothing else Just (s, s + 1)) 1
 
bestcase :: Int
bestcase = go 1 0 where
  go i c = if i == n then c + i else go (i+1) (c+i)
 
--main = print $ bestcase
--main = print $ mysum x1
--main = print $ mysum x2
 

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