Identity in haskell...

Foo.hs

module Main where

import Unsafe.Coerce

-- ideq is strict in its parameters
ideq :: a -> a -> Bool
ideq a b = a `seq` b `seq` aid == bid
  where aid = unsafeCoerce a :: Int
        bid = unsafeCoerce b :: Int

ones :: [Int]
ones = 1 : ones

yksikaksi :: [Int]
yksikaksi = 1 : 2 : yksikaksi

nan :: Double
nan = 0/0

main :: IO ()
main = do
  print $ ones `ideq` ones
  print $ ones `ideq` yksikaksi
  print $ ones `ideq` tail ones
  print $ yksikaksi `ideq` tail yksikaksi
  print $ yksikaksi `ideq` tail (tail yksikaksi)
  print $ nan == nan
  print $ nan `ideq` nan

output.txt

True
False
True
False
True
False
True